Journal of Petrology Advance Access originally published online on March 7, 2006
Journal of Petrology 2006 47(6):1177-1220; doi:10.1093/petrology/egl007
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Post-collisional, Potassic and Ultrapotassic Magmatism of the Northern Tibetan Plateau: Constraints on Characteristics of the Mantle Source, Geodynamic Setting and Uplift Mechanisms
1 INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, P.O. BOX 9825, BEIJING 100029, PEOPLE'S REPUBLIC OF CHINA
2 INSTITUTE OF GEOPHYSICS AND TECTONICS, SCHOOL OF EARTH AND ENVIRONMENT, UNIVERSITY OF LEEDS, LEEDS LS2 9JT, UK
RECEIVED JANUARY 10, 2005; ACCEPTED FEBRUARY 6, 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Cenozoic, post-collisional, potassic and ultrapotassic igneous rocks in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes of the northern Tibetan Plateau are distributed along a semi-continuous, east–west-trending, volcanic belt, which is over 1200 km in length. Spatially, there is a close association with major strike-slip faults, thrust faults and pull-apart basins. The ages of these magmatic rocks range from 45 Ma to the present (the youngest known eruption occurred in 1951); they are shoshonitic, compositionally similar to K-rich subduction-related magmas, and range in SiO2 from 44 to 66 wt %. There is a relative enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) in the most primitive magmatic rocks (MgO >6 wt %) in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes; correspondingly, the primitive magmas have higher 87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in the North Qiangtang terrane than in the Songpan–Ganzi and North Kunlun terranes. The dominant factors that control the geochemical characteristics of the magmas are an enriched asthenospheric mantle source composition, the degree of partial melting of this source, and the combined processes of crustal assimilation and fractional crystallization (AFC). Enrichment of the asthenosphere is considered to have occurred by incorporation of subducted sediments into the mantle wedge above a subducted slab of Indian lithosphere during India–Asia convergence. Continental lithospheric mantle, metasomatically enriched during earlier episodes of subduction, may have also contributed a source component to the magmas. Trace element modelling indicates that the mantle source of the most primitive magmas in the North Qiangtang terrane contained higher amounts of subducted sediment (0·5–10%) compared with those in the Songpan–Ganzi and North Kunlun terranes (<2%). The degrees of partial melting required to generate the primitive potassic and ultrapotassic magmas from the enriched mantle sources range from <0·1% to
15% in the three major basement terranes. Energy-constrained AFC model calculations show that the more evolved magmatic rocks (MgO <6 wt %) are the results of AFC processes in the middle crust in the North Qiangtang terrane and the upper crust in the Songpan–Ganzi and North Kunlun terranes. We propose that the ultimate driving force for the generation of the post-collisional potassium-rich magmatism in north Tibet is the continuous northward underthrusting of the Indian continental lithosphere following India–Asia collision. This underthrusting resulted in upwelling of hot asthenosphere beneath north Tibet, squeezed up between the advancing Indian lithosphere and the backstop of the rigid Asian continental lithosphere. Asthenospheric upwelling may have also contributed to uplift of the northern Tibetan Plateau. KEY WORDS: Tibetan Plateau; potassic and ultrapotassic magmatism; enriched asthenospheric mantle source; EC-AFC modelling; geodynamics
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Cenozoic, post-collisional, potassium-rich igneous rocks form a nearly east–west-trending, semi-continuous, magmatic province in the northern Tibetan Plateau, mainly distributed within the North Kunlun, Songpan–Ganzi and North Qiangtang terranes (Fig. 1). The magmatism is considered to be an indicator of evolving mantle dynamics associated with subduction of Indian continental lithosphere and uplift of the northern Tibetan Plateau following India–Asia collision (Arnaud et al., 1992
; Turner et al., 1993, 1996a; Deng, 1998; Pan et al., 1998; Liu, 1999; Ding et al., 2003). Although there have been many studies of these magmatic rocks since 1946 (e.g. Norin, 1946; Pearce & Mei, 1988; Turner et al., 1996a; Lai et al., 2003; Deng et al., 2004; Williams et al., 2004), their petrogenesis and geodynamic setting remain controversial. Magma generation has been ascribed to convective removal of the lower part of the lithospheric mantle (e.g. Turner et al., 1993, 1996a; Williams et al., 2004) and to intracontinental subduction of the Tarim and Qaidam terranes (Fig. 1) beneath the northern Tibetan Plateau (e.g. Pearce & Mei, 1988; Deng, 1989, 1991, 1998; Arnaud et al., 1992; Willet & Beaumont, 1994; Jin et al., 1996; Meyer et al., 1998; Tapponnier et al., 2001; Ding et al., 2003). Williams et al. (2004) have suggested that the magmatism in north and south Tibet may have a different petrogenesis; magma generation in north Tibet may be triggered by convective removal of the lower part of the lithospheric mantle, whereas that in south Tibet may be controlled by subducted slab break-off, probably implying differences in uplift mechanism between the northern and southern parts of the Tibetan Plateau. Post-collisional igneous rocks spanning 45 Myr (Fig. 2) make the Tibetan Plateau magmatism truly unique. The lack of detailed studies of the temporal and spatial changes in the petrological and geochemical characteristics of the magmatic rocks, particularly given the relative rarity of primitive compositions (MgO >6 wt %), has precluded further constraints on the nature of their mantle source region, petrogenesis, tectonic setting, and the uplift mechanism of the northern Tibetan Plateau.
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This study focuses on potassium-rich magmatism in 25 volcanic fields in north Tibet, extending across a region some 1200 km from west to east and 350 km from north to south (Fig. 1). Emphasis is placed on the magmatism within the North Qiangtang, Songpan–Ganzi and North Kunlun terranes, for which there are relatively few published geochemical data, mainly because of the inaccessibility of the area. The samples studied include a wide range of rock types and cover the complete age range of post-collisional magmatism from
45 Ma to AD 1951 in the different basement terranes (Table 1 and Fig. 2). Six of the samples studied (JH23, JH6, XT8, KX91, KX84 and KX67; Table 2), with MgO >6 wt % (Table 3 and Electronic Appendix), contain mantle xenoliths, indicating short residence times of these magmas in the crust. Compared with previous studies (e.g. Arnaud et al., 1992; Turner et al., 1993, 1996a; Williams et al., 2004), which were predominantly based on igneous rocks with MgO <6 wt %, we have much better coverage of the more primitive magma compositions over a much wider geographical area. We present new major and trace element analyses for 50 samples and Sr–Nd–Pb isotopic data for a subset of 30 samples. On the basis of these and previously published geochronological, geochemical and geophysical data, we develop a model to explain the petrogenesis and geodynamic setting of the post-collisional potassium-rich magmatism.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Tertiary–recent potassium-rich magmatism in the northern Tibetan Plateau
The potassic and ultrapotassic magmatic rocks that form the basis of this study are located in the North Kunlun, Songpan–Ganzi and North Qiangtang terranes (Fig. 1). The ages of these rocks (Fig. 2) range from c. 45 Ma to the present (i.e. AD 1951) on the basis of five thermoluminescence (TL) ages, two 14C ages, one single-zircon U–Pb age, 78 K–Ar and 36 40Ar/39Ar ages, as well as historical records of an active volcanic eruption in Ashikule in AD 1951 [Harris et al., 1988b
; Pearce & Mei, 1988; Deng, 1989, 1991, 1993, 1998; Li et al., 1989; Liu, 1989, 1999; Liu & Xie, 1989; Liu & Maimaiti, 1990; Arnaud et al., 1992; Sun, 1992; Turner et al., 1993, 1996a; Xinjiang Bureau of Geology and Mineral Resources (XBGMR), 1993; Zhang & Zheng, 1994; Deng et al., 1996; Chi et al., 1999; Ding et al., 1999, 2003; Tan et al., 2000; Wu et al., 2001; Luo et al., 2003; Williams et al., 2004; Wang et al., 2005]. The magmatism post-dates the onset of India–Asia continental collision in southern Tibet at 65–70 Ma (Yin & Harrison, 2000), showing that it was generated in a post-collisional tectonic setting. Temporally, magmatism has been semi-continuous since 45 Ma; spatially, it becomes progressively younger from south to north (Fig. 2a). Magmatic activity is characterized by scattered, small-volume lava flows, pyroclastic deposits, cinder cones and volcanic necks. The volcanic fields studied have outcrops that vary from >1000 km2 (e.g. the Duogecuoren and Qiangbaqian fields) to <2 km2 (e.g. the Dahongliutan field); most are relatively small (Table 1). Some of the volcanic rocks studied here (e.g. the Xiongyingtai, Jingyuhu and Kangxiwa fields; Tables 2 and 3) contain mantle xenoliths (Luo et al., 2000; Wu et al., 2001), implying that the host magmas are relatively primitive.
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Phanerozoic history of the basement terranes in north Tibet
Because the Tertiary–recent potassium-rich magmatism of north Tibet spans a number of different basement terranes that may contribute to the petrogenesis of the magmas, we first need to review the geological history of these terranes; particular emphasis is placed on identifying tectono-magmatic events that might have caused enrichment of the mantle part of the lithosphere and on the characteristics of the crust.
The northern Tibetan Plateau is a collage of several allochthonous continental terranes whose boundaries are marked by tectonic suture zones (Fig. 1). From north to south, these are the Kudi suture (I), the South Kunlun suture (II), the Jinsha suture (III) and the Lancang suture (IV). Correspondingly, from north to south, the four main continental terranes are the North Kunlun terrane, South Kunlun terrane, Songpan–Ganzi terrane and North Qiangtang terrane (Fig. 1 and Table 4).
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Within the northern Tibetan Plateau, Precambrian basement mainly crops out in the North Kunlun and South Kunlun terranes (Dewey et al., 1988
; Pan & Bian, 2000; Xiao et al., 2002); this basement is composed of gneisses, schists, migmatites, stromatolite-bearing limestones, marbles, pelites, greywackes and cherts. A granite body, which has a zircon U–Pb age of 2261 Ma (Xu et al., 2000), intrudes the basement of the North Kunlun terrane, providing a minimum Proterozoic age for the basement.
The Qiangtang terrane contains the most extensive exposures of metamorphic rocks in the interior of Tibet, forming an east–west-trending belt 500 km long by 100 km wide (Pan et al., 1998; Kapp et al., 2003). These form a tectonic mélange, interpreted by Kapp et al. (2000) as a metamorphic core complex, comprising a strongly deformed matrix of metasedimentary and mafic igneous schists enclosing less strongly deformed blocks of metabasite, Carboniferous–Triassic metasedimentary rocks and Early Palaeozoic gneisses. Both the blocks and matrix exhibit greenschist, epidote-blueschist and, locally, epidote-amphibolite metamorphic mineral assemblages consistent with exhumation from >35 km depth. Kapp et al. (2003) have proposed that this mélange was produced by underthrusting of oceanic crust and its sedimentary cover 200 km beneath the Qiangtang terrane during southward, Early Mesozoic, low-angle subduction along the Jinsha suture. Low-angle subduction is consistent with the absence of a well-developed Triassic magmatic arc in central Tibet. This model has important geodynamic implications in that it predicts that a significant portion of the central Tibetan mantle lithosphere must have been removed during the Early Mesozoic, and thus would not be available to act as a source component for the Cenozoic magmas. Additionally, the model implies that the lower crust of central Tibet is dominantly composed of the same tectonic mélange. Hacker et al. (2000) have reported the presence of metasedimentary and mafic metaigneous, granulite-facies, crustal xenoliths in 3 Ma shoshonitic lavas from central Tibet, which would be consistent with the Kapp et al. (2003) model. These xenoliths record a thermal gradient reaching 800–1000°C at a depth of 30–50 km and a late-stage heating event consistent with injection of magma into the lower crust at 1300°C. Controversy remains as to whether or not the Qiangtang terrane has any Precambrian basement rocks. There are Precambrian basement exposures in Changdu to the east and the Karakorum to the west, which are thought to be extensions of the Qiangtang terrane (Desio, 1979; Tahirkheli, 1982; Pan et al., 1998).
The Phanerozoic cover sequences of the northern Tibetan Plateau mainly comprise clastic deposits, carbonates and interbedded volcanic rocks [BGMRXAR (Bureau of Geology and Mineral Resources of Xizang (Tibet) Autonomous Region), 1993; Pan et al., 1998; Zhao et al., 2001]. There is no evidence for the existence of Archaean crust in this region (Dewey et al., 1988; Pan et al., 1998; Pan & Bian, 2000).
Previous geological studies (e.g. Pan, 1996; Pan et al., 1998; Pan & Bian, 2000; Xiao et al., 2002) indicate that the southern margin of the North Kunlun terrane was a passive continental margin from Late Proterozoic times, with an ocean (Proto-Tethys) to the south separating it from the South Kunlun terrane. Subduction of Proto-Tethys oceanic lithosphere beneath the South Kunlun terrane occurred during Late Proterozoic to Early Palaeozoic times (Matte et al., 1996; Pan, 1996; Pan & Bian, 2000). The closure of the Proto-Tethys ocean during the Late Ordovician to Early Silurian, around 450 Ma (Xu & Pan, 1993; Pan et al., 1998), resulted in the collision of the North Kunlun and South Kunlun terranes and the formation of the Kudi suture (I). The Kudi suture (Fig. 1) is represented by an ophiolitic assemblage that includes serpentinized harzburgites, dunites, gabbros and pillow basalts, interbedded with deep-water cherts (Matte et al., 1996; Pan, 1996; Pan et al., 1998; Sobel & Arnaud, 1999; Mattern & Schneider, 2000; Pan & Bian, 2000; Xiao et al., 2002).
The South Kunlun and Songpan–Ganzi terranes probably formed a continuous continental block to the south of the Proto-Tethys ocean prior to the onset of subduction (e.g. Pan et al., 1998; Pan, 2000). A back-arc basin (South Kunlun ocean) gradually developed between the South Kunlun terrane and the Songpan–Ganzi terrane as a consequence of southerly subduction of Proto-Tethys oceanic lithosphere (Pan, 2000). The South Kunlun suture (II), located between the South Kunlun terrane to the north and the Songpan–Ganzi terrane to the south (Fig. 1), represents the closure of this back-arc basin. It comprises dismembered ophiolitic assemblages including ultramafic blocks, diabase, gabbro, serpentinized harzburgite, pillow lavas and chert (Molnar et al., 1987; Jiang et al., 2000; Pan & Bian, 2000). The age of the basalts is 260 Ma on the basis of a whole-rock Rb–Sr isochron (Yang et al., 1996). Previous studies (Matte et al., 1996; Guo et al., 1998; Pan et al., 1998; Pan & Bian, 2000; Xu et al., 2000) have suggested that the South Kunlun ocean, now represented by the South Kunlun suture (II), may have subducted to the north during Late Palaeozoic times, based on the presence of contemporaneous subduction-related granitoids and volcanic rocks.
Wang et al. (2000) have proposed that the Palaeo-Tethys ocean basin opened in the Early Devonian. The closure of the main branch of Palaeo-Tethys, the Lancang ocean, is marked by the Lancang suture (IV) [Li, 1987; Zhou et al., 1989; YBGMR (Yunnan Bureau of Geology and Mineral Resources), 1990; Mo et al., 1991, 1993, 2003; Li et al., 1995; Yang, 1998; Wang et al., 2000], separating the North and South Qiangtang terranes (Fig. 1). The precise location and geodynamic setting of the Lancang suture, however, remain controversial (Hsü et al., 1995; Li et al., 1995; Kapp et al., 2000, 2003; Deng et al., 2002). The suture turns into a NW–SE-trending structure to the SE of the study area [Fig. 1b; also see (Yang 1998, fig. 1) and (Wang et al. 2000, fig. 1) for further discussion]. The Lancang suture is marked by dismembered Devonian–Carboniferous ophiolitic assemblages, which comprise serpentinite, harzburgite, gabbro, diabase, mafic pillow lavas, marine sediments and cherts (Li, 1987; Li et al., 1995; Yang, 1998; Wang et al., 2000). Paired with the ophiolitic suite, a Permo-Triassic arc volcanic rock sequence exists to the north and NE of the suture (Mo et al., 1991, 1993; Yang, 1998). These arc volcanic rocks consist of tholeiitic, calc-alkalic and shoshonitic series. There is a geochemical polarity, characterized by an increase in potassium and other incompatible trace elements (e.g. Rb and Ba), towards the NE within the volcanic belt (Mo et al., 1993; Yang, 1998). These indicate a mature volcanic arc, suggesting that the Langcang oceanic crust subducted northwards beneath the North Qiangtang terrane (Yang, 1998; Wang et al., 2000). It should be noted, however, that some workers have disagreed with the presence of the Lancang suture within the Qiangtang terrane (e.g. Deng, 1998; Pan et al., 1998).
Wang et al. (2000) have shown that the North Qiangtang terrane was connected with the southern Songpan–Ganzi terrane, forming an integrated continental block, during the initial stage of Lancang ocean subduction. A back-arc extensional basin gradually formed between the North Qiangtang terrane and the Songpan–Ganzi terrane as a consequence of northward subduction. This phase of back-arc extension is considered to be a precursor of the Jinsha ocean now represented by the Jinsha suture zone (III) (Mo et al., 1991, 1993; Wang et al., 2000), which lies between the North Qiangtang terrane and the Songpan–Ganzi terrane (Fig. 1). The Jinsha suture is marked by mélanges with ophiolitic assemblages including ultramafic blocks, gabbros, picrites, pillow lavas and pelagic sediments (Pan, 1984; Dewey et al., 1988; Pan et al., 1998; Yang, 1998; Pan & Bian, 2000). Early Carboniferous and Early Permian radiolaria have been reported from chert and siliceous limestone interbeds within pillow basalts from the ophiolitic mélange (Wu, 1993; Feng et al., 1997; Pan & Bian, 2000), indicating that the Jinsha oceanic lithosphere was generated at this time. The subduction direction of the Jinsha oceanic crust, however, remains controversial; this has been proposed to be southward (Dewey et al., 1988; Pearce & Mei, 1988; Yin & Nie, 1996; Yang, 1998; Pan & Bian, 2000; Wang et al., 2000; Yin & Harrison, 2000), northward (Coward et al., 1988; Wu et al., 1989; Li et al., 1995), or both (Leeder et al., 1988). There is a contemporaneous arc volcanic belt to the south of the Jinsha suture (Pearce & Mei, 1988; Pan et al., 1998); however, the field relationships are complex because the suture has been strongly modified by post-collisional thrust systems.
The upper crust of the Songpan–Ganzi terrane comprises a 10–15 km thick sequence of Late Triassic flysch sediments deposited in a series of sedimentary basins on the passive continental margin of the North China block (Kapp et al., 2003). A change from marine to non-marine sedimentation during the Early Jurassic is considered to record the accretion of the Qiangtang terrane to the Eurasian margin (Dewey et al., 1988).
Based on the above, it is likely that the lithosphere beneath the North Qiangtang, Songpan–Ganzi and South Kunlun terranes has been geochemically enriched by both southward subduction of the Proto-Tethys oceanic lithosphere and northward subduction of the Palaeo-Tethys oceanic lithosphere. Because both the South Kunlun ocean (marked by the South Kunlun suture) and the Jinsha ocean (recorded by the Jinsha suture) (Fig. 1) initiated as back-arc extensional basins (Pan, 2000; Wang et al., 2000), the lithosphere beneath the Songpan–Ganzi terrane could have been metasomatized during the early periods of the subduction of both the Proto-Tethys and the Palaeo-Tethys oceans, prior to the formation of the back-arc basins, although the Songpan–Ganzi terrane was far from the contemporaneous subduction zones.
The final closure of the South Kunlun, Jinsha and Lancang oceans in the Late Triassic led to the amalgamation of the South Kunlun, Songpan–Ganzi, North Qiangtang and South Qiangtang terranes (Pan et al., 1998; Pan & Bian, 2000; Wang et al., 2000). These continent–continent collisions gave rise to structural deformation, regional metamorphism of varying degrees, disruption of the ophiolitic mélanges and intracontinental subduction along previous zones of oceanic crust subduction (Pan et al., 1998). Triassic synorogenic (syn-collisional) granitoids and intermediate to acidic arc volcanic rocks are mainly distributed within the South Kunlun terrane, the Qaidam terrane, the Songpan–Ganzi terrane and the North Qiangtang terrane (Harris et al., 1988a; Pan et al., 1998; Pan & Bian, 2000; Zhang et al., 2000). Following this period of terrane amalgamation, the whole northern part of the Tibetan Plateau experienced strong intracontinental deformation from the Late Triassic.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The igneous rocks studied are aphyric to weakly porphyritic with <10% (by volume) phenocrysts; mafic samples contain <5% (by volume) phenocrysts. The phenocryst assemblage in the more mafic rocks consists mainly of olivine, orthopyroxene, clinopyroxene, phlogopite, plagioclase and alkali feldspar set in a microcrystalline matrix of plagioclase, alkali feldspar, pyroxene, biotite, apatite and Fe–Ti oxides. The phenocryst minerals in the more evolved rocks include sanidine, quartz, subordinate plagioclase and biotite; the groundmass comprises alkali feldspar, quartz, biotite, Fe–Ti oxides and glass. Some strongly undersaturated rocks (e.g. samples from the North Qiangtang terrane) contain phenocrysts of leucite, clinopyroxene, olivine, nepheline, nosean, haüyne, phlogopite and Fe–Ti oxides; their groundmass consists of sanidine, clinopyroxene, nosean, haüyne, apatite, titanite, Fe–Ti oxides and glass (Table 2).
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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All of the analysed samples are petrographically fresh and show no evidence of significant hydrothermal alteration or weathering. Samples 2–3 kg in weight were cut into several thin slices. Fresh slices were cleaned three times using deionized water, dried, and then crushed in a tungsten carbide swing mill. To minimize contamination, only pieces of samples that did not come directly into contact with the mill were powdered in an agate mortar for subsequent major element, trace element and Sr–Nd–Pb isotope analysis.
For major element analyses, sample powders (1·2 g) were fused with Li2B4O7 (6 g) in a CLAISSEFLUXER VI (Canada) fusion furnace at 1050°C for 20 min. Whole-rock major element oxide contents were analysed on fused glass discs with a Phillips PW1400 sequential X-ray fluorescence spectrometer (XRF) at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing (IGGCAS). The analytical precision was better than 2% relative. Loss on ignition (LOI) was determined after ignition at 1000°C for 10 h of 2 g rock powder. The detailed analytical procedure follows that reported by Zhang et al. (2002). Representative data are presented in Table 3. The complete dataset is included as an Electronic Appendix, which may be downloaded from the Journal of Petrology website at http://www.petrology.oxfordjournals.org/.
For rare earth element (REE) and trace element analyses, whole-rock powders (40 mg) were weighed and dissolved in distilled 1 ml HF and 0·5 ml HNO3 (HNO3 : H2O = 1 : 1) in 7 ml Savillex Teflon screw-cap capsules and then were ultrasonically stirred for 15 min. Subsequently, the solutions were evaporated at 150°C to dryness and the residue was digested with 1·5 ml HF and 0·5 ml HNO3 (HNO3 : H2O = 1 : 1) in Teflon screw-cap capsules. Then, the solutions were heated at 130°C initially and at up to 170°C for 24 h by gradually increasing the temperature during this time. The solutions were then heated at 170°C for 10 days, dried and redissolved in 2 ml HNO3 (HNO3 : H2O = 1 : 1) in the capsules. The solutions were heated at 150°C for 5 h and then evaporated, dried and redissolved in 2 ml HNO3 (HNO3 : H2O = 1 : 1) and 2 ml 1% HNO3 at 150°C for 5 h in screw-cap capsules, to ensure that the samples were completely dissolved. The solutions were put into plastic beakers and then 1 ml 500 ppb In was added as an internal standard. Finally, the solutions were diluted in 1% HNO3 to 50 ml before analysis.
The REE and trace element contents of the sample solutions were analysed by inductively coupled plasma mass spectrometry (ICP-MS) at IGGCAS using a Finnigan MAT system. A blank solution was prepared; the total procedural blanks were <50 ng for all the trace elements reported in Table 3. (Representative data analysed are presented in Table 3. The complete dataset is included as an Electronic Appendix, which may be downloaded from the Journal of Petrology website at http://www.petrology.oxfordjournals.org/.) During the analytical runs, frequent standard calibrations were performed to correct for instrumental signal drift following the procedure of Gao et al. (1999). Four replicates and two international standards (BHVO-1 and AGV-1) were prepared using the same procedure to monitor the analytical reproducibility. The discrepancy, based on repeated analyses of samples and international standards, is <5% for all the elements given in Table 3. Analyses of the international standards are in excellent agreement with the recommended values (Govindaraju, 1994), and deviate <6% from the published values (see the Electronic Appendix). The detailed analytical procedures follow those of Jin & Zhu (2000) and Guo et al. (2005).
For Rb–Sr and Sm–Nd isotope analyses, whole-rock chips of <20 mesh size were used. Before being ground to 200 mesh (75 µm) in an agate mortar, the chips were leached in purified 6N HCl for 24 h at room temperature to minimize the influence of surface alteration or weathering, especially for Sr isotopic ratios. Sample powders (60 mg) were spiked with mixed isotope tracers (87Rb–84Sr for Rb–Sr isotope analyses and 149Sm–150Nd for Sm–Nd isotope analyses), then dissolved with a mixed acid (HF : HClO4 = 3 : 1) in Teflon capsules for 7 days at room temperature. Rb and Sr and REE fractions were separated in solution using AG50Wx8 (H+) cationic ion-exchange resin columns. Sm and Nd were separated from the other REE fractions in solution using AG50Wx8 (H+) cationic ion-exchange columns and P507 extraction and eluviation resin. The collected Sr and Nd fractions were evaporated and dissolved in 2% HNO3 to give solutions for analysis by mass spectrometry. Isotopic measurement was performed on a VG354 mass spectrometer (UK) at IGGCAS; the data are presented in Table 5. The mass fractionation corrections for Sr and Nd isotopic ratios were based on 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219, respectively. The international La Jolla standard yielded 143Nd/144Nd = 0·511862 ± 7 (n = 12, 2
) (the recommended value is 0·511859) and international standard BCR-1 yielded 143Nd/144Nd = 0·512626 ± 9 (n = 12) (the recommended value is 0·512638). The international standard NBS987 gave 87Sr/86Sr = 0·710254 ± 16 (n = 8) (the recommended value is 0·710240) and international standard NBS607 gave 87Sr/86Sr = 1·20032 ± 30 (n = 12) (the recommended value is 1·20039). The whole procedure blank is less than 2 x 10–10 g for Rb–Sr isotopic analysis and 5 x 10–11 g for Sm–Nd isotopic analysis. Analytical errors for Sr and Nd isotopic ratios are given as 2 in Table 5. The 87Rb/86Sr and 147Sm/144Nd ratios were calculated using the Rb, Sr, Sm and Nd concentrations obtained by ICP-MS. The initial 87Sr/86Sr and 143Nd/144Nd ratios were calculated using average ages of the samples based on 40Ar/39Ar, K–Ar dating and other analytical methods (Table 1).
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For Pb isotope measurements, in order to minimize contamination from the atmosphere during the crushing process, 100 mesh powders of samples were used. 150 mg whole-rock powder was weighed and dissolved in Teflon capsules using concentrated HF at 120°C for 7 days. Pb was separated from the silicate matrix and purified using AG1x8 anionic ion-exchange columns with dilute HBr as eluant. The whole procedure blank is less than 1 ng. Pb isotopic ratios were measured with a VG354 mass spectrometer (UK) at IGGCAS. During the period of analysis repeat analyses of the international standard NBS981 yielded 204Pb/206Pb = 0·059003 ± 0·000084 (n = 6) (the certified value is 0·058998), 207Pb/206Pb = 0·91449 ± 0·00017 (n = 6) (the certified value is 0·914598), and 208Pb/206Pb = 2·16691 ± 0·00097 (n = 6) (the certified value is 2·168099). Pb isotope fractionations were corrected using correction factors from the certified values of the international standard NBS 981. The average 2
uncertainty for measured ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb is 0·6%, 0·4% and 0·5% per a.m.u. (atomic mass unit), respectively. The Pb isotope data are presented in Table 6. Detailed sample preparation and analytical procedures for the Sr–Nd–Pb isotope measurements follow those of Zhang et al. (2002) and Fan et al. (2003).
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major and trace element geochemistry
The whole-rock geochemical data (Table 3 and Electronic Appendix) indicate that all of the analysed samples are potassic [K2O/Na2O (wt %) >1·0], except for two samples (QS30 and HS07) with K2O/Na2O <1 (0·92 and 0·95). Eight samples (QS22, YS72, DG08, G98-1, JC975, G98-11, JC973, JC978) have K2O/Na2O ratios >2; their MgO and K2O contents are >3 wt %. These eight samples are considered to be ultrapotassic based on the criteria of Foley et al. (1987)
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The compositions of the analysed samples are plotted in a total-alkali vs silica classification diagram (Le Bas et al., 1986; Le Maitre et al., 1989) in Fig. 3a and subdivided into three groups based on their locations within the North Qiangtang, Songpan–Ganzi and North Kunlun terranes. Samples from these terranes overlap and define scattered trends that lie almost totally within the trachybasalt–basaltic trachyandesite–trachyandesite–trachyte and tephrite–phonotephrite–tephriphonolite–phonolite fields. A plot of K2O vs SiO2 (Fig. 3b) shows that the rocks belong to the shoshonitic magma series, except for one highly differentiated sample, which plots within the high-K calc-alkaline field.
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Mg-numbers [ = molar Mg/(Mg + Fe2+) ratio, calculated assuming Fe2O3/(FeO + Fe2O3) = 0·20] range from 0·22 to 0·72 (Table 3 and Electronic Appendix). Abundances of the compatible elements (e.g. MgO, Fe2O3, CaO, TiO2, Ni, Sc, Cr) in the magmatic rocks of the different terranes display similar variation trends (Fig. 4). SiO2 and Al2O3 increase, whereas CaO, Fe2O3, Ni and Cr (not shown) decrease, with decreasing MgO (Fig. 4a–d and l). This may be explained by fractional crystallization of clinopyroxene and olivine, which is consistent with petrographical observations that olivine and clinopyroxene phenocrysts predominate in the more mafic rocks (Table 2). However, no correlations exist between incompatible trace elements (e.g. Ba, Rb, Sr, Nb, Zr and Pb) and MgO (Fig. 4f–k). Contents of the incompatible trace elements Ba, Rb, Sr and Pb are clearly higher in most samples from the North Qiangtang terrane than in those from the Songpan–Ganzi and North Kunlun terranes (Fig. 4f–h and k). Samples from the North Qiangtang terrane with high Ba, Rb, Sr and Pb concentrations do not show corresponding high LOI values, although the LOI values in the potassium-rich magmatic rocks must also reflect the presence of hydrous mineral phases (e.g. phlogopite). Moreover, the samples with high Ba, Rb, Sr and Pb concentrations are petrographically fresh (e.g. fresh plagioclase), further precluding the possibility of sample alteration.
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Almost all of the samples with MgO >6 wt % have Mg-numbers >0·60; the highest Mg-number is 0·72 (sample G98-1; Table 3 and Electronic Appendix). This suggests that these samples represent relatively primitive magmas. On this basis, we define samples with MgO >6 wt % as primitive and those having MgO <6 wt % as evolved. To minimize the effects of magmatic differentiation and crustal contamination, only primitive samples were used to investigate the conditions of generation of the primary, mantle-derived magmas. Chondrite-normalized REE patterns (Fig. 5) and primitive mantle-normalized incompatible element diagrams for the primitive samples (Fig. 6) show strong incompatible element enrichment. Primitive mantle-normalized concentrations range from several times primitive mantle for heavy REE (HREE), Ti and Y to several hundred and even >1000 times for large ion lithophile elements (LILE) such as Rb, Ba, Th, U, K and Pb (Fig. 6). The mantle-normalized incompatible trace element patterns are distinguished by significantly negative Nb–Ta–Ti and positive Pb anomalies, despite the generally high contents of the elements Nb, Ta and Ti (Table 3 and Electronic Appendix). The average concentrations of Rb, Ba, Th, U, La, Ce, Pb, Sr, Nd, Sm and Eu are higher in the samples from the North Qiangtang terrane than in those from the Songpan–Ganzi and North Kunlun terranes; however, average contents of the high field strength elements (HFSE; Ta, Zr and Ti) are similar in the magmas of the three terranes (Table 3 and Electronic Appendix). Positive correlations are evident between La/Yb and La (Fig. 7a), and Ce/Pb and Ce (Fig. 7b); the slope defined by the samples from the North Qiangtang terrane is different from that for samples from the Songpan–Ganzi and North Kunlun terranes.
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Sr–Nd–Pb isotope geochemistry
Sr–Nd–Pb isotope data for the primitive and evolved samples are plotted in Figs 8 and 9, respectively. All samples have relatively high values of (87Sr/86Sr)i (0·706521–0·710461), low (143Nd/144Nd)i (0·511937–0·512526) and high (207Pb/204Pb)i (15·601–15·804) and (208Pb/204Pb)i (38·485–39·534) at a given (206Pb/204Pb)i (18·572–19·102) (Tables 5 and 6). The Sr–Nd isotope compositions exhibit a negative correlation (Figs 8a and 9a). The Sr–Nd–Pb isotopic compositions of Global Subducting Sediment (GLOSS; Plank & Langmuir, 1998
) and previously published data for potassium-rich igneous rocks from the three terranes (Arnaud et al., 1992; Turner et al., 1993, 1996a; Ding et al., 2003; Williams et al., 2004) are shown for comparison (Fig. 9). In plots of (208Pb/204Pb)i vs (206Pb/204Pb)i, and (207Pb/204Pb)i vs (206Pb/204Pb)i (Figs 8d, e and 9d, e), the samples studied define a diffuse array above and subparallel to the Northern Hemisphere Reference Line (NHRL; Hart, 1984). In all isotope projections, the potassium-rich magmatic rocks broadly overlap the field of GLOSS.
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The primitive samples have relatively higher 87Sr/86Sr and 206Pb/204Pb, and lower 143Nd/144Nd ratios in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes (Fig. 8). In the Pb isotope diagrams (Fig. 8d and e), these samples define diffuse linear arrays within the GLOSS field. The evolved samples have similar Sr–Nd–Pb isotope ratios in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes, although some of the evolved magmas have higher 206Pb/204Pb ratios in the North Qiangtang terrane compared with those in the Songpan–Ganzi and North Kunlun terranes (Fig. 9). In all Sr–Nd–Pb isotope diagrams, the data for the evolved samples from this study broadly overlap those of previous studies.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The major and trace element characteristics and Sr–Nd–Pb isotope compositions of the primitive and evolved magmatic rocks in north Tibet can provide constraints on the nature of the mantle source, processes of partial melting, crustal-level evolution of the magmas and geodynamic setting. Moreover, there may be a relationship between the petrogenesis of the post-collisional K-rich magmas and the uplift of northern Tibet.
Role of crustal contamination processes in the petrogenesis of the magmas
Combined assimilation and fractional crystallization
The broad negative correlation between (87Sr/86Sr)i and MgO (wt %) (Fig. 10) for the North Qiangtang and Songpan–Ganzi terranes suggests operation of combined processes of crustal assimilation and fractional crystallization (AFC) in the petrogenesis of the magmas. The range in (87Sr/86Sr)i for the primitive samples from the North Qiangtang terrane suggests that their mantle source was isotopically heterogeneous (Fig. 10a). In contrast, the primitive samples from the Songpan–Ganzi terrane are isotopically more homogeneous; however, the more evolved magmatic rocks appear to be extremely heterogeneous when data from the literature are included for comparison (Fig. 10b). In Fig. 11, we show the variation of (87Sr/86Sr)i vs Sr (ppm) for the magmatic rocks from the North Qiangtang and Songpan–Ganzi/North Kunlun terranes. The data define distinctly different arrays, suggesting that different crustal contamination (AFC) processes may have operated in the individual terranes.
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Modelling energy-constrained assimilation and fractional crystallization
The energy-constrained assimilation and fractional crystallization (EC-AFC) model (Bohrson & Spera, 2001
; Spera & Bohrson, 2001) provides a rigorous approach to the simulation of AFC processes. The rationale behind our EC-AFC model calculations is as follows. First, we select reasonable ranges of the initial parameters required in the EC-AFC model based on the isotopic and geochemical characteristics of the magmas in north Tibet (Tables 3 and 5, and Electronic Appendix), previous knowledge about upper mantle partial melting processes and the likely composition of the local crust. Then, using selected values of these initial parameters within their ranges, we perform iterative calculations using the EC-AFC calculation program (http://magma.geol.ucsb.edu), progressively changing the set of parameters within their ranges until we obtain the best fit to the Sr contents and Sr isotopic compositions of the northern Tibetan magmatic rocks (Fig. 11). The output from our EC-AFC simulations provides model curves to fit the correlation trends between (87Sr/86Sr)i and Sr (Fig. 11) and also optimized values of the parameters that form the basis of the EC-AFC model (Tables 7 and 8). These parameters provide important constraints on the characteristics of the primitive magmas, the nature of the crustal contaminant and the relative depth at which AFC processes operate in each of the terranes.
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Ranges of the initial parameters of EC-AFC model
There are two types of initial parameters in the EC-AFC model: thermal and compositional parameters (Bohrson & Spera, 2001
). Our approach to the selection of initial values of the thermal parameters is as follows. We selected the magma liquidus temperature to be within the range 900–1600°C, assuming that the primary/parental K-rich magmas were generated by partial melting of the upper mantle beneath north Tibet; we also assumed that the initial temperature of the magma at the site of crustal contamination is equal to the liquidus temperature. We adopted the approach of Bohrson & Spera (2001) and selected the assimilant liquidus temperature, solidus temperature and initial temperature in the ranges 500–1400°C, 300–1200°C and 100–1000°C, respectively. The equilibration temperature (Teq) is the temperature at which the EC-AFC processes cease. This is constrained to lie between 1600°C and 300°C; following Bohrson & Spera (2001), this value must be between the magma liquidus temperature and the solidus temperature of the assimilant. Values for the isobaric specific heat of the magma, the isobaric specific heat of the assimilant, crystallization enthalpy and fusion enthalpy are taken from Bohrson & Spera (2001) and are assumed to be constant (Table 7).
The compositional parameters required for calculation of the EC-AFC model are based on the isotopic and chemical characteristics of the primitive magmas and likely crustal assimilants. We have modelled the relationship between (87Sr/86Sr)i and Sr concentrations for EC-AFC processes in northern Tibet and compared them with the model curves of Bohrson & Spera (2001). For the North Qiangtang terrane, we selected the initial Sr content of the magma to range between 1000 and 9000 ppm, extending significantly beyond the range of primitive compositions observed (Table 3 and Electronic Appendix), and Sr isotope ratios from 0·7070 to 0·7090 based on the (87Sr/86Sr)i ratios of the primitive magmas (Table 5). For the Songpan–Ganzi and North Kunlun terranes, we selected the initial Sr content of the magma to range from 500 to 2000 ppm, according to the data in Table 3 and the Electronic Appendix, and Sr isotope ratios from 0·7060 to 0·7090 based on the (87Sr/86Sr)i of the primitive magmas (Table 5). For the North Qiangtang, Songpan–Ganzi and North Kunlun terranes, we selected both the residue/melt bulk distribution coefficient for Sr in the partial melt of the assimilant (Da) and the phenocryst/melt bulk distribution coefficient for Sr in the primitive magma (Dm) to range from 0 to 5·5 on the basis of previous studies of EC-AFC processes (Bohrson & Spera, 2001). Assimilant Sr contents and Sr isotope ratios in the three basement terranes are taken from Liu (1999), Pan (2000) and Deng et al. (2002) (Table 7).
Calculated results of the above parameters in the EC-AFC model
Based on the above ranges and values of the initial parameters (Table 7), we performed iterative EC-AFC calculations by changing the initial parameters from minimum to maximum values within their respective ranges [for a detailed discussion of the EC-AFC model the reader is referred to Bohrson & Spera (2001) and Spera & Bohrson (2001)]. When the calculation results of the EC-AFC model provided the best fit to the actual data trends (i.e. in Fig. 11 the model curves provided a good fit to the correlation of 87Sr/86Sr vs Sr for the individual terranes), we terminated the iteration and recorded the final values of the thermal and compositional parameters (Table 8).
For the North Qiangtang terrane, the best fit to the data shows that the assimilant liquidus temperature, solidus temperature and initial temperature are 1150°C, 900°C and 600°C, respectively. The magma liquidus temperature is 1300°C and the equilibration temperature (Teq) is 1100°C (Table 8). The majority of the data can be explained by an EC-AFC model in which three primitive magmas (M1–M3), with Sr contents of 1000 ppm, 2000 ppm and 4000 ppm, respectively, and corresponding initial Sr isotope ratios of 0·70720, 0·70805 and 0·70835, assimilate granulite-facies crust assimilant Q. This is consistent with the observation that the most primitive magmas in the North Qiangtang terrane (MgO >8 wt %) are isotopically heterogeneous (Fig. 10a). An important result of our iterative calculations is that the residue/melt bulk distribution coefficient for Sr for the partial melt of the assimilant is <1·0 (0·05), indicating that Sr is incompatible and the residue is feldspar-free. Likewise, the phenocryst/melt bulk distribution coefficients for Sr in the primitive magmas are significantly less than 1·0 (0·00001–0·06). This indicates that there are no fractionating phases in which Sr is compatible.
For the Songpan–Ganzi and North Kunlun terranes, the best fit EC-AFC results indicate that the assimilant liquidus temperature, solidus temperature and initial temperature are 1000°C, 800°C and 300°C, respectively. The magma liquidus temperature is 1280°C and the equilibration temperature (Teq) is 980°C (Table 8). The calculations show that composition of the primitive magma is relatively homogeneous (Sr = 1800 ppm; 87Sr/86Sr = 0·7070), which is consistent with the observation from Fig. 10b. Conversely, the results of the iteration show that a range of crustal assimilants (S1–S3) are required, to fit the range of Sr isotopic compositions and Sr concentrations observed in the magmatic rocks. The calculated residue/melt bulk distribution coefficients for Sr for the partial melts of the assimilants are all >1·0 (1·0–1·5), whereas the bulk distribution coefficients for Sr in the primitive magma are in the range 0·9–1·5 (Table 8). This requires the presence of feldspar in the residue and also indicates feldspar fractionation.
Interpretation of the parameters of the EC-AFC model
The best-fit calculation results of the EC-AFC model confirm that the correlated trends of (87Sr/86Sr)i vs Sr (ppm) for the K-rich magmas are caused by different processes in the North Qiangtang terrane (Fig. 11a) from those in the Songpan–Ganzi and North Kunlun terranes (Fig. 11b). Moreover, the calculated thermal and compositional parameters are also different between the two terranes (Table 8). This suggests differences in the depths of crustal contamination, the nature of the assimilant and the composition of the primitive magmas. In detail, the calculated assimilant liquidus temperature, solidus temperature and initial temperature in the North Qiangtang terrane during the EC-AFC process are higher compared with those in the Songpan–Ganzi and North Kunlun terranes (Table 8). This implies that the site at which EC-AFC processes occurred in the North Qiangtang terrane was deeper in the crust than in the Songpan–Ganzi and North Kunlun terranes. The inference that the residue of partial melting of the assimilant in the North Qiangtang terrane was feldspar-free may be interpreted to indicate a depth of assimilation beyond the field of feldspar stability. Conversely, the presence of feldspar in the residue of partial melting of the assimilant in the Songpan–Ganzi and North Kunlun terranes is consistent with a shallower depth of contamination in the crust. Moreover, the calculated results of the EC-AFC model show that the best-fit assimilant is a mafic granulite (Q) in the North Qiangtang terrane and a pelite (S1, S2) or calc-pelite (S3) in the Songpan–Ganzi and North Kunlun terranes.
Hacker et al. (2000) proposed that thermal gradient was 17°C/km in the Qiangtang terrane based on temperature estimates for crustal xenoliths entrained in 3 Ma shoshonitic rocks. According to this, the depth at which AFC processes occurred in the North Qiangtang terrane and Songpan–Ganzi and North Kunlun terranes was 35 km and 18 km, respectively, based on the calculation results for the assimilant initial temperature (Table 8). Owens & Zandt (1997) suggested that the crustal thickness of the North Qiangtang and Songpan–Ganzi terrane is 65 km and 55 km, respectively. This indicates that EC-AFC processes occurred in the middle crust (about 35 km) in the North Qiangtang terrane, but in the upper crust (about 18 km) in the Songpan–Ganzi and North Kunlun terranes, using the terms middle and upper crust to indicate relative depth.
Sr is compatible during the differentiation of the magmas in the Songpan–Ganzi and North Kunlun terranes (Dm = 0·9, 1·0, 1·5; Table 8), but is incompatible in those of the North Qiangtang terrane (Dm = 0·06, 0·006 and 0·00001; Table 8). This is consistent with the presence of plagioclase phenocrysts in most of the samples from the Songpan–Ganzi and North Kunlun terranes, and their absence in those from the North Qiangtang terrane (Table 2).
For the three basement terranes, it is not possible to fit the correlated trends of (87Sr/86Sr)i vs Sr (ppm) (Fig. 11) for the potassium-rich magmatic rocks with a single composition of assimilant and primitive magma. This suggests that the crustal assimilants and/or parent magmas are heterogeneous in terms of their trace element and Sr isotopic compositions in the three terranes. For the samples from the North Qiangtang terrane, our attempts to simulate EC-AFC processes using a parent magma with low Sr content (M1) required an unrealistically high concentration of Sr in the assimilant (6000 ppm), and the 87Sr/86Sr ratios of the most evolved magmas simulated by the EC-AFC model were lower (0·709; Fig. 11a) than those of the most extreme samples analysed (0·710). Irrespective of changes in the initial parameters of the parent magma, the 87Sr/86Sr ratios of the more evolved magmas calculated by the EC-AFC model were always lower than 0·710 and the model curve (dashed curve line labelled R in Fig. 11a) could not be forced to fit the trend of the analysed data. This requires that the least differentiated parent magmas were compositionally heterogeneous in the North Qiangtang terrane. For the Songpan–Ganzi and North Kunlun terranes, simulation of EC-AFC processes using a single initial composition for the parent magma, combined with variable compositions for the assimilant (Table 7), could fit the correlated trends of (87Sr/86Sr)i vs Sr (ppm) in Fig. 11b, indicating that the assimilants were compositionally and isotopically heterogeneous.
Previous studies (Owens & Zandt, 1997; Unsworth et al., 2004) have proposed that there is pervasive partial melting of the crust beneath the northern Tibetan Plateau based on the high Poisson's ratio of the crust of the North Qiangtang and Songpan–Ganzi terranes. Such pervasive partial melting zones are probably the sites at which EC-AFC processes have taken place. The average thickness of the crust is greater beneath the North Qiangtang terrane (65 km) than beneath the Songpan–Ganzi terrane (55 km) (Owens & Zandt, 1997). This is consistent with the results of our EC-AFC modelling, which indicate that crustal contamination of the rising magmas happened in the middle crust in the North Qiangtang terrane, but in the upper crust in the Songpan–Ganzi and North Kunlun terranes (Fig. 12). The migration with time from the middle crust to the upper crust is probably related to thermal maturation of the crust. The eruption of some primitive magmatic rocks indicates that not all the magmas were affected by EC-AFC processes (Fig. 12).
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The evolved magmatic rocks in the three terranes (North Qiangtang, Songpan–Ganzi and North Kunlun) of north Tibet have broadly similar ranges of Sr–Nd–Pb isotope compositions (Fig. 9). However, the compositions of the most primitive magmas in the different terranes are variable based on their trace element and Sr–Nd–Pb isotope compositions. We consider that the convergence in the geochemical characteristics of the evolved magmas in north Tibet is caused by the operation of EC-AFC processes. Our model results indicate that the composition of the crustal assimilant and/or the primitive mantle-derived magmas was variable in the three terranes (Tables 7 and 8). For example, the primitive magmas in the North Qiangtang terrane have higher contents of LILE (Table 3 and Electronic Appendix), higher (87Sr/86Sr)i and (206Pb/204Pb)i ratios and lower (143Nd/144Nd)i ratios (Fig. 8) than those in the Songpan–Ganzi and North Kunlun terranes. The primitive magmas in the North Qiangtang terrane underwent contamination by middle crustal rock types (Fig. 11a), which have relatively low 206Pb/204Pb ratios and LILE contents (Table 7) compared with likely upper crustal contaminants (Liu, 1999
; Pan, 2000; Deng et al., 2002). The EC-AFC process decreased the 206Pb/204Pb ratios and LILE contents of the resultant evolved magmas compared with the primitive magmas in the North Qiangtang terrane. In contrast, the primitive magmatic rocks in the Songpan–Ganzi and North Kunlun terranes have lower LILE contents (Table 3 and Electronic Appendix), lower (87Sr/86Sr)i and (206Pb/204Pb)i ratios, and higher (143Nd/144Nd)i ratios than those in the North Qiangtang terrane (Fig. 8). The primitive magmas in the Songpan–Ganzi and North Kunlun terranes appear to have undergone contamination by upper crustal rocks (Fig. 11b), which have relatively high 206Pb/204Pb ratios and LILE contents (Table 7) compared with the middle crustal rocks (Liu, 1999; Pan, 2000; Deng et al., 2002). The EC-AFC process increased the 206Pb/204Pb and LILE contents in the resultant evolved magmas, compared with the primitive magmas, in the Songpan–Ganzi and North Kunlun terranes. Thus, the EC-AFC processes resulted in similar 206Pb/204Pb and LILE contents in the evolved magmas of the three terranes in north Tibet. The similarities in 87Sr/86Sr and 143Nd/144Nd ratios in the evolved magmas of the three terranes are because of the higher concentrations of Sr and Nd in the partial melts of the assimilants compared with those of the least differentiated basaltic magmas during EC-AFC processes. This leads to the 87Sr/86Sr and 143Nd/144Nd ratios of the resultant evolved magmas in the three terranes being close to those of the assimilants (e.g. increasing 87Sr/86Sr). Similarities in the 87Sr/86Sr and 143Nd/144Nd ratios of the partial melts of the assimilants in the different terranes (Table 7) may result in convergence in the 87Sr/86Sr and 143Nd/144Nd ratios of the resultant evolved magmas in different basement terranes. Consequently, EC-AFC processes appear to have reduced the systematic differences in the trace element and isotope compositions of the evolved magmas, compared with those in the primitive magmas, between the North Qiangtang terrane and the Songpan–Ganzi and North Kunlun terranes.
The compositions of the evolved magmas in north Tibet have clearly undergone significant modification by crustal contamination during EC-AFC processes. The trace element and Sr–Nd–Pb isotope compositions of the evolved magmas are different from those of the primitive magmas. Only the primitive magmas can be used to investigate the characteristics of the mantle source region and its evolution. Thus, conclusions about the nature of the mantle source reached by previous workers (e.g. Turner et al., 1993, 1996a; Williams et al., 2004), which were dominantly based on studies of the evolved magmas, need to be re-evaluated.
Nature of the mantle source region of the parental magmas
Enrichment of the mantle source
Oceanic basalts [i.e. ocean island basalts (OIB) and mid-ocean ridge basalts (MORB)] typically display positive Nb–Ta–Ti anomalies and negative Pb anomalies in primitive mantle-normalized trace element diagrams (e.g. Hofmann, 1986, 1988, 1997). In contrast, all of the primitive potassic and ultrapotassic magmatic rocks in north Tibet exhibit strongly negative Nb–Ta–Ti anomalies and positive Pb anomalies (Fig. 6), indicating that they were not derived from normal MORB- or OIB-source mantle. The significant difference between the primitive potassium-rich igneous rocks in north Tibet and MORB and OIB sources in the Sr–Nd isotope correlation diagram (Fig. 8a) supports this inference. Moreover, the average Ce/Pb ratios (7·64) of the primitive samples are considerably different from those of oceanic basalts (OIB and MORB) (25; Hofmann, 1988, 1997), suggesting that they are not derived from normal asthenospheric mantle. The presence of residual phlogopite in the mantle source region of the northern Tibetan magmas could not cause the significant reduction in their Ce/Pb ratios relative to those of MORB and OIB because the partition coefficients for Ce and Pb in phlogopite are 0·0007 and 0·019 (Williams et al., 2004), respectively, both of which are far less than unity. Consequently, residual phlogopite in the mantle source should increase the Ce/Pb ratio in the magmas rather than decrease it. In addition, there are differences in the average Ce/Pb ratios of the primitive magmas from each of the three terranes; these are 6·22, 8·83 and 8·43 in the North Qiangtang, Songpan–Ganzi and North Kunlun terranes, respectively, based on the data in Table 3 and the Electronic Appendix. The differences in the Ce/Pb ratios of the primitive magmas in the Songpan–Ganzi and North Kunlun terrane are relatively small. Nevertheless, it seems clear that the mantle source of the primitive magmas must differ in composition between the North Qiangtang terrane and the Songpan–Ganzi and North Kunlun terranes in terms of its trace element composition.
The significantly high abundances of incompatible trace elements (e.g. Rb, Ba, Th, U, Pb and Sr; Figs 4 and 6), and high light REE (LREE)/HREE (Fig. 7a), require enrichment of the mantle source region of the magmas before the onset of partial melting. The generation of such extreme trace element enrichment in the North Qiangtang parental magmas (e.g. Ba >6000 ppm; Fig. 4f and g) by partial melting of normal asthenospheric mantle (i.e. MORB-source mantle) would require vanishingly small degrees of partial melting. For example, if the concentration of Ba in the melt was 7627 ppm (e.g. sample G98-14; Table 3), the degree of partial melting would be 0·0023%, assuming a bulk partition coefficient D = 6 x 10–5 (http://www.earthref.org/) and C0 = 0·63 ppm [normal MORB (N-MORB) mantle source; Sun & McDonough, 1989], based on a modal batch partial melting model (Wilson, 1989). The enrichment of the mantle source is also shown by increases in Th/U, U/Pb, and Rb/Sr and a decrease in Sm/Nd relative to depleted mantle values. Additionally, the high 87Sr/86Sr ratios and relatively low 143Nd/144Nd isotopic ratios (Table 5 and Fig. 8) of the primitive samples from north Tibet reflect a mantle source region with a time-integrated history of enrichment in Rb and LREE.
On the basis of the combined trace element and Sr–Nd–Pb isotope data, we conclude that the parental magmas of the potassic and ultrapotassic magmatic rocks in north Tibet were derived from an enriched mantle source rather than the normal asthenospheric (i.e. N-MORB-source) mantle.
Two end-member components of the mantle source
Experimental data (Brenan et al., 1995; Keppler, 1996) and geochemical studies (Gill, 1981; Pearce, 1982; Pearce & Parkinson, 1993; Pearce & Peate, 1994; Hawkesworth et al., 1993, 1997a; Turner, 2002; Turner et al., 2003) indicate that island arc and active continental margin mafic magmas, petrogenetically related to subduction of oceanic lithosphere, are characterized by significant enrichment in LILE relative to HFSE, with strongly negative Nb–Ta–Ti anomalies and positive Pb anomalies in primitive mantle-normalized incompatible trace element diagrams. These characteristics are also shown by the primitive potassium-rich magmatic rocks from the northern part of the Tibetan Plateau (Fig. 6). Additionally, the Sr–Nd–Pb isotopic compositions of the primitive magmatic rocks in north Tibet plot within the range of isotopic compositions of Global Subducting Sediment (GLOSS; Plank & Langmuir, 1998) (Fig. 8), consistent with a subduction-related origin for the mantle source enrichment.
Previous geological studies have shown that the basement terranes (North Kunlun, South Kunlun, Songpan–Ganzi and North Qiangtang; Fig. 1) in north Tibet experienced multiple episodes of subduction of Proto- and Palaeo-Tethys oceanic lithosphere from Proterozoic to Middle–Late Triassic times, upon which were superimposed the effects of subduction of the back-arc basin lithosphere of the South Kunlun ocean and Jinsha ocean, respectively (Pan et al., 1998; Mattern & Schneider, 2000; Pan, 2000; Wang et al., 2000; Xiao et al., 2002). Migration of subduction-related melts and/or fluids would undoubtedly have contributed to the localized enrichment of the mantle lithosphere beneath north Tibet, generating low melting-point domains that would be easily melted during subsequent phases of lithospheric thinning and reheating, resulting in the generation of potassium-rich magmas with geochemical signatures similar to those of magmas derived from the mantle wedge above subduction zones.
Zhou & Murphy (2005) have provided evidence based on seismic tomography studies for wholesale underthrusting of Indian continental lithospheric mantle beneath the entire Tibetan Plateau and the presence of a thin asthenospheric mantle wedge above the subducted slab of Indian lithosphere. These data provide an alternative model for the petrogenesis of the most primitive magmas by partial melting of this asthenospheric wedge, which has been enriched by melts and/or fluids derived from Indian passive margin sediments subducted into the shallow mantle as a consequence of northward underthrusting of the Indian continental lithosphere. Partial melting of this enriched asthenospheric mantle wedge could produce magmas geochemically similar to subduction-related magmatic rocks. Logically, both the asthenospheric wedge and the base of the Asian continental lithosphere could be potential mantle sources for generation of the primitive magmas in north Tibet.
Previous studies have shown that supra-subduction zone enrichment of the mantle wedge can be broadly attributed to migration of: (1) aqueous fluids derived from dehydration of either altered oceanic crust (Tatsumi et al., 1986; Hawkesworth et al., 1993, 1997a; Turner et al., 1996b, 1997; Turner, 2002) or subducted sediments (Ryan et al., 1995; Class et al., 2000; Elburg et al., 2002); (2) partial or bulk melts of subducted sediments or even the subducted crust itself (Hawkesworth et al., 1993, 1997a; Vroon et al., 1993; Elliott et al., 1997; Elburg et al., 2002).
Important geochemical distinctions have been recognized between subduction-related magmatic rocks in which the magma sources are modified by a subduction-related aqueous fluid and those that are enriched by bulk subducted sediment or a partial or a bulk melt thereof (e.g. Hawkesworth et al., 1997a, 1997b; Elburg et al., 2002). The fluid, which carries very little REE and HFSE (e.g. Ce, Th, Nb, Zr and Ta), introduces significant amounts of LILE (e.g. Rb, Ba, Sr) and other fluid-mobile trace elements (e.g. U, Pb) into the mantle wedge. However, subducted sediment (or partial or bulk melt) is characterized by relatively high Th and LREE contents. Thus, those subduction-related magmatic rocks whose source was strongly metasomatized by a fluid component are likely to have higher U/Th (Hawkesworth et al., 1997a), Ba/Th (Hawkesworth et al., 1997a; Turner et al., 1997) and Sr/Th (Hawkesworth et al., 1997a) ratios than rocks whose source was enriched by bulk sediment or a partial or bulk melt of subducted sediment (Hawkesworth et al., 1997a; Elburg et al., 2002). In contrast, those subduction-related magmas with a strong imprint of a partial melt or bulk melt of subducted sediment or subducted sediment itself in their source region have higher Th/Ce (Hawkesworth et al., 1997a, 1997b; Elburg et al., 2002), 87Sr/86Sr and 206Pb/204Pb ratios (Hawkesworth et al., 1997b; Turner et al., 1997), higher Th contents, and lower 143Nd/144Nd ratios (Hawkesworth et al., 1997a, 1997b) than those related to fluid metasomatism of their mantle source. Both fluid- and melt- (or subducted bulk sediment) induced metasomatism could account for the negative Nb–Ta–Ti anomalies of the north Tibet potassium-rich rocks (Fig. 6).
The combination of incompatible trace element and isotope ratios has been effectively used as a fingerprint in identifying the origin of metasomatic components in the source of subduction-related magmas (Hawkesworth et al., 1997a, 1997b; Turner et al., 1997). Previous studies have shown that fluids generated by dehydration of hydrothermally altered subducted oceanic crust have low 87Sr/86Sr (0·7035), but high Ba/Th (> 170) ratios (Turner et al., 1996b, 1997; Hawkesworth et al., 1997a, 1997b; Elburg et al., 2002; Guo et al., 2005). The north Tibet primitive samples have higher (87Sr/86Sr)i ratios (>0·706) and lower Ba/Th ratios than fluids derived from altered subducted oceanic crust, and lower Ba/Th ratios than fluids derived from subducted sediments (Fig. 13). The (87Sr/86Sr)i ratios of the primitive magmas in north Tibet fall almost exclusively within the range of GLOSS (Plank & Langmuir, 1998; Fig. 13), which may suggest that their mantle source region was modified by mixing in bulk subducted sediment or metasomatized by a partial melt or a bulk melt of subducted sediment prior to the generation of the Cenozoic potassium-rich magmas.
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It is thought unlikely that bulk subducted sediment can be added to the mantle source of subduction-related magmas (Hawkesworth et al., 1997a
; Turner et al., 1997; Elburg et al., 2002). The north Tibet primitive samples display a broad, linear, positive correlation between Th/Ce and Th/Sm (Fig. 14a), which could suggest simple two-component mixing. One end-member has a composition similar to Indian MORB-source mantle (Rehkämper & Hofmann, 1997) with low Th/Ce and Th/Sm. The average composition of GLOSS (Plank & Langmuir, 1998), however, lies within the linear trend; most samples plot beyond the range of Indian MORB-source mantle and the GLOSS average (Plank & Langmuir, 1998). This argues against average GLOSS as the second end-member for the two-component mixing. The sediment-derived end-member has to have much higher Th/Ce and Th/Sm ratios than the GLOSS average. Th/Ce and Th/Sm fractionation is likely during partial melting of subducted sediment, leading to higher Th/Ce and Th/Sm ratios in the partial melt than in the subducted bulk sediment (i.e. GLOSS) or a bulk melt of the subducted sediment. This indicates that the second end-member for the two-component mixing in Fig. 14a is probably a partial melt of subducted sediment. Simple two-component (i.e. partial melt of the sediment and Indian MORB-source mantle) mixing calculations support this inference; the model curve broadly approximates the analysed data trend (Fig. 14a). The departure of the model curve from the trend of the actual data may be interpreted as reflecting differences in the fractionation of Th/Ce and Th/Sm during partial melting of the subducted sediment. The model curve suggests that the degree of partial melting of average GLOSS is 4%, and that the proportion of the partial melt of average GLOSS in the primitive magmatic rocks of the North Qiangtang terrane (4–15%) is higher than that in the Songpan–Ganzi and North Kunlun terranes (<8%). The linear arrays in Sr–Nd–Pb isotope space defined by the primitive samples (Fig. 8a, d and e) also support such a two-component mixing model.
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If a partial melt of subducted sediment was added to the mantle source, the Ba/Th ratios of the primitive magmas should be slightly higher than those of bulk subducted sediment (i.e. GLOSS), because of Ba/Th fractionation during partial melting of the sediment. However, it is possible that phlogopite in the mantle source region may have retained Ba to variable degrees, resulting in relatively low and variable Ba/Th ratios in the primitive magmas in north Tibet (Fig. 13).
The combined constraints of incompatible trace element and Sr–Nd–Pb isotope ratios show that the mantle source region of the primitive magmas in north Tibet resulted from metasomatism of Indian MORB-source mantle by a partial melt of subducted sediment.
Heterogeneity of the mantle source beneath the different terranes
The differences in the proportion of partial melt of subducted sediment (GLOSS) in the primitive magmas of the North Qiangtang terrane and the Songpan–Ganzi and North Kunlun terranes (Fig. 14a), indicated by the two-component mixing model calculations, suggest regional compositional differences in the mantle source regions. Moreover, the primitive magmatic rocks from the North Qiangtang terrane have higher (87Sr/86Sr)i and (206Pb/204Pb)i, and lower (143Nd/144Nd)i ratios compared with those from the Songpan–Ganzi and North Kunlun terranes (Fig. 8). This is consistent with an increased amount of subducted sediment-derived melt added to the mantle source beneath the North Qiangtang terrane relative to the North Kunlun and Songpan–Ganzi terranes.
The slopes of the positive correlations between La/Yb and La (Fig. 7a), and Ce/Pb and Ce (Fig. 7b) are markedly different in the primitive magmatic rocks from the North Qiangtang terrane compared with those from the Songpan–Ganzi and North Kunlun terranes. This indicates that the primitive magmas in the different terranes of north Tibet were derived by variable degrees of partial melting of different mantle sources. The results of non-modal batch melting modelling (Fig. 7a) indicate that the mantle source region of the primitive magmas beneath the North Qiangtang terranes contains 5% sediment melt, whereas that beneath the Songpan–Ganzi terrane contains 1% sediment melt; degrees of partial melting of the mantle source range from 0·1% to 15%. This supports the inference that the composition of the mantle source region beneath the North Qiangtang terrane was different from that beneath the North Kunlun and Songpan–Ganzi terranes.
Residual minerals in the mantle source region of the parental magmas
The positive correlations between La/Yb and La (Fig. 7a), and Ce/Pb and Ce (Fig. 7b) show that partial melting is the dominant control factor on the compositional variations of the most primitive potassium-rich magmatic rocks in the northern part of the Tibetan Plateau. The following residual minerals in the mantle source region might retain their compatible elements during partial melting.
Spinel
The relatively high middle REE (MREE) and HREE abundances (Table 3 and Electronic Appendix) of the primitive samples (e.g. Yb contents that are >10 times chondritic abundances; Fig. 5) preclude garnet as a residual mineral in the mantle source region. The relatively flat HREE distribution patterns in chondrite-normalized diagrams (Fig. 5a–c), showing that HREE fractionations are weak, also do not support the presence of garnet in the mantle source region. Instead, the patterns are consistent with the presence of spinel in the mantle source.
Phlogopite
It has been shown that fractionation of incompatible elements during partial melting will happen if mineral phases that retain them are residual in the source region at low degrees of melting (e.g. Wilson, 1989). Because the mantle source region of the primitive magmas in north Tibet is inferred to be a mixture of Indian MORB-source mantle and a partial melt of subducted sediment, the Rb/Ba ratios of the primitive magmas should lie between those of Indian MORB-source mantle and partial melts of subducted sediment, if no residual minerals retained Rb and Ba, because both Ba and Rb are strongly incompatible during mantle partial melting. However, most of the primitive samples studied here have lower Rb/Ba ratios than both Indian MORB-source mantle and the GLOSS average (Fig. 14b). This requires that a mineral phase has preferentially retained Rb relative to Ba, which, therefore, must have a higher partition coefficient for Rb than Ba. The depletion of Rb relative to Ba in the primitive mantle-normalized incompatible element patterns of the primitive magmatic rocks also supports this inference (Fig. 6). These characteristics are consistent with the presence of phlogopite, which has a higher partition coefficient for Rb than Ba, rather than amphibole, because amphibole has a higher partition coefficient for Ba than Rb (Green, 1994; Foley et al., 1996).
No correlation exists between (Rb/Ba)PM and MgO (wt %) in the studied samples (not shown), suggesting that there is no fractionation of phlogopite during magmatic differentiation (PM in the subscript represents the primitive mantle-normalized value). There is, however, a strong negative correlation between (Rb/Ba)PM and La/Yb in the primitive magmatic rocks, which indicates a decrease in the extent to which Rb is retained by residual phlogopite in the mantle source region with increasing degrees of partial melting (Fig. 15a and b); that is, phlogopite preferentially enters the melt during partial melting. The primitive magmas in the Songpan–Ganzi terrane have very uniform K contents (Fig. 6b), indicating that concentrations of this element are strongly buffered by residual phlogopite in the mantle source. However, the primitive magmas in the North Qiangtang terrane have variable K contents (Fig. 6a). This suggests that K is not as strongly buffered by phlogopite in the mantle source as in the Songpan–Ganzi terrane, or that the mantle source is heterogeneous (Fig. 10a), or both.
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Two Ti-bearing mineral phases
Th/Nb ratios in the primitive magmas of north Tibet should lie between the Indian MORB-source mantle end-member and that of a partial melt of subducted sediment, based on the inference that the mantle source region of the magmas was a mixture of Indian MORB-source mantle and a partial melt of subducted sediment (Fig. 14a), only if the primitive magmas inherit the Th/Nb of their mantle source. However, most of the primitive samples have higher Th/Nb ratios than both Indian MORB-source mantle and the GLOSS average (Plank & Langmuir, 1998
; Fig. 14b). This indicates that the addition of subducted sediment melts to the mantle source cannot totally be responsible for the high Th/Nb ratios; these are, instead, probably caused by residual Ti-bearing phases in the source region. These phases must have higher partition coefficients for Nb than Th, and preferentially retain Nb over Th. The positive correlation between Nb (ppm) and TiO2 (wt %) in the primitive rocks (not shown), combined with strong negative anomalies in Nb, Ta and Ti in mantle-normalized trace element diagrams (Fig. 6), supports the presence of Ti-rich residual mineral phases in the mantle source region. In addition, the relatively flat correlation of TiO2 vs MgO for the samples (Fig. 4e) also suggests that there might be residual Ti-bearing phases buffering the TiO2 contents. No correlations exist between Ti/Ti*, Nb/Ta and MgO in the Tibetan magmatic rocks (not shown), indicating that there is no fractionation of Ti-bearing phases (Ti* is the expected concentration of Ti assuming no Ti anomaly in the mantle-normalized incompatible element diagram shown in Fig. 6). Based on the above, we conclude that the primitive mantle-normalized incompatible element patterns (Fig. 6) result from both additions of melt of subducted sediments to the mantle source, which produces enrichment in LILE (Fig. 14a), and the presence of Ti-rich residual mineral phases retaining HFSE, which causes depletions in these elements. Previous studies (e.g. Williams et al., 2004) identified only one Ti-rich residual mineral phase (i.e. rutile). However, below we show that there were at least two residual Ti-rich mineral phases in the mantle source.
Both rutile and titanite have high partition coefficients for the HFSE Nb, Ta and Ti (Green, 1994; Foley et al., 2000; Green et al., 2000). These phases may retain the HFSE during progressive mantle partial melting, leading to strong depletion of these elements in the resultant magmas (Foley & Wheller, 1990; Sheppard & Taylor, 1992; Pearce & Parkinson; 1993; Pearce & Peate, 1994). Rutile has a higher partition coefficient for Nb (136) than Ta (16·6), whereas titanite has a higher partition coefficient for Ta (142) than Nb (4·6) (data from: http://www.earthref.org/). Some samples from north Tibet exhibit depletions in Nb that are stronger than those of Ta in the mantle-normalized trace element patterns (Fig. 6a and b); this suggests that rutile may be the dominant residual mineral phase in the mantle source. However, other samples show greater depletions of Ta than Nb (Fig. 6a and b), suggesting that titanite may be the dominant residual mineral phase in the mantle source. Other samples show broadly similar depletions of Ta and Nb (e.g. Fig. 6c), which might be well explained by approximately equal proportions of titanite and rutile in the mantle source region. This is further confirmed by the correlation between (Nb/Ta)PM and La/Yb (Fig. 15e and f), which shows a broadly decreasing trend in (Nb/Ta)PM ratios with decreasing La/Yb in the primitive magmas of the North Qiangtang and Songpan–Ganzi terranes. In detail, some of the primitive magmatic rocks have (Nb/Ta)PM ratios >1·0, suggesting that both titanite and rutile were present in the mantle source, and that titanite is more important than rutile in terms of retaining the HFSE; however, other primitive samples have (Nb/Ta)PM ratios <1·0, which suggests that in their petrogenesis titanite had progressively melted out and only rutile was residual in the mantle source, differentially retaining the HFSE because of differences in the partition coefficients for Nb and Ta in rutile and titanite. This suggests that the influence of residual rutile on the Nb and Ta contents of the primitive magmas becomes progressively stronger than that of titanite with increasing degree of partial melting. Thus, the negative correlation between (Nb/Ta)PM and La/Yb (Fig. 15e and f) suggests that the Ti-bearing residual mineral phases in the mantle source included both rutile and titanite, followed by rutile only with increasing degree of partial melting.
Ti/Ti* ratios show a progressive increase with increasing degree of partial melting (decreasing La/Yb) in the primitive magmas of the North Qiangtang and Songpan–Ganzi terranes (Fig. 15c and d). The broad positive correlation between (Rb/Ba)PM and Ti/Ti* (Fig. 16) suggests that residual phlogopite in the mantle source region may also influence the Ti budget in the primitive magmas.
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Apatite
No correlation exists between P/P* (the meaning of P/P* is similar to that of Ti/Ti*) and indices of differentiation (e.g. MgO wt %), which would be present if apatite fractionation was important. Thus, we assume that apatite was not involved in fractional crystallization. The negative P anomalies in the mantle-normalized incompatible element diagrams (Fig. 6a and b) probably reflect the presence of residual apatite in the mantle source region. The broad negative correlation between P/P* and La/Yb indicates increases in the P contents of the primitive magmas with increasing degree of partial melting of the mantle source (Fig. 15 g and h). This indicates that residual apatite in the mantle source region preferentially enters the melt during the partial melting process.
Trace element modelling
On the basis of the above discussion we propose that the dominant constraints on the composition of the primitive magmas in north Tibet are the degree of partial melting, the amount of subducted sediment melt added to the mantle source and the presence of residual phases in the source (e.g. phlogopite, rutile, titanite, apatite). Compared with subduction-derived fluids, partial melts of subducted sediments should have higher Th contents (Hawkesworth et al., 1997a; Elburg et al., 2002). To evaluate quantitatively the relative roles of the degree of partial melting and the amount of subduction-derived sediment melt induced metasomatism, and simultaneously to minimize the effects of fractional crystallization on the composition of the potassium-rich primitive magmas in north Tibet, we have selected the incompatible trace element ratios La/Yb and Th/Sm as simulated calculation parameters; these ratios may be considered indicative of the degree of partial melting and the amount of subducted sediment partial melt added, respectively. Both La/Yb and Th/Sm ratios are not significantly changed by small amounts of fractional crystallization of olivine and clinopyroxene, which are predicted by the major and trace element variations (Fig. 4a–d and l) and AFC modelling (Fig. 11). The model assumes that the primitive magmas in north Tibet were generated through two main processes: (1) enrichment of a depleted mantle source component with a partial melt of subducted sediment, followed by (2) partial melting of the resultant enriched mantle.
In developing an appropriate mantle melting model for the petrogenesis of the potassic and ultrapotassic magmas it is clear that we cannot assume modal melting because minor phases such as phlogopite, titanite, spinel, apatite and clinopyroxene preferentially enter the melt with respect to olivine, orthopyroxene and rutile during mantle partial melting process (Wilson, 1989; Guo et al., 2005). Moreover, the distinctive and systematic trends in trace element abundances and their ratios (Figs 6 and 15) exhibited by the primitive magmas suggest that some minerals (e.g. phlogopite, apatite and titanite) may be preferentially consumed ahead of others (e.g. olivine, orthopyroxene and rutile) with increasing degree of partial melting of the mantle source region. Thus, the proportions of the different mineral phases in the mantle source region will vary during progressive partial melting. Because we do not have enough evidence to quantitatively determine whether there was a continuous change in the bulk composition of the system during the mantle partial melting process, we have not adapted a fractional melting model. Consequently, a non-modal batch melting model (Wilson, 1989) was used to simulate the mantle partial melting process and to constrain changes in melt composition as some phases become preferentially exhausted in the residue as the degree of the partial melting increases.
Although the composition of the mantle beneath the three basement terranes (i.e. the North Qiangtang, Songpan–Ganzi and North Kunlun terranes) in north Tibet, prior to modification by the subducted sediment melt, appears to have been close to that of Indian MORB-source mantle, based on the trace element and Sr–Nd–Pb isotope compositions of the most primitive magmas (Figs 8 and 14a), the concentrations of La (1·9–4·1 ppm), Th (0·079–0·205 ppm), Sm (2·0–3·5) and Yb (2·3–3·5 ppm) in Indian MORB are rather variable (Rehkämper & Hofmann, 1997). Therefore, in our modelling we have used normal MORB (i.e. N-MORB; Sun & McDonough, 1989) source mantle instead as the source material; this is assumed to have 0·1 times N-MORB (Sun & McDonough, 1989) concentrations of the relevant trace elements (i.e. Th 0·012 ppm, La 0·250 ppm, Sm 0·263 ppm and Yb 0·305 ppm) based on the assumption that MORB magmas are results of 10% partial melting of their mantle source (Wilson, 1989).
Ratios of the incompatible trace elements (e.g. La/Yb, Th/Sm) in the primitive Tibetan magmas should not be significantly modified by partial melting processes, although these ratios in the partial melt of the subducted sediment may be slightly increased compared with the subducted bulk sediment because of the slightly different incompatibility between La and Yb, and Th and Sm. We have used the La/Yb and Th/Sm ratios of average GLOSS (Plank & Langmuir, 1998) to approximate those of a partial melt of the subducted sediment. The concentrations of the trace elements Th, La, Sm and Yb in the subducted sediment are 6·91 ppm, 28·8 ppm, 5·78 ppm and 2·76 ppm, respectively.
Johnson et al. (1990) systematically studied residual abyssal peridotites collected from the American–Antarctic and Southwest Indian Ridge and indicated modal proportions of clinopyroxene ranging from 2 to 12%. Because clinopyroxene is preferentially incorporated into the melt during mantle partial melting, compared with olivine and orthopyroxene, the initial proportion of clinopyroxene in MORB-source mantle should be higher than 12%. Johnson (1998) and Hellebrand et al. (2002) indicated that the spinel peridotite modes in MORB-source mantle are olivine (53%), orthopyroxene (27%), clinopyroxene (17%) and spinel (3%). Similarly, Bizimis et al. (2000) suggested the following modal mineralogy for MORB-source mantle: olivine (55%), orthopyroxene (25%), clinopyroxene (18%) and spinel (2%). Infiltration of a subducted sediment-derived, silica-rich partial melt into the mantle will clearly change the modal mineralogy, and would increase the content of orthopyroxene at the expense of olivine (Kelemen et al., 1998). In addition, considering the formation of new mineral phases (e.g. phlogopite, titanite, rutile and apatite) as a result of metasomatism of the mantle beneath the three terranes in north Tibet, we have assumed that the resultant minerals and their modal proportions in the metasomatized mantle source region after introduction of the subduction-derived sediment partial melt are olivine (45%), orthopyroxene (30%), clinopyroxene (18%), spinel (2%), phlogopite (2%), titanite (2%), rutile (0·5%) and apatite (0·5%). The mineral–melt partition coefficients for Th, La, Sm and Yb in the mineral phases used in the model (Table 9) are based on those from the GERM (Geochemical Earth Reference Model) website (http://www.earthref.org/). Phlogopite phenocrysts are present in the K-rich magmatic rocks in north Tibet (Table 2), which suggests hydrous partial melting conditions. Following Bizimis et al. (2000), the melting mineral mode for the hydrous melting condition was considered to be: clinopyroxene (0·56), olivine (–0·10), orthopyroxene (0·52) and spinel (0·02). Because additional mineral phases (e.g. phlogopite, titanite, rutile and apatite) were formed during mantle metasomatism in north Tibet, and phlogopite, apatite, spinel, clinopyroxene and titanite are considered to be completely consumed into the melt before rutile, olivine and orthopyroxene based on the above interpretations (Figs 15 and 16), we, therefore, assumed the following mineral melting mode (i.e. the proportion of the mineral phases entering the melt): apatite (0·015), clinopyroxene (0·55), olivine (–0·10), orthopyroxene (0·40), phlogopite (0·062), rutile (0·003), spinel (0·02) and titanite (0·05).
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On the basis of the above data, following the approach of Guo et al. (2005)
, we carried out non-modal batch melting model calculations. The results of the trace element modelling show that the degree of mantle partial melting required to fit the data ranges from <0·1% to 15%, and that the proportion of subducted sediment-derived partial melt added to the mantle source ranges from 0% to around 10% for the primitive magmas (Table 10 and Fig. 17). Moreover, generation of the primitive magmas in the North Qiangtang terrane requires higher amounts (0·5–10%) of the sediment partial melt added to the mantle source compared with the Songpan–Ganzi and North Kunlun terranes (<2%). Thus, compared with the North Qiangtang terrane, both the Songpan–Ganzi and North Kunlun terranes appear to have experienced less supra-subduction zone metasomatism prior to the generation of the K-rich magmas. It should be noted that the actual proportions of subducted sediment-derived partial melt added would be slightly lower than the model results (Fig. 17) because the La/Yb and Th/Sm ratios are likely to be slightly higher in the subducted sediment-derived partial melt than in the bulk sediment.
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The model results support the inferences made earlier based on the trace element (Figs 6 and 7) and Sr–Nd–Pb isotope data (Fig. 8), which confirm the heterogeneity in composition of the mantle source regions of the primitive potassium-rich magmatic rocks beneath the three terranes in north Tibet. It should be noted that the modelling results depend on the selection of a number of initial parameters, for example, the mantle source and subducted sediment melt compositions, and the partition coefficients and the proportions of residual mineral phases. However, sensitivity calculations indicate that the effect of variation in these parameters (e.g. composition of the mantle source and subducted sediment melt and the partition coefficients of the residual minerals for Th, La, Sm and Yb) on the final results of the modelling is not significant; this is 1–4% for Th/Sm and 3–7% for La/Yb ratios. The variation trends of La/Yb and Th/Sm ratios are similar even when the initial parameters are changed significantly (e.g. proportions of residual mineral phases). The subducted sediment melt has higher contents of K and Ba than N-MORB-source mantle (Fig. 6d). The average concentrations of K and Ba in the primitive magmatic rocks of the North Qiangtang terrane are higher than those in the Songpan–Ganzi and North Kunlun terranes (Table 3 and Electronic Appendix). This, combined with higher proportions of the sediment melt added to the mantle source beneath the North Qiangtang terrane than beneath the Songpan–Ganzi and North Kunlun terranes (Fig. 17), suggests that the subducted sediment melt is a good agent for producing K (and also Ba) metasomatism of the mantle source of the K-rich (and also Ba-rich) magmas in north Tibet.
Petrogenesis and geodynamic setting of the potassium-rich magmatism
As mentioned above, there are two relatively popular hypotheses for the petrogenesis of the post-collisional potassic and ultrapotassic magmas in north Tibet. The first involves southward intracontinental subduction of the Tarim and Qaidam terranes beneath the northern Tibetan Plateau (e.g. Pearce & Mei, 1988; Deng, 1989, 1991, 1998; Arnaud et al., 1992; Tapponnier et al., 2001; Ding et al., 2003). However, the results of the trace element modelling suggest that increasing amounts of subducted sediment partial melt were added to the mantle beneath the northern Tibetan Plateau from north (North Kunlun and Songpan–Ganzi terranes) to south (North Qiangtang terrane). Moreover, there is a semi-continuous increase in age of the post-collisional K-rich magmatism from north to south in the northern part of the Tibetan Plateau (Fig. 2a). This is not easily explained by south-dipping intracontinental subduction of the Tarim and Qaidam terranes. In addition, recent deep seismic reflection profiling (Gao et al., 2001; Xiao et al., 2001) and petrological (e.g. Deng et al., 2004) studies have shown the absence of south-dipping intracontinental subduction in north Tibet. The second hypothesis attributes the petrogenesis of the potassic and ultrapotassic magmatic rocks to convective removal of the lower part of previously thickened lithospheric mantle beneath north Tibet (e.g. Turner et al., 1993, 1996a; Williams et al., 2004). However, this hypothesis is inconsistent with the following observations. (1) Recent studies show little or no sign of lithospheric removal or surface rebound in north Tibet since the Eocene, based on geological, geochronological and geophysical results (Tapponnier et al., 2001). (2) Geophysical studies have indicated that the northern Tibetan upper mantle is characterized by inefficient regional S-wave (Sn) propagation (Ni & Barazangi, 1983; McNamara et al., 1995) and low regional P-wave (Pn) velocity (McNamara et al., 1995), broadly beneath the region of post-collisional potassic and ultrapotassic magmatism studied here (Fig. 1). This region of the mantle is considered to be hotter than that of the adjacent regions to a depth of 410 km (Owens & Zandt, 1997; Kosarev et al., 1999; Xu et al., 1999; Unsworth et al., 2004); thus, it would also appear to be physically difficult to explain by delamination of cold lithosphere because the sinking of delaminated cold lithosphere would decrease the asthenospheric temperatures in this region by thermal conduction. (3) As metasomatism is likely to occur predominantly at deeper levels within the mantle lithosphere (Niu et al., 2002), the remaining upper part of the lithosphere may be barren or less metasomatized. To partially melt such lithosphere is thermally difficult, but if it did happen, it would result in trace element and isotopically depleted magmas, rather than enriched potassic and ultrapotassic magmas. Thus, if the sub-continental lithospheric mantle were thickened, the post-collisional potassic and ultrapotassic magmas would have to be generated by partial melting of the lower part of the thickened lithosphere (because this is the zone of enrichment), rather than the upper part of the lithosphere. (4) Spatially, the trend of the potassic and ultrapotassic magmatic province in the different terranes of north Tibet is roughly parallel to the Himalayas (Fig. 1); this suggests that the generation of the potassic and ultrapotassic magmas is more likely to be related to northward subduction of the Indian continent, post-India–Asia collision, than to convective thinning of the lithospheric mantle (Tapponnier et al., 2001). (5) The age of the post-collisional potassic and ultrapotassic magmatism in north Tibet ranges semi-continuously from 45 Ma to the present day (Fig. 2); this is unlikely to be explained by a petrogenetic model involving wholesale and sudden convective thinning of the lithospheric mantle (Yin & Harrison, 2000) on a time scale of about 5 Myr (Lenardic & Kaula, 1995), although some modifications of the convective thinning model have been proposed (e.g. Conrad & Molnar, 1999). Moreover, the decreasing trend in the age of the post-collisional potassium-rich magmatism in north Tibet from south to north (Fig. 2a) is also difficult to explain by convective thinning of the mantle lithosphere. (6) Geomorphological studies have shown that there is no evidence for sudden uplift, which is predicted by the model of convective thinning of the lithosphere (Meyer et al., 1998). (7) Deng et al. (2004) proposed that the average lithospheric thickness beneath north Tibet has been 150 km since initial collision of India and Asia at 65–70 Ma. There is little evidence to support the substantial Tertiary thickening of the lithosphere (250 km), proposed by Turner et al. (1996a), beneath north Tibet, which is required by models involving subsequent convective thinning of the mantle lithosphere. An earlier version of the second hypothesis, proposed by Turner et al. (1993), was based on limited geochemical data for the magmatic rocks in north Tibet, which suggested that the volcanism was younger than 13 Ma. However, the results of many new geochronological and geochemical studies of the magmatic rocks (summarized here), together with extension of the sampling area, require an alternative interpretation for the petrogenesis of the K-rich magmatism in north Tibet because the previous hypotheses can no longer explain all of the new data.
Studies of the shear-wave anisotropy of the mantle indicate that the direction of fast polarization is roughly east–west beneath Tibet (McNamara et al., 1994, 1995; Hirn et al., 1995; Guilbert et al., 1996; Lave et al., 1996; Sandvol et al., 1997). Some studies have ascribed this shear-wave anisotropy to deformation of the lithospheric mantle (McNamara et al., 1994; Silver, 1996), suggesting that this is a result of finite strain accumulated within the lithosphere over geological time (e.g. Davis et al., 1997). The strong shear-wave anisotropy beneath the northern part of the Tibetan Plateau, including the North Qiangtang and Songpan–Ganzi terranes, suggests that the mantle lithosphere beneath these areas is highly strained (McNamara et al., 1994). The sub-parallel relationship between the post-collisional potassic and ultrapotassic volcanic belts within the various terranes (Fig. 1) and the fabric indicated by the seismic anisotropy data may indicate a relationship between the deformation of the Tibetan lithospheric mantle and K-rich magma generation processes. In addition, Cenozoic tectonic movements in north Tibet are characterized by large-scale strike-slip faults (Tapponnier & Molnar, 1977), some of which are located within older tectonic suture zones [e.g. South Kunlun suture, Jinsha suture; for further discussion see Xu et al. (1999) and Yin & Harrison (2000)]. There are a number of pull-apart basins and push-up structures along these large strike–slip faults in north Tibet, which are associated with movements along the faults (Deng, 1998; Xu et al., 1999; Yin & Harrison, 2000). Geophysical studies have shown that these strike-slip faults may penetrate the lithospheric mantle (e.g. Tapponnier et al., 2001). Detailed field studies have indicated that most of the potassic and ultrapotassic magmatism in the northern part of the Tibetan Plateau is distributed along these huge fault systems, which are either transpressional or strike-slip (e.g. Altyn-Tagh fault) in nature; some of the volcanic fields are located within pull-apart basins (e.g. the Ashikule volcanic field; Fig. 1) or at intersections between extensional or transtensional structures and compressional or strike-slip faults [QBGMR (Qinghai Bureau of Geology and Mineral Resources), 1991; Deng, 1998; Chi et al., 1999; Lai & Liu, 2001]. This suggests that the lithospheric mantle exerts a strong control over post-collisional K-rich magma generation processes in the northern Tibetan Plateau.
Previous studies (e.g. Lai et al., 1996; Owens & Zandt, 1997; Wu et al., 2001; Luo et al., 2003) have suggested that during the Cenozoic the sub-continental lithospheric mantle beneath north Tibet (i.e. the North Qiangtang and Songpan–Ganzi terranes) was relatively thin and weak compared with the lithosphere beneath the Indian continent to the south and that beneath the Tarim and Qaidam terranes to the north (Fig. 1). Moreover, geophysical studies indicate that the base of the lithosphere is between 90–120 km (An et al., 1993) and 140–160 km (Zhu et al., 2002) beneath north Tibet. Deng et al. (2004) proposed that the average lithosphere thickness beneath north Tibet and the terranes of Tarim and Qaidam (Fig. 1) has been 150 km and 200 km, respectively, since the initial collision of India and Asia in south Tibet at 65–70 Ma, based on geophysical, petrological and tectonic studies. Zhou & Murphy (2005) pointed out that the Indian continental lithosphere is stronger and thicker (>200 km) relative to the Tibetan mantle lithosphere. Compared with the relatively thick, rigid and strong lithosphere beneath the stable Tarim and Qaidam terranes (or cratons) to the north and the Indian continental craton to the south, the sub-continental lithospheric mantle beneath north Tibet is relatively thin and weak. This probably results from earlier episodes of subduction-related magmatism and metasomatism, leading to easier deformation of the lithospheric mantle beneath north Tibet during continental collision and the northward underthrusting of the Indian continental lithosphere beneath the Tibetan Plateau lithosphere (Owens & Zandt, 1997; Kosarev et al., 1999; Zhou & Murphy, 2005). The advancing, relatively thick, Indian continental lithosphere is considered to have pushed the asthenosphere beneath Tibet northwards until it was stopped by the thick Tarim and Qaidam continental lithosphere in the north of Tibet (Fig. 18). The relatively hot asthenosphere was squeezed up between the advancing Indian continental lithosphere front and the backstop of the older Asian continental lithosphere (Owens & Zandt, 1997; Kosarev et al., 1999), causing localized partial melting. Such a model is supported by the low P-wave velocities in the upper mantle beneath the Tibetan Plateau (Xu et al., 1999; Zhou & Murphy, 2005). The larger volumes (Table 1) and denser outcrops (Fig. 1) of the post-collisional K-rich magmatic rocks between 85°E to 93°E (longitude) in north Tibet compared with those in the western and eastern portions may be further explained by a steeper angle of underthrusting of the Indian continental lithosphere beneath central Tibet than beneath the west and east Tibetan Plateau; this has been verified by recent seismic tomography results (Zhou & Murphy, 2005). An asthenospheric mantle wedge appears to exist between the overlying northern Tibetan lithosphere and the underlying subducted Indian slab based on seismic tomography data (Zhou & Murphy, 2005). As discussed in previous sections, partial melts of subducted Indian passive margin sediments have probably metasomatized this asthenospheric wedge, forming the enriched mantle source region of the primitive magmas in north Tibet. The metasomatized asthenospheric wedge geochemically comprises two components: MORB-source mantle (normal asthenospheric mantle beneath north Tibet) and subducted sediment melt (partial melt of the subducted Indian passive margin sediments). In many respects, this may be indistinguishable from lithospheric mantle metasomatized during earlier episodes of subduction. Thus we cannot unequivocally distinguish between asthenospheric and lithospheric mantle contributions to the source of the primary magmas. Partial melts formed by upwelling of the metasomatized asthenospheric mantle would infiltrate enriched domains within the overlying Tibetan mantle lithosphere, inducing further partial melting. An alternative possibility is that the mantle source region was located at the boundary between the lithosphere and asthenosphere, including both enriched domains at the base of the lithosphere and the underlying metasomatized asthenospheric mantle. For the Qiangtang terrane, it is likely that there was a minimal contribution from the mantle lithosphere if the model of Kapp et al. (2003) for Late Triassic–Early Jurassic flat slab subduction is accepted.
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Relatively low degrees of partial melting of highly enriched mantle sources (in both lithosphere and asthenosphere) resulted in potassic and ultrapotassic magmas that escaped to the surface along reactivated diffuse palaeo-plate boundaries (lithospheric shear zones) and/or strike-slip faults. This explains the generally small volume yet widespread distribution of the potassic and ultrapotassic magmatism in north Tibet (Fig. 1a and Table 1). Geophysical studies (Owens & Zandt, 1997
; Chen & Ozalaybey, 1998; Kosarev et al., 1999; Xu et al., 1999; Yin & Harrison, 2000) indicate that the northern edge of the subducted Indian continental lithosphere beneath Tibet currently lies between the North Qiangtang and South Qiangtang terranes (at 33°N), and that there is upwelling of hot metasomatized asthenosphere beneath the North Qiangtang terrane, Songpan–Ganzi terrane and the South Kunlun suture in the northern part of the Tibetan Plateau (Figs 1 and 18). This has caused Pliocene–Quaternary magmatism in this area, including an active volcano at Ashikule (Fig. 1 and Table 1), located along the South Kunlun suture at the northern edge of the Tibetan Plateau. The zone of upwelling asthenosphere, inferred from seismic tomographic data, beneath north Tibet roughly overlaps with the distribution of the potassic and ultrapotassic magmatism in the northern part of the Tibetan Plateau (Fig. 1a; Wittlinger et al., 1996; Owens & Zandt, 1997; Deng, 1998; Kosarev et al., 1999; Xu et al., 1999; Zhou & Murphy, 2005).
As discussed above, the sub-continental lithospheric mantle beneath the Qaidam and South Kunlun terranes probably also experienced metasomatism as a consequence of subduction of the Proto-Tethys and South Kunlun oceans; however, no outcrops of post-collisional potassium-rich magmatic rocks have been found in these terranes (Fig. 1a). The plausible reasons are that (1) the sub-continental lithospheric mantle beneath the Qaidam and South Kunlun terranes is rigid and relatively thick and/or (2) the upwelling of hot metasomatized asthenosphere has not yet spread northwards beneath them (Owens & Zandt, 1997; Kosarev et al., 1999). However, the Pulu volcanic field, which is located in the North Kunlun terrane (Fig. 1a), is located on the intersection between the Altyn Tagh strike-slip fault, which is inferred to reach the base of the lithosphere (Tapponnier et al., 2001), and a series of north–south-trending extensional faults (Deng, 1998). This may have induced a zone of decompression partial melting (Yin & Harrison, 2000). Thus, two factors are requisites for the generation of the post-collisional potassic and ultrapotassic magmatic rocks in north Tibet: one is the presence of thin and weak (i.e. easily deformed and disrupted) lithosphere that has previously been metasomatized and enriched, and another is the upwelling of hot metasomatized asthenosphere beneath the lithosphere.
Timing and mechanism of uplift of the northern Tibetan Plateau, constrained by the K-rich magmatism
Upwelling of the asthenosphere, resulting from the northward underthrusting of the Indian continental lithosphere, may have led to the uplift of the overlying lithospheric mantle beneath the northern Tibetan Plateau. This, in turn, resulted in uplift of north Tibet. The progressively northward underthrusting of the Indian continental lithosphere may have resulted in continuous upwelling of the asthenosphere beneath north Tibet since the initial collision of India and Asia in south Tibet at 65–70 Ma. This in turn would lead to progressive uplift of the northern Tibetan Plateau since 65–70 Ma, generating the highest plateau in the world. As a consequence of upwelling of the asthenosphere, K-rich magmatism has formed with an approximately continuous age spectrum from 45 Ma to the present day (Fig. 2).
Lithospheric convergence between India and Asia in Tibet since 65–70 Ma has resulted in underthrusting of the Indian continental lithosphere, pushing the asthenosphere beneath north Tibet progressively further northwards (Owens & Zandt, 1997); however, the Tarim and Qaidam lithosphere has remained stable as there is no south-dipping intracontinental subduction in north Tibet. Consequently, uplift of the North Qiangtang terrane should have been earlier than that of the Songpan–Ganzi and Kunlun terranes. This inference may explain the progressive decrease in the age of the potassic and ultrapotassic magmatism in north Tibet from south to north (Fig. 2a) in terms of the northward migration of the region of partial melting of the enriched asthenospheric mantle. This suggests that both the post-collisional, K-rich magmatism and uplift of the northern Tibetan Plateau resulted from the upwelling of the asthenosphere. If this inference is correct, the age of the magmatism might indicate the timing of relatively rapid and large-scale uplift of the northern Tibetan Plateau.
Figure 19 indicates the volume of magmatism as a function of the age of the K-rich rocks in north Tibet. This shows that post-collisional magmatism occurred continuously between 45 Ma and the present day with peaks at 45–38 Ma, 30–24 Ma and 18 Ma–present. This may suggest that rapid uplift of the northern Tibetan Plateau occurs in similar pulses. Because the sub-continental lithospheric mantle beneath north Tibet is thinner and weaker compared with the lithospheric mantle beneath the Tarim and Qaidam terranes (or cratons) to the north and the Indian continental plate to the south, the uplift of the lithosphere beneath north Tibet, as a consequence of the upwelling of the asthenosphere, may have been earlier than indicated by the age of the K-rich magmatism, because a period of time would be required for partial melting of the enriched mantle source, AFC processes in the crust and upward migration of the magmas. This inference is consistent with our detailed field observations; some volcanic fields are located on the top of the high mountains at an average elevation of 5000 m above sea level (e.g. Ashikule), whereas other volcanic fields are located on erosion plains (e.g. Yulinshan and Bamaoqiongzong; Fig. 1a). Well-preserved outcrops in the volcanic fields indicate that volcanism must have occurred after the formation of the high mountains and the erosion plains; otherwise, the effusive volcanic rocks would have been removed by subsequent strong erosion and weathering.
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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On the basis of major and trace element and Sr–Nd–Pb isotope data, combined with previous geochronological and geophysical studies, we propose a petrogenetic model to explain the spatial and temporal variations in the distribution and geochemical characteristics of the post-collisional potassic and ultrapotassic magmatism in the northern part of the Tibetan Plateau. This model advocates an important role for enrichment of the asthenospheric mantle beneath the northern part of the plateau as a consequence of the underthrust Indian continental lithosphere. Locally, there may also be a contribution from lithospheric mantle enriched during earlier episodes of subduction of the Proto- and Palaeo-Tethys oceans. Partial melts derived from subducted sediments, including Indian passive margin sediments and Proto- and Palaeo-Tethyan continental margin sediments, were the main agents that caused the enrichment of the asthenospheric mantle source of the primitive northern Tibetan magmas. The mantle-normalized incompatible element patterns of the most primitive mafic magmas (MgO >6 wt %) in north Tibet are comparable with those of subduction-related magmatic rocks; these are attributed to a combination of addition of subducted sediment melts to the mantle source and the presence of two Ti-rich residual mineral phases. The amount of subducted sediment melt added to the mantle source region of the potassic and ultrapotassic magmas in the North Qiangtang terrane is higher than that in the source of the magmas in the Songpan–Ganzi and North Kunlun terranes, resulting in compositional differences between the primitive magmatic rocks in the North Qiangtang terrane and those in the Songpan–Ganzi and North Kunlun terranes. The subducted sediment melt is a good agent for producing K (and also Ba) metasomatism of the mantle source of the K-rich (and also Ba-rich) magmas in north Tibet. The composition of the mantle source prior to the introduction of the subducted sediment-derived melts appears to be similar in all three terranes, corresponding to that of Indian MORB-source mantle (i.e. asthenospheric mantle). The increasing input of partial melt of subducted sediment from north to south before the generation of the Tertiary–recent potassic and ultrapotassic magmas, together with the southward increase in the age of the post-collisional potassic and ultrapotassic magmatic rocks, cannot be easily explained by south-dipping intracontinental subduction of the Tarim and Qaidam terranes. However, this is consistent with northward underthrusting of the Indian continental lithosphere. The evidence presented here does not support the hypothesis that the post-collisional K-rich magmas were generated by convective removal (delamination) of the lower part of a previously thickened lithospheric mantle root beneath north Tibet. Our preferred model involves the partial melting/upwelling of hot metasomatized asthenosphere beneath north Tibet as a result of the northward underthrusting of the Indian continental lithosphere and rapid convergence between the Indian and Asian lithosphere. The uplift of the northern Tibetan Plateau is also inferred to result from the upwelling of the asthenosphere and may be periodic at 45–38 Ma, 30–24 Ma and 18 Ma–present. AFC processes, operating in the middle crust in the North Qiangtang terrane and in the upper crust in the Songpan–Ganzi and North Kunlun terranes, can explain the range of evolved (MgO <6 wt %) magmas types, which have similar compositions in the three terranes of north Tibet.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGPETROGRAPHYANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We thank J. C. Lin, N. Sun and Q. Zhao for their friendship and guidance during the field work. Further logistic help was provided during the field campaigns by the Qinghai and Tibet Bureaus of Geology and Mineral Resources of China. This study was financially supported by a joint project (2003–2005) between the Royal Society of London and the National Natural Science Foundation of China (NSFC). Yaoling Niu is particularly acknowledged for his help in this study. This work was also supported by grants from the NSFC (40372045 and 40473023). Julian Pearce, Helen Williams and Wendy Bohrson are acknowledged for their constructive reviews of an earlier version of the manuscript. We also thank the editor, John Gamble, for his support.
* Corresponding author. Present address: Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, People's Republic of China. Telephone: 0086 10 62007334. Fax : 0086 10 62010846. E-mail: zhengfu{at}earth.leeds.ac.uk; zfguo{at}mail.iggcas.ac.cn
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Y. Gao, Z. Hou, B. S. Kamber, R. Wei, X. Meng, and R. Zhao Lamproitic Rocks from a Continental Collision Zone: Evidence for Recycling of Subducted Tethyan Oceanic Sediments in the Mantle Beneath Southern Tibet J. Petrology, April 1, 2007; 48(4): 729 - 752. [Abstract] [Full Text] [PDF]
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