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Journal of Petrology | Volume 44 | Number 10 | Pages 1833-1865 | 2003
© Oxford University Press 2003; all rights reserved
Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction
1 INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, BEIJING 100029, PEOPLE'S REPUBLIC OF CHINA
2 DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, AZ 85721-0077, USA
* Corresponding author. Telephone: (520) 626-8763. E-mail: pkapp{at}geo.arizona.edu
RECEIVED MARCH 15, 2002; ACCEPTED APRIL 14, 2003
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONGeochronological (K–Ar or 40Ar/39Ar), major and trace element, Sr–Nd–Pb isotopic and mineral chemical data are presented for newly discovered Cenozoic volcanic rocks in the western Qiangtang and central Lhasa terranes of Tibet. Alkali basalts of 65–45 Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mg-numbers [100 x Mg/(Mg + Fe)] (54–65), Cr (204–839 ppm) and Ni (94–218 ppm) contents, and relatively low ratios of 87Sr/86Sr (0·7046–0·7061), 206Pb/204Pb (18·21–18·89), 207Pb/204Pb (15·49–15·61) and 208Pb/204Pb (38·42–38·89), and high ratios of 143Nd/144Nd (0·5124–0·5127). In contrast, younger volcanic rocks in the western Qiangtang terrane (
30 Ma) and the central Lhasa terrane (23, 13 and 8 Ma) are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source. They are characterized by very high contents of incompatible trace elements, negative Ta, Nb and Ti anomalies, and radiogenic Pb isotopic compositions (206Pb/204Pb = 18·43–19·10; 207Pb/204Pb = 15·64–15·83; 208Pb/204Pb = 39·14–39·67). 87Sr/86Sr (0·7088–0·7092) and 143Nd/144Nd (0·5122) ratios of the western Qiangtang terrane potassic lavas are similar to those of 45–29 Ma potassic volcanic rocks in the north–central Qiangtang terrane, whereas 87Sr/86Sr (0·7167–0·7243) and 143Nd/144Nd (0·5119) ratios of central Lhasa terrane lavas are similar to those of 25–16 Ma ultrapotassic volcanic rocks in the western Lhasa terrane. The 65–45 Ma alkali basalts in the western Qiangtang terrane, along with widespread calc-alkaline volcanic rocks of this age in the Lhasa terrane, may be related to roll-back of a previously shallow north-dipping slab of Tethyan oceanic lithosphere beneath Tibet. Subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in Tibet because of its occurrence in distinct, east–west-trending belts (45–29 Ma in the Qiangtang terrane; 25–17 Ma in the northern Lhasa terrane; 16–8 Ma in the southern Lhasa terrane) and temporal and spatial relationships with major thrust systems. KEY WORDS: Tibet; geochemistry; Indo-Asian collision; sodic and potassic volcanism; continental subduction
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INTRODUCTION |
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The Tibetan plateau is commonly considered to be the archetypal collisional orogen; nevertheless, significant questions remain regarding its tectonic evolution. Perhaps the most fundamental is whether convergence between India and Asia was accommodated in the continental mantle lithosphere mainly by homogeneous thickening (e.g. Molnar et al., 1993) or subduction (e.g. Tapponnier et al., 2001). Geochemical, isotopic, and geochronological studies on Cenozoic volcanic rocks in Tibet, when integrated with constraints on the timing and distribution of crustal deformation, have the potential to address this question.
The oldest Cenozoic volcanic rocks that have been studied in detail in Tibet occur in the southern Lhasa terrane (Fig. 1). They are represented by the widespread and voluminous Linzizong Formation (Fig. 1), which ranges in age from 65 to 49 Ma near Lhasa (Maluski et al., 1982; Xu et al., 1985a; Coulon et al., 1986; Zhou et al., 2001) and from 54 to 37 Ma in southwestern Tibet (Miller et al., 2000). Calc-alkaline granitoids of similar (Harrison et al., 2000; Miller et al., 2000), and even younger age (Miocene; D'Andrea et al., 1999) have also been documented within the Gangdese (also referred to as Transhimalayan) batholith. Although significant uncertainty remains, it appears that the initiation age of Indo-Asian collision was between 70 and 50 Ma and diachronous along strike (younging from west to east) [see Yin & Harrison (2000) for discussion and references]. Igneous rocks older than the initiation age of Indo-Asian collision are attributed to north-dipping subduction of Tethyan oceanic lithosphere (e.g. Dewey & Bird, 1970; Coulon et al., 1986; Debon et al., 1986). The extent to which this oceanic subduction modified the structure of the Asian mantle lithosphere is poorly understood, in part because there are few constraints on how the dip-angle of the Tethyan slab changed with time. Also unclear is why calc-alkaline magmatism persisted in the southern Lhasa terrane long after initiation of the Indo-Asian collision.
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Potassic volcanic rocks are widely distributed across the Tibetan plateau and their ages span that of the Indo-Asian collision (50 Ma to Recent) (Fig. 1; see references in caption). Although substantial differences exist within and between localities, the lavas generally exhibit negative Nb, Ta and Ti anomalies, strong enrichment in incompatible elements, and relatively radiogenic Sr and Pb and unradiogenic Nd isotopic ratios. The latter characteristics are generally interpreted to indicate that parental magmas were derived from an enriched continental mantle source that was isolated from convecting asthenosphere since at least Proterozoic time (e.g. Turner et al., 1996), although the possibility of a contribution from a mafic granulitic or eclogitic lower-crustal source has been raised (Hacker et al., 2000; Cooper et al., 2002).
The two most widely proposed mechanisms to explain the genesis of Tibetan potassic volcanism are: (1) mantle melting following rapid removal of large portions of previously thickened and therefore gravitationally unstable mantle lithosphere (Molnar et al., 1993; Turner et al., 1993, 1996; Chung et al., 1998; Miller et al., 1999; Williams et al., 2001); (2) melting related to intracontinental subduction (Deng, 1991; Arnaud et al., 1992; Meyer et al., 1998; Tapponnier et al., 2001; Wang et al., 2001). The first mechanism predicts that potassic magmatism should be regionally widespread and post-date major crustal shortening (e.g. Houseman et al., 1981), whereas the second mechanism predicts that there should be distinct belts of magmatism in the hinterlands of major thrust systems, with magmatism being coeval with thrusting. More recently, models involving slab break-off (DeCelles et al., 2002; Kohn & Parkinson, 2002; Maheo et al., 2002) and decompression melting during extension/transtension and concomitant mantle attenuation (Wang et al., 2001; Cooper et al., 2002) have been put forward. They can be placed into the end-member subduction vs distributed deformation models for the continental mantle lithosphere, and also make distinct predictions regarding the temporal and spatial relationships between volcanism and deformation.
Additional constraints on the age, distribution and petrology of Cenozoic volcanic rocks in Tibet, especially those within the poorly studied plateau interior, are necessary to distinguish between the different tectonic models for their petrogenesis. Detailed studies on earliest Tertiary volcanic rocks are restricted to the southern Lhasa terrane. It is unclear whether a recently defined belt of Eocene–Oligocene potassic volcanic rocks and high-K calc-alkaline granitoids in the eastern Qiangtang terrane (Chung et al., 1998; Roger et al., 2000) extends westward across the Tibetan plateau; only two localities of Eocene–Oligocene volcanic rocks have been studied in detail in the Qiangtang terrane of central Tibet (Duoge-Cuoren and Bamaoqiongzong; Fig. 1; Deng, 1993, 1998; Hacker et al., 2000; Tan et al., 2000). Likewise, it is uncertain whether 25–16 Ma volcanic rocks in westernmost Tibet (Turner et al., 1996; Miller et al., 1999) and 15–10 Ma volcanic rocks near Lhasa (Majiang; Fig. 1; Coulon et al., 1986) are spatially restricted or part of much more extensive east–west-trending belts.
This paper presents results of field, geochronological, geochemical and isotopic studies of Cenozoic volcanic rocks in the western Qiangtang and central Lhasa terranes. They provide the first documentation of 60 Ma alkali basalts and 30 Ma potassic volcanic rocks in the western Qiangtang terrane and 23, 13 and 8 Ma potassic to ultrapotassic volcanic rocks in the central Lhasa terrane. The geochemical and isotopic characteristics of these volcanic rocks are compared with those of previously studied volcanic rocks in Tibet. A tectonic model is presented for the petrogenesis of Cenozoic magmatism, which integrates recent constraints on the crustal deformation history and present-day mantle structure of Tibet.
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GEOLOGICAL BACKGROUND |
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From north to south, the interior of the Tibetan plateau comprises the roughly east–west-trending Songpan–Ganzi, Qiangtang and Lhasa terranes (Fig. 1; e.g. Chang & Zheng, 1973; Dewey et al., 1988; Yin & Harrison, 2000). The Songpan–Ganzi terrane comprises the largest volume of Triassic strata on Earth (2·2 x 106 km3; Nie et al., 1994). These strata include multi-kilometer-thick sequences of Late Triassic flysch (Rao et al., 1987; Liu, 1988; Hou et al., 1991) that were deposited variably on continental (Burchfiel et al., 1995; Zhou & Graham, 1996) and oceanic crust (Sengör, 1987), and were derived mostly from the Triassic Qinling–Dabie orogen to the NE (Nie et al., 1994; Yin & Nie, 1996; Zhou & Graham, 1996; Bruguier et al., 1997). The change from marine to nonmarine sedimentation within the Songpan–Ganzi terrane during the earliest Jurassic is taken to record final accretion of the Qiangtang terrane to the Eurasian margin following ocean closure along the Late Triassic–Early Jurassic Jinsha suture by south-dipping subduction (Pearce & Mei, 1988; Yin & Harrison, 2000), north-dipping subduction (Coward et al., 1988; Li et al., 1995), or both (Leeder et al., 1988). The Songpan–Ganzi terrane and Jinsha suture have been modified significantly by major Tertiary thrust systems and associated non-marine basins of primarily Eocene–Oligocene age (e.g. Coward et al., 1988; Wang et al., 2002). Cenozoic volcanic rocks in the Songpan–Ganzi terrane are volumetrically minor but widely distributed (Fig. 1), mainly mafic–potassic to ultrapotassic in composition, and range in age from 17 Ma to Recent (Deng, 1989; Turner et al., 1993, 1996; Zheng et al., 1996; Cooper et al., 2002). An exception is in the Ulugh Muztagh area (Fig. 1), where
4 Ma rhyolites have been documented (Burchfiel et al., 1989; McKenna & Walker, 1990). In the central Qiangtang terrane, a >500 km long east–west belt of blueschist-bearing mélange (Fig. 1) lies structurally beneath Paleozoic to Mesozoic, mainly shallow marine, strata in the footwall of major Late Triassic–Early Jurassic domal low-angle normal faults (Kapp et al., 2000). The mélange is interpreted to represent either a distinct suture zone separating a northern Qiangtang terrane of Cathaysian affinity from a southern Qiangtang terrane of Gondwanan affinity (Li et al., 1995) or materials that were thrust beneath the Qiangtang terrane during early Mesozoic south-dipping oceanic subduction along the Jinsha suture and then exhumed in an intracontinental setting by Late Triassic–Early Jurassic normal faulting (Kapp et al., 2000, 2003b). Early Tertiary (60–45 Ma) alkali basalts occur in the Aksayqin area of the northwesternmost Qiangtang terrane and overlie Cretaceous marine limestone (Deng, 1998). East–west-trending Paleogene non-marine basins are widely distributed across the Qiangtang terrane. They contain a fill of fluvial sandstone and conglomerate, and lacustrine limestone and mudstone, that ranges from Paleocene to Oligocene in age based on biostratigraphic studies and K–Ar (Wang et al., 1983; Xu et al., 1985b; Cheng & Xu, 1986) and 40Ar/39Ar (Kapp et al., 2002) dating of interbedded volcanic rocks. The Paleogene basins are in most places bounded on their northern margins by north-dipping thrust faults (Lei et al., 1996; Luo et al., 1996; Kapp et al., 2000, 2002, 2003b). Eocene–Early Oligocene (50–29 Ma) potassic to ultrapotassic volcanic and subordinate intrusive rocks are widely distributed throughout the Qiangtang terrane (Fig. 1 and references in caption).
The Lhasa terrane and at least the southern portion of the Qiangtang terrane were contiguous along the margin of Gondwana during late Paleozoic time (Yin et al., 1988; Li & Zheng, 1993). Rifting of the Qiangtang terrane from the Lhasa terrane, and opening of the intervening Bangong ocean, occurred during Permo-Triassic (Sengör, 1984) or Early Jurassic (Yin et al., 1988) time. Subsequent closure of the ocean along the Bangong suture occurred by northward subduction beneath the Qiangtang terrane and perhaps one or more oceanic island arc terranes during Middle Jurassic to Early Cretaceous time (Girardeau et al., 1984; Tang & Wang, 1984; Pearce & Deng, 1988). Ocean closure was followed by major Early Cretaceous continental collision between the Lhasa and Qiangtang terranes (Murphy et al., 1997). Localized Tertiary thrusts and associated Paleocene to Oligocene non-marine basin fill along the Bangong suture (Coward et al., 1988; Leeder et al., 1988; Yin & Harrison, 2000; Kapp et al., 2002, 2003a) suggest that this collision may have continued during Paleogene time.
Granitoids in the Lhasa terrane have been divided into two belts: the mainly dioritic Cretaceous–Tertiary Gangdese (Transhimalaya) plutonic belt in the southern Lhasa terrane, and a belt in the northern Lhasa terrane that includes Early Cretaceous peraluminous granites (Xu et al., 1985a; Harris et al., 1990). Emplacement of the Gangdese batholith is attributed to northward subduction of Tethyan oceanic lithosphere beneath the southern Lhasa terrane, along the Indus suture, before collision with India at 50 Ma (e.g. Dewey & Bird, 1970; Tapponnier et al., 1981; Allègre et al., 1984; Debon et al., 1986). Magmatism of similar geochemistry to pre-50 Ma batholith rocks (high-K calc-alkaline) continued in the Gangdese belt until Late Miocene time, suggesting that fluid and thermal conditions typical of arc-type settings persisted in the southern Lhasa terrane >40 Myr after initiation of Indo-Asian collision (D'Andrea et al., 1999; Miller et al., 1999; Harrison et al., 2000). The petrogenesis of the northern Lhasa terrane belt has been attributed to: (1) crustal anatexis during Lhasa–Qiangtang continental collision (Xu et al., 1985a; Pearce & Mei, 1988); (2) high-temperature crustal anatexis related to asthenospheric upwelling and mantle attenuation (Harris et al., 1990); (3) low-angle, northward-dipping subduction of the Tethyan oceanic lithosphere (Coulon et al., 1986).
The Linzizong Formation is widely distributed in the southern half of the Lhasa terrane and consists of up to 2500 m of calc-alkaline andesitic flows, tuffs and breccias, and dacitic to rhyolitic ignimbrites (Wang, 1980; Coulon et al., 1986; Pearce & Mei, 1988; Miller et al., 2000; Zhou et al., 2001; Dong, 2002). Near Lhasa, the volcanic rocks are between 60 and 49 Ma and are locally intruded by the 53 Ma Lhasa granite (Maluski et al., 1982; Xu et al., 1985a; Coulon et al., 1986). In SW Tibet, the volcanic rocks tend to be younger (54–37 Ma; Miller et al., 2000). Additional, poorly studied volcanic sequences of latest Cretaceous–earliest Tertiary age extend into the northern Lhasa terrane (Liu, 1988; Murphy et al., 1997) and across the Bangong suture zone (Kapp et al., 2002). Throughout much of the southern Lhasa terrane, the gently folded Linzizong Formation rests unconformably on strongly deformed Cretaceous and older rocks (e.g. Tapponnier et al., 1981; Burg et al., 1983; Allègre et al., 1984; Burg & Chen, 1984; Pan, 1993; Murphy et al., 1997). This relationship demonstrates that upper-crustal shortening in the Lhasa terrane largely pre-dates the Indo-Asian collision, at least where Linzizong volcanic rocks are preserved (Fig. 1). However, to the south, the Indus suture zone was strongly modified by the Late Oligocene (30–23 Ma) north-dipping Gangdese thrust system and the Miocene (19–10 Ma) south-dipping Great Counter thrust system (Yin et al., 1994; Quidelleur et al., 1997; Yin et al., 1999a; Harrison et al., 2000). Cenozoic potassic to ultrapotassic volcanic rocks have been documented previously only in the western Lhasa terrane (25–16 Ma; Turner et al., 1996; Miller et al., 1999) and in the Majiang area near Lhasa (15–10 Ma; Coulon et al., 1986).
Despite continuing convergence between India and Asia, active deformation of the Tibetan plateau interior is characterized by roughly north-striking normal fault systems and kinematically linked strike-slip faults (Tapponnier & Molnar, 1977; Armijo et al., 1986, 1989; Yin et al., 1999b; Taylor et al., 2003). There appears to be a gradual change from normal-fault-dominated to strike-slip-dominated deformation from south to north across the plateau. Rift systems in the Lhasa and Qiangtang terranes are suggested to have initiated between 14 and 4 Ma (Coleman & Hodges, 1995; Harrison et al., 1995; Yin et al., 1999b; Blisniuk et al., 2001).
The Tibetan lithosphere exhibits major north–south variations in structure and geophysical properties (Zhao et al., 1993; Hirn et al., 1995; Nelson et al., 1996; Owens & Zandt, 1997; Kosarev et al., 1999; Kind et al., 2002).The crustal thickness decreases south to north, from 80–70 km in the Lhasa terrane to 65 km in the Qiangtang terrane, and to 65–55 km in the Songpan–Ganzi terrane (Nelson et al., 1996; Owens & Zandt, 1997; Zhao et al., 2001; Kind et al., 2002). Analysis of teleseismic P waves suggests that Indian mantle lithosphere may have underthrust beneath southern Tibet to as far north as the Bangong suture (Fig. 1), where it appears to be subducting steeply (Owens & Zandt, 1997; Kosarev et al., 1999). In contrast to southern Tibet, the northern Qiangtang and Songpan–Ganzi terranes are characterized by an upper mantle with inefficient S-wave propagation and low P-wave velocities (e.g. Bird & Toksoz, 1977; Chen & Molnar, 1981; Brandon & Romanowicz, 1986). In addition, the north–central Tibetan crust exhibits anomalously high Poisson's ratios and zones of low seismic velocities (Owens & Zandt, 1997). The geophysical properties of north–central Tibet have been interpreted to reflect the presence of partial melts within both the crust and mantle (Owens & Zandt, 1997; Wei et al., 2001). Both Deng (1998) and Hacker et al. (2000) suggested that the lower crust (50–30 km depth) of the Qiangtang terrane is composed of anhydrous metasedimentary and mafic granulites based on studies of xenoliths in Cenozoic volcanic rocks. Supracrustal lithologies could have been emplaced into the lower crust of the northern Qiangtang terrane by southward flat-slab oceanic subduction along the Jinsha suture during early Mesozoic time (Kapp et al., 2000) and major Tertiary underthrusting along reactivated Mesozoic suture zones (Hacker et al., 2000; Yin & Harrison, 2000; Tapponnier et al., 2001).
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CENOZOIC VOLCANIC ROCKS IN WESTERN QIANGTANG AND CENTRAL LHASA TERRANES |
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Paleocene to Middle Eocene volcanic rocks were studied at two localities within the western Qiangtang terrane. Lagala basaltic lavas are located
40 km north of the Bangong suture and extend over 50 km2 (Figs 1 and 2). They crop out as low-relief hills and were emplaced onto Jurassic flysch and limestone. Grey–white sandstones have been baked to a brick-red colour where they are in contact with the lava flows. In the Bangdaco area, 200 km north of the Bangong suture (Fig. 1), is a small flat-lying basaltic lava sheet that lies unconformably on Cretaceous marine limestone. The Cretaceous limestone is strongly folded and occurs to the north along the Jinsha suture zone in the footwall of a north-dipping thrust, with upper Paleozoic sandstone and limestone in the hanging wall. Similar basaltic flows, 150 km along strike to the west in the Aksayqin area (Fig. 1), have been dated at 60–45 Ma by K–Ar and 40Ar/39Ar methods (Deng, 1998).
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Flows of ultrapotassic basaltic trachyandesite, trachyandesite and trachyte have previously been reported to be interbedded with nonmarine sandstone and conglomerate within the Paleogene Kangtuo basin of the southwestern Qiangtang terrane (Fig. 2). The volcanic rocks have been dated at
31 Ma by the K–Ar method (no uncertainty provided; Cheng & Xu, 1986). This paper reports results from a newly discovered locality of Early Oligocene potassic volcanic rocks within the Yulinshan area of the west–central Qiangtang terrane (Figs 1 and 3). The volcanic rocks consist of sheeted lavas that erupted over an area of 200 km2 and conformably overlie red non-marine conglomerate and sandstone. Three distinct lava sheets, separated by two weathered horizons, have been recognized (Fig. 3): the oldest is a sequence of basaltic trachyandesite and trachyandesite; the second consists of leucite tephriphonolite, trachyandesite and trachyte; the youngest consists of trachyte. The sedimentary basin fill and volcanic rocks are broadly folded, and 10 km north of the Yulinshan area, sandstone and conglomerate occur in the footwall of the north-dipping Buergahu thrust fault (Fig. 3). The non-marine sedimentary basin developed variably on top of blueschist-bearing mélange, Carboniferous sandstone and limestone, and Mesozoic sandstone.
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Oligocene–Miocene potassic volcanic rocks were also discovered along the active, north-striking Wenbu–Chazi rift system of the central Lhasa terrane (Figs 1 and 4). Approximately 30 km SW of Wenbu, a horst block within the Wenbu–Chazi graben exposes a volcanic neck that intrudes early Tertiary andesite of the Linzizong Formation (Fig. 4). Just east of the town of Chazi,
300 km2 of sheeted lavas lie on top of the eastern flank of the Wenbu–Chazi rift (Figs 1 and 4).
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ANALYTICAL TECHNIQUES |
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Mineral separates (grain size 0·3–0·5 mm) for K–Ar and 40Ar/39Ar analyses were obtained using standard mineral separation techniques and hand-picked under a binocular microscope to be >99% pure. K–Ar ages (Table 1) were obtained at the Institute of Geology, China Seismological Bureau. K abundances (wt %) were determined using a HG-3 flame-photometer. Radiogenic 40Ar was measured using the isotope dilution method and an MM-1200 mass spectrometer. K–Ar age calculations were made using the formulation of Dalrymple & Lanphere (1969). During the course of K–Ar analyses, repeated analyses of the internal standard, ZBH (biotite, 133·3 Ma), yielded an average age of 133·5 ± 1·5 Ma (1
). Analytical uncertainties are estimated at ±3%.
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Mineral separates for 40Ar/39Ar analysis were irradiated for 37 h at the Beijing Nuclear Research Institute Reactor. Also irradiated were Fish Canyon sanidine (27·8 Ma; Renne et al., 1994) to calculate J factors, and K2SO4 and CaF2 to determine correction factors for interfering neutron reactions. All samples were step-heated using a radio-frequency furnace. Argon isotope analyses were conducted on an RGA-10 mass spectrometer in the Laboratory of Isotope Geochronology at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGCAS). Age calculations were made using the decay constants given by Steiger & Jäger (1977) and the formulations of Wang et al. (1985) and Wang (1992). Argon isotopic results are summarized in Table 2 and uncertainties cited in the text are at the 1
level.
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Mineral assemblages in 33 samples of Tibetan volcanic rocks, identified using a petrographic microscope, are listed in Table 3. Compositions of minerals in seven samples were determined using a CAMECA SX51 electron microprobe at the IGCAS. Multiple analyses were obtained for most of the minerals; only representative analyses are listed in Table 4. The accelerating voltage was 21 kV and the sample current was 10 nA. Beam diameters ranging from 1 to 10 µm were used, depending on the content of volatile components within the mineral being analyzed. Counting times for all elements were 10 s.
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Homogeneous and unaltered rock samples were selected for chemical and isotopic analyses and results are presented in Tables 5 and 6. Five samples were selected from the Lagala (n = 3) and Bangdaco (n = 2) basaltic lavas. Eighteen samples were selected from the Yulinshan area, and include rocks from each of the three lava sheets. Five samples were selected from both the Wenbu volcanic neck and four lava sheets in the Chazi area.
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For chemical analyses, hand specimens were crushed in a tungsten carbide swing mill, sieved, ultrasonically cleaned several times in deionized water and then ground in an agate mortar. Rock powders (
1·2 g) were then dissolved with Li2B4O7 (6 g) in a TR-1000S automatic bead fusion furnace at 1100°C for 10 min. Major element abundances (wt %) were determined on whole-rock powder pellets by X-ray fluorescence (XRF) using an XRF-1500 sequential spectrometer at the IGCAS. Analytical uncertainties are 1–3% for major elements. Loss on ignition was obtained by weighing after 1 h of calcination at 1100°C. For rare earth element (REE) and trace element analyses, rock powders (50 mg) were dissolved using a mixed acid (HF:HClO4 = 3:1) in capped Savillex Teflon beakers at 120°C for 6 days, and subsequently dried to wet salt and redissolved in 0·5 ml HClO4. The solutions were then evaporated to wet salt at 140°C and redissolved in 1 ml HNO3 and 3 ml water for c. 24 h at 120°C. The solutions were diluted in 2% HNO3 for analysis. REE and trace element concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a PQ2 Turbo system at the IGCAS. Uncertainties based on repeated analyses of internal standards are ±5% for REE and ±5–10% for trace elements.
Rb–Sr and Sm–Nd isotopic analyses were conducted on a VG354 mass spectrometer at the IGCAS. Whole-rock powders (70–50 mg) were dissolved for 7 days using a mixed acid (HF:HClO4 = 3:1) in Teflon bombs, and isotopes were separated by AG50WX8 (H+) exchangeable ion poles. Blank contributions are (2–5) x 10-10g for Rb–Sr and 5 x 10-11g for Sm–Nd. Within-run isotope fractionation was corrected by using 146Nd/144Nd = 0·7219 and 86Sr/88Sr = 0·1194. Eight analyses of the NBS 987 Sr standard (86Sr/88Sr = 0·710240) yielded an average 86Sr/88Sr value of 0·710254 ± 0·000014, and 12 analyses of the La Jolla Nd standard (143Nd/144Nd = 0·511859) yielded an average 143Nd/144Nd value of 0·511862 ± 0·000007. Depleted mantle model ages, T(Nd)DM were calculated using 147Sm/144Nd = 0·222 and 143Nd/144Nd = 0·513114 (Michard et al., 1985). The Nd model ages probably represent minimum ages for enrichment because the Sm/Nd ratio of a melt is generally lower than that of its source. 
Nd values were calculated relative to CHUR. Maximum uncertainties for 87Rb/86Sr ratios are estimated to be ±1%.
For Pb analyses, whole-rock powders (200 mg) were dissolved for 2 weeks using a mixed acid (HF: HClO4 = 10:1) in Teflon vessels. Pb was separated from the silicate matrix using AGIX8 anion exchangeable poles. Isotopic ratios were measured on a VG 354 mass spectrometer at the IGCAS. Overall blank contributions were 1–3 ng and not corrected for. Within-run Pb isotope fractionations were corrected using correction factors derived from measurements of the international standard NBS 981 (206Pb/204Pb = 16·937, 207Pb/204Pb = 15·491 and 208Pb/204Pb = 36·721; Catanzaro et al., 1968). Five analyses of the NBS 981 Pb standard yielded 206Pb/204Pb = 16·927 ± 0·014, 207Pb/204Pb = 15·476 ± 0·015 and 208Pb/204Pb = 36·624 ± 0·042. Calculated mass fractionation was 0·2% per atomic mass unit.
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GEOCHRONOLOGY |
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A sample of Lagala basalt (98T03) yielded a whole-rock K–Ar age of 59·2 ± 2·1 Ma (Table 1). This age should be considered tentative until additional data become available, but is reasonable considering that volcanic rocks of similar age and lithology occur
400 km to the NW in the Aksayqin area (60–45 Ma; Deng, 1998) and <45 km to the north (total gas 40Ar/39Ar age of 65·0 ± 0·7 Ma) and south (40Ar/39Ar plateau age of 64·4 ± 0·7 Ma) of the Lagala area (Kapp et al., 2002) (Fig. 1). K–Ar analyses of leucite phenocrysts from three samples of Yulinshan leucite phonolites (98T44, 98T53 and 98T70) yield apparent ages of 30·3 ± 0·3, 30·7 ± 0·3 and 30·7 ± 0·3 Ma (Table 1). It is unlikely that the three samples were affected by excess argon or argon loss as a result of alteration, as they yield consistent K–Ar ages and a linear correlation on a K–Ar isochron diagram (isochron age of 30·8 ± 0·6 Ma). 40Ar/39Ar analyses of sanidine phenocrysts from two samples of Yulinshan trachytes (98T46 and 98T69) yield two well-defined plateau ages of 28·9 ± 0·4 and 29·2 ± 0·4 Ma (Table 2 and Fig. 5). From the above results, it is concluded that Yulinshan volcanism was active during the time interval of 31–29 Ma.
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40Ar/39Ar analyses were conducted on sanidine phenocrysts from two samples from the Wenbu volcanic neck (99T60 and 99T62; Table 2 and Fig. 5). Sample 99T60 yields a plateau age of 13·4 ± 0·5 Ma during the initial 14% 39Ar released and an older, slightly monotonically increasing plateau with a weighted mean 40Ar/39Ar age of 22·9 ± 0·7 Ma (increments 8–14; 82% of 39Ar released). Sample 99T62 also yields two distinct 40Ar/39Ar age plateaux; a younger plateau at 13·2 ± 0·4 Ma (increments 2–8; 49% of 39Ar released) and an older plateau at 22·5 ± 0·5 Ma (increments 9–12; 50% of 39Ar released). For the two samples, the older plateau ages of
23 Ma are interpreted to be the best estimate for the age of volcanism. The younger plateau ages at 13 Ma are interpreted to represent the time when the volcanic neck was cooled to below the closure temperature for Ar in the smallest sanidine crystals. This interpretation is consistent with the presence of 25–17 Ma volcanic rocks in the western Lhasa terrane (Miller et al., 1999) and extension-related denudation in Tibet at 13 Ma (Coleman & Hodges, 1995; Blisniuk et al., 2001; Williams et al., 2001). However, additional 40Ar/39Ar analyses on sanidine separates of variable sizes are needed to test this interpretation.
Sanidine and phlogopite phenocrysts from two samples of lavas from the Chazi area (99T132 and 99T154) yield similar 40Ar/39Ar plateau ages of 13·3 ± 0·4 Ma (increments 4–8; 64% of 39Ar released) and 13·1 ± 0·3 Ma (increments 6–11; 87% of 39Ar released), respectively (Fig. 5). Sanidine phenocrysts from a sample of a volcanic cone (99T145) within the Chazi graben yield a younger 40Ar/39Ar plateau age of 8·2 ± 0·5 Ma (increments 3–8; 79% of 39Ar released). These plateau ages are interpreted to represent the age of volcanism and are similar to the ages of volcanic rocks in the Majiang area, 300 km along strike to the east (Fig. 1) (15–10 Ma; Coulon et al., 1986).
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PETROGRAPHY AND MINERALOGY |
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Lagala and Bangdaco
Studied samples of Lagala and Bangdaco volcanic rocks are weakly altered and dark green in colour. Phenocrysts (1–5% by volume) are composed entirely of olivine (1–2 mm in diameter), and the groundmass is composed of olivine + clinopyroxene + plagioclase + Fe–Ti oxides + glass (Table 3). Olivine phenocrysts in Lagala lavas have Fo contents of 84–70 wt %, whereas those in Bangdaco lavas have Fo contents of 85–80 wt %. Groundmass clinopyroxene has MgO contents of 15–13 wt %, with Al2O3 contents being higher for the Bangdaco lavas (9–8 wt %) than the Lagala lavas (6–4 wt %). Plagioclase in the groundmass is andesine or labradorite, with An55–45 for Lagala lavas and An52–50 for Bangdaco lavas.
Yulinshan
Yulinshan volcanic rocks can be divided into three main groups on the basis of phenocryst mineral assemblage: those that include (1) clinopyroxene + sanidine, but no leucite, (2) leucite + clinopyroxene + sanidine, and (3) either no phenocrysts or phenocrysts of mainly sanidine. As shown in the next section, these three groups correspond to trachyandesites (one of which is basaltic), tephriphonolites and trachytes, respectively.
Clinopyroxene- and sanidine-bearing samples that lack leucite may also include nosean, haüyne, nepheline and biotite phenocrysts. Their groundmass is composed of sanidine + clinopyroxene + nosean + apatite + glass. Some samples contain calcite amygdules that either exhibit chabazite reaction coronas with the groundmass or are completely replaced by chabazite. Clinopyroxene phenocrysts occur as aggregates and are chemically zoned. Rim compositions (En18Fs30Wo52) are characterized by lower MgO and higher Na2O than core compositions (En23Fs26Wo51). Biotite phenocrysts are rich in TiO2 (9 wt %) with mg-numbers [100Mg/(Mg + Fe)] of 65. Sanidine is of the composition Or65–75Ab19–30An1–5.
Leucite-bearing samples (leucite = 50–20 vol. %) include clinopyroxene (20–10 vol. %) and subordinate sanidine + haüyne + nosean + nepheline + sphene + biotite phenocrysts. The groundmass consists of clinopyroxene + sanidine + haüyne + nepheline + nosean + sphene + Fe–Ti oxide + glass. Leucite phenocrysts (2–10 mm in diameter) usually have two sets of polysynthetic twins with analcite reaction rims. Na2O content in the leucite phenocrysts is <0·1 wt %. Nepheline phenocrysts (2–4 mm in diameter) are a solid solution of nepheline and kalsilite, with the chemical composition Na2O 16–17 wt % and K2O 4–5 wt %. Sanidine phenocrysts (2–5 mm in diameter) consist of Or60–75 Ab25–40An1. Clinopyroxene phenocrysts (2–4 mm in diameter) exhibit distinct chemical zoning characterized by rims high in sodium. The chemical compositions of matrix minerals are similar to those of the phenocrysts.
Samples that contain no phenocrysts or phenocrysts of mainly sanidine exhibit typical trachytic textures. The groundmass is green-coloured and consists of microcrystalline sanidine + sodalite + aegirine + nepheline + magnesioriebeckite + glass. The composition of K-feldspar is similar to that of sanidine in the leucite-bearing assemblages. One sample contains phenocrysts of aegirine–augite, which are characterized by jadeite contents of up to 10 wt %.
Wenbu–Chazi
All five samples of the Wenbu volcanic neck contain phlogopite + clinopyroxene + sanidine ± leucite phenocrysts. The groundmass is composed of the same minerals as the phenocrysts, in addition to apatite + Fe–Ti oxide ± glass. Clinopyroxene phenocrysts (Wo47En47Fs6 to Wo45En48Fs7) are characterized by low Al2O3 (0·2–0·4 wt %). Phenocrystic and groundmass phlogopite are characterized by high mg-numbers (75–85).
Lavas in the Chazi area are characterized by phlogopite ± clinopyroxene ± sanidine phenocrysts and a glass-rich groundmass. Phenocrystic and groundmass phlogopite are characterized by high mg-numbers (80–85). Phenocrystic and groundmass clinopyroxene contain low TiO2 (0·3–0·5) and Al2O3 (1·0–1·5) contents.
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MAJOR AND TRACE ELEMENT GEOCHEMICAL DATA |
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Major elements
Major element data are summarized in Table 5. Lagala volcanic rocks have Na2O (3·53–3·76 wt %) >K2O (0·50–0·67 wt %), high MgO (8·80–9·48 wt %), and low SiO2 (47·20–48·42 wt %) contents. They are slightly nepheline normative and classified as alkali basalts (Fig. 6a). Bangdaco lavas are characterized by low SiO2 (
45 wt %) contents, moderate mg-numbers (54–55), and high Na2O (4·6 wt %) and TiO2 (2·1–2·2 wt %) contents. They are trachybasalts (Fig. 6a), and are further classified as sodic hawaiite because Na2O – 2 > K2O.
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Yulinshan volcanic rocks are rich in K2O (3–10 wt %) and relatively low in MgO (0·5–3·7 wt %). SiO2 contents are variable (47·4–57·6 wt %) and decrease, whereas CaO (2·6–9·7 wt %) increases, with increasing MgO. Recalculated to 100% on an anhydrous basis, the rocks plot as basaltic trachyandesite, trachyandesite, tephriphonolite and trachyte on a K2O + Na2O vs SiO2 diagram (Fig. 6a), and within the leucitite and shoshonitic fields on a K2O vs SiO2 diagram (Fig. 6b). On a K2O vs Na2O diagram, the Yulinshan volcanic rocks plot in the ultrapotassic and shoshonitic fields (Fig. 6c). However, according to the definition of Foley et al. (1987), 98T52 is the only sample with high enough MgO (3·7 wt %) to be classified as ultrapotassic.
Wenbu volcanic rocks are characterized by high SiO2 (57–60 wt %) and K2O (8·6–11·7 wt %) contents, high mg-numbers (52–63) and high ratios of K2O/Na2O (3·8–9·2). All samples, except for 99T53 (MgO < 3 wt %), are ultrapotassic as defined by Foley et al. (1987). Chazi volcanic rocks are characterized by slightly lower K2O (6·2–8·2 wt %) and a wider range in SiO2 (50–69 wt %). MgO decreases from 7 wt % to 1·1 wt % with increasing SiO2 content. Recalculated to 100% on an anhydrous basis, the Chazi lavas plot in the fields of phonotephrite, trachyandesite and trachyte on a K2O + Na2O vs SiO2 diagram (Fig. 6a).
Trace elements
Trace element data are presented in Table 5. Lagala and Bangdaco volcanic rocks are slightly enriched in light rare earth elements (LREE; Lan/Ybn = 8–20) and large ion lithophile elements (LILE). They exhibit chondrite-normalized REE variation patterns that are similar to those of Linzizong calc-alkaline volcanic rocks (Fig. 7a). A major difference, however, is the absence of a negative Eu anomaly for Lagala and Bangdaco volcanic rocks, which suggests either that plagioclase was not a major phase controlling magmatic differentiation or elevated fO2. Ta and Nb anomalies are weakly negative (Lagala) or absent (Bangdaco) and P and Ti anomalies are slightly positive (Fig. 7a). High concentrations of some of the compatible elements, such as Cr (Lagala, 768–839 ppm; Bangdaco, 204–215 ppm) and Ni (Lagala, 189–218 ppm; Bangdaco, 94–97 ppm) are suggestive of primitive magmatic characteristics.
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Yulinshan volcanic rocks are characterized by very high REE and LILE (e.g. 96–604 ppm Rb; 5150–15880 ppm Sr; 1570–15970 ppm Ba; 33–219 ppm Th) contents—higher than most of the previously studied Cenozoic potassic volcanic rocks in Tibet (Fig. 7b). They have steep REE patterns (Lan/Ybn = 55–76), although they are more enriched in heavy rare earth elements (HREE), such as Yb (>3 ppm), than other Tibetan potassic lavas. Compatible element contents are relatively low; Ni and Cr contents are in the range of 1–25 ppm and 16–61 ppm, respectively. The primary mantle-normalized variation diagrams are characterized by significant negative anomalies at Ta, Nb, P and Ti, despite their high concentrations (Fig. 7b).
Similar to the Yulinshan volcanic rocks, the Wenbu and Chazi lavas are highly enriched in LREE and LILE (e.g. 416–676 ppm Rb; 552–1760 ppm Sr; 1256–5160 ppm Ba; 108–283 ppm Th), with steep REE patterns (Lan/Ybn = 41–139) (Fig. 7c). However, compatible element contents are higher than those of the Yulinshan lavas; Cr and Ni contents are in the range of 68–181 ppm and 44–117 ppm, respectively. Except for being characterized by higher LILE contents, the normalized REE and trace element variations for Wenbu and Chazi lavas, which include negative anomalies at Ta, Nb, P and Ti, are similar to those of ultrapotassic volcanic rocks in the western Lhasa terrane (Miller et al., 1999).
Sr, Nd and Pb isotopes
Sr, Nd and Pb isotopic analyses are presented in Table 6. For Bangdaco, Wenbu, Chazi and two Yulinshan (99T15 and 99T71) samples, ratios of 87Sr/86Sr(i) and 143Nd/144Nd were age-corrected. Age-corrected Sr and Nd isotopic ratios differ by 0·01–0·07% and 0·001–0·002% from the uncorrected ratios, respectively. Sr and Nd isotopic ratios were not age-corrected for the Lagala and the remainder of the Yulinshan samples, for which 147Sm/144Nd and 87Rb/86Sr ratios were not measured. Likewise, Pb isotopic ratios for all samples were not age-corrected because 238U/204Pb and 232Th/204Pb ratios were not measured. These age-corrections would be expected to be minor, however, considering the young ages (60 Ma and 30 Ma) and low ratios of Rb/Sr (0·007–0·071), Sm/Nd (0·125–0·206) and U/Pb (0·04–0·18) for the samples.
Lagala volcanic rocks are characterized by 87Sr/86Sr and 143Nd/144Nd ratios of 0·7060 and 0·5124, respectively, and the following Pb isotopic ratios: 206Pb/204Pb 18·81–18·89, 207Pb/204Pb 15·56–15·60 and 208Pb/204Pb 38·89–38·96. The Bangdaco lavas exhibit less radiogenic 87Sr/86Sr (0·7040–0·7054), 207Pb/204Pb (15·50–15·53), 206Pb/204Pb (18·21–18·43) and 208Pb/204Pb (38·41–38·70), and higher 143Nd/144Nd ratios (0·5126). The Sr, Nd and Pb isotopic ratios of Lagala and Bangdaco lavas are roughly similar to those of early Tertiary Linzizong volcanic rocks near Lhasa (Figs 8 and 9; Zhang, 1996; Dong, 2002). For Bangdaco lavas, Nd model ages relative to depleted mantle range from 0·57 to 0·64 Ga.
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Yulinshan volcanic rocks are characterized by restricted and relatively high 87Sr/86Sr (0·7088–0·7092) and low 143Nd/144Nd (0·5121–0·5123) ratios that are similar to those of other potassic volcanic rocks in the Qiangtang terrane (Fig. 8). Nd model ages relative to depleted mantle were determined for two samples of Yulinshan lavas and are 1·03 Ga and 0·98 Ga. Pb isotopic ratios are radiogenic: 206Pb/204Pb = 18·91–19·10, 207Pb/204Pb = 15·64–15·83 and 208Pb/204Pb = 39·14–39·69 (Fig. 9). On plots of 207Pb/204Pb and 208Pb/204Pb vs 206Pb/204Pb (Fig. 9), Yulinshan lavas yield steep arrays that lie significantly above the Northern Hemisphere Reference Line (NHRL), parallel to, but shifted to higher values of 206Pb/204Pb than the Geochron (4·55 Ga; Fig. 9a). The steep linear correlations between the different Pb isotopic ratios could be a result of within-run fractionation that was not adequately corrected for by factors determined through analysis of the international standard, NBS 981. However, similar linear correlations between Pb isotopic ratios have been determined for Tibetan potassic volcanic rocks by previous workers (e.g. Turner et al., 1996; Miller et al., 1999). Therefore, the uniform 206Pb/204Pb ratios and more variable 207Pb/204Pb and 208Pb/204Pb ratios are interpreted to be a characteristic of Tibetan potassic lavas as opposed to fractionation during analysis.
Wenbu and Chazi volcanic rocks are characterized by significantly higher 87Sr/86Sr (0·7165–0·7239) and lower 143Nd/144Nd (0·5118–0·5119) ratios than those of the Yulinshan volcanic rocks, but these values are similar to those of ultrapotassic volcanic rocks in the western Lhasa terrane (Fig. 8). Nd model ages relative to depleted mantle range between 1·31 and 1·35 Ga for Wenbu lavas and between 1·36 and 1·70 Ga for Chazi lavas. Ratios of 206Pb/204Pb (18·42–18·94), 207Pb/204Pb (15·69–15·77) and 208Pb/204Pb (39·27–39·63) for Wenbu and Chazi lavas are radiogenic and similar to those of the western Lhasa terrane rocks (Fig. 9).
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DISCUSSION |
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Petrogenesis of Cenozoic volcanic rocks in central and southern Tibet
Lagala and Bangdaco
The Lagala and Bangdaco alkali basalts are relatively primitive, as demonstrated by their high contents of MgO (
9 wt % and 7 wt %, respectively) and compatible elements such as Cr and Ni. Their parental magmas were clearly derived by partial melting of a mantle source. In contrast to the northern Qiangtang Bangdaco lavas, the southern Qiangtang Lagala lavas show slightly negative Nb and Ta anomalies (Fig. 7). This may indicate that the southern Qiangtang terrane mantle source included a subduction-related component. The latter could also account for the more radiogenic Sr and Nd isotopic ratios of the Lagala lavas relative to Bangdaco lavas. Alternatively, the Lagala lavas could have been derived from a mantle source that was isolated from the convecting asthenosphere for a longer period of time than that of the Bangdaco lavas.
On a Yb vs La/Yb diagram (Fig. 10) the Lagala, Bangdaco and Aksayqin (Deng, 1998) basalts lie roughly along a mixing trend (Langmuir et al., 1978) between 4% partial melt of a hypothetical spinel-facies mantle and 0·1% partial melt of a hypothetical garnet-facies mantle. Quantitative modeling of REE abundance variations suggests that if the Lagala and Bangdaco magmas were derived from 3–4% melting of an assumed enriched mantle composition (La 1·19 ppm; Yb 0·31 ppm), the La contents of the partial melts would be 25–35 ppm (close to what is observed) for both the garnet-bearing and spinel-bearing mantle sources. However, the Yb contents would be 0·5 ppm and 3–4 ppm for melts of a garnet-bearing and spinel-bearing mantle, respectively. Given these results, the Yb contents of Lagala (2·9–3·2 ppm) and Bangdaco (2·1–2·4 ppm) lavas may indicate that the parental magmas were derived largely from partial mantle melting within the spinel stability field. Minimal garnet in the mantle source is further supported by the similar chondrite-normalized concentrations of HREE (Er/Yb 1), as garnet fractionates Er from Yb and would be expected to yield Er/Yb >1.
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Yulinshan
The low mg-numbers (4–47) and low Cr and Ni contents of the Yulinshan volcanic rocks (Table 4) indicate that they were derived from highly evolved magmas. On a plot of La/Yb vs La (Fig. 11), the small variation in La/Yb over a large range of La concentrations suggests that fractional crystallization played a major role in controlling chemical variation. Plots of La and Rb vs Ba (Fig. 12) allow evaluation of the relative importance of sanidine, leucite, nosean and clinopyroxene in the evolution of Yulinshan magmas. Significant decreases in Ba with moderate increases in La and minimal increases in Rb contents suggest that the compositions of the tephriphonolite, trachyandesite and highly evolved trachytic lavas were strongly influenced by fractional crystallization of sanidine. Analysis of tephriphonolite magmas suggests that their compositions may also have been influenced by fractional crystallization of clinopyroxene, leucite and perhaps nosean.
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Ratios of a highly compatible element to an incompatible element as a function of the highly compatible element can be used to evaluate the extent of fractional crystallization in the petrogenesis of Yulinshan lavas (e.g. Sc/Yb vs Sc; Fig. 13). Sc and Ba are strongly compatible elements in clinopyroxene and incompatible in sanidine, whereas Yb is an incompatible element in clinopyroxene, sanidine and leucite. If the trachyandesite and tephriphonolite magmas are regarded to have undergone fractional crystallization from the most basic magma (assumed to be represented by sample 98T52), quantitative modelling of Sc/Yb vs Sc shows that trachyandesite and tephriphonolite lavas could be derived by 15–50% fractional crystallization of clinopyroxene and sanidine in ratios of Cpx/(Cpx + Sa) of 0·15–1·0 (Fig. 13).
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Sr and Nd isotopic ratios are not affected by fractional crystallization. Therefore the uniform 87Sr/86Sr (
0·709) and 143Nd/144Nd (0·5122) ratios for the Yulinshan lavas (Fig. 8) probably represent the isotopic characteristics of their source region. The extremely high contents of Sr and Nd would make the latter ratios insensitive to crustal contamination. The MgO content (3·6%) of the most mafic Yulinshan sample (basaltic trachyandesite) is sufficiently high to argue against the possibility that the potassic magmas were derived by partial melting of crustal rocks. For instance, the rhyolitic rocks of the Ulugh Muztagh area (Fig. 1), which are interpreted to be partial melts of pelitic lithologies, exhibit much lower MgO (<0·3%) as well as higher initial 87Sr/86Sr ratios (0·715; McKenna & Walker, 1990). Furthermore, the extremely high concentrations of incompatible elements are not characteristic of typical upper crust or upper-crustal melts (Pearce et al., 1984; Taylor & McLennan, 1985). On a La/Nb vs Rb/Sr diagram the Yulinshan lavas plot close to fields for mantle sources and do not show significant trends toward compositions of Cretaceous crustal-derived leucogranites in the northern Lhasa terrane (Fig. 14; Harris et al., 1990; Ding & Lai, 2003). The extremely high concentrations of Sr and the absence of negative Eu anomalies suggest that plagioclase was not present in the source, and therefore probably reflect enrichment in the mantle source (e.g. Turner et al., 1996).
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The rather high K2O, La and Ba contents of the least evolved Yulinshan lava (98T52; K2O 3·2%, La 390 ppm and Ba 7600 ppm; Table 5) imply that high K2O and LILE contents are also characteristic of their mantle source region (e.g. Turner et al., 1996; Miller et al., 1999). Possible potassic phases in the mantle source include phlogopite and K-rich amphibole, both of which can incorporate LILE in their structures (Konzett & Fei, 2000). As suggested for other Tibetan potassic lavas, the parent magmas of Yulinshan lavas were probably derived from small-degree partial melts of a strongly enriched mantle (Turner et al., 1993, 1996; Deng, 1998; Miller et al., 1999). The similar chondrite-normalized concentrations of HREE for the Yulinshan lavas suggest that garnet did not play a major role in their petrogenesis, as garnet fractionates Er from Yb to yield Er/Yb >1.
Wenbu–Chazi
The southern Tibetan Wenbu and Chazi ultrapotassic volcanic rocks are characterized by radiogenic Sr and unradiogenic Nd isotopic ratios, similar to those of ultrapotassic rocks in the western Lhasa terrane (Miller et al., 1999). Their high mg-numbers (60–65) and high contents of compatible elements such as Cr (120–180 ppm) and Ni (70–120 ppm) indicate that they are more primitive than the Yulinshan lavas. As suggested for the western Lhasa terrane volcanic rocks (Miller et al., 1999), it is unlikely that crustal assimilation and fractional crystallization processes (DePaolo, 1981) played a major role in the chemical evolution of the Wenbu and Chazi volcanic rocks. Assimilation of crustal material is inconsistent with the very high LILE and LREE contents and relatively low ratios of Rb/Sr for the Wenbu, Chazi and western Lhasa terrane ultrapotassic rocks (Fig. 10). Furthermore, the latter volcanic rocks yield a steeply sloping array on a La/Yb vs La diagram, which is characteristic of a partial melting trend and not a fractionation trend (Fig. 11). It is therefore concluded that the major and trace element compositions of the volcanic rocks were largely inherited from and reflect those within the mantle sources, with variations between samples being attributed to variable degrees of partial melting of the mantle source region. On a Yb vs La/Yb diagram (Fig. 14) the Wenbu volcanic rocks, along with the inferred parental magma of Yulinshan lavas, lie along a mixing trend between relatively small-degree partial melts of a spinel-facies phlogopite-bearing mantle (0·1%) and a garnet-facies phlogopite-bearing mantle (0·001%). The Chazi and western Lhasa terrane lavas may have been produced by higher degrees of partial melting, and lie roughly along a mixing trend between partial melts of a spinel-facies phlogopite-bearing mantle (2%) and a garnet-facies phlogopite-bearing mantle (0·1%).
Nature and age of mantle metasomatism
The negative Nb, Ta and Ti anomalies in the incompatible element patterns of the Tibetan post-collisional potassic rocks (Fig. 7) suggest that subduction processes were probably responsible for metasomatism of their continental mantle source region (Arnaud et al., 1992; Turner et al., 1993, 1996; Deng, 1998; Miller et al., 1999). The radiogenic Sr and Pb and unradiogenic Nd ratios of the potassic lavas depart drastically from those of the convecting asthenosphere, and the high La/Nb (2–10) and low Nd (-5 to -15) values for most of the lavas cannot be produced by simple mixing of continental crust, oceanic island basalt (OIB) and asthenospheric magmas (Fig. 15). The geochemical and isotopic characteristics of Tibetan potassic lavas require strongly metasomatized mantle sources that were enriched in incompatible elements.
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It has been suggested that the Tibetan mantle lithosphere experienced a multi-stage history of metasomatism during Precambrian time, as inferred from Proterozoic Nd (0·9–1·3 Ga) and older Pb (2·2–3·5 Ga) model ages for Tibetan potassic volcanic rocks (e.g. Turner et al., 1996; Miller et al., 1999; this study). However, the possibility that an enriched Precambrian mantle lithosphere remained chemically isolated and physically intact beneath Tibet throughout Phanerozoic time is strongly questioned when the Phanerozoic tectonic and magmatic evolution of Tibet is considered.
The Tibetan terranes accreted to the southern margin of Asia primarily during Mesozoic time, following closure of major intervening ocean basins (e.g. Dewey et al., 1988; Yin & Harrison, 2000). Northward subduction of Tethyan oceanic lithosphere along the Indus suture occurred over a time interval of at least 60 Myr before Indo-Asian collision (Xu et al., 1985a; Miller et al., 2000). It seems likely that subduction of multiple oceanic basins beneath Tibet during the Mesozoic would have significantly altered the chemical composition of any pre-existing Precambrian mantle lithosphere. Additionally, Triassic flysch of the Songpan–Ganzi terrane is interpreted to have been deposited largely on oceanic lithosphere (e.g. Sengör, 1987), and is suggested to have thrust southward directly beneath Qiangtang continental crust during early Mesozoic flat-slab oceanic subduction along the Jinsha suture (Kapp et al., 2000, 2003b). Therefore if the Qiangtang and Songpan–Ganzi terranes were underlain by enriched Precambrian mantle lithosphere during potassic volcanism, the above tectonic interpretations require the Precambrian lithosphere to be allochthonous and have underthrust this portion of Tibet subsequent to early Mesozoic time. Mid-Cretaceous northward flat-slab subduction of Tethyan oceanic lithosphere along the Indus suture has also been suggested (Coulon et al., 1986), and could have tectonically removed significant portions of the Lhasa terrane continental mantle lithosphere. An important consideration is the lack of pre-Eocene potassic volcanic rocks in Tibet; Cretaceous–Eocene volcanic rocks in the Lhasa terrane are calc-alkaline in composition (e.g. Coulon et al., 1986; Miller et al., 2000) whereas Paleocene–Eocene volcanic rocks in the Qiangtang terrane are alkaline (Deng, 1998; this study). There was ample opportunity for partial melting of Tibetan continental mantle lithosphere during Mesozoic tectonism. Oceanic slabs that were subducting during Mesozoic time probably experienced episodes of slab roll-back and break-off (e.g. Davies & von Blanckenburg, 1995), which would have provided heat sources for partial melting of the overlying mantle lithosphere. At least the Lhasa and Qiangtang terranes of Tibet experienced major crustal shortening (in excess of 50% for large regions) during Cretaceous time (Murphy et al., 1997; Kapp et al., 2002, 2003a). Early Cretaceous leucogranites in the northern Lhasa terrane represent high-temperature (>850°C) crustal melts and have been attributed to elevated heat input during attenuation of mantle lithosphere, which was previously overthickened during Mesozoic shortening (Harris et al., 1990).
It is here suggested that the Tibetan mantle lithosphere was metasomatized during Cenozoic time by subduction of ancient continental crust beneath Tibet during the Indo-Asian collision. The Precambrian Nd and Pb model ages inferred for Tibetan mantle lithosphere may not reflect the time when the lithosphere became isolated from asthenospheric convection, but rather isotopic signatures inherited from fluids and melts derived from subducted ancient continental crust. The concept of mantle metasomatism during Phanerozoic subduction of ancient continental material has previously been suggested to explain volcanic rocks with enriched and apparently ancient mantle sources in other collisional orogens (e.g. Hergt et al., 1989; Li, 1994; Jahn et al., 1999; Peccerillo, 1999).
On an Nd vs 87Sr/86Sr diagram (Fig. 8), the Qiangtang alkali basalts and Tibetan potassic volcanic rocks lie roughly along mixing lines between (1) the two end-member lavas analyzed in this study: Bangdaco (02T394) and Wenbu (99T60), and (2) the depleted mantle and crust with high 87Sr/86Sr and low 143Nd/144Nd ratios, defined by the isotopic characteristics of the High Himalayan Crystallines (HHC; data from Harris, 1995; Parrish & Hodges, 1996; Robinson et al., 2001; Yang, 2002; Ding & Lai, 2003). This trend may suggest that the lithospheric mantle sources for the Tibetan potassic volcanic rocks were metasomatized by a continental crustal component (fluids and/or melts), similar in composition to the HHC. The isotopic compositions of Qiangtang (45–29 Ma) and Lhasa (25–8 Ma) terrane potassic magmas are consistent with their mantle sources having incorporated 5–10% and 15–20% of a continental crustal component, respectively (Fig. 8). Deviating from the mixing trend are the Linzizong calc-alkaline volcanic rocks, which provide an array toward more radiogenic Sr at lower Nd isotopic ratios (Zhang, 1996; Dong, 2002) (Fig. 8), characteristic of many island arc volcanic rocks (White & Patchett, 1984, and references therein). This observation is consistent with the hypothesis that the Lhasa terrane mantle lithosphere was not significantly metasomatized by continental material before the Indo-Asian collision. It is noted that the apparent mixing trend does not require mixing; it could be a consequence of the volcanic rocks having been derived from a Tibetan mantle lithosphere that is laterally heterogeneous in composition and history of metasomatism. However, the hypothesis of mantle metasomatism during Cenozoic continental subduction is consistent with: (1) the abrupt transition between alkaline and potassic volcanism in the Qiangtang terrane during Eocene time; (2) the trend toward more radiogenic Sr and less radiogenic Nd with decreasing age of the volcanic rocks in southern and central Tibet (more input of a continental component into the mantle source is expected to increase as Indo-Asian collision progresses); (3) the spatial and temporal distribution of Tertiary shortening and volcanism in Tibet (discussed in the next section). Additionally, it is unlikely that the apparent mixing trend is due to significant assimilation of crustal material into mantle-derived melts. This is because crustal assimilation would result in lower LILE and LREE contents than those measured for Tibetan potassic lavas and is inconsistent with the observation that LILE and LREE contents do not, in general, correlate negatively with mg-number (Turner et al., 1996; Miller et al., 1999).
Tectonic model for Cenozoic magmagenesis in Tibet (Fig. 16)
Before Indo-Asian collision, the Tethyan oceanic lithosphere may have been subducting northward beneath Tibet at a low angle (Coulon et al., 1986; Fig. 16a). Shortly before or during the initial stages of collision between India and Asia (poorly constrained to be between 70 and 50 Ma; see discussion by Yin & Harrison, 2000), the Tethyan oceanic slab may have undergone roll-back to steeper angles. Upwelling asthenosphere during slab roll-back may have initiated melting of Qiangtang upper mantle and produced the alkali basalts near the Lagala, Bangdaco and Aksayqin areas of the Qiangtang terrane, and the Linzizong volcanic rocks in the Lhasa terrane (Fig. 16b). This hypothesis of Cretaceous flat-slab oceanic subduction followed by earliest Tertiary roll-back is appealing because it would have provided a mechanism to remove significant portions of the Tibetan continental mantle lithosphere, which in turn would have facilitated India's northward underthrusting of southern Tibet during the Indo-Asian collision.
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The convective thinning model of the lithosphere (Houseman et al., 1981; England & Houseman, 1986) has been widely used to explain the genesis of Tibetan post-collisional potassic volcanic rocks (McKenna & Walker, 1990; Molnar et al., 1993; Turner et al., 1993, 1996; Chung et al., 1998; Miller et al., 1999; Williams et al., 2001). According to this model, the ancient and enriched Tibetan continental mantle lithosphere was homogeneously thickened, along with the crust, during Cenozoic shortening. As a result of gravitational instability (mantle lithosphere is more dense than asthenosphere), portions of the continental mantle lithosphere were rapidly detached and replaced by upwelling of hot asthenosphere, which resulted in partial melting of the remaining sub-continental mantle lithosphere and regionally distributed potassic volcanism, as well as significant surface uplift. This model requires major crustal shortening to have pre-dated volcanism and a rather complicated temporal and spatial history of mantle removal to explain the age distribution of Tibetan potassic volcanic rocks (Fig. 1).
An alternative mechanism is fluid-fluxed melting of Tibetan mantle lithosphere during subduction of continental lithosphere along major Tertiary thrust systems (Deng, 1991; Arnaud et al., 1992; Meyer et al., 1998; Tapponnier et al., 2001; Wang et al., 2001). This mechanism predicts that there should be distinct belts of potassic magmatism that are parallel to and coeval with major thrust systems. Results of this study, when integrated with previous studies, strongly suggest that Middle Miocene and older Tibetan potassic volcanic rocks occur as roughly east–west-trending belts that are spatially associated with major thrust systems of similar age.
The documentation of 30 Ma potassic volcanic rocks in the Yulinshan area of the western Qiangtang terrane helps better define an Eocene--Early Oligocene magmatic belt that is restricted to the Qiangtang terrane (Fig. 1). The Yulinshan volcanic rocks are folded and overlie conglomerate and sandstone that occur in the footwall of a thrust fault to the north (Fig. 2). Under the assumption that non-marine sedimentation was induced by thrust faulting, these structural observations suggest that upper-crustal shortening was occurring coeval with potassic volcanism. Major Paleocene to Oligocene thrusts systems and associated basinal strata (including volcanic interbeds) have also been documented along the Jinsha suture and in the northern Qiangtang terrane (Hoh Xil, Fenghuo Shan, and Nangqian–Yushu fold–thrust belts; Coward et al., 1988; Liu & Wang, 2001; Liu et al., 2001; Horton et al., 2002) and in the southern Qiangtang terrane and along the Bangong suture (Luo et al., 1996; Ai et al., 1998; Kapp et al., 2002, 2003a). The timing and spatial relationships between volcanism and thrusting favor a model in which Eocene–Oligocene potassic volcanism in the Qiangtang terrane is related to northward subduction of Lhasa terrane continental lithosphere and southward subduction of Songpan–Ganzi terrane continental lithosphere by reactivation of the Mesozoic Bangong and Jinsha sutures, respectively (Fig. 16c). Strong coupling between the Indian and Asian lithospheres could have been the driving mechanism for suture zone reactivation. Fluids and melts from subducted Songpan–Ganzi and Lhasa terrane crust could have metasomatized and catalyzed partial melting of mantle to produce the potassic lavas. Eocene–Oligocene potassic lavas in the central Qiangtang terrane differ from younger potassic lavas in that they are interpreted to have experienced major fractional crystallization (Yulinshan, Bamaoqiongzong, and Duoge–Cuoren areas; Lai & Liu, 2001; this study). This fractional crystallization probably occurred in crustal magma chambers, with rapid transport of magmas to the surface being impeded by coeval contraction within the upper crust.
The 25–17 Ma ultrapotassic rocks in the western Lhasa terrane and Wenbu area (23 Ma) may be a part of a more extensive east–west-trending belt of magmatism within the northern Lhasa terrane. Major thrust systems of this age have not been documented in the northern Lhasa terrane, but are present along the Indus suture (Gangdese and Great Counter thrust system; e.g. Yin et al., 1994, 1999a; Harrison et al., 2000) and within the Himalaya (Tethyan thrust belt and Main Central Thrust; Ratschbacher et al., 1994; Hodges et al., 1996). Therefore, northward subduction of Indian continental lithosphere could have produced the 25–17 Ma ultrapotassic volcanism in the northern Lhasa terrane (Fig. 16d). The scarcity of upper-crustal contraction in the northern Lhasa terrane may have allowed mantle melts to transport rapidly through the crust without being affected by significant crustal assimilation or fractional crystallization.
The 8 Ma and 13 Ma volcanic rocks in the Chazi area are potassic, in contrast to the 16 Ma and 15–10 Ma calc-alkaline volcanic rocks that have been documented in SW Tibet (Miller et al., 1999) and near Majiang (Coulon et al., 1986), respectively. All of the volcanic rocks are associated with north-striking late Cenozoic rift systems (Pulan rift in SW Tibet, Murphy et al., 2002; Wenbu–Chazi rift, this study; Majiang rift, Kidd et al., 1988). Whereas rifting is suggested to have initiated in southern Tibet as early as 18 Ma (Williams et al., 2001) or 14 Ma (Coleman & Hodges, 1995), thermochronological studies near the Pulan (Murphy et al., 2002), Wenbu–Chazi (Stockli et al., 2002) and Majiang (Harrison et al., 1995) areas suggest that extension initiated later (10–8 Ma), subsequent to volcanism. Perhaps more significant than the structural setting of the volcanic rocks is the southward younging trend in volcanism, from 25–17 Ma in the northern Lhasa terrane to 16–8 Ma in the southern Lhasa terrane. This change in the age of magmatism may indicate steepening in the angle of the northward-subducting Indian lithosphere (Fig. 16e). The calc-alkaline volcanism may have occurred in areas of greater asthenospheric upwelling during slab roll-back.
Middle Miocene to Recent potassic volcanism is widely distributed in the Songpan–Ganzi terrane, and locally in the Bamaoqiongzong area of the northern Qiangtang terrane (Fig. 1). This volcanism was coeval with major crustal shortening within the Kunlun–Qaidam terrane to the north, which is proposed to have been accommodated in the continental mantle by southward subduction beneath the Songpan–Ganzi terrane (Deng, 1991; Meyer et al., 1998; Tapponnier et al., 2001). This inferred southward-dipping slab of Asian continental lithosphere has been recently well-imaged using seismic receiver functions (Kind et al., 2002), and probably played a major role in the metasomatism and partial melting of the northern Tibetan mantle lithosphere (Fig. 16e). The wide distribution of the northern Tibetan volcanism is striking, however, and may indicate that additional processes, such as convective thinning of the continental mantle lithosphere (e.g. Arnaud et al., 1992; Turner et al., 1996), may have acted to provide a more regional heat source.
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CONCLUSIONS |
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The Lagala alkali basalts and Bangdaco hawaiites crystallized at 60–45 Ma and have high mg-numbers and high Cr and Ni contents characteristic of relatively primitive magmas. The Yulinshan volcanic rocks in the west–central Qiangtang terrane include basaltic trachyandesite, trachyandesite, tephriphonolite and trachyte, and crystallized at
30 Ma. The Yulinshan lavas were derived from highly evolved magmas that experienced significant fractional crystallization. The Wenbu (23 Ma) and Chazi (13 Ma and 8 Ma) ultrapotassic volcanic rocks in the central Lhasa terrane are less evolved than Yulinshan lavas, with high mg-numbers. The Yulinshan, Wenbu and Chazi volcanic rocks are characterized by radiogenic Sr and Pb, unradiogenic Nd isotopic ratios and strong enrichment in LILE and LREE. These characteristics probably reflect the enriched nature of their lithospheric mantle sources. Paleocene–Eocene alkaline volcanism in the Qiangtang terrane and calc-alkaline volcanism in the Lhasa terrane are attributed to earliest Tertiary roll-back of an initially shallow slab of northward subducting Tethyan oceanic lithosphere. A tectonic model in which the Tibetan mantle lithosphere was metasomatized and partially melted during Cenozoic subduction of ancient continental lithosphere is consistent with the timing and spatial distribution of post-collisional potassic volcanism and crustal shortening in Tibet. |
ACKNOWLEDGEMENTS |
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We thank R. Xu, R. Zhang, H. Sang, J. Cao and P. Xu, Institute of Geology and Geophysics, Chinese Academy of Sciences, for assistance with major element, trace element, isotopic and electron microprobe analysis. D. M. Li at the Institute of Geology, China Seismological Bureau, assisted in K–Ar analysis. This work was supported by a grant from the Chinese National Key Project (1998040800) and Chinese Academy of Sciences grant (KZCXZ-SW-119). We thank A. Peccerillo, S. L. Chung and A. Yin for constructive comments on an early draft of this manuscript. We thank Tiffany Barry, Nick Rogers and an anonymous reviewer for their constructive reviews.
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