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
ZHENGFU GUO1,2,*,
MARJORIE WILSON2,
JIAQI LIU1 and
QIAN MAO1
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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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 SiO
2 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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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
, 1996
a; 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., 1996
a; 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
, 1996
a; 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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Fig. 1. (a) Simplified map showing the distribution of Cenozoic post-collisional potassic and ultrapotassic magmatism in the northern part of the Tibetan Plateau (modified from Deng, 1998; Yang, 1998; Liu, 1999; Wang et al., 2000; Yin & Harrison, 2000; Xiao et al., 2002; Ding et al., 2003; Mo et al., 2003). The dot–dashed line represents an area of inferred high upper mantle temperatures based on inefficient regional S-wave (Sn) propagation (Ni & Barazangi, 1983; McNamara et al., 1995) and low regional P-wave (Pn) velocity (McNamara et al., 1995; Owens & Zandt, 1997; Kosarev et al., 1999; Xu et al., 1999). (b) Regional map showing the position of the study area in relation to the main tectonic sutures in the Tibetan Plateau (modified from Liu, 1999; Deng et al., 2004). The bold rectangle shows the position of the study area. The continuous lines with Roman numbers (i.e. I, II, III, etc.) represent the main tectonic sutures. The filled triangles on the continuous lines point in the direction of subduction of oceanic lithosphere. The Roman numbers correspond to those in (a). (c) Map showing the distribution of post-collisional magmatic rocks in northern and southern Tibet (modified from Chung et al., 1998; Williams et al., 2004). The rectangle shows the location of the study area. The Roman numbers correspond to those in (a) and (b).
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Fig. 2. Variation of the ages of the potassium-rich magmatism in the terranes of north Tibet according to latitude and longitude based on 123 dated samples [i.e. K–Ar (78 ages), 40Ar/39Ar (36 ages), thermoluminescence (five ages), 14C (two ages), historical record (one age), single-zircon U–Pb dating by SHRIMP II (one age)]. Sources of the data are given in Table 1. The open symbols denote samples that have MgO >6 wt %; the filled symbols denote samples that have MgO <6 wt %. (a) Latitude (degrees) vs age (Ma). (b) Age (Ma) vs longitude (degrees).
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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
, 1996
a; 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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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., 1988
b; 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
, 1996
a; 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 km
2 (e.g. the Duogecuoren and Qiangbaqian
fields) to <2 km
2 (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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Table 3: Major and trace element analyses of the representative potassic and ultrapotassic magmatic rocks in north Tibet
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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).
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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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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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).
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 AG1
x8 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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Major and trace element geochemistry
The whole-rock geochemical data (
Table 3 and Electronic Appendix)
indicate that all of the analysed samples are potassic [K
2O/Na
2O
(wt %) >1·0], except for two samples (QS30 and HS07)
with K
2O/Na
2O <1 (0·92 and 0·95). Eight samples
(QS22, YS72, DG08, G98-1, JC975, G98-11, JC973, JC978) have
K
2O/Na
2O ratios >2; their MgO and K
2O contents are >3
wt %. These eight samples are considered to be ultrapotassic
based on the criteria of Foley
et al. (1987)
.
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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Fig. 3. (a) K2O + Na2O (wt %) vs SiO2 (wt %) for the potassic and ultrapotassic igneous rocks. All data plotted have been recalculated to 100 wt % on a volatile-free basis (see also Table 3 and the Electronic Appendix). Classification boundaries are from Le Bas et al. (1986) and Le Maitre et al. (1989). Filled and open symbols represent, respectively, data from this study and the published data of Arnaud et al. (1992), Turner et al. (1993, 1996a), Ding et al. (2003) and Williams et al. (2004). Rock types shown by letters are as follows: S1, trachybasalt; S2, basaltic trachyandesite; S3, trachyandesite; T, trachyte; U1, tephrite; U2, phonotephrite; U3, tephriphonolite; Ph, phonolite; O1, basaltic andesite; O2, andesite; O3, dacite. (b) K2O (wt %) vs SiO2 (wt %) diagram for the same samples as plotted in (a). Data have been normalized to 100 wt % volatile-free as indicated in Table 3 and the Electronic Appendix. The dividing lines show the classification boundaries from Rickwood (1989). Data sources and symbols are as in (a).
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Mg-numbers [ = molar Mg/(Mg + Fe
2+) ratio, calculated assuming
Fe
2O
3/(FeO + Fe
2O
3) = 0·20] range from 0·22 to
0·72 (
Table 3 and Electronic Appendix). Abundances of
the compatible elements (e.g. MgO, Fe
2O
3, CaO, TiO
2, Ni, Sc,
Cr) in the magmatic rocks of the different terranes display
similar variation trends (
Fig. 4). SiO
2 and Al
2O
3 increase,
whereas CaO, Fe
2O
3, 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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Fig. 4. Selected major element oxide (wt %) and trace element (ppm) concentrations vs MgO content (wt %) illustrating the broad compositional range of the potassium-rich magmatism in north Tibet. All the major element data have been recalculated to 100 wt % on a volatile-free basis (Table 3 and Electronic Appendix). Data sources and symbols are as in Fig. 3a.
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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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Fig. 7. (a) La/Yb vs La (ppm) diagram. (b) Ce/Pb vs Ce (ppm). To minimize the effect of magmatic differentiation and crustal contamination, only primitive samples with MgO >6 wt % are plotted. Arrowed lines A and B in (a) represent non-model batch melting trends for the Songpan–Ganzi and North Qiangtang terranes, respectively. Tick marks in (a) indicate the degree of partial melting in per cent. The detailed calculation procedures are explained in the footnote to Table 10. No Pb partition coefficient data for residual rutile and titanite in mantle source are available in the literature. Consequently, the trends labelled A and B in (b) are purely schematic. The symbols are as in Fig. 3a.
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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
, 1996
a; 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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Fig. 8. (a) (143Nd/144Nd)i vs (87Sr/86Sr)i. (b) (87Sr/86Sr)i vs (206Pb/204Pb)i. (c) (143Nd/144Nd)i vs (206Pb/204Pb)i. (d) (208Pb/204Pb)i vs (206Pb/204Pb)i. (e) (207Pb/204Pb)i vs (206Pb/204Pb)i. To minimize the effect of magmatic differentiation and crustal contamination, only primitive samples with MgO >6 wt % are plotted. No previously published data for primitive magmatic rocks with MgO >6 wt % are available. All data shown here are from this study. The symbols are as in Fig. 3a. Field for Central Indian MORB is from Mahoney et al. (1989) and Hofmann (1997). Field for Atlantic + Pacific MORB is from White et al. (1987) and Hofmann (1997). The continuous line outlines the field of Global Subducting Sediment (GLOSS; Plank & Langmuir, 1998). The open star shows the average composition of GLOSS (GLOSS average; Plank & Langmuir, 1998). The NHRL (Northern Hemisphere Reference Line; Hart, 1984), EM1 and EM2 (enriched mantle end-members; Zindler & Hart, 1986; Hofmann, 1997; Zou et al., 2000), and MORB and OIB fields (Wilson, 1989; Hofmann, 1997) are shown for reference. BSE is Bulk Silicate Earth.
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Fig. 9. (a) (143Nd/144Nd)i vs (87Sr/86Sr)i. (b) (87Sr/86Sr)i vs (206Pb/204Pb)i. (c) (143Nd/144Nd)i vs (206Pb/204Pb)i. (d) (208Pb/204Pb)i vs (206Pb/204Pb)i. (e) (207Pb/204Pb)i vs (206Pb/204Pb)i. To show the effect of magmatic differentiation and crustal contamination on Sr–Nd–Pb isotope ratios, only relatively evolved samples with MgO <6 wt % are plotted. The diagonal shaded field is that of the primitive magmas with MgO >6 wt % defined in Fig. 8. Field for Central Indian MORB is from Mahoney et al. (1989) and Hofmann (1997). Field for Atlantic + Pacific MORB is from White et al. (1987) and Hofmann (1997). The continuous line outlines the field of GLOSS (Plank & Langmuir, 1998). The open star shows the average composition of GLOSS (GLOSS average; Plank & Langmuir, 1998). The NHRL (Northern Hemisphere Reference Line; Hart, 1984), EM1 and EM2 (enriched mantle end-members; Zindler & Hart, 1986; Hofmann, 1997; Zou et al., 2000), and MORB and OIB fields (Wilson, 1989; Hofmann, 1997) are shown for reference. BSE is Bulk Silicate Earth. The symbols are as in Fig. 3a.
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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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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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Fig. 10. Variation of (87Sr/86Sr)i vs MgO (wt %). The symbols are as in Fig. 3a. (a) Magmatic rocks in the North Qiangtang terrane. (b) Magmatic rocks in the Songpan–Ganzi and North Kunlun terranes.
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TOP
ABSTRACT
INTRODUCTION
