Journal of Petrology Advance Access originally published online on October 1, 2004
Journal of Petrology 2005 46(1):33-78; doi:10.1093/petrology/egh061
Journal of Petrology vol. 46 issue 1 © Oxford University Press 2004; all rights reserved
Potassic Magmatism in Western Sichuan and Yunnan Provinces, SE Tibet, China: Petrological and Geochemical Constraints on Petrogenesis
ZHENGFU GUO1,2,*,
JAN HERTOGEN3,
JIAQI LIU1,
PAUL PASTEELS4,
ARIEL BOVEN4,
LEA PUNZALAN4,
HUAIYU HE1,4,
XIANGJUN LUO1,4 and
WENHUA ZHANG1,4
1 INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, PO BOX 9825, BEIJING 100029, CHINA
2 SCHOOL OF EARTH SCIENCES, LEEDS UNIVERSITY, LEEDS LS2 9JT, UK
3 AFDELING FYSICO-CHEMISCHE GEOLOGIE, KATHOLIEKE UNIVERSITEIT LEUVEN, CELESTIJNENLAAN 200C, B-3001 LEUVEN, BELGIUM
4 LABORATORY OF GEOCHRONOLOGY, VRIJE UNIVERSITEIT BRUSSELS, B-1050 BRUSSELS, BELGIUM
RECEIVED
NOVEMBER 7, 2003;
ACCEPTED
JULY 21, 2004
 |
ABSTRACT
|
|---|
Potassic volcanism in the western Sichuan and Yunnan Provinces,
SE Tibet, forms part of an extensive magmatic province in the
eastern Indo-Asian collision zone during the Paleogene (40–24
Ma). The dominant rock types are phlogopite-, clinopyroxene-
and olivine-phyric calc-alkaline (shoshonitic) lamprophyres.
They are relatively depleted in Na
2O, Fe
2O
3, and Al
2O
3 compared
with the late Permian–early Triassic Emeishan continental
flood basalts in the western part of the Yangtze craton, and
have very high and variable abundances of incompatible trace
elements. Primitive mantle-normalized incompatible element patterns
have marked negative Nb, Ta and Ti anomalies similar to those
of K-rich subduction-related magmas, although the geodynamic
setting is clearly post-collisional. Spatially, some incompatible
trace element abundances, together with inferred depths of melt
segregation based on the Mg-15 normalized compositions of the
samples, display progressive zonation trends from SW to NE with
increasing distance from the western boundary of the Yangtze
craton. Systematic variations in major and trace element abundances
and Sr–Nd–Pb isotope compositions appear to have
petrogenetic significance. The systematic increases in incompatible
trace element abundances from the western margin to the interior
of the Yangtze craton can be explained by progressively decreasing
extents of partial melting, whereas steady changes in some incompatible
trace element ratios can be attributed to changes in the amount
of subduction-derived fluid added to the lithospheric mantle
of the Yangtze craton. The mantle source region of the lamprophyres
is considered to be a relatively refractory phlogopite-bearing
spinel peridotite, heterogeneously enriched by fluids derived
from earlier phases of late Proterozoic and Palaeozoic subduction
beneath the western part of the Yangtze craton. Calculations
based on a non-modal batch melting model show that the degree
of partial melting ranges from 0·6% to 15% and the proportion
of subduction-derived fluid added from
0·1% to
0·7%
(higher-Ba fluid) or from 5% to 25% (lower-Ba fluid) from the
interior to the western margin of the Yangtze craton. Some pre-existing
lithospheric faults might have been reactivated in the area
neighbouring the Ailao Shan–Red River (ASRR) strike-slip
belt, accompanying collision-induced extrusion of the Indo-China
block and left-lateral strike-slip along the ASRR shear zone.
This, in turn, could have triggered decompression melting of
the previously enriched mantle lithosphere, resulting in calc-alkaline
lamprophyric magmatism in the western part of the Yangtze craton.
KEY WORDS: Tibet; potassic magmatism; lithospheric mantle; metasomatism
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INTRODUCTION
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Cenozoic igneous activity in the western Sichuan and Yunnan
Provinces (WSYP) of SE Tibet, SW China, forms a part of a semi-continuous
potassium-rich magmatic province in the eastern Indo-Asian collision
zone (Chung
et al., 1998
b
; Liu, 1999
; Wang
et al., 2001
;
Fig. 1).
Despite many studies, the petrogenesis of the rocks remains
controversial and has been variously attributed to the collision
of India and Asia (Liu, 1999
, 2000
), intraplate deformation
(Deng
et al., 1998
a, 1998
b; Zhong
et al., 2000
), rift magmatism
(Zhu
et al., 1992
; Xie & Zhang, 1995
; Xie
et al., 1999
;
Zhang
et al., 2000
), convective removal of the mantle lithosphere
(Chung
et al., 1998
b), and subducted slab break-off (Flower
et al., 1998
).
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Fig. 1. Simplified map of the distribution of Cenozoic potassic igneous rocks in the western margin of the Yangtze craton (modified from YBGMR, 1990; SBGMR, 1991; Xu et al., 2001b). The traverse is a broad zone about 300 km wide rather than a narrow linear traverse, extending from the Janchuan volcanic field in the SW of the traverse, through the Lijiang, Ninglang, Yanyuan, Muli and Yalojiang volcanic fields, to the Xichang field at the NE end. In addition, we also studied the volcanic fields located to the south of the traverse, including Liuhe, Xiangyun, Yao'an, Yongren, Panzhihua, and Yanbian.  , cities;  , villages. A rectangle drawn with bold continuous lines in the inset shows the position of the study area. The dashed line in the inset represents the northwestern, western and southwestern edges of the Yangtze craton.
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The tectonic–magmatic activity in the eastern Indo-Asian
collision zone has been divided into two episodes: an earlier
phase, localized along the Ailao Shan–Red River (ASRR)
shear zone, which marks the western boundary of the Yangtze
craton (
Fig. 1), is thought to have been generated as a consequence
of synchronous continental lithosphere transpression (42–24
Ma); the later phase (16–0 Ma) is thought to be related
to east–west extension (Wang
et al., 2001
). However, the
lack of detailed field sampling, and petrological and geochemical
data has precluded further constraints on the petrogenesis of
the magmas, including their tectonic setting and relationship
to the geodynamic evolution of the eastern Indo-Asian collision
zone.
This study focuses on a SW–NE-trending, 270 km long, traverse perpendicular to the western margin of the Yangtze craton (Fig. 1). It extends from the Janchuan volcanic field in the SW of the traverse (closest to the western margin of the Yangtze craton), through the Lijiang, Ninglang, Yanyuan and Muli volcanic fields to the Xichang field at the NE end (Fig. 1). In addition, we also studied some other volcanic fields located to the south of the traverse, including Liuhe, Xiangyun, Yao'an, Yongren, Panzhihua, and Yanbian (Fig. 1). The traverse thus represents a broad zone, about 300 km wide, rather than a narrow linear traverse.
The magmatic rocks are mainly Eocene–Oligocene, small volume, mafic potassic dykes, sills and hypabyssal intrusions. Whole-rock major and trace element and Sr–Nd–Pb isotope data are used to define compositional changes along the SW–NE traverse and adjacent areas (Fig. 1). These data are complementary to previous studies that predominantly focused on either the petrogenesis of individual volcanic fields (Zhu et al., 1992; Huang et al., 1997; Deng et al., 1998a, 1998b; Xie et al., 1999) or the compositional variations of igneous rocks mainly along the ASRR shear zone (i.e. along the western margin of the Yangtze craton) in the eastern Indo-Asian collision zone (Chung et al., 1998b; Zhang & Scharer, 1999; Zhang et al., 2000; Wang et al., 2001). In addition, this study, for the first time, recognizes a compositional zonation of trace element abundances and ratios from the western margin (at the SW end of the traverse) to the interior (at the NE end of the traverse) of the Yangtze craton.
Although there have been many studies of regional compositional changes in subduction-related magmatic rocks, most of these have focused on the geochemical zonation across island arcs (e.g. Dickinson & Hatherton, 1975; Leeman et al., 1990; Ishikawa & Nakamura, 1994; Barragan et al., 1998; Hochstaedter et al., 2000; Walker et al., 2000; Elburg et al., 2002). The nature of the compositional zonation of post-collision potassic magmatic rocks across palaeo-subduction zones is poorly understood, probably because crustal contamination and magma differentiation have obscured these zonation characteristics. The near-primitive nature of the WSYP magmatic rocks makes it possible to reveal the regional compositional zonation of post-collision potassic magmatic rocks in SE Tibet.
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GEOLOGICAL SETTING
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The studied potassic igneous rocks are located along the western
margin of the Yangtze craton (
Fig. 1). The basement of the Yangtze
craton consists of Archaean high-grade metamorphic rocks and
early–middle Proterozoic metasedimentary rocks [Zhai &
Yang, 1986
; Wu & Cong, 1988
; Yunnan Bureau of Geology and
Mineral Resources (YBGMR), 1990
; Sichuan Bureau of Geology and
Mineral Resources (SBGMR), 1991
; Gao
et al., 1999
; Qiu
et al.,
2000
]; the cover sequences comprise Phanerozoic clastics and
carbonates. Previous geological studies (YBGMR, 1990
) indicate
that the WSYP volcanic rocks are located within Proterozoic
mobile belts in the western part of the Archaean Yangtze craton
in regions that underwent lithospheric accretionary events associated
with Proterozoic oceanic subduction beneath the western Yangtze
craton. The Kangding granitoid body crops out within the western
mobile belts of the Yangtze craton (Cong, 1988
; SBGMR, 1991
;
Xu
et al., 1995
). This granitoid body extends from Kangding,
which is
260 km north of Xichang (
Fig. 1), to Yuanmou, which
is
120 km south of Huili (
Fig. 1) (Xu
et al., 1995
). The Kangding
granitoid body is oriented approximately north–south and
is associated with contemporaneous intermediate–acidic
arc volcanic rocks (He & Chen, 1988
; Feng
et al., 1990
).
Two
40Ar/
39Ar ages of amphibole from the granitoid body are
740 Ma and 763·6 Ma (Wang
et al., 1987
), whereas 64 zircon
U–Pb ages range from 548 Ma to 1095 Ma, most of which
are within the range 700–800 Ma (Xu
et al., 1995
). A recent
study showed that the age of the Kangding granitoid is 706 ±
36 Ma on the basis of a Sm–Nd whole-rock isochron of five
samples (Chen
et al., 2001
). The granitoid and volcanics are
calc-alkaline (Cong, 1988
; Wu & Zhang, 1990
; YBGMR, 1990
;
SBGMR, 1991
; Xu
et al., 1995
) and their presence suggests that
there was oceanic subduction beneath the western margin of the
Yangtze craton during the late Proterozoic (Xu
et al., 1995
).
The Palaeo-Tethys ocean basin opened when the Indo-China block and the Yangtze craton rifted and separated from the northern margin of Gondwanaland in the Devonian (Wang et al., 2000). The main branch of Palaeo-Tethys was the Lancang ocean, now indicated by the Lancang suture between the Indo-China block and the Sibumasu block (inset in Fig. 1) (YBGMR, 1990; Mo et al., 1991, 1993; Yang, 1998; Wang et al., 2000). The suture is marked by dismembered Devonian–Carboniferous ophiolitic assemblages (serpentinite, harzburgite, gabbro, diabase and mafic lavas) and Permo-Triassic arc volcanic belts (Mo et al., 1991, 1993; Yang, 1998; Wang et al., 2000). The dismembered ophiolitic sequences are believed to have been emplaced mainly into Carboniferous marine sediments (YBGMR, 1990; Mo et al., 1993; Yang, 1998). The Permo-Triassic arc volcanic rocks are well exposed to the east of the Lancang suture zone (Yang, 1998); these include tholeiitic, calc-alkalic and shoshonitic series. There is a geochemical polarity, which displays an increase in potassium and other incompatible trace elements (e.g. Rb and Ba) towards the east within the volcanic belt (Mo et al., 1993; Yang, 1998). This indicates a mature volcanic arc and suggests that the oceanic crust subducted towards the east.
A recent study has demonstrated that the Indo-China block was connected with the western Yangtze craton forming an integrated continental block during the initial stage of Lancang ocean subduction, and that subduction occurred beneath the present-day western margin of the Yangtze craton (Wang et al., 2000). Following initial sea-floor spreading in the early Devonian, deposition of middle–late Devonian oceanic ribbon-bedded cherts (Liu et al., 1991, 1993) occurred. Subduction of the Lancang oceanic crust beneath the integrated Indo-China block and the western margin of the Yangtze craton in the Middle–Late Devonian to earliest Carboniferous eventually led to back-arc extension and rifting between the Indo-China block and the western Yangtze craton (see Wang et al., 2000, fig. 10). The occurrence of Devonian–Carboniferous graptolite-bearing deep-water clastics along the western margin of the Yangtze craton has been considered to be the result of this back-arc extension and rifting (Wang et al., 2000). This phase of extension is considered to be a precursor of the Jinsha ocean, now represented by the Jinsha suture zone (YBGMR, 1990; Mo et al., 1991, 1993; Yang, 1998; Wang et al., 2000), which lies between the Indo-China block and the Yangtze craton (inset in Fig. 1). The exact age and geodynamic setting of the Jinsha suture zone remain controversial (Mo et al., 1991, 1993; Yang, 1998; Wang et al., 2000, and references therein). It is marked by mélanges containing ultramafic blocks, ophiolitic assemblages, pillow lavas and pelagic sediments (Mo et al., 1991, 1993; Yang, 1998). Plagiogranite is widely found within the mélanges, which is thought to be a typical oceanic granitoid (Wang et al., 2000); the plagiogranites occur in tectonic contact with the ultramafic blocks. Two zircon U–Pb ages of 340 Ma and 362 Ma for the Shusong and Shuanggou plagiogranites suggest that the Jinsha oceanic lithosphere between the Yangtze craton and the Indo-China block was generated in late Devonian to early Carboniferous times. Moreover, early Carboniferous radiolarians and conodonts have been reported from chert and siliceous limestone interbeds within pillow basalts from the mélange (Wu, 1993; Feng et al., 1997). This indicates the presence of Jinsha oceanic sediments of early Carboniferous age in this area.
The collision between the Indo-China block and the Yangtze craton, and between the Indo-China and Sibumasu blocks (inset in Fig. 1), occurred in middle–late Triassic times as a consequence of the closure of the Palaeo-Tethys oceans (i.e. Lancang and Jinsha oceans) between these continental blocks (also see Wang et al., 2000, fig. 10). These continent–continent collisions led to structural deformation, regional metamorphism of varying degrees and disruption of the ophiolitic mélanges. Latest Permian–middle Triassic synorogenic (or syn-collision) granitoids, such as the Xumai (238 Ma) and Zhongmu (255–227 Ma) granitoids, are distributed along the collisional orogenic belt (Wang et al., 2000). Late Triassic molasse unconformably overlies early Palaeozoic metamorphic rocks in the suture zone and marks the cessation of tectonic activity.
The western part of the Yangtze craton has experienced intracontinental deformation since the middle–late Triassic, but no further oceanic plate subduction has occurred beneath the western part of the craton (Yang, 1998; Wang et al., 2000). This strong intracontinental deformation makes it difficult to determine the subduction direction of the Palaeo-Tethys ocean lithosphere merely based on the position of arc volcanics and opholitic belts.
During late Permian to early Triassic times a sequence of flood basalts, the Emeishan basalts, considered to be the consequence of mantle plume activity (Chung & Jahn, 1995; Chung et al., 1998a; Xu et al., 2001a), were erupted, covering the western part of the Yangtze craton (inset of Fig. 1). Previous studies (e.g. Chung et al., 1998a) suggested that the plume centre (or head) was to the east of the study area. The exposed area of this large igneous province is around 250 000 km2; basaltic rocks distributed in Sichuan and Yunnan Provinces constitute the western part of the Emeishan flood basalts (Chung et al., 1998a; Xu et al., 2001a).
The 55–50 Ma collision of India and Asia in south Tibet (inset of Fig. 1) led to the formation of the ASRR shear zone in SE Tibet, reactivating the Devonian–Carboniferous suture separating the Yangtze craton and the Indo-China block, which was driven to the SE as a consequence of tectonic escape (Tapponnier et al., 1990; Scharer et al., 1994; Leloup et al., 1995; Zhang & Zhong, 1996; Zhang & Scharer, 1999; Fig. 1). More than 300 km of Tertiary left-lateral strike-slip movement has occurred along the ASRR shear zone (Tapponnier et al., 1990; Scharer et al., 1994; Leloup et al., 1995). This tectonic activity has intensely modified the ophiolitic sequences and the geometry of this suture and overwhelmed (even erased) many of the signatures of the earlier volcanic arc. Coincident with movement along the ASRR shear zone and associated fracture systems, a progressive rise of isotherms resulted in greenschist- to amphibolite-facies regional metamorphism and associated magmatic activity in Eocene–Oligocene times in the western part of the Yangtze craton (YBGMR, 1990; Liu, 1999; Qian, 1999). Numerous Paleogene dykes, sills, and small hypabyssal bodies were emplaced within Cenozoic pull-apart basins (YBGMR, 1990; SBGMR, 1991; Liu, 1999; Qian, 1999), predominantly located at the intersection zones of several different directions of faults in the eastern Indo-Asian collision zone. These basins are typically bounded by high-angle normal faults or strike-slip faults (YBGMR, 1990; SBGMR, 1991; Tan, 1999).
This paper presents detailed geochemical and Sr–Nd–Pb isotopic analyses of magmatic rocks collected from these volcanic–sedimentary basins along a 270 km long traverse extending from 99°50'E, 26°35'N to 102°18'E, 27°51'N (Fig. 1). Most of the samples are Eocene–Oligocene in age, ranging from 24 to 40 Ma based on 44 K–Ar ages and 21 40Ar–39Ar ages (YBGMR, 1990; SBGMR, 1991; Zhu et al., 1992; Zhang & Xie, 1997; Chung et al., 1998b; Deng et al., 1998a; Liu, 1999; Wang et al., 2001). Two new 40Ar–39Ar ages obtained for lamprophyres as part of this study are consistent with this age range (Table 1 and Electronic Appendix; the latter can be downloaded from the Journal of Petrology website at http://www.petrology.oupjournals.org).
|
PETROGRAPHIC CHARACTERISTICS
|
|---|
The igneous rocks studied display many of the typical petrographic
characteristics of shoshonitic (calc-alkaline) lamprophyres
(Rock, 1984
; Rock
et al., 1991
): (1) porphyritic textures with
panidiomorphic phenocrysts and microcrystalline groundmass;
(2) presence of castellated phlogopite phenocrysts and the absence
of orthopyroxene, quartz and feldspar phenocrysts; (3) presence
of globular felsic structures consisting dominantly of feldspars.
The studied samples do not contain titanian richterite or groundmass
tetraferriphlogopite, or accessory priderite, wadeite and perovskite
(Xie & Zhang, 1995
; Deng
et al., 1998
b; Lu & Qian, 1999
;
Qian, 1999
), indicating that they are not typical lamproites
(Mitchell & Bergman, 1991
; Sheppard & Taylor, 1992
),
and have variable mineralogical and textural characteristics.
The more mafic rocks have more olivine (1–4%) and phlogopite
(2–5%) phenocrysts, and relatively less clinopyroxene
(0–3%) set in a microcrystalline matrix. The more evolved
samples have more clinopyroxene (1–5%) phenocrysts and
relatively less olivine (0–2%) and phlogopite (1–3%)
phenocrysts in an augite and feldspar groundmass (
Table 2).
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ANALYTICAL TECHNIQUES
|
|---|
All of the analysed samples are petrographically fresh and show
no evidence of significant hydrothermal alteration.
40Ar/
39Ar
dating (
Table 1) was performed at the Laboratory of Geochronology,
Vrije Universiteit Brussels. Argon step-heating analyses were
performed on a MAP216 mass spectrometer operating in a static
mode. Incremental heating was performed by induction heating
(accuracy is ±10°C). Age calculations were made using
the average of
J-factors obtained from repeat measurements of
the LP-6 biotite standard, with a reference age of 128·1
± 0·2 Ma (relative to 162·9 Ma for SB-3
biotite; Baksi
et al., 1996
). Errors were multiplied by the
square root of the MSWD. The detailed procedures follow the
description of Boven
et al. (2002)
.
Samples 2–3 kg in weight were cut into thin slices. Several fresh slices were cleaned three times using deionized water, dried, and then crushed and powered in an agate mill for major and trace element analyses. For samples CH9716, CH9722, CH9723, CH7F002, CH7F007, CH9783, CH9786, CH9803 and CH9854 (Table 3), the major element and trace element concentrations were analysed at the Department of Afdeling Fysico-chemische Geologie, Katholieke Universiteit Leuven (KUL). Major elements (Si, Ti, Al, Fe, Mg, Ca and P) were determined by atomic absorption spectrometry (AAS), whereas Na, K and Mn were analysed by atomic emission spectrometry (AES). The analytical precisions of both AAS and AES were 
2%. Loss on ignition (LOI) was determined after ignition at 1000°C for 10 h on 2 g samples. The trace elements Rb, Sr, Nb, Y, Pb, and Zr were analysed on pressed-powder discs by X-ray fluorescence (XRF) spectroscopy using a Kevex 0700. Other trace element concentrations [Sc, Cr, Co, Ba, rare earth elements (REE), Hf, Ta, Th and U] were determined by instrumental neutron activation analysis (INAA). The analytical precisions of the XRF and INAA data are 5%. The analytical procedures follow those of Hertogen et al. (1985) and Mulder et al. (1986). For the remaining samples (SH14, SH15, EX08, EX06, GZ0049, YD0089, YN0015, WH79, WH76, YS0073, NL0031, WH91, XH10, YL12, GZ0014, GZ0024, GZ0064, GZ0029, GZ0035, GZ0046, ZH0026, HD92, JH22, JH21, LH14 and LH15), the major element and trace element compositions were analysed at the Institute of Geology and Geophysics (IGG), Chinese Academy of Sciences, Beijing. Whole-rock major element oxides were analysed with a Phillips PW1400 sequential X-ray fluorescence spectrometer. Fused glass discs were used and the analytical precision was better than 2% relative. The analytical procedures in detail follow those of Zhang et al. (2002).
For trace element analyses, whole-rock powders (100 mg) were
weighed and then dissolved in distilled HF–HNO
3 in 15
ml Savillex Teflon screw-cap capsules at 100°C for 2 days,
dried and then digested with 6 M HCl at 150°C for 4 days.
Powders were spiked with Rh, In and Bi before dilution to 3%
HNO
3. Dissolved samples were diluted to 100 ml before analysis.
The trace element contents of the sample solutions were analysed
by inductively coupled plasma mass spectrometry (ICP-MS). A
blank solution was prepared and the total procedural blanks
were <50 ng for all the trace elements reported in
Table 3.
Three replicates and two international standards (BEN, BHVO-1)
were prepared using the same procedure to monitor the analytical
reproducibility. The discrepancy, based on repeated analyses
of samples and standards, is less than 5% for all the elements
given in
Table 3. Analyses of the international standards are
in agreement with the recommended values (Govindaraju, 1994
),
which deviate less than 3% from published values. The detailed
analytical procedures follow those of Jin & Zhu (2000)
and
Zhang
et al. (2002)
. The major and trace element concentrations
of the samples analysed at KUL were reanalysed using the above
procedure; the results are within analytical error, except for
the Rb concentrations of samples CH9716, CH9722, CH9723 and
CH7F002 and the Ba content of sample CH7F002, which are outside
analytical error (see Appendix A). It is possible that the samples
that were sent to KUL from Beijing for analysis were too small,
leading to unrepresentative analyses. For the four anomalous
samples (i.e. the Rb concentrations of samples CH9716, CH9722,
CH9723 and CH7F002 and the Ba content of sample CH7F002), we
used the ICP-MS analyses instead.
Sr–Nd–Pb isotope compositions of selected samples were analysed at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. For Rb–Sr and Sm–Nd isotope analyses, rock chips of less than 20 mesh size were used. Before being ground to 200 mesh in an agate rill mill, 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 were spiked with mixed isotope tracers, then dissolved in Teflon capsules with HF + HNO3. Sr and REE fractions were separated in solution using cationic ion-exchange resin columns. Nd was separated from REE fractions using cationic ion-exchange columns and P507 extraction and eluviation resin (Richard et al., 1976; Zhang et al., 2002; Fan et al., 2003). 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 VG-354 mass spectrometer. 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 standard La Jolla yielded 143Nd/144Nd = 0·511863 ± 10 (n = 16) and international standard NBS987 gave 87Sr/86Sr = 0·710242 ± 11 (n = 14). The whole procedure blank is less than 4 x 10–10 g for Sr and 8 x 10–11 g for Nd. Analytical errors for Sr and Nd isotopic ratios are given as 2
. 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 a mean 40Ar/39Ar age of 35 Ma.
For Pb isotope measurements, to minimize the contamination from the atmosphere during the crushing process, <100 mesh powders of samples were used; 200 mg of powder was weighed into a Teflon cup, spiked and dissolved in concentrated HF at 800°C for 72 h. Pb was separated and purified by conventional anion-exchange techniques (AG1X8, 200–400 resin; Zhang et al., 2002) with dilute HBr as eluant. The whole procedure blank is less than 0·4 ng. Pb isotopic ratios were measured with a VG-354 mass spectrometer. During the period of analysis, repeat analyses of international standard NBS981 yielded 206Pb/204Pb = 16·9506 ± 2 (n = 20), 207Pb/204Pb = 15·5072 ± 6 (n = 20), and 208Pb/204Pb = 36·6786 ± 4 (n = 22). Each sample was measured at least twice to ensure reproducibility of the analyses and the results given are averages. The average 2 uncertainty for measured ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb is 0·2
, 0·8 and 0·6 per a.m.u. (atomic mass unit), respectively. 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). The results are presented in Tables 4 and 5.
|
GEOCHEMICAL CHARACTERISTICS
|
|---|
The geochemical data (
Table 3) show that the WSYP lavas are
potassic (K
2O > Na
2O) to ultrapotassic, with K
2O >3 wt
%, MgO >3 wt % and K
2O/Na
2O ratios ranging from 2·1
to 4·2 (wt %), based on the criteria of Foley
et al.
(1987)
. The compositions of the WSYP rocks are plotted in a
total-alkali vs silica classification diagram (Le Bas
et al.,
1986
; Le Maitre
et al., 1989
) subdivided into groups based on
their distance from the western margin of the Yangtze craton
(
Fig. 2a). Samples from different distances from the margin
of the craton normally overlap and define a scattered trend
(
Fig. 2a) that lies almost totally within the basalt–trachybasalt–basaltic
trachyandesite–trachyandesite fields. A plot of K
2O vs
SiO
2 (
Fig. 2b) shows that the samples plot in the shoshonitic
series field, and also within the field for minettes as defined
by Rock
et al. (1991)
. Moreover, Ce/Yb vs Sm variations (
Fig. 3)
also indicate that most rocks plot in the field of calc-alkaline
lamprophyres (Rock
et al., 1991
).
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Fig. 3. Ce/Yb vs Sm (ppm) diagram. The dashed line outlines the field of CAL (calc-alkaline lamprophyres) after Rock et al. (1991). The symbols are as in Fig. 2a.
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The lavas are not peralkaline because the molar (K + Na)/Al
ratios of the samples vary between 0·40 and 0·99.
Mg-values [= molar Mg
x 100/(Mg+Fe
2+) ratio, calculated assuming
Fe
2O
3/(FeO + Fe
2O
3) = 0·15] range from 66 to 78 (
Table 3).
SiO
2, Na
2O and Al
2O
3 increase, whereas CaO, Fe
2O
3, Ni, and
Cr decrease, with decreasing MgO (
Fig. 4 and
Table 3). However,
K
2O forms a near-horizontal trend when plotted against SiO
2 (
Fig. 2b) and MgO (
Fig. 4f). The abundances of the compatible
elements (e.g. MgO, Fe
2O
3, CaO, Ni, Sc, Cr) in the WSYP igneous
rocks from different distances from the western margin of the
Yangtze craton display similar variation trends (
Fig. 4). Compared
with the Permo-Triassic Emeishan flood basalts in the western
part of the Yangtze craton, which have been considered a consequence
of mantle plume activity (Chung
et al., 1998
a; Xu
et al., 2001
a),
the WSYP magmatic rocks generally have lower contents of Fe
2O
3,
Na
2O, CaO, Al
2O
3 and TiO
2 at a given MgO (>6%) content (
Fig. 4a–e).
The Zr–Nb diagram (
Fig. 5) shows that the
WSYP lavas are similar to Western Mexican minettes, which were
generated in an active subduction-related tectonic setting (Luhr
et al., 1989
; Carmichael
et al., 1996
). The most silica-poor
samples from the Yanyuan and Xichang volcanic fields have relatively
high contents of compatible elements (e.g. MgO 7·5–14·5%,
Cr 277–708 ppm, Sc 15–24 ppm, and Ni 90–585
ppm). The low contents of phenocrysts of these rocks, which
possess about 1–4% olivine phenocrysts (
Table 2), make
it unlikely that these characteristics result from the accumulation
of earlier crystallizing ferromagnesian minerals, such as olivine,
clinopyroxene and Cr-spinel, although the sample with the highest
Ni content (585 ppm) might have accumulated some olivine.
Primitive mantle-normalized incompatible element diagrams show
the strongly enriched nature of the WSYP potassic rocks (
Fig. 6).
The normalized patterns range from several times primitive
mantle for heavy REE (HREE), Ti, and Y, to several hundred times
for large ion lithophile elements (LILE), such as Ba, Rb, U,
K, Pb, and Sr. The trace element patterns are distinguished
by negative Nb, Ta and Ti anomalies, despite the generally high
contents of these elements (
Table 3 and
Fig. 6). All of the
rocks have moderately to slightly positive Ba anomalies. Almost
all of the samples show slightly positive Sr anomalies relative
to Ce and P. Either positive or negative anomalies for P, Zr
and Hf are exhibited in some samples. Rocks located in the northeastern
part of the traverse (
Fig. 1) show slightly negative K anomalies
relative to La and U (
Fig. 6c and d); however, most rocks in
the southwestern part of the traverse (
Fig. 1) have slightly
positive K anomalies (
Fig. 6a and b). Th, light REE (LREE) and
some high field strength elements (HFSE; e.g. Zr and Hf) have
relatively flat patterns compared with the strong enrichment
in Rb, Ba, U, K, Pb and Sr (
Fig. 6), resulting in overall high
LILE/HFSE ratios of the WSYP igneous rocks.
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Fig. 6. Primitive mantle-normalized incompatible element diagrams calculated with normalization factors from Sun & McDonough (1989). The WSYP potassic samples can be subdivided into those that have high Ba/La and Sr/Ce ratios (a), those that have intermediate Ba/La and Sr/Ce (b, c), and those that have low Ba/La and Sr/Ce ratios (d). Distances from the craton margin are indicated. The symbols are as in Fig. 2a.
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The most striking characteristics of the WSYP rocks are their
relatively high contents and considerable variations in the
abundance of LILE and LREE (
Table 3 and
Fig. 7). For example,
Ba abundances range from 1300 to 6600 ppm (
Fig. 7), Rb from
120 to 290 ppm, Sr from 1000 to 3500 ppm, Ce from 60 to 540
ppm, and La from 30 to 260 ppm. In element–element variation
diagrams, no correlation exists between incompatible trace element
contents (e.g. Ba, Sr, Rb, La, Sm) and MgO, and the plots show
considerable scatter (
Fig. 4g–k). The contents of some
incompatible trace elements (e.g. Ba, Th, Sr) increase significantly
with distance from the western margin of Yangtze craton (
Fig. 7).
Variations in the ratios of incompatible elements (e.g.
Ba/La, La/Yb, Sr/Ce, Ba/Rb) also show similar correlations (both
positive and negative). MgO contents show relatively little
variation with distance from the western margin of the Yangtze
craton (
Fig. 7). Because no systematic magma differentiation
trends were observed with the distance from the western margin
of the craton, combined with the relatively high MgO, Ni, and
Cr contents of the WSYP rocks (
Table 3 and
Fig. 4), the compositional
zonation probably reflects the characteristics of the mantle
source region from which the WSYP magmas were derived and the
degree of partial melting.
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Fig. 7. Geochemical variation with increasing distance D (km) from the western margin of the Yangtze craton. The symbols are as in Fig. 2a. NE represents samples located in the NE end of the traverse (see Fig. 1), which is located in the interior of the Yangtze craton; SW represents samples located in the SW end of the traverse (see Fig. 1), which is located in the western margin of the Yangtze craton. Arrow shows the change in composition from NE to SW in the western part of the Yangtze craton.
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On the basis of discontinuities in incompatible trace element
contents (e.g. Ba, Sr, Th, Pb), combined with variations in
their ratios (e.g. Ba/La and Sr/Ce), the WSYP rocks may be broadly
divided into three subgroups: (1) a subgroup with relatively
high LILE/LREE ratios (e.g. Ba/La and Sr/Ce); (2) a subgroup
with intermediate LILE/LREE ratios (e.g. Ba/La and Sr/Ce); (3)
a subgroup with relatively low LILE/LREE ratios (e.g. Ba/La
and Sr/Ce). These three subgroups are mainly related to the
distance from the western margin of the Yangtze craton. The
characteristics of each subgroup are summarized as follows.
Subgroup with relatively high LILE/LREE ratios
The samples with high Sr/Ce and Ba/La are located along the ASRR shear zone (Fig. 1), closest to the western margin of the Yangtze craton (Fig. 7); these include the Xiangyun, Liuhe, Lijiang and Beiya volcanic fields and rocks from the Janchuan area (Figs 1 and 7). Their Sr/Ce and Ba/La ratios range from 12·8 to 19·3 and 43·0 to 57·1, respectively (Table 3 and Fig. 7), among the highest ratios found in the WSYP rocks. The rocks have positive Sr and positive or negative P anomalies in primitive mantle-normalized trace element diagrams, relatively low abundances of Ba, Sr, Pb, Th, U, LREE and Zr, and the lowest LREE/HREE and highest LILE/HFSE ratios among the WSYP rocks (Figs 6a and 8). They have similar incompatible element ratios (e.g. Ba/Th, Sr/Ce, Ce/Pb, La/Yb) to the active continental margin minettes of Western Mexico (Luhr et al., 1989).
Subgroup with relatively low LILE/LREE ratios
The low Sr/Ce and Ba/La samples have slightly negative P and
positive or negative Sr anomalies in primitive mantle-normalized
trace element diagrams, and the lowest Sr/Ce (5·8–8·8)
and Ba/La ratios (23·4–38·7) among the WSYP
samples (
Figs 6d and
7). These lamprophyres are located at the
NE end of the traverse (
Figs 1 and
7), far from the ASRR shear
zone, and include the Yanyuan volcanic field and rocks from
the Xichang area (
Fig. 1). By contrast with the subgroup with
relatively high LILE/LREE ratios, the absolute abundances of
Ba, Th, U, Pb, LREE, Sr, and Zr are very high (
Table 3 and
Fig. 7).
Likewise, they have the highest LREE/HREE (
Fig. 7) and lowest
LILE/HFSE ratios among the WSYP lamprophyres (
Fig. 8).
Subgroup with intermediate LILE/LREE ratios
These rocks are transitional between the high LILE/LREE and low LILE/LREE subgroups. The lavas have intermediate Sr/Ce (7·5–14·2) and Ba/La ratios (31·0–44·6) (Figs 6b, c, and 7), and intermediate LILE abundances and moderate LILE/HFSE and LREE/HREE ratios (Figs 7 and 8). Spatially, they are distributed in the middle part of the traverse (Figs 1 and 7), predominantly in the Yao'an, Yongren and Ninglang volcanic fields (Fig. 1).
Sr–Nd–Pb isotope compositions
The WSYP samples display relative high values of 87Sr/86Sr(i) (0·704848–0·709640), low 143Nd/144Nd(i) (0·511987–0·512793), and high 207Pb/204Pb(i) (15·42–15·65) and 208Pb/204Pb(i) ratios (37·98–38·64) at a given 206Pb/204Pb(i) ratio (17·68–18·53) (Tables 4 and 5). These isotope ratios display considerable variation among the three subgroups of the WSYP rocks. The Sr–Nd isotope compositions exhibit a strong negative correlation (Fig. 9a). The Sr–Nd–Pb isotopic compositions of Globally Subducted Sediment (GLOSS, Plank & Langmuir, 1998) and those of potassic rocks from the ASRR shear zone (Zhang et al., 2000; Wang et al., 2001) are shown for reference in Fig. 9. Depleted Mantle Nd model ages (TDM; Table 4) were calculated based on the assumption of extraction of the WSYP lavas from an upper-mantle reservoir with present-day 143Nd/144Nd and 147Sm/144Nd of 0·51315 (Peucat et al., 1988) and 0·225 (McCulloch et al., 1983), respectively. The model ages of the WSYP samples range from 0·5 to 1·6 Ga (Table 4). In plots of 207Pb/204Pb(i) vs 206Pb/204Pb(i), and 208Pb/204Pb(i) vs 206Pb/204Pb(i) (Fig. 9c and d), the WSYP potassic rocks generally plot above the Northern Hemisphere Reference Line (NHRL; Hart, 1984). In all isotope projections, the fields of GLOSS, together with the isotope compositions of the potassic rocks along the ASRR shear zone (Zhang et al., 2000; Wang et al., 2001), overlap those of the high LILE/LREE ratio subgroup of the WSYP rocks.
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Fig. 9. (a) (143Nd/144Nd)i vs (87Sr/86Sr)i. (b) (87Sr/86Sr)i vs (206Pb/204Pb)i. (c) (207Pb/204Pb)i vs (206Pb/204Pb)i. (d) (208Pb/204Pb)i vs (206Pb/204Pb)i. The continuous line bounds the field of previously published isotope data for Cenozoic potassic lavas along the ASRR shear zone in each diagram (Zhang et al., 2000; Wang et al., 2001). The dotted line outlines the field of global marine sediments (Plank & Langmuir, 1998). The WSYP igneous rocks show a linear array broadly plotting between Indian MORB-source mantle and marine sediments, probably reflecting two-component mixing. The NHRL (Northern Hemisphere Reference Line; Hart, 1984), EM1 and EM2 (enriched mantle end-members; Zindler & Hart, 1986; Hofmann, 1997; Zou et al., 2000) are shown for reference. Data for Pacific MORB are from Ferguson & Klein (1993). Data for Indian MORB are from Michard et al. (1986), Price et al. (1986) and Rehkämper & Hofmann (1997). BE, Bulk Silicate Earth. The symbols are as in Fig. 2a.
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DISCUSSION
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The primary magmas and enrichment processes of the mantle source region
It has been suggested that primary magmas generated by equilibrium
partial melting of mantle peridotite should have high Mg-numbers
(around 70 or more), along with high Ni and Cr concentrations
(Frey
et al., 1978
). The high compatible element abundances
(in particular, sample CH9723, which has >500 ppm Ni, >500
ppm Cr and an Mg-number of 78;
Table 3) in most of the WSYP
rocks, combined with the presence of mantle-derived xenoliths,
suggest that they probably are candidates for near-primary melts
of mantle peridotite. The predominance of olivine and clinopyroxene
phenocrysts in the WSYP magmatic rocks indicates that the lower
MgO contents in some rocks may be explained by minor fractionation
of these phases. Although most of the WSYP magmatic rocks are
consistent with primitive magma compositions, some samples have
the characteristics of moderately evolved magmas (relatively
low MgO, Ni, Cr;
Table 3). To estimate the WSYP melt segregation
depths, primitive melt compositions with an assumed 15 wt %
MgO were calculated using an extrapolation method by fitting
least-squares linear regression lines to the composition arrays
at different distances from the western margin of the Yangtze
craton. The rationale for this calculation, following Scarrow
& Cox (1995)
, Turner & Hawkesworth (1995)
and Hoang
& Flower (1998)
, is as follows. (1) Relatively low MgO,
Ni and Cr contents in the evolved magmas probably were caused
by combined fractionation and crustal assimilation (AFC) processes,
but extrapolation along the observed data arrays back to 15
wt % MgO should minimize the effects of these processes. (2)
The parent magmas are likely to be more magnesian than the most
Mg-rich erupted lavas because of shallow-level fractionation
processes. The most Mg-rich sample analysed from the WSYP has
a bulk-rock MgO content of 14·45 wt % (
Table 3); this
sample has
3% olivine phenocrysts based on petrographic observation
(
Table 2). The MgO content of the melt in equilibrium with the
olivine phenocrysts would therefore be
13 wt %, assuming that
the olivine has
40 wt % MgO. This suggests that the most primitive
magmas in the WSYP probably possess at least 13 wt % MgO. Such
liquids could be in equilibrium with mantle olivine (Fo
85–90)
(Roeder & Emslie, 1970
). (3) Experimental studies show that
primary peridotite melts fall within the range 12–17 MgO
wt % (Hirose & Kushiro, 1993
; Hoang & Flower, 1998
).
The advantage of this method is that it provides a more robust
basis for estimating primitive melt segregation depths by minimizing
the effects of shallow-level fractionation processes.
Based on the data in Table 3, the slopes, intercepts and correlation coefficients from least-squares linear regression equations for the variation between MgO and SiO2 for the various volcanic fields are given in Table 6. The contents of SiO2 at an assumed 15 wt % MgO, referred to subsequently as Si15, were calculated for each of the volcanic fields by extrapolation using these linear regression equations (Table 6). The relationship between Si15 and pressure of melting has been quantified by Hoang & Flower (1998) for both hydrous and anhydrous mantle melting conditions. Because of the presence of phlogopite phenocrysts in the WSYP rocks, we used the equation between Si15 and pressure for hydrous conditions (Hoang & Flower, 1998) to calculate melt segregation pressures for the volcanic fields according to the relation
where Si
15 denotes the
content of SiO
2 in primitive melts with an assumed 15 wt % MgO
as shown in
Table 6.
P represents the melt segregation pressure
of the primitive magmas generated under hydrous conditions (
Table 6).
On the basis of the calculations of melt segregation pressure,
melt segregation depths in the various volcanic fields were
estimated assuming 1 kbar
3·1 km depth (
Table 6).
Calculated results (
Table 6) suggest that the melt segregation
depths increase from SW to NE with increasing distance from
the western margin of the Yangtze craton (
Fig. 10a); these depths
range from 81 to 88 km. This is consistent with the presence
of a northeastward deepening of the low-velocity zone from 70
km to 90 km beneath the western margin of the Yangtze craton
on the basis of geophysical studies (Liu
et al., 1989
; Zhong
et al., 2000
). The calculated results imply that the WSYP magmas
were derived from the spinel stability field. The presence of
spinel-bearing mantle xenoliths in the Yanyuan and Xichang rocks
(Liu, 1999
) supports this inference.
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Fig. 10. (a) Melt segregation depth (km) of the WSYP rocks vs distance (D, km) from the western margin of the Yangtze craton. Data are from Table 6. The meaning of NE and SW is as in Fig. 7. (b) La/Yb vs La (ppm) diagram. The pronounced correlation between La/Yb and La shows that partial melting was the dominant control on the compositional variation of the WSYP rocks. The meaning of NE, SW and the arrow is as in Fig. 7. The symbols are as in Fig. 2a.
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Figure 4 suggests that the source region of the WSYP magmas
is relatively refractory compared with that of the earlier mantle
plume-related Emeishan basalts, which were also erupted within
the western part of the Yangtze craton, because at a given MgO
content the samples have overall lower abundances of components
probably contributed from mantle clinopyroxene, such as Al
2O
3,
Fe
2O
3, Na
2O, CaO and Sc, and slightly higher Cr/S
TOP
ABSTRACT
INTRODUCTION
