shrimp zircon age and geochemical constraints on the origin of lower jurassic volcanic rocks from...
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SHRIMP Zircon Age and GeochemicalConstraints on the Origin of LowerJurassic Volcanic Rocks from the YebaFormation, Southern Gangdese, SouthTibetDi-Cheng Zhu a , Gui-Tang Pan b , Sun-Lin Chung c , Zhong-Li Liao b
, Li-Quan Wang b & Guang-Ming Li ba China University of Geosciences, Beijing, Chinab Chengdu Institute of Geology and Mineral Resourcesc National Taiwan UniversityPublished online: 06 Aug 2010.
To cite this article: Di-Cheng Zhu , Gui-Tang Pan , Sun-Lin Chung , Zhong-Li Liao , Li-Quan Wang& Guang-Ming Li (2008): SHRIMP Zircon Age and Geochemical Constraints on the Origin of LowerJurassic Volcanic Rocks from the Yeba Formation, Southern Gangdese, South Tibet, InternationalGeology Review, 50:5, 442-471
To link to this article: http://dx.doi.org/10.2747/0020-6814.50.5.442
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International Geology Review, Vol. 50, 2008, p. 442–471. DOI: 10.2747/0020-6814.50.5.442Copyright © 2008 by Bellwether Publishing, Ltd. All rights reserved.
0020-6814/08/999/442-30 $25.00 442
SHRIMP Zircon Age and Geochemical Constraints on the Origin of Lower Jurassic Volcanic Rocks from the Yeba Formation, Southern Gangdese, South Tibet
DI-CHENG ZHU,1
Chengdu Institute of Geology and Mineral Resources, 610082, Chengdu, China and State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 100083 Beijing, China
GUI-TANG PAN,Chengdu Institute of Geology and Mineral Resources, 610082, Chengdu, China
SUN-LIN CHUNG,Department of Geosciences, National Taiwan University, Taipei 106, Taiwan
ZHONG-LI LIAO, LI-QUAN WANG, AND GUANG-MING LI
Chengdu Institute of Geology and Mineral Resources, 610082, Chengdu, China
Abstract
We present SHRIMP zircon dating, bulk-rock geochemical, and Sr-Nd-Pb isotopic results forYeba volcanic rocks and a mafic dike from Southern Gangdese (SG), southern Tibet, in order toconstrain their tectonic setting and origin. Yeba volcanic rocks span a continuous compositionalrange from basalt to dacite, although andesites are minor, and mafic and felsic rocks are volumetri-cally predominant. New SHRIMP zircon dating for a dacite coupled with previous SHRIMP zircondating for a mafic dike and fossil constraints for the sedimentary sequence indicate that Yeba volca-nic rocks were emplaced in the Early Jurassic (174–190 Ma). Yeba tholeiitic mafic rocks possesscompositional diversity and are divided into three groups based on concentrations of MgO, Al2O3,and La. Mafic samples are all characterized by marked negative Nb, Ta, and Ti anomalies and pos-itive εNd(T) values (+ 2.4 to + 4.5). Yeba calc-alkaline felsic rocks are characterized by coherent,concave-upward MREE patterns and negative anomalies in Nb, Ta, P, and Ti, with positive εNd(T)values (+ 0.3 to + 2.6). Sr-Nd-Pb isotopes overlap among the different groups of Yeba mafic rocks;Pb isotopic compositions in both mafic and felsic rocks are nearly identical. These features areconsistent with a subduction-related origin, most likely in an arc built on thin, immature continentalcrust. Yeba volcanic rocks are interpreted as having been created by northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time. Geochemical signatures and quantitative modelingindicate that fractional crystallization and crustal assimilation played insignificant roles in thegeneration of Yeba mafic magmas, and that their geochemical diversity was probably produced byvariable degrees of partial melting from a common but heterogeneous mantle source, which had beenmetasomatized by variable contributions of sediments/fluids released from the subducted Neo-Tethyan oceanic crust. Yeba felsic rocks were probably generated by moderate degrees of partialmelting of juvenile basaltic lower crust, which consists of dominant underplated magmas (similar toYeba mafic rocks in composition) and variable contributions from ancient lower crust beneath theGangdese Back-Arc fault uplift belt (GBAFUB).
Introduction
THE GANGDESE BELT (GB) is here defined by theBangong Tso–Nujiang suture zone (BNSZ) in thenorth, and the Indus–Yarlung Zangbo suture zone
(IYZSZ) in the south (Fig.1). It is widely acceptedthat the GB is an archetype collisional orogen,related to the Indo-Asian collision. Widespreadmagmatism in this belt has been well documentedfor many years (Maluski et al., 1982; Xu et al., 1985;Coulon et al., 1986; XBGMR, 1991; BGMRXAR,1993). However, only Cenozoic magmatism generally1Corresponding author; email: [email protected]
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FIG
. 1.
Tec
toni
c fr
amew
ork
of t
he G
angd
ese
belt
show
ing
the
maj
or t
ecto
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and
the
dis
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with
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Tec
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ed fr
om P
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. Abb
revi
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ns: S
MLM
F =
Sham
olei
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omai
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; N. G
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. Gan
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dle
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elt;
S. G
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= So
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an e
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the
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f Fig
ures
1 a
nd 2
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444 ZHU ET AL.
ascribed to collisional processes (e.g., Linzizongvolcanic successions, 64.47–40.84 Ma, Mo et al.,2006) between India and Asia, and originating froman enriched subcontinental lithospheric mantle orpost-collisional thickened Tibetan lower crust (25–10 Ma) (Turner et al., 1996; Miller et al., 1999;Chung et al., 2003, 2005; Ding et al., 2003; Hou etal., 2004) have been widely studied. The GB is alsogenerally regarded as an Andean-style convergentmargin related to the northward subduction of theNeo-Tethyan oceanic lithosphere before onset ofIndia-Asia collision along the Indus–YarlungZangbo suture zone. Knowledge about pre-colli-sional magmatism, however, remains limited. Previ-ous studies suggested that the earliest arc-typemagmatism on the GB occurred in the Early Creta-ceous (Maluski et al., 1982; Xu et al., 1985; Coulonet al., 1986; XBGMR, 1991; Wang et al., 2000; Dingand Lai, 2003); however, a few recent works haveindicated that the island-arc setting on the GB canbe traced to as early as Late Triassic to Early Juras-sic (Li et al., 2003; He et al., 2006; Chu et al., 2006;Dong et al., 2006). Although the Early Mesozoicisland-arc setting on the GB has been identified,details of this tectonic setting and the origin of theMesozoic magmatism remain unclear.
In this paper, we first introduce the GB, summa-rizing both the literature and new observations fromregional geological surveying (1:250,000). We thenpresent our new SHRIMP zircon dating andgeochemical data for poorly constrained Yeba volca-nic rocks well exposed in the Dazi area, southernGangdese (Fig. 1). Our goal is to more accuratelydefine their tectonic setting and origin.
The Gangdese Belt
Although the GB was shortened by at least 180km as a result of the Qiangtang–Lhasa terrane colli-sion during the Jurassic–Cretaceous (Murphy et al.,1997), several tectonic units (shown below) are wellpreserved (Fig. 1). The purpose of the followingreview is to introduce in a generalized way the geol-ogy of different tectonic units in the GB, sheddinglight on Mesozoic igneous rocks that have rarelybeen noted before but are very important in thereconstruction of the geodynamic setting; this is alsothe objective of the discussion of Yeba volcanicrocks in this paper.
Indus–Yarlung Zangbo suture zone
The southern boundary of the GB is the Indus–Yarlung Zangbo suture zone (IYZSZ), which extendsabout 2000 km along the Yarlung Zangbo River inTibet (Fig. 1). The IYZSZ is marked by a continuousbut tectonically disturbed ophiolitic mélange zonerelated to the Neo-Tethyan ocean. Previous studieshave suggested that the IYZSZ comprises Jurassic–Cretaceous ophiolites (mainly Late Jurassic to EarlyCretaceous) and marks the location where the Neo-Tethyan oceanic domains were consumed by north-ward subduction under the Gangdese during theCretaceous (Marcoux et al., 1982; Girardeau andMercier, 1988; BGMRXAR, 1993; Wang et al.,2000; Zhou et al., 2002). These observations havebeen confirmed by recent geological mapping; how-ever, it should be stressed that some Mid–LateTriassic radiolarian assemblages have also beendiscovered from chert sequences within the IYZSZexposed near Jinlu village in Zetang County (Wanget al., 2002) and Tangga village in Lazi County (Panet al., 2004; Zhu et al., 2005), providing significantevidence for the existence of Triassic ophiolitewithin the IYZSZ. These Triassic radiolarian assem-blages that have not been noticed previously suggestthat opening of the Neo-Tethyan ocean can be tracedto as early as Middle Triassic time.
Southern Gangdese
The northern boundary of Southern Gangdese(SG) defined here is the Shamolei–Maila–Luobadui–Milashan fault (SMLMF), extending E-W more than1000 km (Pan et al., 2006, Fig. 1). The SG is domi-nated by the Cretaceous–early Tertiary Gangdesebatholith (also known as the Trans-Himalayanplutonic belt in southern Tibet) that was generallyascribed to the northward subduction of the Neo-Tethyan ocean (Yin and Harrison, 2000 and refer-ences therein). The sedimentary sequence exposedin the SG consists mainly of Upper Triassic–MiddleJurassic to Eocene siliciclastic rocks with abundantvolcanic rocks. The volcanic sequence in the UpperTriassic strata (namely, Yeba Formation) has previ-ously been interpreted as a consequence of rifting(Pierce and Mei, 1988; Yin and Grant-Mackie,2005), possibly related to its separation from India.However, this explanation may be problematic,mainly due to the absence of geochemical data. Thevolcano-sedimentary sequence of the Sangri Group(Upper Jurassic–Lower Cretaceous, Fig. 1) in thisbelt is generally thought by previous works to rep-resent the earliest magmatism resulting from the
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northward subduction of Neo-Tethyan ocean(XBGMR, 1991; Wang et al., 2000; Ding and Lai,2003). The well-known Linzizong volcanic succes-sion widely distributed in this belt is distinctly flat-lying on underlying strata (e.g., Upper CretaceousSexing Formation), implying that the GB has experi-enced insignificant Cenozoic N-S shortening in theupper crust (Yin and Harrison, 2000). The signifi-cant Miocene Cu-, Mo-, Au-bearing porphyry withadakite affinity (Chung et al., 2003, 2005; Hou etal., 2004) intruded within the Gangdese batholiths,Linzizong volcanic successions, and/or underlyingsedimentary formations in the SG are interpreted ashaving formed in an active continental collisionzone (Chung et al., 2003).
Gangdese back-arc fault uplift belt
The Gangdese back-arc fault uplift belt(GBAFUB; Zhou and Cao, 1984), which wasrenamed the Lunggar-Nyainqêntanglha complexpaleo-island arc belt by Pan et al. (2006), isbounded to the north by the Gar–Lunggar–ZhariNam Tso–Comai fault (GLZCF) and to the south bythe SMLMF and is geographically distributed alongthe principal crest of the Gangdese-Nyainqên-tanglha range (Fig. 1). The GBAFUB is mainly com-posed of the Neoproterozoic Nyainqêntanglha Group(748–787 Ma; Hu et al., 2005) and the Carbonifer-ous–Permian metasedimentary sequence with conti-nental shelf facies in a shallow sea and the minorTriassic sedimentary sequence (Pan et al., 2006).Some Carboniferous–Permian volcanic rocks withisland-arc affinity have been identified within theCarboniferous–Permian metasedimentary sequencefrom eastern Bomi County and central LinzhouCounty (Pan et al., 2006). The widespread Carbonif-erous–Permian metasedimentary sequence wasepisodically intruded by the Late Triassic megapor-phyritic granodiorite north of Namling (Fig. 1, 217Ma; Li et al., 2003) or the biotite-hornblende grano-diorite and biotite monzogranite with features of I-type granitoid in the Mamba area (215–207 Ma; Heet al., 2006). These have been interpreted as havingformed in an island-arc setting. The Early Jurassicmuscovite monzonitic granites (193.7–190.8 Ma)from Ningzhong (Fig. 1) are thought to have derivedmainly from the melting of upper crust source rocks(Liu et al., 2006). Tectonically, the GBAFUB wasdistinctly thrust southward over the SG along theShamolei–Maila–Luobadui–Milashan fault(SMLMF) during or prior to the Indo-Asian collision.
Middle Gangdese
The Middle Gangdese (MG) lies to the north ofthe GLZCF and south of the Shiquanhe–Laguo Tso–Yongzhu–Nam Tso–Jiali Ophiolitic Mélange Zone(SLYNJOMZ). It comprises the Jienu Group (Mid–Upper Jurassic), the Zenong Group (Upper Juras-sic–Lower Cretaceous), and the Jiega Formation(Lower Cretaceous) and creates a belt of basalts torhyolites, interbedded with sedimentary, largelyvolcaniclastic rocks. It extends more than 1000 kmE-W (Fig. 1). Triassic and Lower Jurassic sequencesare absent (Pan et al., 2006). The most intensivevolcanic activities (Fig. 1) in the MG started about130–120 Ma and probably continued into the LateCretaceous (Zhu et al., 2006), resulting in a vol-cano-sedimentary sequence with a huge thicknessof up to 10 km (Pan et al., 2006). Basalt lavas in theZenong Group (Fig. 1) are believed to have beenderived from the partial melting of mantle wedgematerials induced by fluid from subducted sedi-ments and/or subducted basaltic crust. They arethen thought to have undergone assimilation andfractional crystallization (AFC process) duringascension, whereas felsic rocks are dominantlyrelated to crustal remelting (Zhu et al., 2006). Con-temporaneous granites are also widely distributed inthe MG from west to east (Mo et al., 2005). Paleo-cene volcanic rocks are absent, while significantNeocene volcanic rocks are mainly distributed westof the MG. Neocene volcanic rocks are ultrapotassicand are interpreted as originating from the partialmelting of metasomatized subcontinental lithos-pheric mantle (Turner et al., 1996; Miller et al.,1999; Nomade et al., 2004).
The Shiquanhe–Laguo Tso–Yongzhu–Nam Tso–Jiali Ophiolitic Mélange Zone
The Shiquanhe–Laguo Tso–Yongzhu–Nam Tso–Jiali Ophiolitic Mélange Zone (SLYNJOMZ) extendsfor ~ 2000 km SE-NW across the northern part ofthe GB (Fig. 1) and was identified during recentregional geological mapping. It has been tectoni-cally disturbed by a large strike-slip and thrust faultand is composed of the Shiquanhe, Laguo Tso,Yongzhu–Nam Tso and Jiali–Bomi ophioliticmélange zone from west to east (Pan et al., 2006).The Shiquanhe ophiolitic mélange zone has beenhighly deformed and was thought to have formed ina backarc basin during Early Cretaceous time. TheLaguo Tso ophiolitic mélange zone consists mainlyof peridotite, gabbro, pillow basalt, plagiogranite, aLate Jurassic–Early Cretaceous radiolarian chert
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sequence and turbidite, although there is no ophi-olitic matrix. Rock assemblages of ophiolite are wellpreserved (although disturbed) in the Yongzhu–NamTso ophiolitic mélange zone that is interpreted ashaving formed in a backarc basin (Ye et al., 2004,2005). Middle Jurassic–Early Cretaceous radio-larian assemblages (Pan et al., 2006), zircon U-Pbdating (178 ± 10 Ma), and Rb-Sr isochron dating(173 ± 10 Ma) in a gabbro dike (Ye et al., 2004)provide probable constraints on the development ofthis backarc basin from Middle Jurassic to EarlyCretaceous time. Only a few ophiolitic relics can betraced in the Jiali–Bomi ophiolitic mélange zone,which is thought to have formed in a backarc settingduring the Triassic time (Pan et al., 2006).
Northern Gangdese
The Northern Gangdese (NG) is bracketed by theBangong Tso–Nujiang suture zone (BNSZ) in thenorth and the SLYNJOMZ in the south (Fig. 1). Pre-dominantly Lower Cretaceous volcano-sedimentarysequences (Wumulongqianbo Formation, DuoniFormation, and Woronggou Formation from west toeast) and the secondary Mid–Upper Jurassic JienuGroup, Lagongtang Formation, and Upper Creta-ceous Jingzhushan Formation are exposed in theNG. The Jienu Group and Lagongtang Formationconsist mainly of quartz sandstone, siltstone, ruffite,and bioclastic limestone, interbedded with island-arc andesite and rhyolite, with variable volcaniclas-tic contents. The widespread Early Cretaceous igne-ous rocks are medium-K calc-alkaline with island-arc affinity and have been dated to 125–110 Ma,coeval with the Early Cretaceous magmatic activi-ties in the MG (Pan et al., 2006; Zhu et al., 2006).These calc-alkaline igneous rocks are widely over-lapped by the Jingzhushan Formation molasseswith angular unconformity. Volcano-sedimentarysequences in this belt are intruded by syn-collisional granite (90–78 Ma) and post-collisionalgranite porphyry (Pan et al., 2006).
Bangong Tso–Nujiang suture zone
The Bangong Tso–Nujiang suture zone (BNSZ)extends for ~ 2800 km E-W with a width of 20–120km (Pan et al., 2006) across the central TibetanPlateau (Fig. 1). It is traditionally thought to markthe location where the Mesozoic Tethyan oceanicdomains (Yin and Harrison, 2000, with references)have been consumed by northward subductionunder the Qiangtang terrane (Coulon et al., 1986;Kapp et al., 2003; Ding et al., 2003; Ding and Lai,
2003) or by southward subduction under the GB(Hsü et al., 1995; Pan et al., 2004, 2006; Mo et al.,2005; Zhu et al., 2006). However, Pan et al. (2006)argued that this Tethyan ocean may have developedas early as in Paleozoic time, based on the identifi-cation of paleo-Tethyan oceanic relics in this suturezone (Wang et al., 2003; Chen et al., 2005).
Field Occurrence and Petrography
The Yeba Formation represents a volcano-sedi-mentary sequence. It extends for ~ 250 km E-Wwith a maximum width of 30 km from Dazi County toGongbo Gyamda County in the SG, eastern Lhasa(Fig. 1). Yeba volcanic rocks consist mainly ofbasalts and felsic lavas (e.g., dacite, rhyolite) as wellas felsic volcaniclastic rocks (e.g., tuffs, breccias,agglomerates), with a few andesites. The thick-nesses ranges from tens of meters to ~3000 m for thebasalts (e.g., east of Baiding village) and from 2000m to 7000 m for the felsic lavas and volcaniclasticrocks. The felsic rocks have been metamorphosed togreenschist, whereas the basalts only underwent rel-atively slight metamorphism. Yeba sedimentaryrocks are composed of ruffite, limestone, and sand-stone, interbedded with siliceous rock. All theserocks have been metamorphosed to greenschistfacies. The Yeba metavolcano-sedimentarysequence is overlapped by the Duodigou Formation(Upper Jurassic) and Menzhong Formation (Creta-ceous) with angular unconformity and is intruded byEarly Tertiary subvolcanic rocks, Eocene monzog-ranite, and granodiorite (Fig. 2). Some mafic dikesare also observed within the Yeba volcano-sedimen-tary sequence.
Basalts with massive or microvesicle structuresfilled by chlorite and epidotes are well exposed eastof the villages of Baiding and Segang (Fig. 2) aroundthe Dazi area. The basalts underwent variable alter-ation; however, original textures appear to havebeen preserved. In hand specimens, most of thebasalts are largely aphyric, and epidote is abundant.In thin section, the basalts exposed near Baidingvillage have an aphyric to phyric texture and consistof plagioclase (~ 8–10%) and clinopyroxene (5–8%)phenocrysts with groundmasses of altered fine-grained plagioclase, clinopyroxene, secondaryaltered minerals (e.g., chlorite, epidote), and minorilmenite plus magnetite. Basalts exposed near thevillage of Segang are generally composed of thesesame minerals with the exception of clinopyroxene,which is absent. A mafic dike sample (DZ07-2)
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shows a blastoporphyritic texture, in which predom-inantly plagioclase phenocrysts have been altered tocarbonate, chlorite, and epidote, with secondary cli-nopyroxene phenocrysts (15–20%) also altered butwith pseudomorphs preserved. A few andesite sam-ples are largely aphyric with groundmasses ofaltered fine-grained plagioclase and other alteredminerals (e.g., chlorite, epidote) as well as Feoxides. Metafelsic rocks exhibit massive or schis-tose structures in hand specimens and show bandedstructure, fluidal structure, and blastoporphyritictexture in thin section. These rocks consist predom-inantly of euhedral plagioclase (10–15%) and minorK-feldspar plus quartz phenocrysts with a ground-mass of sericite, micro-grained felsic minerals, andepidote, chlorite, as well as biotite, showing direc-tional arrangement.
Analytical Procedures
In this paper, relatively fresh samples of basalts,andesites, and felsic rocks within the Yeba Forma-tion, well exposed around the Dazi area (Fig. 2),have been analyzed for major elements, trace ele-ments, and Sr-Nd-Pb isotopes after petrographicobservation. One dacite sample (DZ05-1) with
phyric texture from the southern Dazi Bridge wasselected for zircon SHRIMP dating.
For U-Pb zircon dating, zircons were separatedfrom sample DZ05-1 using standard density andmagnetic separation techniques at the Special Lab-oratory of the Geological Team of Hebei Province,China. Zircon grains, together with the zircon U-Pbstandard TEMORA (Black et al., 2003), were cast inan epoxy mount, which was then polished in order tosection the crystals in half for analysis. Zircons weredocumented with transmitted and reflected lightmicrographs as well as with cathodoluminescence(CL) images to reveal their internal structures, andthe mount was vacuum-coated with a 500 nm layerof high-purity gold. Under the guidance of zircon CLimages, the zircons were analyzed for U-Pb isotopesand U, Th, and Pb concentrations using a SHRIMPII ion microprobe at the Beijing SHRIMP Center,Chinese Academy of Geological Sciences. U-Th-Pbratios were determined relative to the TEMORA stan-dard zircon corresponding to 417 Ma 206Pb/238U =0.0668 (Black et al. 2003), and the absolute abun-dances were calibrated to the standard zircon SL13.Analyses of the TEMORA standard zircon wereinterspersed with those of unknown grains, followingoperating and data processing procedures similar tothose described by Williams (1998). The reference
FIG. 2. Geological map of the studied area showing sample locations (Pan et al., 2005, unpubl.).
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zircon was analyzed after every fourth analysis.Measured compositions were corrected for commonPb using the 204Pb method, and data processing wascarried out using Isoplot (Ludwig, 2001). Uncertain-ties in individual analyses are reported at the 1σlevel; mean ages for pooled 206Pb/238U results arequoted at the 95% confidence level. The U-Pb zir-con data are presented in Table 1.
Major element data were collected using X-rayfluorescence (XRF) on fused glass beads using aRigaku ZSX100e spectrometer in the AnalyticalCenter, Chengdu Institute of Geology and MineralResources. The analytical uncertainty is typically<5%. The same whole-rock powders were used fortrace element concentrations and Sr and Nd isotopicratios. Trace element concentrations were deter-mined at the National Geological Analytical Center,Chinese Academy of Geological Sciences, Beijingusing a Perkin Elmer Elan 6000 inductively cou-pled plasma mass spectrometer (ICP-MS) for aciddissolution. The analytical procedures were similarto those described by Li (1997). The analyticalprecision is generally within 5%. Whole rock majorand trace element data for the analyzed samples aregiven in Table 2.
Sr and Nd isotopic ratios were measured on aFinnigan MAT-262 thermal ionization mass spec-trometer (TIMS) at the Laboratory for RadiogenicIsotope Geochemistry, Institute of Geology andGeophysics, Chinese Academy of Sciences, Beijing.
About 100–150 mg of bulk-rock powder was com-pletely decomposed in a mixture of HF-HClO4 forSr and Nd isotopic analysis. Sr and REE were sepa-rated in quartz columns with a 5 ml resin bed of AG50W-X12, 200–400 mesh. Nd was separated fromother REEs on quartz columns using 1.7 ml Teflon®
powder as cation exchange medium. Proceduralblanks were <200 pg for Sr and <50 pg for Nd. Forthe measurements of isotopic composition, Sr wasloaded with a Ta-HF activator on a single W fila-ment, and Nd was loaded as phosphates and mea-sured in a Re-double-filament configuration. Theconcentrations of Rb, Sr, Sm, and Nd were analyzedby isotopic dilution. The NBS-987 standard mea-sured during the course of analyses gave values of0.512149 ± 3 (2σ) for the 143Nd/144Nd ratio and0.710244 ± 4 (2σ) for the 87Sr/86Sr ratio. 143Nd/144Nd ratios were normalized to 146Nd/144Nd =0.7219 and 87Sr/86Sr ratios to 86Sr/88Sr = 0.1194.Raw data obtained were calculated using the Isoplotprogram (Ludwig, 2001), giving a 2σ error. Techni-cal details on chemical separation and measurementare described in Chen et al. (2002).
Pb isotopic ratios were measured at the NationalGeological Analytical Center, Chinese Academy ofGeological Sciences, Beijing. Pb fractions were sep-arated and purified by HBr and anion exchangeresin and then coated on a single rhenium filamentusing the classic H3PO4 and silica gel method. Massfractionation corrections of Pb isotopic compositions
TABLE 1. Zircon SHRIMP Analysis of a Yeba Dacite, Southern Tibet1
Spot
206Pbc,%
U,ppm
Th,ppm Th/U
206Pb*,ppm
206Pb/238U(Ma ± 1σ)
207Pb*/206Pb* ± % 207Pb*/235U ± % 206Pb*/238U ± %
DZ05-1-1 1.03 302 287 0.95 7.65 185.1 ± 5.8 0.0518 9.1 0.208 9.6 0.02913 3.2
DZ05-1-2 2.45 264 216 0.82 6.75 184.2 ± 5.7 0.0392 18 0.157 19 0.02898 3.1
DZ05-1-3 1.88 439 314 0.71 11.1 183.5 ± 5.6 0.0431 15 0.172 15 0.02888 3.1
DZ05-1-4 4.34 169 128 0.76 4.06 170.7 ± 5.9 0.037 32 0.135 32 0.02684 3.5
DZ05-1-5 4.19 133 90 0.68 3.21 172.0 ± 11 0.038 35 0.141 35 0.0270 6.7
DZ05-1-6 4.11 181 134 0.74 4.16 162.7 ± 5.6 0.039 30 0.136 30 0.02557 3.5
DZ05-1-7 2.75 144 87 0.60 3.50 175.2 ± 5.9 0.0404 23 0.153 23 0.02756 3.4
DZ05-1-8 1.76 130 75 0.58 3.01 168.6 ± 5.6 0.0572 8.8 0.209 9.4 0.02650 3.4
DZ05-1-9 4.50 167 121 0.72 4.13 175.0 ± 6.1 0.037 40 0.140 40 0.02752 3.5
DZ05-1-10 2.31 142 134 0.94 3.33 169.7 ± 5.7 0.0467 17 0.172 17 0.02667 3.4
DZ05-1-11 2.41 105 69 0.66 2.44 167.6 ± 6.0 0.064 19 0.232 19 0.02633 3.6
1Sample DZ05-1: average weighted 206Pb/238U age of 11 spots = 174.2 ± 3.6 Ma (MSWD = 1.7). 206Pbc (%) represents the percentage of common 206Pb in total 206Pb; * denotes radioactivity lead.
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LOWER JURASSIC VOLCANIC ROCKS 449
for the samples were made relative to InternationalStandard NBS-981, which was determined at thesame temperature (1250°C) as the samples. TheNBS-981 gives average 206Pb/204Pb, 207Pb/204Pb,and 208Pb/204Pb ratios of 16.8839 ± 0.0008,15.4207 ± 0.0016, and 36.4753 ± 0.0029 (errorsare 2σ), respectively, during the period of sampleanalysis.
Results
U-Pb zircon geochronology
Cathodoluminescence images and concordiaplots for the analyzed zircons are shown on Figure 3.U-Pb zircon SHRIMP analytical data are listed inTable 1. Zircons are mostly euhedral, relativelytransparent, and colorless, showing concentric zon-ing or straight rhythmic stripes with length/widthratios of about 2:1 to 3:1 (Fig. 3A). Euhedral con-centric zoning or straight rhythmic stripes, coupledwith high Th/U ratios, are indicative of magmaticzircons (Hoskin and Black, 2000). Eleven spotsyield 206Pb/238U ages ranging from 162.7 to 185.1Ma, with a weighted mean age of 174.2 ± 3.6 Ma(MSWD = 3.6) (Fig. 3B). This age is taken to repre-sent the emplacement age of felsic magma in theDazi area.
Geochemistry
Classification. Yeba volcanic rocks span a con-tinuous basalt to dacite suite with a compositionalrange from 46.38 to 68.92 wt% SiO2, with the excep-tion of samples BD01 and BD21, which have con-siderably low contents of SiO2 (42.3 and 40.9 wt%,
respectively). This can be largely ascribed to a highdegree of alteration, judged from sample BD21which presents the highest LOI value (9.17 wt%) ofall analyzed samples. Na2O and K2O concentrations(Table 1) are not considered for classification,because alteration probably played a significant rolein controlling the budget of alkaline elements fornearly all rocks. Yeba volcanic rocks are predomi-nantly basalts and felsic rocks (rhyodacite/dacite)with minor andesites according to the Zr/TiO2versus Nb/Y classification diagram (Fig. 4A). TheFeO*/MgO versus SiO2 diagram indicates that bothYeba basalts and andesites are tholeiitic, whereasYeba felsic rocks (rhyodacite/dacites) are calc-alkaline (Fig. 4B).
Crawford et al. (1987) and Kersting and Arculus(1994) have defined high-magnesia basalt (HMB)(≤54 wt% SiO2, ≥7 wt% MgO, and <16 wt% Al2O3)and high-alumina basalt (HAB) (≤54 wt% SiO2,≤7wt% MgO, and ≥16 wt% Al2O3) to constrain thecompositional variations and petrogenesis of volca-nic rocks from various eruptive settings. La iswidely used as a discriminator of magmatic process,as its concentration significantly increases duringfractional crystallization. Combined with La abun-dance based on the definition of HMB and HAB,Yeba mafic samples are divided into three composi-tional groups (Fig. 4C): (1) Group 1 is defined by lowLa abundance (7.2–15.1 ppm) with relatively highMgO (7.45–10.51 wt%) and low Al2O3 contents(14.03–16.01 wt%), showing similar features toHMB; (2) Group 2 presents low La abundance(10.7–15.9 ppm), relatively low MgO (5.86–7.39wt%), and high Al2O3 contents (16.66–18.41 wt%),
FIG. 3. Cathodoluminescence image and concordia plot of zircon SHRIMP dating for Yeba dacite in the SG, southTibet.
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3
450 ZHU ET AL.
TAB
LE 2
. Maj
or (w
t%) a
nd T
race
Ele
men
t (pp
m) C
hem
istr
y of
Yeb
a Vo
lcan
ic R
ocks
, Sou
ther
n Ti
bet1
Sam
ple:
BD
01B
D21
DZ1
3-1
DZ0
7-2
BD
04-1
BD
04-2
BD
13B
D16
YB
5-2
YB
5-3
DZ0
9-1
DZ1
1-1
BD
05B
D08
BD
19R
ock
type
:G
roup
1G
roup
1G
roup
1G
roup
1G
roup
2G
roup
2G
roup
2G
roup
2G
roup
2G
roup
2G
roup
2G
roup
2G
roup
3G
roup
3G
roup
3
SiO
242
.340
.950
.36
47.4
47.5
648
.66
47.4
50.5
48.2
250
.546
.86
46.3
848
.52
49.5
6Ti
O2
1.01
0.84
0.88
0.66
1.06
0.73
0.78
1.01
1.09
0.92
0.83
0.71
0.8
0.87
Al 2O
317
.77
15.6
216
.01
14.3
17.9
416
.66
16.8
716
.92
18.4
117
.87
16.9
220
.29
15.9
818
.02
ΣFe 2O
3*12
.03
9.61
8.8
7.86
11.5
29.
859.
7610
.02
11.3
89.
9410
.06
12.7
99.
869.
7M
nO0.
270.
190.
160.
140.
240.
170.
170.
170.
150.
220.
210.
240.
160.
29M
gO10
.51
7.84
7.84
7.45
7.39
6.41
5.86
6.68
6.64
7.39
6.08
3.13
7.96
6.55
CaO
7.5
13.1
77.
758.
385.
1110
.54
12.3
13.
373.
672.
416.
7510
.85
11.5
35.
62N
a 2O2.
652.
922.
522.
144.
923.
533.
276.
186.
154.
523.
311.
323.
074.
35K
2O0.
040.
510.
021.
040.
140.
410.
250.
090.
050.
192.
122.
540.
120.
05P 2O
50.
40.
160.
190.
130.
410.
290.
30.
310.
390.
270.
270.
20.
420.
43LO
I5.
289.
176.
0511
.07
3.82
2.59
2.72
54.
056.
087.
452.
032.
134.
28To
tal
99.1
100.
4510
0.3
100.
1510
0.11
99.4
399
.38
100.
2510
0.2
100.
3110
0.31
100.
1310
0.06
99.2
4Sc
30.5
31.1
27.6
21.9
35.8
33.2
32.4
31.8
35.4
35.8
29.6
22.7
28.3
31.9
33.5
V22
325
120
318
327
427
627
426
921
921
123
521
614
125
128
3C
r21
626
719
127
414
413
128
627
212
914
213
558
.714
643
119
8C
o46
.933
.737
31.1
40.5
37.8
35.9
34.5
43.7
4638
.129
59.7
4130
.9N
i94
.296
.881
.312
082
.178
.399
98.1
106
87.7
79.3
34.6
68.1
139
82.8
Rb
0.5
9.0
0.8
50.2
6.5
6.3
6.2
3.8
2.5
0.5
7.1
81.6
89.1
1.8
0.8
Sr44
245
947
439
753
850
451
583
136
534
321
351
670
990
355
6Y
22.2
19.0
520
.113
.922
.522
.120
.520
.120
.524
.719
.817
.025
.119
.125
.1Zr
67.6
83.3
81.8
77.1
109
104
80.6
75.3
7897
.810
682
.670
.887
.795
.5N
b5.
17.
26.
63.
46.
75.
93.
24.
34.
95.
25.
75.
56.
35.
66.
1C
s1.
01.
30.
67.
41.
31.
01.
41.
30.
50.
20.
83.
18.
70.
70.
4B
a64
.165
98.
6619
612
012
757
923
496
.795
.583
.249
440
461
.761
.8La
15.1
12.7
10.9
7.2
15.9
15.6
12.8
12.8
10.7
12.9
14.2
12.2
2021
.918
.4C
e32
.726
.324
.215
.832
.732
27.1
27.3
23.6
28.6
28.9
26.5
36.5
44.9
39.8
Pr4.
33.
43.
22.
14.
24.
23.
53.
53.
44.
03.
63.
44.
85.
55.
1N
d19
.314
.814
9.0
19.2
18.6
15.5
15.5
14.7
17.4
15.5
14.8
20.3
23.6
21.2
Sm4.
43.
43.
42.
24.
54.
33.
73.
63.
33.
93.
73.
55.
04.
94.
9E
u1.
41.
21.
10.
81.
51.
41.
31.
31.
11.
31.
11.
11.
91.
52.
0G
d4.
33.
53.
42.
44.
24.
13.
63.
63.
43.
93.
63.
34.
74.
14.
8Tb
0.7
0.6
0.6
0.4
0.7
0.7
0.6
0.6
0.6
0.7
0.6
0.5
0.7
0.6
0.8
Dy
4.3
3.5
3.7
2.6
4.1
3.9
3.8
3.7
3.5
4.2
3.5
3.1
4.6
3.7
4.7
Ho
0.9
0.7
0.7
0.5
0.8
0.8
0.8
0.8
0.7
0.8
0.7
0.7
0.9
0.7
1.0
Er
2.5
2.1
2.3
1.6
2.4
2.3
2.2
2.3
2.2
2.6
2.1
1.9
2.6
2.1
2.8
Tm0.
30.
30.
30.
20.
30.
30.
30.
30.
30.
40.
30.
30.
30.
30.
4Y
b2.
31.
92.
11.
52.
22.
12.
12.
12.
02.
42.
01.
82.
31.
92.
5Lu
0.3
0.3
0.3
0.2
0.3
0.3
0.3
0.3
0.3
0.4
0.3
0.3
0.4
0.3
0.4
Hf
2.3
2.3
2.4
2.1
2.6
2.5
2.3
2.2
2.2
2.7
2.5
2.1
2.4
2.4
2.8
Ta0.
20.
20.
30.
10.
30.
30.
20.
20.
30.
30.
30.
10.
30.
20.
3Pb
6.8
42.3
13.3
208.
911
.412
.422
.85.
55.
711
.816
.635
.27.
232
.9Th
1.2
1.9
1.2
1.4
3.9
3.6
1.6
1.5
1.0
1.3
2.1
1.2
2.1
2.3
1.6
U0.
30.
80.
50.
50.
60.
60.
50.
40.
30.
40.
40.
50.
60.
60.
4M
g#63
.662
64.1
65.5
56.2
56.6
54.6
57.2
53.8
59.8
54.7
32.9
61.8
57.5
Eu/
Eu*
1.02
1.04
1.00
1.03
1.09
1.05
1.06
1.07
1.01
1.04
0.94
0.98
1.20
1.04
1.24
Tabl
e co
ntin
ues
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LOWER JURASSIC VOLCANIC ROCKS 451TA
BLE
2. C
ontin
ued
Sam
ple:
BD
22Y
B5-
1B
D09
DZ1
0-1
DZ1
0-2
DZ0
1-2
DZ0
2-1
DZ0
3-1
DZ0
3-2
DZ0
3-3
DZ0
4-1
DZ0
5-1
DZ0
7-3
DZ0
7-4
Roc
k ty
pe:
Gro
up 3
Gro
up 3
And
esite
And
esite
And
esite
Dac
iteD
acite
Dac
iteD
acite
Dac
iteD
acite
Dac
iteD
acite
Dac
ite
SiO
248
.68
46.9
457
.44
56.4
65.9
861
.78
65.7
864
.14
66.5
263
.368
.92
67.1
264
.72
TiO
21.
250.
920.
790.
770.
590.
60.
490.
530.
470.
590.
40.
530.
49A
l 2O3
18.3
919
.18
15.6
718
.46
15.8
619
.79
18.5
615
.26
14.7
315
.314
.94
15.0
215
.77
ΣFe 2O
3*12
.22
119.
675.
523.
883.
152.
094.
464.
024.
683.
073.
553.
76M
nO0.
210.
160.
150.
190.
110.
100.
040.
100.
100.
110.
090.
100.
11M
gO4.
962.
092.
171.
871.
251.
031.
001.
921.
411.
671.
091.
231.
18C
aO2.
256.
329.
764.
612.
631.
570.
854.
323.
343.
522.
672.
373.
92N
a 2O5.
835.
981.
192.
995.
916.
343.
283.
174.
132.
33.
984.
532.
54K
2O0.
031.
370.
422.
981.
653.
124.
652.
42.
623.
522.
822.
143.
14P 2O
50.
560.
440.
140.
280.
110.
170.
150.
10.
140.
170.
110.
150.
13LO
I5.
285.
672.
486.
361.
471.
972.
753.
821.
944.
231.
682.
974.
58To
tal
99.6
610
0.07
99.8
810
0.43
99.2
399
.61
99.6
310
0.06
99.4
299
.39
99.6
999
.57
100.
21Sc
3933
.425
.412
.913
.69.
957.
577.
211
.410
.211
.66.
946.
996.
83V
228
353
166
106
110
45.6
34.5
58.4
89.7
59.6
108
41.9
49.4
54.7
Cr
142
31.1
214
20.8
206.
412
.59.
3416
.253
.138
.45.
344.
585.
38C
o42
.634
.441
.86.
866.
55.
595.
576.
8210
.37.
9114
.19.
736.
897.
08N
i75
.217
.314
95.
365.
612.
893.
323.
637.
119.
2814
.32.
782.
672.
38R
b1.
148
.39.
997
.996
.826
80.9
149
68.4
66.2
95.7
80.2
7811
3Sr
101
679
689
121
127
445
248
149
400
355
173
305
290
198
Y27
.927
.520
.422
.323
.229
24.7
21.4
20.9
19.7
19.7
2021
.521
.5Zr
129
95.3
107
141
157
186
270
231
187
208
183
192
209
207
Nb
9.7
4.8
6.7
8.5
7.8
11.4
10.3
9.4
11.1
7.5
7.3
9.0
11.2
10.5
Cs
0.3
4.8
2.2
3.9
4.1
1.0
5.1
8.8
4.4
2.4
4.1
1.5
4.9
7.9
Ba
60.5
254
128
379
369
723
693
883
654
661
735
798
585
487
La19
.319
.320
.418
.920
.326
.531
.426
.122
.321
.520
.323
.323
.326
.3C
e43
.139
.738
.937
.138
.852
.357
.748
.542
.339
.137
.143
46.6
55.9
Pr5.
55.
44.
64.
44.
66.
36.
25.
34.
84.
44.
24.
85.
35.
5N
d24
.223
.019
.318
.319
24.6
23.6
20.5
18.2
17.1
16.6
17.6
19.5
20.4
Sm5.
45.
04.
14.
14.
25.
34.
74.
13.
93.
53.
53.
54.
04.
0E
u1.
62.
01.
31.
51.
61.
51.
31.
11.
01.
11.
21.
01.
11.
3G
d5.
34.
83.
84.
14.
14.
84.
43.
83.
33.
33.
43.
13.
33.
4Tb
0.9
0.8
0.6
0.7
0.7
0.8
0.7
0.6
0.6
0.6
0.6
0.6
0.6
0.6
Dy
5.1
4.5
3.6
4.0
4.1
5.0
4.1
3.6
3.7
3.3
3.3
3.5
3.6
3.7
Ho
10.
90.
80.
80.
81.
10.
90.
80.
70.
70.
70.
70.
80.
8E
r3.
22.
72.
22.
52.
63.
32.
92.
52.
32.
22.
22.
32.
52.
4Tm
0.4
0.4
0.3
0.4
0.4
0.5
0.4
0.4
0.4
0.3
0.3
0.3
0.4
0.4
Yb
2.9
2.5
2.2
2.5
2.4
3.4
3.2
2.6
2.3
2.3
2.3
2.3
2.5
2.4
Lu0.
40.
40.
30.
40.
40.
50.
50.
40.
40.
40.
40.
40.
40.
4H
f3.
22.
72.
73.
73.
95.
06.
55.
65.
05.
04.
45.
15.
65.
3Ta
0.5
0.3
0.4
0.5
0.5
0.5
0.8
0.7
0.5
0.5
0.5
0.5
0.7
0.6
Pb3.
911
.825
.17.
57.
756
.718
.38.
928
.315
.413
27.1
34.4
22.5
Th1.
41.
83.
14.
23.
84.
76.
611
5.1
7.9
4.8
6.8
7.1
6.0
U0.
40.
60.
70.
90.
81.
02.
21.
81.
21.
91.
41.
61.
21.
0M
g#44
.827
.531
.040
.439
.239
.648
.946
.341
.241
.741
.640
.938
.6E
u/E
u*0.
941.
250.
971.
151.
160.
890.
860.
840.
871.
031.
110.
880.
891.
09
1 LO
I =
loss
on
igni
tion;
tota
l iro
n as
ΣFe
2O3*
, Mg#
= 1
00 ×
Mol
ar M
g2+/(M
g2+ +
tota
l Fe2+
)], c
alcu
late
d by
ass
umin
g to
tal F
eO =
0.9
× Σ
Fe2O
3*; E
u/E
u* =
Eu N
/(Sm
N ×
Gd N
)1/2 ,
with
the
subs
crip
t N
deno
ting
norm
aliz
ed to
cho
ndri
te (S
un a
nd M
cDon
ough
, 198
9).
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452 ZHU ET AL.
and is comparable to HAB in composition; and (3)Group 3 is characterized by higher La abundance(18.4–20.0 ppm) and variable MgO contents (2.09–7.96 wt%) relative to Group 2. Note that sampleBD08 is abnormal regarding its high La abundance(21.9 ppm) and will be discussed later.
Major elements. Group 1 basalts have Mg num-bers2 ranging from 62.0 to 64.1; the tholeiitic dike(DZ07-2) exhibits the highest Mg number (65.6) inall analyzed samples. These samples are close tocompositions of primitive arc magmas (Leat et al.,2002; Fig. 5A), indicating near primary mantle-derived compositions with limited fractional crystal-lization. Group 2 basalts are moderately fractionatedwith a narrow range of Mg numbers, 53.8–59.8,whereas Group 3 basalts are variably and highlyfractionated as indicated by their wide, low Mgnumbers (27.5–57.5). Major element oxides versusMg# plots of both Groups 1 and 2 are scattered(Figs. 5A–5E), suggesting that the compositionalvariations from Group 1 to Group 2 cannot beascribed to fractional crystallization. Mg numbers(31–40.4) of two Yeba andesites are comparable tothose of highly fractionated basalts of Group 3,suggesting that these andesites probably were notgenerated from basalts by fractional crystallization.The same is true for Yeba felsic rocks (rhyodacite/dacites), based on their larger Mg# (38.6–48.9) thanandesites and scattered plots on major elementoxides versus Mg# diagrams (Figs. 5A–5E). Themajority of Yeba volcanic rocks define a negativetrend on Al2O3 versus Mg# (Fig. 5C).
Compatible trace elements. Chromium and Niconcentrations (Table 1, Fig. 6) in Yeba mafic rocksare low compared to primary basalts (Ni = 300–400ppm, Cr = 300–500 ppm; Frey et al., 1978). It isprobable that Yeba mafic rocks do not derive from ahomogeneous melt by simple fractionation, mainlydue to the relatively large scatter on the Cr versus Niplot. In detail, Group 1 rocks are moderately frac-tionated by clinopyroxene (cpx) or combined olivine(ol) and spinel (sp) for their moderate, narrow Cr andNi concentrations that define a rough negativetrend, consistent with the presence of many clinopy-roxene phenocrysts observed in thin section. Group2 basalts display a trend with Cr contents dramati-cally decreasing from 286 ppm at an Ni abundanceof 99 ppm to 129 ppm at an Ni abundance of 106ppm, indicating a dominant sp fractionation ratherthan ol separation. Sample DZ11-1 has the lowestCr and Ni concentrations in Group 2 and is relatedto extensive fractionation of cpx and/or combined oland sp. The concentrations of both elements inGroup 3 basalts correlate negatively (Fig. 6), sug-gesting a common parental magma and variable,
FIG. 4. Geochemical classification and association forYeba volcanic rocks. A. Nb/Y-Zr/TiO2 diagram of Winchesterand Floyd (1977). B. FeO*/MgO against SiO2 (wt%) diagramof (Miyashiro, 1974). C. MgO (wt%)–Al2O3 (wt%)–La (ppm)triangle diagram. See text for details. Symbols in all the laterdiagrams are the same as in the legend for Figure 4A, exceptas noted.
2Mg# = molar 100×Mg/(Mg + Fetotal).
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LOWER JURASSIC VOLCANIC ROCKS 453
FIG. 5. Representative major and trace elements plotted against Mg number of Yeba volcanic rocks. It should benoted that the major and the trace elements do not show consistent linear arrays with decreasing Mg#. Fields of primitivearc magma (PAM) and boninite in Figure 5A are from Leat et al. (2002).
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454 ZHU ET AL.
substantial cpx and/or combined ol and sp fraction-ation (Pfänder, 2002). One Yeba andesite (BD09)has abnormally high Cr (214 ppm) and Ni (149 ppm)concentrations, comparable to those of mafic rocks;probably this andesite was not derived from Yebabasaltic magma by mafic mineral fractionation.
Rare earth and other incompatible trace elements.No crystal fractionation trend is observed betweenMg# and REE and HSFE abundances (Figs. 5F–5J)for the basalts in Group 1. These samples show sub-parallel REE patterns (Fig. 7A), with slightly posi-tive Eu anomalies (Eu/Eu* = 1.0–1.04), althoughvarying slightly in absolute abundance. Primitivemantle–normalized trace-element spidergrams (Fig.7B) show that Group 1 samples are well character-ized by coherent negative Nb, Ta, and Ti anomaliesand a positive Sr anomaly, typical of island-arcmagmas. It is important to note that dike sampleDZ07-2 exhibits subparallel REE patterns and traceelement spidergrams, although it has the lowestabsolute REE and other incompatible trace elementabundances, e.g., La = 7.17 ppm, Nb = 3.4 ppm, inGroup 1 (Figs. 7A and 7B).
Group 2 sample plots are scattered on Mg# ver-sus REE and HSFE diagrams (Figs. 5F–5J). Thesesamples also exhibit subparallel REE patterns (Fig.7C) with positive Eu anomalies (Eu/Eu* = 0.94-1.09, averaging 1.07). Group 2 samples also havesignificantly negative Nb, Ta, and Ti anomalies inprimitive mantle–normalized trace-element spider-grams (Fig. 7D), indicating an affinity with island-
arc magma. A positive P anomaly typifies Group 2basalts.
Trace element concentrations of Group 3 samplesdo not show systematic behavior with decreasingMg# (Figs. 5F–5J). Three samples in Group 3exhibit considerable positive Eu anomalies (Eu/Eu* = 1.2–1.25), resulting largely from plagioclaseaccumulation, consistent with the observation on theAl2O3 versus Mg# diagram (Fig. 5C). Samples(except BD08) in Group 3 present subparallel REEpatterns (Figs. 7E and 7F) and primitive mantle–normalized trace-element spidergrams. Features ofisland-arc volcanic rocks (e.g., negative Nb, Ta, andTi anomalies) in Group 3 are also present. SampleBD08 is characterized by an abnormal enrichmentof La (21.9 ppm) and elevated (La/Yb)N (8.3), result-ing in an LREE pattern that is inconsistent withother samples at comparable Mg#. This can beascribed to significant assimilation by continentalcrust, inasmuch as crustal material is enriched inLREE.
Yeba tholeiitic andesites are minor in volumeand only three samples were analyzed in this work.Their La concentrations (18.9–20.3 ppm) compareclosely with those of basalts in Group 3 (Fig. 5F).Yeba andesites are characterized by significantlypositive Eu (Eu/Eu* = 0.97–1.16) and by negativeMREE anomalies (Fig. 7G). Geochemical featuresof island-arc volcanic rocks (e.g., negative Nb, Ta,and Ti anomalies) are also observed in Yeba andes-ites (Fig. 7H).
FIG. 6. Abundances (ppm) of Cr versus Ni diagram in Yeba volcanic rocks showing either substantial clinopyroxenefractionation or combined olivine and spinel fractionation.
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LOWER JURASSIC VOLCANIC ROCKS 455
FIG. 7. Chondrite-normalized REE patterns and primitive mantle–normalized trace element spectra for Yeba volca-nic rocks. Data for chondrite and primitive mantle–normalized values and plotting order are from Sun and McDonough(1989).
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456 ZHU ET AL.
Yeba felsic volcanic rocks (rhyodacite/dacites)exhibit high chondrite-normalized La values [(La)N =85.7–132.5, averaging = 103.6]. Trace element con-centrations of Yeba felsic samples do not showconsistent linear arrays with decreasing Mg# (Figs.5F–5J), suggesting that these samples, collected ina restricted area, are probably not related by crystalfractionation. Yeba felsic rocks have coherent, iden-tical REE patterns (Fig. 7I), with negative or posi-tive Eu anomalies (Eu/Eu* = 0.84-1.11). Yeba felsicsamples exhibit marked concave-upward MREEpatterns. Primitive mantle–normalized trace-elementconcentrations show that Yeba felsic rocks havesimilar multi-element profiles (Fig. 7J). These pat-terns are characterized by significant enrichmentsin all the large-ion lithophile elements (LILE, e.g.,Rb, Ba, Th, U) and the LREE, relative to the HFSE(e.g., Nb, Ta, Ti, Y) and HREE. These rocks there-fore exhibit negative anomalies in Nb, Ta, P, and Ti.However, Zr and Hf are enriched with respect to theMREE. The enrichments in LILE and depletions inNb, Ta, P, and Ti are indicative of island-arc volca-nic rocks.
In summary, major and trace element concentra-tions of Yeba tholeiitic mafic rocks, andesites, andcalc-alkaline felsic rocks are variable and scatteredand do not show systematic crystal fractionationtrends among and with each other, although variabledegrees of mafic mineral separation (ol, cpx, and sp,etc.) appear to be required prior to emplacement intheir generation. Yeba volcanic rocks havegeochemical affinity with island-arc volcanic rocks,and the majority shows high Al2O3 contents withpositive Eu anomalies. It is important that Yebacalc-alkaline felsic rocks from the Dazi area ofsouthern Tibet present significant middle REEdepletions.
Sr, Nd, and Pb isotopic data. Sr, Nd, and Pb iso-topic compositions were determined for 19 bulk-rock samples of Yeba volcanic rocks from the Daziarea in the SG, southern Tibet (Table 3). Initialisotopic ratios were calculated for the ages of 190Ma and 174.2 Ma for Yeba mafic rocks and felsicvolcanic rocks according to the results of SHRIMPzircon dating for the Yeba mafic dike (Xu et al., pers.commun.,) and Yeba dacite in this contribution,respectively. Sr-Nd and Nd-Pb isotopic correlationplots are shown in Figure 8.
Group 1 basalts have narrow initial Sr, Nd, andPb isotopic compositional ranges as follows: 87Sr/86Sr = 0.70433–0.70454; εNd(T) = +2.5 to +3.2;206Pb/204Pb = 18.35–18.59, 207Pb/204Pb = 15.64–
15.67, and 208Pb/204Pb = 38.54–38.82. The tholei-itic dike (DZ07-2) with the nearest primitive man-tle–derived composition has the highest initial Sr,Nd, and Pb isotopic ratios (0.70497, +3.5, 18.68,15.68, and 38.92, respectively) in Group 1. Group 2basalts (except for DZ11-1) present relatively nar-row ranges of initial Sr, Nd, and Pb isotopic compo-sitions, varying from 0.70427 to 0.70505 for Srisotopic ratios, +2.7 to +4.2 for εNd(T) values,18.24–18.48 for 206Pb/204Pb, 15.57–15.62 for207Pb/204Pb, and 38.22–38.56 for 208Pb/204Pb,respectively. Sample DZ11-1 has elevated initial Sr(0.70635) and Pb isotopic compositions (18.64,15.65, and 38.80, respectively) at a similar εNd(T)value (+2.7) to other basalts in Group 2; its high ini-tial Sr isotopic ratio is probably related to seawateralteration (Fig. 8A). Two samples in Group 3 exhibitlarge variations of initial Nd values (εNd(T) = +2.4to +4.5) and Pb isotopic ratios (206Pb/204Pb = 18.39–18.54, 207Pb/204Pb = 15.54–15.70, 208Pb/204Pb =38.34–38.93) at restricted initial Sr isotopic compo-sitions (0.70461–0.70483). Yeba felsic rocks (exceptfor DZ03-1) have variable εNd(T) values rangingfrom +2.6 to +0.3 and limited initial Sr isotopicratios (0.70402 to 0.70438), resulting in a distincttrend toward lower crust or EM1 and plotting in themantle array on the Sr-Nd isotopic correlation dia-gram (Fig. 8A). Their initial Pb isotopic ratios, rang-ing between 18.28 and 18.64 for 206Pb/204Pb, 15.60to 15.68 for 207Pb/204Pb, and 38.44 to 38.91 for208Pb/204Pb, significantly overlap the mafic rocks.Sample DZ03-1 exhibits the lowest initial Sr(0.70338), 207Pb/204Pb (15.57), and 208Pb/204Pb(38.04) isotopic ratios with the highest εNd(T) value(+3.4) of all of the felsic rocks. Its comparativelyhigh measured 87Rb/86Sr ratio (2.41, Table 3) maycarry a relatively large uncertainty and thereforeyield unreasonably low Sr isotopic ratios (Jahn,2004), suggesting that the abnormally low Sr isoto-pic ratio of this sample has less petrogenetic signif-icance. The same is possible for sample DZ07-4,which also has a relatively high measured 87Rb/86Srratio (1.86, Table 3).
Our first new isotopic dataset shows that the Sr,Nd, and Pb isotopic compositions overlap differentgroups of Yeba mafic rocks (Figs. 8A and 8B); thePb isotopic compositions in both Yeba mafic andfelsic rocks are approximately identical (Table 3,Fig. 8B). Excluding the three possibly abnormalsamples (DZ11-1, DZ03-1, and DZ07-4), the Sr-Ndisotopic ratios show an unusual positive correlationor an approximate vertical trend toward the
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LOWER JURASSIC VOLCANIC ROCKS 457TA
BLE
3. S
r, N
d, a
nd P
b Is
otop
ic C
ompo
sitio
ns o
f Yeb
a Vo
lcan
ic R
ocks
, Sou
ther
n Ti
bet1
Sam
ple
no.
Gro
upin
g87
Rb/
86Sr
87Sr
/86Sr
(± 2σ)
(87Sr
/86Sr
) T
147 S
m/
144 N
d
143 N
d/14
4 Nd
(± 2σ)
(143 N
d/14
4 Nd)
TεN
d(T)
T DM
,M
aT 2D
M,
Ma
206 P
b/20
4 Pb
207 P
b/20
4 Pb
208 P
b/20
4 Pb
(206 P
b/20
4 Pb)
T
(207 P
b/20
4 Pb)
T
(208 P
b/20
4 Pb)
T
BD
01G
roup
10.
0038
0.70
4339
± 1
20.
7043
30.
1406
0.51
2699
± 1
00.
5125
22.
618
.46
15.6
438
.66
18.3
515
.64
38.5
4
BD
21G
roup
10.
0404
0.70
4455
± 1
20.
7043
50.
1471
0.51
2718
± 1
20.
5125
42.
818
.44
15.6
438
.65
18.3
915
.64
38.6
2
DZ1
3-1
Gro
up 1
0.00
440.
7045
50 ±
11
0.70
454
0.15
160.
5127
48 ±
13
0.51
256
3.3
18.6
715
.67
38.8
818
.59
15.6
738
.82
DZ0
7-2
Gro
up 1
0.36
570.
7059
56 ±
11
0.70
497
0.15
510.
5127
67 ±
10
0.51
257
3.5
18.7
315
.69
38.9
718
.68
15.6
838
.92
BD
04G
roup
20.
0244
0.70
4881
± 1
40.
7048
20.
1411
0.51
2723
± 1
20.
5125
53.
018
.39
15.5
838
.53
18.2
415
.57
38.2
2
BD
13G
roup
20.
0377
0.70
4618
± 1
30.
7045
20.
1472
0.51
2789
± 1
10.
5126
14.
218
.44
15.6
238
.61
18.3
615
.62
38.5
2
BD
16G
roup
20.
0080
0.70
4334
± 1
00.
7043
10.
1434
0.51
2708
± 1
30.
5125
32.
718
.39
15.6
138
.56
18.3
915
.61
38.5
6
YB
5-2
Gro
up 2
0.01
210.
7045
53 ±
13
0.70
452
0.14
910.
5127
67 ±
12
0.51
258
3.7
18.5
715
.60
38.6
418
.45
15.5
938
.51
YB
5-3
Gro
up 2
0.00
560.
7042
89 ±
15
0.70
427
0.14
570.
5127
73 ±
11
0.51
259
3.9
18.5
015
.58
38.5
118
.36
15.5
838
.34
DZ0
9-1
Gro
up 2
0.07
220.
7052
49 ±
14
0.70
505
0.14
120.
5127
19 ±
11
0.51
254
2.9
18.5
715
.60
38.6
018
.49
15.6
038
.47
DZ1
1-1
Gro
up 2
0.34
060.
7072
72 ±
90.
7063
50.
1416
0.51
2707
± 1
20.
5125
32.
718
.70
15.6
538
.85
18.6
415
.65
38.8
0
BD
19G
roup
30.
0059
0.70
4843
± 9
0.70
483
0.13
880.
5126
87 ±
11
0.51
252
2.4
18.5
715
.70
38.9
718
.55
15.7
038
.93
YB
5-1
Gro
up 3
0.12
850.
7049
58 ±
10
0.70
461
0.14
060.
5128
00 ±
12
0.51
263
4.5
18.5
015
.55
38.4
418
.39
15.5
438
.34
DZ0
1-2
Dac
ite0.
1732
0.70
4811
± 1
30.
7043
80.
1303
0.51
2693
± 1
30.
5125
52.
683
875
318
.67
15.6
838
.96
18.6
415
.68
38.9
1
DZ0
2-1
Dac
ite0.
7170
0.70
5833
± 1
20.
7040
618
.78
15.6
538
.95
18.5
415
.63
38.7
2
DZ0
3-1
Dac
ite2.
4112
0.70
9351
± 1
20.
7033
80.
1156
0.51
2720
± 1
20.
5125
93.
467
268
418
.76
15.5
938
.83
18.3
515
.57
38.0
4
DZ0
3-2
Dac
ite0.
4621
0.70
5466
± 1
20.
7043
20.
1280
0.51
2643
± 1
00.
5125
01.
690
682
918
.57
15.6
038
.70
18.4
815
.60
38.5
8
DZ0
5-1
Dac
ite0.
7963
0.70
6303
± 1
20.
7043
30.
1209
0.51
2627
± 1
10.
5124
91.
586
284
218
.40
15.6
138
.60
18.2
815
.60
38.4
4
DZ0
7-4
Dac
ite1.
8614
0.70
8627
± 1
20.
7040
20.
1205
0.51
2566
± 1
20.
5124
30.
395
893
818
.73
15.6
638
.90
18.6
415
.65
38.7
3
1 T
= ag
e-co
rrec
ted
initi
al is
otop
ic r
atio
s. C
orre
cted
form
ula
as fo
llow
s: (87
Sr/86
Sr) T
= (87
Sr/86
Sr) m
+ 87
Rb
/86Sr
(eλT
–1)
, λ =
1.4
2 ×
10–1
1 a–1
; (14
3 Nd/
144 N
d)T
= (14
3 Nd/
144 N
d)m
+ (14
7 Sm
/14
4 Nd)
m ×
(eλT
– 1
), εN
d(T)
= [(
143 N
d/14
4 Nd)
m/(14
3 Nd/
144 N
d)C
HU
R(T
) – 1
] × 1
04 , (14
3 Nd/
144 N
d)C
HU
R(T
) = 0
.512
638
– 0.
1967
× (e
λT –
1),
T DM
= 1
/λ ×
ln {1
+ [(
(143 N
d/14
4 Nd)
Sam
ple –
0.
5131
5)/((
147 S
m/14
4 Nd)
Sam
ple –
0.2
137)
]}, λ
Sm-N
d =
6.54
× 1
0–12
a–1; T
2DM
is c
alcu
late
d us
ing
the
sam
e as
sum
ptio
n fo
rmul
atio
n as
Ket
o an
d Ja
cobs
en (1
987)
. (20
6 Pb/
204 P
b)T
= (20
6 Pb/
204 P
b)m
+ 23
8 U/20
4 Pb
× (eλ1
T –1)
, λ1
= 1.
5512
5 ×
10–1
0 a–1
; (20
7 Pb/
204 P
b)T
= (20
7 Pb/
204 P
b)m
+ 23
5 U/20
4 Pb
× (eλ2
T –1)
, λ2
= 9.
8485
× 1
0–10
a–1; (
208 P
b/20
4 Pb)
T =
(208 P
b/20
4 Pb)
m +
232 U
/20
4 Pb
× (eλ3
T – 1
), λ3
= 0
.494
75 ×
10–1
0 a–1
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3
458 ZHU ET AL.
proposed compositions of ancient lower crust in theCretaceous Gangdese (Wen et al., unpubl. data)from Yeba mafic rocks to felsic rocks on the Sr-Nddiagram (Fig. 8A). Although the Pb isotopic ratiosare more sensitive than the Sr isotopic compositionsduring seawater alteration and analysis, the presentdata provide a rough range varying from 18.24 to18.68 for 206Pb/204Pb ratios of Yeba volcanic rocks.
Discussion
The age of Yeba volcanism
The time of volcanism has been a subject ofdebate over the past two decades because of thenear absence of fossils in the thick Yeba Formationmetasedimentary strata. At least three different ageshave been proposed to account for the emplacementof Yeba magmas: (1) previous investigators proposedthat Yeba volcanism erupted in the Cretaceousbased on K-Ar dating ranging from 137.2 Ma to 62.3Ma (XBGMR, 1991); (2) Gou (1994) regarded Yebavolcanism as Mid–Late Jurassic based on a numberof bivalves in the Yeba Formation, such as Pronella(Gythemon) sp. nov, Protocardia Stricklandi, etc.;and (3) Yin and Grant-Mackie (2005) recognizedthree bivalve fossil assemblages from the YebaFormation, including the Trigonodus-Isocyprina(Rhaetian–Hettangian, 210–202 Ma), the Lhasa-nella-Propeamussium (Toarcian-Bajocian, 190–169Ma), and the Jurassicorbula-Neomiodon assemblage
(Middle Jurassic); they therefore defined an ageranging from the latest Triassic to Middle Jurassic.SHRIMP zircon dating of a Yeba dacite reportedhere provides reliable chronological constraints(~174 Ma) for Yeba felsic volcanism, which canundoubtedly be placed at the end of Early Jurassictime. Considering that the mafic dike intruded theYeba metavolcano-sedimentary sequence at 188.1 ±3.4 Ma (Xu et al., pers. commun.), we conclude thatthe age of Yeba volcanism can be placed in the EarlyJurassic (174–190 Ma), largely consistent with thenew bivalve fossil studies (Yin and Grant-Mackie,2005) and coeval with extensive granitoid magma-tism near the Ando and Nyarong area (Guynn et al.,2006).
Effects of alteration and low-grade metamorphism on elemental mobility
Yeba volcanic rocks are ≥174 Ma in age, experi-enced regional greenschist-facies metamorphism,and have therefore been altered to various degreesafter eruption/emplacement, judged from petro-graphic observation and variable LOI listed in Table2. Hence, in order to address questions on mantle-source geochemistry and magmatic processes basedon trace-element and isotope data, the effects ofalteration and metamorphism have to be considered.
A general consensus exists that transition metals(e.g., Cr, Ni), rare-earth elements (REEs), and high-field-strength elements (HFSEs) as well as Th andTi in mafic rocks are relatively immobile during
FIG. 8. Sr-Nd and Nd-Pb isotopic diagrams for Yeba volcanic rocks. Data sources are as follows: proposed ancientlower crust (87Sr/86Sr = 0.7045, εNd(T) = –10.2, Sr = 348 ppm, Nd = 11 ppm; Wen et al., unpubl. data); Luobusha MORB(LB98-3G, Zhang et al., 2005) dated at 177 Ma (Zhou et al., 2002) and two extreme 206Pb/204Pb values of Indian oceanpelagic sediments (Ben Othman et al., 1989) are treated as proxies for the depleted Neo-Tethyan mantle and the Neo-Tethyan oceanic sediments, respectively.
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alteration and low-grade metamorphism (Bienvenuet al., 1990; Staudigel et al., 1996). Indeed, aspresented above (Fig. 7), Yeba mafic samples arecharacterized by subparallel patterns of REE andHFSE concentrations in individual groups, indicat-ing that Yeba mafic rocks still preserve their originalREE and HFSE signatures. The same is true forYeba felsic rocks, judging by their coherent pat-terns. However, the original concentrations of alkalimetals (e.g., Na, K, Rb) and alkaline earth metals(e.g., Sr, Ba) were likely modified by alteration andgreenschist-facies metamorphism, as indicated bytheir variable patterns (e.g., element Rb in Fig. 7).In this case, only immobile elements such as thehigh-field-strength elements (Ti, Zr, Y, Nb, Ta, Hf),Th, and REE are used in the following discussionsto identify the geodynamic setting and origin of Yebavolcanic rocks.
Strontium is mobile during alteration and meta-morphism, and for mantle-derived mafic rocks ashift in Sr concentration caused by alteration istypically accompanied by elevated (87Sr/86Sr)T ratiosat a constant εNd(T) value. The elevated Sr isotopiccomposition measured for sample DZ11-1 canlargely be accounted for by this explanation. How-ever, the initial 87Sr/86Sr vs. εNd(T) diagram illus-trates that the majority of Yeba volcanic rocks(except for DZ11-1) plot within the mantle arrayfield (inset in Fig. 8A), indicating that the effects ofhydrothermal and seawater alteration on the Sr, Ndisotopic system of Yeba volcanic rocks were mini-mal. It is difficult to evaluate the reliability of Pbisotopic ratios in Yeba volcanic rocks inasmuch asPb, U elements that are used to correct the mea-sured Pb isotopic compositions are generally mobileduring alteration and metamorphism. Nevertheless,despite the variations of Pb isotopic ratios, a roughrange for Pb isotopic ratios of Yeba volcanic rocks(around 18.5 for 206Pb/204Pb) can, to a certainextent, be estimated. In this paper, Pb isotopic ratiowill only be discussed in combination with the Ndisotopic compositions of Yeba volcanic rocks (Fig.8B).
The tectonic setting of Yeba volcanic rocks
Yeba mafic rocks share a marked arc-type fea-ture on primitive mantle–normalized trace-elementspidergrams (Figs. 7B, 7D, and 7F)—for instance,negative Nb, Ta, and Ti anomalies that are typical ofsubduction-related igneous rocks. This island-arcaffinity for Yeba mafic rocks can be strongly sup-ported by the Group 2 and Group 3 basalts with high
Al2O3 contents (≥16 wt%), which are common inisland-arc settings (Crawford et al., 1987). Zr/Yratios (2.8–4.6) of Yeba mafic rocks are higher thanthe value of 3.0 that was taken to separate the conti-nental (Zr/Y > 3) and oceanic-arc basalts (Zr/Y < 3;Pearce, 1983), suggesting that Yeba mafic rockswere probably constructed in a continental island-arc setting. As illustrated in Figures 7H and 7J,Yeba andesites and felsic rocks have important arc-type features, which are also observed in both conti-nental island-arc and active continental margins, aswell as in syn-collisional settings. In this case, it isimportant to explore which tectonic setting was mostlikely involved in their generation during the EarlyJurassic time. Bailey (1981) proposed that the La/Yb ratio may be taken as a measure of the extent towhich continental crust is involved in magma gene-sis, and that low-K oceanic island-arc andesites,“other” oceanic island-arc andesites, continentalisland-arc andesites, and Andean (active continen-tal margin) andesites can be distinguished on thebasis of their La/Yb and Sc/Ni ratios. Moderate La/Yb ratios and elevated Sc/Ni ratios of Yeba andes-ites and felsic rocks clearly indicate a continentalisland-arc setting origin (Fig. 9A). With the excep-tion of one Yeba felsic sample (DZ01-2) possessingabnormal Rb concentrations, the Yeba felsic rocksshow a narrow range of coherent Rb concentrations(Figs. 7H and 7I). We are therefore certain that theRb concentrations in the majority of the felsic rockscan be used to identify their tectonic setting. Yebafelsic rocks were probably erupted on a volcanic-arcsetting according to the Rb-Yb+Ta diagram (Fig.9B) rather than a syn-collisional setting.
Geochemical criteria for Yeba volcanic rockspresented above strongly suggest a continentalisland-arc setting in the SG. In other words, our newSHRIMP zircon dating and geochemical data con-firm the existence of the Early Jurassic continentalisland-arc setting, which was not understood by pre-vious investigations (Jin and Zhou, 1978; Pierce andMei, 1988), but which has also been identified byzircon SHRIMP and zircon Hf isotopic studies (Chuet al., 2006; Dong et al., 2006). Previous investiga-tions suggested that the earliest arc-affinity volcanicrocks of the Neo-Tethyan ocean are indicated by theSangri Group volcanic rocks (Fig. 1) dated from theLate Jurassic to Early Cretaceous time (XBGMR,1991; Wang et al., 2000; Ding and Lai, 2003) andthat the Late Jurassic–Early Cretaceous volcanicrocks exposed in the MG and NG originated fromsouthward subduction of the Bangong Tso–Nujiang
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oceanic crust (Pan et al., 2006; Zhu et al., 2006). Inthis case, the role of the southward subduction in thegeneration of the volcanic rocks should be first con-sidered. Yeba volcanic rocks in the SG are nowexposed ~ 250 km to the south of the BNSZ (Fig. 1).This distance, combined with at least 180 km ofJurassic–Cretaceous shortening in the GB (Murphyet al., 1997), suggest that Yeba volcanic rocks werelocated south of the BNSZ at a distance of ~430 kmor more before the Middle Jurassic. This distance isnot compatible with the modern observation of a150–300 km distance from trench to arc if weassume that Yeba volcanic rocks are related to thesouthward subduction of the Bangong Tso–Nujiangoceanic crust. Alternatively, new observations indi-cate that the Neo-Tethyan ocean basin probablystarted to spread as early as Middle Triassic time.Considering that Yeba volcanic rocks now lie ~50km north of the Yarlung Zangbo suture zone (Fig. 1),we believe that they are most probably related to thenorthward subduction of Neo-Tethyan oceanic crust.This inference is supported by a two-stage Nddepleted–mantle model age (T2DM, Keto and Jacob-sen, 1987) calculated for Mesozoic granitoids in theSG and the GBAFUB. Yeba felsic rocks (Table 3)and Early Jurassic granitoid rocks at Wuyu (Chu etal., 2006) show a Mid–Late Proterozoic Nd modelage (T2DM <1.0 Ga) that is considerably youngerthan that of the GBAFUB with the Mid–Early Prot-erozoic residence age (majority T2DM >1.5 Ga, Fig.1). In other words, the crustal residence age sug-gests an apparent Neo-Tethyan oceanward youngingfrom the GBAFUB to the SG, providing additionalevidence for the hypothesis that Early Jurassic
magmatism in the SG was probably created by thenorthward subduction of Neo-Tethyan oceanic crust.
Provenance of Yeba mafic rocks
As discussed before, Yeba mafic rocks collectedaround the Dazi area are further subdivided intothree groups, where Group 1 shows similar featuresto high-magnesia basalt, whereas in contrast bothGroups 2 and 3 display characteristics of high-alumina basalt, the most common basalt type inmany arc volcanic centers. The most striking featureof the geochemistry of Yeba mafic rocks is the scat-ter plots on diagrams of major and trace elementconcentrations versus Mg# within an individualgroup or with each other (Fig. 6). This geochemicaldiversity generally results from different processes(e.g., fractional crystallization, crustal assimilation)and heterogeneous mantle source materials by vari-able degrees of partial melting. In this section, wewill shed light on the possible processes that oper-ated during the generation of Yeba mafic rocks.
Fractional crystallization and crustal assimila-tion processes. Fractionation of olivine, clinopyrox-ene, and/or combined spinel is evident from largevariations in Mg#, Ni, and Cr (Table 2, Fig. 7) andprobably controls the compositions of compatibletrace elements. However, the contents of majorelements in both Groups 1 and 2 were probably notsignificantly affected by such fractional crystalliza-tion, as they have relatively primitive compositions(>5.86 wt% MgO), while this mechanism mayaccount for three samples in Group 3 given their rel-ative low MgO contents (<5.0 wt%). Amphibolefractionation was insignificant because of the
FIG. 9. Tectonic discrimination diagrams for Yeba volcanic rocks. A. La/Yb–Sc/Ni diagram for andesitic rocks(Bailey, 1981). B. Rb-Yb+Ta diagram for felsic volcanic rocks (Pearce et al., 1984).
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absence of MREE depletion (Fig. 8). The scatteredTiO2 versus Mg# plots suggest little or no magnetitefractionation. Elevated Al2O3 contents with decreas-ing Mg# evident in Figure 6C and the presence ofpositive Eu anomalies may very well result fromplagioclase accumulation, which is an importantprocess within island-arc crust (Crawford et al.,1987; Vukadinovic, 1993). In general, Groups 1 and2 in Yeba mafic rocks, whether within an individualgroup or with each other, appear not to be signifi-cantly affected by fractional crystallization, as indi-cated by the lack of correlation between major, traceelement, isotopic composition, and Mg# (Figs. 6,10A, 10B); in contrast, some samples in Group 3(e.g., YB5-1) may have experienced important frac-tional crystallization of mafic mineral.
Continental island-arc magmas generally assim-ilate some of the crust through which they ascend,resulting in depletions of high-field-strength ele-ments (e.g., Nb, Ta, and Ti; Pearce, 1982; Davidson,1996), which are also observed in Yeba mafic rocks.These mafic rocks have low Th abundances rangingfrom 1.01 to 3.88 ppm (averaging = 1.84 ppm),which is considerably lower than that of averagemiddle (6.5 ppm) and upper crust (10.5 ppm; Rud-nick and Gao, 2003), indicating that Yeba maficmagmas may have experienced insignificant crustalassimilation during ascent. This suggestion can alsobe demonstrated by the correlations between isoto-pic ratios and Mg#, which can effectively help todelimit crustal assimilation processes because shal-low-level assimilation of continental material wouldcause an increase of 87Sr/86Sr and a decrease inεNd(T) in the magma suites (Rogers et al., 2000). It
is clear that Sr-Nd isotopic compositions of Yebamafic rocks are incompatible with a crustal assimi-lation trend on both (87Sr/86Sr)T versus Mg# andeNd(T) versus Mg# diagrams (Figs. 10A and 10B).This suggests that shallow-level assimilation of con-tinental crust was not significantly involved in theirgeneration.
Therefore, it is likely that fractional crystalliza-tion and crustal assimilation processes played minorroles in the generation of Yeba mafic magmas, andthat the geochemical diversity observed in theserocks was probably governed by their sourcethrough partial melting rather than AFC processes.If this argument is correct, incompatible elementconcentrations and ratios could be used to tracesource heterogeneity and/or variations in degrees ofpartial melting, and ratios of elements of similarincompatibility should mainly reflect source hetero-geneities.
Mantle source heterogeneity. Mantle sourceheterogeneity is common and may be an intrinsicfeature or can be produced by element transfer fromthe downgoing slab to the mantle wedge via: (1)partial melts produced by heating of the downgoingslab (Rapp et al., 1999); (2) fluids (Hawkesworth etal., 1997); and (3) the accretion of solid or partiallymolten subducted sediments to the mantle wedge(Johnson and Plank, 1999), or by a combination ofthese processes. As a consequence of fluid transferfrom the slab to the mantle wedge, its solidus will belowered, leading to the formation of primary melts(Tatsumi, 1989).
For Yeba mafic rocks, there is no correlation (notshown) between Sr and Nd isotope ratios and incom-
FIG. 10. Initial Sr and Nd isotopic compositions versus Mg# diagrams for Yeba volcanic rocks showing possiblefractional crystallization–crustal assimilation processes.
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patible element ratios such as Th/Ta, which isbelieved to reflect slab contribution, indicating thatcomponents directly derived from subducted Neo-Tethyan basaltic oceanic crust were not significant.It is difficult to constrain the influence of fluidphases released from the downgoing Neo-Tethyanoceanic slab by geochemical tracers because Yebamafic rocks have been altered to various degrees,and consequently the original concentrations ofhydrophile elements are not preserved. However,fluids probably existed in the magma source region,as inferred from the fact that some Yeba mafic rocksshow vesicular structures. It has been noted thatstrong depletion of Nb and Ta relative to Ce, anddepletion of Ti relative to other HFSE, characteristicof Yeba mafic rocks, is also a pattern displayed bysubducting sediments (Ben Othman et al., 1989;Plank and Langmuir, 1998). This similarity indi-cates that the subducted Neo-Tethyan oceanic sedi-ments cannot be ruled out in the generation of Yebamafic rocks. To account for the geochemical diver-sity of magma types observed, Rolland et al. (2002)and Bignold and Treloar (2003) have proposed thatseveral percentages of sediments were entrainedinto the magma source regions of the CretaceousLadakh arc and the Cretaceous Chitin island arc,respectively. For Yeba mafic rocks, a similar quanti-tative evaluation of the contribution of sedimentscan be made, using a mixing model between aYarlung Zangbo MORB component (177 Ma; Zhouet al., 2002; Zhang et al., 2005) and Indian oceanicpelagic sediment end-members, in εNd(T) versus(87Sr/86Sr)T (arrow M1 in Fig. 8A) and εNd(T) versus(206Pb/204Pb)T (Fig. 8B). The results of two-compo-nent mixing indicate that Yeba mafic rock bulk com-positions can be explained by mixing withcontributions of 7–12% and 5–10% from Indianoceanic sediments to reach the measured Sr-Nd andNd-Pb isotopic compositions. Note that there are nosignificant differences within individual groups ofYeba mafic rocks in terms of contributions fromsubducted Tethyan oceanic sediments. We thereforeconclude that the source contaminant was at leastpartly a sedimentary component.
In summary, the mantle source region of Yebamafic rocks has probably been metasomatized byvariable contributions of sediments/fluids derivedfrom the subducted Neo-Tethyan oceanic crust.Contributions directly derived from subductedTethyan basaltic oceanic crust in the generation ofYeba mafic rocks probably were insignificant.
Partial melting of metasomatized garnet-peridot-ite for the generation of Yeba mafic rocks. As shownin Figure 7, Yeba mafic rocks exhibit subparallelREE patterns and primitive mantle–normalizedtrace-element spidergrams within individualgroups, indicating that each group could have beengenerated from a relatively common magma sourceregion. This argument can also be supported by Lu/Hf and Sm/Nd ratios. The Lu/Hf versus Sm/Nd dia-gram offers a good opportunity to address the effectsof partial melting, chemical composition of thesource, and crustal assimilation on the petrogenesisof the mafic rocks because these ratios minimize theeffects of magmatic differentiation (Melluso et al.,2003). With the exception of the primitive sample(DZ07-2) and the assimilated sample (BD08), differ-ent compositional groups within Yeba mafic rockshave limited Lu/Hf (0.12–0.15) and Sm/Nd (0.22–0.24) ratios and generally overlap each other (Fig.11A), although varying slightly in detail. Allègreand Minster (1978) showed that the ratio of the con-centration of a highly incompatible element (e.g.,Th) to a moderately incompatible element (e.g.,REE) can be used to identify partial melting trends.Analyses of Yeba mafic rocks define a linear trendon a Th/Sm versus Th diagram (Fig. 11B). Theseobservations seem to indicate that Yeba mafic rocksprobably were generated from a common magmasource region by partial melting.
Generally, when a spinel-lherzolite undergoespartial melting, the mantle and the melt producedwill have similar Sm/Yb ratios, whereas the La/Smratios decrease with increasing degrees of partialmelting. Melting of a spinel-lherzolite source willtherefore create a relatively horizontal meltingtrend. In contrast, small (or moderate) degrees ofpartial melting of a garnet-lherzolite source (withgarnet residue) produce melt with significantlyhigher Sm/Yb ratios than the mantle source. Inconsequence, the garnet-lherzolite melting trend isdisplaced from the horizontal mantle array to higherSm/Yb ratios on a Sm/Yb versus La/Sm diagram.Figure 12A shows that variable degrees of partialmelting of a garnet-lherzolite source can explainthe compositional diversity of Yeba mafic rocks,inasmuch as they plot along the garnet-lherzolitemelting trend (arrow M1), and that garnet tends toremain in the source region.
In summary, trace element ratios suggest that thegeochemical diversity observed in the mafic rocksprobably originated from a variable degree of partialmelting of a common (although heterogeneous)
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mantle source in which garnet remained in the res-tite. This interpretation for the generation of Yebamafic rocks is similar to the conclusions of Castilloand Newhall (2004), who suggested that the compo-sitional variations of Mayon and Taal lavas fromsouthern Luzon in the Philippines are probablyintrinsic to a common, though compositionally het-erogeneous source of the lavas. A similar observa-tion is also made within the volcanic rocks from theLesser Antilles, whose arc presents large chemicaland isotopic heterogeneities. These are explained bythe nature of the sources, which are generated by themixing of a mantle component and variable amountsof subducted sediments with elevated Pb and Srisotopic compositions (Labanieh et al., 2006). Interms of the origin of high-alumina basalts versushigh-magnesia basalts, our bulk-rock data suggestthat no systematic distinction between Group 1 andGroup 2 basalts can be made. We therefore proposethat this relationship is likely helpful in evaluatingsome aspects of the petrogenetic problems of high-magnesia and high-alumina basalts (Kersting andArculus, 1994; and references therein) when moredata are available.
Can Yeba felsic rocks be generated fromamphibole-rich lower crust by partial melting?
Felsic arc magmas may form either by fractionalcrystallization processes from basaltic magmas(Grove and Donnelly-Nolan, 1986; McCulloch et al.,1994; Gertisser and Keller, 2000; Haase et al.,
2006) or by melting of amphibolite in the lower crust(Beard, 1995; Smith et al., 2003). Experimentalwork indicates that both processes lead to similarmajor element compositions of broadly dacitic com-positions (Juster et al., 1989). For the generation ofYeba felsic rocks, several lines of evidence suggestthat they probably originate from amphibole-richlower crust by partial melting: (1) major element andtrace element concentrations versus Mg# plots ofYeba volcanic rocks (Fig. 5) do not show systematiccrystal fractionation trends from the mafic to felsicsuites, indicating that Yeba felsic rocks were notgenerated by fractional crystallization from basalticmagmas; (2) a well-defined linear trend on the Th/Sm versus Th diagram (Fig. 11B) suggests that thefelsic rocks cannot be simply explained by a processother than batch partial melting; (3) the large vol-umes of Yeba felsic magma could not be generatedby fractional crystallization from basaltic magmas.The following discussion places further constraintson the generation of Yeba felsic rocks.
The concave-upward MREE-depleted patterns(Fig. 7I) observed in Yeba felsic rocks are similar topatterns characteristic of calc-alkaline granitoids ofthe North Cascades (Tepper et al., 1993). There thedistinct patterns are interpreted to have been gener-ated from large degrees of partial melting (35–45%)of basaltic lower crust under variable water fugacity,leaving an amphibole-rich residuum (Tepper et al.,1993; Borg and Clynne, 1998). Amphibole-rich
FIG. 11. Plots of Sm/Nd versus Lu/Hf and Th/Sm versus Th to address the nature of magma source region and thepartial melting for Yeba volcanic rocks. Data sources: N-MORB, primitive mantle (Sun and McDonough, 1989); uppercrust (UC), middle crust (MC), and lower crust (LC) (Rudnick and Gao, 2003); subcontinental lithospheric mantle(SCLM) (McDonough, 1990), Luobusha MORB (Zhang et al., 2005). Note that Yeba basalts define a restricted fieldclose to lower crust (Fig. 11A) and that a strong linear correlation for Yeba andesites and felsic rocks is present (Fig.11B).
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FIG. 12. Selected REE ratio diagrams and REE modeling patterns for Yeba volcanic rocks showing melt curves (orlines) obtained by batch partial melting. A. Sm/Yb versus La/Sm diagram, showing the effects of partial melting ofgarnet-bearing mantle source (arrow M1) to generate Yeba basalts and from Yeba basalts to reproduce Yeba andesitesand felsic rocks (arrows M2 and M3). B. Felsic sample DZ01-2 can well be reproduced by 33% partial melting from thebasalt sample BD21 with initial mineralogy assemblages as Amp 70% + Plag 20% + Opx 10%. C. Andesite sampleDZ10-1 can be generated by higher degree (68%) of partial melting from the sample BD21 under similar conditions asin B.
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lower crust derived from underplating by basalticarc magmas was postulated to exist beneath thesouthernmost Cascade arc, and the partial meltingof this juvenile lower crust was proposed to accountfor the generation of felsic calc-alkaline magmas(Borg and Clynne, 1998). These explanations seemapplicable to the generation of Yeba felsic rocksaccording to the REE modeling. We use batch par-tial melting REE modeling under amphibolite-facies conditions to calculate REE abundances inthe felsic rocks. The partition coefficients for REEare from McKenzie and O’Nions (1991). SamplesBD01 and BD21 within the Yeba mafic rocks wereused as proxies of juvenile lower crust beneath theEarly Jurassic Southern Gangdese arc. Meltingmodels reproduce the compositions of Yeba felsicrocks (Fig. 12A) and demonstrate the plausibility ofgenerating felsic magmas by partial melting ofsamples BD01 and BD21, if we assume the initialmineralogy assemblages are Amp: Plag: Opx =70:20:10. REE modeling results evidently indicatethat variable, large degrees of partial melting (20–40%) of juvenile lower crust compositionally similarto Yeba basalts are required to reproduce the lowMREE and HREE abundances measured in themajority of Yeba felsic rock (arrows M2 and M3 inFig. 12A), further confirming that the generation ofthese rocks was actually controlled by amphibole-rich juvenile lower crust through partial melting.For example, sample DZ01-2, which has similar Sr-Nd isotopic compositions to Yeba mafic rocks (Fig.8A), can be well reproduced from basalt sampleBD21 by 33% partial melting (Fig. 12B).
The unusual correlation of Sr-Nd isotopic com-positions typical of Yeba volcanic rocks is quitedistinct from those of typical island-arc volcanicrocks from the Andean arc and other island arcs,which tend to parallel the negative correlation of themantle array and toward the upper crust, but it isvery similar to that of Quaternary lavas from theCentral American volcanic front (Feigenson andCarr, 1986). These authors have argued that theunusual positive correlation of Sr-Nd isotopic com-positions is related to a two-component mixing ofmantle wedge and lower crust, and that the lowercrust played a “density filter” role in the formationof the magmas. We have no samples of the lowercrust beneath the SG arc, but if we treat the pro-posed composition of Cretaceous lower crust (Wen etal., unpubl. data) as a proxy of lower crust beneaththe Early Jurassic SG arc, it is evident that Yebafelsic rocks define a trend from Yeba mafic rocks to
this proposed lower crust, indicating that a two-component mixing similar to the Quaternary lavasfrom Central America (Feigenson and Carr, 1986)was probably involved in the generation of Yebafelsic rocks. Generally, the contributions of ancientlower crust in the production of Yeba felsic rocksinvolving the observed Sr-Nd isotopic compositionscan be quantitatively estimated using the juvenilelower crust with features of Yeba mafic rocks (e.g.,sample BD01 or BD21) and the proposed ancientlower crust as two end-members. The results of two-component mixing indicate that the felsic rocks canbe explained by mixing with contributions of 0–10%from the ancient lower crust to reach their Sr-Ndisotopic compositions (arrow M2 in Fig. 8A). Theappreciable positive correlation between εNd(T)and Mg# observed from Yeba mafic to felsic rocks(Fig. 10B) is therefore plausibly interpreted asresulting from the presence of ancient lower crustalcomponents within the felsic rocks. We infer thatthis assimilation of ancient lower crust probablyoccurs in basaltic magma underplating in thecreation of juvenile lower crust, because the felsicrocks exhibit similar trace element spectra (Fig. 8Iand 8J).
It is difficult to constrain the origin of Yebaandesites because only limited data are available.However, some lines of evidence suggest a probableorigin for Yeba andesites that are generated from thejuvenile basaltic lower crust similar to Yeba maficrocks by higher degrees of partial melting. Theseinclude the following: (1) a linear trend of partialmelting links trace element concentrations andratios (Fig. 11B); (2) Sm/Yb and La/Sm ratios aretransitional between Yeba mafic and felsic rocks,compatible with a trend of high degrees of partialmelting of Yeba mafic rocks (Fig. 12A)—for exam-ple, sample DZ10-1 can be generated from sampleBD21 by 68% partial melting (Fig. 12C).
In summary, geochemistry and REE modelingindicate that Yeba felsic rocks were probably gener-ated from the amphibole-rich lower crust composi-tionally similar to the Yeba mafic rocks by moderatedegrees of partial melting (20–40%). The Sr-Ndisotopic variations of Yeba felsic rocks can beaccounted for by a two-component mixing of pre-dominantly juvenile lower crust (with features simi-lar to Yeba mafic rocks) and variable amounts ofancient lower crust. Although only limited data areavailable, a higher degree of partial melting of juve-nile lower crust similar to the Yeba mafic rocks isprobably the closest process involved in the genera-
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tion of the Yeba andesites. We propose that felsicmagmas were generated by partial melting of juve-nile basaltic lower crust, which is mainly composedof a series of underplated magmas that havegeochemical and isotopic compositions that are sim-ilar to Yeba mafic rocks exposed on the surface andvariable contributions from ancient lower crust. Itshould be emphasized that the REE modeling iseffective in demonstrating the importance of vari-able degrees of partial melting from juvenile basal-tic lower crust in controlling the compositionalvariations of Yeba felsic rocks, but not in quantita-tively constraining the degree of partial melting.
A model for the generation of Upper Jurassic Yeba magmatism
Considering the petrological and geochemicalstudies presented above, we propose a two-stepmodel of combined Neo-Tethyan oceanic subduc-tion and underplating of mafic magmas to explainthe origin of Upper Jurassic Yeba magmatisms in theSG (Fig. 13).
1. Partial melting of a metasomatized mantlewedge peridotite generated the Yeba mafic magmasand created juvenile lower crust beneath the SG.The downgoing Neo-Tethyan oceanic crust carriedsubducted sediments/fluids into the mantle wedge,
resulting in the initiation of partial melting of peri-dotite (Elliott et al., 1997). Consequently, variabledegrees of partial melting of metasomatized peridot-itic mantle wedge yielded magmas of a chemicallydiverse spectrum, traversing the relatively primitivecompositions (HMB) to high-alumina magmas withtholeiitic affinities as observed in Yeba mafic rocks(Groups 1, 2, and 3). It is probable that only a mod-est amount of such magmas erupted on the Earth’ssurface (Cull et al., 1991), as indicated by the rela-tively small volume of Yeba mafic compared to felsicrocks. Most remained in the lower crust (Bergantz,1989) and were variably assimilated by ancientlower crustal material beneath the GBAFUB; subse-quently, juvenile lower crust was created as illus-trated by the fact that Yeba mafic rocks have similarcompositions (Fig. 12A) to average lower crust(Rudnick and Gao, 2003).
2. Partial melting of juvenile lower crust yieldsEarly Jurassic felsic magmas. The continuouslyunderplated basaltic magma retained in the lowercrust was hotter, and its temperature (approximately1200°C) was 600–700°C above the ambient temper-ature in the lower crust (Huppert and Sparks, 1988).It therefore became the principal heat source forpartial melting of the juvenile lower crust. Experi-mental studies indicate that partial melting of mafic
FIG. 13. A two-stage model combining Neo-Tethyan oceanic subduction and underplating of mafic magmas pro-posed for the generation of the Early Jurassic Yeba magmatism in the SG. Step 1. Partial melting of modified mantlewedge peridotite creates Yeba mafic magmas and juvenile lower crust beneath the SG. Step 2: Partial melting of juvenilelower crust yields Yeba felsic magmas.
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rocks recently emplaced into the lower crust canproduce voluminous felsic magmas with isotopicratios similar to their mafic sources (Ratajeski et al.,2005). Therefore, it is probable that the voluminousEarly Jurassic felsic magmas in the SG were gener-ated from the juvenile lower crust by moderatedegrees of partial melting.
Conclusions
1. Yeba volcanic rocks span a continuous compo-sitional range from mafic to dacite, although andes-ites are minor, and mafic and felsic rocks arevolumetrically predominant. New SHRIMP zircondating for a dacite coupled with previous SHRIMPzircon dating for a mafic dike and fossil constraintsfor the sedimentary sequence indicate that Yeba vol-canic rocks in the SG were emplaced in the EarlyJurassic (>174 Ma). Yeba tholeiitic mafic rocks arecharacterized by compositional diversity and aredivided into three groups based on their bulk-rockconcentrations of MgO, Al2O3, and La; no isotopicdistinction is observed within individual groups orwith each other. Yeba calc-alkaline felsic rocks arecharacterized by subparallel, concave-upwardMREE patterns and negative anomalies in Nb, Ta, P,and Ti, as well as positive εNd(T) values (+ 0.3 to +2.6).
2. Yeba volcanic rocks present geochemical fea-tures suggesting a subduction-related origin, mostlikely in an arc built on thin, immature continentalcrust. These volcanic rocks are interpreted to havebeen created by the northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time.
3. The geochemical diversity in Yeba mafic rockswas probably produced by variable degrees of par-tial melting from a common but heterogeneous man-tle source, which had been metasomatized byvariable contributions of sediments/fluids releasedfrom the subducted Neo-Tethyan oceanic crust.Yeba felsic rocks were probably generated by mod-erate degrees of partial melting (20–40%) of juve-nile basalt ic lower crust , which consistedpredominantly of underplated magmas similar toYeba mafic rocks in composition, and variable con-tributions from ancient lower crust beneath theGangdese back-arc fault uplift belt (GBAFUB).
Acknowledgments
We thank Drs. Y. R. Shi and Z. Q. Li for help withSHRIMP dating, Dr. F. K. Chen, C. F. Li, and Ms.
H. M. Han for help with Sr-Nd-Pb isotopic analyses.Constructive discussions by Drs. J. F. Xu, H. Y.Lee,D. J. Wen, and M. F. Chu in improving an earlymanuscript are much appreciated. Gary Ernstreviewed an early version of the manuscript. Thisstudy benefitted from financial support by theProgramme of Excellent Young Scientists of theMinistry of Land and Resources, the NationalKey Project for Basic Research of China (Project2002CB412600), NSFC projects (40503005,40572051, 40473020), the State Key Laboratoryof Geological Processes and Mineral Resources,China University of Geosciences, Beijing,China (GPMR0539), and the Integrated Studyof Basic Geology of Qinghai–Tibetan Plateau(1212010610101).
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