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Ridge subduction and crustal growth in the Central Asian Orogenic Belt

Ridge subduction and crustal growth in the Central Asian Orogenic Belt:Evidence from Late Carboniferous adakites and high-Mg diorites in the western Junggar region,northern Xinjiang (west China)

Gongjian Tang a ,b ,Qiang Wang a ,c ,?,Derek A.Wyman d ,Zheng-Xiang Li c ,Zhen-Hua Zhao a ,Xiao-Hui Jia a ,b ,Zi-Qi Jiang a ,b

Key Laboratory of Isotope Geochronology and Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,PR China b

Graduate School of Chinese Academy of Sciences,Beijing 100049,PR China c

The Institute for Geoscience Research (TIGeR),Department of Applied Geology,Curtin University of Technology,GPO Box U1987,Perth,WA 6845,Australia d

School of Geosciences,Division of Geology and Geophysics,The University of Sydney,NSW 2006,Australia

a b s t r a c t

a r t i c l e i n f o Article history:

Received 15April 2010

Received in revised form 5August 2010Accepted 10August 2010Editor:R.L.Rudnick Keywords:

Ridge subduction Slab window Adakite

Crustal growth

Central Asian Orogenic Belt Xinjiang

The Central Asian Orogenic Belt (CAOB)is a natural laboratory for the study of accretionary tectonics and crustal growth owing to its massive generation of juvenile crust in the Paleozoic.There is a debate,however,on the mechanism of this growth.In the Baogutu area of the western Junggar region,northern Xinjiang (west China),diorite –granodiorite porphyry plutons and dikes are widely associated with Cu –Au mineralization.In this study,we present new results of zircon U –Pb geochronology,major and trace elements,and Sr –Nd –Pb –Hf isotope analyses for two diorite –granodiorite porphyry plutons and two dikes from this https://www.wendangku.net/doc/767785637.html,-ICP-MS zircon U –Pb analyses of four plutonic and dike samples yield Late Carboniferous ages of 315–310Ma.The Baogutu diorite –granodiorite porphyries exhibit low-Fe and calc-alkaline compositions.They are also characterized by high Sr (346–841ppm)contents,low Y (9.18–16.5ppm)and Yb (0.95–1.60ppm)contents,and relatively high Sr/Y (31–67)ratios,which are similar to those of typical adakites.In addition,some samples have relatively high MgO (2.35–8.32wt.%)and Mg #(48–75),and Cr (22.7–291ppm)and Ni (32.0–132ppm)values,which are similar to those of high-Mg andesites.All rock samples exhibit mid-oceanic ridge basalt (MORB)-like Nd –Sr –Pb –Hf isotope features:high εNd (t)(+5.8–+8.3)and εHf (t)(+13.1–+15.7)values,and relatively low (87Sr/86Sr)i (0.7033to 0.7054)and (206Pb/204Pb)i (17.842–18.055).The Baogutu adakitic rocks also contain reversely zoned clinopyroxene phenocrysts,which have low MgO cores and relatively high MgO rims.Geochemical modeling indicates that the Baogutu adakitic rocks could have been derived by mixing ~95%altered oceanic crust-derived melts with ~5%sediment-derived melts.Taking into account the regional geology,I-and A-type granitoids and Cu –Au mineralization,and the presence of Carboniferous ophiolite mélanges in northern Xinjiang,we suggest that the Baogutu adakitic rocks were most probably generated by partial melting of a slab edge close to a subducting spreading ridge in the Late Carboniferous.Ridge subduction and the resultant slab window probably caused strong extension in the overlying lithosphere,extensive melting of subducting oceanic crust,mantle and juvenile lower crust,and interaction between slab-derived melts and the mantle.Thus,events associated with ridge subduction are likely to have played an important role in crustal growth in the CAOB in addition to previously recognized accretion of subduction and arc complexes and post-collisional crustal melting.

1.Introduction

A common tectonic feature in accretionary orogens is ridge subduction accompanied by ridge –trench interaction (Windley et al.,2007).Ridge subduction has been documented at a number

of places along the modern Paci ?c Rim (Sisson et al.,2003;McCrory and Wilson,2009).However,only a few cases have been well-documented in the pre-Cenozoic geologic record,indicating that this process is grossly underrepresented in tectonic syntheses of plate margins in the ancient geologic record (Sisson et al.,2003).

Ridge subduction and ridge –trench interaction may impact strongly on magmatic activity,metamorphism and mineralization near convergent plate margins (Cole and Basu,1992;Haeussler et al.,1995;Sisson et al.,2003;Chadwick et al.,2009;McCrory and Wilson,

Chemical Geology 277(2010)281–300

?Corresponding author.Key Laboratory of Isotope Geochronology and Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,PR China.

E-mail address:wqiang@https://www.wendangku.net/doc/767785637.html, (Q.

10.1016/j.chemgeo.2010.08.012

Contents lists available at ScienceDirect

Chemical Geology

j o u r n a l h o me pa g e :w w w.e l s ev i e r.c o m/l o c a t e /c h e mg e o

2009).When a ridge intersects with the subduction zone,a“slab window”may form between the subducted parts of the diverging oceanic plates(Dickinson and Snyder,1979;Thorkelson,1996).The “Blowtorch effect”(Delong et al.,1979)resulting from the upwelling of asthenospheric mantle through the slab window can produce a wide variety of magmas.Typical magmatic rocks resulting from the subduction of slab windows include adakites,tholeiite,high-Mg andesites(e.g.,Hole et al.,1991;Abratis and Worner,2001;Rogers et al.,1985;Guivel et al.,1999;Breitsprecher et al.,2003).The upwelling of asthenospheric mantle through the slab window not only provides high heat?ow that can induce partial melting of the slab edge,overlying mantle wedge and/or upwelling asthenospheric mantle and crustal rocks(e.g.,Yogodzinski et al.,2001;Thorkelson and Breitsprecher,2005),but also causes an extensional stress?eld above the slab window,leading to the formation of alkaline magmatic rocks or A-type granites(Mortimer et al.,2006;Hung et al.,2007; Anma et al.,2009).

The Central Asian Orogenic Belt(CAOB)(Fig.1a)is one of the largest orogens in the world and comprises island arcs,seamounts, accretionary wedges,oceanic plateaus and,possibly,microconti-nents accreted during the closure of the Paleo-Asian Ocean(Seng?r et al.,1993;Jahn et al.,2000,2004;Xiao et al.,2008;Windley et al., 2007).There is evidence for possible ridge subduction events resulting from the Paleo-Asian Ocean closure.Windley et al. (2007)suggested that the CAOB contains many key features(e.g., adakites,boninites,near-trench magmatism,Alaskan-type ma?c–ultrama?c complexes and high-temperature metamorphic belts) that are explicable by ridge–trench interactions and that this new perspective may provide a promising approach for resolving many aspects of the orogenic belt's evolution.Based on data for Paleozoic ophiolites,tectonics and magmatism,several possible cases of ridge subduction have been proposed in the CAOB.Windley et al.(2007) focused mainly on the broad diagnostic features of ridge–trench interaction in the CAOB and suggested that gold deposits in eastern Tianshan may be related to slab window subduction.Based on geochronological data for ophiolites,Jian et al.(2008)suggested that a ca.430–415Ma ridge-subduction event was recorded in Inner Mongolia in the southeastern CAOB.Sun et al.(2009)suggested Early Paleozoic ridge subduction in the Altai area,CAOB.Geng et al. (2009)and Yin et al.(2010)proposed that a ridge-subduction model could account for the geochemical characteristics of granitoids and coeval ma?c rocks from the western Junggar region.Here,we further document such a case in the western Junggar region based on Cu–Au mineralization-related adakites as well as regional geology and the presence of I-and A-type granitoids.

The mechanism of Paleozoic crustal growth in the CAOB has been the subject of dispute(e.g.,Seng?r et al.,1993;Gao et al.,1998;Jahn et al.,2000,2004;Zhou et al.,2004;Liu and Fei,2006).In the western Junggar region of northwestern Xinjiang,there are widespread Late Paleozoic magmatic rocks,typically I or A-type granite batholiths with highly depleted isotopic signatures(εNd(t)of+6.4to+9.2) (Chen and Arakawa,2005;Han et al.,2006;Su et al.,2006).They have been attributed to either subduction-related sources in an island arc setting(Zhang et al.,2006;Xiao et al.,2008)or to depleted mantle contributions in a post-collisional extensional setting(Chen and Arakawa,2005;Han et al.,2006;Su et al.,2006).These models clearly have signi?cantly different implications for both granite petrogenesis and crustal growth.

In this study,we report on Late Carboniferous adakitic and high-Mg diorite–granodiorite porphyry plutons and dikes associated with Cu–Au mineralization in the Baogutu area of the western Junggar region(Fig.1b and c).On the basis of these results and previous regional studies,we suggest that ridge subduction played an important role in the generation of the large Late Carboniferous–Early Permian magmatic suite,contemporary Cu–Au mineralization, and crustal growth in the western Junggar region of the CAOB.

2.Geological background

The CAOB,also known as the Altaid Tectonic Collage(Seng?r et al., 1993;Jahn et al.,2000,2004;Windley et al.,2007),extends from the Urals in the west,through Kazakhstan,northern China,Mongolia,and southern Siberia to the Okhotsk Sea along the eastern Russian coast (Fig.1a)(Seng?r et al.,1993;Xiao et al.,2004).It is located between the Siberian Craton to the north and the North China and Tarim Cratons to the south(Fig.1a).The CAOB was mainly formed by the progressive subduction of the Paleo-Asian Ocean and the amalgam-ation of terranes of diverse origins(Coleman,1989;Xiao et al.,2004; Windley et al.,2007).The most outstanding feature of the CAOB is the vast expanse of granitoids and volcanic rocks,which are characterized by positiveεNd(t)and young T DM model ages(e.g.,Jahn et al.,2000; Wu et al.,2000).

The western Junggar region is surrounded by the Altai orogen to the north,the Tianshan orogen to the south,the Kazakhstan plate to the west,and the Junggar basin to the east(Fig.1a).No metamorphic basement has been documented in this area and the oldest rocks are ophiolitic ma?c–ultrama?c types of Cambrian–Ordovician age (Fig.1b).Post-Cambrian(Ordovician–Quaternary)sedimentary rocks,particularly Devonian and Carboniferous volcanic–sedimentary rocks,are abundant in the region(Fig.1b).The Devonian strata are mainly distributed in the northwest and the Carboniferous strata are mainly in the southeast;both contain extensive volcanic rocks (Fig.1b).The Carboniferous strata are principally composed of the Tailegula Formation,the Baogutu Formation,and Xibeikulasi Forma-tion,in an upward sequence(Fig.1c)(Shen and Jin,1993).Both Jin et al.(1987)and Song et al.(1996)reported Late Carboniferous deep-water sedimentary rocks close to the Baogutu area in the eastern part of the western Junggar region(Fig.1b),and suggested that a deep-marine basin persisted there until that time.Based on paleogeo-graphic data,Wang(2006)also suggested a Late Carboniferous deep-marine environment in that region,in contrast to a contemporary shallow-marine environment to the west and northwest(Fig.1d–f). During the Early Carboniferous,however,a deep-marine environment is believed to have existed across the entire region(Fig.1d–f)(Wang, 2006).These observations suggest that the western Junggar might have been a growing accretionary prism during the Carboniferous, with a northwest(in present coordinates)directed subduction system (Fig.1d–f).

Many ophiolitic ma?c–ultrama?c rocks occur in the western Junggar region,and their ages range from the Cambrian to Late

Fig.1.(a)Simpli?ed tectonic divisions of the CAOB(after Jahn et al.,2000).(b)Geological map of the western Junggar region(modi?ed after XBGMR,1993).(c)Simpli?ed geological map of the Baogutu deposits(after Cheng and Zhang,2006).WDH—Wudehe;E-KGSY—Eastern Kuogeshaye;W-KGSY—Western Kuogeshaye.Age data for ma?c–ultrama?c or ophiolitic rocks are from Beijing SHRIMP Unit(2005),Xu et al.(2006a)and Xiao et al.(2008).Age data for granite intrusions and volcanic rocks are from Han et al.(2006),Su et al.(2006),Wang and Zhu (2007),(Zhou et al.,2008),Geng et al.(2009)and this study.Age data for Baogutu diorite–granodiorite porphyry plutons and dikes are from this study.(d–f)Paleogeographic maps of the western Junggar region during the Carboniferous to Early Permian(344–290Ma)(from Wang,2006):344–323Ma,turbidite facies pyroclastic rocks,tuffaceous sandstones,limestones, radiolarian cherts and tuff layers were deposited in the Liushugou area.Fusulinid,brachiopod,coral and gastropods fossils occur in~400m thick limestones.Marine clastic rocks including a ~2500m thick limestone sequence,occur in the Tuoli area and contain brachiopods,coral,gastropod and plant fossils.323–303Ma,semi-abyssal facies pyroclastic rocks including~1500m of andesite,basalts and cherts,are abundant in the central area of the western Junggar region and radiolarian fossils are widespread.To the north,marine and continental facies sandstone, mudstone and shales with plant fossils occur and have a combined thickness of3000m.303–290Ma,semi-abyssal to abyssal environments occurred in the Loushugou area where clastic sedimentary rocks contain cherts with radiolaria and have a combined thickness of~750m.Contemporaneously,however,a continental environment was present in the northwest part of the western Junggar region.

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Carboniferous(Fig.1b)(Beijing SHRIMP Unit,2005;Xu et al.,2006a). Recently discovered ophiolites located to the northeast of Keramay city have a SHRIMP zircon U–Pb age of332±14Ma and are the youngest known in the area(Xu et al.,2006a;Xiao et al.,2008).

Carboniferous to Early Permian magmatic rocks occur widely in the western Junggar region and are referred to here as the Keramay arc magmatic rocks(Fig.1b)(see discussion in Section6).Carbon-iferous to Early Permian granitoid intrusions show systematic changes in both compositions and ages from north to south(Fig.1b).

In the north of the western Junggar region(Hebukesaier region) (Fig.1b),the intrusions mainly consist of I-type granitoids,which have an age range of338Ma to313Ma(Han et al.,2006;Zhou et al., 2008).They are characterized by the enrichment of light rare earth elements(LREE)and depletion of high?eld strength elements(HFSE) (e.g.,Nb,Ta and Ti)(Zhou et al.,2008).

In the central part of the magmatic province(i.e.,the Keramay–Hatu–Tuoli area),large batholiths and relatively small stocks intrude Paleozoic strata(Fig.1b).These are predominantly315–300Ma A-type intrusions and minor300–290Ma I-type granitoids(Fig.1) (Chen and Jahn,2004;Chen and Arakawa,2005;Han et al.,2006;Su et al.,2006).The I-type granitoids are characterized by enrichment of large ion lithophile elements(LILE)and LREE,depletion of HFSE and relatively high Mg#(100×Mg2+/(Fe2++Mg2+))(41–63)(Chen and Jahn,2004;Chen and Arakawa,2005),i.e.,geochemical characteristics similar to typical arc magmatic rocks.The more voluminous A-type granites are characterized by high SiO2(71–75wt.%),alkalis and Fe/ (Fe+Mg)values,low Al2O3and CaO contents,enrichment in LILEs (Rb,Th and K)and HFSEs(Nb,Ta,Zr and Hf),and obvious negative anomalies in Eu,Ba and Sr(Chen and Jahn,2004;Chen and Arakawa, 2005;Su et al.,2006;Geng et al.,2009),similar to typical A-type granites(Loiselle and Wones,1979).

In the southern part of the region(Tangbale–Mayle area),there are fewer scattered intrusions,which mainly consist of quartz monzonite, biotite granite and diorite,similar to I-type granitoids(Fig.1b).They are considered to have formed in the late Early Carboniferous (Xinjiang Bureau of Geology and Mineral Resources(XBGMR), 1993).Thus,granitoids in the central part of the area are slightly younger than those in the northern and southern areas.Almost all these granitoids exhibit highεNd(t)(+5.4to+9.2)but the central A-type granites have slightly higherεNd(t)(+6.4to+8.4)(Chen and Jahn,2004;Chen and Arakawa,2005;Su et al.,2006).

Volcanic rocks and dikes are also abundant in the central part of the magmatic province(Fig.1b).The volcanic rocks consist of andesitic basalt,andesite,felsic tuff and minor quartz or olivine tholeiite and occur as part of the Carboniferous Tailegula Formation(Shen and Jin, 1993).A U–Pb zircon SHRIMP age of328±2Ma has been reported for felsic tuffs in the Baobei gold deposit from the western Junggar region (Wang and Zhu,2007).Several hundred intermediate-basic dikes in this area mainly strike NW(280°–300°),and cut both granitoid intrusions and Carboniferous strata(Li et al.,2004;Yin et al.,2010).

The western Junggar region is one of the most important copper and gold producing areas in northwestern China(Shen and Jin,1993; Rui et al.,2002;Zhu et al.,2007).Our study area(Baogutu)is located in the centre of the western Junggar region,and is40km southeast of the Keramay city(Fig.1).Porphyry Cu–Au deposits and orogenic (vein)Au deposits in the area(Fig.1b–c)are hosted in about twenty small plutons and many NW and NE-trending dikes,including the Wudehe No.2(outcrop area of~3km2)and No.5(outcrop area of ~0.84km2)plutons in the north,and the Kuogeshaye dikes in the south(Fig.1c).The plutons intrude the Xibeikulasi and Baogutu formations,which consist mainly of tuffaceous breccias and siltstone (Fig.1c).The Wudehe No.5porphyry copper deposit,containing about1.115×106ton resource averaging0.28%Cu,N0.01%Mo,and 0.25ppm Au(Shen et al.,2009),is the second largest porphyry copper deposit in Xinjiang following the Tuwu–Yandong deposit in eastern Tianshan.The Wudehe Cu(Cu–Mo)mineralization is generally hosted in,or occurs around,small plutons(Fig.1b).The eastern Kuogeshaye orogenic gold deposit(Rui et al.,2002)is the largest gold deposit in the western Junggar area,with an estimated total reserve of4.2tons. The eastern and western Kuogeshaye gold deposits are located in the southern part of the Baogutu area(Fig.1b),with lensoid or ribbon-like gold-bearing veins generally hosted in wall rocks(tuff and tuffaceous siltstone)close to the dioritic porphyrite dikes(Fig.1b).

3.Petrography

The Wudehe plutons mainly consist of quartz diorite and granodi-orite or corresponding porphyries whereas the Kuogeshaye dikes are mainly dioritic porphyrite.The intrusive rocks in the Wudehe No.2 pluton exhibit?ne-medium and medium-coarse-grained inequigranu-lar or porphyry texture and mainly contain plagioclase(30–40vol.%), hornblende(15–20vol.%),biotite(5–15vol.%)and quartz(b5vol.%). Accessory minerals include pyroxene,magnetite,titanite,zircon and apatite.Plagioclase,up to3mm in size,is the most abundant phenocryst phase in all of the Wudehe No.2intrusive rocks.They are generally euhedral and lath-shaped,and some of them show polysynthetic twinning.Hornblende occurs as euhedral phenocrysts with300–600μm in size,but some crystals are altered.Biotite phenocrysts, 100–500μm in size,are transparent brown and subhedral.The intrusive rocks contain minor quartz,which has a round shape caused by resorption.Pyroxene is rare in the intrusive rocks.Fe–Ti oxides appear as microphenocrysts and microlites in the matrix,or as inclusions in other phases(e.g.,hornblende).

The porphyries in the Wudehe No.5pluton contain phenocrysts up to2mm.They contain plagioclase(30–45vol.%),hornblende (15–20vol.%),biotite(10–15vol.%),pyroxene(3–6vol.%),quartz (b5vol.%),and minor titanite,rutile,apatite,magnetite,and zircon. Plagioclase phenocrysts are generally euhedral with polysynthetic twinning.Hornblende commonly occurs as transparent euhedral phenocrysts.Brown biotite occurs as euhedral phenocrysts.Some biotite granules are altered to chlorites and epidote.Pyroxene phenocrysts are sometimes altered to hornblende along grain boundaries.

The Kuogeshaye dioritic porphyrites consist of plagioclase(15–30vol.%),hornblende(10–20vol.%),biotite(5–15vol.%),pyroxene(5–15vol.%),and accessory minerals similar to those in the Wudehe intrusive rocks.Pyroxene phenocrysts in the Kuogeshaye dioritic porphyries are abundant relative to those of the Wudehe intrusive rocks.They occur as transparent and euhedral phenocrysts that have diameters of50–300μm.

4.Analytical methods

Zircons were separated using conventional heavy liquid and magnetic separation techniques.Cathodoluminescence(CL)images were obtained for zircons prior to analysis,using a JEOL JXA-8100 Superprobe at the Guangzhou Institute of Geochemistry,Chinese Academy of Sciences(GIGCAS),in order to characterize internal structures and choose potential target sites for U–Pb https://www.wendangku.net/doc/767785637.html,-ICP-MS zircon U–Pb analyses were conducted on an Agilent7500ICP-MS equipped with a193-nm laser,housed at the National Key Laboratory of Geological Processes and Mineral Resources,Faculty of Earth Sciences,China University of Geosciences(Wuhan).Zircon91500was used as the standard(Wiedenbeck et al.,1995)and the standard silicate glass NIST610was used to optimize the machine,with a beam diameter of30μm.Raw count rates for29Si,204Pb,206Pb,207Pb,208Pb, 232Th and238U were collected and U,Th and Pb concentrations were calibrated using29Si as the internal calibrant and NIST610as the reference material.207Pb/206Pb and206Pb/238U ratios were calculated using the GLITTER program(Jackson et al.,2004).Measured207Pb/ 206Pb,206Pb/238U and208Pb/232Th ratios in zircon91500were averaged over the course of the analytical session and used to

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calculate correction factors.These correction factors were then applied to each sample to correct for both instrumental mass bias and depth-dependent elemental and isotopic https://www.wendangku.net/doc/767785637.html,mon Pb was corrected by ComPbCorr#3151(Andersen,2002)for those with common206Pb N1%.Further detailed descriptions of the instrumentation and analytical procedure for the LA-ICP-MS zircon U–Pb technique can be found in Gao et al.(2002)and Liu et al.(2008, 2010).Uncertainties in the ages listed in Appendix1are cited as1σ, and the weighted mean ages are quoted at the95%con?dence level. The age calculations and concordia plots were made using Isoplot(ver 3.0)(Ludwig,2003).LA-ICP-MS U–Pb zircon data are presented in Appendix1.

Major element analysis and back-scattered-electron imaging for clinopyroxene and plagioclase were carried out at Guangzhou Institute of Geochemistry,Chinese Academy of Sciences(GIGCAS) using a JXA-8100electron microprobe.An accelerating voltage of 15kV,a specimen current of3.0×10?8A,and a beam size of1–2μm were employed.The analytical errors are generally less than2%.The analytical procedures were described in detail in Huang et al.(2007). The results are listed in Appendix2.

Rock samples were examined by optical microscopy and unaltered or least-altered samples were selected for geochemical analysis. Major element oxides were determined using the standard X-ray ?uorescence(XRF)method(Li et al.,2006).Trace elements were analyzed by inductively coupled plasma mass spectrometry(ICP-MS), using a Perkin-Elmer Sciex ELAN6000instrument at GIGCAS. Analytical procedures are the same as those described by Li et al. (2006).Analytical precision for most elements is better than3%.The results are listed in Appendix3.

Sr and Nd isotopic analyses were performed on a Micromass Isoprobe multi-collector ICP-MS at the GIGCAS,using analytical procedures described by Li et al.(2006).Sr and REE were separated using cation columns,and Nd fractions were further separated by HDEHP-coated Kef columns.Measured87Sr/86Sr and143Nd/144Nd ratios were normalized to 86Sr/88Sr=0.1194and146Nd/144Nd=0.7219,respectively.The reported 87Sr/86Sr and143Nd/144Nd ratios were respectively adjusted to the NBS SRM987standard87Sr/86Sr=0.71025and the Shin Etsu JNdi-1standard 143Nd/144Nd=0.512115.

For Pb isotopic determination,about100mg powder was weighed into the Te?on beaker,spiked and dissolved in concentrated HF at 180°C for7h.Lead was separated and puri?ed by conventional cation-exchange technique(AG1×8,200–400resin)with diluted HBr as an eluant.Total procedural blanks were less than50pg Pb.Isotopic ratios were measured using a VG-354mass-spectrometer at the GIGCAS following procedures described by Zhu et al.(2001).Repeated analyses of SRM981yielded average values of206Pb/204Pb=16.9±4(2σ),207Pb/ 204Pb=15.498±4(2σ)and208Pb/204Pb=36.728±9(2σ).

Hf isotopic analyses were conducted using a multi-collector Thermo Electron Neptune MC-ICP-MS system in the Institute of Geology and Geophysics,Chinese Academy of Sciences(Beijing).The analytical methods are similar to those of Li et al.(2006).176Hf/177Hf measurements were normalized to179Hf/177Hf=0.7325.During the period of data acquisition,standard BCR-2was also processed for Hf isotopes,which gave a ratio of0.282881±8(2σm)for176Hf/177Hf,in agreement with the recommended value(Bizzarro et al.,2003).The analyzed results,with the calculated initial isotopic values and model ages,are listed in Appendix4.

5.Results

5.1.Zircon geochronology

To determine the emplacement ages of the ore-related porphyries,4 representative samples were chosen for LA-ICP-MS zircon U–Pb dating, one each from the Wudehe No.2and No.5plutons,and the eastern and western Kuogeshaye dikes.Zircons have a size range of30–150μm with a length/width ratio of1:1–3:1.Cathodoluminescence images of zircon grains used for LA-ICP-MS analysis show micro-scale oscillatory zoning (Fig.2).These zircon grains also exhibit high Th/U ratios(0.31–1.46), suggesting a magmatic origin(Belousova et al.,2002).Tera-Wasserburg diagrams and representative CL images of analyzed zircon are shown in Fig.2,and the U–Pb age data are given in Appendix1.

Twenty-one analyses of zircons from the dominant rock type, quartz dioritic porphyry,were obtained for sample06XJ145from the Wudehe No.2pluton(84°27′10″N,45°29′57″E)give a weighted mean 206Pb/238U age of315±4Ma(2σ)(Mean square weighted deviation (MSWD)=0.76)(Appendix1;Fig.2a).The remaining?ve analyses give206Pb/238U ages ranging from361Ma to375Ma,with a weighted mean206Pb/238U age of367±9Ma(2σ;MSWD=1.08)(Fig.2a), interpreted as the age of inherited zircons.

The twenty-two analyses of zircons from sample06XJ147,a quartz dioritic porphyry typical of the Wudehe No.5pluton(Fig.1c,84°32′28″N, 45°28′27″E),result in a single age population with a weighted mean 206Pb/238U age of311±4Ma(2σ;MSWD=0.38)(Fig.2b).This age is interpreted to be the best estimate of the crystallization age of the Wudehe No.5porphyritic intrusion.

Seventeen of the twenty zircon analyses from the dioritic porphyry dike sample(06XJ-153)in the east Kuogeshaye area(Fig.1c,84°26′25″N,45°23′54″E)give a weighted mean206Pb/238U age of314±4Ma (2σ;MSWD=1.4)(Fig.2c).This age is interpreted to be the best estimate of the crystallization age of the east Kuogeshaye dike. Analysis number4gives the oldest206Pb/238U age of~761Ma (Appendix1).Moreover,two other analyses give206Pb/238U ages of 344Ma and364Ma,respectively.The old(~761–344Ma)zircons were likely inherited or entrained from the wall rocks during dike emplacement.Minor761Ma zircons were most probably derived from shallow-level sedimentary wall rocks because no Archean–Proterozoic metamorphic rocks have been found in the western Jungger and southern Altay areas(Hu et al.,2000;Sun et al.,2008,2009).

All30analyses of zircons from the west Kuogeshaye dioritic porphyry(06XJ-156,84°26′14″N,45°23′24″E)share similar206Pb/ 238U ratios and give a weighted mean206Pb/238U age of310±3Ma(2σ; (MSWD=0.46))(Appendix1;Fig.2d).This age is regarded as the age of crystallization.

5.2.Mineral compositions

Clinopyroxenes from the Baogutu porphyries exhibit variable SiO2 (49.4–53.3wt.%),Al2O3(0.85–3.42wt%)and MgO(13.9–15.5wt.%) contents and Mg#(69–85).Most clinopyroxene crystals are augite and diopside(Fig.3).A prominent feature of the porphyries is the presence of simple reversely zoned clinopyroxene phenocrysts,which have low MgO and Mg#cores and relatively high MgO and Mg#rims (Fig.4;Appendix2).The cores also have high FeO and Na2O contents compared to the rims(Fig.4e and f).No multiply zoned clinopyroxene has been observed in the Baogutu samples.Plagioclase phenocrysts occur as euhedral weakly zoned crystals,with An(anorthite)contents of46–60%in the core and An contents of33–52%in the rim. Plagioclases in the matrix show a large variation in An contents(3–46%)(Appendix5).Large plagioclase crystals may occasionally partly enclose or contain some clinopyroxene grains(Appendix5),indicat-ing that the crystallization of clinopyroxene occurred prior to that of plagioclase.

5.3.Major and trace elements

All of the studied porphyries share similar major element contents.The samples all plot within the calc-alkaline and low-Fe ?elds on a SiO2versus FeO total/MgO diagram(Fig.5a),and conform to a medium-K calc-alkaline trend on a SiO2–K2O diagram(Fig.5b). Most Baogutu porphyry samples plot in the?eld of gabbroic diorites, diorites and granodiorites(Fig.5c)(Middlemost,1994).Porphyries

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0611

14317Ma

317M a 319Ma 320Ma

320Ma

316Ma

375Ma

310Ma

361Ma 376Ma 316Ma

314Ma

319Ma

314Ma

0912

1716

311Ma

307Ma 310Ma

309Ma

310Ma

308Ma

315Ma

309Ma

313Ma 310Ma

308Ma

316Ma 22

304Ma

310Ma

312Ma

308Ma

314Ma

2328

305Ma 298Ma

311Ma

308Ma 309Ma

304Ma 312Ma

306Ma

309Ma 307Ma

309Ma

305Ma

312Ma

306Ma

0908

319Ma 305Ma

307Ma

316Ma

317Ma

309Ma

761Ma

313Ma

317Ma 320Ma

342Ma

313Ma

314Ma

238

U/206Pb

238

U/206Pb

238

206

238

U/206Pb

207

P b /206P b

207

P b /206P b

207

P b /206P b

207

P b /206P b

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from the Wudehe plutons show a large range of SiO 2contents from 50to 70wt.%and MgO contents from 0.47wt.%to 8.70wt.%,although they are dominantly dioritic (or andesitic)(SiO 253.59–61.49wt.%;MgO 3.39wt%–7.79wt.%)(see Appendix 3;Fig.5c).The Kuogeshaye dikes show smaller ranges of SiO 2(57.08wt.%–67.88wt.%)and MgO (1.48wt%–4.14wt.%)contents (see Appen-dix 3;Fig.5c).The Wudehe plutons and the Kuogeshaye dikes have total alkali (Na 2O +K 2O)contents ranging from 4.17to 8.52wt.%and Na 2O/K 2O ratios from 1.80to 9.30(see Appendix 3),indicating their sodium-rich compositions.

The Baogutu samples exhibit geochemical characteristics of typical adakites (Kay,1978;Defant and Drummond,1990;Martin et al.,2005).They are characterized by fractionated rare earth element (REE)patterns with La/Yb ratios (3.0–17)higher than those of normal arc magmas (Fig.6a),and negligible Eu and strongly positive Sr anomalies (Fig.6b).They also show Nb,Ta and Ti depletions,however,which are similar to the Keramay I-type granitoids (Fig.6b).They have high Al 2O 3(14.98–18.32wt.%)and Sr (346–841ppm)contents (see Appendix 3).Except for sample 06XJ-143,all samples have low Y and Yb contents (9.18–16.5ppm and 0.95–1.60ppm,respectively),and high Sr/Y ratios ranging from 31to 67,and plot in the ?eld of typical “adakites ”(Fig.5d).

In addition,some gabbroic diorite and diorite samples with SiO 2contents less than 64wt.%are characterized by elevated MgO (2.35–8.32wt.%)and Mg #(48–75)values (Fig.5e –f),and higher Cr (22.7–291ppm)and Ni (32.0–132ppm)contents (Appendix 3).On SiO 2versus MgO and Mg #diagrams (Fig.5e and f),they partially overlap with high-Mg andesites from the Setouchi Volcanic Belt,Japan (Shimoda et al.,1998;Tatsumi et al.,2006;Tatsumi,2006,and reference therein).The Setouchi high-Mg andesites as a whole,however,are more magnesian and have signi ?cantly higher Mg #values than the Baogutu intrusive rocks.5.4.Nd –Sr –Pb –Hf isotope compositions

The Baogutu samples have isotopic features broadly similar to those of MORB.Both their initial 87Sr/86Sr isotopic ratios (0.7033to 0.7054)and εNd (t)values (+5.8–+8.3,average +6.7)are compara-ble to those of the East Paci ?c Rise (EPR)basalts (Fig.7a;Appendix 4).The Baogutu samples have slightly lower εNd (t)values than the Early Carboniferous (345Ma)volcanic rocks in the Junggar Basin and Late

Carboniferous –Early Permian (315–290Ma)granitoids in the western Junggar region (Fig.7a).The Baogutu samples have high εHf (t)values (+13.5to +15.7)and positive ΔεHf (t)values (+0.9to+4.3)[where ΔεHf (t)=εHf (t)?(1.59εNd (t)+1.28)]and plot close to or above the mantle array [εHf (t)=1.59εNd (t)+1.28]of Chauvel et al.(2008)in the εHf (t)versus εNd (t)diagram (Appendix 4;Fig.7b).In addition,they have variable Pb isotopic compositions (206Pb/204Pb i =17.842–18.055;207Pb/204Pb i =15.411–15.466;208Pb/204Pb i =37.316–37.313)(Appendix 4).6.Discussion 6.1.Petrogenesis

Adakites were originally considered to form by melting of subducted young and hot oceanic crust (Mechanism A;Defant and Drummond,1990).Later studies suggested that adakitic magmas could be produced by alternative mechanisms:(a)partial melting of thickened basaltic lower crust (Mechanism B;Atherton and Petford,1993;Rudnick,1995;Petford and Atherton,1996;Chung et al.,2003;Condie,2005;Wang et al.,2005,2007a );(b)partial melting of delaminated lower crust (Mechanism C;Kay and Kay,1993;Rudnick,1995;Xu et al.,2002b;Gao et al.,2004;Wang et al.,2006a;Huang et al.,2008);(c)partial melting of subducting continental crust (Mechanism D;Wang et al.,2008);(d)assimilation –low-pressure fractional crystallization from parental basaltic magmas (Mechanism E;Castillo et al.,1999;Li et al.,2009);(e)high-pressure crystallization (involving garnet)of typical subduction-related magmas derived from melting of the mantle wedge (Mechanism F;Macpherson et al.,2006);(f)magma mixing,and combined assimilation –fractional crystallization of felsic and basaltic magmas (Mechanism G;Streck et al.,2007).We consider these alternative processes in the following sections with speci ?c reference to the Baogutu adakitic porphyries.6.1.1.Mechanisms B –G

The geochemical characteristics of the Baogutu adakitic porphyries are inconsistent with partial melting of thickened or delaminated continental lower crust (Mechanisms B and G).Commonly,adakitic rocks derived by melting of thickened lower crust are characterized by relatively low MgO or Mg #values (Fig.5e and f),which are similar to those of experimental melts from metabasalts and eclogites (Sen and

MgSiO 3

Fig.3.CaSiO 3–MgSiO 3–FeSiO 3diagram showing the compositions of pyroxene (Morimoto et al.,1988)from the Baogutu adakitic rocks in the western Junggar region,NW China.

https://www.wendangku.net/doc/767785637.html,-ICP-MS U –Pb zircon Tera-Wasserburg diagrams with CL images for (a)quartz dioritic porphyry (06XJ-145),(b)quartz dioritic porphyry (06XJ-147),(c)dioritic porphyrite dike (06XJ-153)and (d)dioritic porphyrite dike (06XJ-156)from the Baogutu Cu –Au deposits,western Junggar,CAOB.Circles indicate locations of analyzed sites,with numbers in the circles representing spot numbers.The age for each spot is given.Scale bars all represent 100μm.

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Dunn,1994;Rapp and Watson,1995;Rapp et al.,1999).In contrast,the Baogutu adakitic rocks possess distinctly higher MgO contents and Mg #values than those of experimental melts from metabasalts and eclogites (Fig.5e and f)(e.g.,Rapp et al.,1999).

Recently,Wang et al.(2008)proposed that adakitic rocks in some collisional orogenic belts (e.g.,Tibet)could have been formed by partial melting of subducted continental crust (Mechanism D).Adakitic rocks formed from subducted continental crust also commonly have higher K 2O contents (N 3wt.%)than those of the Baogutu adakitic rocks (b 3wt.%)(Fig.5b).Moreover,adakitic rocks formed in this way have low εNd (t)values (generally below ?3)(Wang et al.,2008),whereas the Baogutu adakitic rocks have εNd (t)values of between +5.8and +8.3.In addition,in the case of the Baogutu adakitic rocks,sedimentary data suggest that an oceanic setting persisted into the Late Carboniferous in the western Junggar region (Jin et al.,1987;Song et al.,1996;Wang,2006),indicating that no continental collision took place at that time.

The Baogutu adakitic rocks are also dif ?cult to explain by low-pressure assimilation –fractional crystallization (AFC)from parental basaltic magmas (Mechanism E).If AFC processes could account for the petrogenesis of the Baogutu adakitic rocks,then the most probable candidate for a parental magma would be that of the 345Ma Kexia basalts with high εNd (t)(+6.8–+9.6)in the western Junggar Basin (Zheng et al.,2007).If olivine and pyroxene fractionated from the Kexia basalts,then the derived magma would show a clear decrease in Mg #values as well as MgO contents,but this does not occur (Fig.5e –f).In addition,fractionation of olivine and pyroxene is not consistent with the depletion of HREE (e.g.,Yb).These

0.20.3

0.4

0.5

0.6

N a 2O (w t %)

810

F e O (w t %)

μm

13.5

14.014.5

15.0

15.5M g O (w t %)

MgO (wt%)MgO (wt%)

M g O (w t %)

μm

A’

Fig.4.(a –b)Two back-scattered electron images of typical cores and rims in reversely zoned clinopyroxene phenocrysts from the Baogutu adakitic rocks.(c –d)Composition variations (MgO)in clinopyroxene phenocrysts along the A –A ′and B –B ′sections from a and b,respectively.(e)FeO versus MgO diagram for clinopyroxene phenocrysts.(f)Na 2O versus MgO diagram for clinopyroxene phenocrysts.

288G.Tang et al./Chemical Geology 277(2010)281–300

minerals are unable to incorporate HREE elements,which generally leads to concave-upward HREE in the chondrite-normalized REE concentration patterns (Castillo et al.,1999).Low-pressure horn-blende fractionation may drive magmas toward adakitic Sr/Y ratios (Castillo et al.,1999)but even the most Mg-rich samples in the Baogutu suite display this feature (e.g.,MgO =8.7–7.4;SiO 2=51.7–54.0;Sr/Y =23–39).The Sr/Y and La/Yb ratios can also not be associated with a crystal fractionation assemblage involving plagio-clase,given that Eu anomalies are absent and Sr/Y remains high across the entire range of SiO 2contents (Figs.5d and 8b ,d).Where differentiation of high-Mg adakitic rocks has been observed else-where,Sr/Y and La/Yb have been reported to increase with higher MgO content (Danyushevsky et al.,2008),but these trends are not present for the Baogutu rocks (Fig.8b).Moreover,most of the Baogutu samples display lower εNd (t)than the Miaoergou and Keramy granite and Kexi basalt samples,which is inconsistent with a crust

F e O T /M g O

SiO 2(wt.%)

SiO 2(wt.%)SiO 2(wt.%)

N a 2O +K 2O (w t .%)

50556065707580

K 2O (w t .%)

M g O (w t .%)

S r /Y

Y(ppm)

M g #

Fig.5.(a)SiO 2versus FeO T /MgO diagrams (Miyashiro,1974).Dashed boundaries for the tholeiitic and calc-alkaline ?elds are after Miyashiro (1974).Discriminate boundaries (grey lines)between low-,medium-,and high-Fe suites are after Arculus (2003).(b)SiO 2–K 2O plot (Peccerillo and Taylor,1976).(c)SiO 2–K 2O +Na 2O plot (Middlemost,1994).(d)Y versus Sr/Y diagram (after Defant and Drummond,1993).Crystal fractionation paths of the primary minerals are from Castillo et al.(1999).(e)SiO 2versus MgO diagram.(f)SiO 2versus Mg #diagram.Mantle AFC curves,with proportions of assimilated peridotite indicated,are after Stern and Kilian (1996)(Curve 1)and Rapp et al.(1999)(Curve 2),peridotite melts and crust AFC curves from Stern and Kilian (1996).Data for metabasaltic and eclogite experimental melts (1–4.0GPa),and peridotite-hybridized equivalents,are from Rapp et al.(1999)and references therein.Data for high-Mg andesites of SW Japan are from the following references:Shimoda et al.(1998),Tatsumi et al.(2006),Tatsumi et al.(2006),and references therein.Data for the Keramay I-type granitoids and Miaoergou series A-type granites are from Chen and Jahn (2004),Chen and Arakawa (2005),Gao et al.(2006)and Su et al.(2006).Data for the Kexi basalts are from Zheng et al.(2007).Data for the Baogutu samples are from Appendix 3.The ?elds of subducted oceanic crust-,delaminated lower crust-,and thickened lower crust-derived adakites are after Wang et al.(2006a).

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assimilation model (Fig.8e).The relatively high MgO rims of reversely zoned clinopyroxene phenocrysts (Fig.4)also argue against crustal contamination.

The Baogutu adakitic rocks are also unlikely to be the result of high-pressure crystallization (Mechanism F).The high Sr/Y and Dy/Yb ratios can only be achieved by extensive fractionation of amphibole (30–85%)and garnet (15–20%)from the parental basalt (Fig.8a).However,this con ?icts with experimental and natural evidence (Müntener et al.,2001),which suggests that amphibole and garnet only occur at higher degrees of crystallization,and high-pressure fractionation of hydrous basalts would essentially form pyroxene-rich lithologies.In addition,an important feature of high-pressure crystallization is that some key ratios,such as Sr/Y and Dy/Yb,would increase with increasing SiO 2contents (Macpherson et al.,2006),because garnet is involved in crystallization.However,the data for the Baogutu adakitic rock do not show such trends (Fig.8b –c).

It is also unlikely that magma mixing of felsic and basaltic magmas (Mechanism G)can account for the Baogutu adakitic rocks.Candi-dates for mantle-derived basaltic and crust-derived felsic end-members in the area are most plausibly represented by the~345Ma Kexia basalts (Zheng et al.,2007)and the ~290Ma Miaoergou granites,which have high SiO 2(71.39–78.81wt.%)and low MgO contents (0.01–0.85ppm)(Han et al.,2006;Su et al.,2006).The Baogutu adakitic rocks have an average εNd (t)value of +6.7,which is slightly lower than those of the Kexia basalts (average value=+7.9)and the Miaoergou granites (average value=+7.2)(Fig.8e),sug-gesting that the adakitic rocks are not products of mixing between the felsic and basaltic end-members.In addition,adakitic rocks formed by magma mixing generally have relatively uniform compositions with SiO 2contents that span a narrow range (Streck et al.,2007).In

contrast,the Baogutu adakitic rocks display a wide range of SiO 2contents from 52.46to 70.65wt.%(Appendix 3).Magma mixing between felsic and basaltic magmas is also inconsistent with the petrographic evidence.Such a process should commonly be accom-panied by multiply zoned phenocrysts (Troll and Schmincke,2002),however,the clinopyroxenes from the Baogutu samples only display simple reverse zoning (Fig.4).The normally zoned plagioclase (Appendix 5)also contrasts with the complex zoning caused by magma mixing (Troll and Schmincke,2002).Although ma ?c micro-granular enclaves (MMEs)were found in the Keramay plutons (Chen and Arakawa,2005),no MMEs were discovered in the Baogutu adakitic rocks,implying that magma mixing between mantle-and crust-derived magmas is unlikely to have played an important role in the genesis of the Baogutu samples.

6.1.2.Partial melting of subducted oceanic crust (Mechanism A)

We suggest that the Baogutu adakitic rocks were generated by partial melting of subducted oceanic crust based on both geological and geochemical evidence,as follows:

(1)The Baogutu adakitic rocks are geochemically similar to slab-derived adakites.They are medium-K calc-alkaline and have relatively low K 2O contents (Fig.5b),similar to slab-derived adakites in Paci ?c Cenozoic arcs (Stern and Kilian,1996)and the North Tianshan ranges (Wang et al.,2006b,2007b )formed by partial melting of young and hot subducted oceanic crust.They also have high MgO,Mg #,Cr and Ni values,similar to metabasaltic and eclogite melts hybridized by assimilation of 10–20%peridotite (Fig.5e –f).Moreover,on the discrimination diagrams for low-SiO 2-adakites (LSA)and high-SiO 2-adakites (HSA)of Martin et al.(2005)(Fig.9),the Baogutu adakitic rock samples mainly plot in the ?eld of the HSA derived from the interaction between slab-derived melts and mantle peridotites.Therefore,the Baogutu adakitic rocks likely resulted from interactions between mantle peridotite and slab melts during their ascent (e.g.,Rapp et al.,1999;Martin et al.,2005).

(2)There is growing evidence for the existence and subduction of a

Carboniferous ocean in the Junggar area.Huang et al.(1990)proposed that a “Carboniferous Asian Ocean ”or “North Tianshan Ocean ”existed in the south Junggar region in the Carboniferous.Xiao et al.(1992)also suggested that a Carboniferous ocean existed here,which they named the “North Tianshan Ocean ”.Recently,several Carboniferous ophiolites have been con ?rmed in the area,such as the Keramay ophiolites in which gabbro samples yield SHRIMP zircon ages of 332±14Ma (Xu et al.,2006a ).The Darbut ophiolite formed at 346–347Ma as dated by the SHRIMP zircon U –Pb method (Beijing SHRIMP Unit 2005annual report).The Bayingou ophiolites,from which a gabbro sample yielded a zircon U –Pb age of 344.0±3.4Ma (Xu et al.,2006b ),developed in response to the opening of the Carboniferous Ocean (Huang et al.,1990).Wang (2006)also argued that there was a Carboniferous shallow to deep ocean setting in the western Junggar region,based on a sedimentary and paleogeographic analysis.In addition,recently indenti ?ed Carboniferous ada-kite,high-Mg andesite and Nb-enriched basalt suites in northern Tianshan (Wang et al.,2006b;2007b )suggest subduction of Carboniferous oceanic crust along the southern margin of the Junggar Basin.

(3)The occurrence of the Early Carboniferous Darbut,Keramay and

Bayingou ophiolites (Fig.1b)suggests the presence of young oceanic crust,the subduction of which could produce the 315–310Ma Baogutu adakitic rocks.The Nd –Sr isotopic composi-tions of the Baogutu adakitic rocks are similar to those of the basaltic rocks from the Bayingou ophiolites (Xu et al.,2006c ),and also overlap those of the slab-derived adakites resulting

R o c k s /C h o n d r i t e s

Rb Th Nb La Sr Hf Sm Ti Dy Er Lu

R o c k s /p r i m i t i v e M a n t l e

Fig.6.Chondrite-normalized REE patterns (a)and primitive mantle normalized trace element diagrams (b)for Baogutu samples from the western Junggar compared with the Keramay I-type granitoids and Miaoergou series A-type granites (data sources as for Fig.5).Chondrite and primitive mantle normalized values are from Sun and McDonough (1989).

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from subduction associated with the “Carboniferous North Tianshan ocean ”in the North Tianshan ranges (Wang et al.,2006b,2007b ).In addition,the Baogutu adakitic rocks display a limited εNd (t)–εHf (t)?eld overlapping with those of typical MORB (Fig.7b).

(4)Reverse zoning in clinopyroxene phenocrysts in the Baogutu

adakitic rocks (Fig.4)re ?ects a signi ?cant MgO or Mg #increase and Na 2O content decrease in the melt,which is consistent with reaction between the melt and mantle peridotite (e.g.,Yogodzinski and Kelemen,1998;Gao et al.,2004,2008;Xiong et al.,2006).Therefore,the Baogutu adakitic rocks are most probably derived by the interaction between ascending sub-ducted oceanic crust-derived adakitic melts and mantle wedge peridotites,based on the petrographic evidence.

(5)It is well-documented that subduction zone magmatism is

controlled by contributions from the subducted ma ?c oceanic crust,the overlying subducted sediments,and the mantle wedge (e.g.,Kay,1978;Defant and Drummond,1990;Plank and Langmuir,1993;Hawkesworth et al.,1997;Chauvel et al.,2008).Adakites are generally produced by partial melting of subducted ma ?c ocean crust,followed by the interaction of this melt with the mantle (Kay,1978;Defant and Drummond,

1990).If subducted basaltic ocean crust begins to melt,however,then subducted sediments must also undergo partial melting (Kelemen et al.,2003).

In Fig.10a,Th/Ce ratios are plotted against initial 143Nd/144Nd ratios for Baogutu adakitic rocks.This diagram is sensitive to the addition of a sediment-derived melt because the subducted sediments have low 143Nd/144Nd and high Th/Ce ratios relative to mantle wedge (Hawkesworth et al.,1997).The Baogutu adakitic rocks show a broad negative trend,which is consistent with the addition of a sediment melt.The Baogutu adakitic rocks have high Th/La ratios,although they do not show particularly high Th contents (Fig.10b).They are similar to the high-Mg andesites from the Setouchi Volcanic Belt,Japan (Fig.10b),which were generated by the partial melting of subducting sediments,and subsequent melt –mantle interaction (Shimoda et al.,1998;Tatsumi,2001;Hanyu et al.,2006).Plots of Th/La versus Sm/La for arc volcanic rocks show mixing toward local sediments (Plank,2005)and it is evident in Fig.10c that the Baogutu adakitic rock compositions can be produced by a mixture of basalts from the Bayingou ophiolites and subducted sediments.

To model partial melting of subducted oceanic crust and its overlying sediment,we used a Kexia basalt sample (Zheng et al.,

(207P b /204P b )i

(208P b /204P b )i

(

206

Pb/

204

Pb)i

(

206

Pb/

204

Pb)i

-15

-10-5051015

-20-15

-10-5051015

2025(b)

0.700

0.7050.7100.7150.720

-15

-10

-5

εN d (t )

εNd (t)

εH f (t )

(87Sr/86Sr)i (a)

(d)

(c)

Fig.7.(a)εNd (t)versus (87Sr/86Sr)i ,(b)εHf (t)versus εNd (t),206Pb/204Pb i versus 207Pb/204Pb i (c)and 208Pb/204Pb i (d)plots.Data for marine sediments and global subducting sediment (GLOSS)are from Plank and Langmuir (1998)and Chauvel et al.(2008).Data for Tethyan Ocean MORB are from Xu et al.(2002a),Xu and Castillo (2004)and Zhang et al.(2005).Subducted oceanic crust-derived adakites,thickened and delaminated ma ?c lower crust-derived adakitic rocks are after Wang et al.(2006a,2007a),Huang et al.(2008)and references therein.Subducted continental crust-derived adakites are after Wang et al.(2008).Basalts of the Bayingou ophiolite mélanges are after Xu et al.(2006c).Data for the Junggar basin volcanic rocks are from Zheng et al.(2007).The high-Mg andesites of SW Japan are from Hanyu et al.(2006)and reference therein.EPR:East Paci ?c Rise basalts,using the Petdb database (Petrological database of the ocean ?oor,https://www.wendangku.net/doc/767785637.html, ).The mantle array and ?elds for MORB and OIB are from Chauvel et al.(2008).Data for the Baogutu samples are from Appendix 4.Other data sources and symbols are the same as for Fig.5.

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2007)and a Bayingou ophiolite gabbro sample (Xu et al.,2006c )as the subducted Carboniferous Junggar altered ocean basaltic crust (AOC),and the average Global Subducting Sediment (GLOSS:Plank and Langmuir,1998)composition as the overlying sediments.Based on these starting materials,the Baogutu samples can be modeled as 99:1to 90:10AOC melt:sediment melt mixture on Sr,Nd and Pb isotopic variation diagrams (Fig.11a –c).An average composition of the Baogutu rocks can be derived by mixing at a 95:5ratio of AOC and sediment melts (large red circles in Fig.11a –c).Its primitive mantle normalized trace element pattern can also be reproduced by this simple mixing scenario,except for the ?uid-mobile elements (Rb and Ba)(Fig.11d).This suggests that these highly soluble elements were

lost in a ?uid phase prior to melting.Thus,the Baogutu adakitic rocks were most probably produced by partial melting of subducted oceanic crust and a thin veneer of overlying sediments (Fig.11),and subsequent melt –mantle interaction (Fig.5e –f).6.2.Geodynamic processes

Two competing viewpoints for the tectonic evolution of the western Junggar area stress either island arc or post-collisional processes (Chen and Arakawa,2005;Han et al.,2006;Su et al.,2006;Zhang et al.,2006;Xiao et al.,2008).The most appropriate tectonic model for the western Junggar area must account for the diverse

4550

65707580

SiO 2(wt.%)

Dy/Yb

SiO 2(wt.%)

SiO 2(wt.%)SiO 2(wt.%)

Nb/La

(87Sr/8645

εNd (t)

La/Yb

Yb(ppm)

Sr/Y

50556065707580

Fig.8.(a)Nb/La versus Sr/Y;(b)SiO 2versus Sr/Y;(c)SiO 2versus Dy/Yb;(d)Yb versus La/Yb,crystal fractionation paths of the primary minerals are from Castillo et al.(1999).(e)SiO 2versus εNd (t);and (f)SiO 2versus (87Sr/86Sr)i plots.HPFC,high-pressure fractional crystallization involving garnet (Macpherson et al.,2006);LPFC:crystal fractionation of an island arc tholeiite series basalt (Danyushevsky et al.,2008).Fractional crystallization models with different proportions of fractionating amphibole and garnet are shown in (b),using sample 06XJ143as initial magma (SiO 2=50.24wt.%;MgO =8.46wt.%).Partition coef ?cients are after Mori et al.(2007)and references therein:D Nb =0.4945,D La =0.0513,D Sr =0.1095,D Y =1.0291.Data sources and symbols are the same as in Fig.5.

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compositional characteristics of the voluminous Carboniferous to Early Permian magmatic rocks,particularly granitoids.

6.2.1.Three types of granitoids in the central western Junggar area

The central western Junggar area hosts many Late Carboniferous to Early Permian granitoids (ca.315to 290Ma)(Fig.12a).Distinct from the dominantly I-type granitoids of the northern and southern areas (Fig.1b),the central area contains a diverse range of intrusive rocks,which can be classi ?ed into three groups based on their geochemical and geochronological features (Figs.5–7,12).From the southeast to the northwest of the central area,these are the Baogutu adakitic rock group (Group 1),the Keramay I-type granitoid group (Group 2),and the Miaoergou A-type granite group (Group 3),respectively (Fig.1b).Group 3granitoids include the Miaoergou,Akbastao,Hongshan and Hatu plutons (Fig.1b).The three groups exhibit systematic variations in terms of ages and geochemical characteristics (Fig.12).Their ages decrease slightly from Groups 1to 3,although Groups 1and 2overlap signi ?cantly (Fig.12a).Mg #and Sr/Y values clearly decrease from Groups 1to 3(Fig.12c –d),whereas zircon saturation temperatures increase from southeast (Group 1)to northwest (Group 3;Fig.12e).If the transition from high to low Sr/Y ratios relates to the presence of garnet and plagioclase in their respective sources,and the variation in Mg #s re ?ects the proportion of mantle component involved in their petrogenesis (e.g.,Rapp et al.,1999),then the differing compositions suggest that granitoid source depths become shallower from Group 1in the southeast to Group 3in the northwest.

6.2.2.Ridge subduction and slab window model

During the Early Carboniferous,the Paleo-Asia Ocean (the Carboniferous Junggar Ocean)may have been subducting beneath the Keramay arc (Fig.13a).During this process,upwelling slab-derived ?uids would have triggered partial melting of the mantle wedge (Fig.13a).These 338–316Ma volcanic rocks and I-type granitoids with “island-arc type ”geochemical compositions likely originated from partial melting of ?uid-metasomatized mantle wedge (Fig.13a).Based on several lines of evidence,as will be discussed later,we suggest that subduction of a spreading centre during the Late Carboniferous resulted in a slab window,which led to the formation of 315–290Ma magmatism in the central area of the Keramay arc.

Ridge subduction can readily provide enough heat for partial melting of a young oceanic crust (≤25Ma)or old oceanic crust (N 25Ma),generating adakitic rocks (Defant and Drummond,1990;Aguillon-Robles et al.,2001;Kelemen et al.,2003).Such melting occurs because the resultant slab window permits the upwelling of hot asthenospheric mantle (e.g.,Kay et al.,1993;Abratis and Worner,2001;Breitsprecher et al.,2003;Kelemen et al.,2003;Windley et al.,2007).In this scenario,the Baogutu low Mg (Mg #b 48)adakitic rocks resulted from partial melting of a slab edge and high-Mg (Mg #N 48)gabbroic diorites and diorites originated from subsequent interaction between slab melts and the mantle (Fig.13b).

The Baogutu samples generally have slightly lower Sr (346–841ppm)contents,Sr/Y (31–67ppm)values and higher HREE and Y contents relative to typical adakites derived from melting of subducted oceanic crust in non-ridge-subduction settings (Kay,1978;Defant and Drummond,1990;Kay et al.,1993;Stern and Kilian,1996)(Fig.5d).This is similar to other adakites (e.g.,Vizcaino Peninsula,Mexico)formed by ridge subduction (Aguillon-Robles et al.,2001).In addition,the Baogutu samples also show relatively low Nb/Ta ratios (10.7to 18,with an average of 13.4).This feature,coupled with higher Zr/Sm ratios (16.9to 75.7and average 40.2;

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https://www.wendangku.net/doc/767785637.html,parison between the geochemical characteristics of the Baogutu adakitic rocks and the key geochemical parameters used by Martin et al.(2005)to highlight the differences between high-SiO 2(HSA)and low-SiO 2(LSA)adakites.(a)K ppm versus Rb ppm;(b)Sr versus CaO+Na 2O;(c)Cr/Ni versus TiO 2;(d)Sr/Y versus Y.

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Appendix 3),is similar to Archean TTGs (Condie,2005).These trace element patterns indicate that slab (AOC+sediments)melting took place at relatively shallow depths,but mainly with a garnet amphibolite residuum (Fig.11d).These depth and source require-ments further suggest that special circumstances,such as mantle upwelling by ridge subduction,provided the anomalous heat source for slab melting (e.g.,Aguillon-Robles et al.,2001).Interestingly,the zircon saturation temperatures (Watson and Harrison,1983)(642–778°C)for the Baogutu adakites are distinctly lower than those (729–976°C)of the Miaoergou A-type granites (Fig.12e),but similar to those (671–726°C)of the southern Costa Rica adakites formed by subduction of the Cocos Ridge (Abratis and Worner,2001).The temperatures may re ?ect adakite formation by hydrous partial

melting of slab edges saturated with water (Martin,1999)and generation of A-type granites under H 2O-absent conditions.

The Keramay I-type granitoids are characterized by relatively high MgO and Sr contents (Fig.5d –e),high Mg #,MORB-like Nd –Sr isotopic compositions and young Nd model ages (T 2DM )of 300–550Ma (Figs.5f,7a and 12b ),indicating that they were derived from a juvenile source (Chen and Arakawa,2005).Experimental melts of basaltic rocks generally exhibit Mg #lower than 40,whereas the Mg #of the Keramay series are signi ?cantly higher than 40(Figs.5f and 11c ).Accordingly,the melts were not directly derived by partial melting of basaltic (or equivalent)rocks.The Keramay series are also unlikely to have been generated by direct melting of a mantle source,given that their silica contents are as high as 70wt.%(Fig.5).Chen and Arakawa (2005)proposed that the parent magma to the Keramay rocks originated from a depleted lithospheric mantle source that had been metasomatized by ?uids released from a subducting slab.We suggest that the astheno-sphere ascending from the slab window as a result of ridge subduction provided the heat for such partial melting (Fig.13b).The mantle-derived magma may then have evolved into the Keramay I-type granitoids by fractional crystallization (Chen and Arakawa,2005).Thus,the “island-arc type ”characteristics of the Keramay I-type granitoids were probably inherited from their mantle sources (Fig.13a –b).

Geochemical and isotopic characteristics of the Miaoergou series A-type granites suggest that they formed above a slab window (Fig.13b).The granites have low initial Sr isotopic ratios and MgO contents and Mg #values,positive εNd (t)values and young Nd model ages (T 2DM )(Figs.5e –f,7a and 12b –c ).Their very high zircon saturation temperatures (729–976°C)are characteristic of A-type granites (Skjerlie and Johnston,1993).Accordingly,it has been suggested that they were generated by partial melting of juvenile (oceanic?)lower crust (Chen and Arakawa,2005;Geng et al.,2009).We propose that such partial melting occurred because of the upwelling asthenosphere through a slab window (Fig.13b).In addition,the compositional variations of the Miaoergou series A-type granites suggest minor fractional crystallization after the A-type granite magmas were generated.The A-type granites have pronounced negative Eu,Sr and Ba anomalies (Fig.6),indicating the possible fractionation of plagioclase or K-feldspar in the evolution of the A-type granite magmas (Wu et al.,2002).The A-type granites have relatively high Yb (1.96–6.56ppm)and Y (16.3–80.4ppm)contents,precluding heavy rare earth element-rich garnet in the residue and indicating partial melting under low-pressure (b 15kbar)conditions (Pati?o Douce,2005).In summary,we suggest that they were most likely derived from partial melting of juvenile lower crust above a slab window at low pressure (b 15kbar),followed by minor fractional crystallization of plagioclase or K-feldspar.

There is additional evidence for ridge subduction in the region.First,charnockites occur as giant enclaves in the Miaoergou batholith,suggesting a hotter than normal geotherm at that time (Fig.13b).Zhang et al.(2004)proposed that the charnockites were produced by partial melting of juvenile lower crust,heated by mantle upwelling.A slab window would facilitate this upwelling (Fig.13b).Second,numerous NW-extending basic-intermediate dikes intrude the gran-ite batholiths and the Carboniferous strata.The dikes are only slightly younger than the rocks that they intrude (Fig.1b).They display typical “subduction zone ”characteristics and may have formed by partial melting of metasomatized lithosphere mantle in an exten-sional environment.Intrusion of ma ?c dikes into the accretionary prism is a typical feature of ridge subduction and slab window scenarios (Sisson et al.,2003;Windley et al.,2007),and the trend of these dikes may be parallel to that of the subducted oceanic ridge (Fig.13b).Third,other recent studies also document Late Carbonif-erous adakitic diorites (Geng et al.,2009)and high-Mg diorite (or sanukitoid)dikes (Yin et al.,2010)in the west Junggar Basin outside of our study area and also suggest the possible subduction of oceanic ridge (Geng et al.,2009;Yin et al.,2010).Fourth,there are

Th/Ce

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Fig.10.(a)Th/Ce versus 143Nd/144Nd ratios diagram,illustrating a negative correlation for the Baogutu samples.The data set was ?rst ?ltered to exclude all samples with SiO 2N 64wt.%,in order to eliminate late possible AFC processes (Turner et al.,2003).(b)Plot of Th versus Th/La.(c)Sm/La versus Th/La plot after the concept of Plank (2005).Data sources of the Baogutu samples,Bayingou ophiolite mélanges,high-Mg andesites of SW Japan,marine sediments and GLOSS are the same as for Figs.5and 7.

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0.700

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Fig.11.Simple mixing modeling results of subducted oceanic crust and overlying sediments for the studied rock suites for (a)Sr –Nd,(b)Sr –Pb and (c)Nd –Pb isotopic spaces,and model primitive mantle normalized trace element pattern compared with an average Baogutu adakitic rock (d).The trace element pattern of an average Baogutu adakitic rock

is closely matched by a mixture of 95%AOC melt and 5%sediment melt (red circles),except for the ?uid-mobile elements.AOC b =Bayingou ophiolite mélanges in the North Tianshan (Xu et al.,2006c );AOC k =Kexi basalt in the western Junggar Basin from Zheng et al.(2007).Bulk solid/melt partition coef ?cients of andesitic –dacitic melts in equilibrium with a garnet amphibolite residuum (45%Cpx (clinopyroxene),5%Gt (garnet),and 50%Amph (amphibolite)).Individual mineral K d values are from Rollinson (1993)and references therein,Barth et al.(2002),Klemme et al.(2005),and the Geochemical Earth Reference Model (GERM)(https://www.wendangku.net/doc/767785637.html, ).Bulk solid/melt partition coef ?cients for sediment melting are from Johnson and Plank (1999).Composition of AOC and GLOSS melts are based on the assumption of 5%and 10%batch melting,respectively.The Baogutu samples and GLOSS are the same as in Fig.5.

Age(Ma)

Mg #

Tzr s aturation (°c)

Sr/Y TNd 2DM (Ma )

Fig.12.Variation of zircon U –Pb ages (a),Nd isotopic model ages (b),Mg #,(c),Sr/Y (d),and zircon saturation temperatures (e)versus distance along a southeast to northwest traverse in the central zone of the western Junggar magmatic province.Zircon U –Pb ages are from Han et al.(2006),Su et al.(2006),Geng et al.(2009)and this study.Other data sources are the same as for Fig.5.

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Carboniferous primitive tholeiitic basalts in the Hatu area with typical N-MORB geochemical characteristics (Shen and Jin,1993).Such magmas are a common manifestation of spreading ridge subduction (Guivel et al.,1999),and the Hatu basalts were probably derived from partial melting of upwelling mid-ocean-ridge-like depleted mantle above a slab window.

The rock suite in the Keramay arc area is similar to those known to be associated with ridge subduction and slab window formation elsewhere.For example,in the south-central Alaska Range,broadly contemporaneous adakites,I-type granitoids,and A-type granite suites are associated with ridge subduction and slab window formation (Cole et al.,2007;Hung et al.,2007).Adakites there (e.g.,the Jack River pluton)were generated at 62.7±0.4Ma (Cole et al.,2007),I-type granitoids (e.g.,the Composite plutons)at 67–69Ma,and A-type granites (e.g.,the McKinley pluton)at 51±1Ma (Hung et al.,2007).Moreover,these A-type granites are considered to

have

Fig.13.Suggested tectonic model for the Carboniferous to Early Permian magmatic province in the western Junggar region.(a)338–316Ma:A Paleo-Asia Ocean plate subducting northwestward beneath the Keramay arc;?uids released from the subducted oceanic crust led to partial melting of mantle wedge and the formation of 338–316Ma volcanic rocks and I-type granitoids.(b)315–290Ma:Formation of a slab window in the central area of the Keramay due to a spreading ridge subduction.(1)Partial melting of subducted slab edge,and subsequent melt –mantle interaction,producing the Baogutu low Mg (Mg #b 48)adakitic rocks and high-Mg (Mg #N 48)diorites,respectively.(2)Fractional crystallization of a mantle-derived magma,which was metasomatized by ?uids released from subducted oceanic slab,to generate the I-type granitoids.(3)Partial melting of the juvenile lower crust triggered by upwelling asthenosphere through a slab window,to form the A-type granites.Other symbols are the same as in Fig.1b.

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been generated through partial melting of lower crust by upwelling asthenosphere above a slab window(Mortimer et al.,2006;Hung et al.,2007;Anma et al.,2009).In this case,the A-type granites are ca. 10–20Ma younger than the adakites,which is a similar relationship to that found in the Keramay arc(Fig.12a).

The short duration(315–290Ma)of magmatism in the Keramay arc area is also similar to that known to be associated with ridge subduction and slab window formation elsewhere.Cole et al.(2006) suggested the Caribou Creek volcanic rocks from the southern Talkeetna Mountains were related to a slab window,which existed between59Ma and35Ma.Kinoshita(2002)also documented a similar period of magmatism(~20Ma)above a slab window that formed in response to spreading ridge subduction beneath southwest Japan.The Princeton Group in south-central British Columbia formed by remelting of arc basalt above a slab window,and geochronological results indicate that they were erupted between approximately53 and47Ma(Ickert et al.,2009).Anma et al.(2009)demonstrated that the Taitao granites were related to the subduction of a short segment of the Chile ridge spreading centre that started~6Ma ago.Therefore, one of the most distinctive features of ridge subduction is the short duration of magmatism(~20m.y.)above the resulting slab windows, which is consistent with a ridge-subduction model for the315–290Ma magmatism in the Keramay arc area.

In addition,some plutons in the western Junggar area are rounded to ellipse-shaped,indicating that they are likely to have undergone little or no deformation.Many ridge-subduction-related plutons exhibit similar characteristics.For example,the McKinley Sequence and associated Plutons(62–50Ma)in the Central Alaska Range,which are also related to ridge subduction(Hung et al.,2007),show no deformation and are mostly rounded to ellipse-shaped.The Taitao granite pluton(southern Chile)formed due to subduction of the Chile ridge and is undeformed and ellipse-shaped(Anma et al.,2009).In fact,in the ridge-subduction model,the upwelling of asthenosphere through a slab window would cause widespread crustal extension (Wilson et al.,2005;Cole et al.,2006;Cole and Stewart,2009), generally resulting in undeformed plutons,in contrast to those found in a normal(i.e.,non-ridge)subduction setting.

6.3.Implications for crustal growth in the CAOB

The most remarkable feature of the three groups of magmatic rocks in the central area of the western Junggar region is the fact that they are characterized by positiveεNd(t)values(+5.4–+9.2)and very young Nd model ages(T2DM)of300–600Ma(Figs.7a and12b),which is also a common feature of the CAOB.These features indicate that the CAOB crust is made up of young mantle-derived material(Jahn et al.,2004). Two competing viewpoints of CAOB crust growth(arc and post-collision related)have frequently caused dispute(Seng?r et al.,1993;Jahn et al., 2000,2004).Seng?r et al.(1993)hypothesized that nearly half of the CAOB was derived from the mantle by successive accretion of arc complexes and subduction accretion during the Paleozoic.Conversely, many researchers suggest that Phanerozoic CAOB granitoids were generated by basalt underplating in a post-collisional setting(Jahn et al., 2000;Wu et al.,2000).More recently,Jahn et al.(2004)argued that both processes probably played equally important roles.

We suggest that ridge subduction and slab window formation also played an important role in the growth of CAOB crust.Fig.13 illustrates the most plausible geodynamic scenario:(1)Oceanic crust subduction and?uid release leading to mantle wedge partial melting and resultant338–316Ma volcanic rocks and I-type granites in an arc setting(Fig.13a).(2)Direct partial melting of subducted basaltic slab (AOC+sediments),generating adakitic magmas(Process1:see caption in Fig.13b).(3)Melting of metasomatised mantle wedge in the slab window to produce post-315Ma I-type granitoids(Process2 of the Fig.13b).Process3in Fig.13b also illustrates crustal growth via partial melting of the basaltic lower crust by underplating of mantle-derived magmas or direct melting of depleted mantle to produce A-type granites and N-MORB type basalts,respectively.These events were triggered by upwelling of asthenospheric mantle in a ridge-subduction setting(Fig.13b).

Windley et al.(2007)proposed that diagnostic features of ridge subduction are abundant throughout the geological record of the CAOB.Such processes probably accompanied the successive closure of branches of the Paleo-Asian Ocean,given that most subduction systems eventually interact with a spreading ridge(Sisson et al., 2003).Although our study was con?ned to a relatively small area in the western part of the CAOB(Fig.1),it supports the view that ridge subduction was common during formation of the entire belt.If so, then ridge subduction likely made major contributions to crustal growth in the CAOB in addition to those made by accretion of subduction and arc complexes(Seng?r et al.,1993)and post-collisional crustal melting(Jahn et al.,2000,2004).

7.Conclusions

(1)The Baogutu porphyries in the western Junggar region are calc-

alkaline quartz dioritic and granodioritic plutons and dioritic porphyrite dikes.They exhibit geochemical and petrologic characteristics that are typical of adakites;some of them also have the geochemical characteristics of high-Mg andesites. (2)LA-ICP-MS zircon U–Pb dating suggests that the Baogutu

plutons and dikes have similar crystallization ages of315–311Ma and314–310Ma,respectively,and were generated in the Late Carboniferous.

(3)The Baogutu adakitic rocks were most likely generated by

partial melting of a slab edge(containing ca.95%basaltic oceanic crust and ca.5%overlying sediments)close to a subducting spreading ridge in the garnet amphibolite faces as a result of a ridge subduction,and subsequent interactions between mantle peridotite and slab melts during their ascent.

(4)Events associated with ridge subduction are likely to have

played an important role in crustal growth in the CAOB in addition to previously recognized accretion of subduction and arc complexes and post-collisional crustal melting.

Supplementary materials related to this article can be found online at doi:10.1016/j.chemgeo.2010.08.012.

Acknowledgements

We sincerely thank Professors R.Rudnick and M.Sun and two anonymous reviewers for their constructive and helpful reviews on this manuscript.We are also grateful to Professor Liu Yongsheng,Chen Haihong,Liu Ying,Hu Guangqian,Ma Jinlong,Tu Xianglin and Xinjiang 305Project Of?ce for their help in the analytical and?eldwork.This study was jointly supported by the Major State Basic Research Program (973Program)of People's Republic of China(No.2007CB411308),the National Natural Science Foundation of China(Grant No.40721063),the Knowledge Innovation Program of Chinese Academy of Sciences (KZCX2-YW-128),and the Institute for Geoscience Research(TIGeR) at Curtin University of Technology.This is contribution No.IS-1240from GIGCAS and TIGeR publication#233".

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