Petrogenesis of the Xuanwoling mafc–ultramafc intrusion in the northeastern Tarim Block(Northwest China)

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ORIGINAL PAPER

Petrogenesis of the Xuanwoling mafc–ultramafc intrusion in the northeastern Tarim Block(Northwest China)

Zhao-De Xia?Chang-Yi Jiang?Ming-Zhe Xia

Beishan Terrane,located in the northeast of the TarimBlock,innorthwestChina,hasdevelopeda500-kmlong and 100-km wide belt of Permian mafc–ultramafc intrusions. One of these mafc–ultramafc intrusions,the Xuanwoling Intrusion,is composed of dunite,troctolite,olivine gabbros, and gabbros,with cumulate texture and rhythmic layering. The crystallization sequence is olivine+spinel→plagioclase→pyroxene,indicating that the crystallization pressure islowerthan0.5–0.8 GPaandthattheintrusionhasundergone variable degrees of crustal contamination,increasing from dunite to gabbros.The olivines found in the Xuanwoling Intrusion have high Fo values(up to 90),suggesting a primary magma with a high composition of mg.It is likely that this high-mgmagmawasproducedatextremelyhightemperatures (1,330–1,350°C),andasaresult,Nd–Srisotopiccompositions similar to oceanic island basalts are found in the Xuanwoling Intrusion,which we propose arose from the mantle plume.

Mafc–ultramafc·Parental magma· Pressure·Temperature·Xuanwoling·Tarim Block

1 Introduction

Large igneous provinces(LIPs)composed of continental food basalts(CFB)or oceanic island basalts(OIB)and associated mafc–ultramafc intrusions,such as dykes,sills, and layered intrusions,are usually interpreted as the products of mantle plumes(Campbell and Griffths 1990; Mahoney and Coffn 1997;Pirajno 2000).It is possible to determine how LIP was generated and its geodynamic setting by investigating the geochemical features of its mafc–ultramafc intrusions(Ernst and Buchan 2003).

In general,the Tarim Block is composed of the Tarim Basin,the Beishan Terrane,the Kumutag Basin,and the north slope of Altun Mountain(BGMRXUAR 1993).The Early Permian basalts found in the Tarim Basin in northwestern China have been recognized as LIP,and are distributed over an area of 250,000 km2(Yang et al.1996, 2005;Jiang et al.2004a,b;2006a,b,2006a;Zhang et al. 2008;Pirajno et al.2009;Zhou et al.2009;Tian et al. 2010).Permian basalts and contemporaneous mafc–ultramafc intrusions are also found in the Beishan Terrane and its adjacent area.Mafc–ultramafc intrusions are usually zonally distributed,and in the Beishan area,they form a zone 500 km long and 100 km wide,from the Luodong–Pobei Intrusion in the east Lop Nur,through the Cantoushan,Hongshishan,Xuanwoling,Bijiashan,and Bijiashandong intrusions to the Heishan–Guaishishan Intrusion in Liuyuan(Fig.1).Recent geochronological studies have shown that the zircon U–Pb ages of these intrusions were mainly in the range of 270–286 Ma(Jiang et al.2006b;Li et al.2006;Ao et al.2010;Su et al.2011,Qin et al.2011). The peak of published age data of these intrusions is 280 Ma,which probably represents the time of the mantle plume activity(Qin et al.2011).The zircon U–Pb age of the Xuanwoling Intrusion was 260.7±2.0 Ma(Su et al. 2010),but it is unclear whether this intrusion is related to the mantle plume temporally.This paper presents detailed petrological descriptions and mineral compositions of the Xuanwoling mafc–ultramafc intrusion in the Beishan area(Figs.1,2)to constrain the petrogenesis of the intrusion and to discuss its possible relationship with a mantle plume.

2 Geological setting

The Beishan Terrane lies in the northeastern Tarim Block and is bounded by the Kuruktag Terrane to the north and by Dunhuang Terrane to the south.The oldest exposed rocks of the Beishan Terrane include the Beishan group from the Paleoproterozoic and the Gutongjing,Yangjibulake,and Aierjigan groups from the middle-to late-Proterozoic.The sedimentary covers include Sinian,Ordovician,Carboniferous,Permian,and Cretaceous strata(BGMRXUAR 1993).In the Carboniferous,the Beishan Terrane developed calc-alkalic volcanics,acid intrusions,and ultramafc intrusions accompanied by clastic rocks and carbonate rocks.In the Early Permian,the Beishan area was still in an active period and hundreds of mafc–ultramafc intrusions, voluminous basic volcanics,and fragmentary granite intrusions were formed.From west to east,the Permianmafc–ultramafc intrusions are distributed as follows:Luodong,Pobei,Cantoushan,Hongshishan,Xuanwoling, Bijiashan,and Bijiashandong.

Fig.1 a The location and b the distribution of the mafc–ultamafc intrusions in the northeastern Tarim Block;and c regional geological map of the studied areas

Fig.2 Simplifed sketch of the Xuanwoling Intrusion,northeastern Tarim Block

2.1 The Xuanwoling mafc–ultramafc intrusion

The Xuanwoling mafc–ultramafcsintruded into the sandstone and granite of the Later Carboniferous Hongliuyuan Formation in the northern Beishan Terrane.The intrusion is irregularly fat-shaped and has a length of 4 km and a maximum width of 2 km,with an areal extent of about 5 km2.From surface surveys,from base to top,the Xuanwoling Intrusion is stratigraphically composed of a dunite–troctolite unit,a olivine gabbro unit,and a gabbro unit(Fig.2).This layered intrusion has obvious single mineral lamina and rhythmic layering(Fig.3).Gabbros and olivine gabbros always have plagioclase and clinopyroxene layers,with variable thickness from 1 to 30 cm.In the Xuanwoling Intrusion,the thickness of the rhythmic layering between the gabbro and the olivine gabbro is 0.2–2 m,and between the dunite and the troctolite the rhythmic layering is about 2–3 m.

The dunite–troctolite unit is distributed in the lower part of the surface,comprising about 10 vol%of the intrusion, and displays generally medium-grained poikilitic textures. A typical dunite sample consists of 94%olivine,4% interstitial plagioclase,1%spinel,and minor amounts of orthopyroxene and sulfde.In this intrusion,subhedral olivine is usually surrounded and embayed by postcumulus plagioclase and bronzite(Fig.4a).Olivine grains are generally altered along fractures to a fne-grained mixture of magnetite and serpentine-group minerals.Spinel occurs as euhedral to subhedral grains in interstices and as inclusions in olivines.Plagioclase grain sizes are 0.2–1 mm,and some grains have changed to chlorite.The troctolite shows an obvious single,mineral-rich layer and a form-banded structure of olivine–plagioclase–olivine–plagioclase. Troctolite can be divided into three types:(1)mela-troctolite,(2)troctolite,and(3)leuco-troctolite.A typical sample of mela-troctolite consists of 75%olivine,23% interstitial plagioclase,and lesser amounts of clinopyroxene and chromite.Leuco-troctolite samples consist of 80% cumulus plagioclase,15%cumulus olivine,and a small amount of pyroxene.Troctolite samples include similar amounts of plagioclase and olivine.The cumulus crystals of mela-troctolite and troctolite are mainly olivine,whereas the cumulus crystals of leuco-troctolite are mainly plagioclase(Figs.3d,4b–d).

The olivine gabbro unit comprises about 30%of the Xuanwoling Intrusion and includes olivine norite,olivine gabbronorite,and olivine gabbro.The typical olivine norite sample is characterized by cumulus texture and is composed of 45%orthopyroxene,15%olivine,35%interstitial plagioclase, and ＜5% clinopyroxene. Theorthopyroxene generally occurs as 0.1–0.5 mm formations and is short-prismatic crystalline;the olivine generally occurs as approximately 0.5 mm formations and exists as a fne-grained mixture of magnetite and serpentine-group minerals along fractures.The cumulus olivine and orthopyroxene are surrounded and embayed by postcumulus plagioclase(Fig.4e).The typical olivine gabbronorite sample is composed of 20%–35%olivine,20%–50% plagioclase,15%–30%clinopyroxene,5%–10%orthopyroxene,and minor amounts of hornblende.The olivine generally occurs as approximately 0.5 mm formations and has partially serpentinized,while the plagioclase grains are elongated and 0.5–3 mm long.The clinopyroxene grains are interstitial to olivine and plagioclase.A typical sample of olivine gabbro is composed of 55%plagioclase,35% clinopyroxene,and 10%olivine.The olivine generally occurs as approximately 0.5 mm formations and has partially serpentinized.The clinopyroxene is interstitial to olivine and plagioclase,and the larger clinopyroxene crystals contain many smaller plagioclase and olivine crystals(Fig.4f).

Fig.3 Field photos showing the rhythmic layers of the Xuanwoling intrusion:a rhythmic layers for gabbro and leucogabbro.b Single mineral layer in gabbro.c Rhythmic layers for gabbro and olivine gabbro.d Rhythmic layers in troctolite,T-1 and T-3 for olivine-rich cumulus mineral and T-2 for plagioclase-rich cumulus mineral)

The gabbro unit,composed of gabbro and leuco-gabbro, covers 60%of the Xuanwoling Intrusion.The gabbro is characterized by a mosaic texture with plagioclase grains that are usually equigranular or similar to clinopyroxene.A typical sample of gabbro consists of 55%plagioclase, 40%clinopyroxene,＜5%ilmenite,and a lesser amount of orthopyroxene and hornblende.Some plagioclase grains have been corroded by clinopyroxene.The ilmenite is interstitial to the plagioclase and the pyroxene and has corroded them(Fig.4g).The leuco-gabbro is characterized by gabbro-diabase texture and consists of 75%plagioclase,20%clinopyroxene,＜5%olivine,and a lesseramount of orthopyroxene and hornblende.Some plagioclase grains are poikilitic in clinopyroxene,which mainly appears in the space between plagioclase grains(Fig.4h).

3 Analytical methods

3.1 Electron microprobe analyses

Mineral compositions were determined using the JXI-8100 EPMA at the Key Laboratory of Western China’s Mineral Resources and Geological Engineering of Ministry of Education at Chang’an University.Operating conditions were 15 kv and 10 nA with a beam diameter of 1 μm.Natural mineral standards were used for calibration,and a PAP correction procedure was applied to the data.The precision of the analyses is better than 5%for major oxides.

3.2 Major and trace element analyses

Rock samples for whole rock analyses were crushed and then pulverized in an agate mill.The whole rock major elementswere analyzed through X-ray fuorescence (RIX2100)on fused glass beads at the Key Laboratory of Continental Dynamics at Northwestern University in China.The analytical precision was between 1%and 5%.

AThermo-X7ICP-MSwasusedtoanalyzetraceelements atthe KeyLaboratoryofWestern China’sMineral Resources and Geological Engineering of Ministry of Education at Chang’anUniversityinChina.Samplepowdersinamountsof 50 mg were dissolved in high-pressure Tefon bombs using a HF–HNO3mixture.Rh was used as an internal standard to monitor signal drift during ion counting.The analytical precision for most elements was better than 5%.

3.3 Nd–Sr isotope analyses

Sr and Nd isotopes were measured with a MC-ICP-MS in the Guangzhou Institute of Geochemistry at the Chinese Academy of Sciences.Measured87Sr/86Sr and143Nd/144Nd ratios were normalized to 0.1194 and 0.7219,respectively. During the period of analysis,the NBS SRM 987 standard yielded an average value of87Sr/86Sr=0.71026±10,and the La Jolla standard gave an average value of143Nd/144Nd=0.511861±10.

4 Analytical results

4.1 Mineral chemistry

Tables 1,2,and 3 shows representative analyses of the minerals found in the Xuanwoling Intrusion.The Fo [Fo=100×Mg/(Mg+Fe)]values of the Ol ranged from 79.4 to 90.1,and decreased gradually from dunite to troctolite to olivine gabbro.For example,the Fo value of olivine was high,(up to 90.1),in dunite,but was only 79.4 in olivine gabbro.The Ol had NiO and MnO contents of 0.11–0.45 and 0.12–0.32 wt%,respectively,which also decreased from dunite to olivine gabbro.

The spinel had compositional ranges from 17.39 to 43.39 wt%FeOtand 2.28to15.56 wt%MgO;from29.21to 45.75 wt%Cr2O3;and from 11.9 to 39.33 wt%Al2O3,with Cr#[Cr#=100×Cr/(Cr+Al)]ranging from 29 to 69 (Fig.5).The TiO2contents of spinel were 0.32–2.50 wt% and the NiO contents were 0.01–0.18 wt%.(Table 2).

Cumulus Pl occurs mainly in troctolites and belongs to bytownite with An[An=100×Ca/(Ca+Na)]ranging from 80 to 87,while the postcumulus Pl is labradorite with An ranging from 67 to 70.In other rocks,plagioclases are bytownite and labradorite.

In troctolite and olivine norite,Opx is bronzite with MgO contents ranging from 26.17 to 31.76 wt%,and in gabbro,Opx ishypersthenewith MgO contentfor 22.90 wt%.Cpx is augite and diopside,salite and subcalcic augite,with TiO2contents ranging from 0.13 to 1.24 wt% and with Al2O3contents ranging from 1.56 to 4.35 wt%.

4.2 Major element

The mafc–ultramafc rocks of the Xuanwoling Intrusion have SiO2contents ranging from 37.26 to 50.38 wt%, Al2O3contents ranging from 5.89 to 23.19 wt%,and CaO contents ranging from 2.50 to 13.21 wt%(Table 4).The rocks’MgO contents range from 6.40 to 32.29 wt%,and their Fe2O3Tcontents range from 3.46 to 11.85 wt%with an Mg#[Mg#=Mg/(Mg+Fe)]of 0.64 to 0.87.They have low Na2O+K2O (0.51–2.22 wt%)and TiO2(0.08–0.46 wt%,except for one sample with 4.73 wt%). These results show that the rocks belong to the tholeiitic series.

4.3 Trace element

The rocks of the Xuanwoling Intrusions have low REE concentrations (ΣREE=3.71×10-6–48.89×10-6), which gradually increase from dunite to troctolite to olivine gabbro to gabbro.Most of the rocks display enrichment in LREE with(La/Yb)Nratios that range from 1.04 to 3.27; however,three troctolite samples have chondrite-normalized REE pattern depletion in LREE with(La/Yb)Nranging from 0.90 to 0.94.All of the olivine gabbros,except for one sample(δEu=0.99),show positive Eu anomalies (δEu=1.02–2.30)(Table 4;Fig.6).

The rocks have lower trace element concentrations of one to ten times that of the primitive mantle.The primitivemantle––normalized trace element diagram shows that all rocks are enriched in large ion lithophile fuid mobile elements(i.e.,Cs,Rb,Sr,U)and are depleted in high feld strength elements(HFSE,i.e.Zr,Hf,Nb,Ti)(Fig.6).

Table 1 Composition of olivine and spinel grains of the Xuanwoling Intrusion(wt%)

4.4 Sr and Nd isotope

The rocks display a wide range of isotopic compositions (Table 5)with initial87Sr/86Sr ratios ranging from 0.7034 to 0.7096 and143Nd/144Nd ratios ranging from 0.512414 to 0.512882.The calculated εNd(t)values range from-4.0 to 5.7.The initial87Sr/86Sr ratio gradually increases from olivine gabbronorites to olivine gabbros to gabbronorites to gabbros while the corresponding εNd(t)values gradually decrease.Figure 7 shows three samples plotted in the ocean island basalts feld and one other sample plotted at the edge of the OIB feld,with results indicating a trend of depleted mantle and Enriched Mantle 2 mixing.

Table 2 Composition of pyroxene grains of the Xuanwoling Intrusion(wt%)

Table 3 Composition of plagioclase of the Xuanwoling Intrusion(wt%)

Table 3 continued

Fig.5 The compositions of the Xuanwoling spinels:a Cr2O3versus TiO2diagram,b 100 Mg/(Mg+Fe2+)versus 100 Cr/(Cr+Al),and c triangular diagrams of Cr–Al–Fe3+.Data for layered intrusions from Barnes and Roeder(2001),the Jinchuan Intrusion from Barnes and Tang (1999),Sudbury Intrusion from Zhou and Robinson(1997),and Jinbaoshan Intrusion from Wang et al.(2005)

5 Discussion

5.1 Parental magma

The Cr#’s(48–69)of the spinels found in the Xuanwoling Intrusion are similar to those of spinels found in other layered intrusions,such as the Bushveld and the Stillwater Intrusions(Barnes and Roeder 2001),and the Jinchuan Intrusion(45–80;Barnes and Tang 1999).The comparison of the Xuanwoling spinels with the spinels from other intrusions reveals that the Xuanwoling spinels crystallized from mafc magmas.The composition of the parental magmas can be calculated based on the primary spinel and olivine compositions.If the spinel enclosed within the olivine with the highest Cr2O3content(45.7 wt%)in the Xuanwoling Intrusion retains the initial compositions,the Al2O3contents of the melt-in equilibrium with the spinel can be estimated by using the following formula(Maurel and Maurel 1982):in wt%.The calculated Al2O3content of the melt-in equilibrium with the Xuanwoling spinel is about 13.7 wt%.

The Xuanwoling mafc–ultramafc rocks have high levels of MgO–olivine with maximum Fo compositions of olivine up to 90.1.This differs from the mafc–ultramafc rocks in the Huangshan and Jinchuan Intrusions(maximum Fo＜90)(Zhou et al.2004;Tang et al.2006),but is similar to olivine compositions of the picrite in ELIP (Fo=84.5–91.6)(Zhang et al.2006)and Greenland (Fo=77.4–93.3)(Larsen and Pederson 2000;Thompson and Gibson 2000).After assuming a molar Mg–Fe distribution constant (Kd=(Fe/Mg)Ol/(Fe/Mg)magma) of 0.30±0.03(Roeder and Emslie 1970),we estimated that the parental magmas of the Xuanwoling Intrusion are high-MgO(12.58–13.84 wt%)picritic magma.These results are similar to the parental magmas of the Hongshishan Intrusion but higher than those found in the Jueluotag–Jingerquan and Mid-Tianshan Intrusions(Su et al.2011).

5.2 Crustal contamination

The slightly negative Nb and Ta anomalies of the rocks in the Xuanwoling Intrusion are indicative of minor crustal contamination.Lu and Yb have similar geochemical characters and Lu/Yb ratios cannot be signifcantly modifed by partial melting or fractional crystallization.Mantlederived magmas are characterized by low Lu/Yb ratios (0.14–0.15)(McDonough and Sun 1995),whereas continental crust has relatively higher Lu/Yb ratios(0.16–0.18).

In addition,the continental crust is rich in LREE and LILE but strongly depleted in Nb and Ta(Rudnick and Gao 2003).The rocks of the Xuanwoling intrusion have somewhat variable,but generally high,Lu/Yb ratios (0.14–0.19)and low Nb/La ratios(0.20–0.73),clearly suggesting some crustal contamination.The mafc–ultramafc rocks have variable initial87Sr/86Sr ratios (0.7034–0.7094)and εNd(t)values(5.75 to -3.99) (Table 4).In the plot of initial143Nd/144Nd values versus initial87Sr/86Sr ratios,the mafc–ultramafc rocks lie between the enriched extension of the mantle array defned by DMM and crustal rocks(Fig.7),suggesting variable degrees of crustal contamination during magma ascent and differentiation;the samples with low εNd(t)values may have undergone variable degrees(up to 11%)of Tarim Proterozoic gneiss granite crustal contamination(Fig.7). The degrees of contamination in the frst stage are much lower for the olivine-bearing ultramafc–mafc rocks than other mafc rocks,which is consistent with variations of olivine Fo contents and mineral assemblages in the intrusion.

Table 4 Major element(wt%)and trace element(ppm)abundance data for the Xuanwoling Intrusion

Table 4 continued

Table 4 continued

5.3 Physical–chemical condition and Magma evolution

Petrological and geochemical studies of igneous rocks underline the importance of differentiation processes operating at various pressures.The anhydrous experimental data for a primitive,mantle-derived tholeiitic basaltic magma provides constraints on phase equilibria, and solid and liquid compositions along the liquid line of descent of magma differentiating.As a result of the shift of the cotectic lines between plagioclase and pyroxene (and olivine)towards the plagioclase apex in the Cpx–Ol–Pl ternary system,Py crystallization precedes Pl crystallization at high pressure(1.0 GPa)(Morse 1994).However,if the slope of the Cpx-in curve in a pressure–temperature section is fatter than the Ol-in and Pl-in curves,Ol and Pl will crystallize at lower pressures.The crossover of the Cpx-in and Pl-in curves is located between 0.5 and 0.8 GPa.Below these pressures,Pl+Pl forms the high-temperature assemblage,producing dunitic and troctolitic cumulates(Villiger et al.2007).In the Xuanwoling Intrusion,these are rare primary hydrous minerals and the cumulate assemblages evolve from dunites,followed by troctolites and olivine gabbros,and fnally gabbros,suggesting the following crystallization sequence:Ol+Sp→Pl→Py.The cumulate assemblages indicate that the Xuanwoling Intrusion may have formed at a low pressure.In addition,the structural variations in the troctolites may have resulted from variations in pressure.A sudden decrease of pressure may cause Pl crystallization to precede Ol crystallization.

The relationship between the composition of Ol and the liquidus temperature of basaltic melts has been wellestablished through the results of research experiments and phase equilibrium analyses:Tliquid=1,066+12.067-Mg#+312.3(Mg#)2(Roeder and Emslie 1970;Weaver and Langmuir 1990).According to the highest Fo value (90.1)of olivine from the ultramafc rock at Xuanwoling, the liquidus temperature of Ol was as high as 1,330°C. Fabrie`s(1979)also suggested Sp–Ol geothermometry in peridotites,based on the partitioning of coexisting Sp and Ol by Mg2+and Fe2+(1979):T(°C)=(4250Cr#+1343)/When spinels occur as inclusions in olivine within dunite with the maximum Cr/ (Cr+Al)value of 0.61,we get the liquidus temperature of 1,350°C.The temperatures calculated using the Ol and Sp–Ol thermometer are very close and could represent the crystallization temperature of the magma.These are rare primary hydrous minerals in the Xuanwoling Intrusion,so the parental magma is anhydrous or approaching anhydrous and the estimated temperatures will be approximated to the actual ones.The calculated temperature values for the Xuanwoling Intrusion are 100–180°C higher than the average temperature of tholeiitic basaltic magma (1,150–1,225°C)(Carmichael et al.1974)and in line with the temperature range of picritic magma,indicating that magma is a high-temperature magma.

5.4 Tectonic implications

Fig.6 Chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns for the Xuanwoling intrusive rocks.The normalizing values are from McDonough and Sun(1995)

It is commonly believed that the Tarim Block was amalgamated to the Central Asian Orogenic Belt in the Late Carboniferous(Shu et al.2000;Xia et al.2003;Li 2006). There is large-scale Permian mantle magmatism in the northeastern Tarim Block,especially in the Beishan tectonic region.The parental magma of the Luodong Intrusion is picritic,with MgO contents of up to 14.7 wt% and a high crystallization temperature of 1,412°C(Ling et al.2011).Su et al.(2011)suggest that the parental magma of the Hongshishan Intrusion was also a hightemperature,high-mg parentalmagma.These high-temperature and high-mg parental magmas in these intrusions suggest that their petrogenesis could be attributed to a Permian mantle plume.The zircon U–Pb age of many intrusions in the northeastern Tarim Block was mainly in the range of 274–280 Ma(Jiang et al.2006a;Li et al.2006; Ao et al.2010;Su et al.2010,2011;Qin et al.2011),which probably represents the main period of mantle plume activity.Karoo–Ferrar magmatism is related to the mantle plume or plumes;the major pulses of Karoo volcanism occurred at 195 and 180–177 Ma and the major pulses of Ferrar magmatism at 193 and 180–170 Ma.The zircon U–Pb age of the Xuanwoling Intrusion was 260.7±2.0 Ma (Su et al.2011),younger than other intrusions in the northeastern Tarim Block by 14–20 Ma,suggesting that the magmatism occurred in two pulses,similar to the Karoo–Ferrar magmatism.Furthermore,the zircon U–Pb age of the Xuanwoling Intrusion is near-synchronous with the Permian plume represented by Emeishan basalts (259±3 Ma).At the same time,Chen and Han(2006) have reported that 257.4±5.3 Ma mafc rocks in the Wuqiagou area,north Xinjiang Autonomous Region, China,are produced by plume-lithosphere interactions. OIB-like Nd and Sr isotopic compositions,and the hightemperature,high-Mg parental magmas for the Xuanwoling Intrusion indicate that the mantle magmatism is related to a mantle plume,indicating that there existed mantle plume activity at～260 Ma in Xinjiang and that the Intrusion may be associated with the Emeishan mantle plume.

Table 5 Rb–Sr and Sm–Nd isotopic bulk-rock compositions of the Xuanwoling Intrusion

Fig.7 εNd(t)versus initial87Sr/86Sr plot(t=260)for the Xuanwoling Intrusion and modeling calculationsofSr–Nd isotopes suggesting that the Xuanwoling Intrusion experienced variable contamination by the Tarim Proterozoic gneiss granite.Data for DMM are from McDonough and Sun(1995)and Tarim Proterozoic gneiss and gneiss granite from Hu et al.(2006).DMM depleted MORB mantle,EM enriched mantle,OIB oceanic island basalt (Zindler and Hart 1986)

Based on their location in the Beishan tectonic region, on their close spatial and temporal association with voluminous layered mafc–ultrmafc rocks and basalts,and on their geochemical characteristics,we suggest that multistage mantle plume activities were involved in the formation of basalts in the Tarim Basin and other igneous rocks in the northeastern Tarim Block.Several further studies are required including systematical geochronology,detailed feld surveys,and a regional geology survey to constrain the activities of the mantle plume.

6 Conclusions

The Xuanwoling Intrusion in the Beishan area of the northeastern Tarim Block is a layered,mafc–ultramafc intrusion that exhibits cumulate texture and rhythmic layering.The rock associations of the Xuanwoling Intrusion are dunite,troctolite,olivine gabbros,and gabbros.The crystallization sequence isolivine+spinel→plagioclase→pyroxene,and the crystallization pressure may be less than 0.5–0.8 GPa.The Xuanwoling Intrusion has OIB-like Nd–Sr isotopic compositions and high-temperature, high-Mg parental magmas,which suggest that the Xuanwoling Intrusion is related to the mantle plume.

AcknowledgmentsThe research is supported by the National Natural Science Foundation of China(Grant No.4130207,41102045), the Fundamental Research Funds for the Central Universities (2014G1271066), and China Regional Geological Survey (12120113043100).Constructive comments from two anonymous reviewers and guidance from the editors are greatly appreciated.

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Received:2 April 2014/Revised:24 June 2014/Accepted:30 June 2014/Published online:23 December 2014 ?Science Press,Institute of Geochemistry,CAS and Springer-Verlag Berlin Heidelberg 2014

Z.-D.Xia(?)·C.-Y.Jiang·M.-Z.Xia

School of Earth Science and Resources,Chang’an University, 126 Yanta Road,Xi’an 710054,China

e-mail:karlde@163.com

Z.-D.Xia·C.-Y.Jiang

Key Laboratory of Western China’s Mineral Resources and Geological Engineering of Ministry of Education,Chang’an University,126 Yanta Road,Xi’an 710054,China