俯冲洋岛玄武岩脱水释放的水流体成分：来自西天山榴辉岩相高压脉的约束

俯冲洋壳的脱水作用对于岛弧火山岩的成因具有重要意义。西天山高压低温变质带中切割主岩蓝片岩/榴辉岩的榴辉岩相脉体记录了古生代俯冲带中的脱水过程。脉体具有典型高压矿物组合：石榴石(＞8%)+绿辉石(15%)+石英(＞55%)+蓝闪石(5%)+冻蓝闪石(10%)±黝帘石±方解石。高压脉及其主岩的岩相学与矿物化学研究表明流体是在俯冲过程中蓝片岩向榴辉岩进变质条件下释放。脉的主量元素成分显示流体富Si。主岩的稀土元素和微量元素表明其原岩为洋岛玄武岩(OIB)。脉和主岩具有相似的微量元素配分模式,并且相对N-MORB都显示富Li、Be、Cs、Rb、Ba、Pb、La的特点。通过模拟计算,在高压脉、与脉平衡的流体以及模拟的原生流体中都富集流体活动性元素(Li、Be、Pb)和LILE(Cs、Rb、Ba),贫HFSE(Nb)和REE(Nd、Sm)。证实了在古生代南天山洋俯冲时洋岛玄武岩脱水释放出富Li、Be、LILE、La和Pb,贫HFSE和HREE的流体。     

Petrology of coesite-bearing eclogite from Habutengsu Valley, western Tianshan, NW China and its tectonometamorphic implication

Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite‐bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite‐bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na‐Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi‐phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet‐clinopyroxene thermometry yields peak temperatures of 420–520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P–T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4–2.7 GPa and 470–510 °C. Modelled modal abundances of major minerals along a 5 °C km^?1 geothermal gradient suggests two critical dehydration processes at ～430 and ～510 °C respectively. Computed garnet composition patterns are in good agreement with measured core‐rim profiles. The petrological study of coesite‐bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP‐LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP‐LT assemblages at most localities.     

新疆西天山异剥钙榴岩地球化学特征及其对俯冲带流体的指示意义

新疆西天山长阿吾子异剥钙榴岩是目前首例发现的在俯冲带形成的原岩为榴辉岩的异剥钙榴岩,与其作用的流体为俯冲带流体。该异剥钙榴岩与大洋底变质作用形成的异剥钙榴岩相比,岩石学特征非常相似,但是元素迁移方面存在一定的差别。为了推断流体成分,我们使用了三种方法。一、测试了异剥钙榴岩化前后主要矿物相石榴石和辉石化学成分。二、在ACF图上,可以看出全岩成分从F端元向C端元演化,代表Fe减少,Ca增加的趋势,而且完全异剥钙榴岩成分落在了前人所做的异剥钙榴岩成分区间内。三、假设Al为不迁移元素,根据Grant的计算方法,做出主量元素、微量元素和REE的等浓度图。综合以上分析结果推断,俯冲带中流体的成分是复杂和变化的:部分异剥钙榴岩化阶段流体富集Ca、Si、Ti、Mn、Nb、Ta、Zr、Hf,贫乏Fe、Mg、Na、K、REE、Tb、Y,氧逸度升高,流体属于不饱和状态,因此导致不平衡结构发育;完全异剥钙榴岩阶段,流体富集Ca、Mg、LREE、Ni、Cr,贫乏Fe、Ti、Rb、Ba、MREE、HREE等元素,此时流体作用强烈,富水流体相的强烈淋滤作用,使得大部分元素发生了迁移。     

Petrology of rodingite derived from eclogite in western Tianshan, China

This study reports a new rodingite type which was derived from eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites, western Tianshan, China. Based on petrographical investigations, rodingite, partial rodingitized rock and completely rodingitized rock are characterized in this paper. These rocks show a continuous variation in their bulk compositions, mineralogy and their textural properties from eclogite to rodingite. The completely rodingitized rocks can be further divided into prehnite rodingites, hydrogrossular‐diopside rodingite and vesuvianite rodingites on the basis of the mineral assemblage and textural character. The chemical potential path of μ(SiO₂)–μ(CaO/MgO) can be used to constrain the evolution of two stages of rodingitization. The first rodingitization possibly started under conditions of 410–430 °C and 7–9 kbar at upper greenschist facies, and resulted from a secondary serpentinization during exhumation of the subducted slab. A second and pervasive rodingitization took place under conditions of 250–350 °C and 2–10 kbar from greenschist to subgreenschist facies. The P–T path presented shows a retrograde evolution from eclogite to rodingite. We conclude that the process of rodingitization may also take place under subduction zone conditions in addition to its more common occurrence under ocean‐floor metamorphic conditions.     

西天山异剥钙榴岩的地球化学研究

西天山长阿吾子的蛇绿混杂岩带的超基性岩及其伴生岩石异剥钙榴岩位于伊犁中天山板块和塔里木板块碰撞造山带.微量元素特征表明退变榴辉岩原岩成因与俯冲带环境有关.异剥钙榴岩中稀土元素和微量元素全面、大幅度降低,反映出来自于蛇纹岩化,作用于异剥钙榴岩化流体的强大的淋滤作用.因而异剥钙榴岩的稀土和微量元素特征都不能反映原始岩浆的特点,无法据此推断原始岩浆产生的大地构造环境.对退变榴辉岩的主元素和微量元素稀土元素的研究表明,长阿吾子异剥钙榴岩的原岩钙碱性玄武岩可能形成于俯冲上板片(SSZ)的岛弧环境.     

俯冲板片中微量元素的活动性:西天山榴辉岩相运输脉的制约

西天山的基性高压变质岩显示互相连接的榴辉岩相脉体网络来源于蓝片岩的进变质脱水作用,通过这些网脉可以洞察在俯冲带高压条件下远程流体流动中的流体-岩石相互作用和元素负载。岩相学证据表明外来流体的渗透作用产生运输脉,而流体持续的沟道式流动导致主岩蓝片岩的淋滤。脉和蓝片岩蚀变带中锂(Li)的含量几乎是蓝片岩主岩中锂的两倍,它支持流体是外部来源的设想。这些流体触发了蓝片岩主岩的榴辉岩化作用而形成蓝片岩蚀变带。由于成脉流体中的微量元素含量低,所以导致与流体发生化学反应的主岩中所有微量元素的强烈淋滤。在此过程中有53%~81%的微量元素被活化,与大离子亲石元素和轻稀土元素的损失相符合,它们的损失量几乎是重稀土元素和高场强元素损失量的两倍。     

Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction

The transition between blueschist and eclogite plays an important role in subduction zones via dehydration and densification processes in descending oceanic slabs. There are a number of previous petrological studies describing potential mineral reactions taking place at the transition. An experimental determination of such reactions could help constrain the pressure–temperature conditions of the transition as well as the processes of dehydration. However, previous experimental contributions have focused on the stability of spontaneously formed hydrous minerals in basaltic compositions rather than on reactions among already formed blueschist facies minerals. Therefore, this study conducted three groups of experiments to explore the metamorphic reactions among blueschist facies minerals at conditions corresponding to warm subduction, where faster reaction rates are possible on the time scale of laboratory experiments. The first group of experiments was to establish experimental reversals of the reaction glaucophane+paragonite to jadeite+pyrope+quartz+H₂O over the range of 2.2–3.5 GPa and 650–820°C. This reaction has long been treated as key to the blueschist–eclogite transition. However, only the growth of glaucophane+paragonite was observed at the intersectional stability field of both paragonite and jadeite+quartz, confirming thermodynamic calculations that the reaction is not stable in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O. The second set of experiments involved unreversed experiments using glaucophane+zoisite ±quartz in low‐Fe and Ca‐rich systems and were run at 1.8–2.4 GPa and 600–780°C. These produced omphacite+paragonite/kyanite+H₂O accompanied by compositional shifts in the sodium amphibole, glaucophane, towards sodium–calcium amphiboles such as winchite (?(CaNa)(Mg₄Al)Si₈O₂₂(OH)₂) and barroisite (?(CaNa)(Mg₃Al₂)(AlSi₇)O₂₂(OH)₂). This suggests that a two‐step dehydration occurs, first involving the breakdown of glaucophane+zoisite towards a paragonite‐bearing assemblage, then the breakdown of paragonite to release H₂O. It also indicates that sodium–calcium amphibole can coexist with eclogite phases, thereby extending the thermal stability of amphibole to greater subduction zone depths. The third set of experiments was an experimental investigation at 2.0–2.4 GPa and 630–850°C involving a high‐Fe (Fe#=Fe_total/(Fe_total+Mg)≈0.36) natural glaucophane, synthetic paragonite and their eclogite‐forming reaction products. The results indicated that garnet and omphacite grew over most of these pressure–temperature conditions, which demonstrates the importance of Fe‐rich glaucophane in forming the key eclogite assemblage of garnet+omphacite, even under warm subduction zone conditions. Based on the experiments of this study, reaction between glaucophane+zoisite is instrumental in controlling dehydration processes at the blueschist–eclogite transition during warm subduction.     

Deep slab subduction and dehydration and their geodynamic consequences: Evidence from seismology and mineral physics

We present new pieces of evidence from seismology and mineral physics for the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the Pacific and Philippine Sea slabs under western Pacific and East Asia. The Pacific slab is subducting beneath the Japan Islands and Japan Sea with intermediate-depth and deep earthquakes down to 600 km depth under the East Asia margin, and the slab becomes stagnant in the mantle transition zone under East China. The western edge of the stagnant Pacific slab is roughly coincident with the NE–SW Daxing'Anling-Taihangshan gravity lineament located west of Beijing, approximately 2000 km away from the Japan Trench. The upper mantle above the stagnant slab under East Asia forms a big mantle wedge (BMW). Corner flow in the BMW and deep slab dehydration may have caused asthenospheric upwelling, lithospheric thinning, continental rift systems, and intraplate volcanism in Northeast Asia. The Philippine Sea slab has subducted down to the mantle transition zone depth under Western Japan and Ryukyu back-arc, though the seismicity within the slab occurs only down to 200–300 km depths. Combining with the corner flow in the mantle wedge, deep dehydration of the subducting Pacific slab has affected the morphology of the subducting Philippine Sea slab and its seismicity under Southwest Japan. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent small mantle plumes, or hot upwelling associated with the deep slab subduction. Slab dehydration may also take place after a continental plate subducts into the mantle.     

Deep slab subduction and dehydration and their geodynamic consequences: Evidence from seismology and mineral physics

We present new pieces of evidence from seismology and mineral physics for the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the Pacific and Philippine Sea slabs under western Pacific and East Asia. The Pacific slab is subducting beneath the Japan Islands and Japan Sea with intermediate-depth and deep earthquakes down to 600 km depth under the East Asia margin, and the slab becomes stagnant in the mantle transition zone under East China. The western edge of the stagnant Pacific slab is roughly coincident with the NE–SW Daxing'Anling-Taihangshan gravity lineament located west of Beijing, approximately 2000 km away from the Japan Trench. The upper mantle above the stagnant slab under East Asia forms a big mantle wedge (BMW). Corner flow in the BMW and deep slab dehydration may have caused asthenospheric upwelling, lithospheric thinning, continental rift systems, and intraplate volcanism in Northeast Asia. The Philippine Sea slab has subducted down to the mantle transition zone depth under Western Japan and Ryukyu back-arc, though the seismicity within the slab occurs only down to 200–300 km depths. Combining with the corner flow in the mantle wedge, deep dehydration of the subducting Pacific slab has affected the morphology of the subducting Philippine Sea slab and its seismicity under Southwest Japan. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent small mantle plumes, or hot upwelling associated with the deep slab subduction. Slab dehydration may also take place after a continental plate subducts into the mantle.     

西天山高压脉及主岩的氧同位素研究——古俯冲带深部流体及俯冲特征的启示

位于中国南天山的西天山高压变质带代表了伊犁-中天山与塔里木两个板块间古生代南天山洋的古俯冲混杂岩带.高压变质带内广泛发育高压脉.为探讨古俯冲深部流体来源及运移特点及板块俯冲特征,对高压脉和主岩的全岩及主要的高压变质矿物的氧同位素进行了分析.高压脉的δ18O值变化于+8.28‰与+10.70‰之间,多数在+9.50‰±1范围内.基性变质岩的主岩与高压脉具相似的氧同位素组成,变化于+9.25‰～+10.10‰之间.高压脉和主岩的全岩δ18O值变化不大.高压脉与相邻主岩间、同一高压脉中间与边部间氧同位素组成的变化没有明显的规律,一般变化不大,对于大多数脉-主岩对,变化小于1‰.与全岩完全不同的是,单矿物氧同位素组成显示出很大的变化范围,石英、石榴石、绿辉石的δ18O值分别为+11.40‰～+15.20‰,+3.59‰～+11.60‰和+8.30‰～+13.05‰,多硅白云母和蓝闪石δ18O的变化较小,分别为+10.00‰～+11.10‰和+9.26‰～+9.94‰.榴辉质岩石中高压变质矿物间氧同位素分馏广泛不平衡.全岩氧同位素组成特征表明,俯冲带深部流体主体来自邻近主岩,外来流体对氧同位素贡献有限.单矿物氧同位素广泛不平衡特征可能指示古俯冲带俯冲板片的快速俯冲和折返以及部分外来流体的参与.     

东天山地区中天山地块内中元古代花岗岩的特征及地质意义

在天山造山带觉罗塔格山系东段的中天山地块内,新识别出一套侵位于长城系星星峡群变质岩中的片麻状花岗质岩体,岩石组合为片麻状闪长岩+片麻状花岗闪长岩+片麻状二长花岗岩+片麻状钾长花岗岩,锆石SHRIM PU-Pb测年表明这套侵入岩的结晶年龄为(1 453±15)~(1 458±40)Ma(207Pb/206Pb表面年龄),是中元古代晚期的产物。岩石学及地球化学特征显示为高钾钙碱性系列,具I型花岗岩组合特征,形成于板块汇聚的环境。这套花岗质侵入岩的厘定,对于可能存在于元古宙的天山洋的闭合事件,从岩浆作用上提供了重要信息,同时对中天山星星峡群的地层时代提供了一个很好的上限。另外,还发现了一颗形成年龄为(2 991±22)Ma的残留锆石,指示该区可能存在太古宙的古老基底。     

南天山区域大地构造与演化

塔里木和中天山之间的南天山造山带,经历了复杂的构造演化与地壳增生过程。综合分析南天山造山带的构造、地层、古生物、岩石、地球化学和同位素年代学等方面的资料,特别是放射虫、蛇绿岩、蓝片岩等方面的最新研究成果,讨论了南天山的区域构造格局和演化过程。南天山主体为一上百公里宽的增生-碰撞混杂带-南天山(蛇绿)混杂带;其北侧为中天山岛弧,是仰冲壳楔;南侧为塔里木陆块,是俯冲壳楔。古南天山洋为一广阔的大洋,南天山碰撞造山作用起始于二叠纪末-三叠纪初,新近纪-第四纪进入陆内造山作用阶段。     

北祁连洋早古生代双向俯冲的花岗岩证据

笔者主要对北祁连山中段的牛心山岩体、民乐窑沟岩体进行了锆石SHRIMP定年研究。结果表明：牛心山花岗岩的年龄为476Ma，民乐窑沟花岗闪长岩的年龄为463Ma。岩石地球化学显示．两岩体均具有大陆活动边缘的岩浆作用特征，结合岩体产出的区域构造位置及区域地质资料，笔者认为早古生代北祁连洋板块分别发生了向南、向北俯冲，其中向南俯冲形成牛心山花岗岩（476Ma），向北俯冲，形成了民乐窑沟花岗岩侵入体（463Ma）。     

Coexistence of garnet blueschist and eclogite in South Tianshan,NW China: dependence of P–T evolution and bulk‐rock composition

Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high‐pressure (HP)–ultrahigh‐pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk‐rock compositions with higher X(CaO) [=CaO/(CaO + MgO + FeO_total + MnO + Na₂O)] (0.33–0.48) and lower A/CNK [=Al₂O₃/(CaO + Na₂O + K₂O)] (0.35–0.56) in eclogite, but with lower X(CaO) (0.09–0.30) and higher A/CNK (0.65–1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core–rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P–T evolutional histories involving a decompression with heating from the P_max to the T_max stage and a post‐T_max decompression with slightly cooling stage, and similar P–T conditions at the T_max stage. The post‐T_max decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P–X(CaO), P–A/CNK and P–X(CO₂) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X(CaO) (<0.28) and higher A/CNK (>0.75) or fluid composition with higher X(CO₂) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO₂). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk‐rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP–UHP belt.     

OH in zoned amphiboles of eclogite from the western Tianshan,NW-China

Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member ( ^B Na₂ ^C M²⁺ ₃ M³⁺ ₂ ^T Si₈(OH)₂) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition ( ^A _0.5 ^B Ca_1.5Na_0.5 ^C M ²⁺ _4.5 M _0.5^{3+ T} Si_7.5Al_0.5(OH)₂) (A = Na and/or K; M ²⁺ = Mg and Fe²⁺; M ³⁺ = Fe³⁺ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm⁻¹) which can be attributed to a local O(3)-H dipole surrounded by ^{M(1) M(3)}Mg₃, ^{M(1) M(3)}Mg₂Fe²⁺ and ^{M(1) M(3)} Fe²⁺ ₃, respectively, an empty A site and ^T Si₈ environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm⁻¹) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)[M(1)–M(3)-site] → Al(T-site) Al[M(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) [M(1)–M(3)-site] → Na(M4-site) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.     

Silica precipitates in omphacite from eclogite at Alpe Arami, Switzerland: evidence of deep subduction

Observations of oriented SiO₂ precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the Alpe Arami garnet peridotite massif, Ticino, Switzerland, and petrological studies of the eclogitic mineral assemblages, suggest that this rock was subjected to higher‐pressure metamorphism than previously realized. We employed various calibrations of the Fe²⁺ ? Mg exchange thermometer and calculations of equilibria with thermodynamic data, considering the calcium–Tschermak's component (CaAl₂SiO₆), of garnet‐pyroxene pairs. From these calculations, it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following: Stage I (prograde) c. 2.4 GPa; 700 °C; Stage IIa (maximum recorded grade) c. 7.0 GPa; 1100 °C; Stage IIb (retrograde) c. 3.7 GPa; 900 °C; Stage III (retrograde) c. 2.1 GPa; 750 °C. Because of the preservation of Stage I, a relatively rapid subduction and exhumation of Alpe Arami eclogite is suggested. The exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the Alpe Arami garnet lherzolite proposed previously by several authors based upon a variety of different observations, although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km.     

Geochemistry and trace element behaviors of eclogite during its exhumation in the Xitieshan terrane,North Qaidam UHP belt,NW China

The Xitieshan terrane, located in the central part of the North Qaidam ultrahigh pressure (UHP) metamorphic belt, China, is mainly composed of orthogneiss and paragneiss and a few intercalated eclogite layers and boudins. Based on their bulk-rock TiO₂-contents, the eclogites can be subdivided into a high-Ti group (TiO₂ > 2%) and a low-Ti group (TiO₂ < 2%). Whole-rock major and trace element analyses revealed that the protoliths of the low-Ti eclogites are normal-type mid-ocean ridge basalts (N-MORB), whereas those of the high-Ti eclogites are either enriched-type mid-ocean ridge basalts (E-MORB) or near ridge seamount basalts, respectively. The Sr–Nd isotopes of eclogites of both groups are similar to those of MORB. Those of the low-Ti eclogites are characterized by positive ε_Nd(T) and restricted I_Sr values and therefore provide further evidence for the formation of the protoliths of the eclogites in an oceanic environment. On the other hand, the Sr–Nd isotopes of high-Ti eclogites show mainly positive but also some negative ε_Nd(T) values and relatively broadly distributed I_Sr values, indicating minor crustal contamination of the ocean floor basalts. Considering available 750–877 Ma protolith ages preserved in zircon cores, it is inferred that some of the eclogites derived from Neoproterozoic protoliths were emplaced onto the crust far ahead of the Paleozoic deep subduction, while the other eclogites originate from a different oceanic crust, e.g., the Paleo-Qilian ocean, indicating multiple orogenies in the geological history of the Xitieshan terrane, China.Whole-rock and in-situ LA-ICP-MS mineral trace element analyses of eclogites revealed two stages of fluid behavior during retrogression that correspond to the two exhumation stages uncovered by phase equilibrium calculations. The mineral scale trace element distributions and trace element inheritance of newly formed amphibole from its precursors indicate that, at the peak metamorphic stage (M1) and at the earlier (eclogite facies) overprint (M2), the fluid was internally controlled by the rock itself. Within a mafic lens, the amount of water-soluble elements (e.g., Rb, Sr, Ba, U, Pb and LREE), observed in the whole-rock compositions as well as in amphiboles, increases from the core (phengite-bearing eclogite) to the rim (amphibolite) and implies an external fluid source for the amphibolite facies retrogression (M3) which should be helpful for the final exhumation of UHP eclogite.     

东天山圪塔山口铜镍矿区镁铁-超镁铁质岩体橄榄石与尖晶石矿物学特征

新近发现的圪塔山口含铜镍矿化镁铁-超镁铁质岩体位于新疆东天山黄山-镜儿泉铜镍矿带东端,共有4个镁铁-超镁铁质岩体,其中Ⅰ、Ⅱ、Ⅲ号岩体均见铜镍硫化物矿化,研究表明其形成时代(282Ma)及岩浆来源与东天山地区其它铜镍矿化镁铁-超镁铁质岩体一致。本文对主要造岩矿物橄榄石及副矿物尖晶石进行了显微镜下观察及电子探针分析,结果表明橄榄石Fo值介于83.1~86.6之间,平均85.2,为贵橄榄石,其Ni含量变化于1273×10-6~2719×10-6,平均1918×10-6;尖晶石根据铝含量的不同可以分为高铝和低铝两种。圪塔山口岩浆为地幔源区发生15.8%~18.8%的部分熔融,并有过剩橄榄石加入的玄武质岩浆经结晶分异作用形成的派生岩浆。对橄榄石分离结晶和硫化物熔离的计算模拟表明橄榄石结晶前,岩浆已经达到S饱和,结晶过程始终伴随硫化物的熔离作用,虽然早期结晶的橄榄石与硫化物熔体间发生了Fe-Ni交换,但仍有很好的铜镍成矿潜力。     

Switching from advancing to retreating subduction in the Neoproterozoic Tarim Craton,NW China: Implications for Rodinia breakup

Geodynamic drivers for the supercontinent cycle are generally attributed to either top-down(subduction-related)or bottom-up(mantle-related)processes.Compiled geochemical data and U-Pb ages and Hf isotopic signatures for magmatic and detrital zircons from the Tarim Craton reveal a distinct change in subduction style during the Neoproterozoic.The subduction cycle is recorded in increasing and decreasing intensity of subduction-related magmatic rocks and time-equivalent sedimentary successions,and converse trends ofεHf(t)values and corresponding changes in crustal incubation time.These trends are consistent with a switch from advancing to retreating subduction.The switch likely occurred at ca.760 Ma when zirconεHf(t)values increase and crustal incubation times decrease following a transitional shift between 800 Ma and 760 Ma.A switch at this time is consistent with Rodinia breakup and may have resulted in the late Neoproterozoic Tarim rift basin.The long-lived(ca.500 Ma)subduction recorded in the Tarim Craton suggests the predominance of a top-down process for Rodinia breakup on this part of its margin.     

A synthesis of iron deposits in the eastern Tianshan,NW China

The northern Xinjiang region is one of the most significant iron metallogenic provinces in China.Iron deposits are found mainly within three regions:the Altay,western Tianshan,and eastern Tianshan orogenic belts.Previous studies have elaborated on the genesis of Fe deposits in the Altay orogenic belt and western Tianshan.However,the geological characteristics and mineralization history of iron deposits in the eastern Tianshan are still poorly understood.In this paper I describe the geological characteristics of iron deposits in the eastern Tianshan,and discuss their genetic types as well as metallogenic-tectonic settings,Iron deposits are preferentially distributed in central and southern parts of the eastern Tianshan.The known iron deposits in the eastern Tianshan show characteristics of magmatic Fe-Ti-V(e.g.,Weiya and Niumaoquan),sedimentary-metamorphic type(e.g.,Tianhu),and iron skarn(e.g.,Hongyuntan).In addition to the abovementioned iron deposits,many iron deposits in the eastern Tianshan are hosted in submarine volcanic rocks with well-developed skarn mineral assemblages.Their geological characteristics and magnetite compositions suggest that they may belong to distal skarns.SIMS zircon U-Pb analyses suggest that the Fe-Ti oxide ores from Niumaoquan and Weiya deposits were formed at 307.7±1.3 Ma and 242.7±1.9 Ma,respectively.Combined with available isotopic age data,the timing of Fe mineralization in the eastern Tianshan can be divided into four broad intervals:Early Ordovician-Early Silurian(476-438 Ma),Carboniferous(335-303 Ma),Early Permian(295-282 Ma),and Triassic(ca.243 Ma).Each of these episodes corresponds to a period of subduction,post-collision,and intraplate tectonics during the Paleozoic and Mesozoic time.