Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

中国大陆科学钻探主孔0～4500米变质岩石锆石中保存的超高压矿物包体

中国大陆科学钻探主孔0-4500米的岩心主要由榴辉岩、斜长角闪岩、副片麻岩、正片麻岩以及少量的超基性岩所组成。岩相学研究结果表明,榴辉岩的围岩普遍经历了强烈角闪岩相退变质作用的改造,峰期超高压变质的矿物组合已完全被后期退变质过程中角闪岩相矿物组合所替代。采用激光拉曼技术,配备电子探针和阴极发光测试,发现主孔224件岩心中有121件(包括榴辉岩、斜长角闪岩、副片麻岩和正片麻岩)样品的锆石中普遍隐藏以柯石英为代表的超高压矿物包体,且不同岩石类型锆石中所保存的超高压矿物包体组合存在明显差异。(含多硅白云母)金红石石英榴辉岩锆石中保存的典型超高压包体矿物组合为柯石英 石榴石、柯石英 石榴石 绿辉石 金红石和柯石英 多硅白云母 磷灰石。黑云绿帘斜长角闪岩锆石中保存的超高压矿物组合为柯石英 石榴石 绿辉石、柯石英 石榴石 多硅白云母和柯石英 绿辉石 金红石,与榴辉岩所保存的超高压矿物组合十分相似,表明该类斜长角闪岩是由超高压榴辉岩在构造折返过程中退变质而成。在副片麻岩类岩石,如石榴绿帘黑云二长片麻岩锆石中,代表性的超高压包体矿物组合为柯石英 多硅白云母和柯石英 石榴石等;而在石榴黑云角闪钠长片麻岩锆石中,则保存柯石英 硬玉 石榴石 磷灰石、柯石英 硬玉 多硅白云母 磷灰石和柯石英 石榴石 磷灰石等超高压矿物包体。在正片麻岩锆石中,标志性的超高压矿物包体为柯石英、柯石英 多硅白云母、柯石英 蓝晶石 磷灰石和柯石英 蓝晶石 榍石等。此外,在南苏鲁东海至临沭一带的地表露头以及一系列卫星孔岩心的锆石中,也普遍发现以柯石英为代表的标志性超高压矿物包体,表明在南苏鲁地区由榴辉岩及其围岩的原岩所组成的巨量陆壳物质(方圆>5000km2,厚度超过4.5km)曾整体发生深俯冲,并经历了超高压变质作用。该项研究对于重塑苏鲁-大别超高压变质带俯冲-折返的动力学模式有着重要的科学意义。     

http://www.sciencedirect.com/science/article/pii/S1674987112000564

High-pressure（HP） granulites widely occur as enclaves within tonalite-trondhjemitegranodiorite （TTG） gneisses of the Early Precambrian metamorphic basement in the Shandong Peninsula, southeast part of the North China Craton（NCC）.Based on cathodoluminescence（CL）,laser Raman spectroscopy and in-situ U-Pb dating,we characterize the zircons from the HP granulites and group them into three main types:inherited（magmatic） zircon,HP metamorphic zircon and retrograde zircon.The inherited zircons with clear or weakly defined magmatic zoning contain inclusions of apatites,and ²⁰⁷Pb/²⁰⁶Pb ages of 2915—2890 Ma and 2763—2510 Ma,correlating with two magmatic events in the Archaean basement. The homogeneous HP metamorphic zircons contain index minerals of high-pressure metamorphism including garnet,clinopyroxene.plagioclase,quartz,rutile and apatite,and yield ²⁰⁷Pb/²⁰⁶Pb ages between 1900 and 1850 Ma,marking the timing of peak HP granulite fades metamorphism.The retrograde zircons contain inclusions of orthopyroxene.plagioclase.quartz,apatite and amphibole.and yield the youngest ²⁰⁷Pb/²⁰⁶Pb ages of 1840—1820 Ma among the three groups,which we correlate to the medium to low-pressure granulite fades retrograde metamorphism.The data presented in this study suggest subduction of Meso- and Neoarchean magmatic protoliths to lower crust depths where they were subjected to HP granulite facies metamorphism during Palaeoproterozoic（1900—1850 Ma）.Subsequently, the HP granulites were exhumated to upper crust levels,and were overprinted by medium to low-pressure granulite and amphibolite facies retrograde event at ca.1840—820 Ma.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

东昆仑早古生代变质演化:来自浪木日石榴斜长角闪岩的记录SCIEI北大核心CSCD

东昆仑造山带近年来被厘定为早古生代高压-超高压变质带。带内广泛出露早古生代的中-高级变质基性岩,这些岩石记录了不同的变质温压和多期的变质年龄,是反演和制约东昆仑早古生代变质演化的重要样品。本文选取东昆仑浪木日地区的石榴斜长角闪岩为研究对象,开展了变质岩石学及锆石年代学研究。石榴斜长角闪岩呈团块状出露在黑云二长片麻岩中,主要组成矿物为石榴子石、角闪石、斜长石、透辉石和石英,含少量黑云母、绿泥石、金红石、钛铁矿和榍石。石榴子石变斑晶的核部含有绿帘石、角闪石、斜长石、金红石和石英包裹体,其成分从核部到边部,锰铝榴石逐渐降低、钙铝榴石和Mg/(Mg+Fe;)比值逐渐升高,为进变质作用形成的环带。岩石中的矿物结构关系和成分特征显示其经历了进变质、峰期变质和退变质三个阶段的变质演化,变质温压分别为:T≈610℃和P≈6.5kbar、T≈700℃和P≈10.5kbar以及T≈650℃和P≈4.5kbar。这三阶段的变质作用构成顺时针的变质P-T轨迹,指示岩石经历进变质升温升压至峰期阶段,随后经历近等温降压的退变质阶段。同时该P-T轨迹特征表明岩石形成于俯冲-碰撞的构造背景。对石榴斜长角闪岩中的锆石进行SIMS U-Pb定年,得到492.8±5.1Ma的谐和年龄。锆石的形态特征与典型的变质锆石一致,其内包裹的石榴子石、角闪石和斜长石组合与岩石的峰期矿物组合一致。因此,锆石在峰期变质阶段结晶,所测年龄~493Ma为角闪岩相峰期变质年龄。本文研究的石榴斜长角闪岩与该区高压-超高压榴辉岩在野外产状、P-T轨迹和变质年龄等方面密切相关,暗示ca.490Ma是该区高压-超高压变质作用的一个重要时间节点。石榴斜长角闪岩和榴辉岩之间的变质差异,表明东昆仑早古生代经历了多阶段的变质作用,不同岩石记录了原特提斯洋俯冲-碰撞过程的不同阶段。本文获得的变质P-T轨迹和变质年龄可为进一步探究东昆仑早古生代高压-超高压变质作用提供限定。     

苏鲁地体超高压矿物的三维空间分布

采用激光拉曼技术,配备电子探针和阴极发光测试,确认苏鲁地体大多数花岗质片麻岩,所有类型片麻岩、斜长角闪岩、蓝晶石英岩和大理岩的锆石中均隐藏以柯石英为代表的超高压包体矿物组合。其中花岗质片麻岩典型超高压包体矿物为柯石英±多硅白云母;副片麻岩为柯石英+石榴子石+绿辉石、柯石英±石榴子石+硬玉+多硅白云母+磷灰石、柯石英+多硅白云母±磷灰石;斜长角闪岩为柯石英+石榴子石+绿辉石±金红石;蓝晶石英岩为柯石英+蓝晶石+金红石+磷灰石、柯石英+蓝晶石+多硅白云母+金红石;大理岩为柯石英+透辉石、柯石英+橄榄石。表明苏鲁地体由榴辉岩及其围岩所组成的巨量陆壳物质曾普遍发生深俯冲,并经历了超高压变质作用。锆石的矿物包体分布特征及相应的阴极发光图像研究表明,在同一样品中,锆石的成因特征存在明显差异。有的锆石显示继承性(碎屑)锆石的核(core)、超高压变质的幔(mantle)和退变质的边(rim);有的锆石则具有超高压的核、幔和退变质的边;而有的锆石却记录了深俯冲的核、超高压的幔和退变质的边。标志着苏鲁超高压变质带各类岩石副矿物锆石均具有十分复杂的结晶生长演化历史。因此,在充分研究锆石中矿物包体性质、分布特征以及相应阴极发光图像的基础上,采用SHRIMP离子探针技术,在锆石晶体的不同     

Genetic Mechanism of Mineral Inclusions in Zircons from the Khondalite Series, Southeastern Inner Mongolia

The early Precambrian khondalite series is widely distributed in the Jining-Zhuozi-Fengzhen-Liangcheng area, southeastern Inner Mongolia. The khondalite series mainly consists of sillimanite garnet potash feldspar (or two-feldspar) gneiss and garnet biotite plagioclase gneiss. These gneissic rocks have commonly experienced granulite-facies metamorphism. In zircons separated from sillimanite garnet potash feldspar gneisses, many mineral inclusions, including Sil, Grt, Ky, Kfs, Qtz and Ap, have been identified by the Laser Raman spectroscopy. Generally, prograde metamorphic mineral inclusion assemblages such as Ky + Kfs + Qtz + Ap and Ky + Grt + Kfs + Qtz are preserved in the core of zircon, while peak granulite-facies metamorphic minerals including Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap are identified in the mantle and rim of the same zircon. However, in some zircons are only preserved the peak metamorphic minerals such as Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap from core to ri     

超高压变质岩的放射性同位素体系及年代学方法

深刻理解同位素在超高压变质及退变质过程中的地球化学行为对获得超高压变质岩准确并有明确意义的年龄值是非常重要的。对 Sm-Nd,Rb-Sr 同位素体系,只有变质矿物同位素体系达到平衡才能给出精确有意义的等时线年龄。研究表明,与副变质岩互层的细粒榴辉岩的高压变质矿物之间,或者强退变质岩石的退变质矿物之间,其 Nd,Sr 同位素可以达到平衡;然而高压变质矿物与退变质矿物之间 Nd,Sr 同位素不平衡。由于全岩样品总是含有数量不等的退变质矿物,因此石榴石 全岩 Sm-Nd 法或多硅白云母 全岩 Rh-Sr 法将有可能给出无地质意义的年龄。通常低温榴辉岩的高压变质矿物之间存在Nd 同位素不平衡。超高压变质岩多硅白云母所含过剩 Ar 主要源于榴辉岩原岩中角闪石在变质分解时释放出来的放射成因 Ar。因此,不含榴辉岩的花岗片麻岩多硅白云母基本不含过剩 Ar。对变质锆石成因的准确判断是正确理解锆石 U-Ph 年龄意义的关键。本文对不同成因锆石的判别标志及年龄意义做了总结,并指出将阴极发光图形,锆石痕量元素组成及矿物包裹体鉴定相结合是进行锆石成因鉴定的有效方法。高压变质或退变质增生锆石组成单一,是理想变质定年对象。然而变质重结晶锆石域常是重结晶锆石和继承晶质锆石的混合区,因而给出混合年龄。只有完全变质重结晶锆石才能给出准确变质时代。     

Stages and conditions of metamorphism of mafic granulites in the Early Precambrian complex of the Angara–Kan terrane (southwestern Siberian Craton)

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~ 900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~ 1.77 Ga) and two-pyroxene granulites (~ 1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~ 1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of ¹⁷⁶Hf/¹⁷⁷Hf values for the core and rims. The positive ε_Hf values (up to + 6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~ 1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).     

SHRIMP U-Pb zircon dating from Sulu-Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh-pressure and retrograde metamorphic ages

Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu‐Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh‐pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark‐luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white‐luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey‐luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark‐luminescent CL images, and contain low‐P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP‐MS trace‐element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U‐Pb dating on these zoned zircon identified four discrete ²⁰⁶Pb/²³⁸U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid‐crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr^?1 and 6.4 km Myr^?1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu‐Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.     

Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust

Zircon from a lower crustal metapelitic granulite (Val Malenco, N‐Italy) display inherited cores, and three metamorphic overgrowths with ages of 281 ± 2, 269 ± 3 and 258 ± 4 Ma. Using mineral inclusions in zircon and garnet and their rare earth element characteristics it is possible to relate the ages to distinct stages of granulite facies metamorphism. The first zircon overgrowth formed during prograde fluid‐absent partial melting of muscovite and biotite apparently caused by the intrusion of a Permian gabbro complex. The second metamorphic zircon grew after formation of peak garnet, during cooling from 850 °C to c. 700 °C. It crystallized from partial melts that were depleted in heavy rare earth elements because of previous, extensive garnet crystallization. A second stage of partial melting is documented in new growth of garnet and produced the third metamorphic zircon. The ages obtained indicate that the granulite facies metamorphism lasted for about 20 Myr and was related to two phases of partial melting producing strongly restitic metapelites. Monazite records three metamorphic stages at 279 ± 5, 270 ± 5 and 257 ± 4 Ma, indicating that formation ages can be obtained in monazite that underwent even granulite facies conditions. However, monazite displays less clear relationships between growth zones and mineral inclusions than zircon, hampering the correlation of age to metamorphism. To overcome this problem garnet–monazite trace element partitioning was determined for the first time, which can be used in future studies to relate monazite formation to garnet growth.     

Early Carboniferous HP metamorphism in the Hida Gaien Belt,Japan: Implications for the Palaeozoic tectonic history of proto‐Japan

We report two new eclogite localities (at Kanayamadani and Shinadani) in the high‐P (HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin‐shaped P–T path from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. Peak P–T conditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation‐exchange thermobarometry, Ti‐in‐zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory‐zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto‐Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench‐fill sediments. In addition, subduction‐related magmatism c. 500–400 Ma is recorded in the crust below proto‐Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc‐trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto‐Japan.     

Age and duration of ultrahigh-temperature metamorphism in the Anápolis–Itauçu Complex, Southern Brasília Belt, central Brazil – constraints from U–Pb geochronology, mineral rare earth element chemistry and trace-element thermometry

Integrated petrographic and chemical analysis of zircon, garnet and rutile from ultrahigh‐temperature (UHT) granulites in the Anápolis–Itauçu Complex, Brazil, is used to constrain the significance of zircon ID‐TIMS U–Pb geochronological data. Chondrite‐normalized rare earth element (REE) profiles of zircon cores have positive‐sloping heavy‐REE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively sloping heavy‐REE patterns. However, in one sample, a core of zircon interpreted as having formed prior to garnet crystallization and a metamorphic zircon formed within microstructures involving garnet breakdown both display elevated heavy‐REE (and Y) with positive‐sloping patterns. D_REE(zrc/grt) partition coefficients suggest an approximation to equilibrium between zircon and garnet cores, although progressive enrichment in heavy REE towards garnet rims occurs in two of the samples investigated. Titanium‐in‐zircon thermometry indicates zircon growth during both the prograde and post‐peak evolution, but not at peak temperatures of the UHT metamorphism. By contrast, zirconium‐in‐rutile thermometry of inclusions armoured by garnet records crystallization temperatures, based on the upper end of the interquartile range of the data, of ～890 to 870 °C and maximum temperature around 980 °C, indicating prograde and retrograde growth close to and after peak conditions. Rutile located in retrograde microstructures records crystallization temperatures of 850 to 820 °C. Rutile intergrown with ilmenite and included within orthopyroxene, which is associated with exsolved zircon, records temperatures ～760 °C, consistent with Ti‐in‐zircon crystallization temperatures. ID‐TIMS U–Pb geochronological data from two of the four samples investigated define upper intercept ages of 641.3 ± 8.4 Ma (MSWD 0.91) and 638.8 ± 2.5 Ma (MSWD 1.03) that correlate with periods of zircon growth along the prograde segment of the P–T path. Individual zircon U–Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of c. 15 Myr for the UHT event. This period is interpreted as recording modest thickening of hot backarc lithosphere located behind the Arenópolis Arc at the edge of the São Francisco Craton consequent upon terminal collision of the Parána Block with the arc during the amalgamation of West Gondwana.     

山东半岛高压麻粒岩中锆石的U-Pb定年及其地质意义

在山东半岛早前寒武纪变质基底中,高压麻粗岩常常呈透镜体或不规则脉状体广泛分布于TTG片麻岩之中.锆石中矿物包体激光拉曼测试、阴极发光图像分析及原位U-Pb定年结果表明,山东丰岛高压麻粒岩中锆石成因十分复杂,可划分为3种类型:第一类锆石显示明显或弱的岩浆结晶环带,部分锆石保存磷灰石(Ap)等矿物包体,U-Pb定年结果显示...     

Behavior of zircon in the upper-amphibolite to granulite facies schist/migmatite transition, Ryoke metamorphic belt, SW Japan: constraints from the melt inclusions in zircon

Behavior of zircon at the schist/migmatite transition is investigated. Syn-metamorphic overgrowth is rare in zircon in schists, whereas zircon in migmatites has rims with low Th/U that give 90.3 ± 2.2 Ma U–Pb concordia age. Between inherited core and the metamorphic rim, a thin, dark-CL annulus containing melt inclusion is commonly developed, suggesting that it formed contemporaneous with the rim in the presence of melt. In diatexites, the annulus is further truncated by the brighter-CL overgrowth, suggesting the resorption and regrowth of the zircon after near-peak metamorphism. Part of the zircon rim crystallized during the solidification of the melt in migmatites. Preservation of angular-shaped inherited core of 5–10 μm in zircon included in garnet suggests that zircon of this size did not experience resorption but developed overgrowths during near-peak metamorphism. The Ostwald ripening process consuming zircon less than 5–10 μm is required to form new overgrowths. Curved crystal size distribution pattern for fine-grained zircons in a diatexite sample may indicate the contribution of this process. Zircon less than 20 μm is confirmed to be an important sink of Zr in metatexites, and ca. 35-μm zircon without detrital core are common in diatexites, supporting new nucleation of zircon in migmatites. In the Ryoke metamorphic belt at the Aoyama area, monazite from migmatites records the prograde growth age of 96.5 ± 1.9 Ma. Using the difference of growth timing of monazite and zircon, the duration of metamorphism higher than the amphibolite facies grade is estimated to be ca. 6 Myr.     

Metamorphic Evolution of High‐pressure Politic Granulite from the Ailaoshan Orogenic Belt,Northeastern Margin of Indochina Block,West Yunnan,China

Ailaoshan orogenic belt located at the northeastern margin of the Indochina block, southeastern Tibet, was formed by subduction and collision between the Indochina and South China blocks in Triassic and slip shearing resulted from the extrusion of the Indochina block in Cenozoic. The high‐pressure pelitic granulite is located at the southeastern margin of the Ailaoshan metamorphic belt, occurs as a slice of about 500～700m in thickness, consists of garnet, sillimanite, feldspar, biotite and quartz with accessory of kyanite, sapphirine, spinel, rutile, ilmenite, zircon and apatite. The petrography and mineral chemistry show that the high‐pressure pelitic granulite had suffered three stages of metamorphism: 1) the prograde metamorphism recorded by the mineral assemblage of garnet, kyanite, feldspar, biotite and rutile; 2) the peak metamorphism shown by the mineral assemblage of garnet, sillimanite, sapphirine, ternary feldspar, K‐feldspar, plagioclase, biotite, spinel, quartz, rutile and zircon mantle; 3) the retrograde metamorphism recorded by the mineral assemblage of biotite, muscovite, plagioclase, quartz and zircon rim. Zircon SHRIMP U‐Pb dating indicates that the protolith of the pelite granulite was deposited before 336 Ma, the prograde to peak metamorphism occurred at P‐T conditions of ≥10.4 kbar at 850～919 °C in 235 Ma, and the retrograde metamorphism occurred at the P‐T condition of 3.5～3.9 kbar at 572～576 °C until to 33 Ma. They are consistent with the times of Indochina separated from Gondwanaland during late Paleozoic, the amalgamation of the south China and Indochina blocks during the Triassic, and the sinistral slip‐shearing since the Early Cenozoic respectively. It is inferred that that the sedimentary rock was subducted to the lower continental crust (30 km) and suffered granulite‐facies metamorphism due to the collision during Indosinian, then exhumed quickly to middle‐upper crust (10–12km) and superimposed retrograde metamorphism since the Cenozoic.     

In situ U–Pb dating and trace element analysis of zircons in thin sections of eclogite: Refining constraints on the ultra high-pressure metamorphism of the Sulu terrane,China

In situ U–Pb dating and trace element analysis of zircons, combined with a textural relationship investigation in thin section, is a powerful tool to constrain the ultra high-pressure stage of high-grade metamorphism. Two types of zircon grains have been identified in thin sections of a retrograde eclogite from the main hole of the Chinese Continental Scientific Drill project in the Sulu UHP terrane. Type 1 zircon grains occur as inclusions in fresh garnet and omphacite, and Type 2 zircon grains were found in symplectite around omphacite. The fresh rims of Type 1 zircons and mantles of a few Type 2 zircons exhibit remarkably lower REE, Y, Nb and Ta contents than the inherited zircon cores, suggesting coeval growth with garnet, rutile and apatite during UHP metamorphism. These may have formed in the UHP metamorphism and survived retrograde metamorphism. The weighted average ²⁰⁶Pb/²³⁸U age of these zircon domains (230 ± 4 Ma, 2σ) agrees well with the published age of coesite-bearing zircon separates (230 ± 1 Ma, 2σ), suggesting that the peak UHP metamorphism in the Sulu terrane may have occurred at ~ 230 Ma.Zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism. Together, they provide a younger weighted average ²⁰⁶Pb/²³⁸U age of 209 ± 4 Ma (2σ). These retrograde zircon domains have similar REE compositions to the ~ 230 Ma UHP zircon domains. These observations imply that the ~ 209 Ma zircon domains could have formed by fluid activity-associated alterations in the amphibolite-facies metamorphism, which could have resulted in the complete loss of Pb but not REEs in these domains.     

Proterozoic anticlockwise P–T path of the Lewisian Complex of South Harris, Outer Hebrides, NW Scotland

The Leverburgh Belt and South Harris Igneous Complex in South Harris (northwest Scotland) experienced high-pressure granulite facies metamorphism during the Palaeoproterozoic. The metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic, quartzofeldspathic and mafic gneisses, especially in pelitic gneisses from the Leverburgh Belt: (1) some coarse-grained garnet in the pelitic gneiss includes biotite and quartz in the inner core, sillimanite in the outer core, and is overgrown by kyanite at the rims; (2) garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims; (3) the Al^VI/Al^IV ratio of clinopyroxene from mafic gneiss increases from core to rim; (4) retrograde reaction coronas of cordierite and hercynite+cordierite are formed between garnet and kyanite, and orthopyroxene+cordierite and orthopyroxene+plagioclase reaction coronas develop between garnet and quartz; (5) a P–T path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet+sillimanite and garnet+sillimanite+hercynite with increasing temperature; and (6) in sheared and foliated rocks, hydrous minerals such as biotite, muscovite and hornblende form a foliation, modifying pre-existing textures. The inferred metamorphic history of the Leverburgh Belt is divided into four stages, as follows: (M1) prograde metamorphism with increasing temperature; (M2) prograde metamorphism with increasing pressure; (M3) retrograde decompressional metamorphism with decreasing pressure and temperature; and (M4) retrograde metamorphism accompanied by shearing. Peak P–T conditions of the M2 stage are 800±30 °C, 13–14 kbar. Pressure increasing from M1 to M2 suggests thrusting of continental crust over the South Harris belt during continent–continent collision. The inferred P–T path and tectonic history of the South Harris belt are different from those of the Lewisian of the mainland.     

大别山变质岩锆石微区稀土元素和Th,U特征

对进行过微区U-Pb定年和阴极发光成像研究的大别山辛店榴辉岩、双河榴辉岩、黄镇榴辉岩和双河硬玉石英岩中锆石,进行了微区核部与边部稀土元素测定.结合U-Pb年龄和CL图象,探讨了超高压变质过程中稀土元素从原岩锆石到变质锆石的变化.结果表明,原岩锆石和变质锆石有很不相同的稀土元素含量,它取决于变质锆石是由变质重结晶还是变质增生作用形成及形成时间.一般说来,边部变质锆石比原岩锆石亏损稀土元素,特别是重稀土,并且有更低的Th/U比.变质锆石的稀土元素和Th/U比可以为变质锆石形成时的物理化学环境和变质锆石成因提供重要信息.     

Petrology and zircon U–Pb dating of well‐preserved eclogites from the Thongmön area in central Himalaya and their tectonic implications

The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1₍₁₎, (b) in the peak eclogite facies [stage M1₍₂₎], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1₍₁₎ is constrained to 19–20 kbar and 640–660°C and M1₍₂₎ occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene.