青藏高原羌塘中部榴辉岩Ar-Ar定年

羌塘榴辉岩产于红脊山-双湖高压变质带中段的片石山地区,是目前青藏高原内部唯一的榴辉岩产地.榴辉岩多硅白云母40 Ar-39 Ar年龄tp=219.3±1.5 Ma;榴辉岩围岩的多硅白云母40 Ar-39 Ar年龄tp=217.2±1.8 Ma,定年结果表明,榴辉岩与围岩经历了相同的高压变质作用,与红脊山-双湖地区蓝闪片岩的高压变质时间是一致的.根据已有高压变质年代学资料,从羌塘的红脊山到双湖、那曲地区的巴青以北、昌都的酉西,直到滇西地区,断续分布长达2000余公里的高压变质带为同一期构造事件的产物,是龙木错-双湖板块缝合带向南延伸到滇西的重要依据.榴辉岩的定年数据进一步揭示,冈瓦纳板块与欧亚(扬子)板块的主体碰撞时间在220 Ma左右,同时也是古特提斯洋消亡的时间记录.     

Subduction channel fluid-rock interaction:Indications from rutile-quartz veins within eclogite from the Yuka terrane,North Qaidam orogen

High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 431
3 Ma,which is consistent with the 432
2 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.     

Petrogenesis of the Maowu pyroxenite–eclogite body from the UHP metamorphic terrane of Dabieshan: chemical and isotopic constraints

The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.708) and inhomogeneous negative _Nd(T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ¹⁸O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial ⁸⁷Sr/⁸⁶Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget.

Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level.     

东昆仑西段夏日哈木-苏海图早古生代榴辉岩的发现及意义

1:25万区域地质调查在东昆仑西段昆中断裂以北的北昆仑变质基底中发现了呈构造透镜分布的榴辉岩及其退变质的榴闪岩,沿小灶火-苏海图-夏日哈木-拉宁灶火断续延伸长达20km.榴辉岩经历了强烈的退变质作用,大部分已退变为榴闪岩,由峰期变质矿物组合石榴石+绿辉石+金红石+钛铁矿和后期退变矿物组合普通角闪石(绿色)+斜长石组成,石榴石边缘发育似针状普通角闪石及微粒状斜长石组成的后合成晶结构,形成宽0.15~1cm的"白眼圈"结构.石榴石成分为Alm_52-53And_4.2Spess_0.7-1.2Gros_21.7-22Pyrope_19-20,绿辉石中硬玉分子量30.14mol%~37.7mol%.估算榴辉岩相变质温压条件P≈20kbar、T≈660~700℃,榴闪岩相退变温压条件P≈7kbar、T≈550℃,岩石经历了明显的降温降压过程.榴辉岩中锆石阴极发光图像显示大部分具有扇形分带或"杉树叶"结构,17颗锆石Th/U极低,在0.007~0.09之间,其(LA-ICP-MS)U-Pb年龄411.1±1.9Ma代表了榴辉岩相峰期变质年龄,结合区域构造岩浆事件,确定本次高压变质的峰期时代为晚志留-早泥盆世,其形成可能与大规模的陆-陆碰撞事件有关,这一发现为研究东昆仑造山带的构造演化具有重要意义.     

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.     

南天山榆树沟高压麻粒岩地体多期变质定年研究

通过详细的矿物学及岩石学研究,特别是对角闪石的系统研究,确定榆树沟麻粒岩地体至少经历过高压麻粒岩相、中压麻粒岩相、角闪岩相和绿片岩相四期变质作用的改造。总结了各期变质作用的期次和特点。在此基础上,采用~(40)Ar-~(39)Ar同位素定年获得368.2±4.8Ma坪年龄和360±10Ma等时线年龄。采用Sm-Nd同位素矿物等时线定年获得Gra+Pl+Ⅰlm+全岩的等时线年龄为310±5Ma。结合已发表的定年结果综合分析认为,前者可代表榆树沟地体峰期变质-高压麻粒岩相变质作用年龄;后者为峰期后经受中压麻粒岩相变质变形的叠加改造年龄。讨论了多期变质作用中同位素的均一化和封闭温度问题。     

Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway

Silica-deficient sapphirine-bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south-east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent.
Fe²⁺/(Fe²⁺+ Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite.
Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene-sillimanite-sapphirine-cordierite-corundum developed during a high-pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium-pressure granulite facies generated various sapphirine-bearing assemblages. The diagnostic assemblage of this stage is sapphirine-cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.
The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H₂O-poor conditions.     

藏西北戈木日榴辉岩岩石学特征及其构造意义

戈木日榴辉岩产出于羌塘中部龙木错-双湖古板块缝合带内,是羌塘中部高压变质岩带的重要组成部分,前人多认为其系洋壳冷俯冲的产物。该榴辉岩的矿物组合可分为四期:第一期为含Fe~(3+)的钙钠闪石+钠云母+钠长石+绿泥石+石英+富含LREE、Th和U的褐帘石+榍石±富Mn的石榴子石。富Mg的石榴子石+贫Fe的绿辉石+不含微量元素的黝帘石+石英+钠云母+金红石+黄铁矿则为第二期矿物组合。第三期为退变质组合,包括钠长石、绿泥石、榍石和阳起石。第四期为磁铁矿、新生绿帘石和针状钛铁矿。热力学半定量模拟表明,石榴子石核部形成于T=644℃,P=13.0kbar,lgfO_2=-16.3~-16.4,而石榴子石幔部则形成于T=695℃,P=16.6kbar,lgfO_2=-19.0~-19.5,表明该榴辉岩属中温榴辉岩,而不是前人所言的冷榴辉岩,这反映了羌塘中部古特提斯洋盆的构造演化历史比较复杂,不仅有冷俯冲的洋壳,还有中温俯冲的洋壳。该榴辉岩在进变质过程中经历了还原作用,早期含Fe~(3+)的钙钠闪石和褐帘石等形成于高氧逸度之下的含水矿物,转变为形成于低氧逸度条件下的石榴子石和贫Fe的绿辉石、黝帘石等矿物,释放出水和轻REE、Th、U、Ba等微量元素。折返阶段新生成的矿物均不含有这些微量元素,表明早期阶段释放的微量元素很可能离开了俯冲板片,随流体进入上覆地幔楔。本文的研究进一步证实了进变质与还原作用共同促进了物质从俯冲板片中活化、转移至上覆地幔楔,这有助于理解具有独特地球化学特征的岛弧岩浆的起源。     

Partial transformation of gabbro to coesite-bearing eclogite from Yangkou, the Sulu terrane, eastern China

An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region of China consists of blocks of eclogite facies metagabbro, metagranitoid, ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss. A gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra-high-pressures was identified in a single 30 m block; metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins. The metagabbro contains an igneous assemblage of Pl+Aug+Opx+Qtz+Bt+Ilm/Ti-Mag; it shows relict magmatic textures and reaction coronas. Fine-grained garnet developed along boundaries between plagioclase and other phases; primary plagioclase broke down to Ab+Ky+Ms+Zo±Grt±Amp. Augite is rimmed by sodic augite or omphacite, whereas orthopyroxene is rimmed by a corona of Cum±Act and Omp+Qtz layers or only Omp+Qtz. In transitional rocks, augite and orthopyroxene are totally replaced by omphacite, and the lower-pressure assemblage Ab+Ky+Phn+Zo+Grt coexists with domains of Omp (Jd_70–73)+Ky±Phn in pseudomorphs after plagioclase. Both massive and weakly deformed coesite-bearing eclogites contain Omp+Ky+Grt+Phn+Coe/Qtz+Rt, and preserve a faint gabbroic texture. Coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz; some intergranular coesite and quartz pseudomorphs after coesite also occur. Assemblages of the coronal stage, transitional and UHP peak occurred at about 540±50 °C at c. 13 kbar, 600–800 °C at ≥15–25 kbar and 800–850 °C at >30 kbar, respectively. Garnet from the coronal- through the transitional- to the eclogite-stage rocks show a decrease in almandine and an increase in grossular±pyrope components; garnet in low-grade rocks contains higher MnO and lower pyrope components. The growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex; the application of garnet zoning to estimate P–T should be carried out with caution. Some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries; these quartz aggregates, as well as other primary and low-P phases, persisted metastably at UHP conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde P–T evolution.     

南大别角闪岩相退变质与热液活动:朱家冲榴辉岩与闪石脉 Ar-Ar年代学研究

南大别山朱家冲榴辉岩中产有一条岩脉。电子探针分析结果表明,朱家冲榴辉岩中较大颗粒的白云母和岩脉中的白云母均为钠云母;而榴辉岩中细小颗粒的白云母则为多硅白云母;岩脉中的蓝绿色柱状矿物为闪石(amphibole)。采用激光阶段加热技术和真空阶段击碎技术,对朱家冲冷榴辉岩体和岩脉进行了Ar-Ar定年研究。选自榴辉岩和闪石脉的两个钠云母激光加热Ar-Ar年龄均约为200Ma,选自闪石脉的闪石真空击碎流体包裹体Ar-Ar年龄也约为200Ma,代表了热液活动钠云母化发生的时间;而提取流体包裹体后的闪石矿物粉末阶段加热Ar-Ar等时线年龄约为243Ma,则可能代表了朱家冲榴辉岩角闪岩相退变质作用发生的最小年龄。     

Geochemistry of eclogites from the Dabie–Sulu terrane, eastern China: New insights into protoliths and trace element behaviour during UHP metamorphism

The major and trace element compositions of nine eclogites from the Dabie–Sulu ultrahigh pressure (UHP) metamorphic terrane in eastern China were determined for both whole rock and the main constituent minerals, garnet and clinopyroxene. The results indicate that the eclogite protoliths originated from a basaltic magma, which formed in a continental setting as shown by isotopic and immobile element data. Based on the garnet REE characteristics, the eclogites can be roughly divided into two groups. Group 1 has LREE enrichment with no Eu anomaly for whole rock, and smooth LREE depletion but HREE enrichment pattern for garnet, whereas group 2 shows a depletion of LREE with a pronounced positive Eu anomaly and flat HREE pattern for both whole rock and garnet. From these features, we suggest that the protoliths for group 2 are Fe–Ti–gabbros with relatively high cumulus plagioclase and Fe–Ti oxide, whereas the group 1 eclogites are probably from basalts. Therefore, the unusual garnet REE pattern observed in group 2 can be considered as an important signature for identifying gabbro protoliths for eclogites. The identification of gabbro protoliths from the eclogites in the Dabie–Sulu terrane provides evidence for Neoproterozoic rift magmatism in the northern margin of the Yangtze craton. During ultrahigh pressure metamorphism in the Dabie–Sulu terrane, LILEs (including Ba, Rb, Th, U, K) had high mobility, but REEs and HFSEs were immobile, and trace element distribution equilibrium was approached between garnet and clinopyroxene. An estimate of mass balance indicates that garnet and clinopyroxene host the majority of HREEs and Y, and clinopyroxene is a significant host for Sr, but minor and accessory minerals predominantly account for LREEs, Th, U, and Zr.     

北大别超高压榴辉岩的快速折返与缓慢冷却过程

岩石学研究表明,北大别超高压榴辉岩经过了超高压和高压榴辉岩相变质作用以及麻粒岩相叠加和角闪岩相退变质作用.其中,高压麻粒岩相和角闪岩相变质阶段形成的后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩在峰期超高压变质作用之后又经历了一个缓慢冷却过程.超高压岩石折返期间的缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压证据的重要原因.因此,本文为大别山不同超高压岩片的差异折返模型的建立提供了新的证据.     

大别超高压变质带的进变质、超高压和退变质时代的准确限定:以双河大理岩中榴辉岩锆石SHRIMP U-Pb定年为例

在大别超高压变质带的双河地区存在一种特殊类型的榴辉岩,该类榴辉岩主要以似层状、条带状以及不规则透镜体赋存于大理岩中。矿物组成主要为石榴石、绿辉石以及少量的金红石、白云石、菱镁矿等。沿石榴石和绿辉石边缘常退变为角闪石+斜长石等,有的岩石完全退变为斜长角闪岩。激光拉曼和阴极发光综合分析表明,该类榴辉岩中的锆石可划分为两种类型:继承性碎屑锆石和变质锆石。继承性碎屑锆石十分少见,阴极发光图象具有明显的双层结构,即强发光的核和弱发光的边,核部和边部的包体矿物分别为Pl+Ap和Qtz+Pl。SHRIMP U-Pb定年结果表明。继承性碎屑锆石核部记录的207Pb／206Pb年龄为2701±15Ma,Th/U比值明显偏高为1．05,稀土元素配分模式显示重稀土明显富集,具有典型岩浆结晶锆石的特点;边部记录的207Pb／206Pb年龄为1801±12Ma-1753±22Ma,Th／U比值则明显偏低,为0．19-0．22之间,稀土元素配分模式显示重稀土相对平坦,具有典型变质锆石的特点。上述特征表明该类继承性碎屑锆石可能来源于太古代的基底,并经历了早元古代变质热事件的改造。新生的变质锆石无论是矿物包体还是阴极发光图象均与继承性碎屑锆石存在明显差异。有的变质增生锆石具有弱发光的核(阴极发光图象呈灰色)和强发光的边(阴极发光图象呈白色)。核部包体矿物组合为Qtz +Grt+Omp+Phe+Dol+Ap,具有典型石英榴辉岩相矿物组合特征,而边部则保存含柯石英的超高压包体矿物组合Coe+Grt +Omp+Mgs+Arg+Ap,表明该类锆石的核部和边部分别形成于俯冲进变质阶段和超高压变质阶段。另一部分变质增生锆石具有强发光的核(阴极发光图象呈白色)和弱发光的边(阴极发光图象呈黑色)。核部保存的标志性超高压包体矿物组合为Coe+Grt+Omp+Mgs+Arg+Ap,边部则保存Qtz+Cal等退变矿物组合,有的则缺乏矿物包体,表明该类锆石自超高压变质阶段开始生长,并经历了后期退变质作用的改造。从不同微区矿物包体组合的性质及其转变特征可以明显看出,自石英榴辉岩相进变质阶段到超高压峰期变质阶段存在下列转变反应:Qtz→Coe和Dol→Mgs+Arg;而自超高压峰期变质阶段到后期退变质阶段则存在下列退变反应:Coe→Qtz和Arg→Cal。SHRIMP U-Pb定年结果表明,含石英榴辉岩相矿物包体的锆石微区记录的206Pb／238U年龄为249-241Ma,加权平均值为244±4Ma,代表了深俯冲石英榴辉岩相进变质阶段的变质年龄;含柯石英等超高压矿物包体的锆石微区记录的206Pb／238U年龄为239-231Ma,加权平均值为234±3Ma,代表超高压阶段的峰期变质年龄;而含石英和方解石的退变边记录的206Pb／238U年龄为219-211Ma,加权平均值为216±6Ma,应代表后期折返阶段的角闪岩相退变质年龄。上述两类变质增生锆石微区的Th／U比值和稀土元素配分模式十分相似,Th／U比值变化于0．02- 0．18之间,稀土元素配分模式显示重稀土相对平坦,稀土元素总量明显低于继承性碎屑锆石,具有典型变质锆石的特点。根据锆石微区矿物包体的化学成分,采用Grt-Omp和Grt-Omp-Phe温压计,结合前人的变质反应实验资料的综合分析,确定榴辉岩的原岩在深俯冲过程的石英榴辉岩相进变质阶段的变质温压条件为T=588-668℃,P=1．7-1．8GPa;超高压峰期阶段的温压条件为T=784-849℃,P>5．5GPa;而构造折返过程中角闪岩相退变质阶段的温压条件为T=550-720℃,P=0．8～1．4GPa。由此可见,大别超高压变质岩的原岩——元古代(部分可能为太古代)的陆壳物质在早三叠纪发生俯冲至55- 60km深处,并经历了石英榴辉岩相变质作用。随后这些变质岩石继续深俯冲至165～175km的地幔深处,于中三叠纪发生了超高压变质作用,石英榴辉岩相矿物组合转变为超高压榴辉岩相矿物组合。最后这些超高压变质岩石发生构造折返,至晚三叠纪抬升到约30km的中下地壳深度,并经历了角闪岩相退变质作用的改造,超高压榴辉岩相矿物组合退变为角闪岩相矿物组合。由此推断,大别超高压变质带俯冲和折返速率分别为11-12km Myr-1和7．5-8．1km Myr-1。该项成果不仅确定了大别超高压变质地体的石英榴辉岩相进变质-超高压榴辉岩相峰期变质-角闪岩相退变质的年代谱系,而且对于重塑大别超高压变质地体的快速俯冲-折返的动力学模式有着重要的科学意义。     

Contrasting metamorphic conditions in the Neoproterozoic collision-related Nappes south of São Francisco Craton, SE Brazil

Metamorphic conditions are described for three major tectonic entities on the basis of geothermobarometry in a huge Neoproterozoic nappe complex that verges toward the southern border of the São Francisco craton. The uppermost Socorro-Guaxupé Nappe, represented by its granulite facies basal portion, yields a maximum temperature and pressure of 890 °C and 11 kbar. Its metamorphic evolution is consistent with heating at the base of the crust as a result of an abnormally high geothermal gradient, probably due to underplating by the lithospheric mantle. The underlying Três Pontas-Varginha Nappe yields two somewhat distinct P–T paths, both characterized by peak assemblages in the kyanite stability field. The basal kyanite-bearing granulites show higher peak pressure values (15 kbar at 840 °C) and a trajectory that continues in the kyanite stability field, whereas the upper sillimanite granulites show higher temperatures (880 °C at 13 kbar) and a steeper path toward the sillimanite stability field. Data for the Carmo da Cachoeira nappe reveal a steep trajectory, in which the elevated maximum pressure (18.5 kbar at 820 °C) was obtained from a garnet amphibolite that lies along its basal contact. The inverted metamorphic pattern previously observed across these sequences is confirmed by our thermobarometric data, which reveal that the highest temperatures were attained toward the top of the pile.     

柴北缘超高压带中锡铁山榴辉岩的变质时代

锡铁山地体位于柴北缘超高压变质带的中部, 是柴北缘超高压变质带的重要组成部分。该地体由花岗质片麻岩、泥质片麻岩和相对较少的榴辉岩透镜体组成。大部分榴辉岩都经历了不同程度的后生合晶和角闪岩相退化变质改造。虽然近年来进行了大量的锆石U-Pb年代学研究,但榴辉岩相高压-超高压变质的时代一直存在争议,并且以前对锡铁山榴辉岩相变质时代的认识一直与相邻的绿梁山、鱼卡和其东部的都兰等地区的超高压变质年龄有明显的冲突。本文通过锡铁山榴辉岩锆石U-Pb年代学的研究,获得榴辉岩相变质锆石的²⁰⁶Pb/²³⁸U加权平均年龄为433±3Ma,与鱼卡地区榴辉岩的形成年龄一致,代表大陆俯冲时期的高压-超高压变质年龄。该研究对进一步了解锡铁山榴辉岩地体的变质演化和大陆地壳深俯冲有重要意义。     

华北克拉通丰镇碳酸岩中榴辉岩捕虏体岩石学研究:现今板块体制古元古代开始启动证据

关于现今板块构造体制何时启动是目前地球科学研究的焦点问题。本文在原报道的古元古代丰镇火成碳酸岩中发现的榴辉岩捕虏体基础上,开展了详细的岩石学研究。该榴辉岩捕虏体分为两种类型:即相对富石榴石的Fz-2和贫石榴石的Fz-16,它们产于同一地点,且具有相同的矿物成分和结构构造特征。Xu et al.(2018)的研究表明该捕虏体具有1839±26Ma和1766±7Ma的独居石U-Th-Pb年龄且具有大洋辉长岩原岩的全岩成分特征。本文通过进一步的岩相学研究发现该榴辉岩至少经历了两期变质阶段:M1,角闪石/绿帘石-榴辉岩阶段;M2,硬柱石-榴辉岩阶段。具有放射状裂纹包裹特征的柯英石假象在石榴石变斑晶和基质绿辉石中以包体形式出现。以蓝晶石与黝帘石共存为特征的柱状硬柱石假象,也偶尔以包体形式存在于石榴石中。变斑晶石榴石分为富含包体的核部和比较干净的边部。石榴石从中心到边部具有明显的镁铝榴石含量增加和钙铝榴石含量降低的环带特征,通过相平衡模拟和等值线投图得到其温压范围为2.6~3.7GPa和655~670℃,记录了从M1到M2的近等温增压的进变质过程。通过石榴石边部-绿辉石-蓝晶石-石英的地质温压计计算得到温压条件为3.0GPa、734℃。金红石中的锆含量温度计也给出了相似的温度条件,即在2.6~3.7GPa压力时为601~685℃。石榴石边部的柯石英假象和硬柱石假象支持了M2硬柱石-榴辉岩阶段的存在,这表明丰镇古元古代榴辉岩可能是目前发现的世界上最古老的低温超高压变质岩。同时,我们得到该榴辉岩代表的进变质过程中的地温梯度为216±35℃/GPa,证明至少在~1.8Ga以来代表现今板块构造体制的板块冷俯冲作用就开始启动了。     

五台山地区原金刚库组中含蓝晶石组合的变质作用及其意义

本文对山西五台山地区与超镁铁质岩密切共生的含蓝晶石的各类片岩进行了研究。研究发现,在蓝晶石铝直闪石片岩中存在一种特殊的冠状体结构,即蓝晶石和铝直闪石被它们之间内圈的十字石十刚玉十绿泥石和外圈的堇青石所包绕。这种特殊的反应边结构,说明岩石曾处于较高的压力条件下（０．９～１．４ＧＰａ）,然后经历了明显的近等温的减压过程。其它类型的含蓝晶石片岩,也包含了高压变质矿物组合,并经历了同样的变质演化过程。这种演化历史与该区经历了古洋壳俯冲,消减,弧陆碰撞和伴随的迅速折返和抬升是密切有关的。     

中国大陆科学钻探（CCSD）榴辉岩磷灰石脉体中铁的氧化物、重晶石和独居石出溶物的发现及其意义

CCSD主孔榴辉岩等UHP岩石中存在团块状到不规则脉体状的磷灰石集合体，它们多与石英脉共生，显微镜下观察发现这些磷灰石脉体存在大量出溶物，电子探针和激光拉曼光谱仪联合测定显示磷灰石主要为氟磷灰石，其中出溶物主要有四类：A．不规则状的磁铁矿和赤铁矿的连生体；B．针状赤铁矿；C．板状到菱型的独居石；D．针状锶重晶石。A类出溶物的延长方向平行于磷灰石的C轴，长度介于10～50μm；B类主要沿磷灰石的C轴方向排列，宽仅为0．5～2μm，多数在1．5μm左右，长度变化较大，为6～50μm；C类独居石出溶体宽约6～10μm，多为6μm，长约50～75μm；D类锶重晶石主要与B类赤铁矿出溶物共生，宽多1．5μm左右，长约60～70μm。其中A，B和D类出溶物是在磷灰石中首次发现。四类出溶物的长轴和生长方向均基本平行于磷灰石之C轴，显示它们可能是基本同时出溶的。CCSDUHP岩石中氟磷灰石脉体的存在及其铁氧化物、磷酸盐和硫酸盐出溶体的发现，显示UHP岩石在折返的过程中，快速减压曾造成矿物的分解和大量出溶体的形成，伴随脱水和SiO2等硅酸盐熔体的出溶，还存在少量的磷酸盐熔体，同时可能曾经有铁氧化物的熔浆的加入。磷灰石中铁氧化物等出溶物的出溶是在氧逸度较高的环境下进行的．     

Petrogenesis of UHP metamorphic rocks from Qinglongshan, southern Sulu, east-central China

Five kinds of UHP metamorphic rocks, including eclogite, orthogneiss, paragneiss, schist and quartzite are exposed in the Qinglongshan roadcut, southern Sulu orogenic belt of eastern central China. They comprise metamorphic supracrustal rocks with bimodal volcanic characteristics and continental affinity, and granitic intrusive associations. The preservation of coesite inclusions and/or its pseudomorphs in eclogite and other rocks indicate that they have been subjected to in-situ UHP metamorphism. Four stages of metamorphism were recognized by combining petrographic observations and compositions of minerals from various UHP rocks. Prograde epidote-amphibolite facies, UHP coesite–eclogite facies, post UHP quartz–eclogite facies, and retrograde amphibolite facies assemblages delineate an inferred P–T path with a clockwise trajectory and a retrograde event characterized by the coupling of decompression with a temperature decrease. Garnet porphyroblasts in UHP eclogites display a complex growth zoning and mineral distribution, and record a crucial segment of the prograde and retrograde metamorphic evolution. The preservation of growth zoning in eclogitic and gneissic garnets suggests that the UHP rocks had a short residence time before retrograde metamorphism and a very high uplift rate in order to preserve the prograde growth zoning.     

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.