Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range,central Australia

Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800–875 °C and >9–10 kbar) and subsequent decompression in a kilometre‐scale, E‐W striking zone of noncoaxial, high‐grade (c. 700–735 °C and 5.8–6.4 kbar) deformation. However, new SHRIMP U‐Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta‐volcaniclastics) yield ²⁰⁶Pb/²³⁸U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield ²⁰⁶Pb/²³⁸U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400–300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475–460 Ma & 400–300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300–1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed.     

Orthopyroxene-bearing, mafic migmatites at Cone Peak, California: evidence for the formation of migmatitic granulites by anatexis in an open system

Abstract Orthopyroxene-bearing migmatites, exposed at the summit of Cone Peak in the Santa Lucia Range, California, offer an opportunity to explore potential links between granulite facies metamorphism and migmatite formation. Geothermobarometry indicates that the metamorphic temperatures and pressures were in the approximate ranges of 700–750° C and 7.0–7.5 kbar. The rocks at the summit comprise three domains: relatively coarse-grained, leucocratic veins; relatively fine-grained, biotite-enriched zones at the margins of the veins; and a biotite–hornblende-bearing host rock. Orthopyroxene is concentrated in the veins, which have also the highest ratio of anhydrous to hydrous minerals of the three rock types. The composition of the veins, together with their textures and modes, suggest that they formed through anatexis involving a dehydration-melting reaction which consumed hornblende and produced orthopyroxene. Variability in mineralogy and composition indicates that there was some local migration of magma along the veins before their final solidification. The biotite-enriched zones formed either by the concentration of residual biotite at the margins of the vein, or through the metasomatic conversion of hornblende (and/or pyroxene) to biotite, or by a combination of the two processes. Significant differences in the chemistry of the neosome (vein + biotite-enriched zone) and the host rock rule out simple dehydration melting in a local closed system. The model that explains best the mineralogical and chemical patterns involves triggering of melting by an influx of a low- a _H2O mixed fluid which added K and Si to and removed Ca from the neosome.     

南秦岭勉略构造带高压基性麻粒岩变质作用及其锆石U-Pb年龄

勉县-略阳地区是勉略蛇绿构造混杂岩带的代表区段,本文在勉县北部徐家坪地区确定了主要矿物为Grt+Cpx+Pl和具有典型"白眼圈"反应结构的两类高压基性麻粒岩,分别对其进行细致的岩相学研究,并利用THERMOCALC3.33程序进行P-T视剖面图计算。一类高压基性麻粒岩的峰期矿物组合为Grt1+Cpx+Pl1+Qz,对应温压条件为T=800～860℃,P=12.4～14.6kbar,晚期退变质矿物组合为Grt2+Hbl+Pl2+Qz。另一类是具有典型"白眼圈"反应结构的高压基性麻粒岩,"白眼圈"结构中斜长石为富Na的钠-更长石,以此推断该高压基性麻粒岩早期矿物组合中含绿辉石,因此其变质峰期矿物组合可能为Grt1+Omp(?)+Qz或Grt1+Cpx(?)+Pl+Qz,对应温压条件分别为T=775～900℃,P>19.2kbar和T=750～850℃,P=16.5～19.8kbar;该岩石后期经历了以矿物组合Grt2+Opx+Hbl1+Pl1+Qz为代表的麻粒岩相及以Grt3+Hbl2+Pl2+Qz为代表的角闪岩相两期退变质作用。造成这两种高压基性麻粒岩峰期变质矿物组合及其温压条件存在差异的原因可能是岩石原始成分的不同。对高压基性麻粒岩及其中的浅色脉体分别进行了LA-ICP-MS锆石U-Pb年代学分析,得到高压基性麻粒岩214±11Ma的变质年龄及脉体215±5Ma的结晶时代,并结合锆石微量元素特征分析,认为214±11Ma的年龄值代表该高压基性麻粒岩角闪岩相的退变质时代,同时获得该高压基性麻粒岩原岩形成时代可能为477Ma。综合两件高压基性麻粒岩的P-T演化轨迹及变质时代,建立高压基性麻粒岩的P-T-t演化轨迹,据此反映秦岭造山带在印支期沿勉略构造带发生俯冲-碰撞造山过程。     

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...     

高温-超高温变质作用成因研究——来自华北克拉通西部孔兹岩带和南非Kaapvaal克拉通西南部Namaqua活动带与Bushveld变质杂岩体的启示

超高温变质作用是在变质地质学领域,继超高压变质作用研究高峰之后的又一重要前缘课题,对于认识地壳构造-热演化具有重要意义。本文总结了华北克拉通西部孔兹岩带和南非Kaapvaal克拉通西南部Namaqua活动带与Bushveld变质杂岩体的高温-超高温麻粒岩的化学成分、矿物组合、变质演化特征,及其相应的变质事件与构造属性。我国的超高温变质作用带,包括华北克拉通西部的孔兹岩带——从内蒙西段的大青山到东段的集宁-凉城地区的超高温变质岩,皆为Al-Mg质和Al饱和体系的超高温变质岩石,常见假蓝宝石+石英、尖晶石+石英的典型超高温变质组合,以及含假蓝宝石±尖晶石、但缺少石英的非典型超高温变质组合。南非Namaqua活动带与Bushveld变质杂岩体分别发现有独特的Fe-Al饱和的铁尖晶石+石英+大隅石、刚玉+高温石英等超高温矿物组合,罕见的高温硼硅酸盐和硅硼铝镁石等超高温矿物组合;以及Ca-Mg质饱和的钙镁橄榄石+镁硅钙石镁黄长石+镁橄榄石等超高温矿物组合的麻粒岩。研究的核心问题是矿物和岩石在高温-超高温条件下的特殊行为方式,不同构造环境和岩石化学成分下的变质反应及其热动力学过程。由此提出超高温变质作用成因研究中的科学问题:包括不同类型和地质属性的高温-超高温麻粒岩的成因特征;麻粒岩的形成条件演化过程和构造背景;高温-超高温变质过程中部分熔融和重新水化过程中流体的作用以及岩体形变过程中的部分熔融;变质反应以及变质作用P-T-t轨迹、元素地球化学和熔体作用行为;岩石保留的可能的变质事件和年代学记录,定量评价高温-超高温过程中变质演化的时间跨度和演化速率。     

河南桐柏地区麻粒岩和片麻岩的岩石学特征及其成因意义

对河南桐柏地区麻粒岩及其周围以前被认为是片麻岩的岩石从野外特征、结构构造、矿物成分、化学成分及峰期变质条件上进行比较，认为它们均相同。似层状、透镜状麻粒岩间以前被认为是高度风化片麻岩的岩石仍是麻粒岩，这是后期应力作用产生差异变形、退变质及风化作用的结果。这种变化不能等同于麻粒岩退变质为片麻岩。因此，桐柏麻粒岩的北侧围岩是与其呈断层接触的大理岩，南侧围岩为郭庄组上段花岗质片麻岩，麻粒岩构成一个约0．5km～2．0km宽的变质带。这对探讨桐柏麻粒岩的形成和演化具有重要意义。     

P-T grids for silica-undersaturated granulites in the systems MAS (n+ 4) and FMAS (n+ 3)-tools for the derivation of P-T paths of metamorphism

Abstract Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al₂O₃-SiO₂ (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low P _H2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7-10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe-Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high-temperature assemblages and by Al-rich cores in orthopyroxene. Application of the FMAS grid to some well-documented granulite occurrences introduces important constraints on their P-T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P-T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and _XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low-temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.     

大别山北部石榴辉石麻粒岩流体包裹体研究

大别山北部石榴辉石麻粒岩是一种基性麻粒岩,岩石中富含有大量CO_2、CH_4、H_2S、H_2O流体包裹体,及含子晶多相包裹体。首次在寄主矿物石榴石、透辉石发现有含固相的多相包裹体。经拉曼光谱及电镜分析,固相部分为石榴石,辉石、重晶石和铁的氧化物;气相成分为H_2O、CO、CO_2、H_2S、H_2、CH_4、C_2H_2等,均一温度分别为950℃、975℃。指示着石榴辉石麻粒岩在形成过程中,在还原气氛下曾经发生过局部深熔作用,产生在组成上近于石榴石、辉石的熔体。这种溶体被正在结晶的石榴石,透辉石所捕获。石榴石中早期的含石榴石子晶+CO_2多相包裹体其CO_2密度为0.65～0.626g/cm~3;二相的 CO_2包裹体,CO_2部分的均一温度26～28.2℃,为晚期捕获的低密度CO_2包裹体,密度为0.648～0.688g/cm~3。石英中三相H_2O-CO_2包裹体,Th=331～410℃,CO_2-水合物溶化温度7.3°～8.6℃,是一种低盐度包裹体,CO_2部分均一温度:25.7～28℃,CO_2密度0.65～0.698g/cm~3为早中期捕获的包裹体。整个岩石中未发现高密度CO_2流体包裹体。表明麻粒岩可以形成在低密度CO_2之中。H_2O-NaCl包体均一温度80～475℃,密度0.36～1.026g/cm~3,主要为晚期包裹体,少数高密度H_2O包裹体,为早期捕获的包裹体。通过流体包裹体等容线确立石榴辉石麻粒岩P-T轨迹为     

Ultrahigh-temperature Metamorphism (1150{degrees}C, 12 kbar) and Multistage Evolution of Mg-, Al-rich Granulites from the Central Highland Complex, Sri Lanka

Mg- and Al-rich granulites of the central Highland Complex,Sri Lanka preserve a range of reaction textures indicative ofa multistage P–T history following an ultrahigh-temperaturemetamorphic peak. The granulites contain a near-peak assemblageof sapphirine–garnet–orthopyroxene–sillimanite–quartz–K-feldspar,which was later overprinted by intergrowth, symplectite andcorona textures involving orthopyroxene, sapphirine, cordieriteand spinel. Biotite-rims, kornerupine and orthopyroxene-rimson biotite are considered to be late assemblages. Thermobarometriccalculations yield an estimated P–T of at least 1100°Cand 12 kbar for the near-peak metamorphism. Isopleths of Al₂O₃in orthopyroxene are consistent with a peak temperature above1150°C. The P–T path consists of four segments. Initialisobaric cooling after peak metamorphism (Segment A), whichproduced the garnet–sapphirine–quartz assemblage,was followed by near-isothermal decompression at ultrahigh temperature(Segment B), which produced the multiphase symplectites. Furtherisobaric cooling (Segment C) resulted in the formation of biotiteand kornerupine, and late isothermal decompression (SegmentD) formed orthopyroxene rims on biotite. This evolution canbe correlated with similar P–T paths elsewhere, but thereare not yet sufficient geochronological and structural dataavailable from the Highland Complex to allow the tectonic implicationsto be fully assessed. KEY WORDS: central Highland Complex; granulites; multistage evolution; Sri Lanka; UHT metamorphism     

A decompressional P–T path, Reinbolt Hills, East Antarctica

Granulites exposed in the Reinbolt Hills, East Antarctica, are part of the extensive Late Proterozoic granulite complex of East Antarctica, which includes the Rauer Group to the east and the northern Prince Charles Mountains to the west. The deformation history includes three pervasive deformation phases. No chemical or mineralogical distinction between these phases has been detected and this is interpreted to be the result of complete re-equilibration at the end of the third deformation phase. Two late deformation phases post-date the metamorphism and record a medium-temperature cooling path. A short segment of the P–T path of these rocks was inferred from mineral reactions that occurred during these late deformation phases. The path passes from 800°C, 7 kbar to 690°C, 5 kbar, indicating strong decompression, which is typical of a thrust-dominated crustal thickening followed by rapid erosion or extensional collapse.