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。     

东南极茹尔群岛超高温麻粒岩的研究进展

茹尔群岛(又称赖于尔群岛)位于东南极普里兹构造带的东部边缘,是一个由太古宙和中元古代岩石组成的复合高级变质地体。中元古代岩石是含有富Fe-Al的含石榴子石-矽线石的费拉副片麻岩组合,经历了格林维尔和泛非两期变质作用。太古宙正片麻岩是含有富Mg-Al的含假蓝宝石的超高温泥质麻粒岩组合(梅瑟副片麻岩组合),主要由经历超高温变质作用的含假蓝宝石的泥质麻粒岩、富Mg的石榴子石-矽线石泥质片麻岩、斜方辉石-矽线石石英岩、含石榴子石镁铁质麻粒岩和钙硅酸盐麻粒岩等组成。其中,含假蓝宝石泥质麻粒岩中石榴子石变斑晶和矽线石集合体(蓝晶石假象)周围分别发育峰期后由假蓝宝石+斜方辉石和假蓝宝石+堇青石后成合晶组成的典型减压结构。含石榴子石镁铁质麻粒岩中石榴子石变斑晶周围则发育峰期后由斜方辉石+斜长石后成合晶组成的典型白眼圈减压结构。不同研究者得出了具有不同超高温峰期条件、峰期前及峰期后演化历史、不同形式的顺时针变质P-T轨迹。对超高温变质事件发生的时间和构造背景的认识也存在较大分歧,有认为超高温变质事件发生于格林维尔期(~1000 Ma)并与碰撞造山和弧岩浆作用有关,也有研究认为发生于泛非期(~590 Ma或~530 Ma)并与普里兹造山及冈瓦纳大陆聚合有关。因此,为理清该区超高温麻粒岩的变质演化历史和构造背景,需要对其进一步进行详细深入的矿物组合-变质结构分析、P-T轨迹重建及高精度的锆石-独居石U-Pb年代学研究,并进行区域上对比。     

青藏高原东南缘哀牢山构造带泥质高压麻粒岩的发现及其构造意义

哀牢山构造带泥质高压麻粒岩主要由石榴石、夕线石、钾长石和斜长石变斑晶及尖晶石、铁假蓝宝石、蓝晶石、石英、金红石和钛铁矿包裹体组成,为确定印支地块和华南地块的边界提供了关键性标志。石榴石-黑云母-斜长石-石英地质温压计(GBPQ)计算结果及标志性高温矿物组合(Spl+Qz)表明泥质高压麻粒岩的形成和演化经历了高压/高温进变质到中温/低压退变质的顺时针P-T演化过程。其中:1)高压/高温进变质阶段的矿物组合为Ky+Sil+Grt1+Kf1+Pl1+Spr+Ter(Kf+Pl)+Bt1+Spl+Qtz+Ilm1+Rut1,形成于850～919℃,≥10.4kbar;2)中温/低压退变质阶段的矿物组合为Grt2+Bt2+Pl2+Ms+Qtz+Ilm2+Rut2,早期和晚期的温压条件分别为664～754℃,4.9～6.5kbar和572～576℃,3.5～3.9kbar。反映陆壳物质在碰撞过程中俯冲到地下深处(≥30km)经高压高温变质后快速折返到中上地壳的动力学演变轨迹。     

胶北日庄石墨结晶与寄主麻粒岩的变质演化关系

日庄镇含石墨泥质麻粒岩出露于莱西石墨成矿带, 详细的岩相学、地球化学、拉曼光谱、年代学等研究显示岩石经历了超高温变质作用。麻粒岩变质演化过程包括三个阶段, 其中峰前M₁阶段矿物仅部分保留; 峰期M₂阶段和峰后M₃阶段矿物组合分别为Grt+Qz+Pl+Kfs+Sil+Rt+Liq和Grt+Qz+Bt+Pl+Kfs+Sil+Rt+Liq; 相平衡模拟结合二长石温度计和石英中Ti温度计研究结果显示峰期M₂阶段的温压条件为1025~1100℃、1.05~1.2GPa。研究样品中石墨结晶程度较高, 石墨拉曼光谱特征与麻粒岩相变质条件下形成的石墨一致, 岩相学特征指示高晶度石墨结晶于M₂向M₃转变过程中。麻粒岩中获得变质锆石年龄为~1.85Ga, 该年龄是寄主岩石峰期变质作用后熔体冷却时间的记录。岩相学和年代学研究表明, 优质高晶度石墨的形成和生长可能与胶-辽-吉活动带区域上碰撞折返过程中发生的深溶事件或混合岩化作用密切相关。

     

冈底斯岩浆弧东段夕线石榴黑云片岩的变质作用P-T轨迹与构造意义

冈底斯岩浆弧东段的中-新生代变质岩是揭示岩浆弧深部组成与形成演化的窗口。本文对该地区的变质沉积岩——夕线石榴黑云片岩进行了岩石学和锆石年代学研究。岩相学和矿物化学研究表明,夕线石榴黑云片岩具有三期矿物组合:(1)进变质矿物组合为Grt+Pl+Bt+Ms+Qz+Ep+Rt+Ilm,其保存于石榴石和斜长石变斑晶核部;(2)峰期矿物组合为Grt+Pl+Bt+Ms+Qz+Rt+Ilm,为包裹在石榴石变斑晶幔部和基质中的矿物;(3)退变质矿物组合是Grt+Pl+Bt+Sil+Qz+Ilm,为石榴石的冠状体或产在基质中。相平衡模拟表明,岩石的进变质、峰期和退变质条件分别为600～650℃和9～13kbar、～820℃和～16kbar、～680℃和～7kbar,表明岩石具有一个顺时针型的变质作用P-T轨迹,其峰期发生在高压和高温麻粒岩相条件下,并经历了明显的部分熔融。锆石U-Pb定年表明,岩石的变质作用发生在～89Ma。本文和现有研究结果表明,夕线石榴黑云片岩作为晚白垩世岩基中的包体记录了大体积幔源岩浆岩底垫和增生导致的岩浆弧地壳显著加厚过程,以及加厚弧根的高温和高压变质与深熔作用。     

阿尔金淡水泉花岗质高压麻粒岩P-T演化及年代学研究

岩相学研究表明阿尔金南缘淡水泉一带出露的花岗质片麻岩的峰期矿物组合为Grt+Kfs+Ky+Rt+Qz,为典型的高压麻粒岩相矿物组合。利用Thermocalc3.33程序计算的P-T视剖面图和矿物温压计,确定其峰期变质温压条件为T=875~925℃,P=14.5~15.9kbar,该岩石后期还经历了以矿物组合Grt+Kfs+Bt+Pl+Sill+Ilm+Qz为代表的麻粒岩相和以Sill+Bt+Pl+Mu+Qz±Ilm为代表的角闪岩相的退变质作用,具有一个早期快速等温降压,后期近等压降温的顺时针型的退变质P-T演化轨迹。锆石的U-Pb同位素定年结果显示其原岩的时代为866±5Ma,峰期变质时代为505±5Ma。其中该岩石原岩的形成时代与阿尔金及其周缘地区广泛发育的由Rodinia超大陆裂解事件导致的岩浆活动时代一致,而峰期变质时代则与南阿尔金高压/超高压变质岩的峰期变质时代(486~509Ma)一致,说明花岗片麻岩原岩的形成与罗迪尼亚超大陆裂解有关,并在早古生代卷入了陆壳深俯冲事件,构成了南阿尔金高压/超高压变质带的一部分。     

辽东岫岩北部王家堡子一带富铝片麻岩的变质作用及其地质意义

高级变质富铝片麻岩出露于辽东岫岩北部王家堡子一带南辽河岩群变质岩石中,通过详细的岩相学和变质作用研究,确定其变质峰期典型矿物组合为Sil+Grt+Crd+Bt+Pl+Kfs+Qz;峰期后存在典型的减压反应Grt+Sil+Qz→Crd。早期进变质阶段以出现十字石、蓝晶石为特征,晚期退变质阶段以石榴石变斑晶和黑云母转变为绿泥石为特征。传统地质温压计估算的富铝片麻岩峰期变质条件为:T=640℃～690℃,P=5. 5～7. 0 kbar;峰期后减压阶段变质条件为:T=560℃～660℃,P=4. 7～4. 9kbar;晚期退变质条件为:T=520℃～560℃,P=2. 9～3. 0 kbar。矿物组合特征、矿物相转变结构及其温压条件变化反映其具有近等温减压的顺时针P--T轨迹演化特征,不同于前人所获得的逆时针P--T轨迹。研究结果认为南、北辽河岩群在古元古代可能经历了相同的变质作用演化过程。     

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。     

喜马拉雅中段高压麻粒岩变质作用、地球化学与年代学

研究的高压麻粒岩发现于西藏亚东以北约40公里的(Zherger-La)、出露在藏南拆离系(STDS)主构造面下盘的高喜马拉雅结晶岩系中,是继喜马拉雅东西构造结的Nanga Barbat、Namjag Barwa和喜马拉雅中段Khatra & Marina地区、定结地区发现的榴辉岩或高压麻粒岩之后,在青藏高原上新近发现的高压麻粒岩.该麻粒岩呈岩片被包裹于花岗质片麻岩中.麻粒岩记录了两期变质作用,早期矿物组合为Grt+Cpx+Pl+Qz,属麻粒岩相变质产物,矿物成分分析显示早期矿物组合达到了平衡,并且没有表现成分扩散;后期矿物组合为Hbl+Pl+Bio或Opx+Pl,指示了较高温但相对压力较低的麻粒岩相退变变质作用,矿物成分分析和结构显示了退变作用没有达到变质平衡.显微结构可以观察到多组变质反应Grt+Cpx+Qtz=Opx+Pl,Grt+Qtz=Opx+Pl,Grt+Cpx+L=Hbl+Pl+Bio+Mt,和Cpx+L=Hbl+Mt.根据矿物平衡关系,利用Grt-Cpx温度计和Grt-Cpx-Pl-Qz压力计估算的早期变质作用温压为T=780～850℃,P=12～15kbar,相对应的地温梯度16℃～18℃/km.借用Hbl-Pl温度计和A1tot in Hbl压力计估算的晚期变质作用温压为T=730～760℃;P=4～6kbar,相当的地温梯度为38℃～50℃/km.变质作用P-T演化呈等温降压轨迹,指示麻粒岩地体从增厚(或俯冲)地壳到减薄增温(或部分熔融)地壳,进而被快速剥露地表的构造过程.初步的地球化学结果表明高压麻粒岩原岩可能相当于大陆拉斑玄武岩.麻粒岩锆石SHRIMP年代学有两组相对集中的年龄分别为98±5 Ma(5 spots)和17.0±0.3 Ma(13 spots).高压麻粒岩的两期变质作用的温度都在700℃以上,略高于锆石U-Pb同位素体系计时封闭温度,推断17 Ma是高压麻粒岩变质后发生折返,随高喜马拉雅结晶岩系剥露冷却的年龄;98.4Ma的测年结果被推测是高压麻粒岩原岩形成的年龄.在喜马拉雅山,高压麻粒岩记录了类似增厚地壳到减薄地壳的转变一方面可能是地壳深部作用机制的转变,另一方面,这种机制与喜马拉雅南坡巨大的降雨量和去顶作用有密切关系,意义重大.     

南阿尔金尤努斯萨依花岗质高压麻粒岩的发现及其地质意义

南阿尔金尤努斯萨依地区首次发现一套高压麻粒岩相特征矿物组合的含蓝晶石石榴子石的花岗质片麻岩,其矿物组合为Grt+Ky+Per(+Ksp)+Ru+Q。依据矿物内部一致性热力学数据,基于THERMOCALC 3. 40程序平台,计算出P-T视剖面图,并结合矿物等值线、矿物组合稳定域及三元长石温度计等,确定其早期变质温压条件为T=970～1010℃,P=23. 2～25. 3kbar,达到了榴辉岩相变质条件。同时,根据岩相学观察获得的矿物共生组合与矿物变质显微结构以及P-T视剖面图,可识别出该岩石在早期变质之后还经历了以矿物组合Grt+Ky+Per(+Ksp)+Pl+Ru+Q为代表的麻粒岩相、以及Grt+Ky+Ksp+Pl+Bi+Ru+Ilm+Q为代表的麻粒岩-高角闪岩相和以Ky+Ksp+Pl+Q+Bi+Mu+Ru+Ilm为代表的角闪岩相三期退变质作用,它们共同构成了一个顺时针型降温降压的退变质P-T演化轨迹,指示出与大陆俯冲-深俯冲相关的高压变质事件和俯冲碰撞后的折返事件。利用LA-ICP-MS进行的锆石原位微区U-Pb定年和微量元素分析表明,该高压岩石峰期变质年龄497. 8±2. 7Ma,与南阿尔金地区已发现的高压-超高压岩石的峰期变质时代完全一致,表明它们应属同一变质岩带;其原岩形成时代为900. 2±4. 1Ma,与南阿尔金已报道的正变质的高压-超高压岩石原岩的形成时代基本一致,指示它们可能具有相同的原岩属性。这套花岗质高压麻粒岩的确定为进一步约束南阿尔金早古生代高压-超高压变质岩带的时空分布规律提供了新的限定。     

Regional-scale Grenvillian-age UHT metamorphism in the Mollendo–Camana block (basement of the Peruvian Andes)

The Mollendo–Camana Block (MCB) is a 50 × 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U‐Pb and U‐Th‐Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K‐feldspar (Kfs), orthopyroxene (X_Mg Opx = 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (X_Mg Grt = 0.60), sapphirine (X_Mg Spr = 0.87), cordierite (X_Mg Crd = 0.92) and biotite (X_Mg Bt = 0.83). The ubiquitous peak mineral assemblage is Opx‐Sil‐Kfs‐Qtz‐(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al₂O₃) and the local coexistence of sapphirine‐quartz, attest to regional UHT metamorphism (> 900 °C) at pressures in excess of 1.0 GPa. Fluid‐absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo‐Cocachacra areas). In this area, a first stage of decompression from 1.1–1.2 to 0.8–0.9 GPa at temperatures in excess of 950 °C, is marked by the breakdown of Sil‐Opx to Spr‐Opx‐Crd assemblages according to several bivariant FMAS reactions. High‐T decompression is also shown by Mg‐rich garnet being replaced by Crd‐Spr‐ and Crd‐Opx‐bearing symplectites, and reacting with quartz to produce low‐Al‐Opx‐Sil symplectites in quartz‐rich migmatites. Neither osumilite nor spinel‐quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt‐Qtz‐(± pl) symplectites in quartz‐rich migmatites (melt + Opx + Sil = Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid‐absent melting, compatible with a second stage of decompression below 900 °C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large‐scale penetration of hot asthenospheric mantle at the base of an over‐thickened crust.     

An example of ultrahigh-temperature metamorphism: orthopyroxene–sillimanite–garnet, sapphirine–quartz and spinel–quartz parageneses in Al–Mg granulites from In Hihaou, In Ouzzal, Hoggar

Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high-grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz→Opx+Sil, (2) Opx+Sil→Grt+Spr+Qtz, (3) Grt+Sil+Qtz→Crd, (4) Grt+Sil→Crd+Spr, (5) Grt+Sil+Spr→Crd+Spl, (6) Grt+Sil→Crd+Spl, (7) Grt+Crd+Sil→Spl+Qtz and (8) Grt+Sil→Spl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe²⁺) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.     

Ultra-high-temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the Palni Hill Ranges, southern India

Sapphirine granulites from a new locality in the Palni Hill Ranges, southern India, occur in a small enclave of migmatitic, highly magnesian metapelites (mg=85–72) within massive enderbitic orthogneiss. They show a variety of multiphase reaction textures that partially overprint a coarse-grained high-pressure assemblage of Bt+Opx+Ky+Grt+Pl+Qtz. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid considerations, records a clockwise P–T evolution with four distinct stages. (1) Equilibration of the initial high-P assemblage in deep overthickened crust (12 kbar/800–900 °C) was followed by a stage of near-isobaric heating, presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives. During heating, kyanite was converted to sillimanite, and biotite was involved in a series of vapour-phase-absent melting reactions, which resulted in the ultra-high-temperature assemblage Opx+Crd+Kfs+Spr±Sil, Grt, Qtz, Bt, coexisting with melt (equilibration at c. 950–1000° C/11–10 kbar). (2) Subsequently, as a result of decompression of the order of 4 kbar at ultra-high temperature, a sequence of symplectite assemblages (Opx+Sil+Spr/Spr+Crd→Opx+Spr+Crd→Opx+Crd→Opx+Crd+Spl/Crd+Spl) developed at the expense of garnet, orthopyroxene and sillimanite. This stage of near-isothermal decompression implies rapid ascent of the granulites into mid-crustal levels, possibly due to extensional collapse and erosion of the overthickened crust. (3) Development of late biotite through back-reaction of melt with residual garnet indicates a stage of near-isobaric cooling to c. 875 °C at 7–8 kbar, i.e. relaxation of the rapidly ascended crust to the stable geotherm. (4) A second period of near-isothermal exhumation up to c. 6–5 kbar/850 °C is indicated by the partial breakdown of late biotite through volatile phase-absent melting reactions. Available isotope data suggest that the early part of the evolutionary history (stages 1–3) is presumably coeval with the early Proterozoic metamorphism in the extended granulite terrane of the Nilgiri, Biligirirangan and Shevaroy Hills to the north, while the exhumation of the granulites from mid-crustal levels (stage 4) occurred only during the Pan-African thermotectonic event, which led to the accretion of the Kerala Khondalite Belt to the south.     

超高温变质作用：以华北内蒙古土贵乌拉地区为例

超高温麻粒岩(富Mg-Al)是指温度高于900℃、压力为0.7~1.3GPa条件下形成的麻粒岩相变质岩,它记录了下地壳超高温极端变质作用的地质信息。富Mg-Al典型超高温矿物组合有:假蓝宝石+石英,尖晶石+石英,大隅石+石榴石,斜方辉石+夕线石+石英,高氟黑云母和钙镁闪石,刚玉+石英。目前世界上发现的超高温麻粒岩带(块)地区有非洲阿尔及利亚、南非、东南极、巴西中部、澳大利亚中部、印度南部和东南部等地。我们在华北克拉通北部内蒙古中南部地区孔兹岩区中发现了超高温麻粒岩,岩性主要为灰黑色条带状夕线石榴黑云片麻岩,其中含有尖晶石+石英、假蓝宝石+石英、斜方辉石+夕线石+石英以及刚玉+尖晶石+石榴石等超高温矿物组合,指示温度达1000℃,压力超过1.0GPa的变质作用。独居石定年获得了(1927±11)Ma以及(1819±11)Ma两个峰期年龄,代表变质年龄。华北克拉通北部超高温麻粒岩的发现对研究华北克拉通与哥伦比亚超大陆的演化关系有重要意义。     

The stability of sapphirine-quartz and hypersthene-sillimanite-quartz assemblages: an experimental investigation in the system FeO−MgO−Al2O3−SiO2 under H2O and CO2 conditions

Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite (Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al₂O₂−SiO₂) under low fO₂ and for various H₂O/CO₂ conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite. Our data do not show any differences in Gt−Cd stability and composition as a function of H₂O, CO₂ and H₂O−CO₂ (±CH₄) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000° C and 11 kbar, under CO₂-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions, cordierite will have a high-P stability field in the presence of CO₂. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H₂O and CO₂ are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd, Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075° C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg₂Al₄SiO₁₀ due to AlAl=MgSi substitutions [100(Al₂O₃/Al₂O₃+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.     

A petrogenetic grid for aluminous granulite facies metapelites in the KFMASH system

Due to the retrograde cation exchange problems experienced by conventional geothermobarometers above their closure temperatures, petrogenetic grids are a potentially powerful alternative to unravelling the P–T evolution of ultrahigh‐T granulite terranes. A new qualitative KFMASH (K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O) petrogenetic grid for Mg–Al rich metapelites containing K‐feldspar, sillimanite and quartzofeldspathic melt that successfully accounts for the majority of assemblages composed of variations of sapphirine, spinel, garnet, orthopyroxene, cordierite, biotite and quartz is developed. Univariant reactions are predicted utilizing a newly derived ‘melt projection’ and these reactions are entirely consistent with algebraically calculated reaction coefficients obtained using a set of standard phase compositions. Based upon observations of commonly associated mineral assemblages in natural lithologies the [Spr, Spl], [Qtz, Spl], [Bt, Spl], [Opx, Spr], [Opx, Qtz] and [Bt, Opx] invariant points are assumed to be stable, whilst the [Grt, Spr], [Grt, Qtz], [Spr, Qtz] and [Crd, Qtz] are assumed to be metastable. Biotite‐bearing assemblages are confined to the lowest temperatures, and sapphirine + quartz to the highest temperatures. Orthopyroxene + sillimanite ± quartz assemblages are confined to the highest pressures, whilst spinel‐bearing assemblages are stabilized by lower pressures. The alternative choice of invariant point stability leads to significant differences between this grid and previously proposed topologies. Spinel cannot be stable along with the orthopyroxene and sillimanite assemblage as previously proposed. Further, more subtle differences in topology result from the treatment of H₂O in the chemographic projection used to deduce univariant reactions, and projecting from a water‐bearing quartzofeldspathic melt does not yield the same reaction coefficients as projection from H₂O. The new grid allows reinterpretation of previously proposed evolutionary P–T paths for Mg–Al rich granulites from the Napier Complex and Rauer Group, East Antarctica, and In Ouzzal, Algeria.     

Models for orthopyroxene–plagioclase and other corona reactions in metanorites, Dahomeyide orogen, West Africa

Coronal reaction textures occur in metanorite and related intrusions in the Pan-African Dahomeyide orogen of West Africa; they apparently formed by retrograde subsolidus reactions during cooling of the intrusions from 900 to 700 °C at c. 9±1 kbar. The coronas that formed around orthopyroxene (Opx) consist of sequential layers of diopside (Dio), hornblende (Hbl), garnet (Grt) and plagioclase (Pl) with inclusions of kyanite (Ky) or sillimanite (Sil). Three well-organized mineral assemblage sequences have been identified and modelled using steady-state diffusion theory for closed and open systems. The mineral sequence Opx|Hbl+Qtz|Hbl|Pl+Ky, formed by diffusion-controlled reactions in a closed system as layer thicknesses are very sensitive to relative component mobilities defined by Onsager diffusion coefficient (L_ii). Models of this corona (type i) that satisfy modes require L_CaCa≥L_MgMg≥L_FeFe>L_AlAl～L_SiSi ; however, small open-system fluxes involving loss of Al and gain of Ca are required to obtain the best fit between model and observed mineral proportions. Under steady-state diffusion, the monomineralic hornblende layer grew by replacing plagioclase whereas the |Hbl+Qtz| grew by Opx replacement. The type ii corona, which consists of the sequence Opx|Dio|Grt+Dio|Pl+Sil, is also stable under steady-state diffusion in a closed system. Modelling results show that the diopside grew by replacing Opx whereas most of Grt+Dio grew by replacing plagioclase. Stable solutions to the closed-system diffusion model that approximate the mode are restricted to the L-ratio regions where Fe, Mg and Ca are more mobile than Si and Al but are unstable when L_AlAl>100 L_SiSi . However, type ii corona mineral proportions were only closely matched when open-system loss of Al and gain of small amounts of Fewere considered in the diffusion models and relative mobilities were L_FeFe≥L_CaCa≥L_MgMg>L_AlAl～L_SiSi . The modelling results indicate that Ca and Mg were the most mobile elements in the formation of type i corona whereas Fe and Ca were the most mobile components in the growth of type ii coronas. A third corona type, consisting of the mineral sequence Act|Hbl|Grt|Pl+Sil, is only stable in open systems and requires large external fluxes involving gain of Fe, Al, Mg and Na and loss of Ca to obtain a solution of the diffusion model that approximates the estimated mineral proportions. Extensive recrystallization of plagioclase to produce sillimanite or kyanite inclusions accompanying corona formation may explain the open-system behaviour indicated by the diffusion models.     

Reaction history of sapphirine granulites and a decompressional P-T path in a granulite complex from the Eastern Ghats

The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz, spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage, several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich (12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene > spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression (up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may have eventually resulted from exhumation of thickened crust.     

The P-T path of metamorphism and age of migmatites from the northwestern Irkut block (Sharyzhalgai uplift of the Siberian Platform)

We study migmatized aluminous gneisses in the northwest of the Irkut granulite complex in the southeastern Sharyzhalgai uplift of the Siberian Platform basement. Migmatized gneisses with the mineral assemblage Grt + Sil + Bt + Kfs + Pl + Qz (+ Crd + Opx + Spl) contain a leucosome and widespread cordierite-bearing (+ orthopyroxene, quartz, and spinel) symplectites developed after garnet and sillimanite. Study of the microstructural relationships of minerals and modeling using the PERPLEX 672 software have shown a retrograde P-T path of metamorphism for the metasedimentary gneisses, close to the isothermal decompression (ITD). The parameters of the peak of metamorphism are T = 850-870 °C and P > 7 kbar. The weighted average age of zircon from the metasedimentary gneisses (1856 ± 13 Ma, SHRIMP) corresponds to the time of metamorphism. The decompression type of retrograde metamorphism of the rocks in the northwest of the Irkut block indicates their formation in the crust extension and thinning setting. The presence of domal structures in the section of the Irkut block on the shore of Lake Baikal suggests that the dome tectogenesis was involved in the exhumation processes. The Paleoproterozoic metamorphism and granite formation were associated with the same stage of collision processes, when the compression setting was changed by an extension one (1.88-1.85 Ga).