南天山榆树沟麻粒岩相构造岩研究

榆树沟麻粒岩相构造岩宏观上呈糜棱岩带和糜棱岩化带产出,发育强烈流变叶理;微观上主要造岩矿物发育各种塑性变形显微构造。变形过程中同种造岩矿物的动态重结晶新晶与塑变残斑（残晶） 之间化学成分变化不明显, 表明在变形期间水活度低,以位错蠕变为主。该构造岩形成于深地壳麻粒岩相变质的晚期阶段,是地体在早期阶段于下地壳底部－上地幔顶部发生高压麻粒岩相变质作用后因构造抬升进入深地壳麻粒岩相环境下发生退变质和强烈韧性剪切变形的产物, 其形成后经历了快速的构造折返过程。     

冀东含紫苏辉石的变质岩与变质作用的关系

河北迁安麻粒岩相铁矿区含紫苏辉石的变质岩可分为三大类:A、糜棱岩状含磁铁(黑云母)石英紫苏辉石岩、二辉磁铁石英岩;B、片麻状(块状)紫苏花岗质麻粒岩;C、黑云(角闪)麻粒岩、二辉麻粒岩。 A类岩石主要分布于铁矿区的韧性变形带中心。韧性变形带宽10—50m,其中夹有不同规模的角闪磁铁石英岩透镜体。该类岩石的矿物组合为:Hy+Q±Mt±Bi;Hy+Q+Mt±     

冀北张宣高变质花岗岩-绿岩带花岗岩类岩石学、地球化学及锆石铀-铅地质年代学

张宣高变质花岗岩－绿岩带内早期花岗质侵入体是以英云间长岩和花岗间长岩为主体的ＴＴＧ岩系，经历了麻粒岩相高级变质作用的改造，已变质成以含紫苏辉石为特征的片麻岩类岩石。麻粒岩相峰期变质作用的时代为２３９０±１９Ｍａ。紫苏花岗岩属典型岩浆成因侵入岩，其侵位成岩时代为２３９０±２４Ｍａ，是区域麻粒岩相峰期变质作用期间长英质熔浆直接结晶的产物。钾质花岗岩类具多种不同的岩石类型，为一系列小规模的富钾质花岗岩侵入体，是与紫苏花岗岩同一构造岩浆活动事件的产物。麻粒岩相峰期变质作用、岩浆活动时代的一致性，表明了本区太古宙末－古元古代早期，由于大规模构造运动，麻粒岩相变质作用由峰期变质条件向相对低温低压条件演化、花岗岩－绿岩带基底岩石由地壳深部向地壳浅部抬升、并同时伴随紫苏花岗岩及钾质花岗岩等大规模岩浆侵入活动的区域地壳演化规律。     

喜马拉雅造山带中段麻粒岩构造侵位过程中变质变形演化

喜马拉雅造山带中段麻粒岩绝大多数呈透镜状、布丁体等弱应变域断续产出,强应变带围岩往往发育糜棱面理和构造片理。部分麻粒岩经历了强烈的韧性剪切构造变形,形成剪切透镜体,并且明显受韧性剪切带控制,显示成带分布、局部集中的特点。根据矿物组合可将产出的麻粒岩分为4种麻粒岩,其主要组成矿物如斜方辉石、单斜辉石、斜长石、角闪石以及石榴石等均不同程度地发生扭折、压扁、拉长扭曲、亚颗粒化及边缘强烈动态重结晶等强烈塑性显微构造变形特征。研究分析表明,麻粒岩的产出与重熔花岗岩的侵位及藏南伸展拆离断层活动有关,在喜马拉雅造山带强烈伸展快速抬升造山的绝热降压大陆动力学过程中,下地壳基性麻粒岩以较快的速率上升到地表,而下地壳层流作用、造山带伸展-隆升-造山导致麻粒岩相变质作用。     

南天山北缘麻粒岩残迹与辉石相韧性变形研究

对新疆北部南天山北缘蛇绿混杂岩带中麻粒岩的岩石学、矿物学、岩石化学、矿物化学、稀土元素地球化学特征及辉石相韧性变形构造的研究表明,榆树沟麻粒岩的共生矿物组合为透辉石-次透辉石-紫苏辉石-石榴石-斜长石,属基性岩类暗色麻粒岩,以低硅高钙为特征,稀土总量低,轻稀土与铕弱亏损,反映高温高压熔融环境和岩浆分异差的特点。麻粒岩及超镁铁岩中的辉石经历了强烈的韧性剪切变形作用,不对称辉石残斑系、动态重结晶辉石亚颗粒集合体、辉石变形双晶纹、鱼尾构造、核幔构造等显微韧变构造广泛发育。宏观和微观运动学分析揭示了一期从南向北的推覆构造。麻粒岩的形成与早古生代大陆地壳拉伸作用有关,辉石相韧性变形则与早古生代末南天山边缘海的挤压关闭和推覆逆冲有关。     

内蒙古中部早元古代造山事件中麻粒岩相低压变质作用

内蒙古中部早元古代麻粒岩相低压变质地壳,根据富铝片麻岩中的长石种类和岩石组合,可划分为南北2个带、6个岩石组合。它们代表着不同的变质层状岩系序列及其所经历的变质作用的温度、压力条件和构造背景。矿物的包裹结构和反应边组构,记录了高温低压矿物组合取代了中温高压矿物组合:石榴子石+石英→紫苏辉石+斜长石,蓝晶石→矽线石及石榴子石+蓝晶石/矽线石+石英→堇青石。岩石的变形组构、矿物组合的转变关系和变质作用的PTt轨迹表明:本区与变质作用同期的高温正滑韧性剪切作用,是麻粒岩相低压变质地壳形成的主要原因。由中温高压变质环境转入高温低压变质环境是造山事件中推覆作用与拉伸作用联合作用的结果。     

内蒙古大青山高级变质岩韧性剪切带及其流变机制

大青山高级变质岩不仅记录华北克拉通早期大陆形成演化历史,也保留了中下部地壳岩石流变信息,它们经历了下部地壳构造层次高角闪岩相-麻粒岩相条件变质变形、深熔作用改造,形成了复杂构造样式和构造要素组合.韧性剪切带是高级变质岩中主要构造形迹,控制着早前寒武纪高级变质岩主体构造格架.依据野外地质产状、变形特征与构造要素叠加改造关系,韧性剪切带划分为早期近水平顺层伸展型和晚期陡倾韧性剪切带.近水平顺层伸展韧性剪切带呈残留状保留在后期变形改造较弱部位上,主要沿着不同地质单元或者岩性层界面上发育,是在伸展变形体制下形成的.晚期陡倾韧性剪切带呈近东西方向展布,规模较大,叠加和改造早期构造形迹,形成于晚期造山挤压构造环境中,以左行滑移为主.这两种韧性剪切带都形成于地壳中深部构造层次高角闪岩相-麻粒岩相条件下,变形机制主要为熔体增强颗粒边界扩散和颗粒流动,使岩石发生大规模的塑性流动.在宏观上形成了不对称流动组构、条纹条带构造、熔融线理、层内流动褶皱等构造形迹,在微观上矿物晶体没有发生明显塑性变形,均匀消光,晶体为三边平衡结构,与静态变质结构相似,形成了地壳深部构造层次上变质构造岩-构造片麻岩.      

华北克拉通孔兹岩带东端孤山剖面石榴石基性麻粒岩的变质作用及年代学研究

大同孤山石榴石基性麻粒岩出露在华北克拉通孔兹岩带与中部带的构造接触部位,以大小不等的透镜体形式产于孔兹岩带内的夕线石榴片麻岩和紫苏二长片麻岩中.根据岩相学观察、矿物化学研究、P-T视剖面图和传统温压计计算结果,揭示孤山石榴石基性麻粒岩经历了4个阶段的变质演化:早期进变质阶段(M1)的主要矿物组合为石榴石核心及其内部包体矿物单斜辉石+角闪石+斜长石+石英+钛铁矿±金红石.反环带斜长石富钠核部记录了早期压力可达11k bar;峰期变质阶段(M2)的矿物组合是石榴石斑晶和基质中的单斜辉石+斜方辉石+斜长石+钛铁矿+石英,记录的温压条件为850～900℃、9～10kbar;峰期后降压阶段(M3)的标志是石榴石外围发育的后成合晶和冠状环,主要有单斜辉石+斜长石、斜方辉石+斜长石和角闪石+斜长石组合,其形成温压条件为760 ～820℃、5～8kbar;晚期低角闪岩相角闪石的生长表明岩石又经历了降温冷却的过程(M4),温度降至690℃以下.石榴石基性麻粒岩记录了含有近等温降压(ITD)阶段的顺时针变质作用P-T轨迹,揭示了阴山地块与鄂尔多斯地块之间俯冲碰撞加厚下地壳的折返过程.石榴石基性麻粒岩的变质锆石LA-ICP-MS U-Pb定年得到了两组变质年龄数据,分别为1945±25Ma和1828±36Ma,它们与阴山地块、鄂尔多斯地块碰撞形成孔兹岩带的时代及华北克拉通东、西陆块碰撞形成中部带的时代一致.结合该地区其他研究结果推断,石榴石基性麻粒岩在～ 1.95Ga鄂尔多斯地块与阴山地块碰撞过程中俯冲进入下地壳底部,经历早期的高压麻粒岩阶段,随后缓慢地抬升到下地壳上部;之后在～1.85Ga东、西部陆块碰撞过程中,石榴石基性麻粒岩折返到中上地壳.     

紫苏花岗岩成因及构造意义

紫苏花岗岩主要以地壳增生作用、玄武岩底侵地壳熔融、构造增厚地壳熔融、地幔下陷增厚地壳熔融几种方式形成，但紫苏花岗岩化与深层次韧性构造变形具有密切的成因关系。无论是缺乏流体(脱水熔融)还是存在流体(富CO2)的热作用都表明，从地壳岩石形成紫苏花岗岩需要低H2O环境。熔融作用高级阶段导致紫苏花岗岩和紫苏花岗岩-花岗岩杂岩的产生，这些条件可能在玄武质岩浆侵入下地壳提供热源的带中最常见。A型紫苏花岗岩形成的重要因素是共生流体相的成分，通常与非造山和造山后构造背景相关。造山带紫苏花岗岩化与深层次韧性剪切变形，特别是与造山作用过程的减压抬升揭顶作用具有密切关系。     

东喜马拉雅构造结高压基性麻粒岩成因与构造意义

东喜马拉雅构造结南迦巴瓦杂岩中存在典型的泥质、长英质和基性高压麻粒岩。但是,高压麻粒岩在南迦巴瓦杂岩中的分布范围、变质条件和变质时间是否存在空间上的变化并不明确。本文对南迦巴瓦杂岩西南部巴嘎地区的高压基性麻粒岩进行了岩石学和年代学研究。研究表明,巴嘎高压基性麻粒岩由石榴子石、单斜辉石、角闪石、斜长石、黑云母和石英组成,石榴子石变斑晶发育生长成分环带。识别出三期矿物组合：进变质矿物组合M1为石榴子石变斑晶核部及其矿物包裹体,包括石榴子石、石英、榍石和磷灰石;峰期矿物组合M2为变斑晶石榴子石边部和基质矿物,即石榴子石+单斜辉石+斜长石+角闪石+石英+金红石+熔体;退变质矿物组合M3呈冠状体或基质产出,其组合为角闪石+斜长石+单斜辉石+黑云母+石英+榍石。高压基性麻粒岩的峰期变质条件约为1. 5 GPa和915 ℃,具有顺时针P- T轨迹,退变质的早期和晚期分别为近等温降压和降温降压过程。高压基性麻粒岩在峰期条件下发生了明显的部分熔融,含~26%（体积）的熔体,其退变质和熔体结晶作用很可能发生在26~14 Ma。本文和研究区现有研究成果表明,东喜马拉雅构造结南迦巴瓦杂岩中的高压麻粒岩广泛分布,从东北部的加拉、直白和派乡延伸到西南部的巴嘎沟,形成了一条长度超过80 km的高压麻粒岩带。整个带中的高压麻粒岩具有类似的变质条件和持续时间,是印度大陆地壳平缓俯冲并经历了高温和高压变质与部分熔融的产物,构成了喜马拉雅造山带的加厚下地壳。大量高压麻粒岩强烈部分熔融产生的熔体可能为喜马拉雅淡色花岗岩提供了源区。     

西藏定结高压基性麻粒岩(退变榴辉岩)的变质P-T轨迹及构造意义

定结（Dinggye）位于藏南高喜马拉雅结晶岩系的中部,研究该区域麻粒岩的变质P-T轨迹对于理解青藏高原的碰撞和抬升过程至关重要.通过对该地区的高压基性麻粒岩（退变榴辉岩）的岩相学观察,确定了4期矿物组合:（1）峰期榴辉岩相矿物组合（M₁）由石榴子石（核部）+绿辉石（假象）+石英+金红石组成;（2）高压麻粒岩相矿物组合（M₂）主要由石榴子石（幔部）+单斜辉石+斜长石+钛铁矿+角闪石+黑云母组成;（3）中压麻粒岩相矿物组合（M₃）由石榴子石（边缘）+斜方辉石+斜长石+钛铁矿+黑云母组成;（4）角闪岩相矿物组合（M₄）主要由角闪石+斜长石组成.在NCFMASHTO体系下,用THERMOCALC软件对该高压基性麻粒岩进行了热力学模拟.结合传统温压计和平均温压计计算结果,求得M₂、M₃、M₄阶段的温压条件分别为786~826 ℃、0.78~0.96 GPa;798~850 ℃、0.71~0.75 GPa;610~666 ℃、0.51~0.60 GPa,这指示了一条以峰期后近等温降压（ITD）为特征的顺时针P-T轨迹.结合已有地质资料,表明定结高压基性麻粒岩（退变榴辉岩）是喜马拉雅碰撞造山的产物,峰期后经历了近等温降压的构造抬升过程.      

U–Pb zircon study of tectonically bounded blocks of 2940–2840 Ma crust with different metamorphic histories,Paamiut region,South-West Greenland: implications for the tectonic assembly of the North Atlantic craton

New fieldwork, map interpretation, petrography and single zircon U–Pb geochronology has allowed the identification of different crustal blocks in the Paamiut region, in the southern portion of the West Greenland Archaean Craton. Changes of metamorphic grade from only amphibolite facies to granulite facies (some subsequently retrogressed) corresponds with zones of Archaean high strain ductile deformation ± mylonites. U–Pb zircon dates are presented for the TTG (tonalite, trondhjemite, granodiorite) protoliths from each block in the Paamiut region, and the southern portion of the previously identified Tasiusarsuaq terrane lying to the north. The southern part of the Tasiusarsuaq terrane contains 2880–2860 Ma TTG rocks and underwent amphibolite facies metamorphism. Structurally underneath the Tasiusarsuaq terrane to the south is the Sioraq block containing 2870–2830 Ma TTG rocks partly retrogressed from granulite facies. Structurally underneath and to the south is the Paamiut block, dominated by 2850–2770 Ma granodioritic rocks that have only undergone amphibolite facies metamorphism. Also structurally overlying the Paamiut block, but cropping out separately from the Sioraq block, is the Neria block. This appears to be dominated by 2940–2920 Ma gneisses that have been totally retrogressed from granulite facies and strongly deformed. In the southernmost part of the region the Neria block overlies the greenschist to lowermost amphibolite facies Sermiligaarsuk block that contains the ⩾2945 Ma Tartoq Group. Rocks from all the blocks record ancient loss of Pb from zircons and some new zircon growth at 2820 Ma, interpreted to indicate a high grade metamorphic event at that time, including granulite facies metamorphism in the Sioraq and Neria blocks. The blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 Ma metamorphism, explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures. The juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions. The contacts are cut by undeformed Palaeoproterozoic dolerite dykes which post-date amphibolite facies metamorphism. These results, together with previously published data from the Godthåbsfjord region (north of Paamiut) shows that the North Atlantic Craton in West Greenland from Ivittuut in the south to Maniitsoq in the north (∼550 km) consists of a mosaic of ductile fault-bounded packages that attained their present relative positions in the late Archaean.     

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.     

Metamorphic evolution of kyanite–staurolite-bearing epidote–amphibolite from the Early Palaeozoic Oeyama belt, SW Japan

Early Palaeozoic kyanite–staurolite‐bearing epidote–amphibolites including foliated epidote–amphibolite (FEA), and nonfoliated leucocratic or melanocratic metagabbros (LMG, MMG), occur in the Fuko Pass metacumulate unit (FPM) of the Oeyama belt, SW Japan. Microtextural relationships and mineral chemistry define three metamorphic stages: relict granulite facies metamorphism (M1), high‐P (HP) epidote–amphibolite facies metamorphism (M2), and retrogression (M3). M1 is preserved as relict Al‐rich diopside (up to 8.5 wt.% Al₂O₃) and pseudomorphs after spinel and plagioclase in the MMG, suggesting a medium‐P granulite facies condition (0.8–1.3 GPa at > 850 °C). An unusually low‐variance M2 assemblage, Hbl + Czo + Ky ± St + Pg + Rt ± Ab ± Crn, occurs in the matrix of all rock types. The presence of relict plagioclase inclusions in M2 kyanite associated with clinozoisite indicates a hydration reaction to form the kyanite‐bearing M2 assemblage during cooling. The corundum‐bearing phase equilibria constrain a qualitative metamorphic P–T condition of 1.1–1.9 GPa at 550–800 °C for M2. The M2 minerals were locally replaced by M3 margarite, paragonite, plagioclase and/or chlorite. The breakdown of M2 kyanite to produce the M3 assemblage at < 0.5 GPa and 450–500 °C suggests a greenschist facies overprint during decompression. The P–T evolution of the FPM may represent subduction of an oceanic plateau with a granulite facies lower crust and subsequent exhumation in a Pacific‐type orogen.     

沂水青龙峪超镁铁质岩石和基性麻粒岩的锆石SHRIMP U-Pb定年

本文主要对沂水青龙峪出露的超镁铁质岩石和基性麻粒岩进行了锆石SHRIMP U-Pb定年研究。超镁铁质岩石以捕掳体形式存在于沂水杂岩中,不发育鬣刺结构,氧化物组成具有超镁铁质科马提岩的高MgO、富CaO、低SiO2、TiO2、K2O和Na2O含量特征;矿物组合以单斜辉石+橄榄石±斜方辉石+铬铁矿为主;变质矿物以角闪石+蛇纹石化为特征;该岩石以稀土元素总含量(∑REE)低、LREE/HREE=3.35～4.40及Ce和Eu负异常为特征。微量元素组成以Ba、Nb、Zr负异常和Nd、Sm正异常为特征。根据锆石SHRIMP U-Pb定年法对该超镁铁质岩石中捕获的早期岩浆结晶锆石和新生的变质锆石进行的研究,年龄值分别为2657～2702Ma和2551～2585Ma,表明该超镁铁质岩石形成年龄为2585～2657Ma。基性麻粒岩的氧化物组成特征表明其属高Mg的洋岛拉斑玄武岩,麻粒岩相——高角闪岩相变质作用与新太古代的深熔和岩浆侵入作用有关,矿物组合以紫苏辉石+单斜辉石±角闪石+斜长石±石榴子石为特征;晚期蚀变作用与辉长岩墙、辉绿岩脉及石英闪长岩买的侵入有关,矿物组合以滑石化+绢云母化+绿泥石化为特征;稀土元素组成以轻重稀土元素无分异和无Eu异常为特征;微量元素组成以Nb、Zr、P、Ti负异常和Sr、K正异常为特征;锆石SHRIMP U-Pb定年结果表明麻粒岩相——角闪岩相变质作用年龄为2498.4±7.6Ma,导致麻粒岩相——角闪岩相变质的深熔和岩浆结晶年龄为2551±24Ma,晚期蚀变作用的年龄分别为2231～2235Ma和1850±19Ma。     

Orthopyroxene–sillimanite–quartz assemblages: distribution, petrology, quantitative P–T–X constraints and P–T paths

Granulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane. Quantitative pressure, temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene + sillimanite + quartz are assessed with reference to calculated phase diagrams. In NCKFMASH and its chemical subsystems, peak assemblages form mainly in high‐variance fields, and most mineral assemblage changes reflect multivariant equilibria. The rarity of orthopyroxene–sillimanite–quartz‐bearing assemblages in granulite facies rocks reflects the need for bulk rock X_Mg of greater than approximately 0.60–0.65, with pressures and temperatures exceeding c. 8 kbar and 850 °C, respectively. Cordierite coronas mantling peak minerals such as orthopyroxene, sillimanite and quartz have historically been used to infer isothermal decompression P–T paths in ultrahigh‐temperature granulite facies terranes. However, a potentially wide range of P–T paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene + sillimanite + quartz, indicating that an individual mineral reaction texture is unable to uniquely define a P–T vector. Therefore, the interpretation of P–T paths in high‐grade rocks as isothermal decompression or isobaric cooling may be overly simplistic. Integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a P–T path, and in turn on tectonic models derived from these paths.     

Pan-African decompressional P-T path recorded by granulites from central Dronning Maud Land, Antarctica

A high-grade metamorphic complex is exposed in Filchnerfjella (6–8°E), central Dronning Maud Land. The metamorphic evolution of the complex has been recovered through a study of textural relationships, conventional geothermobarometry and pseudosection modelling. Relicts of an early, high-P assemblage are preserved within low-strain mafic pods. Subsequent granulite facies metamorphism resulted in formation of orthopyroxene in rocks of mafic, intermediate to felsic compositions, whereas spinel + quartz were part of the peak assemblage in pelitic gneisses. Peak conditions were attained at temperatures between 850–885 °C and 0.55–0.70 GPa. Reaction textures, including the replacement of amphibole and garnet by symplectites of orthopyroxene + plagioclase and partial replacement of garnet + sillimanite + spinel bearing assemblages by cordierite, indicate that the granulite facies metamorphism was accompanied and followed by decompression. The observed assemblages define a clock-wise P-T path including near-isothermal decompression. During decompression, localized melting led to formation of post-kinematic cordierite-melt assemblages, whereas mafic rocks contain melt patches with euhedral orthopyroxene. The granulite facies metamorphism, decompression and partial crustal melting occurred during the Cambrian Pan-African tectonothermal event.     

Metamorphic Characteristics of Granulite Facies Rocks in the Laixi-Pingdu Area in Eastern Shandong Province

Abstract Granulite in eastern Shandong is mainly exposed in Laixi, Pingdu, Changyi and Anqiu, and the diagnostic mineral assemblage is Opx+Cpx+Hb+Pl ± Q ± Sea. The appearance of orthopyroxene and its coexistence with hornblende indicate that the reaction Hb+Q = Opx+Cpx+Pl+H₂O did not proceed completely and therefore these rocks belong to the amphibolite- granulite transition facies, i.e., belonging to hornblende-granulite subfacies. According to the data obtained from such geothermometers and geobarometers as Opx- Cpx, Opx- Hb, Cpx- Hb, Hb- PI, Sca- Pl and Fe- Ti oxides, it has been determined that the temperature of the main metamorphic stage was 720° – 810°C, the pressure 0.5 GPa and fo₂10^?15.5, showing a geothermal gradient of 41–46°C / km, and thus the rocks belong to “low-temperature” and low-pressure granulite facies.     

Low-Pressure Metamorphism of Granulite Facies in an Early Proterozoic Orogenic Event in Central Inner Mongolia1

Abstract According to the kinds of feldspar and rock associations in the Al-rich gneisses, the low-pressure metamorphic crust of the Early Proterozoic granulite facies in central Inner Mongolia can be divided into southern and northern belts which are composed of six rock associations. They represent the relevant rock sequences of the layered metamorphic rock series formed under specific metamorphic temperature and pressure conditions as well as tectonic environments. Mineral inclusions and reaction texture have recorded that the medium-temperature high-pressure mineral assemblages are replaced by the high-temperature low-pressure mineral assemblages, thus, giving rise to: garnet+quartz? hypersthene+plagioclase; kyanite? sillimanite and garnet+ kyanite / sillimanite+quartz? cordierite. The deformation fabrics of the rocks, the change of mineral assemblages and the PTt path of metamorphism indicate that the contempranceous high-temperature normal-slip ductile shearing is the main cause of the formation of the low-pressure metamorphic crust of granulite facies. In the orogenic event, the co-action of thrusting and extension resulted in the change of a medium-temperature high-pressure metamorphic environment into the high-temperature low-pressure metamorphic conditions.     

Petrogenetic significance of orthopyroxene-free garnet + clinopyroxene + plagioclase ± quartz-bearing metabasites with respect to the amphibolite and granulite facies

Orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages represent the paragenetic link between plagioclase‐free eclogite facies metabasites and orthopyroxene‐bearing granulite facies metabasites. Although these assemblages are most commonly developed under P–T conditions consistent with high pressure granulite facies, they sometimes occur at lower grade in the amphibolite facies. Thus, these assemblages are characteristic but not definitive of high pressure granulite facies. Compositional factors favouring their development at amphibolite grade include Fe‐rich mineral compositions, Ca‐rich garnet and plagioclase, and Ti‐poor hornblende. The generalized reaction that accounts for the prograde development of garnet + clinopyroxene + plagioclase ± quartz from a hornblende + plagioclase + quartz‐bearing (amphibolite) precursor is Hbl + Pl + Qtz=Grt + Cpx + liquid or vapour, depending on whether the reaction occurs above or below the solidus. There are significant discrepancies between experimental and natural constraints on the P–T conditions of orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages and therefore on the P–T position of this reaction. Semi‐quantitative thermodynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data.