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1.
柴北缘都兰高压麻粒岩的变质演化及形成的动力学背景 总被引：7，自引：5，他引：2
在柴北缘-阿尔金HP/UHP变质带东端,新识别出一个高压麻粒岩单元.高压基性麻粒岩是高压麻粒岩单元的主体,还包括少量高压中酸性麻粒岩.高压基性麻粒岩主要由平衡共生的石榴子石、单斜辉石、斜长石组成,还含有不等量的蓝晶石、角闪石、石英、金红石、黝帘石/斜黝帘石、钛铁矿、方柱石等矿物.高压长英质麻粒岩主要包括石榴子石、蓝晶石、钾长石、斜长石、石英等矿物,并具有少量的单斜辉石和角闪石.岩石学和矿物学数据显示高压麻粒岩经历了多阶段的变质演化,温压计算获得峰期高压麻粒岩相的变质条件为1.40～1.85GPa和800～925℃.退变质高角闪岩相的变质条件为P=0.80～1.05GPa和T=580～695℃：进一步的退变质作用发生在低角闪岩相/绿片岩相条件下(<0.8GPa和<550℃).岩石学、矿物学及年代学资料研究表明都兰地区的高压麻粒岩具有与相邻榴辉岩不同的变质演化历史,而不是榴辉岩在抬升过程中热松弛作用所致.高压麻粒岩可能形成于与陆壳俯冲相关的造山带增厚的陆壳根部环境,形成的深度为50～70km. 相似文献
2.
大陆碰撞过程中的巴罗式变质作用及原地深熔作用:以南阿尔金为例 总被引：1，自引：1，他引：0
南阿尔金吐拉地区所出露的变质泥质岩和变质基性岩普遍经历了中压麻粒岩相变质作用,其中变泥质岩以出现石榴子石+夕线石+长石+黑云母+石英为特征,而基性麻粒岩则以石榴子石+单斜辉石+紫苏辉石+斜长石+石英为特征,具有典型中压相系的麻粒岩相变质作用矿物组合,即显示"巴罗式"变质作用特征。野外宏观特征显示这套变泥质岩普遍经历了原地深熔作用,并局部发生混合岩化作用。岩相学观察结果显示泥质片麻岩保留了关键的深熔作用显微结构证据:(1)石榴子石内部发育有钾长石、石英和斜长石组成的矿物集合体,可能代表了早期熔体的假象;(2)黑云母颗粒边界发育尖锐的、不规则的微斜长石,而且黑云母边界溶蚀明显,形成锯齿状不规则的边界,指示深熔作用可能与黑云母的分解密切相关,即黑云母可能为深熔作用的主要反应相;(3)石英、斜长石或石榴子石颗粒边界发育圆珠状不规则的钾长石,而且颗粒边界或三联点中尖锐状钾长石与周围矿物的形成较小的二面角,有些甚至相互连通呈网络状,这也与它们继承了熔体结构特征一致;(4)不规则钾长石(或微斜长石)分布在石榴子石和夕线石附近,指示石榴子石和夕线石可能为深熔作用的残留相。锆石U-Pb定年结果显示麻粒岩相变质作用和相关深熔作用时代基本一致,主要发生在~450Ma。因此,吐拉地区的中压麻粒岩相变质作用和深熔作用明显要晚于南阿尔金地区榴辉岩和高压麻粒岩的峰期变质时代40~50Myr,而是与榴辉岩折返过程中麻粒岩相叠加变质作用的时代较为接近。但南阿尔金~450Ma的变质作用、深熔作用和岩浆作用是否为独立的构造热事件抑或深俯冲板片折返阶段的产物,这还需要今后进一步的工作验证。 相似文献
3.
柴北缘都兰高压麻粒岩的锆石U-Pb定年及其地质意义 总被引：3，自引：0，他引：3
在柴北缘高压-超高压变质带的东端都兰地区,高压麻粒岩以透镜体的形式存在于石榴白云母片岩、花岗质片麻岩以及斜长角闪岩中。高压麻粒岩的主体为基性麻粒岩,并含少量中酸性麻粒岩。基性麻粒岩主要由石榴子石、单斜辉石、斜长石和石英等组成,而中酸性麻粒岩峰期矿物组合为:石榴子石+斜长石+钾长石+蓝晶石+石英±单斜辉石。根据显微构造和反应结构特征,主要识别出3期变质作用:①峰期高压麻粒岩相阶段(M1);②退变质高角闪岩相阶段(M2);③绿片岩相/低角闪岩相阶段(M3)。选取典型的中酸性麻粒岩样品进行了锆石LA-ICP-MSU-Pb原位定年分析,获得加权平均年龄为446.9±6.5Ma,且CL图像显示锆石内部发育石榴子石、单斜辉石、斜长石等矿物包体,反映锆石可能形成在峰期高压麻粒岩相变质条件下。岩石学和年代学结果显示都兰高压麻粒岩和邻近的榴辉岩同时形成于同一俯冲带的不同热构造环境,高压麻粒岩并非榴辉岩热松弛作用形成的,两者具有各自独立的变质演化历史。 相似文献
4.
《新疆地质》2020,(3)
南阿尔金吐拉地区基性麻粒岩呈透镜体分布于泥质片麻岩中。基性麻粒岩主要矿物组成为石榴子石、斜方辉石、单斜辉石、角闪石、黑云母和斜长石,显示"巴罗式"变质作用(中压麻粒岩相变质作用)特征。目前南阿尔金吐拉地区发育的"巴罗式"变质岩研究相对薄弱,对岩石学特征、变质演化轨迹及"巴罗式"变质作用成因机制亦不明确。传统矿物温压计指示基性麻粒岩的峰期变质温压条件为T=820℃～870℃,P=8.0～8.5 kPa,退变质阶段温压T=700℃～750℃,P=6～7.5 k Pa,为一个降温降压顺时针型的P-T演化轨迹。锆石U-Pb定年加权平均年龄为～450Ma,结合岩相学研究和锆石特征,认为其代表了峰期麻粒岩相变质时代。基性麻粒岩与区域上深熔作用和花岗质岩浆作用变质年龄相似,较南阿尔金UHP榴辉岩和HP麻粒岩的峰期变质时代晚40～50 Ma,与榴辉岩折返过程中麻粒岩相叠加变质作用时代较相似。说明吐拉地区"巴罗式"变质作用及同时代花岗质岩浆作用和混合岩化作用可能是由于俯冲陆壳发生断离并开始折返上升,在伸展环境下由幔源岩浆上涌加热中地壳形成的。 相似文献
5.
文中对拉萨地块东南缘林芝杂岩中的含榴斜长角闪岩进行了详细的岩相学研究和锆石U-Pb定年。岩相学观察表明,含榴斜长角闪岩经历了峰期麻粒岩相变质和角闪岩相退变质作用。峰期麻粒岩相矿物组合为石榴子石+高Ti角闪石+紫苏辉石+斜长石+石英+金红石,其中的石榴子石、石英和角闪石中含有大量金红石出溶体,说明这些矿物的初始成分具有高Ti含量。角闪岩相退变质矿物组合为低Ti角闪石+斜长石+斜黝帘石+石英+金红石。利用Ti在石英中的含量(TitaniQ)温度计计算得到峰期麻粒岩相变质温度为803～924℃,后期角闪岩相退变质温度为555～732℃。样品中的锆石具有明显的核边结构,核部为典型岩浆型锆石,具有高的Th/U值,强烈富集HREE,明显的正Ce异常和负Eu异常,206Pb/238U年龄为(89.3±0.6)Ma,代表含榴斜长角闪岩原岩结晶年龄。锆石边部呈无环带结构,同核部相比,具有低的Th/U值,低的M-HREE和弱的Eu负异常,为变质峰期生长的锆石,206Pb/238U年龄为(81.1±0.8)Ma,代表麻粒岩相峰期变质年龄。我们认为约81Ma高温麻粒岩相变质作用可能与洋中脊俯冲造成的板片窗导致的软流圈上涌有关。 相似文献
6.
山东半岛高压麻粒岩中花岗质浅色脉体的成因 总被引：11，自引：11，他引：0
山东半岛早前寒武纪变质基底部分熔融现象十分发育,常见新生的花岗质浅色脉体呈形态各异的网脉状、细脉状、不规则的透镜状、雾迷状分布于高压基性麻粒岩中。锆石中矿物包体的激光拉曼鉴定、阴极发光图像分析、锆石原位LA-ICP-MSU-Pb定年以及稀土元素、微量元素分析的综合研究结果表明,山东半岛早前寒武纪变质基底在麻粒岩相变质作用的同时伴随着明显的部分熔融作用。花岗质浅色脉体中的锆石具有完好的自形晶形态,矿物包体主要为石英(Qtz)+钾长石(Kfs)+斜长石(Pl)±磷灰石(Ap),与花岗质脉体矿物组成完全一致,相应的阴极发光图像自核部到边部均显示明显的岩浆结晶环带。这些新生锆石U含量变化较大(31×10-6～779×10-6)、Th含量(0.03×10-6～1.3×10-6)和相应的Th/U比值(<0.0081)异常偏低,其稀土元素配分模式具有重稀土HREE相对平坦、中等-强烈的负Eu异常(Eu/Eu*=0.13～0.65)和显著的正Ce异常(Ce/Ce*=11～32)的特点。新生锆石的上述性质与世界典型地区混合岩中深熔锆石的特征十分相似,充分表明研究区花岗质浅色脉体中的锆石为深熔成因。野外及室内系统的岩相学观察发现,花岗质浅色脉体的寄主岩石——高压基性麻粒岩并未显示含水矿物脱水(如角闪石)熔融和长英质矿物部分熔融的证据,而其围岩如孔兹岩系则保存含水矿物(如黑云母)脱水熔融和长英质矿物部分熔融的确凿证据。此外,高压基性麻粒岩中的花岗质浅色脉体集中分布于构造变形相对较强、破碎较明显的部位。由此可见,高压麻粒岩中的花岗质浅色脉体不是寄主岩石深熔作用的产物,而更有可能来源于围岩变质表壳岩(如孔兹岩系)的部分熔融。深熔锆石LA-ICP-MSU-Pb定年结果表明,四十个相同性质锆石微区记录了十分一致的207Pb/206Pb年龄,集中变化于1870±11Ma至1843±20Ma(2σ)之间,加权平均年龄为1859.6±2.2Ma(MSWD=0.74),应代表研究区早前寒武纪变质基底的深熔时代。该组年龄比研究区变质杂岩峰期高压麻粒岩相变质时代(1900～1866Ma)明显偏新,而与峰后近等温减压中-低压麻粒岩相退变质时代(1855～1830Ma)大致相当,表明研究区深熔作用与碰撞造山带构造"热"抬升减压过程存在密切的成因关系。区域强烈的深熔事件是导致高压麻粒岩相岩石发生明显退变的主要控制因素之一。 相似文献
7.
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。 相似文献
8.
南阿尔金造山带是目前报道的具有最深俯冲记录的大陆超高压变质带,其内出露有高压-超高温麻粒岩,它们对深入理解大陆地壳岩石超深俯冲与折返过程具有重要意义.介绍了对南阿尔金巴什瓦克地区长英质麻粒岩和基性麻粒岩的岩相学、矿物化学、相平衡模拟及锆石U-Pb年代学研究成果.其中基性麻粒岩主要记录了深俯冲大陆地壳折返过程的变质演化:包括高压榴辉岩相、高压-超高温麻粒岩相、低压-超高温麻粒岩相及随后的近等压降温演化阶段;长英质麻粒岩除了记录与基性麻粒岩相似的折返过程外,还记录了从角闪岩相到超高压榴辉岩相的进变质演化过程.结合已有研究资料,确定超高压榴辉岩阶段峰期条件> 7~9 GPa和>1 000℃,可达到斯石英稳定域.锆石年代学显示两种岩石类型的原岩和变质年龄均分别在900 Ma和500 Ma左右.变质作用与年代学研究表明,南阿尔金大陆地壳岩石在早古生代发生超深俯冲至200~300 km后,折返至加厚地壳底部发生高压-超高温变质作用,随后被快速抬升至地壳浅部发生低压-超高温变质作用并经历迅速冷却. 相似文献
9.
华北东南缘五河杂岩的变质演化过程研究有助于揭示研究区前寒武纪变质基底的形成与演化历史.基于对五河杂岩中镁铁质麻粒岩进行的详细岩相学观察、矿物电子探针及锆石LA-ICP-MS U-Pb定年和微量元素分析,识别出古元古代变质演化的3个阶段,重建了峰期后近等温减压及降压冷却的顺时针P-T-t轨迹.峰期高压麻粒岩相变质阶段的代表性矿物组合为石榴子石（富Ca核部）+单斜辉石（富Al）+斜长石+石英+金红石±角闪石（富Ti）,所记录的峰期温压条件为850~900 ℃、1.5 GPa;峰期后近等温减压麻粒岩相变质阶段,富Ti角闪石分解在周围形成石榴子石+斜方辉石+斜长石±单斜辉石的矿物组合,所记录的温压条件为~900 ℃、1.1~1.2 GPa;晚期角闪岩相退变质阶段,石榴子石分解产生角闪石+斜长石±石英,所记录的温压条件为600~680 ℃、0.65~0.75 GPa.锆石U-Pb定年结果表明,高压麻粒岩相、中压麻粒岩相和角闪岩相变质时代分别为~1.90 Ga、~1.85 Ga和~1.78 Ga.因此,研究区镁铁质麻粒岩的变质演化过程与胶北地体可以对比,结合已有的2.1 Ga花岗质岩石的成因和锆石年代学等方面研究成果,进一步证明五河杂岩属于胶-辽-吉带的西延,二者共同构成了华北克拉通东部一条古元古代碰撞造山带. 相似文献
10.
喜马拉雅造山带中段出露的基性麻粒岩是理解印度大陆前喜马拉雅期演化历史和新生代碰撞造山作用的理想研究对象。本文对亚东多庆湖地区的石榴角闪岩进行了岩石学、全岩主微量元素地球化学和锆石U- Pb年代学研究,揭示了其原岩类型和新生代的变质作用过程。石榴角闪岩的原岩很可能为新元古代(~890 Ma)的玄武岩,具有E- MORB型岩石的地球化学特征。石榴角闪岩具有三期矿物组合：① 进变质矿物组合可能为石榴子石+角闪石+斜长石+钛铁矿+石英,即石榴子石核部及其中包裹体;② 峰期矿物组合为石榴子石+角闪石+斜长石+黑云母+石英,即石榴子石边部和基质矿物;③ 退变质矿物组合为角闪石+斜方辉石+斜长石+黑云母+石英,包括退变质域和石榴子石边部的后成合晶矿物。矿物温压计和相平衡模拟表明,石榴角闪岩进变质、峰期和退变质条件分别为609~621℃和0. 59~0. 65 GPa、805~845℃和0. 91~1. 04 GPa、825~840℃和0. 61~0. 68 GPa,经历了峰期高压麻粒岩相的变质作用。锆石U- Pb年代学研究表明,石榴角闪岩的峰期变质时间为34. 8~20. 3 Ma,退变质时间为18. 1~17. 7 Ma,可能经历了一个较长期的部分熔融过程。本文研究认为,亚东石榴角闪岩是印度板块向欧亚板块长期俯冲、地壳增厚成因的基性麻粒岩,原岩可能与Rodinia超大陆拼合相关;其以加热埋藏、近等温降压为特征的顺时针P- T轨迹指示了喜马拉雅造山带中段的大喜马拉雅岩系上部构造层位经历了长期持续的地壳增厚和高温麻粒岩相变质作用,以及早中新世(21~17 Ma)相对快速的减压抬升和随后(17 Ma之后)相对缓慢的折返至地表的演化过程。 相似文献
11.
A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene+clinopyroxene+plagioclase±quartz [1]); and garnet–clinopyroxene granulite (garnet+clinopyroxene+plagioclase+quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet–clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende±garnet+plagioclase+quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression–dehydration reaction: hornblende+orthopyroxene+plagioclase=garnet+clinopyroxene+quartz+H2O↑. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet+clinopyroxene+plagioclase+H2O=hornblende+quartz and plagioclase+hornblende+H2O=zoisite+chlorite+quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet+hornblende+clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal–chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan–Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision. 相似文献
12.
High‐pressure kyanite‐bearing felsic granulites in the Bashiwake area of the south Altyn Tagh (SAT) subduction–collision complex enclose mafic granulites and garnet peridotite‐hosted sapphirine‐bearing metabasites. The predominant felsic granulites are garnet + quartz + ternary feldspar (now perthite) rocks containing kyanite, plagioclase, biotite, rutile, spinel, corundum, and minor zircon and apatite. The quartz‐bearing mafic granulites contain a peak pressure assemblage of garnet + clinopyroxene + ternary feldspar (now mesoperthite) + quartz + rutile. The sapphirine‐bearing metabasites occur as mafic layers in garnet peridotite. Petrographical data suggest a peak assemblage of garnet + clinopyroxene + kyanite + rutile. Early kyanite is inferred from a symplectite of sapphirine + corundum + plagioclase ± spinel, interpreted to have formed during decompression. Garnet peridotite contains an assemblage of garnet + olivine + orthopyroxene + clinopyroxene. Thermobarometry indicates that all rock types experienced peak P–T conditions of 18.5–27.3 kbar and 870–1050 °C. A medium–high pressure granulite facies overprint (780–820 °C, 9.5–12 kbar) is defined by the formation of secondary clinopyroxene ± orthopyroxene + plagioclase at the expense of garnet and early clinopyroxene in the mafic granulites, as well as by growth of spinel and plagioclase at the expense of garnet and kyanite in the felsic granulite. SHRIMP II zircon U‐Pb geochronology yields ages of 493 ± 7 Ma (mean of 11) from the felsic granulite, 497 ± 11 Ma (mean of 11) from sapphirine‐bearing metabasite and 501 ± 16 Ma (mean of 10) from garnet peridotite. Rounded zircon morphology, cathodoluminescence (CL) sector zoning, and inclusions of peak metamorphic minerals indicate these ages reflect HP/HT metamorphism. Similar ages determined for eclogites from the western segment of the SAT suggest that the same continental subduction/collision event may be responsible for HP metamorphism in both areas. 相似文献
13.
The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (XAn = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (XJd = 0.20–0.15), and TiO2 in amphibole (TiM2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (XAn = 0.25–0.32), and grossular content in garnet (XGrs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (TiM2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle. 相似文献
14.
P-T-t Evolution of Ultrahigh-Temperature Granulites from the Saxon Granulite Massif, Germany. Part I: Petrology 总被引：2，自引：0，他引：2
The granulites of the Saxon Granulite Massif equilibrated athigh pressure and ultrahigh temperature and were exhumed inlarge part under near-isothermal decompression. This raisesthe question of whether P–T–t data on the peak metamorphismmay still be retrieved with confidence. Felsic and mafic granuliteswith geochronologically useful major and accessory phases haveprovided a basis to relate P–T estimates with isotopicages presented in a companion paper. The assemblage garnet +clinopyroxene in mafic granulite records peak temperatures of1010–1060°C, consistent with minimum estimates ofaround 967°C and 22·3 kbar obtained from the assemblagegarnet + kyanite + ternary feldspar + quartz in felsic granulite.Multiple partial overprint of these assemblages reflects a clockwiseP–T evolution. Garnet and kyanite in the felsic granulitewere successively overgrown by plagioclase, spinel + plagioclase,sapphirine + plagioclase, and biotite + plagioclase. Most ofthis overprinting occurred within the stability field of sillimanite.Garnet + clinopyroxene in the mafic granulite were replacedby clinopyroxene + amphibole + plagioclase + magnetite. Thehigh P–T conditions and the absence of thermal relaxationfeatures in these granulites require a short-lived metamorphismwith rapid exhumation. The ages of peak metamorphism (342 Ma)and shallow-level granitoid intrusions (333 Ma) constrain thetime span for the exhumation of the Saxon granulites to 相似文献
15.
M. JANÁK N. FROITZHEIM N. GEORGIEV T. J. NAGEL S. SAROV 《Journal of Metamorphic Geology》2011,29(3):317-332
A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocks belonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogite provides important information about the peak–pressure conditions despite strong thermal overprint at low pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarometry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions of omphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasitic amphibole, zoisite and rutile in the Mg‐rich cores (XMg = 0.44–0.46) record a prograde increase in P–T conditions from ～2.5 GPa and 650 °C to ～3 GPa and 700–750 °C. Maximum pressure values fall within the stability field of coesite. During exhumation, the peak–pressure assemblage garnet + omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplectitic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite. The development of spinel (XMg = 0.4–0.45) + corundum + anorthite assemblage in the kyanite‐bearing domains at ～1.1 GPa and 800–850 °C suggests a thermal overprint in the high‐pressure granulite facies stability field. This thermal event was followed by cooling at ～0.8 GPa under amphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole was developed around garnet. Our results add to the already existing evidence for ultra high pressure (UHP) metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unit and show that it is more widespread than previously known. Published age data and field structural relations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpine nappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to the Kimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. This implies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice, during Alpine and pre‐Alpine orogenic events, and that two independent HP/UHP provinces of different age overlap in this area. 相似文献
16.
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. 相似文献