Glaucophane and barroisite eclogites from the Upper Kaghan nappe: implications for the metamorphic history of the NW Himalaya
Glaucophane and barroisite eclogites from the Upper Kaghan nappe: implications for the metamorphic history of the NW Himalaya
复制标题
上可汗推覆体的蓝闪石和巴罗石榴辉岩：对喜马拉雅山西北部变质历史的影响
DOI：
10.1144/gsl.sp.2000.170.01.22
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
R. Compagnoni
中科院分区：
文献类型：
--
作者：
B. Lombardo;F. Rolfo;R. Compagnoni
摘要本文介绍了巴基斯坦喜马拉雅山高喜马拉雅结晶带上喀汗推覆体中含蓝闪石和重晶石榴辉岩的岩石学和矿物化学研究结果，并讨论了这些岩石中记录的P-T轨迹对西北喜马拉雅山构造和变质历史的意义。这里描述的榴辉岩来自Gittidas的一个以前未描述的露头，但属于同一套石榴石绿辉石-多硅白云母-石英-金红石榴辉岩以前在上卡汉推覆体的其他地方描述。蓝闪榴辉岩中的变质峰组合为石榴绿辉石-金红石-石英，巴曲榴辉岩中的变质峰组合为石榴绿辉石-蛇绿岩-金红石±蓝晶石±多硅白云母±铁白云石。大多数样品含有大量的角闪石、白色云母和石英。白色云母可能作为峰组合（多硅白云母）的一部分或作为蓝晶石（钠云母）之后的退变质相存在。角闪石晚于变质峰组合，在大多数样品中为压辉岩，但在相对富铁的榴辉岩中，A位为蓝闪石，具有显著的Na，M4位为Ca，四面体为A1。榴辉岩中石榴子石表现出强烈的变质分带，从核部到边缘Mg含量增加，Fe含量减少。富铁岩核中充满了峰值组合的矿物包裹体，但也发现了早期钠长岩、绿色和蓝绿色角闪石的包裹体。这些值是接近的平衡条件估计北喜马拉雅Tso-Morari穹的榴辉岩。上Kaghan推覆体的变质演化与Kaghan河谷东部Neelum河谷的榴辉岩含岩单元的变质演化之间的强烈相似性表明，Kaghan-Neelum地区的榴辉岩产状确定了区域范围的榴辉岩含岩带，该榴辉岩带在始新世中期经历了高压变质作用。喜马拉雅山西北部榴辉岩的变质演化记录与最近在喜马拉雅山东部发现的麻粒化榴辉岩的变质演化的比较表明，喜马拉雅山范围的榴辉岩变质作用的可能性，前中新世的年龄。因此，高喜马拉雅结晶推覆体的西北喜马拉雅和那些喜马拉雅东部的主要区别似乎在于其变质演化的早期部分比在不同的P-T路径，他们遵循的折返。
Abstract This paper presents the results of a petrographical and mineral chemical study of glaucophane- and barroisite-bearing eclogites from the Upper Kaghan nappe in the Higher Himalayan Crystallines of the Pakistan Himalaya, and discusses the implications of the P-T path recorded in such rocks for the tectonic and metamorphic history of the NW Himalaya. The eclogites described here come from a previously undescribed outcrop at Gittidas, but belong to the same suite as the garnet-omphacite-phengite-quartz-rutile eclogites previously described elsewhere in the Upper Kaghan nappe. The metamorphic peak assemblage is garnet-omphacite-rutile-quartz in glaucophane eclogite and garnet-omphacite-zoisite-rutile±kyanite±phengite±ankerite in barroisite eclogite. Most samples contain a significant amount of amphibole, white mica and quartz. White mica may be present either as part of the peak assemblage (phengite) or as a retrogressive phase after kyanite (paragonite). Amphibole is later than the metamorphic peak assemblage and is barroisite in most samples, but in relatively Fe-rich eclogite it is glaucophane with significant Na in the A site, Ca in the M4 site and tetrahedral A1. Garnet displays strong prograde zoning in the barroisite eclogites, with Mg increasing and Fe decreasing from core to rim. The iron-rich core is crowded with mineral inclusions of the peak assemblage, but inclusions of earlier paragonite, green and blue-green amphibole were also found. Peak metamorphic conditions in the barroisite eclogites have been estimated at T = 610 ± 30 °C and P = 24 ± 2 kbar from Fe/Mg partition in garnet-omphacite pairs, and from the garnet-omphacite-phengite barometer. These values are close to the equilibration conditions estimated for the eclogites of the North Himalayan Tso-Morari Dome. Strong similarities between metamorphic evolution of the Upper Kaghan nappe and metamorphic evolution of the eclogite-bearing units of the Neelum valley just to the east of the Kaghan valley indicate that the eclogite occurrences of the Kaghan-Neelum area define an eclogite-bearing terrane of regional extent, which was subjected to high pressure metamorphism in middle Eocene times. A comparison of the metamorphic evolution recorded in the eclogites of the NW Himalaya with that of the granulitized eclogites recently discovered in the E Himalaya suggests the possibility of a Himalaya-wide eclogitic metamorphism of pre-Miocene age. Therefore, the main difference between the Higher Himalayan Crystalline nappes of the NW Himalaya and those of the E Himalaya appears to lie less in the early part of their metamorphic evolution than in the different P-T paths they followed during exhumation.