Petrology and zircon U-Pb dating of well-preserved eclogites from the Thongmon area in central Himalaya and their tectonic implications
Petrology and zircon U-Pb dating of well-preserved eclogites from the Thongmon area in central Himalaya and their tectonic implications
复制标题
喜马拉雅中部通蒙地区保存完好榴辉岩的岩石学、锆石U-Pb定年及其构造意义
DOI：
10.1111/jmg.12457
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发表时间：
2019
影响因子：
3.4
通讯作者：
Yu Huanglu
中科院分区：
文献类型：
--
作者：
Li Qingyun;Zhang Lifei;Fu Bin;Bader Tomas;Yu Huanglu
据报道，榴辉岩的发现是在喜马拉雅山脉中部Thongmön地区的大喜马拉雅结晶杂岩中发现的，其变质演化通过岩石学研究、假剖面建模和锆石测年来解释。绿辉石首次在变质镁铁质透镜榴辉岩基质中发现。根据主量元素和稀土元素以及矿物包裹体，在Thongmön榴辉岩中鉴定出两组石榴石。石榴石的核心和中间部分代表Grt I，其中主要元素（Ca，Mg和Fe）几乎均匀分布，很少或弱带状。这一Grt I显示出几乎平坦的球粒陨石标准化HREE模式，主要包裹体为角闪石、磷灰石、石英和丰富的绿辉石。Grt II在大的石榴子石颗粒上形成薄的边缘，其特征是向边缘Ca减少，Mg增加，相对于HREE和LREE，MREE富集。Grt Ⅱ中未发现角闪石包裹体，说明角闪石的分解作用对MREE的富集有贡献。由基质矿物和石榴子石和锆石中的包裹体记录的两个变质阶段概述了通蒙榴辉岩的埋藏和逐渐变质过程直至压力峰值：（a）角闪-石榴子石-绿辉石-多硅白云母-金红石-石英组合，其中多硅白云母被Bt+ P1合晶岩所取代，代表了闪长角闪榴辉岩相阶段M1（1），（B）在榴辉岩峰相[阶段M1（2）]中，角闪石消失并开始熔融。根据石榴子石和绿辉石包裹体的成分，M1（1）发生在19-20 kbar，640-660°C; M1（2）发生在>21 kbar，>750°C，出现熔体并包裹在变质锆石中。两个榴辉岩样品中锆石SHRIMP U-Pb定年结果一致，变质年龄分别为16.7 ± 0.6Ma和17.1 ± 0.4Ma。变质锆石在榴辉岩相峰期与Grt Ⅱ同时生长。Thongmön榴辉岩的特征是角闪岩相到榴辉岩相的逆冲变质作用，是在中新世印度板块继续向欧亚大陆俯冲的过程中形成的。
The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1(1), (b) in the peak eclogite facies [stage M1(2)], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1(1)is constrained to 19–20 kbar and 640–660°C and M1(2)occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene.