西藏定结高喜马拉雅石榴辉石岩-镁铁质麻粒岩的岩石特征及其地质意义

廖群安; 李德威; 易顺华; 卢练

西藏定结高喜马拉雅石榴辉石岩-镁铁质麻粒岩的岩石特征及其地质意义

中国地质大学地球科学学院，湖北武汉 430074

基金项目:

中国地质调查局"1∶25万定结幅、陈塘区幅(国内部分) 区域地质调查"项目 20001300009231

详细信息

作者简介:
廖群安（1959－），男，博士，副教授, 主要从事岩石学及岩石地球化学研究. E-mail：qanliao@cug.edu.cn

中图分类号: P58
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出版历程
- 收稿日期: 2003-05-15
- 刊出日期: 2003-11-25

Petrologic and Geologic Significance of Garnet Pyroxenite and Mafic Granulites from High Himalayan Region, Tibet

Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China

摘要

摘要: 在高喜马拉雅带的定日县曲当—扎乡一带出露的高喜马拉雅结晶岩系中, 发现了高压变质的石榴辉石岩及其降压变质的镁铁质麻粒岩组合, 早期高压条件下形成的石榴辉石岩矿物组合为Grt+Cpx (富铝) +Ru+Q, 斜长石已完全消失, 形成温度为845~896℃, 压力大于1.2GPa, 已达到榴辉岩相的压力条件.中期的麻粒岩相组合为Opx+Pl±Cpx±Ga, 其中Opx、Cpx和Pl为石榴石的后成合晶, 形成温度为993~776℃, 压力为0.90~1.21GPa, 为中压麻粒岩相产物, 晚期矿物仅见普通角闪石、斜长石和石英, 是角闪岩相退变质的产物, 表明HHC经历了降压升温-降压降温的快速抬升过程, 证明其抬升作用与地幔热源的参与有关.
- 高喜马拉雅 /
- 聂拉木群 /
- 石榴辉石岩
Abstract: HP garnet-pyroxenite and the mafic granulites formed by decompression have been found in the high-Himalaya crystallinites (HHC) from Kada-Zaxang near Dingri County, Tibet. The mineral assemblage of the HP garnet-pyroxenite in early stage, Grt+Cpx (rich in Al₂O₃) +Ru+Q, without any plagioclase, was formed in 845-896 ℃ and 1.2 GPa, with pressure as high as that in eclogitic phase. While the mineral assemblage of mafic granulite was Opx±Cpx (Al-poor) +Pl (An high up to 90) ±Grt (rim), of which the Opx, Cpx and Pl are symplectite which replaced the early Grt and Al-rich clinopyroxene in decompression condition: t=993-776 ℃, p=1.21-0.80 GPa. The latest mineral assemblage, Hb+Pl+Q, found to have replaced the assemblage of mafic granulite, was formed by retrograde metamorphism in amphibolitic phase. All of this show that the HHC have undergone an HP metamorphism and a stage of decompression retrograde metamorphism and that in early stage the temperature rose with the decompression, but in late stage it dropped with the decompression. Therefore, it can be assumed that the uplifting of the HHC was related to the hot source of the mantle.
- high-Himalaya /
- garnet-Nielamu /
- garnet-pyroxenite

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图 1 工作区地质简图

1.构造分区断层；2.基底与盖层间的折离断层；3.正断层；4.聂拉木群曲乡岩组；5.聂拉木群江东岩组；6.古生代－新生代盖层；7. 拉轨岗日群；8.基性变质岩透镜体；9.样品位置

Fig. 1. Geological sketch map of study area

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图 2 片麻岩中的镁铁质麻粒岩透镜体

Fig. 2. Lens of mafic granulite in gneiss

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图 3 石榴辉石岩及其降压退变质岩石的岩相学特征

a.石榴辉石岩、石榴石和单斜辉石之间发育有Opx+Pl+Cpx后成合晶(正交, 50×); b.石榴辉石岩中石榴石周围的蠕状后成合晶(单偏光, 100×); c.石榴辉石岩在麻粒岩相退变质, 形成以蠕状后成合晶(Opx+Pl+Cpx) 为主的麻粒岩相组合(单偏光, 50×); d.由麻粒岩相组合进一步退变质, 形成具石榴石假象的斜长角闪岩(单偏光, 50×)

Fig. 3. Petrograph of the garnet-pyroxenite and retrograde rocks by decompression

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表 1 辉石的电子探针分析结果

Table 1. Electron microprobe data of pyroxenes

表 2 石榴石、斜长石和角闪石的电子探针分析结果

Table 2. Electron microprobe data of Grt, Pl and Am

参考文献(15)

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