Preliminary report on the metamorphic evolution of the Guanyingou amphibolites, Dunhuang Metamorphic Complex, NW China.
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摘要: 敦煌东南三危山观音沟地区，出露有石榴斜长角闪片麻岩，岩石中保存了三个阶段的变质矿物组合。进变质阶段的矿物组合（M1）以石榴子石变斑晶中的包裹体矿物组合（Hbl1+Pl1+Qtz1）为代表，形成条件约为550~575℃/5.2~5.7kbar（绿帘角闪岩相）。变质高峰期矿物组合（M2）主要由石榴子石变斑晶和基质矿物（Hbl2+Pl2+Qtz2±Cpx2）组成，形成温度>670℃，压力>11.9kbar（角闪岩相）。退变质阶段矿物组合（M3）为后成合晶矿物组合（Hbl3+Pl3+Qtz3），形成条件约为590~640℃/4.3~5.9kbar（角闪岩相）。观音沟石榴斜长角闪片麻岩记录了包含进变质、变质高峰、退变质过程的造山过程“西阿尔卑斯”型P-T轨迹，其中退变质阶段具有明显的近等温降压特征。变质作用P-T轨迹指示本地区可能经历了俯冲-碰撞-快速抬升的造山过程。结合年代学资料，该期造山事件可能发生于泥盆纪。Abstract: Exposed in the Guanyingou area, Mt. Sanweishan, Dunhuang, NW China, the garnet-bearing amphibolites record different metamorphic mineral assemblages formed during three metamorphic stages. The prograde assemblage (M1), enclosed in garnet porphyroblast, is consisted of hornblende, plagioclase and quartz and the P-T conditions were estimated to be 550~575℃/5.2~5.7kbar (epidote amphibolite facies). The metamorphic peak assemblage (M2) is composed mainly of garnet and hornblende, plagioclase, quartz and locally clinopyroxene in the matrix, with P-T conditions estimated to be of >670℃ and >11.9kbar (amphibolite facies). The retrograde assemblage (M3) is the so-called "white-eye socket" symplectite rimming garnet porphyroblast, consisted mainly of hornblende, plagioclase and quartz, and were formed at about 590~640℃/4.3~5.9kbar (amphibolite facies). Micropetrographic investigation and thermobarometric computation show that the amphibolites experienced clockwise, west-Alpine type P-T paths including near isothermal decomposition (ITD) segments, suggesting that the terrane involved in a collisional orogenic process, possibly in the Devonian.
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Key words:
- Amphibolite /
- Geothermometer /
- Geobarometer /
- Metamorphic P-T path /
- Dunhuang Metamorphic Complex
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