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《Acta Petrologica Sinica》 2013-02
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Genetic mineralogy and metamorphic evolution of Al-rich gneisses in the Shiguai area, Daqingshan-Wulashan metamorphic complex belt.

CAI Jia, LIU PingHua, LIU FuLai, LIU JianHui, WANG Fang and SHI JianRong Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China  
Al-rich gneisses as interlayers within mafic granulites are widespread in the Shiguai area, Daqingshan-Wulashan metamorphic complex belt which is belong to the central part of the Khondalite Belt of the North China Craton. Shiguai Al-rich gneisses consist mainly of sillimanite-garnet-cordierite gneiss, hypersthene-garnet gneiss and sillimanite-garnet gneiss. Based on integrated structural, petrographic, mineral compositional features, geothermobarometric estimations and P-T pseudosection modelling in the system NCKFMASHTO, the metamorphic evolution for the Shiguai Al-rich gneisses could be subdivided into four metamorphic stages. Thereinto, early prograde metamorphic stage (M1) is characterized by garnet cores and the mineral inclusions of biotite+quartz+plagioclase±sillimanite ±K-feldspar±spinel. In contrast, mineral assemblage at the peak metamorphic stage (M2) is interpreted to be matrix garnet and coarse-grained sillimanite+biotite+quartz+plagioclase + K-feldspar±magnetite±ilmenite, which formed at P-T conditions of 840~860℃ and 10.0~10.5kbar. Typical decompressional reactions, such as Grt+Sil+Qz→Crd、Grt+Melt→Crd+Bt+Pl and Grt+Melt→Crd+Qz±Pl have been identified at the post-peak metamorphic stage (M3), and new mineral assemblage of garnet+cordierite+biotite+plagioclase+quartz±sillimanite±hypersthene was formed at this stage with P-T conditions of 720~800℃ and 5.6~6.1kbar. The late cooling stage (M4) is characterized by a retrogressive mineral assemblage of garnet+biotite+plagioclase+quartz+magnetite ±K-feldspar±ilmenite, which formed at P-T conditions of 616~661℃ and 3.4~5.2kbar. This indicates that the Shiguai Al-rich gneisses recorded a clockwise near-isothermal decompression (ITD) P-T path following by a near-isobaric cooling (IBC) stage. It can be concluded that the Shiguai Al-rich gneisses within the Daqingshan-Wulashan metamorphic complex belt was indeed involved in the subduction-collision orogenic event between Yinshan block and Ordos block at Paleoproterozoic.
【Fund】: 科技部973项目(2012CB416603);; 国家杰出青年科学基金项目(40725007);; 中国地质调查局地质大调查项目(1212010811065、1212011120150)联合资助
【CateGory Index】: P588.345
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