Geochemical modelling of the Chilas Complex in the Kohistan Terrane,northern Pakistan |
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| Institution: | 1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China;2. Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China;1. State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;2. State Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;1. School of Earth Science and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, China;2. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China;3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2. School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China;3. Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA |
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| Abstract: | The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramafic–mafic–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite).The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma. |
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