Title: Element Geochemistry,Petrogenesis and Tectonic Setting of the Yangfang Pluton in Beijing
Abstract: The Yangfang Pluton exposed in Western Hills of Beijing is dominantly composed of biotite monzogranite,and much less quartz monzonite,alaskite and dioritic enclaves. The high-precise analysis of major and trace elements for these rocks are firstly reported by this study. The biotite monzogranite is a typical high-potassic calc-alkaline granite,and has the characteristics of Rb,Ba,and LREE enrichment,depletion of Th,U,Nb,Ta relative to LREE,and weak negative Eu anomaly. The quartz monzodiorite is high potassic and total alkali,magnesium enrichment and iron poor characteristics,and exhibits enrichment of Rb,Ba,LREE and Sr,as well as high Sr/Y ratio,but depletion of Th,U,Nb,Ta relative to LREE,and no siginificant Eu anomaly. So the quartz monzodiorite is similar to adakite (high Sr low Y type intermediate and acid igneous rocks) in geochemistry. The alaskite has obvious depletion of Sr,Ba,and REE (especially MREE),and negative Eu anomaly. The dioritic enclave is high in magnesium content,Mg# value and Sr/Y ratio,and enriches K,Rb,Ba,LREE,and Sr,but deplets Th,U,Nb,Ta relative to LREE. It does not show any Eu anomaly. The geochemistry of diorite is the same as that of sanukite,and belongs to typical shoshonitic series. The diorite of the Yangfang Pluton was originated by partial melting of metasomatic lithospheric mantle; and the quartz monzonite is the product of mantle-derived magma and lower crust interaction. Meanwhile,the biotite monzogranite and alaskite came from the partial melting of K-rich mafic rocks in the lower crust and upper crustal rocks,respectively. These petrogenetic models imply a very high thermal gradient underneath the western segment of Yanshan orogenic belt during the late stage of Early Creataceous. Geological evidence and wholerock chemistry as well as geochemical discrimination diagrams indicate that the Yangfang Pluton formed during the collapse stage of orogenic evolution.
Publication Year: 2010
Publication Date: 2010-01-01
Language: en
Type: article
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