地质学报2012，Vol.86Issue(7)：1077-1090,14.
西秦岭金厂石英闪长岩的岩浆混合成因:岩相学和锆石U-Pb年代学证据及其构造意义
Magma Mixing Genesis of the Jinchang Quartz Diorite in West Qinling Orogen,Western China: Petrographical and Geochronological Constraints and Their Tectonic Implications
摘要
Abstract
The Jinchang quartz diorite pluton is located in the south part of western Qinling orogen, western China. Mafic microgranular enclaves (MMEs) are widespread in the pluton, and are mostly rounded or irregular elongated in morphology. They commonly have sharp to transitional contact to the host intrusion. Common presence of needle-like apatite in MMEs, which, however, occurs as short crystals in the host rocks, suggests mixing of mafic and felsic magmas in the formation of the Jinchang quartz diorite. Plagioclase in the quartz diorite have oscillatory zoning in terms of An content, which are 30~40 in the core, >50 in the mantle, and less than 10 in the rims. A significant compositional gap was observed between the core and rims. Inclusions of amphibole and biotite were also found in the rims, but not in the core. Amphiboles in host rocks are mostly hornblende and magnesium hornblende, classified as SiO2-saturated type. Amphiboles in MMEs are chemically complex, with both SiO2-saturated magnesium hornblende and SiO2-unsaturated pargasite and pargasitic hornblende. The Al2 O3 and TiO2 contents of hornblende in the MMEs decrease abruptly from the core to the rims, indicating the core formed in a higher-temperature environment compared to the rims. Biotite in the host rocks and MMEs are also different. Biotites in host rocks are either iron or magnesian biotite, whereas they are exclusively magnesian biotite in MMEs. In the diagram of ∑FeO/(2FeO +MgO) vs. MgO for biotite, both the host rock and MMEs samples fall into the transitional field between crust and mantle, with the MMEs samples closer to the mantle field. Collectively, field and petrographic data suggest that magma mixing has been important in the formation of the Jinchang quartz diorite. Laser ablation ICP-MS zircon U-Pb dating yields crystallization ages of 212±2 Ma and 215±1 Ma for the host rocks and MMEs, respectively, indicating contemporaneous formation and providing additional evidence supporting magma mixing. Petrographic evidence and U-Pb ages demonstrate that the Jinchang quartz diorite was emplaced in a post-collisional extensional environment, which could have facilitated partial melting of enriched lithospheric mantle and lower crust, and subsequently mixing of mafic and felsic magmas to form the quartz diorite.关键词
岩浆混合/暗色包体/锆石LA-ICP-MS定年/金厂石英闪长岩/西秦岭Key words
magma mixing/ mafic microgranular enclaves/ zircon LA-ICP-MS U-Pb dating/ Jinchang quartz diorite/ West Qinling引用本文复制引用
刘志鹏,李建威..西秦岭金厂石英闪长岩的岩浆混合成因:岩相学和锆石U-Pb年代学证据及其构造意义[J].地质学报,2012,86(7):1077-1090,14.基金项目
本文为国家自然科学基金项目(编号90814004,41072057)、中央高校基本科研业务费专项(编号CUG090102)、教育部和国家外国专家局高等学校学科创新引智计划(编号B07039)、地质过程与矿产资源国家重点实验室科技部专项基金(编号MSFGPMR201005)资助成果. （编号90814004,41072057）
