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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
引用本文: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.
作者姓名:刘志鹏  李建威
作者单位:广西壮族自治区国土资源规划院地质矿产所,中国地质大学(武汉)
基金项目:国家自然科学基金项目(面上项目,重点项目,重大项目)
摘    要:位于西秦岭南部的金厂石英闪长岩岩体内含有大量镁铁质暗色微粒包体,包体大多呈浑圆状和水滴状,部分呈不规则拉长状,与寄主岩的接触界线截然或呈渐变过渡关系。石英闪长岩中的磷灰石呈短柱状,而包体中的磷灰石则呈细长针状,反映基性岩浆的快速冷凝结晶。石英闪长岩中的斜长石发育振荡环带,核部的斜长石An低,而边部斜长石An先急剧上升,复又下降;核部与边部之间存在明显的间断,同时斜长石边部包裹有暗色矿物,指示其形成时可能有更基性的岩浆注入。寄主岩中的角闪石大多为普通角闪石和镁普通角闪石,属SiO2饱和型,而包体中角闪石一部分为镁普通角闪石,属SiO2饱和型,一部分为韭闪石、韭闪石质普通角闪石,属SiO2不饱和类型。包体中的角闪石自核部到边部,Al2O3与TiO2含量急剧下降,说明核部和幔部相对于边部形成于更高温的环境。寄主岩中黑云母部分为铁质黑云母,部分为镁质黑云母,而包体中黑云母均为镁质黑云母,在∑FeO/(∑FeO+MgO)对MgO图解上寄主岩与包体中黑云母均落入壳-幔混源区。寄主岩和包体中的锆石均为典型的岩浆锆石,LA-ICP-MS锆石U-Pb定年表明它们的形成年龄分别为212±2Ma及215±1Ma(2σ),在误差范围内基本一致,证明二者同时形成。综合以上岩相学和年代学证据认为,金厂石英闪长岩和镁铁质暗色微粒包体是幔源基性岩浆和壳源酸性岩浆混合作用的产物,形成于秦岭造山带中三叠世造山后伸展环境。结合区域上的研究结果认为,中—晚三叠世时期的幔源岩浆底侵和下地壳部分熔融在西秦岭广泛存在。

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
收稿时间:9/7/2011 12:00:00 AM
修稿时间:2012/1/16 0:00:00

Magma mixing in the formation of the Jinchang quartz diorite, Western Qinling orogen, western China: petrographical and geochronological constraints and tectonic implications
Liu Zhipeng and Li Jianwei.Magma mixing in the formation of the Jinchang quartz diorite, Western Qinling orogen, western China: petrographical and geochronological constraints and tectonic implications[J].Acta Geologica Sinica,2012,86(7):1077-1090.
Authors:Liu Zhipeng and Li Jianwei
Affiliation:Land and Resources Planning Institute of Guangxi,China University of Geosciences (Wuhan)
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 Al2O3 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 ΣFeO/(ΣFeO +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.
Keywords:Magma mixing  Mafic microgranular enclaves  Zircon LA-ICP-MS U-Pb dating  Jinchang quartz diorite  West Qinling
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