冈底斯带甲玛矿区花岗斑岩类年代学、地球化学及岩石成因
Geochronology, Geochemistry and Petrogenesis of the Granitoid Porphyries from Jiama Ore Deposit in Gangdese Belt

藏南冈底斯带中新世斑岩成因主要存在残留洋壳的部分熔融、加厚下地壳的部分熔融、陆下岩石圈的部分熔融和俯冲流体交代基性下地壳的部分熔融四种观点.为了进一步阐明该时期的岩浆成因和大地构造背景，对冈底斯带甲玛矿区不同类型的斑岩体进行了岩石学分析和LA-ICP-MS锆石U-Pb测试，并运用X荧光光谱仪和电感耦合等离子体质谱仪分别对样品进行了全岩主、微量元素测试.测试结果显示冈底斯带甲玛矿区的斑岩类形成于16.7～14.4 Ma，总体上具有埃达克质岩石的地球化学特征.其中花岗斑岩类来自于藏南加厚的基性新生下地壳的部分熔融，而辉长闪长玢岩来源于富集的岩石圈地幔.早中新世以来(18～13 Ma)青藏高原处于构造转换阶段，含矿的埃达克质岩浆沿断裂通道上升，并且在上升过程中遭受到了中上地壳物质的混染，演化形成甲玛矿区内石英闪长玢岩、花岗闪长斑岩、二长花岗斑岩和花岗斑岩，而近乎同时期来自于岩石圈地幔的岩浆则演化形成辉长闪长玢岩；矿区内含矿热液流体在岩浆热驱动和构造应力作用下，在林布宗组砂板岩、角岩与多底沟组大理岩、灰岩的层间滑脱带或褶皱的构造虚脱空间就位，形成冈底斯带甲玛矽卡岩型铜多金属主矿体.
The petrogenesis of Miocene porphyries is still under debate, involving four different genetic models, such as, partial melting of residual oceanic crust, partial melting of thickened lower crust, partial melting of sub-continental lithospheric mantle and partial melting of metasomatized mafic lower crust related to subducted fluids. In order to clarify the petrogenesis and tectonic setting, petrological analyses and zircon LA-ICP-MS U-Pb dating in this study were carvied out. In addition, whole-rock major and trace elements were analyzed by means of XRF and ICP-MS methods. The results demonstrate that the porphyries formed at 16.7-14.4 Ma, showing adakitic geochemical features. Geochemical characteristics, trace elemental ratios and discrimination diagrams suggest that magma source of the granitoid porphyries was derived from partial melting of the lower juvenile crustal material, whereas magma source of gabbro diorite porphyrite was sourced from the enriched lithospheric mantle. Together with published data, it is proposed that the Qinghai-Tibetan plateau was in the tectonic transformation from compression to extension or strike-slip during 18-13 Ma in the early stage of Miocene. The ore-bearing porphyries ascended through vertical fractures or faults and mingled with mid-upper crustal material generating granite porphyry, monzonitic granite porphyry, granodiorite porphyry and quartz diorite porphyrite. On the contrary, the coeval magma derived from the lithosphere mantle formed gabbro diorite porphyrite. Ore-bearing hydrothermal fluid was driven by tectonic stress and heat flow from magma took place at the sandstone slate and hornfel of the Linbuzong Formation, and marble and limestone of the Duodigou Formation characterized by inter-bedded tectonic fracture belts and collapse locations of folds, forming skarn-type copper polymetallic ore deposits