青藏高原东缘哀牢山 金沙江构造带糜棱状花岗岩的 LA ICP MS U Pb 定年及其构造意义

哀牢山—金沙江构造带糜棱状花岗岩位于哀牢山深变质带中部。锆石LA-ICP-MS U-Pb定年结果显示两个样品(08Q2-22和08Q2-15)锆石新生边的年龄分别为250.2±2.1Ma(MSWD=2.5)和247.2±2.3Ma(MSWD=0.15)。石英C轴组构测定(EBSD)结果表明韧性变形变质作用形成于角闪岩相条件(500～630℃),远低于锆石结晶温度(675～864℃)和U-Pb体系的封闭温度(800℃),对锆石内部的U-Th-Pb组成不会产生影响,结合锆石的阴极发光图像及U-Pb协和图的特点分析认为锆石LA-ICP-MS U-Pb定年结果(247～250Ma)是对岩浆侵位时代的体现。花岗岩中白云母等过铝质矿物的出现、低钠高钾、A/CNK和A/NK都1.0、稀土和微量元素配分模式与上地壳相似,以及大离子亲石元素和亲岩浆元素强烈富集,Sm和Y明显亏损的特征展示出过铝质、S型花岗岩的地球化学性质。(Yb+Ta)-Rb、Nb-Y、(Y+Nb)-Rb和CaO-(TFeO+MgO)构造环境判别图解及构造带曾经历早中生代俯冲碰撞作用的历史,表明该花岗岩体形成于俯冲碰撞型构造环境。结合前人在哀牢山、点苍山深变质带中相继发现的早三叠世糜棱岩化花岗岩,以及长英质岩石韧性变形变质作用发生于中下地壳(10～15km以下)的特点,分析认为哀牢山—金沙江深变质带是早三叠世及其以前的岩石被俯冲到中下地壳发生韧性变形变质后被挤出或隆升到上地壳的,深变质变形作用形成于中生代或新生代,而非元古宙,它不是,或至少不完全是扬子地块的结晶基底。

Zircon LA ICP MS U Pb Dating for Mylonitized Granite from the Ailaoshan Jinshajiang Tectonic Zone in the Eastern Qinghai Tibet Plateau and Its Tectonic Significance

Mylonitized granite in the Ailaoshan-Jinshajiang tectonic zone is hosted in the center of the Ailaoshan high grade metamorphic belt. LA-ICP-MS U-Pb dating of two zircon samples (08Q2-22 and 08Q2-15) shows the ages of the growth rims of zircon grains are 250.2±2.1Ma(MSWD＝2.5)and 247.2±2.3Ma(MSWD＝0.15), respectively. The lattice preferred orientations by EBSD suggest that the c-axis fabrics of quartz are prism slip, and the ductile deformation formed under the condition of amphibolite facies (500～630℃). This temperature is far lower than the zircon crystallized temperature (675～864℃) in magmatism and the closed temperature (＞800℃) of U-Pb system in zircon, so the deformation and metamorphism can not destroy the U-Th-Pb stabilization of zircon grains crystallized during magma intrusion. The features of zircon Cathodoluminescence images and the U-Pb concordia lines of zircon indicate that the LA-ICP-MS U-Pb dating (247～250Ma) for zircon rims represents the emplacement age of granite. The granite is characterized geochemically by peraluminous minerals (e.g. muscovite), low Na and high K, A/CNK and A/NK＞1.0, similar distribution of REE and trace elements geochemistry similar to those in the upper crust, as well as strong enrichment of large-ion lithophile and magmatophile elements, depletion of Sm and Y. Tectonic discrimination diagrams of (Yb+Ta)-Rb, (Y+Nb)-Rb, CaO-(TFeO+MgO) and Cenozoic subducting collision of the Ailaoshan-Jinshajiang tectonic belt all suggest that the granite formed and emplaced during the stage of collision. Combined with mylonitized granites successively discovered in the Ailaoshan and Dianchangshan high grade metamorphic belt by previous workers, and ductile deformation and metamorphism of felsic rocks occurring in the middle-lower crust (deeper than 10～15km), our study shows that the Ailaoshan-Jinshajiang metamorphic belt was the early Triassic or early rocks which were squeezed out and uplifted to the crust after it was subducted to the lower crust and suffered ductile deformation and metamorphism. Therefore, the strong metamorphism might occur in Mesozoic or Cenozoic, but not Proterozoic, and is not or completely the crystalline basement rock of Yangtze block.