四川石棉县大水沟岩片绿片岩锆石SHRIMP U-Pb年代学及其地质意义

大水沟岩片位于安宁河深大断裂带中段, 川西松潘甘孜造山带与扬子地台结合部位, 因其特殊的构造位置和赋存碲矿床而备受重视.运用SRHIMP U-Pb定年技术对大水沟绿片岩进行年龄测试, 来确定大水沟岩片的形成时代, 为大水沟周缘石棉-冕宁一带乃至川西扬子地台西缘的构造岩浆演化提供有益信息, 同时为大水沟碲铋矿床的形成时代提供线索.大水沟原岩恢复表明其应当有相当部分为沉积岩, 运用SHRIMP U-Pb定年从中获得的5种不同年龄段具有内部结构、外观特征的锆石差异.2467~2358Ma年龄的残留碎屑岩浆锆石说明扬子地台西缘存在太古代-古元古代的物源搬运; 790.5~762.5Ma的岩浆锆石与新元古代早期Rodinia超大陆裂解、地幔柱上涌时的岩浆事件有关; 696.8~642.9Ma锆石年龄反映了大水沟周缘碰撞造山和后造山期岩浆活动和变质作用; 伴随峨眉地裂大规模基性岩浆活动, 在大水沟近源很可能有262.0~220.0Ma碱性杂岩的侵入; 典型热液增生锆石216.5~167.1Ma年龄揭示了该区中侏罗世大规模的岩浆期后热液活动.大水沟岩片可能为异地推覆系统, 岩浆锆石年龄和热液增生锆石年龄限制了大水沟岩片总体定位于220.0~167.1Ma之间. 

SHRIMP U-Pb Dating of Greenschist from Dashuigou Schiefer, Shimian County,Shichuan Province and Its Geological Significance

Located at the central of the Anning River discordogenic fault, the binding site of the Songpan-Ganze orogenic belt and the Yangtze block, Dashuigou schiefer has attracted increasing attention of researchers regarding to its special tectonic site and the occurred tellurium deposit. In order to provide chronologic evidences for reconstructing tectonic and magmatic geochronic evolution of areas around Dashuigou and the western margin of the Yangtze block in Shichuan, as well as for locating time of Dashuigou schiefer, zircon ages of the greenschist from Dashuigou were determined by SHRIMP U-Pb dating technique. Five age groups of the zircons with different characteristics of inner texture and cosmetics are obtained and subdivided from the greenschist of Dashuigou schiefer which should contain great proportions of sediments in the protolith. Ages of 2467-2430Ma from remained detrital magma zircons shows the mass transportation of Archaean-early proterozoic basement, whereas ages of 790.5-762.5Ma probably represent the magmatic event around Dashuigou on the background of Rodinia crack and upwelling mantle in early neoproterozoic. Metamorphism and magmation generated by orogenics and post orogenics are documented by ages of 696.8-642.9Ma from the magmatic zircons. Zircons of 262.0-220.0Ma ages are inferred to have been originated from alkalic rock nearby related to the Emeishan movement.216.5-167.1Ma ages of typical new growth and re-crystallized zircons caused by hydrothermal alteration reveal the post-magmatic hydrothermal activity. Dashuigou schiefer is probably allochthonic thrust sheet, and ages from magma zircons and those thermal genetics bound its general locating time to the interval from 220.0Ma to 167.1Ma.