攀西茨达和太和层状岩体时代

Geochronology of Layered Intrusions in Cida and Taihe Districts,Panxi Area,Sichuan Province

由于对攀西层状岩体中的同一岩体采用不同的定年方法获得的年龄结果有较大差异 ,导致对层状岩体的成岩时代产生争议。对攀西古裂谷带内茨达和太和两个层状辉长岩体进行了黑云母4 0 Ar/39Ar法和锆石ELA ICP MS法定年研究 ,结果表明 :茨达不含矿层状辉长岩中黑云母4 0 Ar/39Ar坪年龄为 2 5 6Ma ,太和含矿层状辉长岩中锆石U Pb年龄为 2 15Ma ,其年龄范围在 2 5 6～2 15Ma ,相当于晚二叠世—晚三叠世 ,属于海西晚期—印支期 ,而且不含矿层状辉长岩侵位时间在前 ,含矿层状岩体侵位时间在后。此外 ,根据此次层状岩体同位素地质年龄测定 ,表明其早期侵位时间并非在裂谷张裂之前的成穹阶段 ,而是和峨眉山玄武岩在晚二叠世—晚三叠世张裂阶段的喷溢具有同步性 ,亦显示层状辉长岩的岩浆作用与地幔柱活动 ,以及断裂构造的密切关系。

Because layered intrusions in Panxi paleorift host large-scale economic V- and Ti-bearing magnetite deposits,some geochronological studies have been done on it since the 1960s'. The dating techniques include K-Ar,Ar-Ar,Rb-Sr,Sm-Nd and U-Pb dating etc.,but different researchers provided different results on the same layered intrusions so that the contentious ages on the timing of layered rocks have been produced. The Cida and Taihe layered intrusions in the Panxi paleorift. were studied in this paper. The biotite 40 Ar/ 39 Ar dating and the zircon ELA-ICP-MS U-Pb dating for these layered intrusions have been performed. The ages obtained are as follows: the Cida rock body has biotite 40 Ar/ 39 Ar age of 256 Ma;and the Taihe rock body has a zircon U-Pb age of 215 Ma. These data suggest that the Cida and Taihe layered rock bodies were formed possibly in the late Variscan-Indosinian period about 256～215Ma ago. Further more,it is established that the ore-lean and low-grade layered gabbro bodies intruded earlier than ore-bearing and high-grade layered gabbro ones. It is believed that the timing of early intrusion emplacement (259-256Ma) is not the timing of pre-rift doming,but is consistent with the timing of rift during the late Permian,which is synchronous with the outpouring of Emeishan basalts during the late Permian-late Triassic Period (256-215 Ma). Consequently,the whole process of layered gabbro magmatism after outpouring is closely related with the fault structure and mantle-plume activity.