滇东南发现新元古代岩浆岩:SHRIMP锆石U-Pb年代学和岩石地球化学证据

Discovery of the Neoproterozoic magmatics in southeastern Yunnan:Evidence from SHRIMP zircon U-Pb dating and lithogeochemistry.

滇东南老君山地区出露了一套穹隆状变形-变质岩系,新近完成的1:5万都龙幅、麻栗坡县幅区域地质矿产调查报告,将其分解为晚志留纪南温河花岗岩、新元古界新寨岩组和古元古界猛硐岩群。首次对猛硐岩群的石英角闪斜长片麻岩进行了SHRIMP锆石U-Pb定年,结合阴极发光图像分析,获得两组岩浆锆石的结晶年龄分别为761±12Ma和829±10Ma,指示滇东南老君山地区存在新元古代岩浆活动,同时从同位素年代学上,表明猛硐岩群是一套前寒武纪的变质沉积-岩浆杂岩。此外,还获得一些年龄为≈1．83Ga的残留锆石,表明该区存在古元古代的结晶基底。石英角闪斜长片麻岩及共存的斜长角闪岩,它们的原岩具有大陆裂谷背景下玄武质-玄武安山质岩浆系列的地球化学特征,可能与新元古代Rodinia超大陆聚合-裂解过程中的岩浆活动有关,本区在新元古代可能位于南华裂谷与康滇裂谷的交汇部位。

The region includes southeastern Yunnan and North Vietnam, which locates among the Nanhua, Yangtze and Indio-China Blocks, is a key target to study the tectonic evolution of South China and Southeastern Asian. There is a deformed-metamorphic dome outcrops in the Laojunshana area, Southeastern Yunnan. It had been divided into the Later Silurian Nanwenhe granite, the Neoproterozoic Xinzhai Formation and the Paleoproterozoic Mengdong Group in the recently 1/50000 regional geological surveys. The SHRIMP zircon U-Pb dating have been done first on the quartz-hornblende-plagiogneiss from the Mengdong Group, and the ages of two magmatic zircon groups are 761 ± 12Ma （ N = 11, MSWD = 0. 85 ） and 829 ± 10Ma （ N = 10, MSWD = 0. 36 ）, respectively. Combined with the crystal features, CL images, concordant of U-Th-Pb ages （ the D% are 2. 6 and 4. 8, respectively） and the correlation index （0. 64 and 0. 19, respectively）, 761 ± 12Ma should present the crystal age, and 829 ± 10Ma should presented the age of an older magmatism. These ages suggested that there should have the Neoproterozoic magmatism in the area, and the Mengdong Group should be a Precambrian metamorphic sedimentary and magmatic complex. Besides these, the inherited zircons with ≈ 1.83Ga U-Pb dating indicated that there should have the paleo-proterozoic basement rocks in this area. The coexist quartz-plagio-amphibolite have rare-earth element and trace element distribution characteristic of OIB, and the quartz-hornblende-plagiogneiss have similar geochemistry characteristics beside low TiO2 , P2O5, K2O, Rb, Sr, Eu contents and moderate Nb-Ta negative anomaly, which should be attributed to crystal fraction and crust-contaminate. The geochemistry and tectonic setting identification of these samples suggested that their protolith should be basaltic to basaltic-andesitic magmatic series related to mantle-plume. Based on these mention above, the Laojunshan area, Southeastern Yunnan should located in the cross region of the Nanhua Rift and the Kangdian Rift during breaking-up of the Rodinian Super-continent in the Neoproterozoic.