广西北部新元古代花岗岩锆石U-Pb年代学及其构造意义

U Pb zircon ages of granites from northern Guangxiand their tectonic significance

广西北部（扬子块体南缘）３个代表性的新元古代花岗岩体（本洞、三防和元宝山岩体）的矿物组合、岩石化学、元素和同位素地球化学特征表明，它们均为过铝质Ｓ型花岗岩。高精度的ＳＨＲＩＭＰ和颗粒级锆石ＵＰｂ定年结果表明，本洞、三防和元宝山岩体的形成时代分别为（８２０±７）Ｍａ、（８２５±６）Ｍａ和（８２４±４）Ｍａ，在误差范围内基本相同，排除了本区存在中元古代末／新元古代初形成的花岗岩。结合野外地质关系和扬子块体东南缘蛇绿岩的定年结果，扬子块体南缘新元古代晋宁期造山运动的时限应为１．０～０．８０Ｇａ，很可能以０．８２Ｇａ的晚造山花岗岩和约０．８Ｇａ的陆相双峰式火山岩的形成而告终，同时，震旦系底界年龄也应小于８２０Ｍａ。

Neoproterozoic granitic magmatism occurred sporadically along the southern margi n of the Yangtze Block. Mineralogical, geochemical and isotopic characters of th ree representative granitic plutons, the Bendong Granodiorite and the Sanfang an d Yuanbaoshan Granites from northern Guangxi indicate that they are peraluminous S type granites. Precise U Pb zircon ages of (820±7) Ma, (825±6)Ma and (824 ±4) Ma are obtained for the Bendong, Sanfang and Yuanbaoshan plutons, respectiv ely, by means of both SHRIMP and conventional dating methods. These age results are strongly in contrast with the previously published ages ranging from 668 to 1 100 Ma for these granites, precluding the existence of any late Mesoproterozoi c (early Jinningian) granitic magmatism in the study area. Based on the precise U Pb zircon age of both granites and ophiolites as well as field relations, the Jinning Orogeny is referred to the Neoproterozoic orogeny in SE China which too k place between 1.0～0.80 Ga, and likely ended at 0.82 Ga. The granite formation age of about 0.82 Ga supports not only the maximum age of 820 Ma for the initia tion of the post orogenic Sinian Basin, but also the basal boundary age of 0.80 Ga for the Sinian System in SE China.