滇西桃花花岗斑岩中新太古代-古元古代锆石年龄信息:对扬子板块西缘基底时代的约束

扬子板块西缘滇西地区是否存在古老基底一直存在争议。本文对滇西桃花地区花岗斑岩进行了岩石学、地球化学和锆石SHRIMP U-Pb年代学研究。形成于晚造山-后碰撞背景的桃花花岗斑岩具岛弧花岗岩地球化学特征,其成因可能与:1)俯冲拆离的洋壳俯冲拆离的洋壳或富集地幔重熔作用;2)加厚的地壳部分熔融。花岗斑岩中的继承锆石有两种类型:一类是发育具有密集振荡环带的岩浆锆石;另一类是次浑圆状锆石。测年结果显示,花岗斑岩的岩浆锆石年龄为36.35±0.35Ma,环带发育的继承锆石年龄介于167~891Ma之间;而次浑圆状继承锆石可以分为两组,其207Pb/206Pb加权平均年龄分别为1851±22Ma与2499±32Ma。新的锆石测年结果表明着滇西桃花地区不仅存在古金沙江洋东向俯冲形成的晚古生代弧岩浆记录,还发现新元古代岩浆活动信息,及早古元古代和新太古代的锆石记录。推测1.8Ga与2.5Ga锆石可能是捕获自地壳或围岩(石鼓片岩),表明滇西地区可能存在古老基底。

Archean-Paleoproterozoic zircons from a granite porphyry in the Taohua area, western Yunnan and their constrain on the basement of western margin of Yangtze plate

The existence of Paleoproterozoic basement in the western Yunnan area has been a hot topic of debate. In this study, large amounts of inherited zircons of the granite porphyry in the Taohua area were selected to use for the SHRIMP U-Pb dating. The Taohua granite porphyry that has geochemical signatures of island arc granite and was formed in a late orogenic to late-collision-process, which possibly related to remelting magmatism induced by resubduction of the detached Jinshajiang oceanic crust and enriched lithospheric mantle domain or by the partial melting of the thickened crust. Inherited zircons were divided into two types: core inherited zircon presented in cores of magmatic zircons with dense oscillatory zones, and subrounded shape inherited zircon. Based on the dating results, the magmatic zircons have an age of approximately 36.35±0.35Ma, the inherited core zircons have ages ranging from 167Ma to 891Ma, whereas the subrounded shape inherited zircons record two group zircons with mean ages of 1851±22Ma and 2499±32Ma, respectively. The new dating results of the zircons from the Taohua granite porphyry indicate that there are not only records of later Paleozoic arc magmatism related to eastward subduction of the Jinshajiang ocean plate and of the Neoproterozoic rift magmatism, but also Early Proterozoic zircon and magmas in the Taohua area, western Yunnan. Moreover, the 1.8Ga and 2.5Ga zircon groups were originated from the ambient Shigu schist, and the Shigu schist possibly has a 2.5~1.8Ga crust basement. This is to say that there is Early Paleoproterozoic basement in the western Yunnan area.