松潘-甘孜地体内花岗岩锆石SHRIMP U-Pb定年及其构造意义

SHRIMP U-Pb dating of zircons from granitoid bodies in the Songpan-Ganzi terrane and its implications.

松潘-甘孜地体总体上是一个三角状褶皱带,其北侧、东南侧及东侧分别与东昆仑-西秦岭构造带、金沙江构造带及龙门山构造带相邻。地体内几乎全部被三叠系浊积岩所覆盖,其中侵位了很多花岗岩体。显然,这些花岗岩的岩浆特征、来源与侵位时代,对研究松潘-甘孜地体基底性质、构造演化等问题具有重要意义。本文报道利用SHRIMP定年技术对一些花岗岩岩体锆石所进行的精确测年数据。测试结果表明:(1)松潘-甘孜地体内的花岗岩体主要形成于晚三叠世,但岩浆活动可延续到早侏罗世晚期;(2)早期花岗岩浆活动与三叠纪系褶皱变形大致同期,指示这一时期的花岗岩浆的形成可能与三叠系下部大型拆离滑脱构造相关;(3)花岗岩结晶锆石普遍具有浑圆状或不规则状较老的继承锆石核,这些核部继承锆石可能代表花岗岩浆在上升过程中从不同地层内捕获的碎屑锆石,或者是下地壳岩石深熔残留锆石。根据SHRING U-Pb定年,这些继承锆石的年龄分别为二叠纪、加里东期、晚元古代和早元古代。元古代碎屑锆石的存在也可能指示松潘-甘孜地体具前寒武系基底。

The Songpan-Ganzi terrane is in general a triangular fold belt, bounded on the north, southeast and east by the eastern Kunlun-western Qinling tectonic belt, the Jinsha river tectonic belt and the Longmenshan tectonic belt, respectively. It is mainly composed of Triassic turbidite, which is intruded by many granitoid bodies. It is obvious that the composition, sources and time of the granitic magma are very important for studing the basement and tectonic evolution of the Songpan-Ganzi terranc. This paper reports some SHRIMP U-Pb dating results for zircons from granitoids in the Songpan-Ganzi terrane. The dating results are as follows： （ 1 ） The granitoid intrusions occurred in the Late Triassic and Early Jurassic; （2） The earl-stage magmatism was nearly simultaneous with the folding of Triassic strata, suggesting that the magmatism might have related with a large detachment under the Triassic strata; （3）Some of the crystal zircons contain rounded or unregular cores in shape, and it is thought that the rounded zircons were captured from the wall rocks during the upwelling of the granitic magma or detrital zircons from partial melting of the lower crust. Based on the SHRIMP dating, the core zircons yield ages of the Permian, early Paleozoic, late Paleozoic, and early Proterozoic, respectively. The present of Proterozoic detrital zircons indicate that the Songpan-Ganzi terrane is presumably floored with Precambrian basement rocks.