青藏高原羌塘中部本松错花岗质片麻岩锆石U-Pb年龄、Hf同位素特征及地质意义

Zircon U-Pb ages and Hf isotopic characteristics of granitic gneiss from Bunsumco, central Qiangtang, Qinghai-Tibet Plateau

本松错花岗岩岩基位于龙木错-双湖缝合带内,是多期中酸性岩浆侵入的产物.通过对花岗岩岩基中的花岗质片麻岩LA-ICP-MS锆石U-Pb定年,获得206Pb/238U加权平均年龄分别为496.2±3.9,497.1±3.0 Ma,该年龄值是迄今在羌塘盆地发现最古老、最可靠的岩浆岩年代学记录,为解决该地区关键地质问题提供了新的线索.获得花岗质片麻岩的锆石Hf(t)均为负值(均值,2.9,4.1),暗示花岗质片麻岩为壳源成因,两阶段模式年龄均值为1648和1723 Ma,花岗质片麻岩是原特提斯洋向冈瓦纳大陆边缘俯冲而引起古老地壳物质部分熔融的产物,是冈瓦纳大陆拼合之后在冈瓦纳大陆的边缘发生的安第斯型造山作用,而不属于冈瓦纳超大陆汇聚过程中发生的陆-陆碰撞形成的泛非造山带.说明冈瓦纳大陆北缘的岩浆弧向北至少延伸到了龙木错-双湖缝合带以南地区,龙木错-双湖缝合带为冈瓦纳大陆的北界.

The Bunsumco granitic batholith in the Longmu Co-Shuanghu suture zone is the product of intermediate-acidic intrusive magmatism over several periods. Using zircon U-Pb dating by laser-ablation inductively coupled mass spectrometry of granitic gneiss from the batholith, we obtained weighted mean 206Pb/238 U ages of 496.2±3.9 and 497.1±3.0 Ma. These are the oldest and most reliable dates for the chronological record of magmatism in the Qiangtang Basin of the Qinghai-Tibet Plateau. These dates provide new clues to solve the key geological problems in this area. The negativeHf（t） values（averages of-2.9 and-4.1） imply a crustal source for the granite, with Hf two-stage model ages of 1648 and 1723 Ma. The Bunsumco granite is the product of partial melting of old crust, initiated by the subduction of the proto-Tethys in an Andean-type orogenic zone after Gondwana assembly, but does not represent a Pan-African orogen during continental collision within the Gondwana terranes. This suggests that the northern Gondwana margin extends northward to the Qiangtang Basin and is represented by the Longmu Co-Shuanghu suture zone.