藏南冈底斯中段大竹卡桥南花岗闪长岩锆石U-Pb定年及地质意义

Zircon U-Pb dating and geological implications for the granodiorites from the Dazhuka region in Xizang

冈底斯岩带中各岩体的准确定年对特提斯洋演化和青藏高原隆升研究有重要意义。日喀则大竹卡桥南花岗闪长岩体位于冈底斯岩带中段，前人曾开展过两次K—Ar法定年，但定年结果差距较大。本文利用LA-ICP—MS锆石u—Ph方法对该花岗闪长岩进行了定年。定年所选锆石发育有规律环带结构，Th／U比值为0．20～1．69，为岩浆锆石，获得80．3±3．6Ma（MSWD=3．2）的加权平均年龄，代表花岗闪长岩的结晶年龄。定年结果反映该岩体为新特提斯洋俯冲成因。岩体南侧紧邻雅鲁藏布江缝合带，其岩浆活动很可能是雅鲁藏布江新特提斯洋向北俯冲所引起的。这一岩体年龄数据的报道为深入探讨晚白垩世雅鲁藏布江洋是否存在平板俯冲提供了新的研究对象。

The precise dating for the granodiorites from the Gangdise zone in Xizang is of great importance to the research of the Tethyan evolution and Qinghai-Xizang Plateau uplift. The granodiorite masses south of Dazhuka Bridge occur in the central part of the Gangdise zone in Xizang. There is the large discrepancy between the preexisting K-Ar age determinations given by the previous workers. The present paper deals with the zircon U-Pb re-dating of the granodiorite masses south of the Dazhuka Bridge with the aid of LA-ICP-MS method. The selected zircons for dating show regular oscillatory zonal structures. The Th/U ratios vary from 0. 20 to 1. 69. These characteristics suggest a magmatic origin of the zircons. The weighted average age of 80.3 ± 6 Ma （ MSWD = 3.2） may represent the crystallization age of the granodiorites. Since the granodiorite masses are very close to the southern Yarlung Zangbo suture zone, the granodiorites may well be originated from the magmatism caused by the northward subduction of the Yarlung Zangbo Neo-Tethys oceanic plate.