松潘-甘孜造山带中部年轮寺北岩体锆石LA-ICP-MS年代学及地质意义

LA-ICP-MS zircon U-Pb ages and geological significance of Nianlunsi rock,middle Songpan-Ganze fold belt

松潘-甘孜造山带中部出露的年轮寺岩体,岩性为辉石黑云闪长岩,采用LA-ICP-MS法对样品进行了锆石U-Pb测年,获得两组加权平均年龄分别为212. 3±1. 4 Ma(MSWD=0. 68,n=13)和199. 2±2. 1 Ma(MSWD=2. 1,n=6)。结果表明,年轮寺岩体岩浆的初始结晶时代为晚三叠世,伴随区域造山作用,经历了两期岩浆活动,其中212. 3±1. 4 Ma为岩浆活动的主要时期,199. 2±2. 1 Ma为区域岩浆活动的次要阶段。松潘-甘孜造山带花岗岩的成岩过程从243±4 Ma持续至181. 2±1. 4 Ma,大多数花岗岩成岩年龄集中于(229±3)～(205±1) Ma,从而暗示松潘-甘孜造山带晚三叠世开始岩浆活动,造山作用主要发生在印支晚期。结合前人研究,认为年轮寺岩体是在后碰撞构造环境下,由西康群新都桥组的板岩、千枚岩、粉砂质板岩等地壳物质的部分熔融而形成。

In the middle of the Songpan-Ganze orogenic belt,there is an Nianlunsi rock mass with pyroxene biotite diorite lithology,which has not yet been chronologically studied.In this article,the method of zircon LA-ICP-MS U-Pb dating was used,and the author yielded the weighted average 206 Pb/238 U age of 212.3±1.4 Ma(MSWD=0.68,n=13)and 199.2±2.1 Ma(MSWD=2.1,n=6).The result shows that the initial crystallization period of the magma of the Nianlunsi rock was at late Triassic,accompanied by regional orogeny,it experienced two stages of magmatic activities.212.3±1.4 Ma was the main stage of magmatic activities,while 199.2±2.1 Ma was the secondary stage of regional magmatic activities.The diagenetic process of granite in the Songpan Ganze orogenic belt continues from 243±4 Ma to 181.2±1.4 Ma,and most of the granite diagenetic ages are concentrated between 229±3 Ma and 205±1 Ma,suggesting that the Songpan-Ganze orogenic belt began magmatic activity at Late Triassic.Orogenic action occurs mainly in the late Indosinian period.According to the previous research,it is the post-collision tectonic environment,that formed the Nianlunsi rock by crustal materials partial melting,such as slate,phyllite,silty slate,and so on,from Xinduqiao Formation of Xikang Group.