新疆西准噶尔红山岩体及其中闪长质岩墙的时代——来自锆石LA-ICP-MS定年的证据

Ages of the Hongshan granite and intruding dioritic dyke swarms,in western Junggar,Xinjiang,NW China:Evidence form LA-ICP-MS zircon chronology

作为一种特殊产状的岩浆岩,岩墙保存有岩浆活动和地球动力学背景等方面的重要信息。为了研究和探讨西准噶尔乃至中亚地区古生代晚期的地球动力学环境,我们对西准噶尔地区侵入花岗岩的暗色岩墙开展了卫星遥感影像解译、地表地质调查和相关室内研究工作。研究发现,新疆西准噶尔红山花岗岩体中发育大量环状和非环状闪长岩岩墙。环状岩墙从中心到外围,岩墙宽度逐渐变窄,矿物粒度也逐渐变细;非环状岩墙切割环状岩墙,宽度和矿物粒度都没有明显变化。为了确定这些岩墙的形成时代,对3个环状岩墙样品、2个非环状岩墙和1个花岗岩样品进行了锆石LA-ICP-MSU-Pb定年,得到环状岩墙的年龄为:304±1Ma、302±1Ma、303±1Ma;非环状岩墙的年龄为:302±1Ma、302±1Ma;花岗岩的年龄为304±1Ma。结合不同类型岩墙及其与岩体的穿切关系,确定红山岩体的形成时代为304Ma左右,环状岩墙的形成时代为303Ma左右,非环状岩墙的形成时代为302Ma左右。这一年代学研究成果揭示出西准噶尔地区在石炭纪末期发育不同类型的岩浆活动,闪长质岩墙所占据的裂隙是在石炭纪末期形成的;为从岩浆活动和由这些岩墙所占据的裂隙研究,探讨该区古生代晚期的地球动力学环境,提供了有力的时间约束。

Dykes preserve important information which indicates the magma intrusion history and geodynamical environment.In order to reconstruct the Late Paleozoic geodynamical conditions of western Junggar and Central Asia,the large number of dark dioritic dykes,emplaced in Hongshan granitic pluton,in western Junggar,were studied by remote sensing interpretation,field investigation and sample analysis.It is revealed that there are two groups of dykes,namely the ring dykes and the non-ring dykes,emplaced in Hongshan granitic pluton.From the ring centre to margin the thickness and grain size of the ring dykes decrease gradually.The non-ring dykes,without obvious variety in thickness and grain size,cut the ring dykes at many outcrops.The zircon 206Pb/238U ages of the ring dykes,the non-ring dykes and the host rock were obtained,by LA-ICP-MS zircon U-Pb dating,including 3 samples from the ring dykes,2 samples from the non-ring dykes and 1 sample from the host rock were analyzed.Their ages are 304±1Ma,302±1Ma and 303±1Ma for the ring dykes,302±1Ma and 302±1Ma for the non-ring dykes,and 304±1Ma for the host rock.Considering these chronology data and the contact relationships among the ring dykes,the non-ring dykes and the host rock,the host rock was formed at 304Ma,the ring dykes are formed at 303Ma,and the non-ring dykes are formed at 302Ma,approximately.These chronology data imply that there were different kinds of magma intrusions in western Junggar and the fractures filled by later dioritic magma were formed in Late Carboniferous,and provide reliable time constrains for further study of Late Paleozoic geodynamical conditions in western Junggar and Central Asia.