西秦岭北缘秦岭杂岩麻粒岩相变质作用:来自相平衡模拟和独居石原位U-Pb定年的制约  

Granulite-facies metamorphism in the northern part of West Qinling:Constraints from phase equilibrium modeling and in-situ U-Pb dating of monazite

作　　者：郭祺 毛小红[1] 张建新[1] 路增龙 周桂生 滕霞 武亚威 GUO Qi;MAO XiaoHong;ZHANG JianXin;LU ZengLong;ZHOU GuiSheng;TENG Xia;WU YaWei(Key Laboratory of Deep-Earth Dynamics,Ministry of Natural Resources,Institute of Geology Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]自然资源部深地动力学重点实验室,中国地质科学院地质研究所,北京100037 [2]北京大学地球空间科学学院,北京100871

出　　处：《岩石学报》2022年第11期3259-3280,I0001-I0003,共25页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42072237、41630207);中国地质调查局地质调查项目(DD20221649);中国地质科学院地质研究所基本科研业务费项目(J1901-2)联合资助.

摘　　要：西秦岭天水北道地区位于西秦岭造山带北缘,地处秦岭造山带和祁连造山带的结合部位。近来,我们在该地区秦岭杂岩中识别出一套麻粒岩相岩石(石榴夕线黑云片麻岩),峰期矿物组合为石榴子石+钛铁矿+金红石+夕线石+钾长石+石英,相平衡模拟计算结果显示其峰期温压条件为P=1.02GPa,T=820℃,经历了进变质阶段以升温升压为特征,峰期之后以大致近等温降压为特征的顺时针P-T轨迹。独居石原位LA-ICP-MS U-Pb年代学结果显示石榴夕线黑云片麻岩经历了~421Ma、~388Ma和368~362Ma三期构造热事件。结合独居石的产出位置,我们推测421Ma代表了麻粒岩相变质作用的时代,这与天水花庙地区获得的麻粒岩峰期年龄一致;388Ma近似代表了峰期之后大致近等温降压至固相线附近的年龄;368~362Ma与区域韧性剪切作用的时代一致,可能记录了与新阳-元龙韧性剪切变形作用相关的热事件的时代。结合区域地质资料和本研究成果(矿物演化特征、顺时针的P-T演化轨迹和独居石原位定年结果),我们推测西秦岭造山带北缘天水北道地区的麻粒岩可能是早古生代陆-陆碰撞造山过程的产物。The Beidao area of Tianshui region,located in the northern margin of the West Qinling orogenic belt,lies at the junction between the Qinling orogenic belt and the Qilian orogenic belt.Recently,we have identified a suite of granulite-facies metamorphic rocks(garnet-sillimanite-biotite gneiss)from the Qinling Complex in this area.The peak mineral assemblage of garnet+ilmenite+rutile+sillimanite+K-feldspar+quartz was modeled to be stable in P-T condition of~1.02GPa and 820℃.It records a clockwise P-T path,characterized by a prograde evolution with an increase in pressure and temperature,and a retrograde evolution with a largely isothermal decompression process.The in-situ monazite U-Pb dating yielded three age populations of~421Ma,~388Ma and 368~362Ma,indicating that multi-stage tectonothermal events took place in the northern part of the West Qinling orogenic belt.Taking into account the occurrence of monazite,we speculate that 1)the age of 421Ma represents the timing of the granulite-facies metamorphism,which is consistent with the reported granulite-facies metamorphic peak age in the Huamiao area of the Tianshui region;2)the age of 388Ma probably represents the retrograde age of largely isothermal decompression near the solidus;and 3)the age population of 368~362Ma may document the tectonothermal events related to the Xinyang-Yuanlong ductile shear deformation.The integrated study of mineralogy,clockwise P-T path,and in-situ monazite dating,in combination with regional geological data,suggest that the Beidao granulite-facies rocks may be formed due to the continent-continent collisional orogeny in the Paleozoic.

关 键 词：西秦岭造山带 麻粒岩相变质作用 独居石原位定年 秦岭杂岩 相平衡模拟 

分 类 号：P588.34[天文地球—岩石学] P597.3[天文地球—地质学]