新疆西准噶尔乌尔禾地区早二叠世A1型花岗岩成因及其地质意义  

Petrogenesis and its geological implications of Early PermianA-type granite in Urho area,Western Junggar,Xinjiang

作　　者：支倩[1] 李永军[1,2] 段丰浩 王盼龙 张誉洋 ZHI Qian;LI Yongjun;DUAN Fenghao;WANG Panlong;ZHANG Yuyang(School of Earth Science and Resources,Chang\an University,Xi’an,Shaanxi 710054,China;Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits,MNR,Xi'an,Shaanxi 710054,China;School of Geological Engineering and Geomatics,Chang'an University,Xi'an,Shaanxi 710054,China)

机构地区：[1]长安大学地球科学与资源学院,陕西西安710054 [2]自然资源部岩浆作用成矿与找矿重点实验室,陕西西安710054 [3]长安大学地质工程与测绘学院,陕西西安710054

出　　处：《地质学报》2021年第8期2453-2470,共18页Acta Geologica Sinica

基　　金：中央高校基本科研业务费专项资金(编号300102270709);国家重点研发计划项目(编号2018YFC0604001)资助成果。

摘　　要：新疆西准噶尔南部地区出露的中酸性花岗岩以I型和A_(2)型花岗岩为主。本文通过对哈拉阿拉特山南麓乌尔禾地区新发现的A_(1)型碱长花岗斑岩进行详细的岩石学、锆石U-Pb年代学及地球化学研究,旨在准确厘定其形成时代,揭示岩石成因和构造背景。LA-ICP-MS锆石U-Pb定年结果显示碱长花岗斑岩形成于295.0±1.8 Ma,为早二叠世初期岩浆活动产物。地球化学研究表明碱长花岗斑岩具有A型花岗岩特征。岩石显示高硅(SiO2=73.72%~76.79%)、富碱(K_(2)O+Na_(2)O=7.70%~8.60%)、高TFeO/MgO(24.1~244,仅2件样品<16)、10^(4)×Ga/Al(3.73~5.20)、(Zr+Nb+Ce+Y)(1010×10^(-6)~2425×10^(-6))和锆饱和温度(978~1039℃),低镁(MgO=0.01%~0.11%)和铝(Al_(2)O_(3)=11.55%~12.01%),强烈亏损Ba、Sr、Eu、P、Ti以及高ε_(Nd)(t)正值(+7.76~+11.9)等特征。与区域晚石炭世晚期—早二叠世早期的A_(2)型花岗岩相比,乌尔禾碱长花岗斑岩相对富Nb(43.4×10^(-6)~122×10^(-6)),且具有较低的Y/Nb(0.46~2.24)比值,属准铝质A_(1)型花岗岩。综合以上数据并结合前人大量研究成果,我们认为乌尔禾A_(1)型碱长花岗斑岩和区域同时期的A_(2)型花岗岩均形成于由俯冲板片后撤诱发的弧后伸展环境。在弧后伸展体制下,热的软流圈地幔上涌,底侵加热由亏损地幔衍生而来的年轻镁铁质下地壳物质,使其发生低程度部分熔融,最终在地壳浅部形成了相对富Nb的乌尔禾A_(1)型碱长花岗斑岩。The intermediate—acid granites exposed in the southern part of the West Junggar(Xinjiang)are dominated by I-type and A_(2)-type granites.This study presents detailed petrology,zircon U-Pb geochronology and geochemistry of the newly discovered A_(1)-type alkali-feldspar granite porphyries in Urho area,southern slope of the Hala’alate Mountain,with the aim of determining their diagenetic age and revealing the petrogenesis and possible tectonic setting.LA-ICP-MS zircon U-Pb dating for the alkali-feldspar granite porphyry yielded crystallization age of 295.0±1.8 Ma,defining an early Early Permian magmatic event in southern West Junggar.The alkali-feldspar granite porphyries are geochemically characterized by high silicon(SiO2=73.72%~76.79%),alkaline(K_(2)O+Na_(2)O=7.70%~8.60%)contents,high TFeO/MgO(24.1~244,only 2 samples<16),10^(4)×Ga/Al(3.73~5.20)ratios and(Zr+Nb+Ce+Y)(1010×10^(-6)~2425×10^(-6))concentrations as well as high zircon saturation temperatures(978~1039℃),but low magnesium(MgO=0.01%~0.11%)and aluminum(Al_(2)O_(3)=11.55%~12.01%)contents with pronounced negative Ba,Sr,Eu,P,Ti anomalies and high positiveεNd(t)values of+7.76 to+11.9,displaying typical A-type granite affinity.Compared with the late Late Carboniferous to early Early Permian A_(2)-type granites in the region,the Urho alkali-feldspar granite porphyries possess significantly higher Nb(43.4×10^(-6)~122×10^(-6))concentrations and lower Y/Nb(0.46~2.24)ratios,categorizing them as metaluminous A_(1)-type granites.Based on our new petrological,geochronological and geochemical results and in combination with previous achievements,it is proposed that both the studied Urho A_(1)-type alkali-feldspar granite porphyries and the contemporaneous A_(2)-type granites in the southern West Junggar were formed in a back-arc extensional setting triggered by roll-back of the subducting Junggar oceanic slab.The back-arc extension led to the upwelling of hot asthenospheric mantle,which resulted in the low degree decompression melting of the juvenile mafic

关 键 词：A1型花岗岩 弧后伸展 早二叠世初期 西准噶尔南部 

分 类 号：P588.121[天文地球—岩石学]