川西江浪穹窿二叠纪变玄武岩锆石U-Pb年代学及地球化学  

Zircon U-Pb Chronology and Geochemistry of the Permian Meta-basalt in the Jianglang Dome,Western Sichuan Province

作　　者：余祥龙 钟康惠[1] 王雄荣 杨光源[2] 王昌南[2] 胡晓[3] YU Xiang-long;ZHONG Kang-hui;WANG Xiong-rong;YANG Guang-yuan;WANG Chang-nan;Hu Xiao(Chengdu University of Technology,Chengdu 610066;Sichuan Liwu Copper Mining Company,Ganzi,Sichuan 626200;Sichuan Agricultural University,Chengdu 611130)

机构地区：[1]成都理工大学,成都610036 [2]四川里伍铜业股份有限公司,四川甘孜626200 [3]四川农业大学,成都611130

出　　处：《四川地质学报》2023年第2期330-338,共9页Acta Geologica Sichuan

摘　　要：江浪穹窿二叠纪大理岩之中发育一套变玄武岩夹层,其主要成分为角闪石(约75%)和斜长石(约25%)。本文进行了锆石U-Pb年代学与地球化学研究,旨在修定原岩的形成年代与探究其成因。定年数据显示,变玄武岩中发育2453.7~258.9Ma的捕获锆石。这一结果表明,变玄武岩的原岩最有可能形成于晚二叠世,并且具有古老变质基底(2453.7~2125.0Ma锆石)、Rodinia超大陆会聚—裂解事件(1144.5~853.5Ma锆石)和加里东期岩浆活动(430.4~427.1Ma锆石)的地质记录。主微量元素分析数据显示,变玄武岩具有低的SiO_(2)含量(45.53%~40.94%)与高的Mg#值(78.4~70.6)、Cr(1670×10#值(78.4~70.6)、Cr(1670×10^(-6)~1310×10(-6)~1310×10^(-6))含量,其稀土((La/Yb)N=3.07~1.15)与微量元素配分型式平坦,表明原岩岩浆很可能起源于高度部分熔融的亏损地幔源区。岩石的La/Sm(2.34~1.16)、(Th/Yb)_(PM)(1.95~1.23)、(Th/Ta)_(PM)(0.74~0.43)和(La/Nb)_(PM)(1.44~0.27)比值很低,表明岩浆演化过程中地壳物质的混染程度微弱。微量元素比值及构造环境判别图解表明,变玄武岩的化学构成同MORB相当,暗示原岩大概在古特提斯洋成熟的弧后盆地背景下形成。另外成分对比显示,江浪穹窿的二叠纪变玄武岩与峨眉山大火成岩省玄武岩浆活动无关。A meta-basalt interlayer is developed in the Permian marble of Jianglang Dome,its major components are amphibole(about 75%)and plagioclase(about 25%).Zircon U-Pb chronology and geochemistry were performed to explore the formation age and petrogenesis of its protolith.Dating data show that trapped zircons from 2453.7Ma to 258.9Ma were developed in the meta-basalt,indicating that the meta-basalt was probably formed in the late Permian.The rock also has the geological records of ancient metamorphosed basement(2453.7 Ma~2125.0 Ma zircons),Rodinia Supercontinent assembly-breakup(1144.5 Ma~853.5 Ma zircons)and Caledonian magmatic activities(430.4 Ma~427.1 Ma zircons).Major and trace element analyses show low Si0_(2) contents(45.53%~40.94%)and high Mg*values(78.4~70.6)and Cr(1670×10^(-6)~1310×10^(-6))abundances,with flat REE(La/Yb)=3.07~1.15)and trace element profiles.These geochemical features indicate that the protolith magma was probably derived from highly partial melting of a depleted mantle.The meta-basalt show extremely low La/Sm(2.34~1.16),(Th/Yb)pM(1.95~1.23),(Th/Ta)pM(0.74~0.43)and(La/Nb)pM(1.44~0.27)ratios,suggesting weakly crustal contamination.Trace element ratios and tectonic discriminations display similar chemical compositions to those of MORB,indicating a mature back-arc basin setting in the Paleo-Tethys ocean.Compared with the Emeishan basalts,the studied meta-basalt in the Jianglang dome has no genetic relationship with the Late Permian Emeishan basaltic magma activities.

关 键 词：二叠纪变玄武岩 亏损地幔 弧后盆地 松潘-甘孜造山带 古特提斯洋 

分 类 号：P597[天文地球—地球化学]