北秦岭松树沟榴闪岩锆石U-Pb定年、矿物包裹体和Lu-Hf同位素特征及其地质意义  

Zircon U-Pb dating,mineral inclusions,Lu-Hf isotopic data and their geological significance of garnet amphibolite from Songshugou,North Qinling

作　　者：钱加慧[1,2] 杨秀清[1,2,3] 刘良[1] 曹玉亭[1] 陈丹玲[1] 杨文强[1] 

机构地区：[1]西北大学地质学系,大陆动力学国家重点实验室,西安710069 [2]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室,北京100871 [3]国土资源部成矿作用与资源评价重点实验室,中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2013年第9期3087-3098,共12页Acta Petrologica Sinica

基　　金：国家重点研究发展规划"973"项目(2009CB825003);大陆动力学重点实验室"秦岭造山带新元古-早中生代演化的几个重要问题研究"项目;西北大学地质学系国家基础科学人才培养基金(XDCX08-03)联合资助

摘　　要：采用LA-ICP-MS锆石U-Pb定年法对松树沟榴闪岩中的锆石进行年代学分析,得到锆石残留核的206Pb/238U加权平均年龄为729±24Ma,对应的Th/U比值为0.70~1.01;变质锆石的206Pb/238U加权平均年龄为496±9.1Ma,对应的Th/U比值为0.02~0.11。通过激光拉曼探针在变质锆石里寻找到斜长石、单斜辉石、石榴石、石英、榍石、磷灰石等矿物包裹体,结合岩石具有的石榴石变斑晶和单斜辉石+斜长石嵌晶,推测变质锆石可能生长于岩石的高压麻粒岩相变质阶段,不排除早期存在榴辉岩相变质的可能性,所获~496Ma的变质时代记录了一期与陆壳深俯冲相关的区域地质事件。利用LA-MC-ICP-MS法测定了变质重结晶锆石的176Hf/177Hf=0.282560~0.282663;以t=730Ma计算出εHf(t)=+8.6^+12.2;Hf模式年龄为867±43Ma。锆石残留核年龄和变质重结晶锆石的t DM(Hf)数值,表明该榴闪岩的原岩应老于729Ma,原岩物质从亏损地幔分离的时间不晚于867±43Ma。结合前人研究成果,认为松树沟榴闪岩是早期侵位的岩石遭受早古生代高压-超高压区域变质作用的产物。Two discrete age groups were obtained through LA-ICP-MS zircon U-Pb dating of Songshugou garnet amphibolite： a residual core age of 729 ＋ 24Ma with Th/U = 0. 70 --1.01 and a metamorphic age of 496 ＋ 9. 1Ma with Th/U = 0. 02 -- 0. 11. Mineral inclusions of plagioclase, clinopyroxene, garnet, quartz, sphene and apatite were identified by laser Raman spectroscopy in metamorphic zircons from the garnet amphibolite. Combined with the garnet porphyroblasts and poikilitic intergrowth of clinopyroxene ＋ plagioclase occurring in the rock,we infer that the metamorphic zircons of garnet amphibolite may grow under high-pressure granulite- facies metamorphism and we cannot rule out the possibility that there was an early eclogite-facies metamorphism experienced by the rock. The obtained metamorphic age of ~ 496Ma recorded a regional metamorphic event associated with continental deep subduction. Hf isotope compositions of reerystallized zircons were measured by LA-MC-ICP-MS. 176 Hf/177 Hf = 0. 282560 -0. 282663 and sm （t） = ＋ 8. 6 - ＋ 12. 2 when calculating with t = 730Ma. Hf model age is 867 ＋ 43Ma. Age of residual zircon cores and tDM （Hf） of recrystallized zircons from garnet amphibolite indicate that the protolith age of the rock should be older than 729Ma and the time for it fractionated from depleted mantle should not be later than 867 ＋ 43Ma. Take the previous research work into account, Songshugou garnet amphibolite was thought to be a product of an Early Paleozoic HP-UHP regional metamorphism on the early emplaced rock.

关 键 词：榴闪岩 U-PB定年 稀土元素 矿物包裹体 LU-HF同位素 

分 类 号：P588.348[天文地球—岩石学] P597.3[天文地球—地质学]