中国大陆科学钻探工程主孔斜长角闪岩中锆石的SHRIMP U-Pb定年及其地质意义

SHRIMP U-Pb dating of zircons from amphibolite in the main hole of the Chinese Continental Scientific Drilling Project

矿物包裹体激光拉曼测试、阴极发光图像分析和SHRIMP U-Pb定年综合研究结果表明,中国大陆科学钻探主孔斜长角闪岩(样品G12)中的锆石可划分为2种类型:第一类颗粒粗大,含有大量矿物包裹体和杂质;第二类颗粒细小,含有少量矿物包裹体且十分干净。第一类锆石具有明显的双层结构,核部阴极发光图像显示典型的岩浆结晶环带.相应的矿物包裹体为单斜辉石(Cpx)+斜长石(P1)+磷灰石(Ap)±石英(Qtz),表明其寄主岩石的原岩为基性岩浆岩;边部阴极发光图像十分均匀,保存柯石英(Coe)+石榴子石(Grt)+绿辉石(Omp)+多硅白云母(Phe)+磷灰石(Ap)等标志性超高压包裹体矿物组合,记录的变质温压条件为T=732～839℃、p=3.0～4.0GPa,表明该类变质增生锆石微区形成于超高压变质阶段。第二类锆石阴极发光图像十分均匀,发光性明显增强,保存角闪石(Amp)+斜长石(P1)等标志性角闪岩相包裹体矿物组合,记录的变质温压条件为T=612～698℃、p=0.76～1.05GPa,表明该类锆石形成于后期角闪岩相退变质阶段。SHRIMP U-Pb定年结果显示,继承性岩浆结晶锆石核部记录的^(206)Pb/^(238)U的年龄为685～650Ma,表明其原岩形成于新元古代;含柯石英等超高压矿物包裹体的边部记录的^(206)Pb/^(238)U年龄为243～225Ma,表明超高压变质时代应归属于晚三叠世。第二类含低压包裹体矿物的锆石微区记录的^(206)Pb/^(238)U年龄为219～210Ma,应代表后期角闪岩相退变质作用的时代。上述研究结果表明,苏鲁地体斜长角闪岩的原岩在晚三叠世深俯冲至上地幔100～120km的地幔深处并经历了超高压变质作用,约15 Ma之后,这些超高压岩石抬升到约25 km的中部地壳,并经历了后期角闪岩相退变质作用的改造,相应的折返速率为0.57 cm/y。

An integrated study of mineral inclusions, trace elements and U-Pb ages was accomplished for complex zircons from amphibolite in the main hole of the Chinese Continental Scientific Drilling Project （CCSD-MH）. Zircon grains separated from amphibolite G12 may be subdivided into two types based on cathodoluminescence （CL） and laser Raman spectroscopy; they are big dusty zircons with an inherited （magmatic） core and a UHP metamorphic rim and small clear zircons. Zoned, grey-white luminescent inherited cores preserve mineral inclusions of clinopyroxene ＋ plagioclase ＋ apatite＋quartz indicative of a mafic igneous protolith. Grey-dark luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage of coesite ＋ garnet ＋ omphacite ＋ phengite ＋ apatite and formed at T = 732-839℃ and p= 3.0-4.0 GPa. In contrast, white luminescent, small clear zircons preserve amphibolitefacies mineral inclusions of amphibole ＋ plagioclase and formed at T = 612-698℃ and p= 0.76-1.05 GPa. Inherited zircons of sample G12 yielded SHRIMP ^206Pb/^238U ages of 685-650 Ma. The UHP rims yielded consistent Triassic ages around 243-225 Ma with aweighted mean of 229±3 Ma. Small clear zircon gave ^206pb/238^U ages around 219-210 Ma with a weighted mean of 214±3 Ma, interpreted as late-stage amphibolite-faeies retrogression. The data suggest deep subduetion of the protoliths of amphibolite in the Sulu terrane to＇the deep interior of the mande at 100-120 km depth in the Late Triassic （-229 Ma）, and -15 Ma later （at-214 Ma）, these UHP rocks was uplifted to the middle crust at -25 km depth and underwent reworking of late-stage amphibolite-faeies retrogressive metamorphism, with an average rate of exhumation of 0.57 cm/y.