康定杂岩Rb-Sr、Sm-Nd同位素系统及其意义

通过对康定—冕宁地区出露的英云闪长岩、黑云角闪斜长片麻岩、角闪变粒岩全岩及其中所分离出的角闪石、黑云母、斜长石、钾长石的Rb-Sr、Sm-Nd同位素的系统测定,结合岩石的锆石U-Pb年龄结果,确定这些变质杂岩由于经历了复杂的形成过程与变质历史,Rb-Sr、Sm-Nd同位素体系难以确定其结晶年龄。由单矿物与全岩Rb-Sr、Sm-Nd体系拟合的~700 M a的等时线年龄反映了角闪岩相-高角闪岩相的变质作用年龄。Sm-Nd同位素体系由于在变质作用过程中的部分开放性,很容易给出无意义的较老的混合年龄。康定杂岩结晶后并没有经历麻粒岩相变质作用,区域上所含的麻粒岩透镜体可能是新元古代(773~721 M a)期间由Rod in ia超大陆裂解产生的新生洋壳向扬子克拉通陆块俯冲消减过程的变质产物。俯冲到一定深度后,由于板片被拉断,软流圈上涌导致变质洋壳板片岩石、先前底侵变质的镁铁质岩石及扬子陆块长英质基底岩石发生部分熔融,以镁铁质岩石熔融产生的熔浆为主(>70%),与长英质基底岩石熔融产生的熔浆混合形成w(Na2O)/w(K2O)>1的TTG组合。

Rb-Sr and Sm-Nd Isotopic Compositions of the Kangding Complexes and Their Significances

Through systematic measurement of Rb-Sr and Sm-Nd isotopic compositions in three sets of hornblende,biotite,plagioclase,K-feldspar separates extracted from tonalite,biotite-hornblende-plagioclase gneiss,and hornblende leptite and their correspond whole rocks in the Kangding-Mianning area,Sichuan Province,together with U-Pb ages of zircons in the rocks,the results indicate that their original crystallization ages of magmas cannot be ascertained by Rb-Sr and Sm-Nd isotopic systematics because of complicated history of formation and metamorphism.Fitted~700 Ma isochrones by Rb-Sr and Sm-Nd isotopic compositions of mineral separates and their whole rocks suggest metamorphism age of amphibolites-upper-amphibolite facies.Because of partial resetting of Sm-Nd isotopic system during the processes,Sm-Nd isochrones give rise to older meaningless mixed age usually.The Kangding complexes had not experienced metamorphism of granulitic facies after magma crystallization.The lenticular granulitic bodies may be the relict products of metamorphosed oceanic crustal component after partial melting during subduction of newly formed oceanic crust underneath the Yangtze craton in Neoproterozoic period by the Rodinia super-continent's break-off.The metamorphosed oceanic crustal rocks,previously underplating mafic rocks,and metamorphosed felsic basement rocks of the Yangtze craton began to partially melt when the subducted oceanic plate was broken off and up-welling of asthenospheric material.TTG(tonalite-trondjemite-granodiorite)association was formed by mixing of dominant(>70%)magma derived from partial melting of metamorphic mafic rocks with that from partial melting of metamorphic felsic basement rocks.