摘要
中国大陆科学钻探工程主孔0~2000岩芯段的片麻岩包括二长花岗质片麻岩和斜长片麻岩两类,其中的Sr同位素组成与元素Rb的丰度关系显示原岩侵位后发生过强烈的Rb加入事件,说明Rb-Sr同位素体系在超高压变质和随后的退变质作用过程中受到了明显的扰动。相反,Sm-Nd同位素体系尚保持着封闭状态,可有效地示踪原岩的成因。二长花岗片麻岩的ε_(Nd)(t)值较低,变化范围也不大(-8.2~-13.0),它们的Nd同位素模式年龄(T_(DM))主要介于2.25到2.54 Ga之间,远大于其原岩的实际侵位年龄,指示其原岩可能是由古元古代的地壳物质在新元古代部分熔融形成的。而斜长片麻岩的ε_(Nd)(t)值变化范围广,从 1.3到-9.4。其中采自大套斜长片麻岩段的样品其ε_(Nd)(t)值从-3.0到-9.4,它们的Nd同位素模式年龄介于2.1~2.4 Ga,与二长花岗片麻岩的T_(DM)值类似,该类岩石的地球化学特征总体类似于扬子地台北缘新元古代双峰式火山岩的酸性端元,指示它们的原岩可能是在新元古代由地幔来源的玄武岩浆上升到地壳层位,引起成分不均一的古元古代地壳物质发生部分熔融形成的,其中可能混入了少量地幔来源的物质。而采自变镁铁质岩"夹层"中的斜长片麻岩显示出接近球粒陨石的Nd同位素特征(ε_(Nd)(t)= 1.3~-2.6),类似或略低于邻近的榴辉岩和斜长角闪岩围岩的Nd同位素组成及大别山-苏鲁造山带内大多数榴辉岩的Nd同位素组成,指示它们与这些变镁铁质岩之间可能有成因联系。它们的Nd同位素模式年龄(T_(DM)=1.28-1.60 Ga)虽然早于其原岩的侵位年龄,但明显小于花岗片麻岩及大套斜长片麻岩的T_(DM)值。我们倾向于认为这些"薄层"状的斜长片麻岩的原岩是由玄武岩浆派生的,伴有一定程度的地壳混染。
Granitic gneiss and plagiogneiss are the predominant rock types in the main hole of the Chinese Continental Scientific Drilling Project(CCSD-MH).Present-day ~(87)Sr/~(86)Sr ratios of these gneisses are obviously supported by Rb,which can be ascribed to add of Rb after the granite intrusion,implying the Rb-Sr systems were highly disturbed during ultrahigh-pressure metamorphism and/or subsequent retrograde metamorphism.On the other hand,the Sm-Nd isotopic systems have not been reset during the entire process of subduction and exhumation of the continental rocks and can be used to constraint on the nature of the protoliths.The granitic gneisses have low and narrow ranges ofε_(Nd)(t)values(-8.2 to-13.0),their Nd model ages from 2.25 to 2.54 Ga,much older than the emplacement ages of their protoliths,indicating their protoliths were probably formed as partial melting of Paleoproterozoic crust rocks in Neoproterozoic.The plagiogneisses in the CCSD-MH have a wide range ofε_(Nd)(t)values,from 1.3 to-9.4.The samples from the mass plagiogneiss part(the second petrological segment)show a wide range ofε_(Nd)(t)values(3.0 to-9.4),their Nd model ages from 2.25 to 2.54 Ga,similar with the T_(DM)values of the granitic gneisses.The geochemical characteristics of the plagiogneisses are similar with those of the acidite end-member(rhyolitic pyroclastic rocks)of the Neoproterozoic bio-model volcanic rocks remained in the northern margin of the Yangtze Craton,indicat partial melting of"old but heterogeneous crust sources"caused by basaltic magma from upper mantle,with some mantle material imput.Whereas the plagiogneisses as thin layers in metamorphic mafic host have near- chondriticε_(Nd)(t)values(ε_(Nd)(t)= 1.3~2.6),similar to or slightly lower than that of the eclogite and amphibolite,indicating a genetic relationship between the protoliths of the plagiogneisses and the metamorphic mafic host or implying that the plagiogneisses were retrograded directly from the eclogites.Their model ages(T_(DM)=1.28 1.60 Ga)are slight older than the emplacement ages of their protoliths,but much later than the T_(DM)Values of the granitic gneisses and the mass plagiogneiss.We tend to think that they was derived from the basaltic magma accompany some degree of crustal contamination during the magma differentiation.