Nd-Sr isotopic constraints on the protoliths of the gneisses from the 100 similar to 2000 m main hole of the Chinese Continental Scientific Drilling Project.

Granitic gneiss and plagiogneiss are the predominant rock types in the main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH). Present-day Sr-87/Sr-86 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 of epsilon(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 of epsilon(Nd) (t) values, from + 1.3 to 9.4. The samples from the mass plagiogneiss part (the second petrological segment) show a wide range of epsilon(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-chondritic epsilon(Nd) (t) values (epsilon(Nd) (t) = + 1.3 similar to - 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.