Comparison of Protolith Assemblages of Metamorphic Rocks in the Dabie-Sulu Orogen and the Late Mesozoic Magmatic Rock Associations in the Coastal Region of Zhejiang and Fujian Provinces:Implications for the Neoproterozoic Tectonic Setting of Northeastern Y

The Dabie-Sulu orogenic belt is tectonically situated in the northeastern margin of the Yangtze Block.This belt is mostly underlain by granitic gneisses(over 80%),and minor eclogites occur as pods or layers within them.The protoliths of the granitic gneisses and eclogites correspond to acidic and basic igneous rocks,respectively,and both of them are of Neoproterozoic ages.In this paper,the protolith assemblages and geochemical characteristics of these metamorphic rocks are systematically compared with those of the Late Mesozoic igneous rock associations in the coastal area of Zhejiang and Fujian Provinces.The results have shown that they share quite similar features which can be summarized as follows:(1)Igneous rock types in both regions are dominated by acidic members with the basites constituting only about 5%,and there is a bimodal rock association with the acidic rocks as the predominant components.(2)The dominant acidic rocks in both regions can genetically be categorized into I-type granitoids,but a few of them show features of A-type granites.There are several plutons containing alkaline mafic minerals and can be ascribed to peralkaline A-type granites.These A-type granites,spatially coexisted with the I-type granites and commonly occurring as I-and A-type composite plutons,can be further grouped into PA or A2 subtype which generally suggested to have been formed in a postorogenic extensional setting.(3)The geochemical characteristics of basites in both regions are also quite similar.Both of them show Nb and Ta depletions on the primitive-mantle normalized spidergrams,and are mostly plotted in the fields of volcanic-arc or continental basalts on the related tectonic discrimination diagrams.Considering that there is generally a consensus concerning the tectonic setting for the Late Mesozoic magmatism in the coastal area of Zhejiang and Fujian Provinces, i.e.,generated in a back-arc extensional setting induced by the subduction of Paleo-Pacific plate,therefore,the highly similar features between the protolith assemblages of metamorphic rocks in the Dabie-Sulu orogen and the Late Mesozoic igneous rock associations in the coastal area of Zhejiang and Fujian Provinces,undoubtedly indicate that the northeastern margin of Yangtze Block during the Neoproterozoic might have been under a back-arc extensional setting along an active continental margin.