大别山前寒武纪变质地体基本组成

THE MAIN COMPOSITION OF THE PRECAMBRIAN METAMORPHIC TERRANE IN THE DABIE MOUNTAINS

本文以新城-圻春断裂为界将大别山前寒武纪变质地体划分为华北陆块南缘和扬子陆块北缘两个次级变质地体。两个次级地体不仅在地球物理、构造变形方面明显不同,而且在物质成分上有显著差异:它们有各自独立的变质地层系统,遭受了不同类型的变质作用,有完全不同的岩浆活动图象。上述差异均可指示华北、扬子两古陆碰撞对接时扬子陆块北缘向北俯冲至华北陆块南缘之下,这可能包括两次敛合作用、从元古代开始至中生代最终结束的长期复杂的造山过程。

The precambrian terrane in the Dabie Mountains is a composite metamorphic terrane formed, most probably, by collision-convergence between the North China and Yangtze continental blocks. The Xingcheng-Qichung fault zone is one of the most important tectonic boundaries within this composite terrane. In the present paper the terrane is divided, into two major tectonic units (second-order metamo-rphic terranes) by that tectonic boundary: the metamorphic terrane on the north margin of the Yangtze continental block (NMY) in the south and the metamorphic terrane on the south margin of the North China continental block (SMNC) in the north. The two metamorphic terranes show significant differences not only in geophysics and tectonic deformation but also in their material composition. The differences in material composition are manifested in metamorphic stratigraphy, metamorphism and magmatism. The two metamorphic terranes are characteristc by their own metamorphic stratigraphic systems undergone different types of metamorphism and by entirely different features of magmatism. The differences in composition of the southern and northern Dabie terranes indicate that during the convergence-collision of the two continental blocks the NMY might have been subducted northward beneath the SMNC. This would be a longterm and complicated orogenic process which had probably begun in the Mid-Late Proterozoic and ended in the Jurassic. The P-t path of the SMNC metamorphic terrane suggests a two-stage convergence. The geological characteristics of eclogite and its associated ultramafic rocks show that some of the eclogite were probably brought up by ultramafic magma from the deep level of a subduction, zone to the lower crust and then emplaced to the shallow level of the crust by tectonic processes.