THE MESO-CENOZOIC CRUST-MANTLE IETERACTIONS AND THE TECTONIC EVOLUTION IN THE NORTH PART OF SOUTH CHINA SEA AND ITS COASTAL AREAS ——TO THE MEMORY OF PROFESSOR CHEN GUODA, THE ADVOCATE OF THE CONCEPT OF CONTINENTAL MARGINAL EXTENSIONAL BELT
ZOU Heping (Department of Earth Sciences, Sun Yat-sen University, Guangzhou, GD 510275, China)
The lithospheric mantle beneath the north part of South China Sea and its coastal areas is mainly composed of lherzolites which are high-thermal, relatively fertile in major elements, and MORB-OIB-like in isotope compositions; however, in the uppermost part there is a relict older lithospheric mantle composed of harzburgites which are low-thermal, relatively depleted in major elements, and EM-like in isotope compositions. In the lowest part of the crust, mafic granulites which were resulted from fractional crystallization and cumulation of mantle-derived magma in Late Mesozoic are distributed. Based on the data for the deep-sourced xenoliths in the Cenozoic volcanic rocks, a tentative model has been put forward to explain the crust-mantle interactions or the lithosphere-asthenosphere interactions and the tectonic evolution in the north part of South China Sea and its coastal areas. The evolution processes can be divided into: (1) episode of the Cathaysian post-platform orogenic belt with a thickened crust and also a thickened lithosphere from Indosinian to earlier Yanshanian; (2) episode of fast thinning (e.g., delamination) of the lithosphere, in company with extension and collapse of the orogenic belt and magma underplating in the lower crust during later Yanshanian; (3) episode of the asthenosphere upwelling again (e.g., as a result of the mantle plume) accompanied by the base-erosion of the lithospheric mantle and also by the attenuation of the crust because of the flexible flowage in the lower crust and the rifting in the upper crust during the period from Eocene to Oligocene; and (4) episode of formation of the "new" lithospheric mantle which was transformed from a part of the asthenosphere with release of the mantle-derived heat in the extensional settings and cooling of the mantle. Therefore, the continental marginal extension along the north part of South China Sea, which was exhibited as vertical thinning and horizontal spreading of the lithosphere, was a result of the deep crust-mantle interaction after the continent crust in the region had evolved into the continental activation orogenic stage. It was different from the back-arc spreading caused by subduction of oceanic plates, and was not controlled by the Atlantic-type seafloor spreading either.