南海北部及其沿岸中、新生代壳幔相互作用与构造演化——纪念“陆缘扩张带”概念的倡导者陈国达教授

新生代火山岩中的深源捕虏体资料反映,南海北部及其沿岸地区岩石圈地幔的主体由主量元素易熔组分相对饱满的、同位素组成类似MORB-OIB型的、高温型的二辉橄榄岩所组成;但在其顶部残留有古老的岩石圈地幔,它由主量元素易熔组分相对贫瘠的、同位素组成类似EM型的、较低温的方辉橄榄岩组成。在下地壳底部,分布着由晚中生代幔源岩浆分离结晶和堆晶的基性麻粒岩。由此提出了该区中、新生代壳 -幔或岩石圈 -软流圈相互作用与构造演化的简略模式: (1)印支期 -燕山早期为地壳岩石圈厚度增大的华夏型后地台活化造山带环境;(2)燕山晚期岩石圈快速减薄(如拆沉作用),造山带拉伸塌陷,地壳深处并发生广泛的底侵作用; (3)始新世 -渐新世软流圈再次上涌(如地幔柱的影响),岩石圈地幔发生底蚀减薄,地壳也因为下部层的塑性流展和上部层的张裂拉伸而减薄; (4)中新世以来,由于地幔热源在拉伸环境中被释放,壳幔发生冷却,部分软流圈地幔转化为“新生的”岩石圈地幔。研究进一步说明,南海北部陆缘扩张是该区大陆构造演化到大陆活化造山带后期,在深部壳 -幔的相互作用下,岩石圈所发生的垂向减薄和侧向伸展,既不同于弧后扩张,也不是受控于大西洋式的海底扩张。

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

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.