南海及其周缘中新生代火山活动时空特征与南海的形成模式

TEMPORAL AND SPATIAL DISTRIBUTIONS OF MESO-CENOZOIC IGNEOUS ROCKS OVER SOUTH CHINA SEA

根据南海海区、华南和中南半岛的地面露头、钻井、拖网及地球物理资料,分析了南海地区火山活动的时空分布特点。在南海陆缘和周边陆区中生代末期花岗岩分布非常广泛。新生代火山岩活动规模较小,主要是海底扩张之后在洋盆扩张脊、北部陆缘的陆洋边界附近、雷琼地区和中南半岛南部的玄武岩。在南海北部陆缘的深部地震调查中发现,在地壳下部存在小规模的高速异常体,结合浅部的晚第三纪—第四纪火山活动,认为该高速体形成于南海扩张之后。这些特征表明,在南海的拉张过程中岩浆供应不丰富,在陆缘未形成大规模的侵入和喷出岩。南海陆缘属于岩浆匮乏型被动大陆边缘。南海海区残留多个刚性断裂陆块,反映了裂谷拉张过程中脆性破裂。根据这些特征,南海形成难以用印藏碰撞引起的软流圈物质上涌导致岩石圈破裂这样的模式来解释。

The temporal and spatial distributions of Mesozoic and Cenozoic magmatism were reviewed with outcrops, drilling, dredge and seismic data. Late Mesozoic granite was widespread over the South China to Indochina; whereas the Cenozoic magmatism was not so vigorous, late basalts occurred mainly in the extinguished spreading ridge, continental-oceanic transitional zone in the northern margin, Leizhou Peninsular-northern Hainan Island and south Indochina after the cessation of sea floor spreading. Small-scale high velocity layer was revealed by deep seismic survey. It was inferred to be a magma intrusion, and most possibly formed after the spreading of the South China Sea considering the presence of late Neogene-Quaternary volcanoes nearby. These features indicated that the margins of the South China Sea were non-volcanic passive continental margins. Remains of large rigid faulted continental blocks reflected fragile fracture through rifting and separation. The paucity of magma supply and the presence of rigid faulted blocks are unfavorable to the mantle flow model previously proposed to interpret the opening of the South China Sea which was supposed to be linked to the escape of Indochina induced by Indo-Eurasia collision.