跳出南海看南海--新特提斯洋闭合与南海的形成演化

The formation of the South China Sea resulted from the closure of the Neo-Tethys:A perspective from regional geology

本文总结了笔者参与基金委重大研究计划"南海深海过程演变"的研究成果。我们发现南海和青藏高原都是新特提斯洋闭合的产物,而非前人所说的南海是由青藏高原碰撞导致的中南半岛逃逸所形成。与青藏高原碰撞隆升机制不同,南海是新特提斯闭合后期弧后拉张的结果。新特提斯洋位于北边的欧亚大陆与南面的非洲、印度和澳大利亚板块之间,呈东宽西窄的喇叭型。在西部,新特提斯洋向北的俯冲可能在侏罗纪就开始了,局部形成了弧后盆。约130Ma前,由于凯尔盖朗等大火成岩省的喷发,新特提斯洋脊也开始向北漂移。由于新特提斯洋东部宽度较大,弧后拉张明显,形成了古南海。新特提斯洋闭合过程中一个重大事件是洋脊俯冲:从菲律宾经福建及两广到青藏高原,均有100Ma左右的埃达克岩产出,是洋脊俯冲的产物。其中,菲律宾、福建、广东埃达克岩形成了斑岩铜金矿床;而在青藏高原,埃达克岩虽有矿化,但没有形成大规模的斑岩铜金矿床。同时期,华南出现了一次短暂的大规模挤压事件,与洋脊俯冲契合。这次挤压事件可能导致了古南海闭合的开始。与此同时,青藏高原冈底斯出现高温岩石——埃达克质紫苏花岗岩;其北面有～110Ma短时间内发生的大规模花岗岩事件。考虑到板块重建的结果,这些埃达克岩和华南短时间挤压事件的时空分布显示新特提斯洋脊在约100～110Ma,近似平行于俯冲带俯冲到了欧亚大陆之下;其前片下沉,扰动软流圈,形成大规模岩浆活动;后片则缓慢后撤,于～80Ma形成了A-型花岗岩。这些A-型花岗岩多属于A2型,受到了还原性板块俯冲的影响而普遍含锡,形成了全球60%的锡矿。俯冲板片的后撤,导致了拉张,可以合理解释南海北缘的"神狐运动"。随着俯冲板片后撤,俯冲角度加大,形成新的弧后拉张,于～33Ma出现洋壳,形成了南海。青藏高原碰撞引起的物质向东、南、北等各方向逃逸,对东亚大陆的构造格局也产生了重要的影响,但是并非南海拉张的主要控制因素。到～23Ma时,东经九十度海岭的俯冲阻挡了青藏高原下方地幔物质向东南方向逃逸,改变了东亚构造格局。同时,由于该海岭俯冲产生的向北东方向的挤压,造成印支半岛向西南挠曲,导致南海洋脊产生向南的跃迁。

This contribution summarizes our new progresses on the formation of the South China Sea(SCS).Our results indicate that the SCS was formed during the closure of the Neo-Tethys Ocean,rather than extrusion of the Indochina Peninsula induced by the collision between Eurasian and Indian continents as previously proposed.The Neo-Tethys Ocean was located between the Eurasian,African,Indian and Australian continents.It was bell-shaped,opening towards the east.The northward subduction of the Neo-Tethys Ocean may have started as early as the Jurassic in the west.At about 130Ma,the eruption of the Kerguelen large igneous province initiated a new ridge to the south of Indian and,consequently the northward drifting of plates and the old spreading ridge of the Neo-Tethys Ocean.Back-arc extension was more developed in the east because the Neo-Tethys Ocean was wider there.The Proto-SCS was most likely one of the back-arc basins.Ridge subduction was a major event during the closure of the Neo-Tethys Ocean,which resulted east-west ward linearly distributed adakites of 100~110Ma,from the Philippines,Fujian,Canton till to the Tibetan Plateau.Adakites to the east of Canton are closely associated with porphyry Cu deposits,whereas these in the Tibetan Plateau are mineralized,but no major deposits have been discovered so far.Considering plate reconstruction results,the ridge subduction was responsible to the adakites.Meanwhile,there was a widely distributed northward compression in the South China block,which is consistent with the subduction of the Neo-Tethys ridge.We suspect that such compression was responsible to the onset of the closure of the Proto-SCS.Interestingly,adakitic charnockite formed in the Gangdese belt,while large scale magmatic flare occurred to the north of the Gangdese at^110Ma,in the Tibetan Plateau.We propose that the Neo-Tethys ridge was roughly parallel to the subduction zone and subducted underneath the Eurasian Continents at 100~110Ma,resulted in compression and adakites.Magmatic flare occurred due to the sinking of the front(north)limb of the subducted ridge.The south limb of the ridge rolled back later on,forming A-type granites of^80Ma.Most of these A-type granites are A2 type,which have been influenced by subduction of plate with reducing matters and are associated with tin deposits that account for 60% of the world's total tin reserves.Slab roll back resulted in extensions,which plausibly explains the"Shenhu"Movement.After the slab rolled back,flat subduction changed to normal subduction,the SCS formed as a result of back-arc extension during the continuous subduction of the Neo-Tethys plate,which eventually formed oceanic crust^33Ma.The collision at the Tibetan Plateau triggered mantle flows and major changes in the tectonic regime of eastern Asia.Such extension,however,was not the controlling factor for the extension of the SCS.At^23Ma,the subduction of the 90° East Ridge blocked the eastward mantle flow and southeastward extrusion of the Indochina Peninsula.Meanwhile,the subduction of the 90° East Ridge also led to the westward bending of the Indochina Peninsula and consequently the southward ridge jump of the SCS.