青藏高原北部新生代火山岩区深部结构特征及其成因探讨

Deep Structure beneath the Cenozoic Volcanic Zone in the Northern Qinghai-Tibet Plateau and Its Cause of Formation Discussion

青藏高原北部发育的大量新生代钾质、高钾质火山岩体的成因一直是个谜。利用布置在青藏高原内部及其周缘的305个临时宽频地震台站和固定地震台站记录到的9649个远震事件,共139021条P波初至到时资料对青藏高原深部结构特征进行了层析成像反演研究。结果显示,印度岩石圈地幔俯冲前缘已经到达了羌塘地体中部之下,在俯冲前缘存在一个从地幔深处延伸至地表的大规模低速体。该低速体可能是由于印度岩石圈地幔前缘俯冲进入软流圈深处而引起地幔热扰动,造成深部软流圈地幔的热物质向上扩散而形成的深部地幔物质上涌通道;该通道为青藏高原北部的新生代钾质、高钾质火山岩体的形成提供了条件。因此,青藏高原北部新生代火山岩可能是印度岩石圈地幔持续北向俯冲的结果。

There are a lot of Cenozoic potassic and ultrapotassic lavas in the northern Qinghai-Tibet Plateau,their existing is an enigma.We have collected all the available arrival time data recorded by the temporary seismic stations and phase reports from the International Seismological Center in Tibet and its surrounding areas.In the tomographic inversion,we have used 139,021 P-wave arrival times from 9,649 teleseismic events recorded by 305 seismic stations.Tomographic images show that the frontier of the lithospheric mantle of Indian plate（ILM） subducted beneath the center of Qiangtang terrane,and there is a great scale vertical low-velocity zone from deep to surface at the frontier of ILM.It supplies a channel for deep mantle upwelling.Hot materials of deep asthenospheric mantle might flow upward along the channel,then potassic and ultrapotassic lavas erupted in the northern Tibet.Cenozoic potassic and ultrapotassic lavas in the north Tibet are the direct result of northward subducting of ILM.