藏北新生代玄武质火山岩起源的深部机制——大陆俯冲和板片断离驱动的地幔对流上涌模式

发布时间：2019-02-14 14:41

【摘要】：近年来地震层析成像揭示出可可西里-西昆仑中新世-第四纪钾质火山岩带下方存在一个深达900km的巨型地幔低速体,空间上与新特提斯洋和印度大陆俯冲断离板片沉降形成的冷地幔下降流共存(Replumaz et al.,2010a,b),两者构成统一的地幔对流体系。研究表明,羌塘古近纪(60~34Ma)钠质玄武岩和高钾钙碱性玄武岩均以富含Ti O2、P2O5和大离子亲石元素为特征,主体具有与OIB相近的微量元素组成和弱亏损的Sr、Nd同位素特征,指示岩浆起源于软流圈的上涌熔融,但Nb、Ta的弱亏损表明岩浆源区有岩石圈地幔熔融组分的贡献。羌塘(32~26Ma)碱性钾质玄武岩与可可西里和西昆仑中新世以来喷发的钾质玄武岩的地球化学性质相近,不相容元素比值和Sr、Nd同位素组成指示岩浆起源于古俯冲地幔楔的低程度熔融。这些特征表明藏北软流圈上涌作用始于古近纪,初始上涌中心位于羌塘地体之下。计算表明藏北古近纪火山岩距离当时的印度大陆北缘的最大和最小距离约为1250km和700km,与现今可可西里地幔低速体的南、北边界与印度大陆北缘的距离相近,支持羌塘古近纪地幔上涌作用也是受藏南冷地幔下降流所驱动。青藏高原在南北缩短过程中不仅表现为软流圈自西向东挤出流动,地幔垂向对流也是其重要的运动形式,在地幔上升流形成的藏北热幔区内,地壳的水平缩短增厚与岩石圈地幔的伸展减薄呈脉动式共存。藏南冷地幔下降流和藏北热地幔上升流的持续北移是导致藏北后碰撞火山岩时空迁移的主要控制因素。
[Abstract]:In recent years, seismic tomography has revealed that there is a giant low-velocity mantle body deep up to 900km beneath the Miocene Quaternary potash volcanic belt in Hochsily-West Kunlun. (Replumaz et al.,2010a,b) coexisted in space with the cold mantle descending current formed by the subduction and off plate subsidence of the New Tethys Ocean and the Indian continent, which constituted a unified mantle convection system. The results show that both the 60~34Ma sodium basalts and the high-potassium calc-alkaline basalts are rich in Ti O _ 2 O _ 2P _ 2O _ 5 and large ion lithophile elements, and the main elements have the characteristics of trace element composition similar to OIB and weakly depleted Sr,Nd isotopes. It indicates that the magma originated from upwelling melting of the asthenosphere, but the weak depletion of Nb,Ta indicates that the magma source region has the contribution of the melting component of the lithospheric mantle. Alkaline kalitic basalts in Qiangtang (32~26Ma) are similar in geochemical properties to those erupting since Miocene in Hohsili and West Kunlun, with incompatible element ratios and Sr,. The Nd isotopic composition indicates that the magma originated from the low melting of the paleo-subduction mantle wedge. These characteristics indicate that the upwelling of the asthenosphere in northern Tibet began in Paleogene and the initial upwelling center was located below the Qiangtang terrane. The results show that the maximum and minimum distance between the Paleogene volcanic rocks in northern Tibet and the northern margin of the Indian continent at that time is about 1250km and 700km, which is close to the distance between the northern and southern boundaries of the lower mantle in Hoh Xili and the northern margin of the Indian continent. The upwelling of the Paleogene mantle in Qiangtang is also driven by the descending flow of the cold mantle in southern Tibet. During the shortening process of the Qinghai-Xizang Plateau, not only is the asthenosphere extruding from west to east, but the vertical convection of the mantle is also an important form of movement, in the hot mantle region of northern Tibet formed by the upwelling of the mantle. The horizontal shortening and thickening of the crust coexist with the extension and thinning of the lithospheric mantle. The downward flow of the cold mantle in the south of Tibet and the continuous northward migration of the upwelling of the thermal mantle in the northern part of Tibet are the main controlling factors for the spatiotemporal migration of the collision volcanic rocks in the northern part of Tibet.
【作者单位】：吉林大学地球科学学院;
【基金】：国家自然科学基金项目(41172056)资助
【分类号】：P542;P588.145

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