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

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。

The formatting mechanism of Cenozoic basaltic volcanic rocks in the northern Tibet:Continental subduction and slab break-off driven by mantle convection and upwelling

In recent years, seismic tomography revealed the existence of a huge mantle low-velocity up to depth of 900km, which is beneath the Kekexili-western Kunlun Miocene-Quaternary potassic volcanic rocks area. Coexist with cold mantle downwelling from Neo-Tethys and India continental subduction break away from the plate sedimentation (Replumaz et al., 2010a, 2010b). Both constitute a unified system of mantle convection. Research shows that Qiangtang sodic basalt and potassium calc-alkaline basalts in Paleogene (60~34Ma) are rich in TiO₂, P₂O₅ and large-ion lithophile elements, and are similar to OIB trace element composition and weak losses of Sr and Nd isotopic compositions, indicates magma is given priority to with upwelling of the asthenosphere melting. But weak loss of Nb and Ta indicate the magma source area mixed with the lithosphere mantle components. The alkaline potassic basalt in Qiangtang (30~26Ma) and the potassic basalt in Kekexili and West Kunlun from Miocene have the similar geochemical properties. The ratios of incompatible elements and isotope composition of Sr and Nd reflected that the magma come from low degree of molten of the mantle wedge in ancient subduction zone. These features indicate that, the mantle in northern Tibet upwell started in the ancient, and the initial upwelling center is located in the Qiangtang block. Calculation shows that northern volcanic rocks in paleogene distance at the time of India continental northern maximum and minimum distance is about 1250km and 700km, and nowadays Kekexili mantle of low-speed south, north close to the distance of India continental northern border, support Qiangtang mantle upwelling in paleogene is also affected by cold mantle flow driven down from south area. The shorten of Tibetan Plateau from the north to the south not only Reflected in the form of asthenosphere flows out from the west to the east, but also the vertical mantle convection. Thickening of the crust and thinning of the lithosphere mantle is coexistence in the northern Tibet hot mantle zone formed in the mantle upwelling, and continued north of hot mantle upwelling northern main controlling factors of the volcanic rocks in space and time migration after the collision. Moving north of cold mantle drop flow in South Tibetan and hot mantle upwelling in the northern Tibet as a result of the space-time migration of Northern Tibet post-collision volcanic.