Geochemical comparison of the Cenozoic high-MgO-potassic volcanic rocks between northern and southern of Tibetan Plateau:difference of the both mantle sources.

Cenozoic high-MgO-potassic volcanic rocks (MgO≥6 wt.%,K_2O/Na_2O1) are widely distributed in the Tibetan plateau,which rocks are generally believed to be derived from a mantle source.These volcanic rocks have some common chemical characteristics such as enrich-K,LILE and depleted-HFSE,but the rocks from northern area of the Tibetan Plateau show geochemical characteristics clearly different from those from southern area.The distribution of Cenozoic volcanic rocks in the Tibetan plateau is not continuous in spatial-temporal.The high-MgO-potassic volcanic rocks in both areas were probably derived from an enriched mantle which is related to metasomatism during earlier subduction events,but mineral association and depths of their mantle sources are different.The geochemical differences of both south and north Tibetan mantle sources are probably caused by the different source composition or/and amount of crustal materials involved into their sources.The high-MgO-potassic volcanic rocks in south have lower ~(143)Nd/~(144)Nd but higher ~(87)Sr/~(86)Sr isotopic ratio relative to those in north of the Tibetan Plateau,which suggests the latter was sourced from a relatively depleted enriched-mantle than the former.Although the convective removal and southern-norhtern-dipping continental subduction models may explain some geological phenomena of uplift and distribution of the Cenozoic volcanic rocks in the Tibetan plateau,as a whole it seems more likely for the combination of both the multiple slab break-off of northern subduction of the India continent and southward-dipping subduction of the Asian continent on the northern margin of the Tibetan plateau,which can better explain the uplift process and origin of Cenozoic volcanic rocks on the Tibetan plateau.