POST-COLLISIONAL, POTASSIC AND ULTRAPOTASSIC MAGMATISM OF THE NORTHERN TIBETAN PLATEAU: CONSTRAINTS ON CHARACTERISTICS OF THE MANTLE SOURCE, GEODYNAMIC SETTING AND UPLIFT MECHANISMS

Abstract

Cenozoic, post-collisional, potassic and ultrapotassic igneous rocks in the North Qiangtang, Songpan - Ganzi and North Kunlun terranes of the northern Tibetan Plateau are distributed along a semi-continuous, east - west-trending, volcanic belt, which is over 1200 km in length. Spatially, there is a close association with major strike-slip faults, thrust faults and pull-apart basins. The ages of these magmatic rocks range from 45 Ma to the present (the youngest known eruption occurred in 1951); they are shoshonitic, compositionally similar to K-rich subduction-related magmas, and range in SiO2 from 44 to 66 wt %. There is a relative enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE) in the most primitive magmatic rocks (MgO >6 p;wt %) in the North Qiangtang terrane compared with those in the Songpan-Ganzi and North Kunlun terranes; correspondingly, the primitive magmas have higher 87 Sr/86 Sr and 206 Pb/204 Pb, and lower 143 Nd/144 Nd ratios in the North Qiangtang terrane than in the Songpan-Ganzi and North Kunlun terranes. The dominant factors that control the geochemical characteristics of the magmas are an enriched asthenospheric mantle source composition, the degree of partial melting of this source, and the combined processes of crustal assimilation and fractional crystallization (AFC). Enrichment of the asthenosphere is considered to have occurred by incorporation of subducted sediments into the mantle wedge above a subducted slab of Indian lithosphere during India-Asia convergence. Continental lithospheric mantle, metasomatically enriched during earlier episodes of subduction, may have also contributed a source component to the magmas. Trace element modelling indicates that the mantle source of the most primitive magmas in the North Qiangtang terrane contained higher amounts of subducted sediment (0·5-10%) compared with those in the Songpan - Ganzi and North Kunlun terranes (<2%). The degrees of partial melting required to generate the primitive potassic and ultrapotassic magmas from the enriched mantle sources range from <0·1% to ~15% in the three major basement terranes. Energy-constrained AFC model calculations show that the more evolved magmatic rocks (MgO <6 wt %) are the results of AFC processes in the middle crust in the North Qiangtang terrane and the upper crust in the Songpan - Ganzi and North Kunlun terranes. We propose that the ultimate driving force for the generation of the post-collisional potassium-rich magmatism in north Tibet is the continuous northward underthrusting of the Indian continental lithosphere following India - Asia collision. This underthrusting resulted in upwelling of hot asthenosphere beneath north Tibet, squeezed up between the advancing Indian lithosphere and the backstop of the rigid Asian continental lithosphere. Asthenospheric upwelling may have also contributed to uplift of the northern Tibetan Plateau. © 2006 Oxford University Press.