DID THE HIMALAYAN CRYSTALLINES EXTRUDE PARTIALLY MOLTEN FROM BENEATH THE TIBETAN PLATEAU?

Abstract

The hypothesis that the Himalayan crystalline core originated by ductile channel flow of partially molten mid-crust from beneath the Tibetan Plateau is critically reviewed. The proposal that widespread shallow anatexis exists beneath southern Tibet today is inconsistent with numerous observations (e.g. 'bright spots' restricted to a single rift and evidence that they represent aqueous fluids rather than molten silicate; the seismogenic southern Tibetan Moho; 3He/4He data indicating the presence of mantle heat and mass in the rift valley; the likelihood that any melt present is due to late Neogene calc-alkaline magmatism; the lack of Tertiary migmatites in the crustal section exposed in the uplifted rift flank of the Yangbajain graben; the lack of Gangdese zircon xenocrysts in the Greater Himalayan Crystallines (GHC); and the broadly coherent stratigraphy in the GHC). Evidence advanced in support of this model is equally or better explained as resulting from localized Neogene calc-alkaline magmatism. A recently developed rapid denudation/channel flow model does explain key petrogenetic and thermochronological features of the Himalaya, but is inconsistent with several geological constraints, most notably the small portion of the collision front over which focused erosion has localized exposure of the GHC. It is concluded that no evidence has yet been documented that requires the existence of partially molten crust flowing in a channel from beneath the Tibetan Plateau to form the Himalaya. © The Geological Society of London 2006.