Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

Abstract

It is generally believed that Miocene adakitic rocks in the Lhasa terrane (block) of Southern Tibet were derived from a thickened lower crust, and that their compositions will therefore reflect the geochemical characteristics of their lower-crustal source(s). Adakitic rocks in the western part of the Lhasa terrane possess geochemical and Sr–Nd isotopic characteristics that are clearly different from those in the eastern part of the Lhasa terrane; the former display a more enriched Nd–Sr isotopic signature together with an older model age relative to the latter, suggesting that the compositions of the lower crust under the western and eastern parts of the Lhasa terrane also differ. In addition, the adakitic rocks in the Himalayas on the southern side of the Indus–Yalu suture (IYS) have similar geochemical characteristics and Sr–Nd isotopic compositions to the adakitic rocks of the western Lhasa terrane, but differ from rocks in the eastern Lhasa terrane and from igneous rocks derived from crust beneath the Himalayas. We suggest that the lower (and/or middle) crustal materials beneath the western Lhasa terrane probably extend to the southern side of the IYS due to channel flow. If mid-lower crustal flow has occurred below Southern Tibet, the southeastward-moving anatectic mid-lower crustal material was probably derived from below the western part of the Lhasa terrane rather than the eastern. Moreover, the emplacement ages of adakitic rocks from Mayum, on the southern side of the IYS, imply that southeastward ductile flow of mid-lower crust beneath western Lhasa may have occurred as early as 17 Ma.