Postcollisional calc-alkaline lavas and xenoliths from the southern Qiangtang terrane, central Tibet

A newly recognized east-west trending province of 43 to 28 Ma volcanic rocks occurs in the southern Qiangtang terrane of central Tibet. The lavas are Na-rich calc-alkaline in composition, relatively primitive, and locally host ultramafic and mafic xenoliths. Foliated mafic granulite xenoliths from ∼ 28 Ma lavas equilibrated at temperatures in the range of 980 to 1260 °C, indicating that the southern Qiangtang terrane lower crust was deformed and heated to very high temperatures during or before the Oligocene. In the northern Qiangtang terrane is a parallel suite of volcanic rocks of coeval age. However, here, the volcanic rocks are (ultra)potassic in composition and underlain by a hot (T > 800 °C) metasedimentary-bearing lower crust. We suggest that both suites of Qiangtang lavas were derived from a primitive mantle source and that the enriched nature of the northern Qiangtang lavas reflects contamination by partial melts of metasedimentary lower crust. This contrasts with the conventional interpretation that Tibetan potassic lavas were solely derived from an ancient, enriched mantle lithospheric source. While removal of lithospheric mantle seems to be required to produce the high temperature melts, Eocene-Oligocene volcanism was coeval with thrust reactivation along bounding suture zones, implying that mantle dynamics were linked to intracontinental subduction.