Post-collisional potassic and ultrapotassic magmatism in SW Tibet:: Geochemical and Sr-Nd-Pb-O isotopic constraints for mantle source characteristics and petrogenesis

Major and trace element, Sr-Nd-Pb-O isotope and mineral chemical data are presented for post-collisional ultrapotassic, silicic potassic and high-K calc-alkaline volcanic rocks from SW Tibet, with 40Ar/39Ar ages in the range 17-25 Ma. The ultrapotassic lavas contain mantle xenocrysts (olivine +/- rutile/armalcolite). Their initial Sr-87/Sr-86 (0.7172-0.7220) and Nd-143/Nd-144 (0.51190-0.51200) ratios suggest that they originated from lithospheric sources enriched in Rb with low Sm/Nd ratios. Initial Pb isotopic compositions (Pb-206/Pb-204 = 18.41-18.51; Pb-207/Pb-204 = 15.68-15.72; Pb-208/Pb-204 = 39.42-39.60) and geochemical features such as high Th/Ta, low Sr/Nd, low Ce/Pb and negative Eu anomalies are consistent with a recycled crustal component. Nd depleted mantle model ages range from 1.3 to 1.9 Ga, whereas Pb model ages record an Archaean event, suggesting that the source had a complex multi-stage evolution. In contrast, the high-K calc-alkaline dacites and rhyolites have less enriched initial Sr (0.7091-0.7097) and Nd (0.51213-0.51225) isotopic compositions. The presence of zircon xenocrysts with a Pb-evaporation age of 471 +/- 33 Ma documents the importance of crustal anatexis in their genesis. Processes responsible for the partial melting of metasomatized lithospheric mantle and post-collisional magmatism in the Lhasa block could be a consequence of (1) convective removal of the lower lithosphere of (2) of slab breakoff.