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 87Sr/86Sr (0.7172–0.7220) and 143Nd/144Nd (0.51190–0.51200) ratios suggest that they originated from lithospheric sources enriched in Rb with low Sm/Nd ratios. Initial Pb isotopic compositions (206Pb/204Pb = 18.41–18.51; 207Pb/204Pb = 15.68–15.72; 208Pb/204Pb = 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 or (2) of slab breakoff.