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 INTRODUCTION chemical data are presented for post-collisional ultrapotassic, silicic The high plateau of Tibet, the Himalaya and the Kapotassic and high-K calc-alkaline volcanic rocks from SW Tibet, rakoram Ranges are the product of the continuing colwith Ar/Ar ages in the range 17–25 Ma. The ultrapotassic lision of India with the southern margin of Eurasia during lavas contain mantle xenocrysts (olivine ± rutile/armalcolite). the past 50 my (e.g. Klootwijk & Radhakrishnamurty, Their initial Sr/Sr (0·7172–0·7220) and Nd/Nd 1981). Despite numerous studies of the region, the pro(0·51190–0·51200) ratios suggest that they originated from cesses responsible for the formation of the plateau and lithospheric sources enriched in Rb with low Sm/Nd ratios. Initial the surrounding mountain ranges are still controversial. Pb isotopic compositions ( Pb/Pb = 18·41–18·51; Pb/ Three hypotheses have been suggested to explain the Pb = 15·68–15·72; Pb/Pb = 39·42–39·60) and crustal thickening and altitude of Tibet. In the first geochemical features such as high Th/Ta, low Sr/Nd, low Ce/ (e.g. Argand, 1924; Powell & Conaghan, 1975; Ni & Pb and negative Eu anomalies are consistent with a recycled crustal Baranzagi, 1983), virtually the entire Tibetan plateau is component. Nd depleted mantle model ages range from 1·3 to 1·9 underthrust by Indian lithosphere. In the second (e.g. Ga, whereas Pb model ages record an Archaean event, suggesting Zhao & Morgan, 1985, 1987), thickening is by inflow of that the source had a complex multi-stage evolution. In contrast, material from India. In the third, the crust of Tibet is the high-K calc-alkaline dacites and rhyolites have less enriched interpreted as having thickened by shortening (e.g. Dewey initial Sr (0·7091–0·7097) and Nd (0·51213–0·51225) iso& Burke, 1973). Quantitative analysis of the third model, topic compositions. The presence of zircon xenocrysts with a Pbassuming a vertically average lithosphere rheology, preevaporation age of 471 ± 33 Ma documents the importance of dicts lithospheric thickening and additional uplift of the crustal anatexis in their genesis. Processes responsible for the partial entire plateau by convective thinning of the lower conmelting of metasomatized lithospheric mantle and post-collisional tinental lithosphere (e.g. England & Houseman, 1988, magmatism in the Lhasa block could be a consequence of (1) 1989). Molnar et al. (1993) argued that a sudden increase convective removal of the lower lithosphere or (2) of slab breakoff. in uplift occurred at ~8 Ma and that the start of post-