Post-Collisional Potassic and Ultrapotassic Magmatism in SW Tibet: Geochemical and Sr–Nd–Pb–O Isotopic Constraints for Mantle Source Characteristics and Petrogenesis

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