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

Abstract

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 40 Ar/ 39 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 87 Sr/ 86 Sr (07172-07220) and 143 Nd/ 144 Nd 1981). Despite numerous studies of the region, the pro-(051190-051200) 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 ( 206 Pb/ 204 Pb = 1841-1851; 207 Pb/ Three hypotheses have been suggested to explain the 204 Pb = 1568-1572; 208 Pb/ 204 Pb = 3942-3960) 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 Nd depleted mantle model ages range from 13 to 19 underthrust by Indian lithosphere. In the second (e.g. Ga, whereas Pb model ages record an Archaean event, suggesting 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 (07091-07097) and Nd (051213-051225) 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). in uplift occurred at ~8 Ma and that the start of postcollisional potassic basaltic volcanism in northern Tibet is a result of rapid heat transfer to the mid-lithosphere caused by removal of the lithospheric root.