Enriched mantle source and petrogenesis of Sailipu ultrapotassic rocks in southwestern Tibetan Plateau: constraints from zircon U-Pb geochronology and Hf isotopic compositions

The Sailipu potassic-ultrapotassic volcanic rocks in western Lhasa block, Tibetan plateau, are trachyandesite with mantlederivecl xenoliths. The high contents of K2O, MgO, Cr, Ni, high K2O/Na2O ratio and Mg-# of the rocks suggest a primitive magma of mantle low-degree partial melting, or some fractionation of olivines, clinopyroxenes and Fe=Ti oxides. The rocks also show extremely enrichment of LILE, LREE, significant negative Nb, Ta and Ti anomalies, enrichment in radiogenic Sr, Pb and Nd, those of which suggest that an enriched mantle source region should have been responsible for the Sailipu magmatism. LA-ICP-MS has been carried out on dating 18 new zircons from Sailipu potassic-ultrapotassic rocks, and gives U-Pb ages between 15. 8Ma and 19. 2Ma. The weighted mean of 11 U-Pb analyses for potassic sample SL0628 is 17.7 +/- 0.3Ma, in agreement with previous 40Ar/39Ar data. These zircons have epsilon(Hf)(t) values from -7.6 to 3. 9, yielding a wide range of crustal Hf model ages (t(DMC) =0. 86 similar to 1. 59Ga), generally showing enriched characteristics except two analyses. This indicates that they were derived from an enriched source with a little depleted mantle materials involved. 49 inherited zircons, obtained from three samples, have U-Pb ages from 20Ma to 1907Ma, wide ranges of Hf isotope (epsilon(Hf) (t) = -25. 9 similar to 5. 3) and crustal Hf model ages (t(DMC) = 0. 79 similar to 4. 08Ga). Among the above 49 zircons, 37 inherited ones, with younger U-Pb ages between 20 to 110Ma, suggest the mantle source may have experienced four stages of magmatism (62. 2 similar to 64.0 Ma, 43.3 similar to 55.1 Ma, 29.5 similar to 37.7Ma and 20. 1 similar to 27.4Ma) and two magmatic gaps (70 similar to 90Ma and 37.7 similar to 43.3Ma). We also identify a significant, previously unknown stage of magmatism between 29. 5Ma and 37.7Ma in Lhasa block, and find a magmatism stage with ages at the end of Linzizong volcanic rocks but enriched Hf isotope (62.2 similar to 64.0 Ma, epsilon(Hf)(t) = -21.2 similar to 3.0). The above Hf analyses of 49 inherited zircons suggest enriched compositions in mantle source were derived from Lhasa block and Indian continental crust. The potassic-ultrapotassic magma in Sailipu was possibly resulted froth the tearing of the Indian continental slab and diachronous subduction along the Indo-Asian collision zone.