Origin of Late Oligocene adakitic intrusives in the southeastern Lhasa terrane: Evidence from in situ zircon U–Pb dating, Hf–O isotopes, and whole-rock geochemistry

Abstract

We describe the whole-rock geochemistry, biotite Ar–Ar and zircon U–Pb geochronology, and in situ Hf–O isotopic compositions of the Linzhi two-mica granitoid in the southeastern Lhasa terrane. The granitoids are calc-alkaline to high-K calc-alkaline in composition, and they have geochemical affinities with adakites. The whole-rock geochemistry, with high SiO2 and Al2O3 contents, low MgO, Cr, and Ni contents, low Mg# values, negative εNd(t) (−6.2 to −2.9), and low 87Sr/86Sr(i) (0.7059–0.7074), together with the in situ zircon Hf–O isotopic data (εHf(t)=−2.7 to 2.7; δ18O=7.2‰–8.8‰), indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the Lhasa terrane. This lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs (<55%). SIMS zircon U–Pb analyses of two samples indicate that the crystallization of the Linzhi granitoid took place around 26Ma, implying that the crust of the Lhasa terrane was already thickened up to 50km prior to emplacement of the adakitic rocks. 40Ar/39Ar ages for biotites, and the presence of magmatic epidote, suggest the granitoid was intruded at mid-crustal depths, indicating that the eastern Lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22Ma. The geochemical features of the Linzhi adakitic rocks are significantly different from those of Eocene adakitic rocks from the north Himalayan belt, and this suggests that we can rule out the possibility of crustal thickening in the eastern part of Lhasa terrane, caused by impingement of the Indian lower crust. Post-collisional (26–10Ma) adakitic rocks occur within an E–W-trending magmatic belt that runs parallel to the Yarlung–Tsangpo suture in the Lhasa terrane. These rocks have distinctive variations in geochemistry along the strike of the Lhasa terrane, indicating that the lower crust is isotopically heterogeneous from west to east along the terrane. These geochemical variations probably result from variable degrees of input, along strike, of juvenile mantle-derived melts into an old lower crust. The middle part of the Lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle-derived melt, underplated during Neo-Tethyan oceanic crust subduction. In contrast, the lower crust below the western and eastern parts of the terrane is relatively ancient. Therefore, crustal thickening caused by underplating of mantle-derived magmas could play an important role in the middle segment of the terrane, whereas crustal compression and shortening as a result of the collision of the Indian and Asian plates could be the major cause for crustal thickening in the western and eastern segments.