Southern Tibetan Oligocene–Miocene adakites: A record of Indian slab tearing

Abstract

Oligocene–Miocene granitoids exposed within the 1500-km-long Gangdese arc of southern Tibet exhibit adakitic compositions. Five plutonic samples from the southeastern Lhasa terrane near Namche Barwa area were analyzed to determine their geochemical characteristics and to better understand the geodynamic evolution of the eastern Himalayan syntaxis orogen. The samples, yielded U–Pb zircon ages in the range of 30–26 Ma, are intermediate to silicic in composition and have elevated K 2 O (1.5–8.9), Th/La ratios (0.14–1.10), low MgO (< 2%) and Mg# (< 50), as well as high initial 87 Sr/ 86 Sr i (0.7062–0.7103) and low εNd(t) (− 10.9 to − 1.0) isotopic compositions. In addition, these granitic and granodioritic rocks have high Sr/Y (66–306) and La/Yb (25–312) ratios, a characteristic of adakitic rocks. In-situ zircon εHf(t) isotopic compositions are in the range of + 8.1 to − 1.9. Within the framework of the Tibetan–Himalayan orogenesis we attribute these rocks to represent partial melting and mixing products of two end-member components of the lower Tibetan crust: the roots of the relatively juvenile Gangdese arc crust and newly-underplated high-potassium mafic magmas. Adakitic magmatism initiated at 30–26 Ma near the eastern Himalayan syntaxis and systematically decreases in age to the west to 18–9 Ma near Shigatse. We attribute this temporal–spatial distribution of adakitic magmatism within the Gangdese arc, along with the regional Oligo-Miocene geology, to the progressive tearing of the Indian plate. Based on the decrease in age of adakitic magmatism from east to west we hypothesize that the tear initiated beneath the eastern Himalayan syntaxis and propagated westward. • Adakitic granitoids formed at 30–26 Ma around eastern Himalayan syntaxis. • Indian crustal materials besides juvenile crust were involved in generation of these granitoids. • Indian slab tearing initiated beneath eastern Himalayan syntaxis and propagated westward.