Mesozoic crustal evolution of southern Tibet: Constraints from the Early Jurassic igneous rocks in the central Lhasa terrane

Abstract

Phanerozoic growth of continental crust on our planet is one of the important research themes in Earth Science. Here, we present the results of a systematic study of newly found and previously reported Mesozoic igneous rocks, including diorite cumulate, granodiorite cumulate, mafic magmatic enclaves (MME) and host granitoids in the central Lhasa terrane, southern Tibet. These igneous rocks give zircon UPb crystallization ages of 199–189 Ma. Based on constituent mineral and bulk-rock compositions, the cumulates are best understood as resulting from amphibole, plagioclase and titanite crystallization from a mafic andesitic magma. The host granitoids also show compositional systematics consistent with amphibole-plagioclase fractional crystallization from andesitic magma. The MMEs share many characteristics with their host granitoids in common, including identical crystallization age, similar mineralogy, mineral chemistry and zircon isotopic compositions, representing earlier cumulate derived from the same magmatic system as their host rocks. The magma parental to the studied Early Jurassic igneous rocks is best explained as resulting from partial melting of hydrated ocean crust together with varying continental material. The increasing zircon εHf(t) values of multiple plutons in the central Lhasa terrane with time during ~215–170 Ma indicate its gradual increase in mantle contribution. We present the Early Mesozoic crustal evolution in the Lhasa terrane.