Zircon U–Pb and Hf isotopic constraints from eastern Transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern Tibet

Abstract

Before the Indian collision with Asia, northward subduction of the Neo-Tethyan oceanic lithosphere produced an Andean-type convergent margin in South Asia characterized by arc magmatism starting from the early Jurassic and lasting until the Eocene. The magmatic arc is now represented by widespread intrusive bodies or the so-called Transhimalayan batholiths in the Lhasa terrane of southern Tibet that have been divided into two main magmatic suites, i.e., the northern plutonic belt and the southern Gangdese Batholith. Their temporal distribution, concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern Himalayan syntaxis, however, remains poorly constrained. Here we report the first combined zircon U–Pb and Hf isotopic study of the Transhimalayan batholiths from the eastern part of the Lhasa terrane (∼ 95–97.5°E and ∼ 28.5–30°N), SE Tibet. Zircon U–Pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the Early Cretaceous (∼ 133–110 Ma) and subordinately in the Paleocene (∼ 66–57 Ma), long after an older granite intrusion in the earliest Jurassic (∼ 198 Ma). The zircon ε Hf( T) values range from + 5 to − 20, yielding Hf crustal model ages ( T DM C) between 0.8 and 2.4 Ga that peak at ∼ 1.7 Ga, suggesting a major episode of crustal growth in the Proterozoic and a predominantly, or even exclusively, crustal source for the granitoid petrogenesis. These zircon U–Pb and Hf isotopic constraints, together with whole-rock geochemical characteristics, allow us to correlate them to S-type granitoids in the northern plutonic belt, rather than to the I-type Gangdese Batholith from the central part of the southern Lhasa terrane. In contrast to the Gangdese magmatism that formed with a significant juvenile mantle input related to the Neo-Tethyan subduction, these S-type granitoids show geochemical similarities to the North American Cordilleran Interior batholiths that did not form directly from a subduction setting. Thus, we attribute the petrogenesis of the widespread Early Cretaceous granitoids in the northern belt and eastern Himalayan batholiths to a postcollisional regime due to the Late Jurassic–Early Cretaceous continental collision between the Lhasa and Qiangtang terranes. Under this framework, the S-type magmatism was generated in response to collision-induced crustal thickening, which may have capabilities of causing crustal anatexis by itself in the region. However, continued interplay with the Neo-Tethyan subduction zone processes such as back-arc extension could also have played a crucial, and long-lasting, role in the magma generation.