Middle Neoproterozoic (ca. 760 Ma) arc and back-arc system in the North Lhasa terrane, Tibet, inferred from coeval N-MORB- and arc-type gabbros

Abstract

The North and South Lhasa terranes in the central Tibetan Plateau were traditionally considered to be one block in the Precambrian and originate from the northeastern India or northern Australia. In this contribution, we report Middle Neoproterozoic (ca. 760 Ma) gabbros in the North Lhasa terrane. The gabbros can be subdivided into two groups. The group-1 gabbros (ca. 766 Ma) are calc-alkaline and exhibit arc-like geochemical features. They have high ISr (0.708 to 0.710) and low zircon εHf(t) (−2.0 to +1.0) and whole-rock εNd(t) (−2.4) values, and were probably generated by partial melting of an enriched subduction-modified lithospheric mantle. By contrast, the group-2 gabbros (ca. 766 and 758 Ma) belong to the tholeiitic series and are geochemically similar to N-MORB. Their low ISr (0.705 to 0.706) and high positive zircon εHf(t) (+5.6 to +8.8) and whole-rock εNd(t) (+2.6 to +3.8) values are indicative of an N-MORB-type depleted mantle source. The presence of these coeval N-MORB- and arc-type gabbros is related to an arc and back-arc system, and indicates an age equivalent to the northern EAO (East African Orogen). Integrating previous studies with the data presented in this contribution, we suggest that the North Lhasa terrane was most likely separated with the South Lhasa terrane in the Precambrian and originated from the northern EAO.