Compositional diversity of continental arc magmatism caused by variable degrees of mantle melting: A Gangdese case study

Abstract

Compositional diversity of magmatism in continental arcs is common and can be collectively attributed to the combination of intra-crustal and extra-crustal processes, but distinguishing their relative contributions remains a challenge. This study investigates processes controlling the compositional diversity in the ca. 220–150 Ma intrusive rocks from the central Gangdese arc (88–92°E) in southern Tibet. New and published zircon Hf isotopic data reveal that assimilation with enriched crustal materials is insignificant for these rocks, but is evident for the coeval intrusive rocks from the eastern Gangdese arc (92–94°E). Our comprehensive dataset reveals progressive increases in highly incompatible elemental (HIEs; e.g., Cs and Rb) concentrations and Rb/Sr ratios in the basaltic rocks from ca. 220 to 150 Ma, coinciding with an increase in crustal thickness from ∼25–30 km to ∼35–40 km and the progressive waning of arc magmatic intensity during this time period. This temporal trend, coupled with the similar HIE enrichment trends recorded by the intrusive rocks from the Kohistan arc, suggest that the decreasing degrees of mantle melting due to progressive mantle cooling played a primary control on the compositional diversity. This work provides a way to trace how varying degrees of mantle melting, in addition to intra-crustal processes, impact the compositional diversity of continental arc magmatism.