The role of clinopyroxene in amphibole fractionation of arc magmas: Evidence from mafic intrusive rocks within the Gangdese arc, southern Tibet

Abstract

Amphibole is thought to be a key control on the differentiation of arc magmas but the fractionation of this mineral remains enigmatic because it is rarely present in erupted lavas. This study presents detailed petrological, mineralogical, and geochronological data for newly identified Late Triassic (217–215 Ma) mafic intrusive rocks from the Gangdese arc of southern Tibet. These gabbros host an amphibole-rich cumulate enclave and clinopyroxene crystal clots, including two mineral assemblages in chemical disequilibrium: (1) amphibole and Type-1 clinopyroxene within the enclave and clots; and (2) small crystals of Type-2 clinopyroxene, plagioclase, and biotite within the host. Textural and geochemical evidence indicate that the enclave and clots represent captured fragments of amphibole- and clinopyroxene-dominated lower crustal crystal mushes, respectively, whereas small host crystals formed by the shallow crystallization of evolved melts extracted from these crystal mushes. The compositional variations between the two types of clinopyroxene also reveal this melt segregation process. Amphibole with similar rare earth element patterns to associated Type-1 clinopyroxene inclusions suggest that the former formed through peritectic reactions consuming clinopyroxene. Enhanced amphibole fractionation may also have occurred as a result of the presence of high-density clinopyroxene inclusions. This indicates that the fractionation of amphibole together with precursory clinopyroxene inclusions within the deep crust controls the early chemical evolution of mafic arc magmas, a process that is consistent with the geochemical variations in lavas from three other active arc volcanoes. The associated cumulates can efficiently delaminate back into the mantle, ultimately leading to the generation of SiO2-rich continental crustal material.