Origin of Amphibole Cumulates at the Base of an Exposed Arc Crustal Section: Perspectives From Fiordland, New Zealand

The formation of amphibole-rich cumulates in arc settings plays a crucial role in influencing the chemistry of ascending arc magmas. However, understanding the processes of formation of these cumulates is challenging due to the subtle nature of their fractionation. This study presents new petrographic and geochemical analyses of amphibole-rich cumulate rocks from outcrops across Fiordland, New Zealand, an exposed section of the lower arc crust. Our findings show two primary styles of formation. Style 1 represents direct fractionation from a melt, and is typified by adcumulate textures, similar rare earth element concentrations, and minor mineral phases. Style 2 amphiboles show greater variations in their rare earth element contents and typically lack adcumulate textures. Our results indicate that direct igneous fractionation from a melt is the primary formation mechanism for amphibole cumulates, with minor contributions from melt–rock interaction, aligning with the amphibole sponge model. This research underscores the significance of amphibole cumulate formation in the evolution of the lower crust in arc systems and illustrates how co-crystallizing minerals and precursor assemblages can influence the chemistry of ascending magmas during metasomatism.