Revisiting the role of amphibole fractionation in generating intermediate-silicic arcs: Geochemical and geophysical perspectives from cumulate roots

Abstract

The formation of intermediate to silicic magmas in subduction zones is fundamental to understanding the genesis of Earth’s continental crust. A prominent hypothesis posits that fractionation of amphibole-rich package from basaltic magmas in the mid-to-lower crust plays a central role in this process. This model assumes the widespread presence of amphibole-rich cumulates—such as hornblendites—within the mid- to lower crust beneath volcanic arcs; however, this assumption remains largely untested. Here, we integrate geochemical analyses and thermodynamic modelling to evaluate the equilibrium melt compositions and geophysical properties of olivine hornblendites from central Tibet and analogous rocks worldwide. Petrological, mineralogical, and isotopic analyses of spatially associated and coevally formed olivine hornblendite, gabbro, and andesite from an Early Cretaceous arc in central Tibet reveal that amphibole fractionation plays a cryptic role in generating some arc andesites, despite amphibole is not present as a phenocryst phase. Melts in equilibrium with amphibole in Tibetan and global hornblendites are predominantly andesitic (mean SiO2 = 62 wt%). Additionally, these olivine and pyroxene hornblendites exhibit relatively high seismic velocities (Vp > 7 km/s) and are primarily rooted in the mid-crust. However, large volumes of such high-velocity cumulates have not detected by geophysical studies beneath the middle crust of central Tibet or other volcanic arcs globally. Our findings indicate that amphibole formed via peritectic reactions produces melts with SiO2 contents rarely exceeding 65 wt%, implying that more silicic magmas likely require additional processes, such as magma mixing and/or partial melting of pre-existing crust.