Different water contents lead to contrasting magmatic differentiation pathways: A case study of two coeval rock suites

Continental magmatism associated with orogenesis is characterized by large geochemical diversity, which is generally attributed to variations in source lithologies, melting condition and differentiation pathway. Water can decrease melt viscosity, reduce solidus temperatures, and control mineral phase equilibria. However, the effects of water on magmatic differentiation are difficult to evaluate, because magmas lose most of their volatiles as they cool and solidify. In this study, we combined the P2O5 and Zr systematics of igneous rocks with thermodynamic modeling of apatite and zircon saturation to constrain magma water content during differentiation. We performed a case study of igneous rocks from an early Paleozoic orogen in South China, and identified two coeval suites of igneous rocks (Series I and II). Unevolved low SiO2 rocks from both series have similar major and trace element compositions; however, as SiO2 increased to intermediate values, Series I evolved to much higher contents of P2O5, TiO2, K2O, and incompatible trace element contents than Series II. Our modeling results show that the two series might have had different initial water contents (-1 wt% vs.-4 wt%), and that the distinct trace element contents between Series I and II rocks can be explained by differentiation of magmas with different initial water contents. (c) 2021 Published by Elsevier B.V.