Quantifying Magma Water Contents: A New Entrainment Model for Charnockite Formation

Abstract

Water is essential for the formation of granites and continental crust, whereas charnockite, being an important component of deep crust, is inferred to be formed in low-water environments. Charnockite is an orthopyroxene-bearing felsic rock, its origin, generation, and preservation remain hotly debated. Quantifying the magma water content of charnockite and further determining the orthopyroxene preservation mechanism is crucial to understanding the petrogenesis of charnockite. Here, we report a ca. 431 Ma peraluminous Gaozhou charnockite with granulitic enclaves in South China. The body displays A-type characteristics with crustal reworking zircon isotopic features (δ¹⁸O = 8.0–9.8 ‰; ε<inf>Hf</inf>(t) = −11.5 to −3.4). The charnockite and its enclaves show identical mineral assemblages and comparable orthopyroxene chemical compositions. The two anhydrous minerals of orthopyroxene and garnet are identified as of peritectic and magmatic origins given their textural features and geochemical compositions. Moreover, petrographic observations and bulk geochemical data argue that the peritectic minerals were derived from the entrainment of their granulitic sources. Crystallization phase modeling indicates orthopyroxene would have been completely hydrated and formed biotite when water contents exceed ∼0.3 wt.% near the solidus. Water-in-zircon analysis and thermodynamic modeling indicate low magma water conditions (∼0.15 wt.%; 135 ppm, zircon water medians) for the Gaozhou charnockite from early crystallization to final solidification. CO<inf>2</inf>-rich fluids flushed the charnockite reservoir further contributing to the stabilization of the orthopyroxene. Therefore, we propose a new entrainment model for the formation of charnockite that requires low-water environments achieved by high-temperature melting of dehydrated lower crust granulitic rocks.