Crystallization conditions of peraluminous charnockites: constraints from mineral thermometry and thermodynamic modelling

Most igneous charnockites are interpreted to have crystallized at hot and dry conditions, i.e. at >800 °C and <3 wt.% H2O and with an important CO2 component in the system. These charnockites are metaluminous to weakly peraluminous and their formation involves a significant mantle-derived component. This study, in contrast, investigates the crystallization conditions of strongly peraluminous, metasediment-sourced charnockites from the Qinzhou Bay Granitic Complex, South China. To constrain the temperature-melt H2O crystallization paths for the studied peraluminous charnockites, petrographic characterization was combined with fluid inclusion compositional data, mineral thermometry, and thermodynamic modelling. The uncertainties of the thermodynamic modelling in reconstructing the crystallization conditions of the granitic magmas have been evaluated by comparison between modelled and experimental phase relations for a moderately evolved, peraluminous granite (~70 wt.% SiO2). The comparison suggests that the modelling reproduces the experimentally derived phase saturation boundaries with uncertainties of 20–60 °C and 0.5–1 wt.% H2O for systems with ≤1–2 wt.% initial melt H2O at ~0.2 GPa. For the investigated natural systems, the thermometric estimates and modelling indicate that orthopyroxene crystallized at relatively low temperature (750–790 ± 30 °C) and moderately high to high melt H2O content (3.5–5.6 ± 0.5 wt.%). The charnockites finally solidified at relatively “cold” and “wet” conditions. This suggests that thermodynamic modelling affords a possible approach to constrain charnockite crystallization as tested here for peraluminous, moderately low pressure (≤0.3 GPa), and overall H2O-poor systems (≤1–2 wt.% H2O total), but yields results with increasing uncertainty for high-pressure or H2O-rich granitic systems.