Microgranitoid enclaves in the felsic Looanga monzogranite, New England Batholith, Australia: Pressure quench cumulates

The quench microstructure and cumulate chemistry of the more mafic enclaves are argued to result from PH2O reduction events within the upper parts of the magma chamber due to roof fracture brought on by the pressure increase imparted to the magma by the fluid release from a water saturated magma. The sudden reduction in fluid pressure results in an increase in the liquidus and solidus temperatures of the water saturated magma and this rather than a drop in temperature produces quench conditions. The reduction in PH2O shifts the cotectic compositions in the Q-Ab-Or system closer to K-feldspar and quartz and this and the heat of crystallisation restrict the amount of these two near solidus minerals that crystallise. The enclaves form as crystal cluster cumulates around minerals already in the magma and/or any other solid substrate available including the magma chamber roof. This pressure-quench-cumulate mechanism explains why the Looanga enclaves are mineralogically similar to the host granite and we suggest that this process may be more widely applicable to enclaves in granitic rocks.