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

Sparse microgranitoid enclaves (MGE) in the leucocratic I-type Looanga monzogranite near Bendemeer, N.S.W. Australia, range from microdiorite to micromonzogranite and all have fine to medium grainsize igneous microstructures. The enclaves that vary from SiO₂ 53 to 69wt.% are all less silicic than the host monzogranite (71-76wt.%). Although compositionally diverse, the enclaves and host monzogranite pluton share a common mineralogy of quartz, oligoclase, ferro-edenitic hornblende, iron-rich (mg ~35) biotite, fluor-apatite and ±K-feldspar. Except for the core of one double enclave, the enclaves have the same ⁸⁷Sr/⁸⁶Sr initial ratio as the host pluton. A characteristic of the enclaves is high MnO/(MnO+MgO+FeO) ratios with MnO abundances of the more mafic enclaves up to 0.8wt.%, higher than any common magma. The enclaves have a wide range of Na₂O/K₂O ratios (0.5 to 2.8) and, in common with the host pluton, contain hornblendes with Na₂O/K₂O ratios varying from 1.5 to 2.3. The hornblendes in two enclaves have lower Na₂O/K₂O ratios than their host enclave, making it unlikely that the hornblende could have crystallised from a melt of the same composition as these enclaves.Chemically and mineralogically the more mafic enclaves have characteristics expected of cumulates formed from a magma of similar composition as the host pluton, in that they contain the same minerals but are enriched in the near-liquidus phases (hornblende, plagioclase and biotite) and depleted in the near-solidus phases (quartz and K-feldspar). Except for some minor replacement of pyroxene by hornblende the minerals do not show microstructural evidence of being made over from other minerals. It is argued that the mineral chemistry of these enclaves is also a primary feature rather than the result of mineralogical equilibration with the host monzogranite magma. The two most felsic enclaves are medium-grained monzogranites (SiO₂ 68 and 70wt.%) and are considered to be compositionally little different from the magmas from which they crystallised. These two exhibit hydrothermal alteration and are considered to be fragments of an earlier roof phase of the intrusion.The quench microstructure and cumulate chemistry of the more mafic enclaves are argued to result from P_H2O 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 P_H2O 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.