Source of biotite in the Wuluma Pluton: Replacement of ferromagnesian phases and disaggregation of enclaves and schlieren

Experimental glasses and naturally quenched melts from pelitic and psammitic protoliths both contain markedly less (FeO* + MgO) than typical S-type granites. The entrainment of residual (peritectic) phases and contamination by country rock material are two mechanisms commonly proposed to account for their higher abundance in granites. This study examines the origin of the ferromagnesian minerals, specifically biotite, contained in the Wuluma Pluton, a heterogeneous granite body in central Australia that lies within its granulite facies pelitic and psammitic source rocks. Heterogeneity at the margins of the Wuluma Pluton arises from abundant enclaves and innumerable dykes of granite separated by screens of country rock. Towards its center heterogeneity arises from abundant biotite-rich, enclaves, schlieren and aggregates. Based on field observations, the microstructural relationships between minerals and the composition of minerals obtained by electron microprobe and LA-ICP-MS analysis, the origin of three of the most common types of schlieren were determined. Enclave-related schlieren are the remains of isolated enclaves that interacted with the host granite; long schlieren developed from screens of country rocks trapped between sheets of granite, and wispy schlieren are derived from porphyroblasts of garnet, cordierite and orthoproxene in the granite. Once incorporated in the granite, all the enclaves, schlieren, porphyroblasts of garnet and cordierite are progressively replaced by biotite. Most of the newly formed biotite and the magmatic biotite subsequently re-equilibrate with the host granite and acquires the same composition. Very little of the biotite retains the compositional traits of the minerals it has replaced. Disaggregation of the enclaves and schlieren that have been replaced by biotite adds biotite to the granite, and this constitutes the major source of biotite in the Wuluma Pluton, that can reach 15 modal%.