Crystal chemistry of biotites from mafic enclaves in the Warburton granodiorite, Lachlan Fold Belt (Australia)

This study details the crystal chemical features of biotite crystals from host granodiorite and mafic microgranitoid enclaves in the Warburton Granodiorite (Lachlan Fold Belt, Australia), a small (~ 80 km²) S-type pluton within the Melbourne basement terrain of the Lachlan Fold Belt. Biotites are the most abundant mafic minerals in both enclaves and host rock, with Fe/(Fe+Mg) ratio ranging from 0.47 to 0.54. Differences in chemical composition concern their Ba and Ti content, which are higher in host granodiorite. The Al for Si substitution, greater than the ideal value, suggests the presence of exchange vectors involving octahedral, tetrahedral and interlayer substitutions. Actually, the exchange vectors ([12])K_-1(+[4])Si_-1(4+[12])Ba(2+[4])Al³⁺, ([6])Mg_-1(2+[4])Si_-2(4+[6])Ti(4+[4])Al₂(3+[6])Al_-1(3+[4])Si_-1(4+[6]) Ti(4+[4])Al³⁺ are observed. Nevertheless, other vectors involving anionic sites may be present. The crystal structure refinements (2.8 ≤ R(obs) (%) ≤ 3.9), carried out in the space group C2/m, indicate that the crystals are mostly 1M polytype. The disordered pattern of octahedral cation distribution may indicate a relatively quickly cooling of Warburton Granodiorite and of enclaves it contains.