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 f rom host granodiorite and mafic microgranitoid enclaves in the Warburt on Granodiorite (Lachlan Fold Belt, Australia), a small (similar to 80 km(2)) S-type pluton within the Melbourne basement terrain of the Lac hlan 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 0.5 4. Differences in chemical composition concern their Ba and Ti content , which are higher in host granodiorite. The Al for Si substitution, g reater than the ideal value, suggests the presence of exchange vectors involving octahedral, tetrahedral and interlayer substitutions. Actua lly, the exchange vectors (K-1+[4]Si-14+[12]Ba2+[4]Al3+)-K-[12], 23+)- Mg-[6],(Al-13+[4]Si-14+[6]Ti4+[4]Al3+)-Al-[6] are observed. Neverthele ss, other vectors involving anionic sites may be present. The crystal structure refinements (2.8 less than or equal to R-obs (%) less than o r equal to 3.9), carried out in the space group C2/m, indicate that th e crystals are mostly 1M polytype. The disordered pattern of octahedra l cation distribution may indicate a relatively quickly cooling of War burton Granodiorite and of enclaves it contains.