BIOTITE PORPHYROBLAST NUCLEATION AND GROWTH - CONTROL BY MICROFRACTURE OF PREEXISTING FOLIATIONS IN SCHISTS IN THE ROBERTSON-RIVER-METAMORPHICS, AUSTRALIA

A macroscopic fold formed during the fourth deformation, D-4, in the R obertson River Metamorphics, north Queensland, Australia. Growth of bi otite, garnet, staurolite and andalusite porphyroblasts also occurred synchronous with D-4. Only biotite porphyroblasts have formed preferre d alignments across the fold, and they define two dominant orientation s. The more common is parallel to D-4 fold axes, which is also paralle l to D-1, D-2 and D-4 intersection lineations. The other varies up to 90 degrees from this within the D-4 axial plane. Mineral elongation li neations do not reflect the extension direction of the deformation in which they formed (D-4), but are a function of pre-existing anisotropi es in the rock parallel to intersection lineations. Porphyroblast grow th is inferred to be a result of microfracturing along favourably orie nted linearan isotropies formed by multiple intersection of foliations , as well as parting of the the S-1 and S-2 foliations.