PHASE-RELATIONSHIPS IN BUCHAN FACIES SERIES PELITIC ASSEMBLAGES - CALCULATIONS WITH APPLICATION TO ANDALUSITE-STAUROLITE PARAGENESES IN THEMOUNT LOFTY RANGES, SOUTH-AUSTRALIA

Low-pressure, medium- to high-temperature (Buchan-type) regional metam orphism of pelitic rocks in the Mount Lofty Ranges, South Australia, i s defined by the development of biotite, staurolite-andalusite, fibrol ite, prismatic sillimanite and migmatite zones. K-feldspar makes its f irst appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovi te, concentrating particularly on well-developed andalusite-staurolite parageneses. In general, the spatial distribution and mineral chemica l variation of these assemblages accord with the predictions of petrog enetic grids and P-T and T-X(Fe) pseudo-sections constructed from the internally consistent data set of Holland and Powell (1990) in the sys tem KFMASH, assuming a(H2O) approximately 1, although analysed white m ica compositions are systematically more aluminous than predicted. Imp ortantly, the stability ranges of most critical assemblages predicted by these grids and pseudo-sections coincide closely with P-T estimates calculated using the data set of Holland and Powell (1990) from the M ount Lofty Ranges assemblages. With the exception of Mn in garnet and Zn in one staurolite-cordierite-muscovite assemblage non-KFMASH compon ents do not significantly appear to have affected the stability ranges of the observed assemblages. An apparent local reversal in isograd zo nation in which andalusite first appears down-grade of staurolite sugg ests a metamorphic field gradient concave towards the temperature axis and, together with evidence for essentially isobaric heating of indiv idual rocks, is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels.