LOW-PRESSURE, HIGH-TEMPERATURE ANTICLOCKWISE P-T PATH REVEALED BY PHASE EQUILIBRIUM MODELLING COUPLED WITH MICROSTRUCTURES IN THE CENTRAL MARY KATHLEEN FOLD BELT, NORTH-EASTERN AUSTRALIA

Metapelitic rocks encompassing a wide range of mineral and bulk rock chemical compositions were studied from the central Mary Kathleen Fold Belt, NW Australia. The study area lies completely within the sillimanite zone, but the metamorphic grade increases from SE to NW as indicated by the modal increase of sillimanite versus andalusite from incipient to near-complete transformation. Calculated peak metamorphic temperatures increase correspondingly from 600°C to 630°C with peak pressures between 4 and 4.8 kbar.

Textural observations in magnesian, low-Al (cordierite-orthoamphibole-dominated) metapelites indicate a 2-stage growth of cordierite. Phase diagram modelling shows that cordierite started to grow below 550°C at the expense of chlorite, muscovite and minor andalusite. Most cordierite grew together with biotite at around 560°C, while orthoamphibole became stable near peak PT conditions.

Non-isobaric temperature-composition phase diagrams constructed along the prograde PT path show that the aluminium-rich metapelites had the same assemblage of chlorite-muscovite-(andalusite)-quartz at low-grade conditions but developed different prograde and peak mineral assemblages due to bulk compositional differences. Fe-rich muscovite-bearing mica schists show simple reaction textures, mostly the transition from andalusite to sillimanite.

Cordierite-bearing mica schists developed different assemblages due to their Mg-richer bulk compositions. During near isobaric heating chlorite and muscovite reacted to form cordierite, biotite and andalusite. Petrographic observations suggest that some mica schists developed a second generation of andalusite at the expense of muscovite together with biotite rimming the early-formed prograde cordierite porphyroblasts due to late prograde increase of P. These rocks had lower chlorite/muscovite ratios at low grade compared to rocks with no late prograde andalusite, therefore chlorite was consumed before muscovite on the prograde path.

During near isobaric cooling kyanite-chlorite (plus quartz) formed at the expense of cordierite around 540 °C at 4.5 kbar. Decompression and continued cooling resulted in the formation of retrograde andalusite. The whole prograde-peak-retrograde P-T path forms an anticlockwise loop characterized by the sequential, multi-stage growth of aluminosilicates.