LOW-PRESSURE, HIGH-TEMPERATURE METAPELITIC ROCKS FROM THE CENTRAL MOUNT ISA INLIER, NW QUEENSLAND, AUSTRALIA: NEW RESULTS FROM QUANTITATIVE PHASE DIAGRAM CALCULATIONS

Highly magnesian and aluminous metapelites from the Rosebud Syncline, central Mount Isa Inlier, (NW Queensland, Australia) were studied using quantitative phase diagram modeling. In the syncline, rocks of the Mid-Proterozoic Corella Formation (Mary Kathleen Group) are exposed, a compositionally variable (meta)sedimentary sequence of calcareous and pelitic compositions. The area has been affected by three distinct deformation events, with the present doubly-plunging, north-south trending synclinal structure being the result of tight to isoclinal D₂ folding, accompanied by inhomogeneous stretching along a vertical axis. The metamorphism is characterized by prograde assemblages of the andalusite-sillimanite facies series.

Throughout the syncline, Ca- and alkali-poor alumino-magnesian rocks are interlayered with highly aluminous schists and calcareous rocks. The dominant prograde assemblage of the Mg-rich types shows cordierite and orthoamphibole with biotite, chlorite, plagioclase, quartz and occasionally talc. The contacts with calc-silicate rocks are sharp, while the contacts with aluminous schists may be sharp, or gradational from muscovite-rich to cordierite-rich ones, then cordierite-orthoamphibole rocks. The peak assemblage of cordierite, orthoamphibole, biotite, plagioclase and quartz yield 626 °C and 4.5 kbar in the NCKFMASH system.

Aluminous, orthoamphibole-free schists have the prograde assemblage andalusite, sillimanite, biotite, plagioclase and quartz, and can be divided into 2 major groups: muscovite-bearing and cordierite-bearing. Rare garnet is only present in Mn-richer muscovite-bearing lithologies. The ratio of sillimanite to andalusite increases from the SW to the E and NE. Both muscovite- and cordierite-bearing schists yield 595-630 °C and 3.8-4.8 kbar in the MnNCKFMASHTO system.

Quantitative phase diagram modeling results thus indicate metamorphic peak conditions of around 600-630 °C and 3.8-4.8 kbar for the entire study area, without any significant difference between the various rock types analyzed. These new pressure-temperature data using individual bulk rock compositions support earlier P-T estimates based largely on petrogenetic grids.