Isograd pattern and regional low-pressure, high-temperature metamorphism of pelitic, mafic and calc-silicate rocks along an east-west section through the Mt Isa Inlier

The southern Mt Isa Inlier is characterised by high-temperature/low-pressure metamorphism with metamorphic grade varying from greenschist to upper amphibolite facies. Metamorphism across the inlier occurred in a series of events during the Isan Orogeny (1610 - 1500 Ma), although some of these may be local in extent and related to various granitic intrusions. Localised events occurred prior to the Isan Orogeny during the emplacement of the Wonga and Sybella Batholiths, at ca 1740 Ma and 1672 Ma, respectively. The overall pattern is of a series of north - south amphibolite facies belts separated by zones of mainly greenschist facies. Isograd surfaces were initially shallowly dipping before being steepened by subsequent folding. Pelitic rocks change with increasing grade through chlorite, biotite, garnet, staurolite and/or andalusite, sillimanite, and sillimanite/K-feldspar zones. More magnesian-rich schists grew cordierite in place of garnet, with or without staurolite at higher grade. In the absence of pelitic rocks, regional metamorphic patterns in calc-silicate rocks were defined by the clinopyroxene isograd, which approximately corresponds to the sillimanite isograd. Estimated maximum P - T conditions for the highest grade zones are 400 - 500 MPa at >650°C with late (retrograde) kyanite + chlorite in some areas indicating anticlockwise P - T - t paths. Multiple stages of porphyroblast growth have been documented locally, suggesting a number of thermal events throughout the Isan Orogeny. The metamorphic peak occurred in most parts of the inlier between ca 1595 and 1580 Ma. Most workers correlate the metamorphic peak with regional D2. There is a broad-scale relationship between granites and amphibolite-facies metamorphic rocks, but with the exception of abundant migmatites in the sillimanite/K-feldspar zones, the granite bodies intruded before or after the metamorphic peak. A number of models have been proposed for metamorphism in the Isan Orogeny. Our preferred model implicates the intrusion of large volumes of tholeiitic magmas into the lower crust and advective transfer to the mid-crust via crustal melts that now comprise the migmatites of the sillimanite/K-feldspar zones. In addition, heat from older granites with relatively high values of heat-producing elements probably made a local contribution to the thermal budget