A newly defined Late Ordovician magmatic–thermal event in the Mt Painter Province, northern Flinders Ranges, South Australia

ABSTRACT

Magmatism, metamorphism and metasomatism in the Palaeoproterozoic–Mesoproterozoic Mt Painter Inlier and overlying Neoproterozoic Adelaidean rocks in the northern Flinders Ranges (South Australia) have previously been interpreted as resulting from the ca 500 Ma Delamerian Orogeny. New Rb–Sr, Sm–Nd and U–Pb data, as well as structural analysis, indicate that the area also experienced a second thermal event in the Late Ordovician (ca 440 Ma). The Delamerian Orogeny resulted in large-scale folding, prograde metamorphism and minor magmatic activity in the form of a small volume of pegmatites and leucogranites. The Late Ordovician event produced larger volumes of granite (the British Empire Granite in the core of the inlier) and these show Nd isotopic evidence for a mantle component. The high-temperature stage of this magmatic–hydrothermal event also gave rise to unusual diopside–titanite veins and the primary uranium mineralisation in the basement, of which the remobilisation was younger than 3.5 Ma. It is possible that parts of the Mt Gee quartz–hematite epithermal system developed during the waning stages of the Late Ordovician event. We suggest that the Ordovician hydrothermal system was also the cause of the commonly observed retrogression of Delamerian metamorphic minerals (cordierite, andalusite) and the widespread development of actinolite, scapolite, tremolite and magnetite in the cover sequences. Deformation during the Late Ordovician was brittle. The recognition of the Late Ordovician magmatic–hydrothermal event in the Mt Painter Province might help to link the tectonic evolution of central Australia and the southeast Australian Lachlan Fold Belt.