The volcanology, petrogenesis, and economic potential of the Mesoproterozoic shallow-water, intra-caldera, lava-like rheomorphic Kathleen Ignimbrite, west Musgrave Province, central Australia

The rhyolitic 1071 ± 5 Ma Kathleen Ignimbrite (Formation) forms part of a ~1 km thick, volcanic-sedimentary rock succession at the base of the Talbot Sub-basin and Pussy Cat Group. This group is one of many thick and extensive bimodal volcanic-sedimentary successions within the failed intra-cratonic rift-dominated Bentley Supergroup, located in the remote Musgrave region of central Australia. The ancient Kathleen Ignimbrite (KI) represents an intra-caldera fill-sequence that resulted from a very large explosive caldera-forming eruption during the Mesoproterozoic evolution of the west Musgrave Province. The Kathleen caldera has an estimated minimum intra-caldera volume of ~190 km3 and could even represent a Volcanic Explosivity Index (VEI) 7 or greater magnitude explosive eruption. The KI is remarkably well-preserved and exhibits well-defined volcanic textures, such as two distinct eutaxitic fiamme populations, one crystal-rich, the other crystal-poor or aphyric, remnants of devitrified glass shards, autobreccia, hyaloclastite and peperite, as well as prominent planar to folded laminations and flow-banding, interpreted to have formed during rheomorphism. Much of the up to 640 m thick deposit is welded and, in particular, the basal 50 m is extremely welded and lava-like in nature and shows evidence of having behaved rheomorphically. The high grades of welding in this deposit have obliterated many of the original vitriclastic textures, but this study demonstrates that the deposit is indeed ignimbritic and constitutes an intra-caldera eruption fill-sequence. The KI constitutes three members: the lower ignimbritic Kurrparu Member consists of a basal, crystal-poor, lava-like rheomorphic facies with a quench-fragmented bottom contact with wet underlying sediments that grades up into a thick poorly-sorted crystal-rich to crystal-poor matrix supported lapilli tuff facies with unequivocal vitriclastic textures, followed by a gradational change to a eutaxitic to upper crystal-poor facies. Overlying the Kurrparu Member are thin interbedded breccias and crystal-rich to crystal-poor, eutaxitic to non-eutaxitic vitriclastic ignimbrites and breccias of the Karlaya Member, which in turn is overlain by the reworked shallow-water turbiditic volcaniclastic upper Kilykilykarri Member. The KI was emplaced in an initial shallow-water marine shelf-type or large lake-type palaeodepositional environment. Large volumes of magma were emplaced as slightly less evolved, coeval and cogenetic, syn-sedimentary, crystal-rich, porphyritic rhyolite intrusions into the sedimentary rocks that host the KI sequence, representing magmatism from a shared, compositionally zoned magma chamber that was contemporaneous with the caldera eruption. The rhyolitic Pussy Cat Group magmas that formed the high-silica KI and its associated intrusions were evolved, anhydrous, and metaluminous (to slightly peraluminous) A-type magmas that were enriched in the rare earth elements (REE; up to 794 ppm total REE) and fluorine (≤0.8 wt%) and are similar to topaz rhyolites of the central western USA. These rhyolitic magmas originated from a common source and were generated by extended fractionation of basaltic magmas. The crude vertical stratification within the KI varies between crystal-rich to crystal-poor and has the two pumice populations in addition to geochemical and textural differences in the eruption sequence. This suggests two possible eruption model scenarios, both of which fit the results: one involving the tapping of an evolved melt-rich aphyric cap of a shallow-level zoned magma chamber with the later extraction and mixing of a more primitive crystal-rich basal cumulate, the other requiring silicic magma recharge from a hotter, more buoyant, aphyric silicic magma originating from a common deeper-seated magma reservoir that triggered the eruption and subsequent mixing of the upper-crustal zoned magma chamber. The physical volcanology, palaeoenvironment and geochemistry (barring higher fluorine concentrations) of the KI are similar to ignimbrites of the Snake River Plain of the central USA, suggesting a new locality for Snake River (SR)-type volcanism. Despite these similarities the KI has lower magmatic crystallisation temperatures (~830 deg. C), lower eruption and deposition temperatures, and lower viscosity values (10^8.16 Pa.s) than the extremely welded SR-type rheomorphic ignimbrites. The lava-like rheomorphism seen in the KI deposit does not appear to be related to one single factor, but instead appears to be related to a combination of rheomorphism-controlling factors that are to date unique to the KI. In addition, the extreme fractionation and associated heavy REE enrichment associated with the formation of evolved, high-silica rhyolite magmas such as those that formed the KI means these types of deposits may be future low-grade, bulk-tonnage REE mineral deposits. This research highlights the high volumes of coeval and cogenetic juvenile mantle-derived felsic magmatism that affected this area and validates the intra-cratonic instabilities suggested for current tectonic setting of west Musgrave Province during the Giles Event and Ngaanyatjarra Rift. It provides additional evidence for the recognition of a large silicic component associated with a likely large igneous province (LIP) within the Talbot Sub-basin of the west Musgrave Province and supports the presence of possible supervolcanos and likely supereruptions in central Australia during the Mesoproterozoic.