The East Australian Potassic Suite: Petrology, Bulk Chemistry, and Origin

Anthony Lanati , Joshua Shea , Stephen Foley, Marthe Klöcking , Arno Rohrbach, Stephan Klemme

The Eastern Australian Potassic Suite (EAPS) is an alkaline volcanic province made up of over 20 widely dispersed outcrops that extend almost 700 km, inferred to form the southern portion of the Cosgrove continental hotspot track. In contrast to the large basaltic volcanic complexes to the east and north, the EAPS occurs exclusively as mafic potassium–rich occurrences with inferred low–volume expressions with outcrop extents on the order of hundreds of metres to a few kilometres. These localities mostly occur on lithosphere thicker than 120 km suggesting that the lithosphere–asthenosphere boundary may have a strong influence on their depth of generation. The primary felsic mineral in these rocks is leucite, which has previously seen the EAPS defined as leucitites in constantly evolving classifications of exotic, but potentially economically significant alkaline rocks. However, this classification does not reflect their chemical or genetic affinity. In this study, we undertake a systematic re–evaluation of these occurrences with the aim of understanding their source enrichment processes and melting conditions. Newly acquired major, trace, and volatile element whole–rock data shows that the EAPS is chemically variable, but exceptionally enriched in potassium compared to other East Australian volcanism, with high K₂O/Na₂O and MgO (Av.: K₂O 4.98 wt%; K₂O/Na₂O 3.23; K/Na 2.12; MgO 12.14 wt%). We report the only complete volatile element data for the EAPS which show the lavas are similarly enriched in nitrogen compared to other lamproites, while being more CO₂-rich despite being partially degassed (N: 44–350 ppm; CO₂: 1129–10274 ppm). Despite not occurring on a craton, the EAPS trace element patterns and chemistry most closely resemble cratonic (anorogenic) lamproites. Additionally, the mineralogy, major and trace element concentrations closely match the classification criteria for lamproites. Trace element ratios of these near-primary mantle melts have a primitive signature generated from a highly enriched source that has previously undergone a degree of mixed silicate–carbonatite metasomatism. The most likely source for these rocks based on these new data is a hydrous phlogopite–bearing and olivine–poor assemblage that originates in the garnet stability field (i.e. phlogopite–garnet–pyroxenite). The inherited titanian affinity and elevated phosphorus contents of these magmas suggest apatite and oxide minerals were also present in the source. This new data helps inform interpretations of regional variations in melt generation and mantle source mineralogy in the highly heterogeneous metasomatised mantle beneath eastern Australia. We further suggest that the mechanisms that generated the EAPS likely include a combination of edge–driven convection and shear–driven upwelling as well as channelised melt flow which contributed to metasomatic depletion and refertilisation cycles. These cycles are synonymous with the initial stages of continental destabilisation that could develop toward rifting.