This contribution presents the distribution and geology of Australian alkaline and related rocks of Paleozoic age, one in a series within the Alkaline Rocks Atlas of Australia that collectively document alkaline rocks across the continent through time.
In general, alkaline and related rocks are a relatively rare class of igneous rocks worldwide. Alkaline rocks encompass a wide range of rock types and are mineralogically and geochemically diverse. They are typically thought to have been derived by generally small to very small degrees of partial melting of a wide range of mantle compositions. As such these rocks have the potential to convey considerable information on the evolution of the Earth’s mantle (asthenosphere and lithosphere), particularly the role of metasomatism, which may have been important in their generation, or to which such rocks may themselves have contributed. Such rocks, by their unique compositions and/or enrichments in their source protoliths, also have considerable metallogenic potential, e.g., diamonds, Th, U, Zr, Hf, Nb, Ta, REEs. It is evident that the geographic occurrences of many of these rock types are also important, and may relate to presence of old cratons, craton margins or major lithospheric breaks. Finally, many alkaline rocks also carry with them mantle xenoliths providing a snapshot of the lithospheric mantle composition at the time of their emplacement.
Accordingly, although alkaline and related rocks comprise only a volumetrically minor component of the geology of Australia, they are of considerable importance to studies of lithospheric composition, evolution and architecture and to helping constrain the temporal evolution of the lithosphere. They are also directly related to metallogenesis and mineralisation, particularly for a number of the critical minerals, e.g., rare earth elements, niobium. In light of this, Geoscience Australia is undertaking a compilation of the distribution and geology of Australian alkaline and related rocks, of all ages, and producing a GIS and associated database of such rocks, to both document such rocks and for use in metallogenic and mineral potential studies.
The broadening of the definition of alkaline rocks within the Alkaline Rocks Atlas herein, to include ultra-high K mafic to felsic silica-saturated rocks (alkaline-shoshonites), which are commonly formed at convergent margin settings, manifests in some divergences in the presentation of alkaline rocks that are particularly relevant to the Phanerozoic, and Paleozoic Australia in particular.
Paleozoic alkaline and related rocks occur throughout eastern Australia, with occurrences in the Northern Territory, and in all States excluding Western Australia. However, with a few exceptions they are principally located within the Tasman Element, and are over-represented in NSW – with respect to other states jurisdictions (based on available data). Paleozoic alkaline rocks range from ultramafic through to felsic, and from strongly alkaline (undersaturated) through to mildly alkaline.
Strongly alkaline rocks – congruent with the outline of alkaline rocks presented above – are comparatively rare in the Paleozoic, and are compositionally diverse incorporating alkali basalt, kimberlite, carbonatite-related rocks, and lamprophyre, with wide-ranging ages.
Overwhelmingly, the Paleozoic alkaline rock compilation is dominated by very high K alkali mafic to felsic silica-saturated rocks. Mafic-intermediate rocks within this grouping typically have an “arc signature” (i.e., low Nb/Y) but incorporate both arc magmas as well as rocks associated with backarc rifting. These rocks typically occur within rock units or packages that comprise a diverse array of rock types and compositions from volcanic rocks, related volcaniclastics and epiclastics through to sedimentary rocks. Igneous rocks within these packages commonly range from subalkaline / calc-alkaline through to mildly alkaline (trachybasalt to trachyandesite, and less commonly trachyte) based on alkali contents. Quartz-saturated felsic alkaline rocks are dominated by near peralkaline–peralkaline A-types and high-temperature transitional I-A compositions, but locally include rarer mildly alkaline (based on HFSE) rocks. The inclusion of whole rock units, which may only incorporate a small volume of alkaline rocks, necessarily means that the volume of these alkaline rocks is both poorly constrained and over-represented with this dataset.
