The Mount Isa Inlier in northwestern Queensland contains Early to Middle Proterozoic volcanics, sediments, and granites which were formed between 1865 and 1610 Ma ago, and were subsequently metamorphosed during intense structural deformation between 1570 and 1450 Ma ago. Four major cycles of volcanism are recognised. The oldest cycle is represented by the Leichhardt Metamorphics which consist of 150 000 km3 of rhyodacitic and rhyolitic ignimbrite, tuff, and agglomerate ranging from low greenschist fades in the north to upper amphibolite facies in the south and west of the centrally exposed basement succession. The Leichhardt Metamorphics are overlain unconformably by the second cycle. In the east, this comprises the Magna Lynn Metabasalt (1000 km3 of meta-andesite, metabasalt, mafic tuff, and minor quartzite), and the Argylla Formation (100 000 km3 of metarhyolite, metadacite, metarhyodacite, siltstone, and sandstone) dated at 1777 Ma. Similar rocks in the west form the Bottletree Formation. The proportion of sedimentary rocks increases with time in this cycle and is particularly high in the east. The third cycle is dominated by approximately 50 000 km3 of continental tholeiite (Eastern Creek Volcanics) in a rift valley in the west of the Inlier. An andesitic stratovolcano (Marraba Volcanics) and oceanic (?) low-K tholeiite (Soldiers Cap Group) occur in the east. The final cycle is best developed in the northwest of the Inlier where at least 1000 km3 of alkaline felsic and mafic volcanics (Fiery Creek Volcanics and Carters Bore Rhyolite) occur. Minor younger volcanism is represented by tuffs in the Mount Isa and McNamara Groups in the west, and by mafic and felsic lavas possibly in the Corella Formation in the east. Terrestrial conditions of the first cycle were interrupted by minor marine incursions in the east during the second cycle. Marine shelf conditions developed during the third cycle in the east and spread westward late in this cycle. Terrestrial conditions returned in the fourth cycle in the west.
Metamorphic mineral assemblages have obliterated most primary textures and minerals. Metamorphic temperatures ranging from 400 to 650°C and pressures ranging from 2 to 6 kb have been determined. A low-pressure facies series is indicated by minerals such as cordierite and andalusite. Biotite in the Leichhardt Metamorphics is strongly Fe-rich but in the younger cycles the biotite becomes progressively more Mg-rich. In the mafic rocks, magnesio-hornblende is dominant but actinolite and primary clinopyroxene occurs in lower grade sequences. Garnet and diopside are developed in high grade areas. Most volcanics in the fourth cycle contain abundant monoclinic potash feldspar and hematite which are attributed to alteration in hypersaline conditions.
The felsic volcanics generally contain more than 4 cg/g K2O, are enriched in LIL elements, and are corundum normative. The first cycle contains moderate contents of the HFS elements (Zr, Nb, Y, HREE, and Ti) but the second cycle is particularly enriched in these elements and Fe. This variation is related to a transition from possible erogenic volcanics to a bimodal suite. Felsic volcanics are rare in the third cycle and those in the final cycle contain >10 cg/g K2O and less than 0.5 cg/g of Na2O, CaO, MgO, and MnO.
Andesite is a minor constituent of all cycles but Is the only non-felsic volcanic in the Leichhardt Metamorphics and is a major constituent of the Magna Lynn Metabasalt (second cycle) and the Marraba Volcanics (third cycle). Tholeiitic volcanics are dominant in the third cycle; those in the east are LIL-depleted, whereas those from the west are LIL-enriched continental basalts.
Alkali olivine basalts are identified in the highly altered fourth cycle by abundant relict olivine phenocrysts, immobile elements contents (Nb/Y>0.8), and highly enriched LREE.
The Leichhardt Metamorphics could have been derived by partially melting shale leaving a residue containing some garnet. The second cycle of felsic volcanics is consistent with partial melting at slightly higher levels in the crust involving similar source rocks which had been dehydrated and veined with LIL-enriched felsic rocks during the earlier cycle. The mafic volcanics are attributed to partial melting of the mantle and they provide a likely heat source for generating the felsic volcanics. Solidified mafic magma and the cumulates from fractional crystallisation in the lower crust, granites in the upper crust, and metamorphism stabilised the Inlier by 1450 Ma.