The Maronan Supergroup: an inferred early volcanosedimentary rift sequence in the Mount Isa Inlier, and its implications for ensialic rifting in the Middle Proterozoic of northwest Queensland

Abstract

Mapping in the Cloncurry-Selwyn region of the Proterozoic Mount Isa Inlier has rationalized the stratigraphy there, and revealed a early volcanosedimentary basinal cycle, which probably pre-dates others in the inlier. The Soldiers Cap Group (sensu stricto) has been expanded to incorporate most of the Kuridala Formation. Quartzofeldspathic rocks in the latter, and in undifferentiated Soldiers Cap Group, are now interpreted to conformably underlie the Soldiers Cap Group, in a 5 km thick sequence termed the Fullarton River Group. The Soldiers Cap and Fullarton River Groups collectively comprise the Maronan Supergroup, an apparently conformable sequence ∼ 10 km thick, consisting of terrigenous and volcaniclastic sediments and bimodal volcanics, which mature texturally and chemically up-sequence, and which were probably deposited in a marine environment. Grain-size indicators suggest an easterly sediment source, but some quartzo-feldspathic material may have been derived from Barramundi Igneous Suite rocks to the west. We interpret the Maronan Supergroup as a complete cycle of basin sedimentation, probably related to ensialic rifting. The supergroup crops out over an area of ∼ 15 000 km 2. It is unconformably overlain in the east and south by Mesozoic sediments, and bounded in the west by the Mount Dore Fault Zone, a major east-dipping structure separating it from the Mary Kathleen Group along the entire length of the contact. The Gin Creek Block is a quartzo-feldspathic inlier tectonically emplaced in the Mary Kathleen Group, and may be stratigraphically equivalent to the Fullarton River Group. The stratigraphic relationship of the Maronan Supergroup to the two cycles of rift sedimentation already recognized in the Mount Isa Inlier needs geochronological justification, but circumstantial evidence suggests it is older than either of these. It is certainly distinct from the other sequences in having, for example, an overall higher metamorphic grade, different geochemical and geophysical signatures for volcanic and volcaniclastic units, and a range of lithologies unique in the Mount Isa Inlier, such as a thick turbidite package in its upper central part, plagioclase-rich sediments, and true banded iron formations (containing Pb + Zn ± Cu mineralization). Assuming the Maronan Supergroup is older than other cover units in the Mount Isa Inlier, we interpret the history of sedimentation in the inlier to have involved three cycles of ensialic rifting, each showing a temporal trend of evolving volcanicity and increasing sediment maturity. Lithologies show a natural three-fold subdivision into distinct packages, based on the dominant lithologies. These are characterized, from oldest to youngest, by: (1) felsic volcanics and associated volcaniclastics; (2) mafic volcanics and maturing coarse- to fine-grained clastic sediments; and (3) fine-grained, clastic and carbonate-bearing sediments. A complete rift cycle displays all three rift packages, which are interpreted to relate to early rift, maturing rift, and sag phases of the cycle. This subdivision relates specifically to the Mount Isa Rift Orogen, but may be applicable elsewhere, as the patterns of evolution of volcano-sedimentation in other eastern Australian Proterozoic basinal sequences show broadly similar characteristics. From existing geochronological data and, where necessary, inferred ages of sedimentation, the three rift cycles have ages of 1880-1810 Ma, 1810-1750 Ma and 1680-1660 Ma. The Barramundi Igneous Suite may be broadly synchronous with initiation of rifting in the Mount Isa Inlier.