The 1800–1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes

Abstract

The chronostratigraphic and magmatic framework for the Proterozoic Mount Isa Inlier, northwest Queensland has been substantially revised to match insights provided by isotopic age constraints accumulated over recent years. This material and existing chronometric constraints is used to construct a new time–space plot for the Inlier and to compare Eastern Succession volcanosedimentary accumulations with temporally equivalent units in the Western Fold Belt. The accumulation of volcanosedimentary sequences in the Eastern Succession occurred in four main periods: namely 1890–1850 Ma, 1800–1750 Ma, 1710–1690 Ma, and 1680–1610 Ma. Temporally equivalent units can be identified in the Western Fold Belt. Periods of sedimentary accumulation were immediately preceded by bimodal igneous activity across the Inlier. The locus of bimodal volcanism migrates from west to east with time. Intracontinental rifting is the most likely tectonic setting for the sedimentary basins. The majority of the Soldiers Cap Group was deposited between ca. 1675 and 1650 Ma with the exception of units in the Boomarra Horst now known to belong to older sequences (deposited ca. 1775–1750 Ma), and the Llewellyn Creek Formation at Snake Creek which may be as old as ca. 1690 Ma. Upper amphibolite facies rock units in the southwest (e.g. the Gandry Dam Gneiss) are likely to be higher metamorphic grade, temporal equivalents of the Soldiers Cap Group. Five rock units previously assigned to the Cover Sequence 2 Mary Kathleen Group; namely, the Answer Slate, Kuridala Formation, Staveley Formation, Marimo Slate and Agate Downs Siltstone are now known to have been deposited between ca. 1675 and 1610 Ma. These units (bar the Kuridala Formation) have been removed from the Cover Sequence 2 Mary Kathleen Group and given the informal name ‘Young Australia Group’. The Kuridala Formation has been included in the Soldiers Cap Group. The rock units within the Young Australia Group are likely to be lateral facies equivalents of the younger units within the Soldiers Cap Group. Rock units within the Mount Albert Group (although undated) are considered likely to be temporal equivalents of the Soldiers Cap Group and the Young Australia Group. The Roxmere Quartzite is included here in the Young Australia Group that it tectonically overlays. The Ballara Quartzite and Mitakoodi Quartzite represent a single regionally extensive sequence deposited at ca. 1755 Ma. The Double Crossing Metamorphics (previously mapped as basement) are likely to correlate with the Argylla Formation and Mitakoodi Quartzite. Most of the Eastern Succession to the west of the Pilgrim Fault is likely to have been a topographic high between ca. 1780 and 1755 Ma, prior to the Ballara Quartzite unconformably overlying the ca. 1780 Ma Argylla Formation. Detrital zircons of ‘Argylla’ and ‘Wonga’ ages indicate that the Mary Kathleen Belt is likely to have been a topographic high at least periodically during accumulation of the Soldiers Cap and Young Australia Groups. Soldiers Cap Group units in the southeast contain numerous detrital zircons giving ages between ca. 1730 and 1710 indicating a possible siliciclastic source to the east. The host rocks to the Pb–Zn–Ag deposits are restricted to Cover Sequence 3 units deposited between ca. 1680 and 1590 Ma and, with the exception of Century, young from the southeast towards the northwest. Cu ± Au deposits and prospects are hosted in units from all the major periods of volcanosedimentary accumulation. However, many of the most significant deposits are found within the younger sequences.