A basin system and fluid-flow analysis of the Zn-Pb-Ag Mount Isa-type deposits of northern Australia: Identifying metal source, Basinal Brine reservoirs, times of fluid expulsion, and organic matter reactions

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Basement framework and geodynamic evolution of the Palaeoproterozoic superbasins of north-central Australia: an integrated review of geochemical, geochronological and geophysical data

A largely convergent setting is proposed for crustal, tectonic and basin evolution of the intracratonic regions of north-central Australia between 1800 and 1575 Ma. The new geodynamic model contrasts with previous proposals of widespread extension during the Leichhardt, Calvert and Isa intervals. Local transtensional to extensional structures exist, but these are best explained by a combination of flexural, thermal and dynamic processes related to an active southern margin. The development of thick accumulations of sediments (superbasins) is linked geodynamically to interpreted active margin processes (subduction and magmatic arcs) in central Australia. A synthesis of geochemical data from the 1870-1575 Ma igneous units from the Arnhem, McArthur and Mt Isa regions of north-central Australia confirms the intracratonic setting of these units and suggests that a long-lived thermal anomaly was responsible for the generation of both mafic and felsic magmas. The geochemical characteristics suggest the igneous units are derived from the lithospheric mantle and are not typical rift- or plume-related melts. A review of the U-Pb SHRIMP ages for the entire region demonstrates the minimum distribution of correlative igneous rocks was widespread. Exotic populations in the 207Pb/ 206Pb isotopic data provide insights into the nature and evolution of the crust throughout north-central Australia. Archaean inheritance is found to be nearly ubiquitous. The data support the temporal subdivision of north-central Australia into the Leichhardt (1800-1750 Ma), Calvert (1750-1690 Ma) and Isa (1690-1575 Ma) intervals which are marked by superbasins and concomitant episodes of igneous activity. A highly heterogeneous pre-superbasin crust is interpreted from regional, newly processed geophysical data. The cratonic portion of north-central Australia is interpreted to consist of three broad northwest-trending belts or elements that are further distinguished into western, central and eastern geophysically distinct provinces. A map of the superbasin distribution is derived and integrated with structural and stratigraphic data to assess the evolution of the basins and the crust through time. The superbasin successions of north-central Australia are synchronous and wide-spread, although not necessarily interconnected. The tectonic model incorporates dynamic tilting of the craton during episodes of subduction and transmission of compressive intraplate stresses through the craton during intervening episodes of orogeny. These processes resulted in flexure, strike-slip deformation and a complex thermal structure. These mechanisms account for the subsidence and basin evolution that results in widespread ramp and strike-slip basins. The model also accounts for the thermal history recorded by magmatic events. The proposed geodynamical model provides a unifying crustal evolution scenario for central and northern Australia for approximately 225 million years of the Proterozoic

Exceptional Preservation of Fossils in an Upper Proterozoic Shale

Late Proterozoic organisms must have been diverse and widely distributed, but in general their fossil record is both taxonomically and environmentally limited. Exceptional preservation of Proterozoic fossils is not unknown, but it is usually associated with silicified carbonates from restricted peritidal or playa lake environments. We report here an exceptionally well preserved and distinctive assemblage of Lake Proterozoic fossils from subtidal marine shales. In additional to the sphaeromorphic acritarchs and cyanobacterial sheaths routinely preserved in Proterozoic rocks, this assemblage includes multicellular algae ('seaweeds'), a diverse assortment of morphologically complex protistan vesicles, and probable heterotrophic bacteria. Thus, it provides one of the clearest and most taxonomically varied views of Proterozoic life yet reported.Organismic and Evolutionary Biolog