The geology, geochemistry and geochronology of the Atnarpa Igneous Complex, S.E. Arunta Inlier, Northern Australia: Implications for early to middle Proterozoic tectonism and crustal evolution

The study area is dominated by the Atnarpa Igneous Complex (AIC), which is associated with a basement supracrustal assemblage and juxtaposed with a cover sequence of the Amadeus Basin. Systematic field, petrographic, geochemical, U-Pb and Rb-Sr isotopic and geochronological studies were focused on the Atnarpa Igneous Complex principally in the Atnarpa area, which have put some constraints on the nature of the geochemical and tectonic evolution of Proterozoic crust in the SE Arunta Inlier. Three major rock units, namely, the Atnarpa Calc-alkaline Suite (ACAS), the Atnarpa Granitic Batholith (AGB) and the Atnarpa Tonalite-Basalt Suite (ATBS), are recognised to be sequentially emplaced during the early to middle Proterozoic time. The Atnarpa Calc-alkaline Suite (ACAS) is considered to be a typical high-level calc-alkaline suite with some transitional trondhjemitic affinities in its felsic differentiates. Its genesis is formulated in terms of high-level fractional crystallization from a parental magma with high oxygen and water fugacities. The parental magma is considered to be mantle-derived, being related to oceanic crust subduction under the continental margin. Its crystallization age is dated by U-Pb zircon isotopics at 1879±11/10 Ma. This is the oldest U-Pb zircon age so far known in the Arnn ta Inlier. The Atnarpa Granitic Batholith (AGB) is composed of the pre-collisional Atnarpa Older Tonalite (AOT) and Leucogranite (AOLG) and minor syn-collisional Atnarpa Muscovite Granite (AMG). The AOLG yields a U-Pb zircon age of 1873±11/10 Ma and the AOT, 1863±33/27 Ma (Sando 1987). The AOT & AOLG are characteristic of low K20, Rb, REE, Th, Rb/Sr and inferred 87 Sr/86Sr initial ratios and high Na20, MgO, Fe20 3tot, Sr, Ba, Sc, and Ni, which is quite distinctive from the Barramundi Association of similar ages (Wyborn 1988; Wyborn and Page 1983). The AOT and AOLG are interpreted as being derived by crystal fractionation from one parental magma. The parental magma is considered to be generated by both partial melting and assimilation of a mafic underplate with a short crustal pre-history due to the addition of heat and material from the ascending less-silicieous mantle-derived calc-alkaline magma. The AMG is geochemically unrelated to the AOT & AOLG. It, however, shows geochemical affinities to the syn-collision peraluminous S-type granite of Harris et al. ( 1986), and is considered to be derived from anatexis of a sedimentary source within supracrustal levels owing to a subduction initiated marginal collison similar to the Cordilleran type. The Atnarpa Tonalite-Basalt Suite (ATBS) is a bimodal high-level suite with geochemistry, including strongly fractionated REE pattern with HREE depletion in the Atnarpa Younger tonalite (AYT), similar to the ubiquitous Archean tonalitic-basaltic bimodal suites. The AYT is dated at 1751±12 Ma, which marks the second magmatic event in this area. Its calculated 87Sr/86Sr initial ratio at 1751 Ma is 0.7017. The ATBS is considered to be formed in response to the opening and closure of a marginal or back-arc basin. Tectonically, whilst the orogenesis of most of the northern Australian Proterozoic terrains has been interpreted to be essentially ensialic and rift-related without the involvement of oceanic crust subduction (Etheridge et al. 1987), the SE margin of the Arunta Inlier is considered to represent a continental margin during the early to mid Proterozoic, analogous to the Cordilleran Orogenic Belt. The study area has undergone two episodes of subduction-related magmatism, probably in response to episodic change in subduction rate or the crustal formation events to the south of the Arunta Inlier, as indicated by the Sm/Nd data from the Musgrave Inlier (McCulloch 1987). The first episode (1860 - 1880 Ma) was contemporaneous to the wide-spread Barramundi Association (Etheridge 1987) and the second episode (e.g. 1751 Ma), to the magmatism in the Entia Dome, Harts Range area (Cooper et al. 1988) as well as other places in the northern Australian Proterozoic terrains (Page 1988). The upper mantle under the SE margin of the Arunta Inlier must have experienced multi-stage differentiation and depletion in order to account for the unusually high positive initial ENd values for the AIC and very low Sr initial ratio in the AYT (0.7017). Rb/Sr isotopic data of whole rocks and micas indicate that the study area has subsequently suffered from several episodic thermal- deformational events. These include: (1). 1670 Ma old event of regional deformation and amphibolite facies metamorphism; (2). fast cooling from 500°C to below 320°C during 1512 - 1469 Ma; (3). substantial crustal uplift at around 1000 Ma probably in response to the subsidence of the Amadeus Basin and (4). the Palaeozoic Alice Springs Orogeny.Thesis (MSc) -- University of Adelaide, School of Physical Sciences, 198

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