J.A. Cooper

Rate of Arunta Inlier evolution at the eastern margin of the Entia Dome, central Australia

Abstract Zircon dating in the Entia Dome of the Arunta Inlier helps to temporally define the multiphase, successive Strangways and Harts Range Orogenies. It appears that the entire activity of both orogenies took place between 1767 ± 2 Ma and 1731 ± 1 Ma, with a major shearing and thrusting event associated with plutonic activity at 1747−2+3 Ma. These comparatively short time spans indicated are quite sufficient for the large horizontal movements required for the intra-continental basin formation and deformation that has been postulated by James and Ding. The earlier of these events are later than those currently proposed for equivalent stages of other Middle Proterozoic inliers of the North Australian Orogenic Province which raises difficulties with detailed correlations in grouping the inliers together. Monazite (UPb), muscovite (RbSr) and biotite (RbSr) isotopic systems had not closed until 360 Ma, 325 Ma and 308 Ma, respectively, indicating uplift and cover removal from > 20 km to

Fractionation of monazite in the development of V-shaped REE patterns in leucogranite systems: Evidence from a muscovite leucogranite body in central Australia

Abstract Geochemical modelling based on Kd values for rare earth (REE) and other trace elements suggests that a Proterozoic muscovite leucogranite from the Atnarpa area, southeastern Arunta Inlier, central Australia was derived from a restite-free melt by simple but extensive crystal fractionation involving feldspars, biotite and accessory REE-rich minerals such as monazite and zircon at a pressure of 0.5–1.0 kbars and a temperature below 750°C. This study indicates that REE-rich accessory minerals, particularly monazite, play an important role in the evolution of the REE in high-Si, low temperature, granitic melt systems.

The geochemical evolution of Proterozoic granitoids near Port Lincoln in the Gawler orogenic domain of South Australia

Abstract Proterozoic granitoids in the Port Lincoln area of the Gawler orogenic domain of South Australia changed in mineralogy and composition through time. Two older suites, the > 1843 Ma Massena Bay orthogneisses and the 1843 Ma Donington pyroxene granitoids, have curvilinear trends on a modified ACF diagram and relatively unfractionated HREE, consistent with compositional evolution through pyroxene- and plagioclase-dominated crystal fractionation. On the other hand, the 1757 Ma Colbert and 1709 Ma Moody suites have modal hornblende, more strongly fractionated HREE and rectilinear trends on the ACF diagram. They probably require hornblende- and plagioclase-dominated fractionates and/or amphibole (± garnet) as a residual phase during partial melting. Superimposed on this two-fold subdivision of older and younger granitoid suites according to degree of HREE fractionation and ACF trends, there are also progressive compositional changes from the oldest to the youngest granitoids. For example, at comparable SiO2 levels the granitoid suites become progressively richer in K, LREE, Zr, Nb, Ti and P, but Mg, Ca, Sc, V, Ni and Cr progressively decrease. These secular compositional patterns are similar to those described for the Mt Isa region of northern Australia, including the Ewen-Kalkadoon magmatic association to which the Donington suite bears many compositional similarities. Normalised incompatible element plots for the Port Lincoln area granitoid suites have similar shapes to penecontemporaneous mafic dykes. Major and trace element modelling allows derivation of the granitoids by partial melting in the lower crust of mafic granulites compositionally analogous to the mafic dykes. Proterozoic crustal growth in the Port Lincoln area probably took place by vertical accretion through underplating of new mafic material to the lower crust and subsequent remelting of this to yield magmas of granitoid composition.

UPb and RbSr geochronology and geological evolution of the Harts Range ruby mine area of the Arunta Inlier, central Australia

Abstract Supracrustal and meta-igneous rock units from the ruby mine area of the Harts Range, eastern Arunta Inlier, central Australia, have been dated by the zircon UPb and RbSr total-rock (TR) and mica methods. A well-defined zircon discordia for a weakly deformed specimen of the Bruna granite gneiss yields an age of emplacement of 1748−4+5 Ma, thereby constraining the minimum age of the Irindina supracrustal assemblage. Metapelitic gneiss within the supracrustals and a meta-igneous ultramafic boudin from the associated Harts Range meta-igneous complex yield highly discordant zircon data, revealing a strong early Palaeozoic overprint. RbSr TR data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed, also apparently during the lower Palaeozoic. However, RbSr model age calculations and the zircon UPb data suggest a maximum age of about 2000 Ma for the supracrustal and meta-igneous rocks, and argue for new Proterozoic crust formation. Zircon UPb data from a deformed pegmatite, emplaced in the meta-igneous complex, yield an emplacement age of 520−4+5 Ma, further pointing to Lower Palaeozoic magmatism and deformation. Correlations of U content and calculated 206 Pb 238 U age for the ultramafic boudin zircons suggest that new growth of low-U zircons occurred during retrogression associated with this event. The Sr-isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of Palaeozoic Sr. Our data suggest lower Palaeozoic (possibly Delamerian) tectonothermal activity to be more important in the evolution of the Harts Range area than previously recognised. On the other hand, RbSr mica ages for deformed and undeformed pegmatites, and TR isochrons for the latter, show that pervasive tectonothernal activity had ceased by about 315 Ma and that regional cooling occurred between about 345 and 325 Ma. Local shear-zone biotite resetting may have persisted to about 300 Ma, consistent with the previously recognised Alice Springs Orogeny. Possible dilational Pb loss in the Bruna zircons occurred at about 103 Ma.

Crustal development in the Agnew region, Western Australia, as shown by Rb/Sr isotopic and geochemical studies

Abstract Rb/Sr geochronology on a folded greenstone-granitoid complex in the Agnew area, Western Australia, yields four distinct ages of igneous activity that conform with stratigraphic and intrusive relationships. They are (using λ 87 Rb = 1.42 · 10 −11 a −1 , NBS 70A = 522 ppm Rb and 65.3 ppm Sr): Ma 87Sr/86Sr initial (IR) (1) Differentiated gabbro-granophyre from a stratigraphically old (Kathleen Valley) greenstone sequence > 2718 ± 50 0.7007 ± 0.0004 (2) Voluminous tonalite, the Lawlers Tonalite 2652 ± 20 0.70152 ± 0.00012 2576 ± 14 0.70218 ± 0.00021 (3) A less voluminous leucogranite, and a large complex pegmatite cutting the Perseverance nickel orebody 2588 ± 18 0.7624 ± 0.0068 (4) Aplitic leucotonalite (very minor volumes but widespread) 2474 ± 14 0.70193 ± 0.00012 Earlier measurements by Roddick et al. (1976), 70 km to the north, of the Mount Keith Granodiorite (2632 ± 17 Ma, IR = 0.70149 ± 0.00015) and Jones Creek Pegmatites (2481 ± 18 Ma, IR = 0.7253 ± 0.0002) appear to fit groups (2) and (4), respectively. The two principal deformational events in the area are bracketed by groups (2) and (3), and by groups (3) and (4), respectively. Strontium IRs and major and trace-element chemistry suggest that the tonalite-granodiorite and the later leucotonalite were obtained by partial melting of tholeiitic material with garnet as a controlling mineral phase. The leucogranite more likely resulted from remelting of tonalite-granodiorite under similar conditions with garnet again a controlling mineral phase. These four magmatic events constitute a massive addition to the crust from the mantle lasting about 250 Ma. About 90% of the silicic material was emplaced 80 Ma after activity began (tonalite-granodiorite stage) and at least 99% had been emplaced after 140 Ma (leucogranite stage).

The Atnarpa Igneous Complex, southeast Arunta Inlier, central Australia: implications for subduction at an Early-Mid Proterozoic continental margin

Abstract Three distinctive igneous suites have been identified from the Atnarpa Igneous Complex, southeast margin of the Arunta Inlier, central Australia. The 1879+11,−10 Ma gabbro-diorite-tonalite-trondhjemite suite (SiO2=49–78%) shows both calc-alkaline and trondhjemitic affinities. The overall depletion of Nb, Ti, Y, K and Rb in this suite indicates that its parental magma must have been depleted in these elements. It is the oldest zircon age so far obtained in the Arunta Inlier. The 1873+11−10 Ma low-Al tonalite-trondhjemite-granodiorite suite is also depleted in K and Rb. However, unlike either the 1879 Ma or the 1751 Ma suites, it has lower Al2O3 and Sr at comparable SiO2 levels and display flat HREEs and negative Eu anomalies. The 1751±12 Ma younger tonalite suite is high in Sr, Sr/Y and Al2O3, and low in Zr, Ti, Nb, Y, Th and U, and displays a strongly fractionated REE pattern with HREE depletion, typical of Archaean high-Al tonalite-trondhjemite-granodiorite suites. Such geochemical features contrast with the 1850–1880 Ma old K, Rb, REE, Th-enriched, Na, Sr-depleted Barramundi Igneous Association, widespread in Proterozoic terrains of northern Australia. The parental magma of the 1879±11 Ma suite is considered to be derived by subduction-related partial melting of a metasomatised-hybridised mantle wedge with phlogopite retained in the residue, the parental magma of the 1873±11 Ma low-Al suite, by relatively low-pressure partial melting of a mafic source with residual plagioclase left in the residue, and the 1751±12 Ma younger tonalite suite, by partial melting of a subducted oceanic crust at mantle depth with residual garnet±amphibole. These processes contrast with models of restite unmixing of a geochemically uniform, but fractionated, preexisting mantle-derived underplate previously proposed for the K-rich Barramundi Igneous Association. Combining geochemical features of the Atnarpa Igneous Complex with reported data from the Harts Range and the Alice Springs areas, we propose that the southeast margin of the Arunta Inlier represents a convergent continental margin during the Early to Middle Proterozoic which has undergone at least two episodes of subduction-related magmatism (1860–1880 Ma and 1730–1770 Ma). Such continental margin subduction could provide additional driving force for crust-mantle delamination and ensialic A-subduction within the preexisting continental crust further north. Both within-plate vertical underplating and marginal lateral accretion could be important processes for Proterozoic crustal evolution.

Isotopic dating and structural relationships of granitoids and greenstones in the East Pilbara, Western Australia

A structural RbSr isotopic study has been made on two small areas in the eastern Pilbara block, Western Australia. The sites were chosen because they showed good interrelationships of granitoid and greenstone units. At Warrery Gap, on the western side of the Corunna granitic dome, the acid volcanics of the Duffer Formation (at the top of the lower Talga-Talga Subgroup of the Warrawoona Group) show some updating, but a good 3506 ± 62 Ma isochron, with initial ratio (IR) of 0.7006 ± 0.0011, was recognized, consistent with the 3452 ± 16 Ma zircon measurement of Pidgeon (1978b). Both a penetrative D1 deformation, and the doming of D2 preceded intrusion of late tectonic granitoids, dated at 3270 ± 22 Ma (IR = 0.7015 ± 0.0003), into both the Duffer Formation and the overlying pillow lavas of the Salgash Subgroup. Thus, the Salgash Subgroup is much older than suggested by Glikson (1979) and the stratigraphic succession cannot contain a hiatus between the apparently conformable Talga-Talga and Salgash Subgroups of the magnitude he proposed. The granitic domes clearly owe their form to the D2 deformation rather than to batholitic intrusion, but near horizontal structural lineations suggest that they were not formed by diapiric movements. A granodiorite and pink feldspar granite from just within the Corunna granitic dome are slightly deformed: pooled isochrons indicate an age of 3232 ± 27 Ma but different IRs of 0.7032 and 0.7009, respectively. At Tambourah, in the eastern Shaw granitic dome, local D2 also deforms an intrusive microadamellite of age 3087 ± 34 Ma and IR = 0.7103 ± 0.0057. There is therefore a real spread in ages of D2 granitoids and D2 deformation between about 3300 and 3100 Ma. Layered megacrystic gneiss, at Tambourah, also intruded by the microadamellite, contains a nebulous foliation argued to be local S1. Layered and homogeneous megacrystic gneiss produce updated RbSr total rock isochrons of 2995 ± 95 and 2779 ± 38 Ma, respectively. The primary age of these D1 gneisses is clearly greater than that of the D2 granitoids and is probably indicated by Pidgeons (1978c) zircon age of 3417 ± 40 Ma from the Shaw granitic dome. If so, Hickmans (1975) “Migmatite Suite” contains both D1 gneisses and D2 granitoids separated in age at Tambourah by ca. 300 Ma, although neither appears to be older than the lower part of the preserved layered sequence. The protocrust on which that sequence was deposited has yet to be directly identified.

Proterozoic mafic dykes near Port Lincoln, South Australia: Composition, age and origin

Igneous‐textured and recrystallized amphibolite or pyroxene granulite dykes cut Proterozoic granitoid gneisses in the Port Lincoln area of South Australia. The igneous‐textured dykes are largely Ol‐normative, and the amphibolites Qz‐normative, tholeiites. The igneous‐textured dykes comprise norites, gabbronorites and plagioclase‐phyric dolerites which may be readily distinguished by their different Hy/Di ratios. All three igneous‐textured groups are LREE enriched, but they are further distinguished by their different degrees of HREE fractionation, distinct Zr/Ti and Zr/Y ratios and different combinations of normalized negative Ti, Nb, P, Sr and V anomalies. The compositional variation within the three groups may be modelled by crystal fractionation of opx + cpx + plag and cpx + plag dominated assemblages. Rb‐Sr isotopic data from the norites and previously published Rb‐Sr and K‐Ar isotopic data from gabbros northwest of the Port Lincoln area collectively suggest mafic magmatism was widespread about 1600 M...

Zircon ages constrain the timing of deformation events in the the Granites‐Tanami region, northwest Australia∗

Dating of extensive granitoids in the The Granites‐Tanami Block of northern Australia has enabled time limitations on deformational events, including five Palaeoproterozoic events (D1‐D5) and one older, almost Neoarchaean event. D1 is present in the basal Tanami Group, D2 to D5 help define other stratigraphic groups in the region. The intrusive rocks have been dated by traditional zircon and stepwise lead evaporation techniques. A post‐D2/pre‐D3 foliated and boudinaged leucocratic tonalite gave 1870 ± 3 Ma. A syn‐D3 foliated and boudinaged tonalite 1858 ± 3 Ma and a post‐D3/pre‐D4 foliated tonalite 1840 ± 11 Ma. Two unmetamorphosed and post‐D5 granitoids gave 1797 ± 1 Ma and 1787 ± 3 Ma, respectively. Some deformation events are separated by as little as 10 to 20 million years. These times of intrusive igneous activity exactly replicate other igneous events, many of which are volcanic, in the Halls Creek Province 400 km to the northwest, which strengthens proposed geological relationships between the two ...

The geology and geochronology of a Proterozoic trachyandesite plug, Murchison Province, Yilgarn Block, Western Australia

A discrete, roughly circular, steep‐sided magnetic anomaly outlines a buried trachyandesite plug at Gearless Well in the Murchison Province of the Yilgarn Block. Agglomeratic texture indicates high level emplacement and probable venting. The mildly alkaline, silica‐saturated, heterogeneous trachyandesite is markedly alumina‐deficient and enriched in large ion lithophile elements and, to a lesser extent, high field strength elements. A seven‐point whole rock linear alignment on the Rb/Sr isochron diagram of samples from one drill hole is interpreted as a two‐component mixing line of meaningless age. Samples from another drill hole indicate the presence of a third component. Two two‐point biotite‐whole‐rock measurements yield similar ages which pool to give 2188 ± 11 Ma, the estimated time of emplacement of the trachyandesite plug. Samples defining the seven‐point line were produced by small to moderate amounts of partial melting of LIL‐enriched (large ion lithophile elements) upper mantle, and the line is ...