Australia contains a series of geographically diverse and economically important Paleo- to Mesoproterozoic crustal blocks. Understanding the crustal evolution and possible correlation of these blocks is important for development of mineral exploration strategies. The Peake and Denison Ranges in South Australia comprise the largest outcrop of Paleo- to Mesoproterozoic rocks in the northern Gawler Craton. They are of potential economic significance as they lie within the same structural corridor as the giant Olympic Dam U-Cu-Au-Ag deposit, and contain volcanic and intrusive rocks of similar age to those found in the metallogenic Mt Isa block. Based on detailed mapping, this thesis documents a study of crustal evolution from a geochemical and isotopic perspective. Important considerations were to determine the timing and character of magmatic and metamorphic events, and to develop the correct use of the geochemical and isotope information to determine magma sources and possible tectonic environments.
The first geological event in the Peake and Denison Ranges is recorded indirectly in the trace element and isotopic composition of a suite of ~1800 Ma year old subalkaline metaigneous rocks in Kingston and Peake and Coppertop Blocks. Modelling of their Sm-Nd systematics demonstrates that their arc signature is unlikely to have resulted from contamination by continental crust or by subduction-derived sediment, but was instead inherited from a mantle source that was metasomatised by subduction-derived fluids at ~2500 Ma. In Kingston Block, subalkaline magmatism was accompanied by the intrusion of alkaline magmas with plume-type chemical signatures. Supracrustal sequences deposited at this time consist of mature sandstone, mudstone and minor carbonate. A second magmatic cycle is recorded in ~1746 Ma felsic volcanism in Davenport Block. Concomitant sedimentation included deposition of sandstone, dolomitic sandstone, calcareous mudstones, minor ironstones, and possible tuffs. Metarhyolites and granitoids associated with this event are weakly to strongly altered but their chemical characteristics suggest that they may be plume-related magmas. Possibly syntectonic intrusion of an unusual clinopyroxene-bearing metatrondhjemite/tonalite body at Lagoon Hills occurred at 1733 ± 13 Ma.
In Kingston and Algebuckina Blocks, metamorphism is evident in the occurrence of migmatised pelites and the presence of relict cordierite and fibrolitic sillimanite in quartzite and metapelite. Geothermobarometry on a sillimanite-cordierite-biotite-quartz gametite indicates that peak metamorphic conditions reached 0.5 GPa and 650°C. The timing of this metamorphic event (M1) was determined by the weighted mean of two separate Pb-Pb garnet isochron dates from Algebuckina and Kingston Blocks ~1718 ± 31 Ma. A second metamorphic event (M2) occurred between 1530-1450 Ma and is recorded in metamorphic rims on zircon (1530 Ma), 207Pb/206Pb ages of titanite and apatite (1507-1446 Ma), and Rb-Sr whole rock-mineral ages (1477 ± 8 Ma and 1451 ± 6 Ma). Mineral assemblages produced by M2 are best developed in Davenport Block (homblende+biotite mantling clinopyroxene, oblique muscovite) and suggest that lower amphibolite facies conditions were reached. The timing of metasomatism, which formed quartz-albite hosted breccias and Na-Ca-Fe alteration in Davenport Block, is unknown, but constrained by field and petrographic evidence to post M1 and pre M2.
This thesis highlights several problems in using trace elements geochemistry to determine the tectonic environment of ancient igneous rocks. Because the arc-type signature of the ~ 1800 Ma subalkaline magmatism was inherited from a ~ 2500 Ma metasomatised mantle source, it cannot be inferred that eruption occurred in a volcanic arc setting. Using tectonic discrimination diagrams is also problematic because they are based on analyses of present day volcanic rocks, but the concentration of elements in the mantle has changed over time as a result of crustal growth. Th, Hf and Ta are the basis of a commonly used tectonic discrimmation diagram (Wood, 1980). Mass balance calculations show that Th/Ta, Th/Hf and Ta/Hf in the depleted mantle have changed through time (84%, 94% and 64% respectively since 2.5 Ga) as a result of extraction of continental crust.
Nevertheless, combined field, petrographic, trace element and isotopic data provide several constraints on the likely tectonic setting of Paleoproterozoic rocks in the Peake and Denison Ranges. Because high grade metamorphism accompanied by intense compressional deformation occurred shortly after volcanism and sedimentation (1718 Ma ± 31), it is unlikely that the tectonic setting was intracratonic. In order for the metasomatised mantle source of subalkaline magmatism to have been preserved for ~700 Ma (prior to eruption), magmatism and sedimentation must have occurred along the margin of a stable craton rather than in a juvenile ocean-arc setting. Metasediments in Kingston and Peake and Coppertop Blocks (mature metasandstone, minor pelite and bimodal volcanic rocks with intercalated carbonate) and in Davenport Block (mature sandstone, calcareous sandstone, dolomitic mudstone, minor ironstone, and metarhyolite) are untypical of active continental margins, but could well have been deposited along the cratonic margin of a back-arc basin or along a reactivated formerly passive continental margin.