Petrological, geochemical and Sr-Nd isotopic studies of metamorphosed mafic rocks in the Wonominta block, northwestern New South Wales

Abstract

The Wonominta Block is located in far north-western New South Wales and consists of composite stratigraphic units. This project is focused on investigation of selected mafic rocks in the lower three sequences, namely the basement, Late Proterozoic and early Cambrian, using petrological, geochemical and isotopic methods, in order to achieve a better understanding of the tectonic evolution of the region. Such an understanding of the region is relevant to understanding continental development at the broader scale but also provides a more sound basis for mineral exploration. Field observation, petrographic and mineralogical investigations of the mafic rocks reveal a complicated metamorphic and deformation history. In general, all samples appear to have experienced burial metamorphism (sensu lato) to various degrees before deformation, and the metamorphism had already reached peak conditions (ca. 500 °C) when deformation occurred. The P-T calculations reveal a hi gh geothermal gradient for the burial metamorphism and a low one for regional metamorphic processes. This resulted in anti-clockwise P-T-t paths up to upper greenschist -lower amphibolite facies for the basement sequence, lower greenschist for Late Proterozoic sequence and pumpellyite-actinolite for early Cambrian sequences. Geochemical studies reveal MORB-like compositions for the basement mafic rocks with significant Th-enrichment and some Nb-depletion. Detailed geochemical stratigraphy also shows systematic changes within the basement sequence: from the Wilandra Inlier in the south to the northern Ponto Mine Inlier, an increasing involvement of crustal materials in magma generation has been inferred. It is also inferred that magma mixing processes had not been important in magmatic evolution due to the facts that several petrographically distinct rocks in various sequences cannot be distinguished geochemically, and that large variations in high-field-strength (HFS) element compositions generally show coherent correlations. The younger rocks (both Late Proterozoic and early Cambrian) have sodic alkaline basalt affinities with no crustal contamination observed. To provide a chronological constraint on the Wonominta Beds, an ion microprobe zircon U-Pb study yielded a preliminary age of 525+8 Ma (20) for the youngest part (the Mt. Wright Volcanics), but the Nd/Sm isotope analyses on basement samples produced only a poorly defined isochron of 1.40 + 0.24 Ga (mean standard weighted deviation (MSWD) = 0.9). The Sr-Nd isotopic investigations indicate low initial 87Sr/86Sr ratios and positive ENd(T) values for all the analysed mafic rocks from the three sequences. For the basement samples, the initial 87Sr/86Sr ratios are various due to alteration but may range from 0.7020 to 0.7041; the enq(T) is from 4.8 to 8.9. These results suggest that there were possibly two depleted mantle sources for the formation of the basement mafic rocks. One of the extremely depleted end-members may have been a “fossil mantle wedge” in the source region. The assimilation-fractional crystallisation (AFC) model calculation using Nd isotope compositions indicates that the contribution of crustal materials to the observed most contaminated sample could be up to 7%. In contrast, the alkaline basalts in the younger sequences, have clear primitive isotope compositions with €yq(T) about 4.7 + 0.4 and an initial 87Sr/86Sr ratio about 0.703. From these petrological, geochemical and isotopic studies, a new tectonic model for the evolution of the Wonominta Block is proposed. In this model, the Ponto Mine Inlier and Wilandra Inlier are proposed to comprise the basement for Late Proterozoic development, and they themselves may have developed in a back-arc basin environment during Early - Middle Proterozoic. Above the basement, the emplacement of alkaline basalts in the Late Proterozoic and Early Cambrian sequences represents the beginning of a rifting event along the cratonic margin, which led to the formation of a deep-water basin where the Early Cambrian Teltawongee beds were deposited. Regionally, the Kanmantoo Trough in South Australia would have been developed during the same tectonic event Such a model requires a reevaluation of the tectonic boundary between the Phanerozoic Tasman Orogen and its Precambrian basement in eastern Australia. It is thus suggested that the Delamerian Foldbelt (or Delamerian - Ross Orogen to be comparable with the Tasman Orogen) represents a transitional tectonics between the Phanerozoic tectonic development and the Precambrian cratons. This transitional tectonics comprises the Adelaide Geosyncline and the Kanmantoo regime, and involved two rifting events that may have been parts of the global rifting which led to the break-up of the Proterozoic supercontinent.