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    Gabbro magmatism in the Lachlan Orogen, southeastern Australia: implications for mafic–felsic associations and granitoid petrogenesis
    Whelan, Joanne Amy ( 2016)
    The nature and role of mafic endmembers in granitoid petrogenesis is poorly constrained in the Lachlan Orogen of southeastern Australia. Most previous studies have focussed on intermediate–felsic magmas and attempted to model the nature of their primitive precursors. Ordovician–Devonian magmatic rocks of gabbroic composition are exposed as rare volumetrically minor intrusions throughout the Lachlan Orogen, and are temporally and spatially associated with more voluminous granitoids. A detailed study of tholeiitic and alkali gabbros exposed in the Kuark Zone of eastern Victoria provides new insights into gabbro petrogenesis and the source regions of such magmas which in turn have implications for the generation of granitoids, particularly I- and A-type magmas, throughout the Lachlan Orogen. The Arte Igneous Complex and Scrubby Flat Gabbro are poorly exposed mafic intrusions spatially associated with A-, I- and S-type magmas. Although major- and trace-element variations do not always show clear evidence for a geochemical link between these units, variations in Sr, Nd and Hf isotope compositions indicate a shared source for at least some of the magmas. Alkali gabbros in particular preserve considerable Sr-Nd isotopic heterogeneity ranging greater than 10 epsilon Nd units within a small geographic area interpreted to represent the root of the Arte intrusion. It is proposed herein that magmatic differentiation occurred via fractional crystallisation and cumulate processes; however, it is argued that much of the Sr-Nd isotopic variation was inherited from a heterogeneous source region. A model involving a small degree (<20%) of partial melting of greenstone basement can explain the variation within the alkali gabbro of the Arte Igneous Complex. Subsequent higher degrees of partial melting (>30%) can explain the more voluminous, less heterogeneous tholeiitic gabbros. Spatially associated A- I- and S-type granitoids and the gabbros is more cryptic, some geochemical and Sr-Nd isotope links are apparent. Importantly, the I-type intrusions are interpreted to have been derived magmas fromed by partial melting of more intermediate compositions within greenstone basement (c.f. ultramafic to mafic for the gabbros), thus they share a similar heterogeneous source region with the gabbro rocks. In contrast, the S-types intrusions have a more complex link to the gabbros and are interpreted to have been derived via partial melting and assimilation of Ordovician turbidites by tonalite magmas of the Arte Igneous Complex. Comparison of the magmatic rocks of the Kuark Zone with other twelve of the 20 known Lachlan Orogen gabbros reveals similar isotopic heterogeneity. This requires that the source heterogeneity is present on a local- and regional-scale. Cambrian greenstone basement exposed in rare fault-bounded belts throughout the Lachlan Orogen have the same isotopic heterogeneity. Moreover, the same heterogeneity is observed in I-type granitoids of the Lachlan Orogen. The implication is that I-type magmas may also be generated by partial melting of greenstone basement rocks, thus both gabbroic and I-type magmas image their source region. There is a correlation between the age of gabbros and Sr-Nd isotope values, with younger gabbros characterised by on average more isotopically juvenile compositions. The ca. 380 Ma Bingie Bingie Suite that approachs compositions of depleted mantle. A number of the gabbros exhibit arc-like trace-element characteristics, however, given that the greenstones were generated in a Cambrian arc environment, these signatures may be inherited from their source (greenstones) rather than the gabbro magmas themselves being generated in a subduction zone setting. The chemical characteristics of most of the gabbros are consistent with the partial melting of greenstone basement in a back arc basin setting under extension. Influx of new mantle-derived magma is only likely to have occurred to produce the youngest mafic rocks (e.g., Mount Buller Igneous Complex). The results of this study provide new insights into the source regions for mafic intrusions of gabbro/diorite composition in the Lachlan Orogen. In light of this new information, these insights present an opportunity to re-examine the petrogenetic models for I- and A-type granitoids in particular.