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ItemGabbro magmatism in the Lachlan Orogen, southeastern Australia: implications for mafic–felsic associations and granitoid petrogenesisWhelan, 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.
ItemThe platinum-group element geochemistry and petrogenesis of the Heazlewood River mafic-ultramafic complex, TasmaniaPeck, David C. ( 1990)The Heazlewood River mafic-ultramafic complex (HRC) comprises well-layered olivine- and orthopyroxene-rich cumulates, gabbronorite dykes, tonalites and low-Ti tholeiitic basalt and boninite lavas. The complex was emplaced as part of a large, low-angle thrust sheet during the middle Cambrian and subsequently deformed during the Devonian, so that the original stratigraphical relationships are obscured. The cumulate succession incorporates two distinct blocks, viz. the western HRC, comprising primitive adcumulates, and the eastern HRC, consisting of more evolved orthocumulates and mesocumulates. These two cumulate blocks are interpreted to represent stratigraphically equivalent parts of a single magma chamber. In this scenario, the western HRC represents an axial part of the intrusion where high heat flows, due to repeated injections of primitive magma, promoted the development of a compositionally zoned magma chamber. In contrast, the eastern HRC is believed to constitute a marginal facies of the intrusion, where sidewall cooling caused rapid crystallisation of successive magma additions and inhibited adcumulate growth and the formation of a compositionally stratified liquid column. Results from a detailed study of the mineral compositions and whole-rock geochemistry of the HRC suggest that all of the cumulates and most of the dykes and tonalites were derived from boninitic parental magmas. This hypothesis is substantiated by empirical models which were calculated using both major and trace element approaches. The models also show that the low-Ti basalts (second-stage melts) and boninites (third-stage melts) were probably derived from component-induced progressive partial melting of a MORB-depleted spinel lherzolite source. Partial melting of the refractory mantle source was initiated and sustained by the continued influx of slab-derived Si02-, LREE-, Zr-enriched hydrous fluids. The proposed petrogenetic model for the HRC is most consistent with an island arc setting for the complex, with melting occurring in MORB-depleted forearc lithosphere overlying a subduction zone. The HRC is not an ophiolite sensu stricto, despite the fact that it is more similar to the upper portions of the so-called 'island-arc ophiolites' (eg. Troodos) than to any other type of ultramafic intrusion. It is best perceived as a high-level boninitic magma chamber which developed immediately beneath a platform of genetically-related submarine lavas. The composition of the boninitic parental magmas was the principal control on the PGE geochemistry of the cumulate sequences. Despite representing PGE-enriched, S-undersaturated second-stage melts similar to the parental (U-type) magmas for the ultramafic portions of the Bushveld complex, the boninites were unable to form a Merensky-reef type PGE deposit because they did not come into contact with S-saturated (A-type) magmas. In the absence of cumulus sulphides, the PPGE (Pt, Pd, Rh) were partitioned into the residual liquids, whereas the IPGE (Os, Ir, Ru) were strongly fractionated into early-formed olivine-chromite cumulates. These features are highlighted by the extremely low IPGE tenor of the boninites, and the relatively high IPGE tenor of the dunites in comparison to the more evolved cumulates. Three types of chromitites are recognised in the HRC. Type I and type II chromitites occur as magmatic schlieren which probably formed during replenishment events. Type III chromitites occur as layers, pods and irregular patches developed in an unusual xenolith-bearing plagioclase peridotite. It is interpreted to have formed due to mixing between ascending xenolith-bearing, hydrous intercumulus liquids and resident ultramafic magma along the floor of the magma chamber. Chromitite occurrences in the HRC are enriched in PGE by up to two orders of magnitude relative to their ultramafic host rocks, and most strongly-enriched in Ru and/or Pt and Rh. Their PGE tenor reflects the early crystallisation of laurite, followed by Pt and Rh sulpharsenides, in response to increasing S and As activities which developed primarily due to magma mixing. The low Os and Ir abundances in the chromitites is believed to reflect their formation from Os- and Ir-depleted boninitic magmas. The HRC and the Adamsfield complex were the world's major suppliers of Os-Ir-Ru alloys during the early part of this century. The alloys occur in alluvial deposits that are spatially associated with primitive olivine-rich cumulate sequences. The latter are commonly suspected to represent the source for the alloys, but recent exploration programs have yet to define a bedrock occurrence of Os-Ir-Ru alloys in Tasmania. The results from the present study provide important constraints on the genesis of these alloys. Silicate inclusions found in the alloys suggest that they formed at mantle temperatures and pressures and were transported to crustal magma chambers by boninitic magmas. The alloys may have crystallised during ascent, or alternatively, represent residual mantle phases which became incorporated into the boninites during partial melting. Most of the observations pertaining to the Os and Ir geochemistry of the HRC suggest that the alloys probably occur in thin magmatic concentrations that were deposited along the base of the intrusion from the most primitive of the boninitic magmas involved in the generation of the cumulate sequences. Future exploration should focus on delineating the cumulate products of these primitive magmas and specifically, in defining the horizons which demarcate fresh influxes of these liquids.
ItemThe geology and geochemistry of the Agnew Intrusion: implications for the petrogenesis of early Huronian mafic igneous rocks in Central Ontario, CanadaVogel, Derek Christian ( 1996-07)The Early Proterozoic Agnew Intrusion is a well-preserved leucogabbronoritic to gabbronoritic layered intrusion that is a member of the East Bull Lake suite of layered intrusions (ca. 2490-2470 Ma) occurring in central Ontario. These intrusions are related to the development of the Huronian Rift Zone, which may be part of a much more widespread rifting event that involved the Fennoscandian Shield. Structural data suggest that these intrusions have been subjected to ductile deformation and are erosional remnants of one or more sill-like bodies originally emplaced along the contact between Archaean granitic rocks of the Superior Province and an Early Proterozoic Huronian continental flood basalt sequence in the Southern Province.