Lachlan Fold Belt Research Articles

Chemical and isotopic investigation of the I-type Bega Batholith, southeastern Australia: Implications for batholith compositional zoning and crustal evolution in accretionary orogens

Cordilleran granitic batholiths represent significant episodes of crustal growth and differentiation, and commonly display lateral isotopic and chemical variations. Establishing the tectono-magmatic processes responsible for generating this compositional asymmetry is important for understanding crustal evolutionary processes throughout the Phanerozoic. The Bega Batholith, an example of a ‘Cordilleran style’ granite batholith, is the largest I-type Siluro-Devonian granite complex in the Lachlan Fold Belt (LFB) of southeastern Australia and comprises seven granite supersuites that display systematic lateral isotopic and chemical asymmetry. From west to east towards the present-day continental margin, an increase in the content of Na2O, Sr, Al2O3, and P2O5, with concomitant decreases in CaO, Sc, Rb, and V are observed. In the same direction, whole-rock initial 87Sr/86Sr decreases from 0.7098 to 0.7039, εNd values increase from −8.3 to +4.4, and δ18O decreases from 10.2 ‰ to 7.9 ‰. Depleted-mantle model ages also decrease from ca. 1800 Ma in the west to 600 Ma in the east. Here, we address whether these chemical and isotopic variations were generated by interaction between two distinct components (mantle-derived magmas and supracrustal sources) or were alternatively produced by partial melting of infracrustal source rocks formed sequentially by much earlier episodes of crustal underplating. Combined whole-rock Nd-Sr-O isotopic and geochemical analyses indicate that several I-type supersuites exhibit chemical and isotopic correlations consistent with two-component magma mixing. This new evidence challenges the long-held view that I-type granites derive exclusively from the melting of infracrustal sources, and that granite terranes represent wholesale crustal reworking rather than new crustal growth. Our results show that the compositional zoning within the Bega Batholith is multifaceted. Firstly, the presence of two discrete mantle sources endows chemically and isotopically distinct eastern and western segments in the batholith. Secondly, within these compositionally distinct regions the lateral compositional changes across supersuites derives from mixing between mantle-derived and supracrustal sources. Finally, progressive extension within a developing back-arc environment regulates the ratio of crust-mantle contributions and compositional architecture of each I-type supersuite.

Fault Structure Reconstruction From Magnetic Anomalies Using an Improved Backtracking Search Optimization Algorithm

AbstractFault structure reconstruction is an important aspect of magnetic prospecting as it contributes to understanding tectonic history, hazard assessment, and infrastructure planning. Metaheuristics have proven useful in estimating fault structure parameters from geophysical anomalies. Recently, a modified version of the Backtracking Search Optimization Algorithm (BSA), named mBSA, has been proposed that combines mutation steps of BSA and Differential Evolution (DE) algorithms to achieve a better balance between exploration and exploitation. Here, we present imBSA as an efficient approach that builds on the improvements of mBSA. It uses an adaptive amplitude control factor in the mutation step derived from a normally distributed random number obtained via an exponential function. The performances of these approaches were tested on some widely used benchmark functions such as Rosenbrock, Rastrigin, Griewank, and Himmelblau. Synthetic magnetic fault anomalies were evaluated with and without noise, as well as three real anomalies from the Dehri (India), Perth Basin, and Lachlan Fold Belt (Australia). Prior to the optimizations, the solvability of the model parameters was examined using some error topography images. The obtained solutions in the synthetic cases were evaluated using principal component analysis. In addition, a novel visualization option for high‐dimensional optimization problems was introduced. The experiments showed that imBSA has a faster convergence rate and better optimization statistics compared to BSA and mBSA. Therefore, it is believed to be an efficient and good alternative tool to widely used metaheuristics such as Particle Swarm Optimization and DE in explaining geophysical anomalies and optimization problems in other geoscientific disciplines.

Geochemical characteristics and structural setting of lithium–caesium–tantalum pegmatites of the Dorchap Dyke Swarm, northeast Victoria, Australia

The Dorchap Dyke Swarm hosts the first recorded occurrence of lithium–caesium–tantalum (LCT) pegmatites in Victoria, Australia. Syn-orogenic emplacement of pegmatite dykes occurred along a northwest-trending shear system during the Benambran Orogeny. Pegmatites are derived from fractionated melt associated with the Mount Wills Granite, which is an S-type, peraluminous granite originating from supracrustal melting of Ordovician sedimentary sequences. A distinct, eastward-oriented fractionation trend across the Dorchap Dyke Swarm has highlighted a 20 × 8 km highly fractionated zone in the northeastern Dorchap Range, which includes spodumene- and petalite-bearing pegmatites. A distinct pattern of elemental enrichment (P > Cs > Be > Nb ≥ Ta > Li > Sn) is observed across the strongly fractionated zone of the Dorchap Dyke Swarm. Subsequent metasomatic fluid movements and hydrothermal overprinting have resulted in redistribution of mobile elements in the Dorchap Range, either as hydrothermal alteration species or in some instances as the development of exomorphic halos. Additionally, a regional alteration overprint, likely associated with subsequent metamorphism of pegmatite dykes has resulted in alteration of primary petalite to spodumene plus quartz, and primary spodumene to cookeite. Bulk rock geochemical data from the Dorchap Dyke Swarm suggest a syn-collisional setting for dyke intrusion, consistent with the inferred tectonic setting of the central Lachlan Fold Belt at the time of pegmatite emplacement. Pegmatite dykes locally contain an overprinted structural foliation, which is consistent with the primary structural trend of deformed metasediments and may indicate that dyke emplacement was syngenetic with regional folding and compression of the surrounding Omeo Metamorphic Complex during the Benambran Orogeny. Subsequent hydrothermal alteration of some dykes likely occurred immediately following the Bindian Orogeny. KEY POINTS Dorchap Dyke Swarm are the first recorded Li–Cs–Ta pegmatites in Victoria, Australia. Pegmatites were emplaced synchronous with or immediately following the Benambran Orogeny. Dorchap Dyke Swarm pegmatites are geochemically correlated with the Mount Wills Granite.

Granite petrogenesis and the δ44Ca of continental crust

Stable Ca isotopes are an increasingly useful tool for understanding the sources and processes leading to the formation of magmatic rocks, yet Ca isotope fractionation during genesis of silicic continental crust is still poorly understood. Here, we present Ca, Sr, and Nd isotope, as well as major- and trace-element whole-rock geochemical data for A-, I-, and S-type granites (n = 30) from Australia/Tasmania, Canada, and France (δ44CaBSE of -0.6‰ to +0.2‰) and compare them to phase-equilibrium models for partial-melting (pelite, greywacke, MORB, enriched Archean tholeiite) and crystallization (hydrous arc basalt, A-type granite) that incorporate novel ab-initio predictions for Ca isotope fractionation in epidote and K-feldspar. The ab-initio calculations predict that epidote has similar δ44Ca to anorthite and that K-feldspar is the isotopically lightest known silicate mineral at equilibrium (Δ44Cakspar-melt of -0.4‰ at 1000 K). Our phase-equilibrium model results suggest that δ44Ca variations in all three granite types can be fully explained through magmatic processes, without necessarily requiring addition of isotopically exotic components (e.g., carbonate sediments). Heavy Ca isotope enrichments in A-type granites from the Lachlan Fold Belt, however, require isotopic disequilibrium between plagioclase and melt, which we use to constrain average plagioclase growth rates in these systems. This also serves to illustrate that whole-rock Ca isotope measurements can be used to estimate crystal growth rates, even in the absence of analyzable phenocrysts. In general, low Ca diffusivities and strong isotopic diffusivity ratios (D44/D40) in low-H2O granitic magmas should lead to resolvable isotopic disequilibrium effects in plagioclase, even at relatively slow growth rates (e.g., > 0.03 cm/yr). Combining our data with those from previous studies, we demonstrate that average granitoids and upper continental crust (with newly estimated δ44CaBSE of -0.25 ± 0.02‰, 2SE) have resolvable low δ44Ca compared to basalts and oceanic crust. Given that pressure has a major influence on Ca isotope fractionation across all of our models, this implies that melts feeding upper crustal granitoids dominantly evolve in the lower crust (10-14 kbar, through either partial-melting or fractional crystallization). This observation also suggests that heavier Ca isotopes are preferentially recycled back into the mantle through subduction and/or lower-crustal delamination events, but this is unlikely to have had a significant influence on the δ44Ca evolution of the upper mantle through geologic time.

Multi-scale isotopic heterogeneity reveals a complex magmatic evolution: An example from the wallundry suite granitoids of the lachlan fold belt, Australia

Open-system magmatic processes are expected to impart various sorts of isotopic heterogeneity upon the igneous rocks they produce. The range of processes under the “open-system” umbrella (e.g., simple two-component mixing, magma mingling, assimilation with fractional crystallization) cannot usually be uniquely identified using data from a single isotope system. The use of bulk-rock, mineral separate and in situ techniques and multiple isotope systems allows the characterization of isotopic variability at different sampling scales, illuminating details of the petrogenesis of a magmatic system. This approach has been applied to granitoids of the Wallundry Suite in the Lachlan Fold Belt, Australia. The Wallundry Suite exhibits variations in mineral assemblage, mineral composition and trends in bulk-rock major and trace element compositions consistent with the involvement of liquid-crystal sorting processes such as fractional crystallization. In situ paired O-Hf isotope data from zircon in six samples show an array indicating the isotopic evolution of the melt phase. Similarly, bulk-rock Sr-Nd-Hf isotope arrays support open-system magma evolution. These data combined with the petrographic observations and major and trace element geochemical variations suggest some form of assimilation-fractional crystallization process in the petrogenesis of the Wallundry Suite. Added complexity is revealed by two observations: 1) the isotopic variations are only weakly coupled to the lithology and major element compositions of the samples; and 2) there are distinguishable differences between the Hf isotope compositions of bulk-rock samples and those of the magmatic zircons they host. To varying degrees the rocks consistently show negative ΔεHfbulk-zrc values (i.e., the bulk-rock compositions have less radiogenic Hf isotope values than their coexisting zircons). The preservation of distinctly low Nd and Hf isotope ratios in an Fe-Ti oxide mineral separate suggests that the bulk-rock vs. zircon discrepancy is caused by the presence of unmelted components derived from a contaminant of continental origin (i.e., a rock with low Sm/Nd and Lu/Hf and thus unradiogenic Nd and Hf). Evidently, a complex interplay of assimilation, crystallization and melt segregation is required to account for the data. This investigation demonstrates that such complexity can, nevertheless, be disentangled through comparison of complementary isotope data at multiple sampling scales.

Dating initial crystallisation of some Devonian plutons in central Victoria and geological implications

Rather than being inherited as detrital grains, many of the rounded and/or embayed cores in zircon crystals from granitic rocks are probably antecrysts, formed early in the magmatic systems. Using laser-ablation, sector-field ICP-MS, we obtained internally consistent weighted-mean concordia ages of 375.3 ± 2.5 Ma and 376.9 ± 2.6 for early crystallisation of two samples from the previously undated Mount Disappointment pluton in the Melbourne Zone. Within uncertainty, our date of 376.4 ± 2.4 Ma for the Baringhup pluton of the Harcourt batholith is the same as the published and our new date for the Mount Alexander pluton, in the same batholith. Using the same techniques, we produced an improved date of 370.9 ± 6.5 Ma for the I-type Ercildoun Granite in the neighbouring Bendigo Zone of the Lachlan Fold Belt. We also obtained dates that confirm published ages for the Mount Bute Granite in the Stawell Zone, the Tynong pluton of the Tynong batholith in the Melbourne Zone and the Oberon pluton of the Wilsons Promontory batholith in the Bassian Zone. These dates confirm the Late Devonian (Fransian) age of most of this widespread plutonism, as well as the reported Emsian age of the Wilsons Promontory batholith. However, part of what has been mapped as the Mount Wombat pluton of the Strathbogie batholith could be significantly older than the rest. KEY POINTS Many rounded or embayed cores in zircon crystals are antecrysts rather than inherited detrital grains. The Mount Disappointment pluton is confirmed as Late Devonian in age, at 375.2 ± 2.5 Ma. At 378.2 ± 2.0 Ma, the Baringhup pluton of the Harcourt batholith is the same age as the Mount Alexander pluton, in the same batholith. Part of the Mount Wombat pluton in the Strathbogie batholith may be older than the rest of the pluton.

U-Pb geochronology of the Silurian-Devonian Bega Batholith, south-eastern Australia: Insights into the origin and development of I-type granites

Earth’s continental crust is primarily composed of buoyant, felsic igneous (granitic) material that has been extracted from the mantle along convergent plate margins. Establishing the timescales of subduction-related granite batholith assembly is key to understanding the processes of magma generation, segregation, and transport that define the dynamics and rates of crustal growth. The ‘Cordilleran’ Bega Batholith is the largest I-type granite batholith in the Lachlan Fold Belt (LFB) of south-eastern Australia. The product of accretionary orogenesis along the Terra Australis Orogen, it has been extensively characterised by isotopic and geochemical studies yet lacks a robust geochronological framework. We present new ID-TIMS and SHRIMP U-Pb zircon ages for twenty-two plutonic rocks of the Bega Batholith. This geochronological survey indicates a prolonged duration for I-type magmatism, with crystallisation ages ranging from ca. 420 Ma in the west, to 385 Ma in the eastern Bega Batholith. These I-type granites collectively occupy ∼90,000 km3 and were emplaced over a period of ∼20–35 Ma, corresponding to a magma production rate of ∼8–15 km3/Ma km−1, considerably lower than that of S-type granites of the region and other subduction-related granite terranes globally. Fundamentally, the temporal overlap between the Bega I-type granites and S- and A-type granites also argues against a hiatus between each phase of granite magmatism in the LFB, suggesting a continuum between the granite types and derivation from common source components. This new geochronological data provides the first comprehensive compilation of U-Pb ages from across the Bega Batholith, establishing initial constraints on the development and evolution of I-type magmatism in south-eastern Australia. Our results demonstrate that I-type granite petrogenesis is linked with crustal growth processes, requiring the modification of global crustal growth curves during the Phanerozoic to account for new continental crust generated within I-type granite terranes of the Paleo-Pacific Gondwanan margin.

Potential field modelling and U–Pb geochronology reveal the pluton emplacement dynamics of the Lower Devonian Tarnagulla Granodiorite, southeast Australia

The Tarnagulla Granodiorite in the western Lachlan Fold Belt is poorly exposed; however, magnetic data reveal its distinct U shape (in plan view) and patterns characterised by multiple nested cusp-shaped anomalies of contrasting magnetic intensity. We interpret this magnetic response to reflect incremental assembly of the pluton above a persistent magma point source that migrated laterally. Here, we use potential field data, geophysical modelling and high-precision zircon U–Pb dating to constrain the shape of the intrusion at depth and deduce the migration path of the magmatism epicentre. Potential field profile modelling across three traverses, constrained by petrophysics, resolves the subsurface geometry of the Tarnagulla Granodiorite. In cross-section, the intrusion is funnel-shaped, with a lower boundary that varies from ∼2 km depth in the north to ∼6 km depth in the southwest, reaching a maximum depth of ∼8 km, which may coincide with a magma feeder zone. New precise U–Pb geochronology provides ages that confirm the interpreted magnetic phases were emplaced sequentially along a clockwise intrusion path that progressed stepwise from the north to the southwest between 406.62 ± 0.27 Ma and 399.3 ± 1.1 Ma. We attribute migration of the epicentre of magmatism within the Tarnagulla Granodiorite to syn-intrusion (Early Devonian) reactivations of the underlying Tarnagulla Fault, with relative displacements between the fault hanging wall (which hosts the pluton) and footwall (which we interpret to have hosted a persistent point-source magma feeder zone during pluton emplacement). Our model explains the intrusion’s U-shaped magnetic response and reveals critical information about the paleo-stress and paleo-strain conditions during emplacement, characterised by three main displacement vectors and fault slip magnitudes. The geophysical characteristics, path of emplacement and mineralisation distribution all suggest that progressive fractional crystallisation contributed to the formation of an enriched final pulse of magma that resulted in Mo, Cu and W mineralisation hosted in the Moliagul Granodiorite. KEY POINTS High-precision U–Pb zircon geochronology reveal that the Tarnagulla Granodiorite was emplaced along a clockwise pathway from the northeast to the southwest between 406.62 ± 0.27 Ma and 399.3 ± 1.1 Ma. Active faulting during pluton emplacement is the suggested explanation for the Tarnagulla Granodiorites’ U-shaped magnetic response. Fractional crystallisation throughout pluton emplacement produced the varying internal geophysical response of the pluton as well as the enrichment of Mo and W at Mt Moliagul.

Holocene heavy mineral assemblages in a coastal compartment in eastern Australia

Heavy minerals along the east coast of Australia form part of a northward transport pathway that moves sediment from southern New South Wales to Queensland, Australia. The proportion and occurrence of heavy minerals from non-local source rocks reflect reworking of quartz sand from the outer-shelf to the inner-shelf and coastal environments during the post-glacial marine transgression. The northward transport system is disrupted by a prominent headland to preserve a locally derived pyroxene-rich heavy mineral suite in the Kiama coastal compartment. The heavy mineral assemblage indicates derivation from multiple Neoproterozoic and Palaeozoic sources in southern Australia and possibly Antarctica, including the Lachlan Fold Belt and Sydney Basin in southeastern Australia. The common pyroxene was derived locally from coastal Permian latite units in the Sydney Basin, Mesozoic tinguaites and the Eocene Robertson Basalt.Five identified mineralogical facies are controlled by spatial variability of heavy mineral assemblages rather than the textural characteristics of the sediment. The upper river facies is dominated by catchment-derived augite. The Minnamurra spit and back-barrier facies has the highest heavy mineral content with abundant ultrastable and metastable minerals consistent with their accumulation as prograding coastal barriers. The two high-energy sand-dominated inner-shelf facies contain a mix of ultrastable and metastable heavy minerals with pyroxene being more common in the inner zone while zircon, rutile, garnet, epidote, andalusite and monazite/apatite are more common in the outer zone where they are affected by the regional northward sand migration. The low-energy mid-shelf facies (depth >55 m) is finer grained and contains the highest proportion of micaceous minerals and hornblende. The adjacent probable tsunami sandsheets at Dunmore have very similar heavy mineral signatures to the coastal barrier and inner shelf deposits and were probably derived from them.

Modelling the Formation of Linear Geochemical Trends Using the Magma Chamber Simulator: A Case Study of the Jindabyne Granitoids, Lachlan Fold Belt, Australia

Abstract Understanding the origins of major and trace element variations and the isotopic character of granite samples in terms of sources and magmatic processes is, arguably, the core of granite petrology. It is central to attempts to place these rocks in the context of broader geologic processes and continent evolution. For the granites of the Lachlan and New England Fold Belts (LFB and NEFB) of Australia there has been great debate between competing petrogenetic models. The open-system view is that the isotopic variability and within-suite compositional trends can be accounted for by magma mixing, assimilation and fractional crystallisation (FC). In contrast, the restite unmixing model views the isotope compositions of diverse granites as a feature inherited from individual protoliths that underwent partial melting to produce magmas entraining varying proportions of residual material in a felsic melt. Reconciling all aspects of the geochemical data in a mixing model is contingent on a plausible fractionation regime to produce the observed consistently linear (or near-linear) trends on Harker diagrams; however, published FC models lack phase equilibria constraints on the liquidus assemblage and do not account for the likely changes in trace element partitioning across the modelled compositional range. The Magma Chamber Simulator (MCS) can be used to model fractional crystallisation alone (FC) or with assimilation (AFC), constraining phase equilibria and accounting for the thermal budget. Here, this tool was used to conduct a case study of the I-type Jindabyne Suite of granites from the LFB, testing whether thermodynamically feasible geochemical trends matching the observed linear variations can arise through FC (with or without assimilation of supracrustal material). The results of 112 MCS models show: (1) that major element liquid lines of descent (LLDs) may be sensibly linear over limited compositional ranges, (2) that the involvement of assimilation extends the range in which trends are relatively simple and near-linear, and (3) that, despite these observations, neither FC nor AFC are able to correctly reproduce the geochemical evolution of the I-type Jindabyne Suite granitoids as an LLD (contrary to existing models)—instead, these processes persistently produce curved and kinked trends. The output of these simulations were further refined to explore models in which: (1) crystal-bearing magmas evolve via FC or AFC (with chemical isolation assumed to be achieved through crystal zoning) and undergo varying degrees of melt-crystal segregation at different stages to produce the sample compositions, and (2) in situ crystallisation occurs via FC within the crystallisation zone, driving the evolution of a liquid resident magma, which the samples represent. These models are able to reproduce the Jindabyne Suite trends reasonably well. The modelling implies that FC, or some variant thereof, is a viable explanation for the linear trends in Jindabyne; however, tendency for grossly non-linear LLDs highlights that it should not be assumed that FC can generally explain linear trends in granites without careful modelling such as shown here.

Dendrogeochemistry and soil geochemistry applied to exploration for alkalic Cu-Au porphyry mineralization under cover at the Racecourse prospect, NSW, Australia

ABSTRACTThe Racecourse Cu-Au porphyry prospect is found within the Macquarie Arc of the Lachlan Fold Belt, in the Lachlan Transverse Zone, a cross-arc structure hosting significant world-class mineralization, including the Cadia and Northparks districts. Several geochemical and geophysical surveys of the prospect have been complimented by a total of 19,819 m of drilling, with only four holes reaching a depth greater than 300 m. Positive lithogeochemistry (fertility indices, comparisons with the Cadia and Northparks systems) subtle alteration, and mineralized intercepts indicate heightened mineral potential, yet the prospect has lacked a comprehensive geochemical survey outlining the extent of the mineralized target at surface. Soil samples and Monterey pine (Pinus radiata) tree cores were collected above and distal to mineralization intercepted by prior drilling in order to outline the ore deposit footprint and test the viability of dendrogeochemistry as an exploration tool for porphyry Cu mineralization. Ultimately, this study documented the spatial extent of the Racecourse target and identified potential areas for additional Cu mineralization. Soil samples were separated with the <250 μm size fraction analyzed and show distinct anomalous populations of Au, Cu, Mo, Pb, and Zn above prior drilling. Tree cores were collected by increment borer and tree rings of the Pinus radiata were counted and measured, with an age interval of 2003–2008 exhibiting the least ring-width variability chosen to chemically analyze. Selected intervals were digested and analyzed and have elevated Cu, Mo, and Zn in an area that overlaps a previously drilled soil anomaly, whereas an anomaly in the southwest of the survey area documents a Cu, Pb anomaly corresponding to localized faulting and tertiary basalt subcrop. Tree roots are directly tapping chemical variability at depth, aided via metal mobilization through faulted fluid conduits. Lead isotope ratios from the Pinus radiata identify distinct groups of lead spatially associated with discrete metal anomalies of varied lithological ages. At the Racecourse target, anomalous Pinus radiata samples yield a similar isotopic signature to the faulted southwestern anomaly, potentially linking the source of these two metal anomalies. When these results are integrated with the current understanding of the mineralized body, geochemical media suggest that mineralization may continue down-plunge at depth.

Geochemistry, origin and anatexis temperature of monzogranite formation in Mount Khalaj (Mashhad, Iran)

Geochemistry, origin and anatexis temperature of monzogranite formation in Mount Khalaj (Mashhad, Iran)

Crustal structure of southeast Australia from teleseismic receiver functions

Abstract. In an effort to improve our understanding of the seismic character of the crust beneath southeast Australia and how it relates to the tectonic evolution of the region, we analyse teleseismic earthquakes recorded by 24 temporary and 8 permanent broadband stations using the receiver function method. Due to the proximity of the temporary stations to Bass Strait, only 13 of these stations yielded usable receiver functions, whereas seven permanent stations produced receiver functions for subsequent analysis. Crustal thickness, bulk seismic velocity properties, and internal crustal structure of the southern Tasmanides – an assemblage of Palaeozoic accretionary orogens that occupy eastern Australia – are constrained by H–κ stacking and receiver function inversion, which point to the following: a ∼ 39.0 km thick crust; an intermediate–high Vp/Vs ratio (∼ 1.70–1.76), relative to ak135; and a broad (> 10 km) crust–mantle transition beneath the Lachlan Fold Belt. These results are interpreted to represent magmatic underplating of mafic materials at the base of the crust. a complex crustal structure beneath VanDieland, a putative Precambrian continental fragment embedded in the southernmost Tasmanides, that features strong variability in the crustal thickness (23–37 km) and Vp/Vs ratio (1.65–193), the latter of which likely represents compositional variability and the presence of melt. The complex origins of VanDieland, which comprises multiple continental ribbons, coupled with recent failed rifting and intraplate volcanism, likely contributes to these observations. stations located in the East Tasmania Terrane and eastern Bass Strait (ETT + EB) collectively indicate a crust of uniform thickness (31–32 km), which clearly distinguishes it from VanDieland to the west. Moho depths are also compared with the continent-wide AusMoho model in southeast Australia and are shown to be largely consistent, except in regions where AusMoho has few constraints (e.g. Flinders Island). A joint interpretation of the new results with ambient noise, teleseismic tomography, and teleseismic shear wave splitting anisotropy helps provide new insight into the way that the crust has been shaped by recent events, including failed rifting during the break-up of Australia and Antarctica and recent intraplate volcanism.

Some factors affecting granitic pluton topography: Pluton topography in the southern Lachlan Foldbelt

Where neotectonics are inactive, granite batholith topography is determined largely by the dissolution rates of the minerals of the component plutons. In southern New South Wales, Australia, plutons in the Kosciusko, Corryong, Berridale, and Murrumbidgee Batholiths allow an assessment of the extent to which faults, joints, grainsize and foliation may control a pluton's topography. Changes in mean elevation of zoned plutons closely follow the changes in mineralogy; more felsic zones invariably occur at higher elevations than more mafic zones. Faults clearly dictate major landscape features, such as linear valleys. Neither grainsize nor foliation show any appreciable effect on these landscapes. Joint spacing in these plutons has a positive correlation with the rock's grainsize, with an intrinsic value varying linearly as the grain size varies. Not all joints at the intrinsic spacing may be evident. In perennially wet valleys most joints become weathered and so visible, leading to joints mostly spaced at that intrinsic value. In quickly drying crests and ridges more incipient joints remain closed, leading to larger boulders and a wider average joint spacing. Thus the evident joint spacing in granites is determined by grainsize and landscape position.

Boron partitioning between zircon and melt: Insights into Hadean, modern arc, and pegmatitic settings

There is a lot of debate surrounding the geodynamic environment of the Hadean Earth, and the only definitively known fragments from the first 500 Ma are detrital zircons. This makes the combination of natural and experimentally obtained data a powerful tool to unravel the chemistry of the early Earth. We have used an incompatible marker element, B, to partially derive the chemistry of the parent melts of the Hadean and modern zircons. We report an experimental calibration for temperature dependent B partitioning between zircon and a hydrous weakly peraluminous granitic melt.log10DBzrc/melt=−1027±372TK−2.011±0.257where DBzrc/melt is the zircon-melt partition coefficient for B and T is temperature in K.This calibration has been applied to natural samples, viz., Hadean zircons from the Jack Hills (JH), Australia, Phanerozoic zircons from the Lachlan Fold Belt (LFB), Australia and three pegmatitic zircons from Seiland Igneous province, Norway, Paicoma Canyon, California and Freeman Mine, North Carolina. Our results present direct evidence of B being present in the Hadean crust. The zircons from JH and LFB are rather poor in B (8–80 ppb), but comparable to each other, while the pegmatites have as much as ten times the [B] (~0.35–0.45 ppm). Application of our experimental calibration yields calculated B melt concentrations of 10–90 ppm for the JH and LFB zircons. Such values for melt [B] (concentrations) are similar to the modern upper continental crust (17 ppm) and volcanic arcs, but are high when to compared to OIBs (0.6–1.8 ppm) and MORBs (1.3 ppm). The Lachlan zircons have [B] values that are broadly similar to the detrital Hadean and Archean zircons, and these have been presented as a point of comparison between Hadean and modern zircons. The pegmatite zircons return calculated melt values (244–701 ppm) that are much higher than the parent melts of the Australian zircons. One of the pegmatite zircons show a variation in calculated Ti-in-zircon crystallization T from the core to the rim (716 °C – 916 °C). Two of the other zircons crystallized at 664 ± 14 °C and 598 ± 13 °C (2 s.e.; not taking into consideration a ~50oT uncertainty due to imperfectly constrained silica/titania activities) and show no intracrystalline variation in [B] or T. Finally, B has been proposed to have been a possible stabilizing agent for ribose aqueous solutions and could have played a part in the formation of carbohydrates and proteins which were building blocks for RNA. Our documented presence of B in the Hadean crust at calculated concentrations similar to a modern volcanic arc setting, makes the role of B in ribose stabilization at least possible on the primordial Earth.

Petrogenesis of granitoids from the Lachlan Fold Belt, southeastern Australia: The role of disequilibrium melting

S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, have been investigated to assess the role of disequilibrium melting in their petrogenesis. Differences between the median initial εHf compositions of magmatic zircon populations and the host bulk-rock (ΔεHfblk-zrc) range from −0.6 to +2.5 ε units, providing evidence for intra-sample (and hence inter-phase) Hf-isotopic heterogeneity. Linear variations on Harker diagrams and O and Hf isotope compositions of magmatic zircon preserved in many I- and S-type granites are inconsistent with assimilation or simple mixing hypotheses. In contrast, isotopic disequilibrium between the melt and a restite assemblage can explain the bulk-rock versus zircon differences observed in these samples.Assuming that magmatic zircon records the melt composition, differences between the bulk-rock εHf and εHf of magmatic zircon (ΔεHfblk-zrc values) measured for I-type granites (0.4–2.5) can largely be explained by disequilibrium amphibole dehydration melting of meta-igneous protoliths that were either isotopically heterogenous at the time they were formed, or perfectly homogeneous before being aged in the crust for 0.4–1.0 billion years prior to partial melting. The Currowong Suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations; however, these observations could be explained by the restite unmixing of magma batches generated from a single source rock if, as modelling has suggested, separate batches contain different melt compositions.By investigating the application of disequilibrium melting to granite genesis, this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source, rather than necessitating the involvement of multiple sources and mixing processes. As a result great care should be taken in the interpretation of isotope data from granitic bulk-rocks or their zircons.

Ca. 1.04 Ga hot Grenville granites in the western Yangtze Block, southwest China

The late Mesoproterozoic to early Neoproterozoic magmatism in the western Yangtze Block, though minor in volume, has offered a rare opportunity for understanding the role of the Yangtze Block in the Grenville orogeny. The Huidong K-feldspar granites have SIMS zircon U-Pb ages of 1048.5 ± 4.9 Ma and 1043.1 ± 5.1 Ma, whereas the Yuanmou biotite granites were dated at 1041 ± 12 Ma by LA-ICP-MS. Their ages are indistinguishable within error, suggesting that they were emplaced contemporaneously. Both Huidong and Yuanmou rocks show geochemical features typical of A-type granites, including high FeO*/MgO, Ga/Al ratios and elevated contents of high-field-strength elements (HFSE). The Huidong granites exhibit positive whole-rock εNd(t) (+0.58 to +4.4) and zircon εHf(t) (+6.0 to +8.3) values close to those of regional coeval mafic rocks. They have a narrow range of zircon δ18O from 6.2‰ to 7.2‰, which is only slightly higher than the ‘mantle zircon’ value of 5.3 ± 0.6‰. High Nb/La and Nb/Th ratios in these rocks overlap with those of regional coeval mantle-derived rocks. Along with high zircon saturation temperatures of >1000 °C, we suggest that the Huidong granites could have formed mainly by extreme fractionation from basaltic parental magmas, or alternatively by partial melting of newly emplaced tholeiite at high temperatures. By contrast, the Yuanmou granites have lower whole-rock εNd(t) (−2.0 to +0.59) and zircon εHf(t) (−1.5 to +5.1) values and a Hf isotope crustal model age of ∼1.56–1.97 Ga, suggesting that its parental magmas may be generated by partial melting of old crust during intrusion of mantle-derived magmas. Ca. 1.04 Ga granites in the region show geochemical features similar to A-type granites in orogenic belts, e.g., the aluminous A-type granites in the Lachlan Fold Belt, Southeast Australia. They are also characterized by high-extremely high (>1000 °C) temperature in their genesis, similar to coeval hot Grenville granites globally. Temporal correlation of metamorphic and magmatic records between South China and the Grenville Province leads us to conclude that the ∼1.04 Ga A-type granites in the western Yangtze Block form parts of the hot Grenville granites emplaced during post-orogenic crustal extension.

Insights into the structure and dynamics of the upper mantle beneath Bass Strait, southeast Australia, using shear wave splitting

We investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in Bass Strait and the surrounding region of southeast Australia. Our dataset includes ∼366 individual splitting measurements from SKS and SKKS phases. The pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current N-S absolute plate motion. In the eastern Lachlan Fold Belt, fast shear waves are polarized parallel to the structural trend (∼N25E). Further south, fast shear wave polarization directions trend on average N25–75E from the Western Tasmania Terrane through Bass Strait to southern Victoria, which is consistent with the presence of an exotic Precambrian microcontinent in this region as previously postulated. Stations located on and around the Neogene-Quaternary Newer Volcanics Province in southern Victoria display sizeable delay times (∼2.7 s). These values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. In the Eastern Tasmania Terrane, nearly all observed fast directions are approximately NW-SE. Although part of our data set strongly favours anisotropy originating from “fabric” frozen in the lithospheric mantle, a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. We suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring, and so variations in anisotropy related to mantle flow may be expected. Alternatively, the pattern of fast polarisation orientations observed around Bass Strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered Cosgrove volcanic track. However, it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs. asthenospheric upper mantle due to poor backazimuthal coverage of the data.

Crustal reworking at convergent margins traced by Fe isotopes in I-type intrusions from the Gangdese arc, Tibetan Plateau

Voluminous magmatism at active continental margins and collision zones is marked by I-type intrusions. Paleocene-Eocene intrusions, exposed in the Lhasa terrane of the Tibetan Plateau, are part of the Gangdese arc that operated during Neo-Tethyan ocean subduction and subsequent India-Asia collision. Arc-like geochemical trace element signatures and radiogenic isotope systematics are indicative of juvenile I-type magmatism with variable silica contents consequent to igneous differentiation. Here, we present the first Fe isotope data for this fossil arc terrane, which have highly variable δ57Fe values (relative to IRMM-014) of −0.05 to +0.57‰. The data show no obvious correlations with major and trace element systematics, except for highly evolved rocks with >70 wt% SiO2. Based on trace element systematics, deuteric fluid exsolution is excluded as a cause for the isotope variations in less evolved rocks. Furthermore, there is no apparent relation between Fe isotope values and the established tectonic evolution from syn- to post-collisional magmatism. Comparison with I-type granites of the Australian Lachlan Fold Belt and Cordilleran Snake River Plain reveals an, on average, lighter δ57Fe in the Gangdese suite in primitive lavas, yet distinctively heavier than intra-oceanic arcs. The heavier δ57Fe values for primitive Lachlan Fold Belt and Snake River Plain rocks are interpreted here as crustal contribution in the form of S-type magmas derived from crustal anatexis, which are absent in the Gangdese arc. Gangdese Belt data yield an average δ57Fe of +0.13 ± 0.02‰ (2σ), which is proposed as the best estimate for juvenile crust at active continental margins.Rhyolite-MELTS modelling suggests that the Gangdese Belt data can be reproduced through fractional crystallization along a liquid line of descent at oxygen fugacity between FMQ = 0 to +2, typical for arc-related melts. Crucially, the majority of data is coherent with partial melts of existing mafic crust as the major contributor to the juvenile component in the intrusions, with minor mantle-derived melts, evidenced through heavier primitive Fe isotopes. This indicates that for the Gangdese Belt samples, the primitive, parental melts are derived from lower crustal successions through predominantly crustal reworking with only subordinate crustal growth. Hence, the Fe isotope data indicates a two-step evolution with melting of underplated mafic material sometime in the geologic past. Although juvenile in nature, these successions are part of the existing crust, supporting scenarios in which crustal reworking in convergent margin intrusions is an important process in evolved magma generation.

Pathfinder Exploration Techniques Targeting Porphyry and Epithermal Alteration SystemsiIn the Temora Copper-Gold Belt

The Temora project is a key exploration target for Sandfire to discover an economic copper-gold deposit in the Lachlan Fold Belt, NSW. The district contains an Ordovician Volcanic centre that has been explored over many years and hosts a number of defined porphyry and epithermal deposits. The availability of historical drill holes meant pulps could be re-analysed for multi element geochemical data and Short Wave Infrared analysis (SWIR) data to be collected across the district. Spectral data from the white micas defined the areas of higher temperature alteration associated with mineralisation across the district. Analysis, logging and re-interpretation of the data led to the definition of a number of new targets where the systems were poorly or untested. A high priority target was defined at the Donnington prospect to the north of a small known porphyry resource on the margins of the Rain Hill intrusive complex. The target showed a potential porphyry zonation systems in the interpreted geology, alteration, SWIR analysis, geochemistry and geophysics. Drilling in the first field season intersected 120m @ 0.3 % Cu and 0.5 g/t Au from 250m with follow up drilling intersecting 77m @ 0.3 % Cu and 0.5 g/t Au from 250m around a small porphyry intrusive. Mineralisation is hosted in quartz-magnetite-feldspar-pyrite-chalcopyrite veins associated with a chlorite-magnetite-carbonate alteration. A later quartz-sericite-pyrite alteration postdates the mineralisation causing a demagnetised zone surrounding a weak magnetic high that has been used in defining the mineralisation. The mineralisation is still open and further drilling is required.