Lachlan Fold Belt Research Articles

Trace inheritance—Clarifying the zircon O-Hf isotopic fingerprint of I-type granite sources: Implications for the restite model

Early to mid Carboniferous I-type granites distributed in a broad meridional belt west of Sydney, southeastern Australia, represent the last phase of granite magmatism in the southern Lachlan Fold Belt. Rare inherited zircon in the granites, in combination with zircon precipitated from the melt phase of the magmas, provides direct evidence of the nature and age of the source rocks from which the granites were derived, and the isotopic compositions of those rocks. Most granites from the north and central parts of the belt, represented by the Wuuluman, Home Rule, Oberon and Lett plutons (334±3, 328±4, 339±2 and 328±2Ma, respectively), are characterized by scarce inherited zircon dominantly of Siluro-Devonian age, consistent with all being derived from a common zircon-poor source rock of mid to late Devonian age. Based on the isotopic compositions of the igneous zircon rims, that source was relatively homogeneous and immature (δ18Ozrn 6–7‰, εHf(t) 0–+4). Two samples of the Tarana pluton (331±2Ma), near the centre of the belt, show evidence for heterogeneity in that source and the presence of a more evolved component that is also more zircon rich. Together with the Lett pluton from the same geochemical suite, their zircon rim compositions define a mixing array between more and less evolved end members (δ18Ozrn 5.5–9.5‰, εHf(t) -4–+4). Only in the southern Chapmans Creek pluton (327±3Ma) is there clear evidence that the source of the granite also contained a minor component with a similar older protolith to the LFB early Palaeozoic sediments that host most of the LFB pre-Carboniferous granites. Presence of that component, which overwhelmingly dominates the inheritance in the pre-Carboniferous granites, is also reflected in a slightly elevated whole rock initial 87Sr/86Sr (>0.7050) and igneous δ18Ozrn (ca. 7.5‰), but not in the igneous zircon εHf(t) (ca. +2). The amount of restitic zircon in granite is determined by not only the proportion of restite present but also the zircon contents of the various source rock components. The process of restite unmixing does not necessarily mean that more mafic, restite-rich granites will contain more inherited zircon.

Structural data constraints for implicit modeling of folds

A recent method for modeling folds uses a fold frame with coordinates based on the structural geology of folds: fold axis direction, fold axial surface and extension direction. The fold geometry can be characterised by rotating the fold frame by the pitch of the fold axis in the axial surface and the angle between the folded foliation and the axial surface. These rotation angles can be expressed as 1D functions of the fold frame coordinates. In this contribution we present methods for extracting and automatically modeling the fold geometries from structural data. The fold rotation angles used for characterising the fold geometry can be calculated locally from structural observations. The fold rotation angles incorporate the structural geology of the fold and allow for individual structural measurements to be viewed in the context of the folded structure. To filter out the effects of later folding the fold rotation angles are plotted against the coordinates of the fold frame. Using these plots the geometry of the folds can be interpolated directly from structural data where we use a combination of radial basis function and harmonic analysis to interpolate and extrapolate the fold geometry. This contribution addresses a major limitation in existing methods where the fold geometry was not constrained from structural data. We present two case studies: a proof of concept synthetic model of a non-cylindrical fold and an outcrop of an asymmetrical fold within the Lachlan Fold belt at Cape Conran, Victoria, Australia.

Ophiolites and Intra‐Oceanic Island Arc Assemblages of Eastern Australia, New Caledonia and New Zealand

Throughout the Phanerozoic the eastern margin of Gondwana and related fragments such as New Caledonia and New Zealand that are now dispersed from it grew through the addition of ophiolites and associated intra‐oceanic island arc assemblages. Exactly how and why this occurred remains controversial with two main competing models referred to as either ‘quantum'or‘accordion’ tectonics. The quantum model envisages continental growth through the additional of discrete intra‐oceanic assemblages analogous to contemporary tectonic settings in Taiwan, Timor and Papua New Guinea (Aitchison and Buckman, 2012). The alternative regards eastern Australia as the type example of a different style of convergent plate margin referred to as an ‘extensional accretionary orogeny’ (Collins, 2002).The oldest Phanerozoic ophiolites and intra‐oceanic island arc assemblages are of Cambrian age and are widely reported from the Lachlan Fold Belt in the eastern Australian states of Victoria and NSW (Spaggiari et al., 2003; Greenfield et al., 2011). Similar rocks are also known from Mount Read in Tasmania (Berry and Crawford, 1988; Crawford and Berry, 1992; Mulder et al., 2016), the Weraerai terrane and its correlatives in the New England orogen further east in northeastern NSW (Aitchison et al., 1994; Aitchison and Ireland, 1995) and Queensland, the Takaka terrane in NW Nelson, New Zealand (Münker and Cooper, 1999) and the Bowers terrane in Northern Victoria Land, Antarctica (Weaver et al., 1984; Münker and Crawford, 2000; Rocchi et al., 2011; Palmeri et al., 2012).The Late Ordovician saw the development of the intra‐oceanic Macquarie island arc (Glen et al., 1998; Glen et al., 2007). This system contains important economic mineral deposits. The way in which these arc rocks developed and were juxtaposedagainst a surrounding suite of Lachlan Fold Belt, eastern Australia remains the subject of investigation (see Aitchison and Buckman, 2012 for discussion). In a similar area, enigmatic rocks of the Tumut ophiolite also crop out (Graham et al., 1996; Belousova et al., 2015).Further to the east in the New England orogeny Siluro‐Devonian rocks of the Gamilaroi terrane and it's along strike correlatives near Mt Morgan in Queensland represent another intra‐oceanic island arc assemblage emplaced onto the Gondwana margin in the Late Devonian (Aitchison and Flood, 1994; Offler and Murray, 2011).The Late Carboniferous‐Permian saw development of significant intra‐oceanic island arc and ophiolitic complexes remnants of which crop out in New Zealand, eastern Australia, and New Caledonia. These include the Brook Street terrane (Spandler et al., 2005; McCoy‐West et al., 2014) and Dun Mountain Ophiolite Belt in New Zealand (Coombs et al., 1976; Stewart et al., 2016), the Gympie terrane in southeast Queensland (Waterhouse and Sivell, 1987; Sivell and Waterhouse, 1988) and the Koh terrane in New Caledonia (Meffre et al., 1996; Ali and Aitchison, 2002).The youngest on‐land association of ophiolitic and intra‐oceanic island arc rocks in the region is of Eocene age. Ultramafic rocks are well exposed in New Caledonia where they structurally overlie continental rocks of Gondwana margin affinity that, in the northeast of the island, have experienced eclogite facies metamorphism (Aitchison et al., 1995). The emplacement of these rocks was a widespread regional event with potentially correlative rocks exposed in Papua New Guinea (Parrot and Dugas, 1980) as well as Northland and East Cape in New Zealand (Whattam et al., 2005; Whattam et al., 2008).

Two different types of granitoids in the Suyunhe large porphyry Mo deposit, NW China and their genetic relationships with molybdenum mineralization

The Suyunhe large porphyry Mo deposit (∼0.57Mt molybdenum), located in the West Junggar, NW China, is the largest known porphyry Mo deposit in Xinjiang. Granitoids in this deposit are mainly characterized by three closely spaced intrusive centers (known as stocks I, II and III respectively). The stocks I and III mainly consist of barren granodiorite porphyry and tonalite porphyry, whereas the stock II is mainly composed of fertile monzonitic granite porphyry and granite porphyry. Based on detailed major and trace element, and Sr–Nd isotopic analyses, two distinct compositional groups can be identified. The first group of high-silica end-members (HSE) is characterized by high SiO2 (mostly >75wt%), low MgO (0.07–0.69wt%) and Mg# (0.19–0.36), significant Eu depletion in the chondrite-normalized diagram, and low Sr/Y and La/Yb, as well as noticeably negative anomalies of Ba, Sr, P and Ti in the primitive mantle-normalized diagram. The second group of low-silica end-members (LSE), however, displays adakite-like features with lower SiO2 (<75wt%), higher MgO (0.52–1.32wt%) and Mg# (0.32–0.52; mostly >0.4), and higher Sr/Y (mostly >20) and La/Yb (>8). The depleted Sr–Nd isotopic characteristics (εNd(T)=3.5–6.4 and Isr=0.7026–0.7055) and young two-stage model ages of HSE and LSE indicate that they were both derived from partial melting of juvenile lower crust that might be triggered by asthenosphere upwelling subsequent to a slab rollback event. However, the depths of initial melting might be different. The current evidence demonstrates that HSE in the Suyunhe deposit formed by partial melting of juvenile crust at depths of less than ∼33km with a plagioclase residue, whereas that for LSE occurred at depths of >40km where a garnet residue existed and the crust was thickened. The lower source depth, as well as subsequently strong plagioclase fractionation, results in the absence of adakite-like characteristics in HSE.The Ce4+/Ce3+and EuN/EuN∗ ratios in zircons of HSE are much lower than ore-forming intrusions from porphyry Cu deposits in the Central Asian Orogenic Belt, but noticeably higher than barren intrusions from the Lachlan fold belt and ore-bearing intrusions from small-intermediate porphyry Mo deposits from the East Qinling–Dabie and the Nanling metallogenic belts, China, indicating that neither too high nor too low oxygen fugacities are favorable for large porphyry Mo deposits. Based on previous studies of adakitic rocks in the world, adakite-like LSE in the Suyunhe deposit are believed to have higher oxygen fugacities, and thus be less fertile than HSE. We finally suggest that adakites and adakite-like rocks are unproductive for porphyry Mo deposits.

Aluminum in zircon as evidence for peraluminous and metaluminous melts from the Hadean to present

AbstractZircon structurally accommodates a range of trace impurities into its lattice, a feature which is used extensively to investigate the evolution of silicate magmas. One key compositional boundary of magmas is defined by whether the molar ratio of Al2O3/(CaO + Na2O + K2O) is larger or smaller than unity. Here we report ∼800 Al in zircon concentrations from 19 different rocks from the Lachlan Fold Belt (southeastern Australia), New England (USA), and Arunachal leucogranites (eastern Himalaya) with Al2O3/(CaO + Na2O + K2O) whole rock values that range from 0.88 to 1.6. Zircons from peraluminous rocks yield an average Al concentration of ∼10 ppm, which distinguishes them from crystals found in metaluminous rocks (∼1.3 ppm). This difference is related to the materials involved in the melting, assimilation, and/or magma differentiation processes; for example, magmas that assimilate Al‐rich material such as metapelites are expected to produce melts with elevated alumina activities, and thus zircons with high Al concentrations. These observations are applied to the Archean and Hadean Jack Hills detrital zircon record. Detrital Archean zircons, with ages from about 3.30 to 3.75 Ga, yield Al in zircon concentrations consistent with origins in peraluminous rocks in ∼8% of the cases (n = 236). A single zircon from the pre‐3.9 Ga age group (n = 39) contains elevated Al contents, which suggests that metaluminous crustal rocks were more common than peraluminous rocks in the Hadean. Weathered material assimilated into these Hadean source melts was not dominated by Al‐rich source material.

A reappraisal of the evolution of the palaeo-Pacific margin of Gondwana from the Pb and Os isotope systematics of igneous rocks from the southern Adelaide fold belt, South Australia

We have undertaken Pb and Os isotopic analyses of igneous rocks from the southern Adelaide fold belt to further understanding of the Neoproterozoic – early Palaeozoic evolution of the palaeo-Pacific margin of Gondwana. The new Pb isotope data and a reappraisal of existing geochronological data suggest that layered mafic intrusions at Black Hill and Cambrai may have intruded much earlier than previously thought. Intrusion may even have occurred prior to deposition of the Normanville and Kanmantoo Group sediments and the onset of what is traditionally regarded as the Delamerian Orogeny. We tentatively term this earlier event the Hallett Cove orogeny. However, equivalent ages are common in the formerly adjacent Ross Orogen. When the Pb isotope signatures of the suites of gabbros, and I-, S-, and A-type granites are compared at 487Ma they all span a very large range (206Pb/204Pbi=15.4–18.7, 207Pb/204Pbi=15.4–15.7, 208Pb/204Pbi=33.0–38.8) compared with other formerly adjacent terranes or the subsequent Lachlan fold belt. Similarly, 187Os/188Osi ratios span 0.122–1.17 but include one gabbro and one rhyolite that have subchondritic 187Os/188Osi ratios. When taken together, the combined data do not support models in which the observed range in initial isotope ratios reflects major and variable mid to upper crustal assimilation of magmas derived from the contemporary asthenospheric mantle. Rather, it appears that magmas parental to the gabbros and A-type rocks were derived from subduction-modified lithospheric mantle that has a median metasomatic age of 1.4Ga. Large-scale Pb isotopic homogenization, as proposed for the Lachlan fold belt did not occur in the Adelaide fold belt.

The molybdenum isotopic compositions of I-, S- and A-type granitic suites

This study reports Mo isotopic compositions for fifty-two Palaeozoic granitic rocks with contrasting source affinities (A-, I- and S-type) from the Lachlan Fold Belt (LFB) and the New England Batholith (NEB), both in SE Australia, and three compositionally zoned plutons (Loch Doon, Criffell, and Fleet) located in the Southern Uplands of Scotland. The results show relatively large variations in δ98Mo for igneous rocks ranging from −1.73‰ to 0.59‰ with significant overlaps between different types. No relationships between δ98Mo and δ18O or ASI (Alumina Saturation Index) are observed, indicating that Mo isotopes do not clearly distinguish igneous vs. sedimentary source types. Instead, effects of igneous processes, source mixing, regional geology, as well as hydrothermal activity control the Mo isotope compositions in these granites. It is found that Mo is mainly accommodated in biotite and to a lesser extent in hornblende. Hornblende and Fe3+-rich minerals may preferentially incorporate light isotopes, as reflected by negative correlations between δ98Mo and K/Rb and [Fe2O3]. There is a positive correlation between initial 87Sr/86Sr and δ98Mo in I-type granitic rocks, reflecting the admixing of material from isotopically distinct sources. Granitic rocks from Scotland and Australia display strikingly similar curvilinear trends in δ98Mo vs. initial 87Sr/86Sr despite the differing regional geology. Localized hydrothermal effects can result in low δ98Mo in granite, as seen in three samples from Loch Doon and Criffell which have anomalously light δ98Mo of <−1‰. Based on this study, an estimate of δ98Mo=0.14±0.07‰ (95% s.e.) for the Phanerozoic upper crust is proposed. This is slightly heavier than basalts indicating an isotopically light lower crust and/or a systematic change to the crust resulting from subduction of isotopically light dehydrated slab and/or pelagic sediment over time.

Evidence for hybridisation in the Tynong Province granitoids, Lachlan Fold Belt, eastern Australia

ABSTRACTThe role of mafic–felsic magma mixing in the formation of granites is controversial. Field evidence in many granite plutons undoubtedly implies interaction of mafic (basaltic–intermediate) magma with (usually) much more abundant granitic magma, but the extent of such mixing and its effect on overall chemical features of the host intrusion are unclear. Late Devonian I-type granitoids of the Tynong Province in the western Lachlan Fold Belt, southeast Australia, show typical evidence for magma mingling and mixing, such as small dioritic stocks, hybrid zones with local host granite and ubiquitous microgranitoid enclaves. The latter commonly have irregular boundaries and show textural features characteristic of hybridisation, e.g. xenocrysts of granitic quartz and K-feldspars, rapakivi and antirapakivi textures, quartz and feldspar ocelli, and acicular apatite. Linear (well defined to diffuse) compositional trends for granites, hybrid zones and enclaves have been attributed to magma mixing but could also be explained by other mechanisms. Magmatic zircons of the Tynong and Toorongo granodiorites yield U–Pb zircon ages consistent with the known ca 370 Ma age of the province and preserve relatively unevolved ϵHf (averages for three samples are +6.9, +4.3 and +3.9). The range in zircon ϵHf in two of the three analysed samples (8.8 and 10.1 ϵHf units) exceeds that expected from a single homogeneous population (∼4 units) and suggests considerable Hf isotopic heterogeneity in the melt from which the zircon formed, consistent with syn-intrusion magma mixing. Correlated whole-rock Sr–Nd isotope data for the Tynong Province granitoids show a considerable range (0.7049–0.7074, ϵNd +1.2 to –4.7), which may map the hybridisation between a mafic magma and possibly multiple crustal magmas. Major-element variations for host granite, hybrid zones and enclaves in the large Tynong granodiorite show correlations with major-element compositions of the type expected from mixing of contrasting mafic and felsic magmas. However, chemical–isotopic correlations are poorly developed for the province as a whole, especially for 87Sr/86Sr. In a magma mixing model, such complexities could be explained in terms of a dynamic mixing/mingling environment, with multiple mixing events and subsequent interactions between hybrids and superimposed fractional crystallisation. The results indicate that features plausibly attributed to mafic–felsic magma mixing exist at all scales within this granite province and suggest a major role for magma mixing/mingling in the formation of I-type granites.

The Early Devonian palaeobiogeography of Eastern Australasia

Biogeographical analyses of Devonian Australasia (Australia and New Zealand) have previously presented a narrative approach to the interrelationships of continental faunal provinces. Useful for the creation of biotic hypotheses, similarities and differences between biotic provinces remains untested. This study seeks to propose the first regional faunal provinces allowing a test of geological and biotic hypotheses using Parsimony Analysis of Endemicity. The relationships of eastern Australasian terranes were tested through the hierarchical analysis of faunal composition. Biochorologic sub-provinces were proposed using geo-spatial computer software and analysed through phylogenetic software that resulted in the formalisation of 20 biochorologic sub-provinces. The relationships between these units show a significant holdover of Silurian species, the interactions between convergent terranes and the continental margin, the distinct history of the Lachlan Fold Belt terranes, and offers biotic support for the existence of the Tasmanian microcontinent Vandieland.

Possible Biosphere-Lithosphere Interactions Preserved in Igneous Zircon and Implications for Hadean Earth.

Granitoids are silicic rocks that make up the majority of the continental crust, but different models arise for the origins of these rocks. One classification scheme defines different granitoid types on the basis of materials involved in the melting/crystallization process. In this end-member case, granitoids may be derived from melting of a preexisting igneous rock, while other granitoids, by contrast, are formed or influenced by melting of buried sedimentary material. In the latter case, assimilated sedimentary material altered by chemical processes occurring at the near surface of Earth-including biological activity-could influence magma chemical properties. Here, we apply a redox-sensitive calibration based on the incorporation of Ce into zircon crystals found in these two rock types, termed sedimentary-type (S-type) and igneous-type (I-type) granitoids. The ∼400 Ma Lachlan Fold Belt rocks of southeastern Australia were chosen for investigation here; these rocks have been a key target used to describe and explore granitoid genesis for close to 50 years. We observe that zircons found in S-type granitoids formed under more reducing conditions than those formed from I-type granitoids from the same terrain. This observation, while reflecting 9 granitoids and 289 analyses of zircons from a region where over 400 different plutons have been identified, is consistent with the incorporation of (reduced) organic matter in the former and highlights one possible manner in which life may modify the composition of igneous minerals. The chemical properties of rocks or igneous minerals may extend the search for ancient biological activity to the earliest period of known igneous activity, which dates back to ∼4.4 billion years ago. If organic matter was incorporated into Hadean sediments that were buried and melted, then these biological remnants could imprint a chemical signature within the subsequent melt and the resulting crystal assemblage, including zircon.

From Migmatites to Plutons: Power Law Relationships in the Evolution of Magmatic Bodies

Magma is generated by partial melting from micrometre-scale droplets at the source and may accumulate to form >100 km-scale plutons. Magma accumulation thus spans well over ten orders of magnitude in scale. Here we provide measurements of migmatitic leucosomes and granitic veins in drill cores from the Estonian Proterozoic basement and outcrops at Masku in SW Finland and Montemor-o-Novo, central Portugal. Despite the differences in size and number of measured leucosomes and magmatic veins, differences in host rock types and metamorphic grades, the cumulative width distribution of the studied magmatic leucosomes/veins follows a power law with exponents usually between 0.7 and 1.8. Published maps of the SE Australian Lachlan Fold Belt were used to investigate the distribution of granitoid pluton sizes. The granites occupy ca. 22 % of the 2.6 × 105 km2 area. The cumulative pluton area distributions show good power law distributions with exponents between 0.6 and 0.8 depending on pluton area group. Using the self-affine nature of pluton shapes, it is possible to estimate the total volume of magma that was expelled from the source in the 2.6 × 105 km2 map area, giving an estimated 0.8 km3 of magma per km2. It has been suggested in the literature that magma batches in the source merge to form ever-bigger batches in a self-organized way. This leads to a power law for the cumulative distribution of magma volumes, with an exponent m V between 1 for inefficient melt extraction, and 2/3 for maximum accumulation efficiency as most of the volume resides in the largest batches that can escape from the source. If m V ≥ 1, the mass of the magma is dominated by small batches; in case m = 2/3, about 50 % of all magma in the system is placed in a single largest batch. Our observations support the model that the crust develops a self-organized critical state during magma generation. In this state, magma batches accumulate in a non-continuous, step-wise manner to form ever-larger accumulations. There is no characteristic length or time scale in the partial melting process or its products. Smallest melt segregations and >km-scale plotuns form the end members of a continuous chain of mergers of magma batches.

Fe isotopes and the contrasting petrogenesis of A-, I- and S-type granite

Abstract We present new Fe isotope data of 42 S-, I- and A-type (ferroan) granites from the Cambrian Delamerian orogen in South Australia, the Palaeozoic Lachlan Fold Belt and Western USA. Interpretation of these data, together with modelling suggests that magmatic processes do result in quite complex Fe-isotopic differentiation trends and can lead to granites with isotopically heavy iron with δ 57 Fe > 0.35‰. By comparison Mid-Ocean Ridge Basalts (MORBs) have δ 57 Fe = 0.15‰ (Teng et al., 2013). These variations are similar to those previously reported (Poitrasson and Freydier, 2005; Heimann et al., 2008; Telus et al., 2012), but, contrary to some interpretations (Beard and Johnson, 2006; Heimann et al., 2008), heavy values are not necessarily the product of late-stage hydrothermal fluid loss, though this process is undoubtedly also an important factor in some circumstances. A-type (ferroan) granites reach very heavy δ 57 Fe values (0.4–0.5‰) whereas I-types are systematically lighter (δ 57 Fe = 0.2‰). S-type granites show a range of intermediate values, but also tend to be isotopically heavy (δ 57 Fe ≈ 0.2–0.4‰). Our results show that the iron isotopic values and trends are signatures that reflect granite generation processes . A modelling using the Rhyolite-MELTS software suggests that contrasting trajectories and end-points in Fe isotope evolution towards granite depend on: oxidation state of the evolving magma and, whether or not the system is oxygen-buffered. Iron isotopic evolution supports an origin of ferroan A-type granite from protracted, closed magma chamber fractionation of moderately reduced mafic magmas. In these systems magnetite saturation is delayed and the ferric iron budget is finite. I-type systems originate with the supply of relatively oxidised, hydrous, subduction-related magmas from the mantle wedge to the upper plate crust. These then experience oxygen-buffered open-system AFC processes in lower crustal hot-zones. S-type magmas are crustal melts that crystallise under reduced conditions initially imposed at source by sulphidic or graphitic sedimentary protoliths. The composition of the resulting melts reflects the domination of partial melting where conditions are hence buffered (open system) followed by subsequent late-stage, closed system fractionation of these extracted, reduced magmas.

Comparing prospectivity modelling results and past exploration data: A case study of porphyry Cu–Au mineral systems in the Macquarie Arc, Lachlan Fold Belt, New South Wales

Mineral exploration is undertaken in stages, with each stage designed to get to the next decision point of whether or not to keep exploring a particular area based on the results at hand. As a general rule, each consecutive exploration stage is more expensive due to the progressively more drill- and study-intensive nature of the work required, in particular after discovery of a potentially economic mineral deposit. As such, the distribution of exploration activities and related expenditures essentially serve as a spatial measure of prospectivity as perceived by mineral exploration companies. In this study we compare historic (1980 to 2002) porphyry Cu–Au exploration activities and expenditures in part of the Ordovician to Early Silurian Macquarie Arc, Australia's most significant porphyry province with total resources greater than 80Moz of Au and 13Mt of Cu, to prospectivity modelling results from a weights of evidence (WofE) model. The outcomes of this spatial and statistical comparison indicate that at 2002 the Macquarie Arc was by no means a mature exploration destination and that past exploration investment outside the main mining areas was not necessarily effective. Moreover, no spatial correlation was apparent between areas of higher exploration expenditure and greater geological potential. For example, of the 692km2 of highly prospective ground covered by the exploration licences examined in this study, only 89km2 (c. 13%) have been explored effectively in that they received some form of drilling. Interestingly, the remaining area (603km2 or c. 87%) had not yet been effectively tested. As such, our analysis confirmed that despite a greater 100year exploration and mining history, much of the prospective ground within the study area remained untested. Taken as a whole, the results of our spatial and statistical comparison are important inputs for assessing the effectiveness of exploration investment and explanation maturity and, therefore, future exploration decision-making. The outcomes also have implications for strategic planning of future government legislation helping to manage and maximise the benefits from exploration investment.

Emplacement and deformation ages of the Wyangala Granite, Cowra, NSW

New geochronological data from the Wyangala Granite in the Hill End Trough, Eastern Lachlan Fold Belt, constrain the timing of granite emplacement and subsequent deformation. SHRIMP U–Pb dating of zircon indicates that the granite crystallised in the late Silurian (425.2 ± 3.5 Ma). Subsequent west-over-east thrusting resulted in the development of mylonite along its eastern margin and cataclasis of megacrystic and groundmass feldspars, especially in pervasive shear zones within the granite. 40Ar/39Ar dating of feldspar from two deformed granite samples yielded ‘plateau’ ages in the range 375–365 Ma that are interpreted to reflect the timing of deformation associated with major shearing. These ages broadly overlap with the proposed age range of the late Middle Devonian Tabberabberan event and some biotite K–Ar and Rb–Sr ages in the nearby Sunset Hills Granite.

Structure of the Millen Schist Belt (Antarctica): Clues for the tectonics of northern Victoria Land along the paleo‐Pacific margin of Gondwana

AbstractNorthern Victoria Land (Antarctica) belonged to the active proto‐Pacific margin of Gondwana, which was the site of convergence during the Paleozoic. This study provides new insights into the structural architecture of northern Victoria Land, focusing on the boundary area between the Bowers and Robertson Bay terranes, i.e., in the Millen Schist Belt. It is a high‐strain equivalent of the adjoining terranes, presently delimited by the Leap Year and the Handler faults. Our study reveals that these two faults overprint a preexisting transitional deformational boundary and are associated with a significant syntectonic circulation of fluids and mineralization. The Millen Schist Belt consists of two lithotectonic packages, juxtaposed along the Crosscut‐Aorangi duplex thrust system, related to late Ross deformation. As there is increasing evidence of a post‐Ross contractional event in northern Victoria Land, we suggest that the structural architecture of the Bowers‐Robertson Bay terrane boundary results from a long‐lasting SW‐NE contractional regime, during the Ross‐Delamerian Orogeny and still active afterward. This points to an extension of the Australian Lachlan Orogeny in Antarctica. The similarity of the structural architecture, the gold mineralization, the rock type, and the age supports the correlation of the Bowers and the Robertson Bay terranes with the Stawell Zone of the Lachlan Fold Belt. In our new tectonic scenario the Lanterman Fault (northern Victoria Land) plays the same role as the Moyston Fault (southeastern Australia), and the Leap Year and Handler faults correlate with the “intra‐zone faults” of the Stawell Zone (e.g., the Ararat‐Stawell Fault Zone).

Magmatism, orogeny and the origin of high-heat-producing granites in Australian Proterozoic terranes

Temperature-dependent processes such as magmatism and orogeny provide key insights into the Earth’s thermal state. We propose an integrated model for the origin of voluminous Proterozoic-aged granitic rocks in northern Australia, suggesting that the observed large range of concentrations of incompatible elements (particularly U, Th and K) was established progressively as a consequence of a hot-plate orogenic style involving little crustal thickening, combined with ‘normal’ juvenile magma additions. Progressive extraction of the heat-producing elements into the mid- to upper crust, where they remain below the erosional base level, resulted in anomalous thermal conditions that modulated the continuing orogenic history. This may represent an important style in post-Archaean lithospheric behaviour. Supplementary materials: Age and calculated heat production data for all Australian Proterozoic granitic rocks (calculated for the time of intrusion and for the present day) and for individual Proterozoic terranes of the North Australian craton, comparison of the range and distribution of calculated heat production of granitic rocks with the North Australian Craton and within the Mount Isa Inlier, and Zr and SiO 2 data for granites of the Lachlan Fold Belt and for the Mount Isa Inlier are available at www.geolsoc.org.uk/SUP18721 .

Comment — Origin of enclaves in S-type granites of the Lachlan Fold Belt

The recent passing of Bruce Chappell is a loss to the geochemical– petrological community, and no less to ourselves who grew up (scientifically)with andwere both inspired and provoked by hiswork. Bruce was no stranger to controversy and we believe that we can best honour his memory by scrutinising what may be the last of his contributions to his very considerable scientific legacy — Chappell and Wyborn (2012). Chappell andWyborn (C&W) present the view that S-type granitic rocks commonly contain twomain texturally and chemically different types of enclaves (rock inclusions). The first kind is foliated (schistose to gneissose) metamorphic rocks, commonly rich in biotite and generally of metapelitic composition. The second kind is essentially unfoliated, fine-grained enclaves with poikilitic and commonly porphyritic textures, and sometimes having a marginal reaction zone, in contact with their host granitoid rock. These are commonly called microgranular enclaves. C&W take the position that both these kinds represent forms of restitic material, which they define as any solid remaining when a rock undergoes partial melting. They point out that this does not mean that these enclaves were 100% solid at the time of inclusion in the host magma, however. We are happy with the proffered definition of restite (“pieces of largely solid material remnant from partial melting, which were transported from the source”, Chappell and Wyborn (2012), p. 236), but note that this does not necessarily imply that a restitic enclave must be the restitic counterpart of the particular enclosing granitoid. We also agree that both the microgranular and the schistose enclaves may have contained both crystalline and molten portions. However,

Reply — Origin of enclaves in S-type granites of the Lachlan Fold Belt

Reply — Origin of enclaves in S-type granites of the Lachlan Fold Belt

Evolution of a Cambrian active continental margin: The Delamerian–Lachlan connection in southeastern Australia from a zircon perspective

The Early Palaeozoic Ross–Delamerian orogenic belt is considered to have formed as an active margin facing the palaeo-Pacific Ocean with some island arc collisions, as in Tasmania (Australia) and Northern Victoria Land (Antarctica), followed by terminal deformation and cessation of active convergence. On the Cambrian eastern margin of Australia adjacent to the Delamerian Fold Belt, island arc and backarc basin crust was formed and is now preserved in the Lachlan Fold Belt and is consistent with a spatial link between the Delamerian and Lachlan orogens. The Delamerian–Lachlan connection is tested with new zircon data. Metamorphic zircons from a basic eclogite sample from the Franklin Metamorphic Complex in the Tyennan region of central Tasmania have rare earth element signatures showing that eclogite metamorphism occurred at ~510Ma, consistent with island arc–passive margin collision during the Delamerian(−Tyennan) Orogeny. U–Pb ages of detrital zircons have been determined from two samples of Ordovician sandstones in the Lachlan Fold Belt at Melville Point (south coast of New South Wales) and the Howqua River (western Tabberabbera Zone of eastern Victoria). These rocks were chosen because they are the first major clastic influx at the base of the Ordovician ‘Bengal-fan’ scale turbidite pile. The samples show the same prominent peaks as previously found elsewhere (600–500Ma Pacific-Gondwana and the 1300–1000Ma Grenville–Gondwana signatures) reflecting supercontinent formation. We highlight the presence of ~500Ma non-rounded, simple zircons indicating clastic input most likely from igneous rocks formed during the Delamerian and Ross Orogenies. We consider that the most probable source of the Ordovician turbidites was in East Antarctica adjacent to the Ross Orogen rather than reflecting long distance transport from the Transgondwanan Supermountain (i.e. East African Orogen). Together with other provenance indicators such as detrital mica ages, this is a confirmation of the Delamerian–Lachlan connection.

Intraplate crustal remelting as the genesis of Jurassic high-K granites in the coastal region of the Guangdong Province, SE China

The origin and tectonic significance of high-K granites (>3wt% K2O at 70wt% SiO2), calc-alkaline I-type granites in particular, remain controversial. This paper takes granitic plutons distributed in the coastal region of the Guangdong Province of southeastern China as examples to explore the genesis of such rocks. SIMS zircon U–Pb geochronological data show that the granites were emplaced at 166–159Ma. These granites can be subdivided generally into two groups on the basis of integrated mineralogical, geochemical, whole-rock Sr–Nd isotopic and in situ zircon Hf–O isotopic studies. The group A granites (SiO2=64–72wt%) are characterised by their common occurrence of amphibole (±titanite) and dominantly metaluminous feature (A/CNK=0.85–1.03). They are high in K2O (3.5–7.0wt%) and K2O/Na2O (>1), and have trace element concentrations (e.g., Nb, Y, Zr and Ga) similar to typical I-type granites in the Lachlan Fold Belt, southeastern Australia. Their whole-rock ISr (0.7057–0.7077) and εNd(t) (−6.46 to −3.13) are less evolved than many coeval granites in this region. As in situ zircon Hf–O isotopes show little evidence of magma mixing, these granites with low zircon δ18O (6.3−7.9‰) and high εHf(t) (−5.9 to −0.2) could have been generated from melting of oxidised high-K basaltic rocks. The group B granites, emplaced to the east of group A granites, are dominantly weakly peraluminous (A/CNK=1.00–1.05). They have higher SiO2 (70–76wt%), less common or absence of amphibole, higher zircon δ18O (6.6−9.0‰) and lower εHf(t) (−11.4 to −5.9) than the group A granites. Zircon Hf–O isotope data reveal that the group B granites contain higher percentage of supracrustal materials than those of the group A, but the variations of major and trace elements do not support an assimilation and fractional crystallization (AFC) model. Instead, the group B granites, with features transitional between typical I-type and S-type granites, were most likely formed in a region where there was physical juxtaposition between infracrustal metaluminous and supracrustal peraluminous source rocks. Thus, granites of both groups represent products of crustal reworking likely due to asthenosphere upwelling and/or underplating and intrusion of mafic magmas. The close association in time and space of these granites with OIB-like basaltic rocks and the secular compositional change of Jurassic basaltic rocks in the region suggest that these rocks probably formed in an intraplate extensional setting resulted from the delamination of a flat-subducted oceanic slab.