Albany-Fraser Orogen Research Articles

Detrital zircon and apatite reveal Paleoproterozoic rifting along the eastern margin of the Yilgarn Craton

Detrital minerals within Proterozoic basins are commonly an extant record of now-destroyed crust and provide valuable constraints on ancient paleogeography and tectonic processes. However, reconstructing basin histories depends on discriminating potentially exotic, far-travelled (allochthonous) versus locally sourced (autochthonous) detritus. Detrital zircon from the Woodline Formation, a component of the Proterozoic Barren Basin at the margin of the Yilgarn Craton, Western Australia, closely matches the age and Hf isotopic signatures of magmatic units in the Paleo- to Mesoproterozoic Albany–Fraser Orogen (AFO). However, the provenance of some 2300–2000 Ma zircon detritus is not readily accounted for by known magmatism in the AFO nor the wider Western Australian Craton. To help resolve this enigmatic source, we present isotopic data (U–Pb, Lu–Hf) and trace element geochemistry for zircon and limited U–Pb data for scarce apatite collected from sand and rocks of the Woodline Formation. Based on Hf isotopic signatures, some of the detritus appears to be derived from the recently identified 2030–2010 Ma felsic-igneous Moonyoora Suite, representing the oldest known magmatic components of the AFO. Moreover, the isotopic data imply an earlier ca. 2250 Ma episode of mafic rift-related magmatism, potentially offering a glimpse into the earliest rifting stages along the Yilgarn Craton’s eastern margin. While Paleoproterozoic detritus is omnipresent in Woodline rocks, it is conspicuously absent from overlaying regolith, demonstrating that cover atop the Woodline Formation is imported and not locally derived. This disparity is important as it highlights that ongoing exploration efforts analysing unconsolidated regolith may not fully capture basement signatures.

Australian critical metal exploration for analogues of Chinese ionic-clay REE deposits

The clean energy transition has focused attention on the critical metals required for manufacture of new energy technologies. The extremely heterogeneous distribution of critical metal mineral deposits requires that countries must make new discoveries of key critical metals to avoid potential future geopolitical risks. Although Australia has REE resources, they are mainly of LREEs, so deposit styles with significant HREEs are key targets. The most obvious are the so-called ionic-clay REE deposits of southern China that are the major global suppliers of HREEs. Mineral exploration in Australia using the Chinese model is producing hitherto unrecognized REE concentrations, here termed regolith-hosted REE mineralization, in a variety of regolith types in several weathering environments, particularly in Western and South Australia. Amongst these has been the discovery of near-surface, regolith-hosted REE mineralization in the Albany-Fraser Orogen which has the potential to complement REE production from the giant Mount Weld carbonatite and other monazite-sand deposits in Western Australia. Widespread near-surface, regolith-hosted REE mineralization is present over an area of 12,000 km2 in the Esperance District. As much of this area is beneath barren Eocene-aged cover clays, all the new REE discoveries have been made, and are being delineated using shallow drilling, to depths of less than 80 m. The exploration implication is that the search space for regolith hosted REE deposits in Western Australia remains immature. These recently discovered deposits have both similarities and contrasts with geological features of ionic clay hosted REE deposits in China. Similarities include their apparent paragenesis and geometry, particularly their blanket morphology, sub-horizontal and gentle dips, and the common, but not exclusive, relationship with felsic basement terranes. However, Western Australian prospects are related to Upper Cretaceous to Eocene climates rather than more recent weathering under sub-tropical conditions. Whether the deposits can be developed to production remains unclear, with project economics tied closely to the efficacy of mineral processing technologies that target high REE extraction rates and recoveries.

Plate tectonic limits on the assembly of cratonic Australia

Continuous Proterozoic plate reconstructions consistent with plate geodynamics were designed to reconcile data, conceptual models, and human imagination as applied to the assembly of the Australian craton. The reconstructions were created using GPlates and considered available magmatic and metamorphic U-Pb zircon data. Two tectonic scenarios for the Paleo- to Mesoproterozoic assembly of the cratonic lithosphere of Australia were tested for their geodynamic and geochronological viability. The scenarios are founded on tectonic events that are relatively well-established and well-supported by data, but also with several periods and areas that are disputed mostly due to scarcity of data. For these scenarios, we test geodynamic criteria to explore a) which possibilities are better supported by our geodynamic criteria, and b) the implications of those models for the larger-scale plate tectonic setting. The time frame in this model set includes assembling two supercontinents, Nuna (1900–1800 Ma) and Rodinia (1300–900 Ma), and the transition between them. Model 1 presents a progressive assembly of the Australian continent between 2000 Ma and 1300 Ma, showing events in the West Australian Craton (WAC), the convergence of the WAC and the North Australian Craton (NAC), and then accretion at the margin of the South Australian Craton and NAC until collision in the Albany-Fraser Orogen (AFO). Model 2 suggests WAC was separate from the rest of the Australian Craton until 1300 Ma and all combined in one orogenic cycle. It proposes Andean-type subduction along the E and NE margins of the WAC for 40–60 million years as the means of final assembly. Both models are geodynamically plausible, yet each lacks sufficient data support to conclusively resolve the differences and deepen our understanding of the Proterozoic assembly of Australia.

Implications of high-grade metamorphism on detrital zircon data sets: A case study from the Fraser Zone, Western Australia

Detrital zircon grains provide a useful tool to determine the maximum age of sedimentary strata and can elucidate tectonic setting and basin evolution. However, high-grade metamorphism can overprint the mineral chemistry of the detrital cargo and reset both primary age and compositional information. Incomplete resetting of the U–Pb isotopic system, fluid alteration, and complex internal structures due to granulite-facies metamorphism makes dating and interpretation of the obtained zircon ages challenging. Here, we use trace element chemistry, U–Pb geochronology (LA-ICP-MS and SHRIMP), textural observations, and inclusion characterization from detrital zircon crystals in the Snowys Dam Formation (Albany–Fraser Orogen; AFO) to deconvolve its strongly overprinted sedimentary history, and to evaluate the likely geodynamic setting for basin genesis. Detrital zircons that survived metamorphic overprinting show that the main detrital load was sourced from Paleoproterozoic lithologies from neighbouring regions, with rare Archean Yilgarn crystals. The youngest detrital grains correlate with the age of plagiogranites in the Madura Province to the east of the AFO and imply a maximum depositional age of c. 1390 Ma, indicating that the Fraser Zone temporally correlates to an oceanic arc and thus best fits a back-arc basin setting. High-grade metamorphism during Stage I of the orogeny (c. 1330–1260 Ma) promoted recrystallization of pre-existing detrital zircons together with neoblastic growth of metamorphic zircon rims. At c. 1245 Ma, during early Stage II of the AFO, anatexis of the metasedimentary rocks produced rare neocrystallized magmatic zircon. Late-stage fluids are indicated by elevated LREE for these grains, which also contain a hydrous inclusion assemblage. High volume sampling techniques such as LA-ICP-MS often drilled through multiple age domains that can hinder geological interpretation. Thus, a lower volume sampling technique (e.g., SHRIMP) and verification with mineral chemistry is highly instructive in accurately resolving primary age groups in high-grade terrains.

A newly discovered 2030–2010 Ma magmatic suite records the dawn of Proterozoic extension on the southern margin of the Yilgarn Craton

Uranium–lead zircon geochronology, zircon lutetium–hafnium isotope analyses and whole-rock geochemistry are used to investigate concealed basement geology and the location of Archean–Paleoproterozoic terrane boundaries in the northeastern Albany–Fraser Orogen (AFO). We identify a new major component of the AFO, and the first compelling evidence for ca. 2030–2010 Ma A-type felsic magmatism in western Australia. Prior to this study, the oldest rocks in the AFO that formed by extension and reworking of the southern Yilgarn Craton margin had protolith emplacement ages of ca. 1810–1800 Ma; our results extend this record by ca. 200 Myr and provide new insights into the crustal and tectonic evolution of the AFO. This phase of magmatism marks the start of a long-lived and widespread Paleoproterozoic extensional regime that prevailed across the West Australian Craton (WAC) for ∼ 400 Myr. Extension in the WAC was coeval with localised magmatism, volcanism and volcano-sedimentary basin formation in the North and South Australian cratons. Globally, our findings also correlate with the onset of prolonged extension and the generation of a vast tract of oceanic crust around the periphery of Nuna. We posit a petrogenetic link between ca. 2030–2010 Ma felsic rocks in the AFO and age–isotope-equivalent ultramafic–mafic rocks in the eastern Yilgarn Craton that are located along-strike and are interpreted to reflect a possible far-field and late-stage magmatic expression of the ca. 2060–2055 Ma Bushveld superplume.

Apatite laser ablation Lu[sbnd]Hf geochronology: A new tool to date mafic rocks

Mafic rocks are the most common type of igneous rocks on Earth, however, constraining the crystallization age of mafic rocks can be challenging. Apatite is a common accessory phase in mafic rocks and is amenable to dating using the U–Pb system. However, the U–Pb system in apatite has a relatively low closure temperature (∼350°-550 °C) and is therefore prone to resetting by later thermal and metasomatic events. Here, a recently developed LuHf dating method using laser ablation reaction-cell mass spectrometry is applied to apatite from mafic rocks. The LuHf system in apatite has a higher closure temperature (∼650°-750 °C) compared to U–Pb, increasing the chances of obtaining primary crystallization ages. Furthermore, the laser-ablation method allows rapid data collection compared to traditional solution-based LuHf dating techniques. Four study areas were selected to compare the LuHf vs U–Pb systematics of apatite in mafic igneous rocks: the Paleoproterozoic Sudbury Igneous Complex (Canada), the Neoproterozoic Borborema Province (NE Brazil), the Paleoproterozoic Fennoscandian Shield (Finland), the Archean Yilgarn Craton and adjacent Mesoproterozoic Albany Fraser Orogen (Western Australia). For all analyzed samples that have apatite trace element compositions typical of an undisturbed primary mafic igneous lithology, the LuHf system retains primary igneous apatite crystallization ages, whereas the U–Pb system in the same grains often records isotopic disturbance or a cooling age. In few cases, the LuHf system has also been disturbed in response to recrystallization, however, such disturbance is readily detected with trace element data. Hence, this study demonstrates the potential of laser ablation apatite LuHf dating to obtain primary crystallization ages for otherwise difficult to date mafic rocks.

Isotopic investigations of the Nova-Bollinger Ni–Cu–Co deposit in the Fraser Zone, Albany-Fraser Orogen, Western Australia

The Fraser Zone of the Albany-Fraser Orogen of Western Australia contains an extensive suite of mafic and mafic–ultramafic intrusions. This includes the Nova-Bollinger intrusions (ca 1300 Ma), which host economic Ni-sulfide mineralisation, a prime example of magmatic sulfide liquids emplaced into the deep crust under ambient granulite facies conditions. Sulfur, Sr and Sm–Nd isotopes, and trace elements have been used to evaluate the role of crustal contamination of the parental magmas to Nova-Bollinger and other sulfide-bearing mafic–ultramafic bodies within the Fraser gabbro suite. All ore types have δ34S values between +1.6 and +8.0‰ VCDT with a positively skewed distribution and a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values near 0‰ VCDT. No significant Δ33S anomalism was observed. The initial 87Sr/86Sr and 147Sm/143Nd ratios of most of the Nova-Bollinger igneous rocks have an age of formation of 1300 Ma, which is similar to those of CHUR1300Ma and record limited contamination by lower continental crustal material. The country rocks of the Snowys Dam Formation, locally enriched in iron and sulfur, are likely candidates as crustal contaminants. These metasedimentary sulfides show similar ranges in values for both δ34S and Δ33S to the ores, but their Sr isotopic range is greater than observed in the host intrusions at Nova-Bollinger and in most of the other sampled intrusions. Bollinger globular sulfides with ɛSr values of 63.1 and 144.5 and ɛNd values of −5.7 and −1.8 are suggestive of localised crustal contamination from adjacent wall rock marbles, but otherwise high ɛSr country rocks do not appear to have been dominant contaminants. Bulk assimilation of the country rocks, as modelled in both silicate and sulfide portions of the system, of up to 30% contamination of a primitive mantle melt by siliceous country gneisses, would have been sufficient to produce the degree of crustal contamination recorded in the Nova-Bollinger ores, and more regionally within the Fraser gabbros. Beyond indications that S was crustally derived, it is not possible to identify a specific source, but Snowys Dam sediments are the most likely. There is no compelling evidence for Archean S in the ores, unsurprisingly given the mid-Proterozoic age. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies. KEY POINTS Nova-Bollinger Ni–Cu–Co sulfide ores have δ34S values between +1.6 and +8.0‰ VCDT, with a median value of +2.3‰ VCDT, anomalous relative to uncontaminated mantle values but similar to the range in the country rock metasediments. Values are consistent with equilibration of sulfide xenomelts at R factors of 200–700. There is no detectable mass-independent fractionation of δ33S in the Nova-Bollinger ores. The initial 87Sr/86Sr and 147Sm/143Nd ratios of most of the Nova-Bollinger igneous rocks at the age of formation of 1300 Ma are similar to those of CHUR1300 Ma and record contamination of a primitive mantle melt by up to 30% lower continental crustal material. Unusual globular ores at Bollinger have anomalous ɛSr indicating localised contamination from adjacent wall rock marble; however, this contamination is not pervasive through the orebody. It is likely that much of the contamination of the magma and generation of sulfide liquid took place during lateral flow through linked sill complexes within the deeply buried Snowys Dam metasediments, prior to the emplacement of the magmas to form the orebodies.

Preservation of basement architecture in zircon Hf isotope maps

AbstractZircon Hf isotopic mapping has been proposed to image large‐scale lithospheric architecture. Here we present zircon Hf isotope maps of the Proterozoic Albany–Fraser Orogen (AFO) in Western Australia, which records a protracted geological history in both extensional and compressional tectonic regimes. The spatial variation in Hf isotope composition of AFO granites is nearly orthogonal to the dominant crustal fabric. Although the 176Hf/177Hf compositions are too juvenile to represent direct melts of the Archaean basement, the variability in 176Hf/177Hf in these AFO magmatic rocks closely mimics the structure of the adjoining Yilgarn Craton. This orogen‐parallel heterogeneity is best illustrated during earliest Palaeoproterozoic magmatic events, which are characterised by low Sr/Y ratios, consistent with tectonic models involving relatively uniform extension along the craton margin during the Palaeoproterozoic. These results demonstrate that regional‐scale zircon Hf variability does not always image coeval lithospheric architecture, but may help illuminate deep basement structure in regions with complex deformation histories.

Enhanced hydrological cycle during Oceanic Anoxic Event 2 at southern high latitudes: New insights from IODP Site U1516

The Oceanic Anoxic Event 2 (OAE 2) represents one of the most profound global environmental disturbances of the Mesozoic Era, which was associated with a positive carbon isotope excursion due to widespread organic carbon burial. However, the character, evolutionary history, and exact driving mechanisms of OAE 2 are still much debated, exacerbated by the lack of records from climate-sensitive southern high latitudes. Here we present a multi-proxy record of a stratigraphically continuous succession spanning the late Cenomanian to early Turonian at the International Ocean Discovery Program Site U1516 in the Mentelle Basin off southwest Australia, which was located at southern high latitudes (~ 60–62°S) during the Late Cretaceous. Sedimentary records at Site U1516 allow the first detailed insights into source-to-sink processes in the Mentelle Basin and associated paleoenvironmental reconstruction in the southern Indian Ocean during OAE 2. Rare earth element abundances and clay-bound neodymium isotopes of the siliciclastic sediment fractions indicate that southwest Australia was the main detrital source of sediment at Site U1516 during OAE 2, with volcanogenic sediments representing only a minor component. A major provenance shift is observed across OAE 2, indicating an increased sediment contribution from distant sources (i.e., Yilgarn Craton and Albany-Fraser Orogen) relative to proximal sources (i.e., Perth Basin and Leeuwin Block) in southwest Australia, interpreted as reflecting a major reorganization of the drainage system due to enhanced fluvial runoff. Based on these findings, we infer that the OAE 2 interval showing the most prominent δ13C excursion was associated with an intensification of the hydrological cycle in the high-latitude southern hemisphere, with potential impacts on regional ocean chemistry and marine productivity. In particular, we suggest that enhanced terrigenous input and riverine nutrient export likely played a key role in controlling siliceous productivity at nearby ocean margins. As a consequence, the intense burial of organic matter (i.e., black shale) at Site U1516, which occurred episodically throughout OAE 2, possibly resulted from the combination of enhanced riverine-driven marine productivity and increased burial efficiency associated with the development of anoxic/euxinic bottom water conditions.

The intraplate Mangaroon Orogeny and its role in the Paleoproterozoic tectonic evolution of the Australian continent

The Mangaroon Orogeny (1695–1620 Ma) recorded in the Capricorn Orogen of Western Australia involved magmatism, deformation and metamorphism that are described mostly in the west of the orogen. New geophysical modelling indicates the orogeny also had significant influence in the eastern orogen from the interpreted presence of the magnetic Durlacher Supersuite in the subsurface that is temporally related to the Mangaroon Orogeny. Deposition of specific sedimentary and metasedimentary intervals in the Capricorn Orogen are also indicated to be in close temporal association with the Mangaroon Orogeny as suggested by detrital zircon geochronology. Existing data and new interpretations of provenance information and geophysical datasets have been integrated to propose a different model to previous authors in which the Mangaroon Orogeny involved prominent strike-slip motion, with longer phases of dextral transtension and significant but short-lived phases of sinistral transpression. The switches between sinistral and dextral phases coincide with geodynamic changes in other parts of the Australian proto continent that help explain these alternating kinematic phases. Changing forces from the plate boundary to the east of the Mount Isa-Calvert Superbasin and another to the south, e.g. the Albany-Fraser Orogen, generated variable stress fields. The model implies that the western and eastern Australian Cratons must have been joined by at least the Mangaroon Orogeny, which has an impact on paleo reconstructions of the Australian continent during the late Paleoproterozoic.

Compositions and Ni-Cu-PGE Tenors of Nova-Bollinger Ores with Implications for the Origin of Pt Anomalies in PGE-Poor Massive Sulfides

Abstract The Nova-Bollinger Ni-Cu-platinum group element (PGE) deposit in the Fraser zone of the Albany-Fraser orogen consists of two main orebodies, Nova and Bollinger, hosted by the same tube-shaped intrusion but having distinctly different Ni tenors of around 6.5 and 4.8 wt %, respectively. Nova is also higher in Pd, but Cu and Pt tenors are similar. Both deposits have very low PGE tenors, with average Pd concentrations of 110 ppb in massive sulfide at Bollinger and 136 ppb at Nova. The Nova and Bollinger orebodies show relatively little internal differentiation overall on deposit scale but show strong differentiation into chalcopyrite-rich and chalcopyrite-poor regions at a meter scale. This differentiation is more prevalent at Nova, where massive sulfide-filled vein arrays are more extensively developed, and in massive ores, particularly veins, than in net-textured ores. Net-textured and disseminated ores have on average Ni and Cu grades and tenors similar to those of massive, semimassive, and breccia ores in the same orebody but a smaller range of variation, largely due to a more limited extent of sulfide liquid fractionation and higher average concentrations of Pt and Pd than adjacent massive ores. Unusually for differentiated magmatic sulfides, there is no systematic positive correlation between Pt, Pd, and Cu. A partial explanation for the lack of a Pd-Cu correlation is that Pd was partitioned into peritectic pentlandite in the middle stages of sulfide liquid solidification. This explanation is not applicable to Pt, as Pt characteristically forms its own phases rather than residing in base metal sulfides. PGE tenors are very low in both orebodies, very similar to those observed in other Ni-Cu-Co sulfide ores in orogenic settings, notably the Savannah and Savannah North orebodies. This depletion is attributed to sulfide retention in the mantle source of the parent magmas rather than to previous fractional extraction of sulfide liquid in staging chambers or feeder networks. The higher Ni and Pd tenors at Nova are attributed to reworking and upgrading of precursor sulfide liquid originally deposited upstream at the Bollinger site. Replicate analyses of multiple jaw-crusher splits returned highly variable Pt and Au assays but much smaller relative errors in the other PGEs. The poor Pt and Au reproducibilities are attributed to nugget effects, explicable by much of the Pt and Au in the samples being present in sparse Pt- and Au-rich grains. This is principally true for Pt in massive rather than disseminated ores, accounting for a strong contrast in the distribution of Pt/Pd ratios between the two ore types. Numerical simulation suggests that Pt is predominantly resident in Pt-rich platinum group minerals with grain diameters of 100 µm or more and that at the low (<100 ppb) concentrations in these ores, this results in most assays significantly underreporting Pt. This is likely to be true in other low-PGE ores, such that apparent negative Pt anomalies in massive ores may in such cases be attributable to sampling artifacts.

An apatite to unravel petrogenic processes of the Nova-Bollinger Ni-Cu magmatic sulfide deposit, Western Australia

The trace element signature and U-Pb systematics of apatite are useful in identifying petrogenic processes and tracking crust through the ∼375–600 °C thermal interval. Furthermore, as apatite is more abundant in mafic rocks than other more conventional U-Pb chronometers (e.g. zircon), it may be better suited to constrain magmatic ore systems. This work focuses on a case study of apatite from the Nova-Bollinger Ni-Cu magmatic sulfide deposit in the Fraser Zone of the Albany Fraser Orogen, Western Australia. LA-ICP-MS analysis is used to characterise the U-Pb systematics and trace element chemistry of apatite across four rock types at Nova-Bollinger. These analyses are combined with whole-rock geochemistry and petrography to resolve the poorly understood thermal evolution of Nova-Bollinger after peak metamorphic conditions.Apatite U-Pb data indicate an initial 15 °C/my cooling rate to 1284 ± 12 Ma from peak metamorphic conditions of ∼850 °C at 1304 ± 22 Ma. After 1284 Ma the cooling rate dramatically slowed to 1.2 °C/my until at least 1225 ± 17 Ma. Most apatite retains an igneous trace element signature where thermal diffusion has not influenced trace element concentrations, suggesting that any exposure to temperatures above ∼670 °C was short-lived or trace element diffusion was slow. Furthermore, trace element chemistry discriminates apatite that crystallised from an evolved intercumulus disequilibrium liquid, and apatite that crystallised in equilibrium with the mineral assemblage of more fractionated rocks. The new results from this study highlight the change from a fast to slow cooling regime in this litho-tectonic zone, likely driven by ebbing regional magma emplacement. This new information enables better understanding of the tectono-magmatic evolution of the Proterozoic Nova-Bollinger deposit and the broader Fraser Zone.

Multiple modes of sulphur cycling within a mineralised orogen: A case study from the Fraser Zone, Western Australia

Sulphur isotope analysis can be used to fingerprint sulphur sources within mineral systems and trace sulphur cycles through geological time. In this work we present petrographic and in-situ sulphur isotope analyses of sulphide minerals from the Zn–Cu–Au–Ag Andromeda volcanogenic massive sulphide (VMS) prospect, Fraser Zone, Albany-Fraser Orogen, Australia. Sulphide mineral assemblages of pyrrhotite, pyrite, chalcopyrite and sphalerite occur within deformed lenses of sulphide breccia at Andromeda, with the presence of sphalerite contrasting with typical magmatic sulphide mineral assemblages recorded elsewhere within the Fraser Zone. The δ34S values of Andromeda sulphides range from 4.50‰ to 6.70‰, characteristic of Archaean and Proterozoic VMS-style deposits worldwide. The Δ33S data show evidence of mass-independent fractionation of sulphur (MIF-S), which indicates that sulphur from Archaean sediments was included within Andromeda sulphides, which formed in close proximity to the Fraser Shear Zone (FSZ). This contrasts with the findings of previous studies of sulphur isotopes within magmatic sulphides elsewhere in the Fraser Zone, which are distal to major shear zones, and in which sulphur derived from Archaean sediments is absent. The absence of a MIF-S signature from the magmatic sulphides is interpreted to reflect cycling of surface-derived sulphur through the ocean prior to incorporation into the magmatic sulphides. The presence of Archaean sedimentary sulphur at Andromeda is attributed to transport from a reservoir of sulphur recording MIF-S within the older Biranup Zone, which is in close proximity to the FSZ, a lithosphere-scale structure. Fluid movement through the FSZ is a viable mechanism that might have transferred sulphur recording MIF-S from Archaean sulphur-bearing pyrite in the Biranup Zone to sulphides at Andromeda. These findings support multiple modes of sulphur cycling within a single Proterozoic mineralised setting and imply that similar polymodal sulphur cycling might have occurred on other Proterozoic continental margins worldwide, with implications for mineralisation and global sulphur cycle systematics.

Early Neoproterozoic Accretion at the Northern Margin of the Chottanagpur Gneiss Complex, Eastern India

AbstractThe study based on structural mapping, monazite chemical age determinations and analyses of P–T pseudosections explores the hitherto undocumented accretion dynamics of the northern margin of the Chottanagpur Gneiss Complex (CGC), Eastern India. In the Gaya‐Chatra‐Koderma sector straddling the northern margin of the CGC, the CGC block comprises a shallow‐dipping carapace of 1.5–1.4 Ga and 1.0–0.9 Ga mylonitized granitoids, intrusive into 1.6–1.5 Ga anatectic basement gneisses, and supracrustal rocks inter‐leaved with recumbently folded gneisses. The carapace, structurally overlying the steeply dipping high‐T fabrics in the basement anatectic gneisses exposed in the tectonic windows, formed due to top‐to‐the‐north thrusting of the allochthonous CGC supracrustal rocks over the ∼1.7 Ga undeformed granitoids and steeply dipping greenschist‐facies metamorphic rocks (∼1.8 Ga) in the northern block. Continued N‐S shortening caused steeply dipping ENE‐striking sinistral and ESE‐striking dextral shear zones that locally modify former structures in the CGC foreland. P–T pseudosection analyses indicate that locally developed Early Neoproterozoic garnet‐bearing leucosomes in a hornblende‐biotite granitoid formed by lower crustal decompression melting, whereas the structurally overlying mica schist in the shallow‐dipping carapace experienced mid‐crustal loading. We suggest the accretion strain accommodated by the foreland thrusts were contemporaneous with extensional detachment zones that exposed lower parts of the thickened orogen. The tectonic events in the CGC appear to correlate poorly with those in the Capricorn Orogen and the Albany Fraser Orogen in Western Australia.

Cryptic excess argon in metamorphic biotite: Anomalously old 40Ar/39Ar plateau dates tested with Rb/Sr thermochronology and Ar diffusion modelling

In 40Ar/39Ar geochronology, excess Ar is 40Ar that does not derive from the in situ radioactive decay of 40K, or from the measurable input of atmospheric Ar, and results in increased daughter-parent ratios that correspond to anomalously old apparent dates without geological age significance. Excess 40Ar is commonly identified by a saddle-shaped 40Ar/39Ar degassing spectrum. However, biotite from the east Albany-Fraser Orogen of Western Australia contains excess 40Ar, but yields well-defined 40Ar/39Ar plateau dates, reproducible upon replicate analysis of biotite from the same sample. This “cryptic excess 40Ar” is inferred where plateau dates (1) are older than existing time constraints on cooling, such as U/Pb zircon ages for amphibolite to granulite facies metamorphism, and (2) vary between multiple samples from the same outcrop that have experienced the same thermal history. Rb/Sr geochronology is used to test the geologic significance of 40Ar/39Ar plateau dates, as the closure temperatures of both chronometers are similar, and the two chronometers are expected to yield similar cooling dates in an undisturbed system.Six Rb/Sr biotite-whole rock isochron ages from three outcrops yield a weighted mean age of 1133 ± 3 Ma (MSWD = 1.13, P = 0.34), and are interpreted to record post-orogenic cooling. 40Ar/39Ar plateau dates from the same samples are 31–394 Ma older, and in six of nine samples cannot be explained by simple monotonic cooling, by low-temperature biotite recrystallisation, or by alteration; instead, biotite contains cryptic excess 40Ar. Diffusion modelling of 40Ar/39Ar step-heating plateaus suggests that Ar, both excess and radiogenic, is homogeneously distributed within the crystal lattice, and was incorporated during biotite crystallisation. Biotite likely crystallised in a rock with a high partial pressure of Ar, possibly due to a high rate of radiogenic 40Ar generation in a K-rich lithology, together with a poorly-connected intergranular fluid network that inhibited Ar loss from the rock volume. This is supported by a homogeneous distribution of Ar at the cm scale, with reproducible biotite 40Ar/39Ar dates from each sample. Patchy fluid circulation during metamorphism led to the pervasive but heterogeneous distribution of excess 40Ar across outcrops.Due to the presence of the 40Ar/39Ar plateau, cryptic excess 40Ar is difficult to identify a priori, without comparison to additional geochronological constraints. Where cryptic excess 40Ar on a regional scale has been identified, such as in the eastern Biranup Zone of the Albany-Fraser Orogen, Rb/Sr geochronology may provide a more robust alternative to constrain the cooling path of rocks from high temperatures.

Regolith characterization and landscape evolution for geochemical exploration of the covered Yamarna Terrane, Western Australia

The easternmost Archaean Yamarna Terrane is the least explored terrane under transported cover in the Yilgarn Craton of Western Australia. Our aims are to understand the evolution of the paleolandscape, determine the weathering history and depositional processes, and the provenance, mineralogy and geochemistry of the Permian to Quaternary transported cover. This is a necessary step for selecting the appropriate sample media to vector towards buried mineral deposits in this terrane. The Permian cover was deposited in broad, structurally controlled valleys that were carved into and partly to completely truncate the previously weathered Archaean rocks. UPb dating of zircon grains indicates that the main provenance of the Permian cover on the Yamarna Terrane is the Albany-Fraser Orogen, with limited contribution from the Yilgarn Craton, Musgrave Province and Antarctica. The Permian cover of the paleovalleys wedges out laterally towards pre-existing, weathered basement paleohighs, capped by lateritic residuum. The Permian cover is deeply weathered, and consists of upper ferruginous (mottled, brecciated, pisolitic and nodular) and lower kaolinitic, poorly lithified sandstones and mudstones with a basal gravel at the bottom. Above the basal unconformity between the Permian and weathered Archaean, the Rb/Zr ratio decreases sharply and the Permian cover shows negative Eu anomalies (Eu/Eu*) compared to positive Eu/Eu* in the underlying weathered and fresh mafic Archaean rocks, suggesting distal felsic source rocks for the Permian cover. In places, where Au increases, the base of the Permian cover still shows positive Eu/Eu* anomalies, suggesting physical dispersion from proximal mineralized mafic rocks.The Post-Permian weathering and erosion processes produced an extensive layer of Au-bearing detrital ferruginous clasts derived from lateritic residuum and ferruginous pisoliths and nodules from the Permian cover. These ferruginous grains occur as loose or cemented by Fe oxides forming ferricrete. Ferricrete is not necessarily related to the underlying weathered dolerite compared to the lateritic residuum. This is shown, in some places, by a sharp decrease in the Ti/Zr ratio, high HFSE, negative Eu/Eu*, high LREE/HREE compared to the underlying weathered dolerite suggest an unconformity and physical dispersion from weathered felsic source rocks. The Au-enrichment in the ferricrete derived from the felsic rocks occurred after deposition as a result of hydromorphic dispersion by groundwater. The weathered Permian cover and Archaean rock are covered by aeolian sand within which Au is concentrated in authigenic pisoliths. These pisoliths were formed by periodical water saturation during seasonal water table fluctuation. The unconformities and the ferruginous regolith materials are relevant to Au exploration through cover in the Yamarna Terrane and are highly recommended for regional-scale systematic sampling.

Nova-Bollinger Ni-Cu Sulfide Ore Deposits, Fraser Zone, Western Australia: Petrogenesis of the Host Intrusions

Abstract The Nova-Bollinger Ni-Cu sulfide ore deposit is the first economic Ni-Cu-Co sulfide deposit to have been discovered in the Albany-Fraser orogen in Western Australia. The host rocks are mafic-ultramafic intrusive cumulates subdivided into two connected intrusions, designated Upper and Lower. The Upper Intrusion is bowl-shaped and modally layered with alternating peridotite and norite mesocumulate layers, with a Basal Series of dominantly orthocumulate mafic lithologies. The Lower Intrusion is a much thinner, semiconformable chonolith (flattened tube-shaped intrusion) consisting of mostly unlayered mafic to ultramafic orthocumulates. The Lower Intrusion hosts all the high-grade mineralization and most of the disseminated ores. A distinctive plagioclase-bearing lherzolite containing both orthopyroxene and olivine as cumulus phases is a characteristic of the Lower Intrusion and the Basal Series of the Upper. The intrusions differ slightly in olivine and spinel chemistry, the differences being largely attributable to the more orthocumulate character of the Lower Intrusion. Sector zoning in Cr content of pyroxenes is observed in the Lower Intrusion and in the lower marginal zone of the Upper and is attributed to crystallization under supercooled conditions. Symplectite pyroxene-spinel-amphibole coronas at olivine-plagioclase contacts are ubiquitous and are attributed to near-solidus peritectic reaction between olivine, plagioclase, and liquid during and after high-pressure emplacement, consistent with high Al contents in igneous pyroxenes and estimates of the peak regional metamorphism. Original cumulus olivines had compositions around Fo86 and were variably Ni depleted, interpreted as the result of preintrusion equilibration with sulfide liquid. The Upper and Lower Intrusion rocks represent cumulates from a similar parental magma, a high-Al tholeiite with MgO between 10 and 12%, low TiO2 (0.5–0.6%), and high Al2O3 (14–17%). Modeling using alphaMELTS indicates a primary water content of around 2 wt %. The cumulates of both intrusions were derived via multiple magma pulses of liquid-olivine-sulfide slurries with variable amounts of orthopyroxene emplaced into the deep crust at pressures of around 0.7 GPa during the peak of regional metamorphism. The intrusions developed initially as a bifurcating sill, the lower arm developing into the ore-bearing Lower Intrusion chonolith and the upper arm inflating into the cyclically layered Upper Intrusion.

Mafic intrusions in southwestern Australia related to supercontinent assembly or breakup?

Variably oriented dolerite intrusions outcrop in the Albany–Fraser Orogen along the south coast of Western Australia with previously unknown ages but where previous studies interpreted Mesoproterozoic to Cretaceous emplacement. Here, we place temporal constraints on seven mafic intrusions across ∼150 km of coast using zircon U–Pb, apatite U–Pb, and plagioclase 40Ar/39Ar geochronology, coupled with whole-rock major and trace-element geochemistry, that reveal late Mesoproterozoic to potentially Early Cretaceous crystallisation ages. Three intrusions metamorphosed to greenschist facies are likely associated with either the emplacement of the ca 1210 Ma Marnda Moorn large igneous province or Stage II Albany–Fraser Orogeny, both of which were associated with the assembly of Rodinia. Three unmetamorphosed dykes have (probable) Neoproterozoic to lower Cambrian emplacement ages, likely associated with the ca 550–500 Ma Kuunga Orogeny during Gondwana assembly. The final sill, also unmetamorphosed, strikes perpendicular to the other six intrusions, shows unusual Pb anomalies and contains inherited zircon that has been reset by a Permian or younger event, pointing towards magmatism in southwestern Australia during the breakup of Gondwana. The new results provide hitherto unrecognised mafic intrusive evidence for modification of Proterozoic crust, potentially associated with Rodinia assembly, Gondwana assembly and Gondwana breakup in southwestern Australia. KEY POINTS Variably oriented mafic dykes in southwest Australia are dated by zircon U–Pb, apatite U–Pb and plagioclase 40Ar/39Ar methods. The dykes are related to Rodinia assembly (ca 1200 Ma), Gondwana assembly (ca 550 Ma) and, probably, Gondwana breakup (ca 135 Ma). These new ages provide evidence for mafic activity clearly linked to the supercontinent cycle.

Reaction Coronas at Olivine–Plagioclase Contacts in Host Rocks from the Nova–Bollinger Ni–Cu–Co Deposit, Albany–Fraser Orogen, Western Australia: Evidence of a Magmatic to Metamorphic Continuum

Abstract A prominent feature of the cumulate rocks of the Nova intrusions is the presence of reaction coronas between olivine and plagioclase. A particular spatial arrangement of mineral textures, typical of olivine–plagioclase coronas in high-P troctolites and olivine gabbros worldwide, is almost universally observed: partially resorbed olivine with a cuspate margin, surrounded by orthopyroxene, surrounded by a zone of symplectite comprising hornblende + clinopyroxene, orthopyroxene and Cr-poor Al–(Fe, Mg) spinel, with a cuspate boundary against plagioclase. Where amphibole-free clinopyroxene–spinel symplectite is developed it is typically as an outer layer against plagioclase. The mineralogy and composition of these coronas has been revealed by a combination of scanning electron microscopy and synchrotron X-ray fluorescence microscopy. There is a close spatial association between reaction symplectites and Bowen reaction series sequences developed by reaction between cumulus phases and fractionated trapped interstitial melt, giving rise to local Cr and Ti enrichment within the coronas relative to olivine and plagioclase, accompanying enrichment in water recorded by stabilization of amphibole. These relationships imply that trapped liquid crystallization and corona formation were part of a process continuum, such that the coronas record mineral reactions close to the solidus temperature of the cumulates, initially developing in the presence of evolved hydrous interstitial liquid and continuing to develop in the solid state below the solidus. The symplectites owe their fine microstructure to diffusion-limited growth of the products of solid-state reactions below the solidus. The initial magmatic stage of corona formation records conditions of 0·76 and 0·96 GPa and around 1035 °C based on THERMOCALC pseudosections calculated for the bulk composition of the corona assemblage. Two-pyroxene assemblages within the symplectites record temperatures around 850–900 °C and pressures between 0·7 and 1·1 GPa. These pressure estimates are in good agreement with those obtained from regional metamorphism studies of the country rock paragneisses, confirming independent lines of evidence that the Nova intrusions were syn-metamorphic, emplaced under granulite-facies peak metamorphic conditions at crustal depths of 28–35 km. The unusually extensive development of the symplectites reflects the cooling history of the intrusions, which remained at temperatures close to solidus temperatures on the timescale of regional tectonic uplift and cooling of the deep roots of the orogen.

Improved full waveform moment tensor inversion of Cratonic intraplate earthquakes in southwest Australia

SUMMARYIn contrast to global observations in stable continental crust, the present-day orientation of the maximum horizontal stress in Western Australia is at a high angle to plate motion, suggesting that in addition to large-scale plate driving forces, local factors also play an important role in stress repartitioning. As a reliable stress indicator, full waveform moment tensor solutions are calculated for earthquakes that occurred between 2010 and 2018 in the southern Yilgarn Craton and the adjacent Albany-Fraser Orogen in southwestern Australia. Due to regional velocity heterogeneities in the crust, we produced two geographically distinct shear wave velocity models by combining published crustal velocity models with new ambient noise tomography results. We applied a full waveform inversion technique to 15 local earthquakes and obtained 10 robust results. Three of these events occurred near Lake Muir in the extreme south of the study area within the Albany-Fraser Orogen. The focal mechanism of the 16th September 2018 Lake Muir event is thrust; two ML≥ 4.0 aftershocks are normal and strike-slip. Our results are consistent with field observations, fault orientations inferred from aeromagnetic data and surface displacements mapped by Interferometric Synthetic Aperture Radar which are all consistent with reactivation of existing faults. The other seven solutions are in the southeastern Yilgarn Craton. These solutions show that the faulting mechanisms are predominantly thrust and strike-slip. This kinematic framework is consistent with previous studies that linked active seismicity in the Yilgarn Craton to the reactivation of the NNW–SSE oriented Neoarchean structures by an approximately E–W oriented regional stress field. Our results suggest that the kind of faulting that occurs in southwest Australia is critically dependent on the local geological structure. Thrust faulting is the dominant rupture mechanism, with some strike-slip faulting occurring on favourably oriented structures.