The Archean alkaline and related igneous rocks of Australia web map service depicts the spatial representation of the alkaline and related rocks of Archean age. All are from the Pilbara and Yilgarn Cratons of Western Australia.

<div>Komatiites are extinct volcanic rocks that formed by partial melting of 20-50% of their mantle source – a unique feature that allows us to understand the composition of the mantle in the early Earth. Due to their high temperature, komatiites incorporate proximal rock types on their way to, and on, the Earth’s surface. In this craton-scale study, we looked to use this property of komatiite magmas to track their interaction with the crust of the Yilgarn Craton. The results yielded Hf and Nd isotope arrays (Figure 1), that potentially have three components. The first likely represents the depleted mantle source of the magmas, and most plot in the region between +2 and +6 εHfi and 0 to +3.3 εNdi. The second source represents a more unradiogenic component, most likely 3.5-3.3 Ga continental crust (minimum). This component is more notable in the 2.8 and 2.9 Ga events with values trending to CHUR and negative εNd. The 2.8 Ga dataset, and particularly the komatiitic basalts from the Marda region, appear to show the most contamination with old crust. This is likely due to this area representing the old nucleus of the craton, as shown in Hf-isotope mapping. The final component represents an ultra-depleted source. Data from Ravensthorpe, Mt Clifford, and Wiluna show trends towards this source referred to as the Early Refractory Reservoir (ERR)(Nebel et al. 2014). We suggest that the 2.7 and 2.9 Ga plumes interacted with refractory Hadean plume residues, which constitute the ERR, within the Yilgarn lithosphere. Isotopic data on crustal rocks suggests the Yilgarn may have formed in the Hadean to Eoarchean, and hence the ERR could represent the residue of the Hadean crust generation process that formed the low Lu/Hf Jack Hills zircons. If correct, this suggests that the ERR survived for much longer than previously thought, as a rare component within the Earth’s oldest cratons.</div> This Abstract was submitted/presented to the 2023 6th International Archean Symposium (6IAS) 25 - 27 July (https://6ias.org/)

Following deep seismic reflection surveys on the Yilgarn and Pilbara cratons by Geoscience Australia with the Geological Survey of Western Australia and on the Superior Craton by the Canadian Lithoprobe program, these cratons are now some of the best surveyed Archean regions on Earth. We present seismic images that highlight how variations in crustal architecture relate to differences in Archean tectonic processes between cratons. All cratons are characterized by a mostly non-reflective 4–12 km-thick uppermost crust due to the presence of large granitoid plutons and gneissic domains. Localized regions of upper crustal seismic reflectivity are typically interpreted as supracrustal rocks and mafic sills or faults and shear zones. The middle and lower Archean crust contains variably complex geometries of relatively high amplitude reflections, though in some regions, such as the Eastern Goldfields Superterrane and the Abitibi Greenstone Belt, the lower crust appears less reflective than the middle crust. Crustal thicknesses vary from 30 km in the eastern Pilbara to 35–40 km across much of the Yilgarn and Superior, though thicknesses as great as 45–52 km occur locally in the latter two cratons. The characteristics of the Archean crust-mantle boundary, or Moho, which is commonly well-defined, differs between cratons, indicating significant variations in the tectonic processes that have driven the final stages of crustal evolution. Dipping reflections in the uppermost mantle linked to convergent crustal structures are interpreted as relict subduction scars. In the southern Superior Craton, Moho offsets and northdipping reflections in the middle and lower crust arose through successive underthrusting of Meso-Neoarchean island arcs, oceanic plateaux and microcontinental fragments, as they accreted against a pre-existing northern nucleus (e.g. North Caribou and Opatica terranes). Seismic reflection lines reveal a doubly vergent orogen above north-dipping mantle reflections that indicate subduction drive accretion. Post-orogenic crustal extension, which is inferred from crustal-scale normal shear zones and dropped greenstone belts, has not erased the original accretionary crustal architecture. In contrast, in the Yilgarn Craton interior, accretionary structures are less clear and there are no prominent offsets in the Moho. In the Youanmi Terrane, which represents the cratonic nucleus, a pervasive fabric of listric east-dipping mid-crustal reflections soles out into the upper part of subhorizontal lower crustal reflections. We interpret this reflective fabric to be the result of widespread crustal collapse during the late stage of craton evolution at c. 2.65–2.6 Ga that also produced subsidence of the upper crust. Though terrane boundaries can be identified in seismic data across the Eastern Goldfields Superterrane, these boundaries have commonly been modified by extension, which also overprinted any accretionary lower crustal structures, perhaps simultaneous with widespread intrusion of post-tectonic melts. Exhumation of moderately reflective, amphibolite to granulite facies crust in the Narryer Terrane above dipping mantle reflectors indicates that shortening along the northwestern edge of the Yilgarn Craton was subduction driven. In the eastern Pilbara Craton, shallowly dipping to subhorizontal reflections in the middle and lower crust preclude crustal-scale vertical tectonic movements and imply that the vertical displacements inferred from surface mapping were largely confined to the upper crust. <div>The abstract accompanies a talk the describes the architecture and and related tectonic processes of several Archean cratons based on reflection seismic interpretations. </div> This Abstract was submitted to & presented at the 2023 6th International Archean Symposium (6IAS) 25 - 27 July (https://6ias.org/)

<div>Archean greenstone belts are a vital window into the tectonostratigraphic processes that operated in the early Earth and the geodynamics that drove them. However, the majority of greenstone belts worldwide are highly-deformed, complicating geodynamic interpretations. The volcano-sedimentary sequence of the 2775-2690 Ma Fortescue Group is different in that it is largely undeformed, offering a unique insight into the architecture of greenstone sequences. In the Fortescue magmatic rocks, geochemical signatures that in deformed belts in the Superior or Yilgarn Cratons might have been interpreted as arc-like, are explained by contamination of rift-related mantle and plume-derived magmas with Pilbara basement crust; understanding the wider geological and structural setting allows a more complete interpretation.</div><div> However, contamination of Fortescue magmas by an enriched sub-continental mantle lithosphere (SCLM) is an alternative hypothesis to the crustal contamination model. If demonstrated, the addition of sediments and fluids to the SCLM, required to form enriched/metasomaytised SCLM, would suggest active subduction prior to the Neoarchean. To test this hypothesis, we collected Hf-O isotopic data on zircons from felsic volcanic rocks throughout the Fortescue Group; if the contamination had a subducted sedimentary component (δ18O>20‰), then the O-isotopes should record a heavy signature.</div><div> The results show that the ca. 2775 Ma Mt Roe Formation has εHfi from 0 to -5.6, and δ18OVSMOW of +4.8- +0.3‰, with the majority of values <+3‰. The ca. 2765 Ma Hardey Formation (mostly sediments) has highly unradiogenic εHfi of -5 to -9.4, and δ18O of +7.8- +6.6‰. The ca. 2730 Ma Boongal Formation displays similar values as for Mt Roe, with εHfi +1.9 to -5.5 and δ18O +3.0 to -0.6‰. The ca. 2720 Ma Tumbiana Formation shows the greatest range in εHfi from +4.9 to -4.6, with δ18O +7.1- +0.7‰, with the majority between +4.5 and +2.5‰. Data from the 2715 Ma Maddina Formation are more restricted, with εHfi between +4.0 and -0.1, and δ18O +5.0- +3.8‰. The youngest formation, the 2680 Ma Jeerinah Formation, has εHfi +2.3 to -6.2, and δ18O +5.1 to -2.1‰.</div><div> Importantly, these data provide little evidence of a cryptic enriched SCLM source in the Fortescue magmas. Furthermore, the dataset contains some of the lightest δ18O data known for Archean zircon, highlighting a ca. 100 Myr period of high-temperature magma-water interaction, with long-term continental emergence implied by the trend to meteoric δ18O compositions. The exception to this is the Hardey Formation, which may have formed via crustal anatexis in a period of reduced heat-flow between the 2775-2665 and 2730-2680 Ma events. Data from the other formations show a broad trend of increasing δ18O and εHf from 2775 to 2680 Ma. We suggest this represents the effects of progressive cratonic rifting, allowing mantle-derived magmas to reach the surface less impeded, and also a decreasing role of meteoric water in the rift zone as the sea invades. As a result, the εHf and δ18O data from the Fortescue Group represent the evolving nature of an Archean rift zone, from an emergent volcanic centre, to a submarine environment.</div><div><br></div>This Abstract was submitted/presented to the 2023 6th International Archean Symposium (6IAS) 25 - 27 July (https://6ias.org/)

<div>The Yilgarn Craton of Western Australia represents one of the largest pieces of Precambrian crust on Earth, and a key repository of information on the Meso-Neoarchean period. Understanding the crustal, tectonic, thermal, and chemical evolution of the craton is critical in placing these events into an accurate geological context, as well as developing holistic tectonic models for the Archean Earth. In this study, we collected a large U-Pb (420 collated samples) and Hf isotopic (2163 analyses) dataset on zircon to investigate the evolution of the craton. These data provide strong evidence for a Hadean-Eoarchean origin for the Yilgarn Craton from mafic crust at ca. 4000 Ma. This ancient cratonic nucleus was subsequently rifted, expanded and reworked by successive crustal growth events at ca. 3700 Ma, ca. 3300 Ma, 3000-2900 Ma, 2825-2800 Ma, and ca. 2730-2620 Ma. The <3050 Ma crustal growth events correlate broadly with known komatiite events, and patterns of craton evolution, revealed by Hf isotope time-slice mapping, image the periodic break-up of the Yilgarn proto-continent and the formation of rift-zones between the older crustal blocks. Crustal growth and new magmatic pulses were focused into these zones and at craton margins, resulting in continent growth via internal (rift-enabled) expansion, and peripheral (crustal extraction at craton margins) magmatism. Consequently, we interpret these major geodynamic processes to be analogous to plume-lid tectonics, where the majority of tonalite-trondhjemite-granodiorite (TTG) felsic crust, and later granitic crust, was formed by reworking of hydrated mafic rocks and TTGs, respectively, via a combination of infracrustal and/or drip-tectonic settings. While this process of crust formation and evolution is not necessarily restricted to a specific geodynamic system, we find limited direct evidence that subduction-like processes formed a major tectonic component, aside from re-docking the Narryer Terrane to the craton at ca. 2740 Ma. Overall, these 'rift-expansion' and 'craton margin' crustal growth process led to an intra-cratonic architecture of younger, juvenile terranes located internal and external to older, long-lived, reworked crustal blocks. This framework provided pathways that localized later magmas and fluids, driving the exceptional mineral endowment of the Yilgarn Craton.</div> This Abstract/Poster was submitted to & presented at the 2023 6th International Archean Symposium (6IAS) 25 - 27 July (https://6ias.org/)

<div>Lead isotopic data implies that Th and U were fractionated from one another in Earth's early history, however, the origin of this fractionation is poorly understood. We report new <em>in-situ</em> Pb isotope data from orthoclase in 144 granites, sampled across the Archean Yilgarn Craton to characterize its Pb isotope variability and evolution. Granite Pb isotope compositions reveal three Pb sources, a mantle derived Pb reservoir and two crustal Pb reservoirs, distinguished by their implied source 232Th/238U (κPb). High-κPb granites reflect sources with high 232Th/238U (~4.7) and are largely co-located with Eoarchean–Paleoarchean crust. The Pb isotope compositions of most granites, and those of VHMS and gold ores, define a mixing array between a mantle Pb source and a Th-rich Eoarchean-Paleoarchean source. Pb isotope modelling indicates that the high-κPb source rocks experienced Th/U fractionation at ~3.3 Ga. As Th/U fractionation in the Yilgarn Craton must have occurred before Earth’s atmosphere was oxygenated, subaerial weathering cannot explain the apparent differences in their geochemical behavior. Instead, the high Th/U source reflects Eoarchean–Paleoarchean rocks that experienced prior high-temperature metamorphism, partial melting, and melt loss in the presence of a Th-sequestering mineral like monazite. Archean Pb isotope variability thus has its origins in open-system high-temperature metamorphic processes responsible for the differentiation and stabilization of Earth’s continental crust.</div> <b>Citation:</b> M.I.H. Hartnady, C.L. Kirkand, S.P. Johnson, R.H. Smithies, L.S. Doucet, D.R. Mole; <i>Origin of Archean Pb isotope variability through open-system Paleoarchean crustal anatexis</i>. Geology 2023; doi: https://doi.org/10.1130/G51507.1

<div>Archean crustal evolution, and its tectonic paradigm, can be directly linked to the evolution of the mantle, the hydrosphere-atmosphere, oxygenation of the Earth, and the formation and storage of ore deposits. Hence, it is vital to understand the evolution of the early crust if we are to understand our planet’s evolution as well as transformational events in its history.</div><div> The collection of vast amounts of isotopic data, especially U-Pb, Sm-Nd, Lu-Hf, and δ18O, over the last 30 years, has significantly advanced our understanding of crustal processes and their timing. However, we rarely look at these data in a spatial context. This study aims to constrain the time-space evolution of the south-east Superior Craton, Canada, by mapping the zircon Hf-O isotopes and trace element data from 148 Archean magmatic rocks (6340 total analyses).</div><div> In Lu-Hf space, the dataset demonstrates the highly juvenile nature of this region, with the majority of values between εHfi +6 and +2. When plotted spatially, the most juvenile data (+4 to +6 εHfi) delineate an E-W oriented zone, broadly in-line and sub-parallel to the Cadillac-Larder Lake and Porcupine-Destor structures. Surrounding this juvenile region is less juvenile crust (0 to +3 εHf). Corresponding δ18O values show that light to mantle-like data (3.0-5.6‰) correlate with the most juvenile crust imaged by the εHf, with heavier δ18O (5.8-7.5‰) plotting to the south, east and west of this zone. Zircon trace element proxies for hydration (Eu/Eu*), oxidation (ΔFMQ using Ti, Ce, U), and continental vs. oceanic origin (Ui/Yb) replicate the pattern observed in the Lu-Hf and δ18O. This suggests that, broadly, the SE Superior consists of a central E-W orientated juvenile zone consisting of the most reduced, least hydrated, least continental, and most high-temperature hydrothermally-altered crust. This zone is surrounded by crust which is more hydrated, oxidised, has a greater supracrustal δ18O component, and is slightly less juvenile. The major ore systems of the Abitibi subprovince, including VMS, gold and komatiite-hosted Ni-Cu-PGE systems, fall within the E-W highly-juvenile zone.</div><div> Current tectonic models for this region of the Superior Craton range from (1) long-lived Neoarchean subduction across the whole Abitibi tectono-thermal ‘event’ (2750-<2695 Ma) – ‘horizontal’ tectonics; and (2) a variety of non-arc processes such as plume-related crustal overthickening (i.e., oceanic plateau), sagduction/drip tectonics, and subcretion, amongst others – ‘vertical’ tectonics. Models combining arc and non-arc processes have also been suggested (i.e., plume-arc interaction), and our data broadly support a combined model. We propose the E-W zone delineated by the various geochemical data represents a paleo-rift zone, driven by ambient mantle or mantle plume processes. The dry, reduced, oceanic character of the zone appears to preclude an arc or back-arc setting prior to ca. 2.7 Ga. However, temporal changes in hydration, oxidation, and the increased heavy δ18O component at ca. 2.7 Ga suggest a major geodynamic shift, potentially marking the onset of subduction and associated compression. This is contribution 2020-050 of the Mineral Exploration Research Centre (MERC) Metal Earth project.</div><div> This Abstract was submitted/presented to the 2023 6th International Archean Symposium (6IAS) 25 - 27 July (https://6ias.org/)

The Archean alkaline and related igneous rocks of Australia web map service depicts the spatial representation of the alkaline and related rocks of Archean age. All are from the Pilbara and Yilgarn Cratons of Western Australia.

<div>The interpretation of AusAEM airborne electromagnetic (AEM) survey conductivity sections in the Canning Basin region delineates the geo-electrical features that correspond to major chronostratigraphic boundaries, and captures detailed stratigraphic information associated with these boundaries. This interpretation forms part of an assessment of the underground hydrogen storage potential of salt features in the Canning Basin region based on integration and interpretation of AEM and other geological and geophysical datasets. A main aim of this work was to interpret the AEM to develop a regional understanding of the near-surface stratigraphy and structural geology. This regional geological framework was complimented by the identification and assessment of possible near-surface salt-related structures, as underground salt bodies have been identified as potential underground hydrogen storage sites. This study interpreted over 20,000 line kilometres of 20&nbsp;km nominally line-spaced AusAEM conductivity sections, covering an area approximately 450,000 km2 to a depth of approximately 500&nbsp;m in northwest Western Australia. These conductivity sections were integrated and interpreted with other geological and geophysical datasets, such as boreholes, potential fields, surface and basement geology maps, and seismic interpretations. This interpretation produced approximately 110,000 depth estimate points or 4,000 3D line segments, each attributed with high-quality geometric, stratigraphic, and ancillary data. The depth estimate points are formatted for Geoscience Australia’s Estimates of Geological and Geophysical Surfaces database, the national repository for formatted depth estimate points. Despite these interpretations being collected to support exploration of salt features for hydrogen storage, they are also intended for use in a wide range of other disciplines, such as mineral, energy and groundwater resource exploration, environmental management, subsurface mapping, tectonic evolution studies, and cover thickness, prospectivity, and economic modelling. Therefore, these interpretations will benefit government, industry and academia interested in the geology of the Canning Basin region.</div>

<div>Convergent margins are a hallmark feature of modern style plate tectonics. One expression of their operation is metallogenesis, which therefore may yield important insights into secular changes in styles of convergence and subduction. A global comparison of metallogenesis along convergent margins of over 20 well-endowed provinces indicates a consistent and systematic progression of mineral deposit types. We term this progression the convergent margin metallogenic cycle (CMMC). </div><div> This CMMC mirrors convergent margin evolution. Each metallogenic cycle begins with the formation of porphyry copper deposits and/or volcanic-hosted massive sulphide deposits, associated with arc construction and back arc basin formation, respectively. When the convergent margin transitions into contraction/orogenesis due to processes such as accretion, flattening of subduction, or continent-continent collision, mineral deposits that form include orogenic gold and structurally hosted base metal deposits. Post-contractional extension is marked by the formation of intrusion related rare metal (tin, tungsten, molybdenum) and gold deposits, pegmatites, and alkaline porphyry copper deposits, closing the CMMC. </div><div> Our analysis of the metallogenic record reveals that prior to ~3 Ga, metallogenesis is episodic and non-systematic, with CMMCs not recognised. From the mid- to late Mesoarchean onwards, CMMCs are observed in all provinces analysed, and display systematic trends through time: the Meso- to Neoarchean metallogenic provinces are characterized by a single metallogenic cycle, whereas in the Paleo- to Mesoproterozoic provinces, both single and multiple metallogenic cycles occur. From the middle Neoproterozoic onwards multiple metallogenic cycles are the rule. This evolution is accompanied by an increase in the duration of metallogenesis, ranging from ~100 to 180 million years in the Meso- to Neoarchean and 220 to more than 400 million years since the late Proterozoic.&nbsp;</div><div> We interpret these trends to reflect secular changes in tectonic processes and Earth evolution. The emergence of CMMCs from ~3 Ga provides independent evidence for the operation of some early form of subduction since this time. The fact that CMMCs are recognized in all provinces of mid-Meso- to Neoarchean age suggests that subduction was the common <em>modus operandi</em> rather than an exception. The first appearance of multiple metallogenic cycles in the Paleoproterozoic may reflect the strengthening of cratonic margins by tectonothermal maturation since formation in the Archean. Long-lived metallogenesis and multiple metallogenic cycles in the Neoproterozoic and Phanerozoic are linked to deep-slab break-off, or modern, subduction in which the internal strength of the subducting slab allows maintenance of slab coherency.&nbsp;&nbsp;</div><div> This Abstract was submitted/presented to the 2023 6th International Archean Symposium (6IAS) 25 - 27 July (https://6ias.org/)