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Highlights • Summary of internal lithological variations of coarse-crystalline mafic bodies from Archean greenstone belts in the Eastern Goldfields Superterrane (Australia) • Comparison of the world famous Golden Mile Dolerite (1700 t Au) to a number of other mineralised and unmineralised bodies. • Comparison of coarse-crystalline mafic bodies to ponded lavas from the literature. • Present an emplacement model for differentiated mafic sills. Abstract Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substantial proportion of Archean greenstone belts and are hosts to major orogenic gold deposits, yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence. We present results from drill core logging of several late Archean coarsely crystalline packages from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, including the Golden Mile Dolerite. These large mafic bodies comprise mostly medium- to coarsely crystalline mafic rocks, but also include a variety of other products of differentiation. The bodies range from ∼75 to 460 m thick and extend for tens of kilometers, are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy. As such, this study excludes mafic dykes and extremely thick and internally complex bodies such as the Bushveld Complex. From base to top, the internal stratigraphy generally consists of a lower unit of aphyric basalt, dolerite/gabbro and peridotite ± pyroxenite, then a voluminous middle dolerite/gabbro, and an upper quartz dolerite/gabbro, and a topmost aphyric basalt. Granophyric veins and domains are common in the upper third of the mafic body. Contacts are gradational between all lithofacies, with the exception of granophric veins, which have sharp contacts. Within this overall stratigraphic framework there is variability in the relative thickness of lithofacies, number of lithofacies, as well as repetition of some lithofacies (several examples also include basalt in the interior). In well exposed examples, the key evidence for distinguishing intrusions from extrusions is provided by the top contact. However, the top contact from these examples is ambiguous. To overcome this gap, we review the character of well-studied thick ponded lavas from the literature. Most aspects of the internal character are not unique to intrusions, although the maximum groundmass grain size (excluding granophyric veins) of the Archean examples (>3 mm) is significantly coarser than that for any known ponded lavas (≪1 mm). The ability to distinguish intrusions from extrusions has important implications for reconstructing Archean greenstone stratigraphy and mineral exploration. [ABSTRACT FROM AUTHOR]

Highlights • Major, trace and 142–143Nd of mafic rocks from WAC. • 143Nd errochron age of 3300 Ma. • No 142Nd anomaly in any of the mafic samples analysed. • Trace element and 142–143Nd data unable to distinguish geodynamical regime. Abstract Determining the extent of mantle mixing during the Hadean-Archean is an integral factor in our understanding of early geodynamic processes as well as the speed and degree of mantle homogenisation. If a dynamic convecting mantle was the predominant regime it would be expected that early formed isotopic anomalies would be quickly homogenised, whilst in a stagnant lid regime these anomalies might last for hundreds of millions of years. Archean terranes are dominated by felsic tonalite, trondhjemite and granodiorite rocks, as such these rocks have been the main rock type studied, even though they are not formed by primary mantle melts. Therefore, in this study, we have focused on mafic rock samples as tracers of primary mantle melting, from the relatively unknown West African craton. The mafic rocks have undergone metamorphism to granulite facies with the majority of samples retrogressed to amphibolite facies. Major element compositions reveal that the protolith was tholeiitic basalt. The majority of trace elements display limited mobility, as primary correlations with Zr are still present. Trace element patterns are typical for Archean mafic rocks, with Nb-Ta and Ti depletions. The meta-basalts yield a 147Sm-143Nd errorchron with an age of 3300 ± 300 Ma, with initial εNd of +4.4. The Nd compositions indicate that the meta-basalts have experienced minimal crustal assimilation and retain primary geochemical attributes. A positive initial εNd for the meta-basalts suggests that the magmatic protolith evolved from an already differentiated source. Nevertheless, 142Nd compositions are within error of modern day terrestrial, as such, the protolith for these samples either did not experience any early differentiation, or that mantle convection had homogenised any anomalous signal. These results highlight the lack of large (>±8 µ142Nd) 142Nd anomalies detected from outside the North Atlantic and Superior Cratons, suggesting that Archean mantle dynamics was complex, with different processes occurring on different parts of Earth, similar to today. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Structural signatures for 2.45 Ga rifting were preserved. • Rifting utilized pre-existing structures of the Archaean basement. • Pull-apart structures controlled deposition and structural evolution of supracrustal belts. • Segmentation of the Archaean crust explained. • Active mode of rifting favoured. Abstract Constraints from restoration of minor rift-related faults preserved from subsequent Palaeoproterozoic overprint along the southern margin of the Peräpohja Belt, northern Fennoscandia indicate local NE-SW extension directions prevailed at the time of 2.44 Ga rifting of the Archaean continent. This palaeostress field is attributed to pull-apart setting of the Peräpohja Belt at a left overstepping zone of major sinistral N-S trending deformation zones. Consequently, the variably overprinted NW-SE trending structures of the Peräpohja Belt, including the NW-SE central graben were originally generated as normal faults. The proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step-over zone between strike-slip faults. By contrast, the faults bounding E-W to ENE-WSW trending basement horsts are the result of reactivation of older basement structures underlying the pull-apart basin. Based on the results, we infer that the final break-up of the Archaean continent in northern Fennoscandia utilized the 2.44 Ga rift-related NW-SE trending crustal anisotropy (parallel with dyke swarms), with pre-existing Archaean N-S trending structures reactivated as second-order shear zones in-between. The voluminous syn-rift 2.5–2.4 Ga and later pulses of mafic magmatism (2.32–1.98 Ga) are considered indicative of active-type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the Superior, Hearne and Karelian (-Kola) cratons. The presented model supports the continuity of the Archaean craton beyond the Caledonian Orogen towards north-west and explains its segmentation, suggesting a model for the localization of the Palaeoproterozoic supracrustal belts of Northern Fennoscandia. Moreover, this paper provides an approach to understand the basement-cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits. [ABSTRACT FROM AUTHOR]

Highlights: [•] We present a depth profile of carbonation in the 2.6Ga seafloor basalts. [•] Carbon isotope ratio of calcite systematically changes with depth. [•] Degree of carbonation is lower than those in the Early and Middle Archean equivalents. [•] Seawater CO2 level likely decreased from the Middle Archean to the Late Archean. [ABSTRACT FROM AUTHOR]

Abstract: Many continental growth models have been proposed over the years to explain geological and geochemical data. Amongst these data, the evolution of the 87Sr/86Sr of marine carbonates has been used as an argument in favour of delayed continental growth models and of a Neoarchean pulse in continental growth. This interpretation requires that continental freeboard and continental hypsometry have remained constant throughout Earth''s history. However, recent studies suggest that Archean sea levels were higher, and Archean relief lower, than present-day ones. To assess the validity of the evolution of the 87Sr/86Sr of marine carbonates as a proxy for continental growth, we have developed a model that evaluates the co-evolution of mantle temperature, continental hypsometry, sea level, ridge depth, emerged area of continental crust and the 87Sr/86Sr of ocean water as a function of continental growth. We show that Archean sea levels were between ∼500m and ∼1800m higher than present-day ones, that Archean mid-oceanic ridges were between ∼700m and ∼1900m shallower than present-day ones, and that the Archean emerged land area was less than ∼4% of Earth''s area. Importantly, the evolution of the area of emerged land, contrary to that of sea level and ridge depth, barely depends on continental growth models. This suggests that the evolution of surface geochemical proxies for felsic lithologies does not constrain continental growth. In particular, the evolution of the 87Sr/86Sr of ocean water predicted for an early continental growth model is in broad agreement with the 87Sr/86Sr data on marine carbonates when changes in continental freeboard and continental hypsometry are taken into account. We propose that the Neoarchean shift in the 87Sr/86Sr of marine carbonates recorded the emergence of the continents rather than a pulse in continental growth. Since the evolution of other geochemical indicators for felsic crust used as proxies for continental growth is equally well explained by continental emergence, we suggest that there could be no need for delayed continental growth models. [Copyright &y& Elsevier]

Abstract: The 2.8–2.6Ga Bulawayan Supergroup of the Zimbabwe craton is inferred to be a passive margin- and plume-related conformable sequence comprising (in part) the felsic volcanic Koodoovale→sedimentary Manjeri→komatiitic Reliance→basaltic Zeederbergs units. The Reliance unit is spatially associated with subjacent komatiitic sills of the Mashaba Ultramafic Suite, the inferred subvolcanic intrusive phase of the Reliance flow-fields. We report U–Pb zircon ages for several units. The Pangani sill, in the Filabusi belt, yielded an age of 2912Ma, indicating it may belong to the 3.0–2.9Ga Belingwean Supergroup. The Koodoovale unit appears to range in age from 2866 to 2788Ma. Inferred syn-eruptive mass flow deposits within the Manjeri basinal shallow-water sub-facies in the Midlands SE belt yield ages of 2877 and 2870Ma. Sills of the Mashaba Ultramafic Suite, together with a previous result of 2743Ma for the Mashaba Igneous Complex, yield a mean age of 2746Ma, which we propose as a proxy age for the Reliance unit and for the Reliance komatiitic event as a whole. The 2.80Ga age for the top of the Koodoovale unit in the western Filabusi and Gweru NW belts confirms (with greater stratigraphic control) earlier precise geochronology and provides an age for the start of sedimentation in the overlying Manjeri basinal deep-water sub-facies. The 2.79Ga felsic sample from within the Reliance unit in the Shangani N belt is likely from a Koodoovale-derived, post-eruptive mass flow deposit. In the Midlands SE belt, the new felsic rock ages support the conformable relationship between the uppermost Koodoovale unit and overlying Manjeri unit but place the transition at c. 2.87Ga, significantly earlier than in belts further west. The new ages for the Koodoovale unit and the revised age for the Reliance unit introduce new perspectives on the development of the Bulawayan Supergroup: (1) The Koodoovale-Manjeri transition is diachronous across the Zimbabwe craton, (2) the duration of Manjeri sedimentation varied from 50Ma to 120Ma in the basinal deep-water and shallow-water sub-facies, respectively, to 80Ma in the platformal facies in the east where sedimentation is estimated to have begun at 2.83Ga; such prolonged sedimentation is controversial but can be explained by extended periods of non-deposition in the basinal deep-water sub-facies, (3) using all published precise ages, Reliance-Zeederbergs volcanism may have lasted 70Ma (2.75–2.68Ga), much longer than Phanerozoic plume systems, and (4) the previously proposed correlation and late Archean ‘nearest neighbour’ relationship between the Zimbabwe and Slave cratons are provided further support. [Copyright &y& Elsevier]

Abstract: Podiform chromitites in ophiolite usually are interpreted as the crystallization products of mafic magmas, contemporaneous with the generation of the ophiolite at mid-oceanic ridges or back-arc spreading centres. However, the real ages of the chromitites can rarely be determined directly, because their chemistry provides few opportunities for geochronology. Here we employ the 187Re–188Os decay system (N-TIMS) to date the chromitite and a cross-cutting troctolite in the Sartohay ophiolite, northwestern China, and we have dated zircons separated from the troctolite by SHRIMP U–Pb methods. Inherited zircons from the troctolite yield a lower intercept age of 0.48±0.08Ga and an upper intercept age of 2.28±0.11Ga; two concordant grains give ages >2.4Ga. Whole-rock Re–Os data for the troctolites and chromites plot between 2.45Ga and 0.43Ga reference isochrons. Plots of TMA vs 187Os/188Os suggest mixing between ultra-depleted chromitite and suprachondritic troctolite, mainly affecting the 187Os/188Os of the troctolites; subsets of troctolite samples scatter around reference isochrons with ages of 0.4–0.5Ga. The zircon data and the Re/Os data are consistent with published Sm–Nd evidence that the troctolites, and hence the Sartohay ophiolite, were formed in Palaeozoic time. However, the 187Os/188Os compositions of the chromites (0.1109±4 to 0.1249±5), give Neo-Proterozoic to Paleo-Archean model ages (T MA =0.8–3.5Ga), indicating extraction from the primitive mantle as early as 3.5Ga, some 3Ga before the formation of the Sartohay ophiolite. A Re–Os apparent isochron age (2.45Ga), the old T Ma model ages and the ancient zircon ages (>2.4Ga) all are consistent with part formation of the Sartohay chromitite in Archean time, and then reworked in the Palaeozoic time. We suggest that a volume of early Archean depleted mantle remained within the Siberian lithospheric mantle for 2.5–3.0Ga; it later became portions of the mantle wedge above the Paleozoic subduction zone of the Paleo-Asian Ocean, and ultimately was incorporated into the overthrust ophiolite. [Copyright &y& Elsevier]

Abstract: This study reports new REE+Y and 3-isotope sulfur data for Archean banded iron formation and volcaniclastic rocks with cherty bed tops in the Neoarchean Abitibi greenstone belt of Canada. The data were analyzed with a view to better constrain Neoarchean ocean chemistry, atmospheric conditions prevalent during weathering and transport, the development of Algoma-type banded iron formation and the overall process of stratigraphic development of greenstone belts. The Abitibi greenstone belt consists of 7 mafic to felsic volcanic cycles, each capped by a sedimentary interface zone consisting of chemical and minor clastic metasediments. We concentrated sampling on the iron formation capping the ca. 2730Ma Deloro assemblage as it occurs over a wide area (300km×600km) and because there is a substantial depositional gap prior to deposition of the overlying volcanic rocks. Volcaniclastic rocks within the ca. 2710Ma Tisdale assemblage were also sampled. Chemical analyses focussed on the SiO2-rich portion of the samples and were conducted by laser ablation ICP-MS. In situ analysis of S isotopes was obtained for pyrite by ion probe. REE data display four types of patterns: (1) hydrothermally influenced marine hydrogenous sediment, (2) contaminated, hydrothermally influenced marine hydrogenous sediment, (3) hydrothermally dominated patterns, and (4) replacement patterns indicating silicification of precursor volcanic units. Contamination and/or the presence of non-chert components were documented with Th, U and Zr content. Non-chert components were defined as: (1) phosphates that led to elevated Th/U, (2) clastic detritus leading to flat shale normalized REE patterns, and (3) volcanic detritus leading to elevated values for Zr. No meaningful difference in REE+Y geochemistry as a function of elevated Th/U was found implying that phosphates have the same REE patterns as the host chert. The cherts within banded iron formation exhibited stratigraphic variation in several localities, progressing from replacement chemistry (flat REE profile) at the base, hydrogenous sediment geochemistry (positive La, Gd, Y/Ho anomalies) in the middle part and hydrothermal patterns (depleted LREE, elevated positive Eu anomalies) in the upper part. The upper parts of some units also display +Ce anomalies possibly reflective of more oxygenated water also supported by S isotope data. A consistent increase in Pr/Yb in the upper parts of units is postulated to reflect shallowing upward of depositional depth to an unknown extent but not above storm wave base. A number of samples with flat REE patterns lacking La and Gd anomalies represent hydrothermal deposition with the largest Eu/Eu* values recorded for Archean iron formation. The main contribution of the Abitibi banded iron formations is that they provide a deeper water perspective on Archean ocean chemistry. The resulting picture is that of slow BIF accumulation, a generally strong hydrothermal input of REE and complex oceanic cycling, possibly involving a chemocline above the sampled water depth. The new sulfur isotope data show a greater extent of mass independent fractionation than previously recorded for the Neoarchean, with Δ33S ranging from −1.4 to +4.2‰. The strongly MIF positive source (Δ33S=+4‰) was apparently similar to Paleoarchean values. [Copyright &y& Elsevier]

Abstract: Numerous studies using various proxies have been conducted to ascertain the temperature of the Archean ocean. If, as some of these investigations indicate, the Archean ocean was relatively hot, with a cooling trend through time to the Phanerozoic, this would have had consequences for sediment erosion, transport and deposition. Substantially higher surface temperatures would cause the viscosity of water, an important variable in defining flow hydrodynamics, to be lowered. In particular, less viscous water would have affected the development of current ripples. These are dependent on the existence of the laminar sublayer, as the sweeps of turbulent eddies impinging on this layer build small mounds that evolve into ripples. The laminar sublayer thins at higher temperatures allowing sand grains of smaller sizes to destroy it. At modern surface temperatures current ripples cannot form in coarse-grained sands, and if a hotter ocean existed in the Archean it would have inhibited current ripple formation in medium-grained sands, and possibly even fine-grained sands. Examination of numerous Neoarchean, deep-water, turbiditic successions in western Superior Province led to the realization that ripple cross-lamination is rare in all these units. As the formation of current ripples is dependent on: the flow depositing sediment of the proper grainsize, velocities in the appropriate range, and existence of a viscous sublayer, and the first two prerequisites were met during deposition from these decelerating turbidity currents, the existence of the viscous sublayer during the majority of flows must be questioned. The occurrence of current ripple cross-lamination in well exposed turbidites of the Neoarchean, McKellar Harbour Formation was quantified and found to be substantially less than the abundance of ripple cross-lamination present in Phanerozoic turbidites, indicating that the Neoarchean deep ocean may have been considerably warmer than the deep ocean of today. [Copyright &y& Elsevier]

Abstract: Subaqueous high-silica rhyolite in Prosser Township, southern Abitibi Subprovince, Ontario, consists of intercalated, thin (<15m; low aspect ratio<0.3) aphyric lavas with brown felsic tuffs, overlain by a pyroclastic pumice- and spatter-rich tuff breccia and are capped by quartz–porphyritic rhyolite. The quartz–porphyritic flows contain ∼10% phenocrysts whereas ‘aphyric’ flows have only sparse phenocrysts, typically 1%. The aphyric flows are characterized by features that are typical of subaqueous rhyolite flows, including well developed flow-top breccia, basal peperite, flow-banding, and hyaloclastite dominated breccia. Mass balance calculations and textural analysis demonstrate that the high-silica aphyric rhyolites were originally glassy having been recrystallized to a pseudo-spherulitic quartz–albite mixture which was subsequently sericitized and chloritized. The REE were moderately mobile (10–40%) whereas the high field strength elements were less mobile (<10%). The major elements within the flow interior are shown to have been relatively immobile and represent a reasonable estimate of the protolith composition. The viscosity of the aphyric lava protolith was estimated to be 7.8×106 Pas based on an eruption temperature of ∼850°C calculated from measurements using the Ti-in-zircon saturation thermometer. This viscosity estimate is similar to the calculated viscosity (∼4×107 Pas) of the subaerial Námshraun rhyolite flow, Iceland. The Prosser and Námshraun rhyolites have comparable dimensions and eruption temperature. It is concluded that the low aspect ratio of the Prosser Township flows may have been controlled by a number of environmental factors including paleo-slope, magma rheology, eruption rate and the insulative effects of the flow breccia. [Copyright &y& Elsevier]

Abstract: The Precambrian terranes of north east Africa are still poorly known. It is generally believed that the oldest rocks in the region are the charnockitic gneisses of the Uweinat massif, located at the triple junction between Libya–Egypt–Sudan, because they have yielded Rb–Sr ages around 2.6Ga and Nd TDM model ages around 3.0–3.2Ga. Here we confirm that these rocks are indeed Archean with SHRIMP U–Pb zircon ages as old as 3.0Ga, but we also report older rocks to the east, in the neighboring Gebel Kamil region. This area contains a metaigneous complex formed of tonalite–trondhjemite–granite (TTG) and gabbro-diorite (GbD) gneisses with a whole-rock Sm–Nd isochron age of 3.16±0.16Ga, average Nd T CR of 3.17±0.04Ga, and average ɛNd(3.2Ga) of 3.4±0.3. The oldest TTG gneisses, which are also the oldest rocks found to date in north east Africa, contain large magmatic zircons with SHRIMP U–Pb crystallization ages peaking at 3.22±0.04Ga, closely matching the Sm–Nd and the Nd T CR model ages. These ages are interpreted to represent arc-magmas produced between 3.1Ga and 3.3Ga. Other TTGs have younger Archean zircon ages that form a continuum between 3.1Ga and 2.55Ga, with three peaks at around 2.97Ga, 2.85Ga, and 2.6–2.7Ga; these rocks were apparently generated from the older TTGs during repeated events of crustal recycling. The crust was stable from 2.55Ga to 2.0Ga, when an intense thermal event generated rims of variable thickness over the Archean zircons. Neither the reworking from 3.1Ga to 2.55Ga, nor the metamorphism at 2.0Ga involved the addition of juvenile material to the crust of this area, which behaved as an almost closed system from 3.1Ga to 0.75Ga, when the intrusion of Pan-African I-type granitoids with a juvenile component began. [Copyright &y& Elsevier]

Abstract: The St Ives Goldfield of Western Australia hosts high grade gold mineralization in a variety of lithologies within ∼2.7Ga greenstone belts. The low salinity CO2-bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones, or from external magmatic sources, and independently sourced abiogenic-CH4 has been suggested as an important control on gold mineralization. In order to better constrain possible fluid sources, we applied a combination of He–Ne–Ar–Kr–Xe isotope and Cl–Br–I analyses to quartz and carbonate veins containing H2O–CO2 fluid inclusions, pyrite±magnetite/hematite; and quartz veins containing CH4-rich fluid inclusions, pyrrhotite±pyrite. Samples containing H2O–CO2 fluid inclusions have higher Br/Cl and I/Cl values and less radiogenic noble gas isotope signatures than suggested for CH4-rich fluid inclusions. The samples containing CH4 fluid inclusions have maximum 40Ar/36Ar of ∼50,000; 20Ne/22Ne of ≤9.8 and maximum 21Ne/22Ne of ∼0.56. In contrast, H2O–CO2 fluid inclusions have maximum 40Ar/36Ar of only ∼20,000 and maximum 21Ne/22Ne of ∼0.14. The use of 40Ar–39Ar methodology to measure K simultaneously with 40Ar/36Ar, enables us to show the highest 40Ar/36Ar ratios are representative of the fluids trapped at ∼2.65Ga, and have not been significantly modified by post-entrapment 40ArR production (40ArR =radiogenic 40Ar). Furthermore, the 40Ar/36Ar ratio is correlated with the 21Ne/22Ne and 136Xe/130Xe ratios, demonstrating that other heavy noble gas isotope ratios are also close to their original composition. The highly radiogenic noble gas isotope signatures are unlikely to have been generated within the greenstone host-rocks, because seawater altered-volcanic rocks have high abundances of non-radiogenic atmospheric noble gas isotopes. In contrast, the strongly radiogenic noble gas isotope signatures are likely to have been generated in ancient basement rocks underlying the greenstones at ∼2.65Ga. The combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced H2O–CO2 and CH4 localized high-grade gold mineralization. We suggest that the fluids were intimately associated with crustal magmatism in the St Ives Goldfield and that abiogenic CH4 could be an important moderator of redox processes in other crustal environments. [Copyright &y& Elsevier]

Abstract: Field and laboratory investigations of a 2690.83Ma (207Pb/206Pb age of Saganaga Tonalite) unconformity exposed in outcrop in northeastern Minnesota, USA, reveal evidence for development of a deep paleoweathering profile with geochemical biosignatures consistent with the presence of microbial communities and weakly oxygenated conditions. Weathering profiles are characterized by a 5–50m thick regolith that consists of saprolitized Saganaga Tonalite and Paulson Lake succession basaltic metavolcanic rocks retaining rock structure, which is cross-cut by a major unconformity surface marking development of a successor basin infilled with alluvial deposits. The regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal (saprock) as well as extrabasinal detritus. Thin-section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars, exfoliation and chloritization of biotite, and weathering of Fe-Mg silicates and Cu-Fe sulfides; weathering of Fe-Ti oxides was relatively less intense than for other minerals and evidence was found for precipitation of Fe oxides. Geochemical analyses of the tonalite, assuming immobile TiO2 during weathering (τ Ti,j ), show depletion of SiO2, Al2O3, Na2O, CaO, MgO, and MnO, and to a lesser degree of K2O, relative to least-weathered parent materials. Significant Fe was lost from the tonalite. A paleoatmospheric pCO2 of 10–50 times PAL is estimated based on geochemical mass-balance of the tonalite profile and assuming a formation time of 50–500Kyr. Interpretations of metabasalt paleoweathering are complicated by additions of sediment to the profile and extensive diagenetic carbonate (dolomite) overprinting. Patterns of release of P and Fe and retention of Y and Cu in tonalite are consistent with recent laboratory experiments of granite weathering, and with the presence of acidic conditions in the presence of organic ligands (produced, for example, by a primitive microbial community) during weathering. Cu metal in the profile may document lower pO2 than present day at the surface. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45–2.22Ga) in Ontario, Canada, reveal general similarities in paleoweathering with our study, as well as important differences related to lower paleoatmospheric pO2 and terrestrial biosignature for the older Minnesota profile. A falling water table in the Alpine Lake locality is presumed to have promoted formation of this gossan-like deep-weathering system that extends to 50-m depth. [Copyright &y& Elsevier]

Abstract: The eastern part of the North China Craton in eastern Hebei Province contains metamorphosed and deformed Neoarchaean to earliest Palaeoproterozoic rocks (∼2550–2490Ma) with some older Archaean rocks. Numerous precise U–Pb zircon ages, structural observations, Nd and Hf isotopic data and whole rock geochemistry (our new data with reassessment and integration of previous work) show that there is not a single protracted event at the end of the Archaean lasting ∼60 million years, but that from east to west there are several separate events with their own unique character. The eastern coastal region is dominated by weakly deformed 2530–2510Ma granites with subordinate granodiorites, diorites and magnesian-gabbros. This suite has inclusions of older 2550–2540Ma plutonic rocks. ∼80km inland to the west, there are amphibolite facies biotite-rich schists, lenses of BIF, siliceous fuchsite bearing rocks with 3880–3540Ma zircons, peridotites and orthogneisses, all intruded by younger granites and monzonites. Three amphibolite–granulite facies gneisses and schists with volcano-sedimentary protoliths have 2548–2534Ma igneous zircon populations, rare inheritance back to >3600Ma and 2506±6Ma zircon equated with granulite facies metamorphism. Older tonalitic to granitic polyphase orthogneisses contain 3287±11Ma to ∼2940Ma igneous components. Late kinematic high Fe/Mg gabbros, monzonites and granites have ages of 2499±7 to 2491±13Ma. 160–100km inland, gneissose 2550–2530Ma quartz-diorite to tonalitic rocks occur as intrusions into, or intercalations with, mafic rocks. These were affected by granulite facies metamorphism but then widely retrogressed. Abundant syn-granulite facies neosome and later shear zones disrupt the early geological relationships. The granulite facies metamorphism is dated at 2503±5Ma from several samples, including a syn-granulite facies pegmatite. The Neoarchaean igneous rocks show marked negative Nb, Ti anomalies, LIL enrichment and enrichment of the light REE relative to the heavy REE across the entire ultramafic, basaltic, tonalitic and granitic compositional spectrum: 2550–2530Ma tonalites and quartz diorites are most important in the west and 2530–2510Ma granites are most important in the east. Their composition resembles those formed at various stages of magmatic arc evolution at a convergent plate boundary, rather than being plume products. The eastern Hebei rocks thus formed in a complex arc with distinct pulses of plutonism at 2550–2540 and 2530–2510Ma, with local incorporation of older continental crust. The 2500–2490Ma thermal event with its associated higher Fe/Mg magmas involved interaction of underplated fractionated gabbroic melt with deep crustal melts, possibly during extension of an already ∼50 million years old arc. [Copyright &y& Elsevier]

Abstract: The Kola Superdeep Borehole, the deepest ever drilled into the Earth''s crust with a depth of 12.2km, is located in the Kola Peninsula, Russia, and consists of high-grade metasedimentary and metaigneous lithologies ranging in age from Archean to Proterozoic. Metabasic rocks and amphibolites from the borehole have δ 18O values in the range from +4.4‰ to +9.2‰ and δD values in the range from −73‰ to −40‰, gneisses and a migmatite have δ 18O values in the range from +7.8‰ to +9.4‰ and δD values in the range from −63‰ to −44‰, and metasedimentary rocks show large variations in δ 18O values from +6.3‰ to +15‰, and have δD values in the range from −57‰ to −33‰. The isotopic compositions of the deep borehole samples are identical with their surface equivalents, indicating that the processes of emplacement and exposure of deep crustal rocks have not affected their H and O isotopic compositions in marked contrast to what has been observed for crust in tectonically active areas. Fluid compositions calculated from the δ 18O and δD values of coexisting amphibole, feldspar, biotite and garnet indicate that two isotopically distinct fluids have interacted with the Kola Superdeep Borehole rocks. One fluid has isotopic compositions consistent with locally rock-buffered regional metamorphic fluids, and the other fluid has compositions expected for Paleoproterozoic seawater. The δ 18O values of co-existing amphibole–feldspar and biotite–garnet pairs in amphibolites and quartzite have apparent equilibration temperatures from ca. 500°C to 800°C, which are in good agreement with temperatures determined by cation exchange geothermometry and co-existing assemblages. Only rocks associated with two major faults and the thrust zone that marks the Proterozoic–Archean boundary indicate isotopic exchange with regional metamorphic fluids circulating along these structures. The stable isotopic compositions of the Kola Superdeep Borehole rocks indicate that there was interaction of the rocks with channelized fluids moving along major faults, but there was no pervasive fluid flow during metamorphism. [Copyright &y& Elsevier]

Abstract: Well-preserved alkaline basalts, bearing relict aegirine, leucite and nepheline mineralogy, are stratigraphically associated with high-Mg basalts in the Neoarchean Penakacherla greenstone belt, eastern Dharwar craton, India. Alkaline basalts (Mg #∼0.70–0.58) are enriched in alkalies (K2O+Na2O∼7wt.%), and TiO2 (2.3–2.1wt.%), and exhibit fractionated REE patterns with (La/Yb)N ranging from 23 to 29. On primitive mantle normalized diagrams they record a downturn from Ce to Th, small negative Nb anomalies relative to La, and Zr/Hf ratios higher than the primitive mantle value, in common with compositional characteristics of Phanerozoic alkaline ocean island basalts (OIB). Some interelement ratios are intermediate between EM1- and HIMU–OIB. Associated high-Mg basalts (MgO 17.2–9.2wt.%) have comparatively lower TiO2 (1.2–0.50wt.%), and flat HREE patterns with slight depletion in LREE, and small positive Nb anomalies. These basalts are compositionally similar to tholeiitic basalts associated with komatiites in many Neoarchean greenstone terranes. Two samples have the conjunction of Nb/Th<8 and fractionated LREE [(La/Yb)N 2.60–2.63] consistent with crustal contamination. On the global array of Phanerozoic to Recent ocean island basalts, in SiO2 versus Nb/Y coordinates, alkaline basalts plot with counterparts from Aitutaki and Heard, whereas high-Mg basalts plot near Iceland tholeiites. Alkaline basalts plot with OIB, and high-Mg basalts near N-MORB, on the Th/Yb versus Nb/Yb MORB-OIB array of Phanerozoic intraplate basalts; accordingly the mantle components of that array were established in the 2.7Ga mantle asthenosphere. Alkaline basalts are rare in Archean volcanic sequences. This occurrence of alkaline basalts indicates subduction, recycling, and incubation of Mesoarchaean oceanic and continental crust in the mantle, and generation of high-Mg and alkaline basalts from a mantle plume at 2.7Ga, possibly analogous to counterparts of Iceland. The mantle plume likely erupted at a thin craton margin, given flat HREE of most high-Mg basalts. [Copyright &y& Elsevier]

Abstract: In situ U–Pb–Hf isotope analyses of detrital zircons from the Alwar quartzite, the oldest member of the North Delhi fold belt in the Aravalli mountain range, indicate that the basement of NW India shares a common Archaean-Palaeoproterozoic evolutionary history, and is a consequence of several crust-forming and crust-reworking events between 3.70 and 1.71 Ga. This is suggested by a zircon age spectrum with most ages at 1.87-1.71 Ga (75%), minor age peaks at 2.5-2.2 and 2.9-2.7 Ga, and Palaeoarchaean ages of 3.27-3.23 Ga. Lu–Hf isotope data reveal that the oldest crust, either in the Aravalli mountain range or in its hinterland, was formed from a depleted mantle source at around 3.7 Ga. Furthermore, the data also indicate that the hinterland of the Alwar sediments was affected by several periods of new “juvenile” crust formation at 3.25, 2.89, 2.67, 2.51, and 1.87-1.80 Ga, while crustal reworking was predominant at 2.62-2.45, 2.2 and 1.87-1.77 Ga. Zircons with xenocrystic cores and metamorphic overgrowths, additionally suggest that the source region was affected by high-grade (?) metamorphic events at 2.5 and 2.2 Ga. Based on our new data, in conjunction with field relationships and existing zircon ages in the Archaean basement of NW India, it can be concluded that the Palaeo- to Neoarchaean detritus (3.3-2.5 Ga) in the Alwar quartzite stems from proximal sources. The predominant late Palaeoproterozoic zircon detritus (1.87-1.71 Ga) was supplied from a nearby basement, which was affected by subduction-accretion tectonics at around 1.85 Ga, and by a first, perhaps post-collisional rift-related phase at 1.77 Ga. The latter event is unrecognized so far, and predates a second phase of rift-related A-type magmatism at 1.72-1.70 Ga. The youngest detrital zircon age of ca. 1.71 Ga suggests that the sedimentation and metamorphism in the North Delhi fold belt postdate the second phase of rift-related A-type magmatic activity. [Copyright &y& Elsevier]

Abstract: This paper discusses the exhumation history of Palaeoproterozoic high-grade units from the Lapland Granulite Belt (northern Finland) through a reappraisal of previous structural and metamorphic data and a new structural analysis. The Lapland Granulite Belt is located between two cratons (Inari Craton and South Lapland Craton). It is marked by a major thrusting event associated with the development of an inverse metamorphic gradient within the underlying Tanaelv and Karasjok Belts. The review of published metamorphic data combined with our new structural analysis suggests that the exhumation history of the Lapland Granulite Belt was accommodated by the development of coeval basal thrusting and early normal-sense shear-zones, and later by distributed erosion. We propose also that the early exhumation stages of the Lapland granulite unit were associated with heating of both the overlying Inari Craton and the underlying South Lapland Craton. The occurrence of coeval thrusts and normal-sense shear-zones combined with the important role of distributed erosion during the final exhumation stages suggest that this Palaeoproterozoic collisional belt may represent an intermediate stage between ultra-hot accretionary orogens involving weak lithospheres and modern orogens involving stiff lithospheres (mixed-hot orogen of ). Our analysis emphasizes that the transition between ancient-type orogens and modern-type ones was gradual in space and time, being firstly dependent on the thermal and rheological state of the sub-continental lithospheric mantle of the lithospheres involved in compression. [Copyright &y& Elsevier]

Abstract: The volcanic stratigraphy of the upper Onverwacht Suite in the southeastern part of the Paleoarchean Barberton Greenstone Belt has been investigated in 18 sections through parts of the Hooggenoeg, Kromberg, and lower Mendon Complexes. The ca. 2700m thick volcanic sequence of the lowest tectonostratigraphic unit – the Hooggenoeg Complex (HC) – can be subdivided into 9 stratigraphic units representing major eruptive phases, each separated by silicified sedimentary and volcaniclastic rocks. The thicknesses of the units vary from <100m to ∼700m. Some units are wedge-shaped and die out over distances of a few kilometre along strike. The volcanic rocks are predominantly basaltic lavas, with minor basaltic komatiite and komatiite in the middle part of the HC. The basaltic lavas of the overlying tectonostratigraphic unit, the Kromberg Complex (KC), occur as screens within intrusives, or are in tectonic contact with adjacent rocks. The basal part (Ncakini section) of the tectonostratigraphically uppermost Mendon Complex (MC), consists of Mg-rich basalt lava. The lavas of the HC and KC are predominantly pillowed and massive flows, whereas those of the Ncakini section are exclusively massive. The massive and pillowed lava are commonly organized in cyclic units from 3m to 32m thick, and consisting of massive lava or large pillows that are succeeded by progressively smaller pillows. Cyclic units are inferred to have resulted from individual eruptions with decreasing rates of effusion. The pillow lavas of both the HC and KC contain <3–5% vesicles indicating eruption at greater depths than ∼2000m. In general vesicular pillows occur in the lower part of both the HC and KC, and non-vesicular (and variolitic) lavas are found at the top, suggesting increasing water depths, and hence eruption in a subsiding basin. The chert layers within the HC include silicified tuffs derived from explosive subaerial or shallow-marine eruptions distant from the deep-water lavas of the HC and KC. [Copyright &y& Elsevier]

Abstract: New U–Pb-data from two supracrustal belts enclosed in Archaean crust in the West Troms Basement Complex (WTBC) of North Norway document the presence of volcanic, subvolcanic and sedimentary rocks formed in the pre-orogenic period of extension and basin formation after 2.0Ga. The Mjelde-Skorelvvatn belt of highly deformed supracrustal rocks is dominated by mafic metavolcanic rocks, including metagabbro dated by zircon at 1992±2Ma. These mafic rocks are temporal correlatives of a number of occurrences in Fennoscandia and Laurentia such as the Pilguyarvi unit in the Pechenga structure in the Kola peninsula and the Purtuniq unit in northern Labrador. The Torsnes belt contains siliciclastic and metavolcanic mafic rocks unconformably overlying Neoarchaean crust. The indicated maximum deposition age of the Torsnes belt is given by the age of 1970±14Ma for the youngest detrital zircon. The absence of 1.95Ga and younger detrital zircons, which are abundant in Svecofennian siliciclastic sequences in Fennoscandia, suggests that the Torsnes belt is a distinct pre-orogenic deposit, and we interpret it as a marginal continental or intracontinental siliciclastic-volcanic basin. The new data from the WTBC can be placed in the context of continental divergence and sea-floor formation in the period between 2.0Ga and the commencement of orogeny in Fennoscandia and Laurentia at 1.92Ga. [Copyright &y& Elsevier]

Abstract: Charno–enderbitic granitoids in the Karelia craton of the Fennoscandian shield have been studied paleomagnetically. The characteristic remanence component has a steep negative inclination and is interpreted to record magnetization at a maximum age of 2684±2Ma. Consistently stable results were obtained from 12 sites in the Koitere area, corresponding to regions with high positive magnetic anomalies and high remanence intensities. Petrographic studies, coupled with rock magnetic investigations, indicate that the remanence resides in fine SD/PSD magnetite grains formed during Neoarchean clinopyroxene alteration. Cross-cutting vertical/subvertical Paleoproterozoic dolerite dykes suggest that the Koitere granitoids are in their original orientations and were not affected by Svecofennian deformation at ca. 1.9–1.8Ga. The Koitere granitoids have an opposite polarity compared to the steep positive inclination remanence direction of the previously studied ca. 2.63Ga Varpaisjärvi enderbites and granulites. The data from Koitere and Varpaisjärvi imply that at ca. 2.7–2.6Ga the Karelia craton was located at high latitudes of 80–60°, whereas previous paleomagnetic data from ca. 2.5Ga formations in the Vodlozero terrane in NW Russia indicate a near-equatorial position. Comparison of paleomagnetic data from the Koitere and Varpaisjärvi granulite-grade rocks with rocks of similar age in the Superior craton shows that at ca. 2.7–2.6Ga the Superior and Karelia cratons were located at high latitudes and in close proximity, although the present data cannot demonstrate that the cratons were amalgamated. However, during the Archean–Paleoproterozoic transition at ca. 2.50Ga both cratons experienced significant rotation and drifting to near-equatorial paleolatitudes, suggesting that the Superior and Karelia cratons may have been attached at that time. [Copyright &y& Elsevier]

Abstract: The two Palaeoarchaean volcano-sedimentary complexes of the Buck Ridge (Barberton Greenstone Belt, South Africa) and Kittys Gap (Coppin Gap Greenstone Belt, East Pilbara, Australia) have a similar geological setting and age (∼3.45Ga). The predominantly volcaniclastic sediments are concentrated at the top of these complexes, and experienced thorough, (very) early diagenetic silicification. In many places the silicification process has led to excellent preservation of the primary sedimentary structures. Elsewhere it has resulted in their obliteration or replacement by diagenetic structures. The Buck Ridge chert forms a regressive-transgressive succession, deposited around base level, with lacustrine and littoral marine facies. Deposition of the Kittys Gap Chert was also close to base level, almost exclusively subaqueous, with tidal influence and a regressive sequential trend. In both volcano-sedimentary complexes, these low-energy sediments are juxtaposed with high-energy breccia pods and layers, with often a high Fe-oxide content. The breccias are interpreted as being the result of explosive hydrothermal activity. Sedimentation was strongly controlled by normal faulting. [ABSTRACT FROM AUTHOR]

Abstract: Geometrical patterns of Paleoproterozoic dyke swarms in the Superior craton, North America, and paleomagnetic studies of those dykes, both indicate relative motion across the Kapuskasing Structural Zone (KSZ) that divides the craton into eastern and western sectors. Previous work has optimized the amount of vertical-axis rotation necessary to bring the dyke trends and paleomagnetic remanence declinations into alignment, yet such calculations are not kinematically viable in a plate-tectonic framework. Here we subdivide the Superior craton into two internally rigid subplates and calculate Euler parameters that optimally group the paleomagnetic remanence data from six dyke swarms with ages between 2470 and 2070Ma. Our dataset includes 59 sites from the Matachewan dykes for which directional results are reported for the first time. Our preferred restoration of the eastern Superior subprovince relative to the western subprovince is around an Euler pole at 51°N, 85°W, with a rotation angle of 14° CCW. Although we do not include data from the KSZ in our rigid-subplate calculations, we can align its dyke strikes by applying a 23° CCW distributed shear that preserves line length of all dykes pinned to the western margin. Our model predicts approximately 90km of dextral transpressional displacement at ca. 1900Ma, about half of which is accommodated by distributed strain within the KSZ, and the other half by oblique lateral thrusting (with NE-vergence) across the Ivanhoe Lake shear zone. We produce a combined apparent polar wander path for the early Paleoproterozoic Superior craton that incorporates data from both western and eastern subplates, and that can be rotated to either of the subplates’ reference frames for the purposes of Archean-Paleoproterozoic supercraton reconstructions. [ABSTRACT FROM AUTHOR]

Abstract: The Archean basement in the northeastern part of the Kaapvaal craton is intruded by a large number of mafic dykes, defining three major dyke swarms, which collectively appear to fan out from the Bushveld Complex. Herein we present U–Pb baddeleyite ages for two of these dyke swarms, the northwest trending Badplaas Dyke Swarm and the east-west trending Rykoppies Dyke Swarm, and infer their correlation with tectonic events in the Kaapvaal craton. We also present a U–Pb baddeleyite age for a noritic phase of the Marginal Zone of the Rustenburg Layered Suite (Bushveld Complex). The age of the Badplaas swarm is determined from two dolerites dated at 2965.9±0.7Ma and 2967.0±1.1Ma. These ages coincide with units of the Nsuze Group lavas (2967–2985Ma), which constitute the world''s oldest preserved rift basin, and suggest that dykes of this swarm are feeders to basaltic units of this group. Similarly, the E–W trending Rykoppies swarm has earlier been interpreted as a potential feeder system to the Bushveld Complex. However, the emplacement ages of six dolerites fall in the range 2.66–2.68Ga, thus ∼600Myr earlier than the intrusion of the Bushveld Complex (herein dated at 2057.7±1.6Ma). Rather, these ages coincide with the Allanridge Formation at the uppermost part of the Ventersdorp Supergroup as well as volcanic rocks of the “protobasinal” sequences preserved at the base of the overlying Transvaal Basin. The Rykoppies Dyke Swarm probably marks the initial stages of rifting of the Transvaal Basin and reflects a major shift from a NW–SE to an E–W trending tectonic setting. The origin of the Rykoppies Dyke Swarm can be linked either to prolonged mantle plume activity or to the onset of back-arc extension associated with south-directed subduction of oceanic lithosphere in a compressional setting along the northern margin of the Kaapvaal craton. [Copyright &y& Elsevier]

Abstract: 431 oriented samples were collected from 27 dolerite dykes at 17 sites, belonging to 2.95, 2.65, and 1.90Ga swarms, that trend SE, E and NE, respectively from the Bushveld Igneous Complex into the eastern Kaapvaal Craton (ages determined by ; Olsson in ). Samples were analyzed for paleomagnetism and also anisotropy of magnetic susceptibility (AMS). For the 2.95Ga SE-trending dykes high temperature/coercivity ‘P’ component has unblocking temperatures up to 590°C and coercivity 40–90mT and demonstrate SSW declination and intermediate positive inclination. Based on positive contact and conglomerate tests we argue for a primary origin of this component. The paleopole (BAD), calculated from ‘P’ component, does not correspond to any of the previously obtained Archean–Paleoproterozoic paleopoles for the Kaapvaal Craton, and represents a new key pole for 2.95Ga. The high-coercivity ‘H’ component for the 2.65Ga-old E-trending dykes has a SSW declination and steep positive inclination. Paleomagnetic pole (RYK), recalculated from this component, is close to the paleopoles, obtained by and for 2.78Ga Ventersdorp volcanics. The third group, NE-trending dykes of the 1.90Ga Black Hill swarm demonstrate an ‘M’ component with dual polarity high-coercivity component with SSE-declination and negative intermediate inclination. The paleopole (BHD), calculated from this component is close to the 1.87Ga pole of the Kaapvaal Craton obtained by . Overprint directions include a very well developed thermo-chemical overprint (Dec=329° Inc=−36°), which is believed to be associated with a ∼0.18Ga regional ‘Karoo’ thermal event. [ABSTRACT FROM AUTHOR]

Abstract: The Archaean block of southern Greenland constitutes the core of the North Atlantic craton (NAC) and is host to a large number of Precambrian mafic intrusions and dyke swarms, many of which are regionally extensive but poorly dated. For southern West Greenland, we present a U–Pb zircon age of 2990±13Ma for the Amikoq mafic–ultramafic layered intrusion (Fiskefjord area) and four baddeleyite U–Pb ages of Precambrian dolerite dykes. Specifically, a dyke located SE of Ameralik Fjord is dated at 2499±2Ma, similar to a previously reported 40Ar/39Ar age of a dyke in the Kangâmiut area. For these and related intrusions of ca. 2.5Ga age in southern West Greenland, we propose the name Kilaarsarfik dykes. Three WNW-trending dykes of the MD3 swarm yield ages of 2050±2Ma, 2041±3Ma and 2029±3Ma. A similar U–Pb baddeleyite age of 2045±2Ma is also presented for a SE-trending dolerite (Iglusuataliksuak dyke) in the Nain Province, the rifted western block of the NAC in Labrador. We speculate that the MD3 dykes and age-equivalent NNE-trending Kangâmiut dykes of southern West Greenland, together with the Iglusuataliksuak dyke (after closure of the Labrador Sea) represent components of a single, areally extensive, radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the North Atlantic craton. Comparison of the magmatic ‘barcodes’ from the Nain and Greenland portions of the North Atlantic craton with the established record from the north-eastern Superior craton shows matches at 2500Ma, 2214Ma, 2050–2030Ma and 1960–1950Ma. We use these new age constraints, together with orientations of the dyke swarms, to offer a preliminary reconstruction of the North Atlantic craton near the north-eastern margin of the Superior craton during the latest Archaean and early Palaeoproterozoic, possibly with the Core Zone craton of eastern Canada intervening. [Copyright &y& Elsevier]

Abstract: A new structural evolution consisting of both extensional and contractional events has been defined for the St Ives Goldfield in the south-central Kalgoorlie Terrane of the eastern Yilgarn Craton in Western Australia. These events shaped the development of the fault architecture, which controlled the location of the regional anticlines, the magmatic centres, and the deposition of the Archaean greenstone successions. The fundamental grain of the St Ives Goldfield is north-northwest-trending. This trend is marked by faults which developed during D1 extension, which was oriented east-northeast–west-southwest. Across these faults we map major stratigraphic changes in the thickness and composition of units, especially of the previously undivided Black Flag Group volcaniclastic rocks. The centre of the St Ives Goldfield is dominated by the Kambalda Anticline. This north-northwest-trending regional fold was likely established early during the D1 extensional history, and was fully established during subsequent east-northeast-oriented D2 contraction. The regional anticline is an important architectural element because (1) magmatism and gold mineralising fluids were focussed into this domed region, and (2) deformation was partitioned across the limbs and crest of this structure. The D3 event involved regional uplift and extension, resulting in the formation of late basins (Merougil Conglomerate locally) and the emplacement of granitoids sourced from a metasomatised mantle wedge (Mafic-type porphyries). The most significant gold event in terms of endowment occurred during D4b sinistral strike-slip shearing and associated thrusting (e.g., Tramways and Republican thrusts). These thrusts were previously interpreted as the first contractional structures to deform the area (‘D1’), but are here reinterpreted as relatively late (D4b). In this D4b event, the north-northwest-trending faults underwent sinistral strike-slip shearing and were linked across the Kambalda Anticline by accommodation structures represented by generally east- to east-northeast-trending thrusts. Reactivation of D1 transfer structures may have influenced the location of these later accommodation structures. Late-stage mineralisation during D5 was the result of dextral strike-slip brittle shearing. [Copyright &y& Elsevier]

Abstract: World-class mineral systems, such as those found in the Archaean eastern Yilgarn Craton, are the product of enormous energy and mass-flux systems driven by lithospheric-scale processes. These processes can create big footprints or signatures in the lithosphere, which can be observed at a range of scales and via a variety of methods: including geophysics, isotopes, tectonostratigraphy and geochemistry. We use these datasets to describe both the architecture (structure) of world-class gold systems of the Yilgarn Craton and the signatures of their formation. By applying an understanding of the most critical elements of the process, and their signatures, new areas, especially undercover, may be targeted more predictably than before. Knowledge of the major architectural elements of the Yilgarn Craton has increased greatly over the last decade, through the collection of a wide range of geophysical, geochemical and isotopic data. These data range in scale from (1) lithospheric-scale, which provide information on the entire craton and their underlying volumes down to depths in excess of 350km, through; (2) regional-scale, which provide information at the province scale and to depths of 30–40km, to; (3) mine- and camp-scale, which provide information on the top few kilometres of the crust. The architecture (structure) in the upper mantle and lower crust can be inferred from broadband seismic tomography and analysis of long-wavelength gravity data. Geophysical data, such as magnetotellurics and deep-crustal reflection seismic, also provide evidence for the signatures of the flow of fluids through this architecture (the pathways) and they illustrate that the scale of the system is many orders of magnitude larger than the immediate deposit itself. Of particular importance is the role of deep-crustal-penetrating shear zones or faults which link the mantle with domes in the upper crust. Together, these data provide first-order insights into the physical and chemical properties of the Yilgarn Craton. These insights provide geodynamic clues and constraints as to why the Yilgarn is so well endowed in metals such as gold (). The intersection of favourable architectural features provides connections between the mantle and the upper crust in the Kalgoorlie region, possibly explaining why this area is so well endowed in comparison to others. [Copyright &y& Elsevier]

Abstract: The thick piles of late-Archean volcaniclastic sedimentary successions that overlie the voluminous greenstone units of the eastern Yilgarn Craton, Western Australia, record the important transition from the cessation in mafic-ultramafic volcanism to cratonisation between about 2690 and 2655Ma. Unfortunately, an inability to clearly subdivide the superficially similar sedimentary successions and correlate them between the various geological terranes and domains of the eastern Yilgarn Craton has led to uncertainty about the timing and nature of the region''s palaeogeographic and palaeotectonic evolution. Here, we present the results of some 2025 U–Pb laser-ablation-ICP-MS analyses and 323 Sensitive High-Resolution Ion Microprobe (SHRIMP) analyses of detrital zircons from 14 late-Archean felsic clastic successions of the eastern Yilgarn Craton, which have enabled correlation of clastic successions. The results of our data, together with those compiled from previous studies, show that the post-greenstone sedimentary successions include two major cycles that both commenced with voluminous pyroclastic volcanism and ended with widespread exhumation and erosion associated with granite emplacement. Cycle One commences with an influx of rapidly reworked feldspar-rich pyroclastic debris. These units, here-named the Early Black Flag Group, are dominated by a single population of detrital zircons with an average age of 2690–2680Ma. Thick (up to 2km) dolerite bodies, such as the Golden Mile Dolerite, intrude the upper parts of the Early Black Flag Group at about 2680Ma. Incipient development of large granite domes during Cycle One created extensional basins predominantly near their southeastern and northwestern margins (e.g., St Ives, Wallaby, Kanowna Belle and Agnew), into which the Early Black Flag Group and overlying coarse mafic conglomerate facies of the Late Black Flag Group were deposited. The clast compositions and detrital-zircon ages of the late Black Flag Group detritus match closely the nearby and/or stratigraphically underlying successions, thus suggesting relatively local provenance. Cycle Two involved a similar progression to that observed in Cycle One, but the age and composition of the detritus were notably different. Deposition of rapidly reworked quartz-rich pyroclastic deposits dominated by a single detrital-zircon age population of 2670–2660Ma heralded the beginning of Cycle Two. These coarse-grained quartz-rich units, are name here the Early Merougil Group. The mean ages of the detrital zircons from the Early Merougil Group match closely the age of the peak in high-Ca (quartz-rich) granite magmatism in the Yilgarn Craton and thus probably represent the surface expression of the same event. Successions of the Late Merougil Group are dominated by coarse felsic conglomerate with abundant volcanic quartz. Although the detrital zircons in these successions have a broad spread of age, the principal sub-populations have ages of about 2665Ma and thus match closely those of the Early Merougil Group. These successions occur most commonly at the northwestern and southeastern margins of the granite batholiths and thus are interpreted to represent resedimented units dominted by the stratigraphically underlying packages of the Early Merougil Group. The Kurrawang Group is the youngest sedimentary units identified in this study and is dominated by polymictic conglomerate with clasts of banded iron formation (BIF), granite and quartzite near the base and quartz-rich sandstone units containing detrital zircons aged up to 3500Ma near the top. These units record provenance from deeper and/or more-distal sources. We suggest here that the principal driver for the major episodes of volcanism, sedimentation and deformation associated with basin development was the progressive emplacement of large granite batholiths. This interpretation has important implication for palaeogeographic and palaeotectonic evolution of all late-Archean terranes around the world. [Copyright &y& Elsevier]

Abstract: New SHRIMP U–Pb zircon geochronology and fieldwork integrated with reappraisal of earlier mapping demonstrates that the so-called ‘southern region’ of the mainland Lewisian Gneiss Complex comprises a package of distinct tectono-stratigraphic units. From south to north these are the Rona (3135–2889Ma), Ialltaig (c. 2000Ma) and Gairloch (ca. 2200Ma) terranes. These terranes were metamorphosed and deformed separately until ca. 1670Ma by which time they had been juxtaposed and were integral with terranes to the north. The northern boundary of the Palaeoproterozoic Gairloch terrane is a shear zone, north of which is the Archaean Gruinard terrane with 2860–2800Ma protoliths and ca. 2730Ma granulite facies metamorphism. In contrast, south of the Gairloch terrane, the Archaean gneisses of the Rona terrane have older protolith ages, underwent an anatectic event at ca. 2950Ma and show no evidence of 2730Ma granulite facies metamorphism. In current structural interpretations the Gruinard terrane forms a structural klippe over the intervening Gairloch terrane. However, the Rona and Gruinard terranes cannot be equivalent on age grounds, and are interpreted as unrelated different entities. Contained within the southern margin of the Gairloch terrane is the Ialltaig terrane, shown here to comprise an exotic slice of granulite facies Palaeoproterozoic crust, rather than Archaean basement as previously thought. The ca. 1877Ma granulite facies metamorphism of the Ialltaig terrane is the youngest event that is unique to a single terrane in the mainland Complex, making it an upper estimate for the timing of amalgamation with surrounding tectonic units. U–Pb titanite ages of 1670±12Ma and ca. 1660Ma for low-strain zones at Diabaig are interpreted to be cooling through the titanite closure temperature after the amphibolite facies reworking of these southern terranes and the southern margin of the Gruinard Terrane. These new data have implications for the tectonic setting of the mainland in relation to the Outer Hebrides and in the wider evolution of the basement in the North Atlantic. [Copyright &y& Elsevier]

Abstract: The voluminous Meso- to Neoarchean rocks exposed in the Beartooth Mountains of the northern Wyoming Province of western North America comprise the Long Lake Magmatic Complex (LLMC), a variably metamorphosed and deformed association of igneous and meta-igneous plutonic rocks with SiO2 ranging from at least 52 to 78wt%. Within this compositional range, rock types include lineated amphibolites to hornblende-bearing gneisses of intermediate composition and multiple generations of foliated to unfoliated granitoids. Emplacement ages range between approximately 2.79 and 2.83Ga, based on U–Pb zircon geochronology (SHRIMP). Field relations, elemental compositions, and geochronology indicate that these rocks do not represent a single fractional crystallization sequence, but rather, the LLMC was constructed by injection of numerous, discrete magmas as sill-like bodies over an ∼40Ma period. Although there is a continuum of compositions in the LLMC, trace element abundances can be used to distinguish distinct sources and petrogenetic processes that can be broadly extrapolated to at least 3 compositional groupings: (1) trondhjemitic to granitic intrusive rocks with SiO2 >70wt%, (2) variably metamorphosed granodioritic orthogneisses with SiO2 between 63 and 70wt%, and (3) amphibole-bearing mafic to intermediate gneisses with SiO2 between 52 and 63wt%. Despite the range of SiO2 contents, maximum LREE abundances are similar across the compositional range and, consequently, exhibit a wide range of (La/Yb) n ratios (∼20–130). All LLMC rocks share a relative depletion in HFSE abundances similar to modern convergent margin magmas. Initial Sr and Nd isotopic compositions across the compositional range are consistently offset from typical bulk silicate earth (BSE) values and preclude unaltered derivation from primitive or depleted mantle. Common Pb isotopic data define a single array that lies above model crustal growth curves and, along with the Nd and Sr data, suggest relatively uniform interaction with, or derivation from, older lithosphere. The combined isotopic and elemental data suggest the LLMC resulted from simultaneous, rapid, and voluminous production of diverse magmas that represent melting of isotopically similar, but compositionally distinct, crustal and mantle sources. Dynamically, Meso- to Neo-archean crustal growth in the northern Wyoming Province appears to require an environment similar to a modern ocean–continent convergent margin with a comparable rate of crustal production and diversity of magma series. The resultant crust and associated mantle lithosphere (keel) appear to have suffered little-to-no modification prior to Laramide (Cretaceous) uplift and exposure. [Copyright &y& Elsevier]

Abstract: The Xiong’er Group is an important geologic unit in the southern margin of the North China Craton. It is dominated by the volcanic rocks, dated at 1763±15Ma, that have SiO2 contents ranging from 52.10wt% to 73.51wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic andesites, andesites and rhyolites. They unexceptionally show enrichment of light rare earth elements (LREE) and share similar trace element patterns. Depletions in Nb, Ta, Sr, P and Ti relative to the adjacent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents (0.29–5.88wt%) and accordingly low Mg# values of 11–53. The Nd isotopes are enriched and show a weak variation with ɛ Nd(t)=−7.12 to −9.63. Zircon Hf isotopes are also enriched with ɛ Hf(t)=−12.02±0.45. The volcanic rocks of the Xiong’er Group are interpreted to represent fractional crystallization of a common mantle source. The volcanic rocks might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the Late Archean. This brings a correlation with the subduction-modified lithospheric mantle in an extensional setting during breakup of the Columbia supercontinent in the late Paleoproterozoic, rather than in an arc setting. The elevated SiO2 contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar Nd isotopic characteristics to the subcontinental lithospheric mantle. The existence of extensive Xiong’er volcanic rocks (60,000km2) indicates an early large-scale subduction-related metasomatism in the area and probably suggest a flat subduction model for the plate-margin magmatism in the Late Archean. [Copyright &y& Elsevier]

Abstract: The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U–Pb zircon analyses to provide an event history for its evolution. The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320±14Ma to 3231±6Ma and 3187±2Ma to 3154±5Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178±2Ma and subject to migmatisation at around 2597±49Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570±18Ma and 2542±5Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531±13Ma and 2513±0.4Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147±6Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton. The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792±130Ma to 763±13Ma and 553±68Ma. These events are interpreted as being the only manifestation of the Pan-African orogeny seen in the craton, which led to the assembly of the tectonic blocks that comprise the island. [Copyright &y& Elsevier]

Abstract: Based on the petrology of hydrothermally altered Archean basaltic greenstones, thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high-temperature (∼350°C) hydrothermal fluid in an Archean subseafloor basalt-hosted hydrothermal system. The results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high-CO2 condition, and predict a generation of SiO2-rich, Fe-poor hydrothermal fluids in the Archean subseafloor hydrothermal system. The chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic-neutral seawater are characterized by inverse pH and chemical polarity to modern hydrothermal systems, leading to extensive precipitation of silica and iron oxyhydroxides on/under the seafloor. Such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike, the mechanism of silicification, and the pH-controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the Archean ocean. Such high-temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life. [ABSTRACT FROM AUTHOR]

Abstract: We report isotopic (Sm–Nd, Rb–Sr and zircon Hf and U–Pb SIMS SHRIMP) data on rocks collected from various localities throughout the southern Prince Charles Mountains known as the Ruker Province. The area is made up of a high-grade metamorphic basement, overlain by variably deformed and metamorphosed supracrustal associations of Proterozoic age. The area comprises two distinct tectonic terranes, experienced major tectonothermal processes in the Archaean (ca. 3400–2800Ma: the Ruker Terrane) or in the Palaeoproterozoic (ca. 2500–2100Ma: the Lambert Terrane). New zircon U–Pb ages of ca. 3180–3150Ma, ca. 2800, and ca. 2500Ma were obtained for various orthometamorphic rocks from the Ruker Terrane and ca. 2200Ma, ca. 1740Ma, and ca. 920Ma for syn-tectonic granitic veins and leucosomes from the Lambert Terrane. The Sm–Nd data provide evidence for the initial separation of the continental crust of the Ruker Terrane from the mantle mainly between ca. 3.2 and 3.4Ga, but up to 3.8Ga, and indicate the presence of two mantle reservoirs which correspond to (1) depleted to slightly enriched, and (2) ultra-depleted mantle. Some material was also derived from the mantle at ca. 2.7–3.0Ga; the crust of this age apparently underlies the central part of the Ruker Terrane (the CumpstonMassif–Mt Newton block), which is also distinguished by younger (ca. 2.5–2.1Ga) zircon ages. The Lambert Terrane contains subordinate ca. 3.6–3.8Ga protoliths, and the bulk of the crust seems to have originated between 2.6–2.9Ga from various mantle sources and it may represent an accretional complex onto the Archaean Ruker Terrane. We also summarize published isotopic data and propose an integrated geological evolution for both terranes of the Ruker Province, discuss its relationships with the bordering Rayner Province, and compare its isotopic features with other Precambrian cratons of Gondwana. The key geological features of the Ruker Terrane suggest a similarity to the Yilgarn Craton, which would imply considerable mineral resource potential of the Prince Charles Mountains. [Copyright &y& Elsevier]

Abstract: We present carbonate Mo isotope data of the 2.64–2.5Ga old Ghaap Group (Transvaal Supergroup, South Africa) to contribute to the debate on early Earth oxygenation. Mo fractionation in carbonates has just recently been introduced as a potential new proxy for ocean redox chemistry and it was suggested that under favorable conditions non-skeletal carbonates closely monitor the ambient seawater δ 98/95 Mo composition. The record presented here is characterized by three phases of moderately to strongly fractionated δ 98/95 Mo with values of up to 1.6‰, interrupted by two shifts towards detrital levels. This near-continuous presence of heavy δ 98/95 Mo suggests a quasi-permanent availability of free atmospheric O2, because fractionation relative to the continental background signature occurs exclusively when Mo is present as dissolved molybdate ions (MoO4 2−). We compare our carbonate data to a previously published black shale Mo record on samples from the same drill core. The general interpretation of both records is identical: a rise in O2 preceded the commonly accepted transition to a consistently oxygenated atmosphere by ∼150Ma. However, in the Ghaap Group carbonates fractionated Mo is found at a time when the coeval black shales still indicate detrital sedimentation. A combination of detritus admixture and/or redox controlled Mo isotope fractionation during shale formation in suboxic to anoxic waters is suggested as cause for a period characterized by a persistent offset between lighter shale- and heavier carbonate δ 98/95Mo. The apparent offset can then be attributed to the different geochemical behavior of the two archives with respect to the local depositional conditions (redox- vs. non-redox-dependent). Such changes in the sedimentary environment might also explain why the carbonates do not reproduce rapid fluctuations found throughout the shale record but show a continuous signature with scarce outliers. Thus, by means of the combined archives the conspicuous δ 98/95 Mo scatter found in the shale record could be attributed to variable depositional conditions during its formation rather than global oxic/anoxic fluctuations. Simultaneously, the carbonates provide a better approximation of the long-term δ 98/95 Mo seawater evolution. This study introduces the Mo isotopic composition of carbonates as a promising new tool for paleo-redox studies, in particular when combined with black shales. [Copyright &y& Elsevier]

Abstract: Tarim craton is one of the major cratons in Central Asia. Basement rocks of the craton, such as TTG gneisses, are dominantly exposed in the northern (Kuluketage area) and the Eastern (Altyn Tagh Mountain). Recent years, only a few reliable Archean ages have been obtained for the basement rocks in the Altyn Tagh Mountain. Geochronological and geochemical studies have been conducted on the orthogneisses exposed in the Kuluketage area in order to unravel complex history of the Tarim craton. Zircon U–Pb dating of the rocks yielded three weighted mean 207Pb/206Pb ages of 2516±6, 2575±13 and 2460±3Ma, indicating that their protoliths were formed in the late Neoarchean to early Paleoproterozoic. The late Neoarchean orthogneisses exhibit low Sr/Y ratios (4–19) and Mg# values (35–44), with depleted Sr contents, negative Nb, Ta and Ti anomalies and strongly fractionated REE patterns ((La/Yb)N =13–62), displaying typical geochemical features of arc igneous rocks. In contrast, the early Paleoproterozoic orthogneisses are characterized by high-SiO2 contents, Sr/Y ratios (37–67) and Mg# values (46–67), with variably fractionated REE patterns ((La/Yb)N =9–89), slight Sr enrichment and positive Eu anomalies, resembling high-SiO2 adakites derived from subducted basaltic slab-melts. The late Neoarchean TTGs have low ɛ Hf(t) values (–5 to +1) and initial Hf compositions (0.280987–0.281160) with Mesoarchean two-stage model ages (T DM2 =2.9–3.3Ga), suggesting that the crustal materials of the basement rocks in this area were initially extracted from a depleted mantle in the late Paleo- to Mesoarchean and were reworked in the late Neoarchean. The history of crustal growth is different from that of the North China and Yangtze cratons and thus implies relatively younger cratonization than the North China and Yangtze cratons. Zircons in the early Paleoproterozoic orthogneisses have high ɛ Hf(t) values (+4 to +10) and young Neoarchean two-stage model ages (2.5–2.7Ga), revealing a juvenile crustal growth event in the late Neoarchean. Because there is no record of coeval juvenile crustal growth in the other two cratons, we suggest a separate crustal evolution for the northern Tarim craton which was stayed far from the other two cratons before the early Paleoproterozoic. [Copyright &y& Elsevier]

Abstract: The Mosquito Creek Basin stands apart in the Archaean granite–greenstone terrane of the east Pilbara Craton because of the rectangular outcrop but crescent-shaped subcrop, and its composition of low-grade metamorphic mainly turbiditic clastic sedimentary rocks. These sediments belong to the c. 2.9Ga Mosquito Creek Formation. Tectonic interpretations of the basin have varied from a simple synclinorium, to an accretionary prism, an extensional passive margin, and more recently into a rift basin separating the Kurrana Terrane from the East Pilbara Terrane (EPT). Combined sedimentological and structural–geological investigations show coarse-clastic fan-delta marginal facies with stacked unconformities, rimming an E–W basin centre with predominantly finer-grained mass flow-dominated sediments. A N–S traverse therefore contains a complete cross-section of the Mosquito Creek Basin, shortened to about 50% of its original width by tight folding and thrusting. The basin fill shows one major FU sequence from fan-delta sandstone to distal turbidites, unconformably overlying an older, partially preserved clastic sequence of comparable facies. A large slump unit occupies the basin axis. The unconformities along the margins interfere with folding of the sediments which also influenced basin-centre turbiditic palaeocurrents. The basin is interpreted as a near-symmetrical, underfilled, compressional, intramontane, clastic sedimentary basin unconformably superposed on a volcanic greenstone belt rock assemblage with enclosed low-angle shear zones. Compression started during sedimentation under conditions of net subsidence within a rim syncline between adjacent domes. This occurred after phases of low-angle extensional and compressional deformation of the substrate not related to the present-day geometry of the granitoid domes. The geological setting of the Mosquito Creek Basin does not differ fundamentally from that of other adjacent E–Woriented greenstone belts of the EPT. The basin is inferred to represent the last, stratigraphically uppermost, and because of subsequent erosion rarely preserved, stage of greenstone belt evolution. [Copyright &y& Elsevier]

Abstract: A group of extremely rare coeval ultramafic–mafic and syenitic dykes was discovered in the Eastern Hebei region of the North China Craton. These dykes intrude the 3.8–2.55Ga old Caozhuang complex. An olivine gabbro dyke and syenite dyke yield, respectively, SHRIMP zircon U–Pb ages of 2516±26Ma and 2504±11Ma, interpreted as the magmatic crystallization age. Their zircons have single-stage Hf model ages 2677Ma and 2705Ma. The olivine gabbros have Mg# values of 59–63, similar to high-magnesian tholeiitic basalt. They show relatively LREE-enriched patterns without Eu anomalies (La/YbN =9.28–9.78, Gd/YbN =2.8, δEu=0.96–1.00), with enrichment in LILEs and depletion in HFSEs. They also contain high Cr and Ni, and have Zr/Hf and Nb/Ta ratios similar to those of the primitive mantle. The syenites are alkaline in composition, with 8.67–8.88wt.% Na2O+K2O, and show high total REE contents (543–854ppm) and strong LREE-enriched patterns with minor Eu negative anomalies (La/YbN =50–101, Gd/YbN =5.2–5.3, δEu=0.80–0.82). Trace element diagrams show strong LILE-enrichment relative to HFSE. Zr/Hf and Nb/Ta ratios are 47–49 and 26–76, respectively, which are much higher than those of primitive mantle and higher than the average Nb/Ta ratio of post-Archean continental crust. The petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source, which implies that the NCC probably had continental crust of considerable thickness at this time. Combined with evidence for ∼2.5Ga granite intrusion and metamorphism in the Eastern Hebei region and adjacent areas, we propose that the NCC has been a present scale craton at the end of Archean. [Copyright &y& Elsevier]

Abstract: The Kalgoorlie, Gindalbie, Kurnalpi, Duketon and Burtville terranes were amalgamated by ∼2658Ma into the Eastern Goldfields Superterrane, which was later accreted Cordilleran-style onto the foreland of a pre-existing continent. Late-stage (synorogenic) basins developed within the Eastern Goldfields Superterrane during the early stage of accretion. The basins are similar to all late-stage basins of Archaean provinces, developing late in the tectonic cycle in a strike-slip regime. They developed in two stages, during the gap in granite emplacement between ∼2658Ma and ∼2648Ma. Basin analysis suggests that the bulk of sediments was derived from rocks readily identifiable today within the superterrane, consistent with the provenance style of all late-stage basins. Sediment derivation from the foreland occurred only during Stage II, and is recorded only from basins proximal to the suture. Detrital-zircon geochronology has corroborated those interpretations. Eleven new SHRIMP U-Pb detrital-zircon datasets and 14 previously published datasets indicate that the bulk of zircon-bearing sediment was derived from the 2715Ma to 2658Ma High-Ca (TTG) granites and volcanic rocks that define the terranes. Moreover, primary modes in the detrital-zircon age-spectra of 2680±3Ma, 2673±Ma, 2666±3Ma and 2660±3Ma coincide with the peaks of emplacement of High-Ca granites. Provenance from across the suture has also been corroborated, with detrital-zircon age-spectra of the Kurrawang Basin, a Stage II basin close to the suture, having no match to rocks of the Eastern Goldfields Superterrane. Defining modes of ∼2730Ma and >3000Ma, some as old as ∼3750Ma and ∼3540Ma, can be matched to an ∼2735Ma calc-alkaline volcanic province and quartzites with 4350–3130Ma detrital zircon in the foreland. The study shows that detrital-zircon age-spectra can be powerful supporters of tectonic analysis, but for the study presented here their interpretation has been greatly assisted by prior basin analysis and a considerable geochronological database of source-rock equivalents. [Copyright &y& Elsevier]

Abstract: U–Pb zircon ages from trondhjemitic gneisses of the Badplaas domain, located in the southwestern parts of the Mesoarchaean Barberton granitoid-greenstone terrain of South Africa, document a hitherto largely unrecognized plutonic event. The geochronological results indicate the addition of juvenile, felsic crust over a period of 60 Myr between ca. 3290 and 3230Ma and prior to the main collisional event in the granitoid-greenstone terrain at 3230Ma. The timing and duration of plutonism, together with the structural and compositional heterogeneity of the Badplaas domain suggest that the Badplaas domain represents part of a convergence-related magmatic arc. On a regional scale, the spatial and temporal relationships between plutonism, metamorphism and deformation are interpreted to preserve an almost complete inventory of a Mesoarchean arc-trench system. This includes (1) the 3290–3230Ma convergence-related magmatic arc, (2) a largely coeval deposition of the back-arc type volcano-sedimentary 3260–3225Ma Fig Tree Group of the Barberton greenstone belt, (3) relics of the underplated crust preserved in high-P, low-T rocks to the immediate east of the magmatic arc, juxtaposed against (4) high-T medium-P rocks of the overlying plate along the major crustal structure of the Inyoni shear zone. These findings corroborate the notion that thermal and rheological conditions on the early Earth supported, at least locally, crustal growth and the extraction of buoyant felsic crust along convergence-related magmatic arcs. [Copyright &y& Elsevier]

Abstract: We report on an inferred Paleoproterozoic soil horizon from the Singhbhum-Orissa craton near Keonjhargarh, Orissa, eastern India, which preserves vertic (Vertisol-like) features. The ∼2–10m thick, white shale consists of quartz–muscovite–pyrophyllite interpreted as a paleosol, overlies ∼3.32Ga granite and is covered by pre-2.0Ga sandstones (Kolhan Group) deposited in marginal marine environments. The paleosol passes downwards through a partly altered granite (saprolith) zone to fresh granite. Abundant pedogenic slickensides, ped structures, gilgai topography, reddish stain on ped surfaces, and sepic-plasmic microfabric in the paleosol were probably related to shrink and swell processes driven by marked seasonal precipitation in a well-drained landscape. An observed decrease of alkali elements and an increase of silica, alumina and CIA values from granite to paleosol, together with relatively constant Ti/Al ratios suggest in situ weathering of granite protolith. However, a paleo-Vertisol necessitates a mafic parent rock, either directly (igneous) or reworked through alluvium. The c. 2.25Ga Malangtoli-Jagannathpur basaltic rocks are relatively widespread and close to the inferred paleosol occurrences, and provide a logical solution. Geochemistry of the highly depleted paleosol, in contrast, particularly when applying the isochon method, strongly supports a granitic parent. The apparent conundrum can possibly be solved through repeated alluvial reworking of both granitic and mafic detritus and accumulation of weathered material with varying proportions of these two components, yet enough smectitic clays (∼30%) to form a paleo-Vertisol. This pre-2.0Ga weathering event in the Singhbhum craton is consistent with a global scenario of Paleoproterozoic weathering which followed an extended global glaciation, and thus has the potential to contribute to understanding of pre-2.0Ga amalgamation of regional and global cratonic blocks. [Copyright &y& Elsevier]

Abstract: A structural and geochronology study on the 2900Ma Lake Johnston Greenstone Belt (LJGB) has defined multiple deformation events. Quantitative U–Pb age constraints have been obtained on the ductile deformation, and peak metamorphic assemblages. The D1LJ event is associated with the development of fold-nappes during interpreted NNE–SSW shortening. U–Pb SHRIMP dating on monazite from a dyke folded by the D1LJ event gives an age of 2634±1Ma, providing an upper age limit on D1LJ. This age could have been reset by the D2LJ metamorphic event. A major metamorphic/thermal event post-dates the D1LJ event and is inferred to be the result of elevated temperatures coeval with the emplacement of granitoid intrusions. The D2LJ event is associated with peak metamorphic temperatures, and reflects NNE–SSW to NE–SW shortening associated with dextral shearing. A dyke that intrudes and crosscuts D2LJ fabrics, yield a U–Pb SHRIMP age on monazite at 2629±1Ma and provides a lower age limit on the D2LJ event. D3LJ is defined by crenulation cleavages developed during E–W shortening. The D4LJ episode is defined by brittle deformation and a series of N- to NE-trending, steep-dipping dextral faults formed during craton-scale shortening under a NE–SW directed far field stress. The brittle D5LJ event is characterized by E–W shortening with dip-slip reverse N–S faults. The new D2LJ deformation age is younger than previously constrained ages for major deformation in the 2900Ma greenstone belts. Deformation within the LJGB was broadly coeval with the waning stage of formation of gold deposits in adjacent greenstone belts. Most peak magmatic and metamorphic events preserved in the greenstone belts have been constrained to ca. 2640–2650Ma. The presence of such a high temperature deformation event recorded at ca. 2630Ma in the LJGB belt has implications for the thermal history, fluid formation, and geodynamic context of the development of these gold systems. The 2630Ma age correlates with late-stage and post-gold emplacement of pegmatite dykes in surrounding terranes. The LJGB is interpreted to have been a mobile belt associated with high grade metamorphism that was inboard (west) of, and active, during the formation of major world class gold systems in the Eastern Goldfields Terranes. The D1LJ to D2LJ changes in the kinematics of the system (i.e. D1LJ to D2LJ), may correlate with a stress switch linked to the influxes of gold-bearing fluids in adjacent greenstone belts. [Copyright &y& Elsevier]

Abstract: The Barberton granitoid-greenstone belt is a classic dome-and-keel province, characterized by kilometer-scale gneiss domes and elongate keels of largely folded supracrustal rocks. Combined U–Pb SHRIMP data and structural mapping demonstrate that the geometry of the Barberton belt reflects events that occurred over ∼30 million year interval, from ca. 3230 and 3203Ma. Early deformation with NW–SE shortening in the upper crust was accompanied by emplacement of tonalite-trondhjemite-granodiorite TTG magmas at 3234±12 and 3226±9Ma. Much of the structural grain of the greenstone belt relates to a long episode of post-orogenic extension, with NE-directed extension in the lower crust leading to exhumation of high-grade gneisses in the southern Barberton terrane. Advective heat transfer during emplacement of kilometer-scale (TTG) plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence. The end of this tectonic cycle is punctuated by the emplacement of the undeformed, 3203±7Ma Dalmein pluton, which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high-grade gneisses. [Copyright &y& Elsevier]

Abstract: A sequence of c. 3.2Ga low-grade sedimentary rocks, bimodal volcanic rocks, and subvolcanic layered mafic-ultramafic sills and dykes occur in greenstone belts that flank the margins of the 3.51–3.24Ga East Pilbara Terrane of the Pilbara Craton, Western Australia. U–Pb SHRIMP zircon data and Sm–Nd model age data suggest that these rocks represent juvenile addition of material to the margins of the craton during a period of major extension, accompanied by the local emplacement of granitic rocks. This extensional event is also represented in adjacent terranes, by thick basalt flows with c. 3.2Ga zircon and Sm–Nd model ages, and by granitic rocks. Based on the available age data, geochemistry of basaltic rocks, and the distribution of this event in space and time within the craton, we interpret the c. 3.2Ga rocks to reflect widespread rifting of the protocraton and associated thinning of the lithosphere. This event resulted in separation of the flanking Kurrana and Karratha Terranes from their parent East Pilbara Terrane, which represents the ancient nucleus of the craton. These terranes were then reassembled during subsequent accretion at 3.07–2.905Ga. Combined, the data suggest the preservation of a Mesoarchean Wilson cycle in the Pilbara Craton. [Copyright &y& Elsevier]

Abstract: The Witwatersrand Basin in South Africa has been the focus of gold exploitation for more than 100 years, and the subject of hundreds of research investigations, principally sedimentological and metallogenic, and recently, geophysical and geochronological. Surprisingly the basin-wide structural character has not been fully investigated despite the fact that gold metallogenic models have been proposed that want structural architectures as conduits for gold mobilisation and/or sites of deposition (i.e., the modified placer and hydrothermal models). These models, which have polarised viewpoints on the source for gold (placer versus metamorphic or hydrothermal source), assume an underlying structural character to the basin and structural relationships between goldfields, but these assumptions have never been tested through evaluation of the existing structural dataset. As corollary, the published data for the structural character of the Free State (Welkom), Klerksdorp, West Wits Line, West Rand, Central Rand, East Rand and South Rand goldfields are summarised and evaluated to search for basin-wide similarities and/or correlations that would support tectonic models for the Witwatersrand basin, and explain its metallogenic character. Unfortunately, the large published historical database (>90 years) proved to be quite limited with respect to geometric, kinematic and/or relative chronological detail, but must be evaluated against an advanced understanding of Earth processes, tectonics and the structural geology approach. The dataset is largely qualitative rather than quantitative, but is useful in establishing broad tectonic trends of the formation and tectonic evolution for the Witwatersrand Basin. It suggests that two deformation events (Umzawami and Ukubambana events) mark the end of deposition in the Witwatersrand and Transvaal basins, respectively, with development of fold–thrust belts at ca. 2.7 and 2.2–2.0Ga and these can be correlated with supercontinental cycles. The intervening period (i.e., 2.7–2.2Ga) was characterised by at least 4 basin-forming events that culminated in the deposition of the volcanic-sedimentary sequences of the Klipriviersberg Group, and the sedimentary sequences of the Platberg, Chuniespoort and Pretoria groups. Basin formation was facilitated by the reactivation of structures formed during the Umzawami Event (or older) as feeder dykes, normal-listric faults, or growth faults. The limited structural data for the Witwatersrand basin unsurprisingly also leads to the conclusion that gold was macroscopically to mesoscopically remobilised from a primary placer source, but remobilisation was localised (goldfields-scale) and not is a basin-wide phenomena. [Copyright &y& Elsevier]

Abstract: A systematic REE+Y study of Archean (ca. 3.0Ga) cherts from the Mount Goldsworthy greenstone belt in the northeastern Pilbara Craton, Western Australia was performed in order to understand their origin and depositional environment. Analyzed samples include microfossil-bearing black cherts from the Farrel Quartzite and a black vein chert from the underlying Warrawoona Group, and laminated to banded chert including carbonaceous chert, jaspilite and banded iron-formation from the overlying, deepening-upward Cleaverville Formation. Laminated to banded cherts from the Cleaverville Formation show a clear stratigraphic trend upsection of increasing Y/Ho and positive Eu-anomalies, with HREE-enrichment and positive La-anomalies. The data comprise a mixing array between two end-member components on a newly proposed Y/Ho–Eu-anomaly diagram. One end-member is interpreted to be Archean seawater, with a super-chondritic Y/Ho ratio (∼100) and a weak positive Eu-anomaly (∼3). The other end-member with a chondritic Y/Ho ratio and a negligible Eu-anomaly is assumed to be non-marine water such as continental run-off, ground water, or geothermal water. Black cherts containing microfossils in the Farrel Quartzite are characterized by a positive La-anomaly, HREE-enrichment, negligible to a slight positive Eu-anomaly, and a chondritic to slightly super-chondritic Y/Ho ratio. They are distinct from vein cherts with a distinct MREE-enrichment and contemporaneous hydrothermal cherts with pronounced Eu-anomaly, and plot close to the supposed non-marine end-member component on the Y/Ho–Eu-anomaly diagram. The black cherts were thus precipitated from a water mass influenced significantly by for example continental run-off, ground water and/or geothermal water, but not from high-T hydrothermal solution, increasing the credibility of microfossils contained in them. [Copyright &y& Elsevier]

Abstract: In a recent issue of Precambrian Research (vol. 170, pp. 73–87), Wang (2009) published a paper entitled ‘Tectonic evolution of the Hengshan–Wutai–Fuping complexes and its implication for the Trans-North China Orogen, in which he proposes that (1) the formation of arc-related igneous rocks in the Hengshan and Wutai Complexes formed by northwest-directed subduction, not by (south)east-directed subduction; and (2) the Wutai granitoids were the products of an intra-oceanic island arc, not formed in a continent-derived arc. However, we consider both the structural and geochemical approaches Wang (2009) adopted to determine the polarity of the subduction and the tectonic setting of the Wutai granitoids are inappropriate or invalid. Firstly, all his structural observations used to determine the polarity of subduction are restricted to the Zhujiafang and Longquanguan ductitle shear zones, both of which developed at the late stage. It is unreliable to use kinematic indicators from these later structures to infer the polarity of the pre-collisional subduction. Secondly, Wang (2009) assumed that the initial positive ɛ(Nd) values (+0.5 to +4.5) of the Wutai granitoids indicate their generation directly from a mantle source, forming in an intra-oceanic arc. Such an interpretation is incorrect because the positive ɛ(Nd) values of the Wutai granitoids merely indicate that the igneous protoliths of these granitoids originated from a depleted mantle source, not the Wutai granitoids themselves. As the products of the earliest arc magmatism in the Wutai arc, the Wutai granitoids could not be generated directly from a mantle source but must have been derived by partial melting of juvenile crust. [Copyright &y& Elsevier]