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Donald R Lowe - One of the best experts on this subject based on the ideXlab platform.
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provenance and paleogeography of archean fig tree siliciclastic rocks in the east central barberton Greenstone Belt south africa
Precambrian Research, 2021Co-Authors: Nadja Drabon, Emily Stoll, Donald R LoweAbstract:Abstract The 3.6–3.2 Ga Barberton Greenstone Belt, South Africa, is a complex terrain divided into multiple structural blocks. The structural deformation and poor geochronological constraints often make correlation among blocks difficult. To overcome structural complexities, provenance proxies including sandstone petrography, shale geochemistry, and detrital zircon geochronology are here used to compare source terrain signatures among multiple structural blocks in the East-Central Domain of the Barberton Greenstone Belt. This will expand our understanding of Paleoarchean paleogeography and the nature of crustal uplift in the southeastern part of the Barberton Greenstone Belt. The Manzimnyama and Mlumati Synclines are composed of 600 m-thick sections of the Fig Tree Group, Mapepe Formation. These synclines have high sedimentary and volcanic lithic contents in sandstones, an upward transition from felsic to mafic signatures in shales, and a detrital zircon age distribution transition from a basal member with a main peak at 3.28 Ga to members with a peak at about 3.45 Ga. There are three other East-Central Domain, fault-bounded blocks made up mostly of Mapepe Formation strata. The Eastern Barite Valley lies northwest of the Manzimnyama Syncline. From older to younger strata, the Eastern Barite Valley progresses from high lithics to more quartz and feldspar in sandstones, from more mafic to felsic signatures in shales, and from a unimodal 3.45 Ga detrital zircon signature at the base to complexly mixed age distributions with a strong 3.24 Ga peak at the top. The Paulus Syncline, southeast of the Manzimnyama Syncline, has a high chert content, mafic to ultramafic geochemistry signatures, and a main detrital zircon peak at 3.28–3.30 Ga with a minor peak at 3.45 Ga. The Emlembe Syncline, southeast of the Paulus Syncline, has a high monocrystalline quartz content, mixed geochemistry signatures, and unimodal 3.28–3.30 Ga detrital zircon ages. The proximity and sedimentology of these structural blocks could suggest deposition in a single basin; however, contrasts in provenance suggests sediments were deposited in different basins, at different times, or in one basin with provenance partitioning. The blocks likely had locally different sources, implying the sequences were not part of a single large source, transport, and depositional fairway.
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the non glacial and non cratonic origin of an early archean felsic volcaniclastic unit barberton Greenstone Belt south africa
Precambrian Research, 2020Co-Authors: Donald R Lowe, Gary R ByerlyAbstract:Abstract Recent studies have suggested that ~3.445 Ga felsic volcaniclastic rocks of member H6 of the Hooggeneoeg Formation in the Barberton Greenstone Belt, South Africa, show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older, granitic continental crust. These conclusions represent fundamental re-interpretations of this sequence, its crustal setting, and processes of deposition. We here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits, storm-influenced shallow-marine units, and turbidity-current-like, storm-driven submarine flows, all of which lack any evidence of a glacial influence during sedimentation. They accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface. The debris consists entirely of volcanic-derived materials. No clasts of plutonic rock are found in the conglomerates. Low-temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline, previously thought to be characteristic of granitic plutonic rocks. This potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the Greenstone Belt. This succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center.
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provenance and tectonic implications of the 3 28 3 23 ga fig tree group central barberton Greenstone Belt south africa
Precambrian Research, 2019Co-Authors: Nadja Drabon, Aleksandra Galic, Paul R D Mason, Donald R LoweAbstract:Abstract Sedimentary rocks provide a valuable record of non-uniformitarian Paleoarchean tectonic processes. The rocks of the 3.28–3.23 Ga Mapepe Formation mark the initiation of orogenesis in the Barberton Greenstone Belt, South Africa. However, reconstruction of the Mapepe basin(s) is challenging because the rocks occur in fault-bounded structural Belts that are characterized by lateral heterogeneity and whose correlation remains uncertain. We have studied the petrography, geochemistry, and detrital zircon geochronology from outcrop and drill core samples from two adjacent structural Belts, the Manzimnyama Syncline and the Eastern Barite Valley area, to evaluate the timing of basin formation, correlate between these two adjacent areas, and provide constraints on the character of tectonic uplift. In the Eastern Barite Valley area sedimentation was initiated at about 3260 Ma. The section includes a lower, deep-water unit of mainly mudstone overlain by a shoaling-upward sequence of fan-delta sandstone and conglomerate. The sandstone throughout the formation was derived by erosion of local uplifts of the underlying Greenstone-Belt sequence except at the top of the section where sedimentation was dominated by dacitic volcaniclastic sediments dated at 3239 Ma. In contrast, rocks of the Mapepe Formation in the Manzimnyama Syncline area, 1–2 km south of the Eastern Barite Valley area, record the initiation of sedimentation at about 3277 Ma with deposition of a rhyolitic tuffaceous unit followed by persistent deep-water sedimentation of banded iron formation, banded ferruginous chert, and turbiditic sandstone. Siliciclastic sediments are immature and were derived by erosion of older supracrustal Greenstone-Belt rocks. No Fig Tree age volcanism younger than 3260 Ma is recorded in this Belt. Overall, Mapepe sedimentary rocks record the formation of numerous local uplifts of underlying supracrustal rocks with periodic volcanic activity, and the dispersal of debris to form mainly small fan-deltas flanked by deep-water systems. The contrasting ages, lithologies, depositional settings, and provenance of the sedimentary rocks in the now adjacent Eastern Barite Valley and Manzimnyama Syncline Belts suggest that they were deposited in either different parts of a large, complex Fig Tree basin or represent separate basins that have been juxtaposed during post-Fig Tree tectonism and shortening.
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sedimentology of the 3 3 ga upper mendon formation barberton Greenstone Belt south africa
Precambrian Research, 2016Co-Authors: Elizabeth J Trower, Donald R LoweAbstract:The Mendon Formation is the uppermost unit of the 3.5-3.26 Ga Onverwacht Group in the Barberton Greenstone Belt, South Africa. It consists of a cyclic stack of komatiitic volcanic units separated by thin cherty sedimentary layers. In most areas, the uppermost Mendon Formation is a sedimentary interval characterized by black chert, banded black-and-white chert, and banded ferruginous chert, although the detailed patterns of lithofacies in different sections are more complex. Previously reported zircon U/Pb ages suggest that Mendon deposition could represent more than 70 Myr of time between ∼3,334 Ma and ∼3,260 Ma. This study presents sedimentological and petrographic observations of the upper Mendon Formation from across the central part of the Barberton Greenstone Belt in order to investigate sediment sources, depositional processes, and environments of sedimentation. The dominant mode of sedimentation was quiet settling of carbonaceous grains and, in the deepest sections below storm wave base, fine ferruginous material, resulting in finely laminated black and grey chert. In situ carbonaceous laminations are rare, suggesting that benthic microbial mat growth had little direct influence on deposition. The hemipelagic background deposition was punctuated by occasional inputs of fine pyroclastic debris, formation and deposition of silica granules, and reworking by infrequent storm events. Storm deposits are represented by coarse-grained, poorly-sorted intraclast breccias, some of which include distinctive intraclasts sampling lithofacies that are not observed in situ. Despite considerable lateral variability, correlative temporal trends are resolvable in many Mendon sections: there is an upward-deepening of the overall depositional setting recorded in the oldest upper Mendon sections, consistent with the previous interpretation that Mendon time was characterized by rifting ( Lowe, 1994a; Lowe, 1999a). Younger Mendon cycles include thick, relatively ferruginous basal sections, interpreted to reflect the deepest water deposition. These sections are capped by black chert and silicified ashes with more evidence of disturbance and reworking by storms, reflecting gradual shoaling. This sedimentological analysis is broadly consistent with previous geochemical and tectonic analyses and provides a better picture of depositional patterns during uppermost Onverwacht time, before the distinct change in tectonic regime marked by impact spherule layer S2 and the onset of Fig Tree Group orogenesis and related siliciclastic deposition.
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sedimentology of the 3 3ga upper mendon formation barberton Greenstone Belt south africa
Precambrian Research, 2016Co-Authors: Elizabeth J Trower, Donald R LoweAbstract:The Mendon Formation is the uppermost unit of the 3.5-3.26 Ga Onverwacht Group in the Barberton Greenstone Belt, South Africa. It consists of a cyclic stack of komatiitic volcanic units separated by thin cherty sedimentary layers. In most areas, the uppermost Mendon Formation is a sedimentary interval characterized by black chert, banded black-and-white chert, and banded ferruginous chert, although the detailed patterns of lithofacies in different sections are more complex. Previously reported zircon U/Pb ages suggest that Mendon deposition could represent more than 70 Myr of time between ∼3,334 Ma and ∼3,260 Ma. This study presents sedimentological and petrographic observations of the upper Mendon Formation from across the central part of the Barberton Greenstone Belt in order to investigate sediment sources, depositional processes, and environments of sedimentation. The dominant mode of sedimentation was quiet settling of carbonaceous grains and, in the deepest sections below storm wave base, fine ferruginous material, resulting in finely laminated black and grey chert. In situ carbonaceous laminations are rare, suggesting that benthic microbial mat growth had little direct influence on deposition. The hemipelagic background deposition was punctuated by occasional inputs of fine pyroclastic debris, formation and deposition of silica granules, and reworking by infrequent storm events. Storm deposits are represented by coarse-grained, poorly-sorted intraclast breccias, some of which include distinctive intraclasts sampling lithofacies that are not observed in situ. Despite considerable lateral variability, correlative temporal trends are resolvable in many Mendon sections: there is an upward-deepening of the overall depositional setting recorded in the oldest upper Mendon sections, consistent with the previous interpretation that Mendon time was characterized by rifting ( Lowe, 1994a; Lowe, 1999a). Younger Mendon cycles include thick, relatively ferruginous basal sections, interpreted to reflect the deepest water deposition. These sections are capped by black chert and silicified ashes with more evidence of disturbance and reworking by storms, reflecting gradual shoaling. This sedimentological analysis is broadly consistent with previous geochemical and tectonic analyses and provides a better picture of depositional patterns during uppermost Onverwacht time, before the distinct change in tectonic regime marked by impact spherule layer S2 and the onset of Fig Tree Group orogenesis and related siliciclastic deposition.
Ali Polat - One of the best experts on this subject based on the ideXlab platform.
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a back arc origin for the neoarchean megacrystic anorthosite bearing bird river sill and the associated Greenstone Belt bird river subprovince western superior province manitoba canada
International Journal of Earth Sciences, 2019Co-Authors: Paul Sotiriou, Ali Polat, Robert Frei, Xueming Yang, James Van VessemAbstract:This study presents extensive new field, petrographic, and whole-rock major and trace element and Sm–Nd and Pb isotope data for the Neoarchean megacrystic anorthosite-bearing Bird River Sill, spatially associated Neoarchean supracrustal rocks of the Bird River Greenstone Belt, and the Mesoarchean Maskwa Lake TTG Batholith I, Manitoba, Canada. Field observations indicate that the 2743 Ma Bird River Sill was emplaced into the Northern Lamprey Falls Formation in an oceanic setting and subsequently intruded by the ca. 2725 Ma Maskwa Lake Batholith II. The Northern Lamprey Falls Formation is in fault contact with the overlying Peterson Creek Formation, which in turn is unconformably overlain by the Bird River Formation. The peridotites of the Bird River Sill were subjected to intense alteration and ductile shearing, resulting in the redistribution of many major and trace elements and resetting of their Sm–Nd and U–Th–Pb isotopic systems. The gabbro, leucogabbro, and anorthosite units of the sill largely retain their primary igneous textures and minerals, REE and HFSE compositions, and Sm–Nd and Pb isotope signatures. These geochemical and petrographic data indicate that these units of the Bird River Sill crystallised from a hydrous, Ca- and Al-rich tholeiitic magma that fractionated from a hydrous primitive arc tholeiitic parental magma. The major and trace element geochemistry and initial eNd values of the Bird River Sill (+ 0.53 to + 4.72) and the Northern Lamprey Falls Formation (+ 1.40 to + 2.66) of the Bird River Greenstone Belt crystallised from parental magma batches that were derived by partial melting of variably-depleted mantle sources. High-magnesian andesites from the Bird River Formation formed by partial melting of subducting oceanic crust and/or slab sediments and subsequent interaction with sub-arc mantle peridotite, whereas the Peterson Creek Formation dacites and Maskwa Lake TTG Batholith I tonalites formed by partial melting of juvenile lower arc crust. The major and trace element geochemistry of the Bird River Sill and Greenstone Belt and the Maskwa Lake TTG Batholith I, and geological relationships and lithological associations point to a transition from continental arc to continental back-arc magmatism along the southern active margin of the North Caribou terrane at ca. 2743 Ma, resulting in the formation of the Maskwa Lake microcontinent that occurred prior to subsequent continental arc magmatism along the southern margin of this microcontinent. Combined field observations and geochemical data indicate that the Bird River Sill and Bird River Greenstone Belt are a dismembered Archean subduction-related ophiolite that marks a suture zone between the Winnipeg River subprovince and the Maskwa Lake microcontinent.
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lithological structural and geochemical characteristics of the mesoarchean târtoq Greenstone Belt southern west greenland and the chugach prince william accretionary complex southern alaska evidence for uniformitarian plate tectonic processes
Canadian Journal of Earth Sciences, 2016Co-Authors: Ali Polat, Timothy M Kusky, Thomas F Kokfelt, Kevin Burke, Dwight C Bradley, Annika Dziggel, Jochen KolbAbstract:The Mesoarchean Târtoq Greenstone Belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by fel...
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the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq Greenstone Belt sw greenland
Lithos, 2008Co-Authors: Ali Polat, Brian J Fryer, Robert Frei, Peter W U Appel, Yildirim Dilek, J C Ordonezcalderon, Zhaoping YangAbstract:The Mesoarchean (ca. 3075 Ma) Ivisaartoq Greenstone Belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The Belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks underwent at least two stages of post-magmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. These alteration processes resulted in the mobilization of many major and trace elements. The trace element characteristics of the least altered rocks are consistent with a supra-subduction zone geodynamic setting and shallow mantle sources. On the basis of geological similarities between the Ivisaartoq Greenstone Belt and Phanerozoic forearc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq Greenstone Belt represents a relic of dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of a lower layer of leucogabbros and anorthosites, and an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and sills, and dunitic to wehrlitic sills. The Sm–Nd and U–Pb isotope systems have been disturbed in strongly altered actinolite schists. In addition, the U–Pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration. Gabbros and diorites have the least disturbed Pb isotopic compositions. In contrast, the Sm–Nd isotope system appears to have remained relatively undisturbed in picrites, pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (eNd = + 4.23 to + 4.97) than pillow lavas, gabbros, and diorites (eNd = + 0.30 to + 3.04), consistent with a variably depleted, heterogeneous mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (> 90%) of Ca-rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas. The anorthositic cumulates have significantly higher initial eNd (+ 4.8 to + 6.0) values than the surrounding gabbroic matrix (+ 2.3 to + 2.8), consistent with different mantle sources for the two rock types.
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field and geochemical characteristics of the mesoarchean 3075ma ivisaartoq Greenstone Belt southern west greenland evidence for seafloor hydrothermal alteration in supra subduction oceanic crust
Gondwana Research, 2007Co-Authors: Ali Polat, Brian J Fryer, Robert Frei, Peter W U Appel, Yildirim Dilek, J C Ordonezcalderon, Yuanming Pan, Julie A Hollis, Johann G RaithAbstract:The Mesoarchean (ca. 3075Ma) Ivisaartoq Greenstone Belt in southern West Greenland includes variably deformed and metamorphosed pillow basalts, ultramafic flows (picrites), serpentinized ultramafic rocks, gabbros, sulphide-rich siliceous layers, and minor siliciclastic sedimentary rocks. Primary magmatic features such as concentric cooling-cracks and drainage cavities in pillows, volcanic breccia, ocelli interpreted as liquid immiscibility textures in pillows and gabbros, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic flows are well preserved in low-strain domains. The Belt underwent at least two stages of calc-silicate metasomatic alteration and polyphase deformation between 2963 and 3075Ma. The stage I metasomatic assemblage is composed predominantly of epidote (now mostly diopside)+quartz+plagioclase±hornblende ±scapolite, and occurs mainly in pillow cores, pillow interstitials, and along pillow basalt-gabbro contacts. The origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration. On the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote-rich aggregates, the stage I metasomatic assemblage is interpreted as relict epidosite. The stage II metasomatic assemblage occurs as concordant discontinuous layered calc-silicate bodies to discordant calc-silicate veins commonly associated with shear zones. The stage II metasomatic assemblage consists mainly of diopside+garnet+amphibole+plagioclase+quartz±vesuvianite±scapolite±epidote±titanite ±calcite±scheelite. Given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric, its origin is attributed to regional dynamothermal metamorphism. The least altered pillow basalts, picrites, gabbros, and diorites are characterized by LREE-enriched, near-flat HREE, and HFSE (especially Nb)-depleted trace element patterns, indicating a subduction zone geochemical signature. Ultramafic pillows and cumulates display large positive initial eNd values of +1.3 to +5.0, consistent with a strongly depleted mantle source. Given the geological similarities between the Ivisaartoq Greenstone Belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq Greenstone Belt represents Mesoarchean supra-subduction zone oceanic crust. © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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geochemical characteristics of the neoarchean 2800 2700 ma taishan Greenstone Belt north china craton evidence for plume craton interaction
Chemical Geology, 2006Co-Authors: Ali Polat, Brian J Fryer, Timothy M Kusky, Joel E Gagnon, Shuanhong ZhangAbstract:Abstract The 2800–2700 Ma Taishan Greenstone Belt, North China Craton, consists of an association of komatiites, pillow basalts, banded iron formations, conglomerates, and greywacke sandstones. The Belt was intruded by high-Al tonalite–trondhjemite–granodiorite (TTG) plutons and deformed under amphibolite-grade metamorphism at about 2700 Ma. At the bottom of the association komatiites are characterized by locally well-preserved spinifex, cumulate, and breccia textures. Spinifex komatiites have uniformly high-MgO (31–33 wt.%), but low-Al2O3 (2.8–5.7 wt.%), TiO2 (0.14–0.24 wt.%), and Zr (4.6–9.6 ppm) contents. Towards the top of the sequence komatiites become less magnesian (MgO = 22–23 wt.%), more aluminous (Al2O3 = 6.2–8.9 wt.%), and display higher concentrations of incompatible elements. Based on [Al2O3] versus [TiO2] molecular relations, these rocks plot mainly in the field of Al-depleted komatiites. Komatiites occurring at stratigraphically lower levels have concave-upward LREE patterns and negative Nb anomalies (Nb / Nb* = 0.19–0.96). Stratigraphically upper komatiites and pillow basalts also share the negative Nb anomalies. There are strong correlations between the initial eNd values and indices of crustal contamination and olivine fractionation, suggesting that the geochemical characteristics of the Taishan komatiites can be best explained by olivine fractionation and assimilation of 2–6% LREE-enriched older continental crust or sediments. Relict olivine grains in the Taishan komatiites have lower Mg-numbers (Fo79–84) than whole-rock (84–90) and serpentine–chlorite–amphibole matrix (90–95), indicating that whole-rock composition is not in equilibrium with olivine composition. This disequilibrium is likely have resulted from the enrichment of Fe in recrystallized olivines during amphibolite metamorphism at ∼2700 Ma. Conglomerates are composed dominantly of gneissic, granitic, and amphibolitic clasts. Grain types and trace element characteristics of sandstones are consistent with a continental source area. There are zircon and apatite xenocrysts in the stratigraphically lower komatiites. Collectively, these geological features are consistent with the deposition of the Greenstone Belt on continental basement. Accordingly, we propose a geodynamic model of plume–craton interaction to explain the geological and geochemical characteristics of the Taishan Greenstone Belt.
Gary R Byerly - One of the best experts on this subject based on the ideXlab platform.
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the non glacial and non cratonic origin of an early archean felsic volcaniclastic unit barberton Greenstone Belt south africa
Precambrian Research, 2020Co-Authors: Donald R Lowe, Gary R ByerlyAbstract:Abstract Recent studies have suggested that ~3.445 Ga felsic volcaniclastic rocks of member H6 of the Hooggeneoeg Formation in the Barberton Greenstone Belt, South Africa, show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older, granitic continental crust. These conclusions represent fundamental re-interpretations of this sequence, its crustal setting, and processes of deposition. We here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits, storm-influenced shallow-marine units, and turbidity-current-like, storm-driven submarine flows, all of which lack any evidence of a glacial influence during sedimentation. They accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface. The debris consists entirely of volcanic-derived materials. No clasts of plutonic rock are found in the conglomerates. Low-temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline, previously thought to be characteristic of granitic plutonic rocks. This potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the Greenstone Belt. This succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center.
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high resolution tephra and u pb chronology of the 3 33 3 26 ga mendon formation barberton Greenstone Belt south africa
Precambrian Research, 2015Co-Authors: Nicholas B Decker, Donald R Lowe, Gary R Byerly, Thompson M Stiegler, Elizabeth J T StefurakAbstract:Abstract The Mendon Formation in the Barberton Greenstone Belt of South Africa marks the boundary between the Onverwacht and Fig Tree Groups. These groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation, respectively. This transition marks the end of komatiitic volcanism in the Barberton Greenstone Belt and is accompanied by numerous impact-related spherule layers. This study characterizes the upper Mendon Formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary. A suite of whole rock and handheld X-ray fluorescence analyses are presented in conjunction with textural information, stratigraphic relationships, and U/Pb ages to create a temporal and chemostratigraphic framework for the Mendon Formation. Local and regional stratigraphic variations, including absence of distinctive layers and variation in layer thickness, seen across the Mendon preclude ascription of a single stratigraphy that accurately describes the >1.2 km of section present in this formation. These variations indicate diachronous deposition of the Mendon Formation over a wide areal extent and into multiple basins or sub-basins by more than one magmatic source. 204 Pb-corrected 206 Pb/ 238 U and 207 Pb/ 235 U concordia model ages of 3279 ± 9.1 Ma and 3287.3 ± 2.9 Ma for two samples from upper portions of the Mendon Formation provide temporal context for deposition. Two samples from the basal 10 m of the Fig Tree Group, above the S2 spherule bed that marks the boundary between the Onverwacht and Fig Tree Groups, give model ages of 3267.8 ± 6.9 Ma and 3261 ± 18 Ma. These ages provide added constraints for the Onverwacht-Fig Tree boundary and confirm that the Weltevreden Formation is roughly age-correlative with the uppermost Mendon Formation. While the Mendon and Weltevreden Formations are in part age-correlative and have similar lithologies, they do not appear to be genetically related. The dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the Mendon and Weltevreden Formations indicate that the primary mode of crustal formation was likely plume-related magmatic accretion and not subduction. The relatively sharp transition within the Barberton Greenstone Belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact-related spherule layers, which suggest that major impacts may have played a role in the evolution of early tectonics to more modern, subduction-related styles.
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geochemistry and petrology of komatiites of the pioneer ultramafic complex of the 3 3 ga weltevreden formation barberton Greenstone Belt south africa
Precambrian Research, 2012Co-Authors: Thompson M Stiegler, Gary R Byerly, Marc Cooper, Donald R LoweAbstract:Abstract The 3.3 Ga Weltevreden Formation, located in the northern part of the Barberton Greenstone Belt, contains multiple complexes that are composed largely of well-preserved, partially serpentinized komatiitic rock. The Pioneer Ultramafic Complex is a 1-km-thick section of massive and differentiated komatiites and spinifex-textured komatiitic basalts that are interbedded with current-structured komatiitic tuffs. Most komatiites were emplaced subaqueously as large sheet flows that would on occasion pond, cool slowly and internally differentiate. Between effusive eruptions, explosive komatiitic volcanism resulted in the deposition of up to 60-m-thick sections of fine-grained ash. Massive komatiites preserve primitive olivines with a maximum Mg# of 92.9, supporting the eruption of a high temperature, high magnesian liquid. Al2O3/TiO2 (19–34) and Gd/YbN (0.84–1.15) ratios indicate that the komatiites and komatiitic basalts originated from Al-undepleted to slightly Al-enriched magma. Fresh pigeonite found in the cores of zoned pyroxenes suggests that crystallization occurred under anhydrous conditions. Similar to komatiitic augites analyzed in other areas of the Barberton Greenstone Belt, those in the Pioneer Complex possess both high Wo contents (0.31–0.46) and high Mg#s (75–90.2) but these features are not attributed to high dissolved water contents in the melt.
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chapter 5 3 an overview of the geology of the barberton Greenstone Belt and vicinity implications for early crustal development
Developments in Precambrian Geology, 2007Co-Authors: Donald R Lowe, Gary R ByerlyAbstract:Publisher Summary This chapter presents an overview of the geology of the Barberton Greenstone Belt and vicinity. Rocks in the 3.55 to 3.22 Ga Barberton Granite Greenstone Terrain (BGGT), South Africa and Swaziland, represent one of the oldest, well-preserved pieces of continental crust on the Earth. Together with similar rocks of nearly identical ages in the Pil-bara Craton of Western Australia, rocks of the BGGT have provided most of the direct geologic evidence on the nature and evolution of the pre-3.0 Ga Earth, its crust, surface environment, ocean, atmosphere, and biota. The BGB includes volcanic, sedimentary, and shallow intrusive rocks ranging in age from >3547 to
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ironstone pods in the archean barberton Greenstone Belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs comment and reply reply
Geology, 2004Co-Authors: Donald R Lowe, Gary R ByerlyAbstract:We respond to de Ronde et al. as per their numbered comments. 1. de Ronde et al. suggest that there are two populations of ironstone pods in the Barberton Greenstone Belt: those on Farm Mendon that are Archean in age and composed primarily of hematite and all other ironstone pods, which are young
Robert Kerrich - One of the best experts on this subject based on the ideXlab platform.
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geochemistry of neoarchean ca 2 55 2 50 ga volcanic and ophiolitic rocks in the wutaishan Greenstone Belt central orogenic Belt north china craton implications for geodynamic setting and continental growth
Geological Society of America Bulletin, 2005Co-Authors: Ali Polat, Robert Kerrich, Brian J Fryer, Timothy M Kusky, Katie PatrickAbstract:Geological investigation of the Neoarchean (2.55–2.50 Ga) Wutaishan Greenstone Belt in the central orogenic Belt of the North China craton has provided new information on the geodynamic origin of this Belt and its mineral deposits. Structural, geochronological, and geochemical characteristics of the Wutaishan Greenstone Belt suggest that it formed in a forearc tectonic environment at ca. 2.55 Ga and accreted to the Eastern continental block at ca. 2.50 Ga. A ridge subduction model is proposed to explain several unique geological features of the Wutaishan Greenstone Belt, such as the generation of dunites and chromitite-hosting harzburgites with U-shaped rare earth element (REE) patterns, formation of volcanogenic massive sulphides (VMS) and banded iron formations (BIF), extrusion of mafic to felsic volcanic rocks, and intrusion of tonalite-trondhjemite-granodiorite plutons (TTG). Anomalously high geothermal gradients in the subarc mantle-wedge beneath the Wutaishan forearc may have increased its buoyancy, resulting in its accretion to the continental crust. We propose that ridge subduction also played an important role in the growth of Archean continental crust. In this model, the origin of Archean TTG is genetically linked to eclogites through partial melting of accreted and/or underplated oceanic plateaus and normal oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of an anomalously hot asthenospheric mantle window resulting from ridge subduction. TTG suites intruding Archean accretionary complexes formed the nuclei of intra-oceanic island arcs; subsequent juxtaposition of these arcs resulted in the lateral growth of Archean continental crust.
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formation of an archean tectonic melange in the schreiber hemlo Greenstone Belt superior province canada implications for archean subduction accretion process
Tectonics, 1999Co-Authors: Ali Polat, Robert KerrichAbstract:The late Archean (circa 2750–2670 Ma) Schreiber-Hemlo Greenstone Belt, Superior Province, Canada, is composed of tectonically juxtaposed fragments of oceanic plateaus (circa 2750–2700 Ma), oceanic island arcs (circa 2720–2695 Ma), and siliciclastic trench turbidites (circa 2705–2697 Ma). Following juxtaposition, these lithotectonic assemblages were collectively intruded by synkinematic tonalite-trondhjemite-granodiorite (TTG) plutons (circa 2720–2690 Ma) and ultramafic to felsic dikes and sills (circa 2690–2680 Ma), with subduction zone geochemical signatures. Overprinting relations between different sequences of structures suggest that the Belt underwent at least three phases of deformation. During D1 (circa 2695–2685 Ma), oceanic plateau basalts and associated komatiites, arc-derived trench turbidites, and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex. Fragmentation of these sequences resulted in broken formations and a tectonic melange in the Schreiber assemblage of the Belt. D2 (circa 2685–2680 Ma) is consistent with an intra-arc, right-lateral transpressional deformation. Fragmentation and mixing of D2 synkinematic dikes and sills suggest that melange formation continued during D2. The D1 to D2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating. The D2 intra-arc strike-slip faults may have provided conduits for uprising melts from the descending slab, and they may have induced decompressional partial melting in the subarc mantle wedge, to yield synkinematic ultramafic to felsic intrusions. A similar close relationship between orogen-parallel strike-slip faulting and magmatism has recently been recognized in several Phanerozoic transpressional orogenic Belts, suggesting that as in Phanerozoic counterparts, orogen-parallel strike-slip faulting in the Schreiber-Hemlo Greenstone Belt played an important role in magma emplacement.
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Boninite series: low Ti-tholeiite associations from the 2.7 Ga Abitibi Greenstone Belt
Earth and Planetary Science Letters, 1998Co-Authors: Robert Kerrich, Derek A. Wyman, J. Fan, Wouter BleekerAbstract:Abstract Boninite series volcanic flows, interfingered with komatiites and tholeiitic basalts, occur at several localities in the ∼2.7 Ga Abitibi Greenstone Belt. Flows from Whitney Township, Ontario, have a compositional range of SiO2 44–60 wt%, MgO 24–7.4, Mg# 83–69, and Ni 930–200 ppm. Low TiO2 (0.14–0.31 wt%) but high Al2O3 (13–25 wt%) contents yield variably high Al2O3/TiO2 ratios of 48–100. These boninite series volcanics are characterized by fractionated HREE where Gd/Ybn 0.3–0.7; positive normalized Zr(Hf)/MREE anomalies, and Zr/Hf > 36; generally negative normalized Nb anomalies; and LREE depletion to enrichment (La/Smn 0.72–1.4). Flows with similar compositional affinities occur in the neighbouring Kidd Volcanic Complex and Tisdale volcanic group. Alteration, and/or contamination by continental crust can be ruled out as the cause of the distinctive and coherent compositions. If the areally extensive komatiite–tholeiite association represents an ocean plateau derived from a mantle plume and the boninite series formed in a convergent margin, then the interfingering of komatiite and boninite series flows may represent interaction of a plume with a subduction zone.
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hfse ree fractionations recorded in three komatiite basalt sequences archean abitibi Greenstone Belt implications for multiple plume sources and depths
Geochimica et Cosmochimica Acta, 1993Co-Authors: Q Xie, Robert Kerrich, J. FanAbstract:Abstract Komatiite-tholeiite sequences (2710–2725 Ma) in the Archean Abitibi Greenstone Belt show systematic differences of rare earth element (REE) patterns and high field strength element (HFSE)/ REE fractionations between volcanic centres. Type 1 komatiites are Al undepleted, and have flat REE patterns, with mantle normalized Nb, Zr, Hf/REE ≈ 1. Type 2 komatiites are relatively MgO rich (16–24 wt%), Al undepleted, with LREE depletion and positive normalized Nb, Zr, Hf/REE fractionations. Type 3 komatiites are Al depleted, Fe rich, HREE depleted, and feature negative normalized Hf, Zr/ REE fractionations. The three types of komatiites may have formed in plumes originating at progressively greater depths: type 1 in undepleted mantle at 700 km, that subsequently migrated to shallower depth and melted at Magnesium tholeiites associated with type 1 and 3 komatiites are likely the fractionation products of komatiites at low and high pressures, respectively, whereas type 2 Mg tholeiites originated in an undepleted source relative to the coexisting komatiites with “perovskite signature.” Early Greenstone Belt evolution was characterized by numerous oceanic plateaus developed from multiple plumes originating at different depths from heterogeneous mantle; the plateaus may have been preferentially obducted.
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the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq Greenstone Belt sw greenland
Lithos, 2008Co-Authors: Ali Polat, Brian J Fryer, Robert Frei, Peter W U Appel, Yildirim Dilek, J C Ordonezcalderon, Zhaoping YangAbstract:The Mesoarchean (ca. 3075 Ma) Ivisaartoq Greenstone Belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The Belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks underwent at least two stages of post-magmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. These alteration processes resulted in the mobilization of many major and trace elements. The trace element characteristics of the least altered rocks are consistent with a supra-subduction zone geodynamic setting and shallow mantle sources. On the basis of geological similarities between the Ivisaartoq Greenstone Belt and Phanerozoic forearc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq Greenstone Belt represents a relic of dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of a lower layer of leucogabbros and anorthosites, and an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and sills, and dunitic to wehrlitic sills. The Sm–Nd and U–Pb isotope systems have been disturbed in strongly altered actinolite schists. In addition, the U–Pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration. Gabbros and diorites have the least disturbed Pb isotopic compositions. In contrast, the Sm–Nd isotope system appears to have remained relatively undisturbed in picrites, pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (eNd = + 4.23 to + 4.97) than pillow lavas, gabbros, and diorites (eNd = + 0.30 to + 3.04), consistent with a variably depleted, heterogeneous mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (> 90%) of Ca-rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas. The anorthositic cumulates have significantly higher initial eNd (+ 4.8 to + 6.0) values than the surrounding gabbroic matrix (+ 2.3 to + 2.8), consistent with different mantle sources for the two rock types.
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field and geochemical characteristics of the mesoarchean 3075ma ivisaartoq Greenstone Belt southern west greenland evidence for seafloor hydrothermal alteration in supra subduction oceanic crust
Gondwana Research, 2007Co-Authors: Ali Polat, Brian J Fryer, Robert Frei, Peter W U Appel, Yildirim Dilek, J C Ordonezcalderon, Yuanming Pan, Julie A Hollis, Johann G RaithAbstract:The Mesoarchean (ca. 3075Ma) Ivisaartoq Greenstone Belt in southern West Greenland includes variably deformed and metamorphosed pillow basalts, ultramafic flows (picrites), serpentinized ultramafic rocks, gabbros, sulphide-rich siliceous layers, and minor siliciclastic sedimentary rocks. Primary magmatic features such as concentric cooling-cracks and drainage cavities in pillows, volcanic breccia, ocelli interpreted as liquid immiscibility textures in pillows and gabbros, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic flows are well preserved in low-strain domains. The Belt underwent at least two stages of calc-silicate metasomatic alteration and polyphase deformation between 2963 and 3075Ma. The stage I metasomatic assemblage is composed predominantly of epidote (now mostly diopside)+quartz+plagioclase±hornblende ±scapolite, and occurs mainly in pillow cores, pillow interstitials, and along pillow basalt-gabbro contacts. The origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration. On the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote-rich aggregates, the stage I metasomatic assemblage is interpreted as relict epidosite. The stage II metasomatic assemblage occurs as concordant discontinuous layered calc-silicate bodies to discordant calc-silicate veins commonly associated with shear zones. The stage II metasomatic assemblage consists mainly of diopside+garnet+amphibole+plagioclase+quartz±vesuvianite±scapolite±epidote±titanite ±calcite±scheelite. Given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric, its origin is attributed to regional dynamothermal metamorphism. The least altered pillow basalts, picrites, gabbros, and diorites are characterized by LREE-enriched, near-flat HREE, and HFSE (especially Nb)-depleted trace element patterns, indicating a subduction zone geochemical signature. Ultramafic pillows and cumulates display large positive initial eNd values of +1.3 to +5.0, consistent with a strongly depleted mantle source. Given the geological similarities between the Ivisaartoq Greenstone Belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq Greenstone Belt represents Mesoarchean supra-subduction zone oceanic crust. © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
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geochemical characteristics of the neoarchean 2800 2700 ma taishan Greenstone Belt north china craton evidence for plume craton interaction
Chemical Geology, 2006Co-Authors: Ali Polat, Brian J Fryer, Timothy M Kusky, Joel E Gagnon, Shuanhong ZhangAbstract:Abstract The 2800–2700 Ma Taishan Greenstone Belt, North China Craton, consists of an association of komatiites, pillow basalts, banded iron formations, conglomerates, and greywacke sandstones. The Belt was intruded by high-Al tonalite–trondhjemite–granodiorite (TTG) plutons and deformed under amphibolite-grade metamorphism at about 2700 Ma. At the bottom of the association komatiites are characterized by locally well-preserved spinifex, cumulate, and breccia textures. Spinifex komatiites have uniformly high-MgO (31–33 wt.%), but low-Al2O3 (2.8–5.7 wt.%), TiO2 (0.14–0.24 wt.%), and Zr (4.6–9.6 ppm) contents. Towards the top of the sequence komatiites become less magnesian (MgO = 22–23 wt.%), more aluminous (Al2O3 = 6.2–8.9 wt.%), and display higher concentrations of incompatible elements. Based on [Al2O3] versus [TiO2] molecular relations, these rocks plot mainly in the field of Al-depleted komatiites. Komatiites occurring at stratigraphically lower levels have concave-upward LREE patterns and negative Nb anomalies (Nb / Nb* = 0.19–0.96). Stratigraphically upper komatiites and pillow basalts also share the negative Nb anomalies. There are strong correlations between the initial eNd values and indices of crustal contamination and olivine fractionation, suggesting that the geochemical characteristics of the Taishan komatiites can be best explained by olivine fractionation and assimilation of 2–6% LREE-enriched older continental crust or sediments. Relict olivine grains in the Taishan komatiites have lower Mg-numbers (Fo79–84) than whole-rock (84–90) and serpentine–chlorite–amphibole matrix (90–95), indicating that whole-rock composition is not in equilibrium with olivine composition. This disequilibrium is likely have resulted from the enrichment of Fe in recrystallized olivines during amphibolite metamorphism at ∼2700 Ma. Conglomerates are composed dominantly of gneissic, granitic, and amphibolitic clasts. Grain types and trace element characteristics of sandstones are consistent with a continental source area. There are zircon and apatite xenocrysts in the stratigraphically lower komatiites. Collectively, these geological features are consistent with the deposition of the Greenstone Belt on continental basement. Accordingly, we propose a geodynamic model of plume–craton interaction to explain the geological and geochemical characteristics of the Taishan Greenstone Belt.
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geochemistry of neoarchean ca 2 55 2 50 ga volcanic and ophiolitic rocks in the wutaishan Greenstone Belt central orogenic Belt north china craton implications for geodynamic setting and continental growth
Geological Society of America Bulletin, 2005Co-Authors: Ali Polat, Robert Kerrich, Brian J Fryer, Timothy M Kusky, Katie PatrickAbstract:Geological investigation of the Neoarchean (2.55–2.50 Ga) Wutaishan Greenstone Belt in the central orogenic Belt of the North China craton has provided new information on the geodynamic origin of this Belt and its mineral deposits. Structural, geochronological, and geochemical characteristics of the Wutaishan Greenstone Belt suggest that it formed in a forearc tectonic environment at ca. 2.55 Ga and accreted to the Eastern continental block at ca. 2.50 Ga. A ridge subduction model is proposed to explain several unique geological features of the Wutaishan Greenstone Belt, such as the generation of dunites and chromitite-hosting harzburgites with U-shaped rare earth element (REE) patterns, formation of volcanogenic massive sulphides (VMS) and banded iron formations (BIF), extrusion of mafic to felsic volcanic rocks, and intrusion of tonalite-trondhjemite-granodiorite plutons (TTG). Anomalously high geothermal gradients in the subarc mantle-wedge beneath the Wutaishan forearc may have increased its buoyancy, resulting in its accretion to the continental crust. We propose that ridge subduction also played an important role in the growth of Archean continental crust. In this model, the origin of Archean TTG is genetically linked to eclogites through partial melting of accreted and/or underplated oceanic plateaus and normal oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of an anomalously hot asthenospheric mantle window resulting from ridge subduction. TTG suites intruding Archean accretionary complexes formed the nuclei of intra-oceanic island arcs; subsequent juxtaposition of these arcs resulted in the lateral growth of Archean continental crust.
