Tectonic Implications From Ural Russia for the Columbia Supercontinent

U–Pb zircon geochronology and geochemistry of Paleoproterozoic magmatic suite from East Sarmatian Orogen: tectonic implications on Columbia supercontinent

Authors:

Terentiev et al

Abstract:

Extensive Paleoproterozoic magmatism occurred in the East Sarmatian Orogen (ESO), located at the junction of the Sarmatian and Volga–Ural segments of the East European Craton, generating numerous felsic to mafic-ultramafic complexes. Here we report geochemical and zircon U-Pb age data from a suite of ultramafic–mafic and diorite–granodiorite intrusive rocks from this orogen. SHRIMP U–Pb dating of zircons yielded the following concordant ages for the magmatic suites: (1) 2085 ± 17 to 2093 ± 20 Ma (sub-volcanic high-Mg intrusions of SHMB-like melanorite–melaquartz-diorite–melagranodiorites and quartz-diorite–tonalite–granodiorites); (2) 2072.4 ± 8, 2073 ± 8.3, and 2068 ± 13 Ma (ultramafic–mafic–diorite layered intrusions); (3) 2053 ± 14 to 2058 ± 22 Ma (magnesian–ferroan quartz-diorite–tonalite–granodiorite intrusions) and (4) 2035 ± 18 Ma (ferroan norite–diorite intrusions). The trace and rare earth element geochemistry indicate arc-related features. The enrichment in LREE and LILE in the mafic-ultramafic and diorite-granodiorite suites is correlated with melting of metasomatised lithosphere mantle along with involvement of crustal components in their petrogenesis. The third magmatic event conforms to the age of the post-collision S- and A-granitoids of the region. Our data indicate two major magmatic events in the post-collisional setting, the former immediately after metamorphism with the formation of schistosity in the host rocks, and the latter two during the culmination of the orogenic cycle in the Voronezh Crystalline Massif.

The duration of the Paleoproterozoic orogeny during the collision of the Sarmatian and Volga–Ural segments of the East European Craton is estimated to be approximately 100 million years from ∼2140 to ∼2035 Ma. We propose that the layered orogenic plutons were derived from relatively deeper mantle source, whereas the sub-volcanic rock series are derivatives of the slightly shallow mantle source. The Paleoproterozoic collision in the region broadly coincides with timing of amalgamation of the global supercontinent Columbia.

Age and Evolution of the Deep Continental Root Beneath the Central Rae Craton, Northern Canada

Age and evolution of the deep continental root beneath the central Rae craton, northern Canada

Authors:

Liu et al

Abstract:

Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton.

Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0 to 2.6 Ga overall; 2.8 to 2.6 Ga for Pelly Bay and 3.0 to 2.7 Ga for Repulse Bay), Paleoproterozoic (2.1 to 1.7 Ga), and “Recent” (< 1 Ga, with model ages similar to the ca. 546 Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7-2.6 Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77-1.70 Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (Snyder et al., 2013, in press) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ∼ 200 km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (Snyder et al., 2013, in press; Spratt et al., 2014). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.

The PaleoPosition of the Amazonian Craton in the Calymmian MesoProterozoic

Reassessment of Aguapeí (Salto do Céu) Paleomagnetic pole of the Amazonian Craton and implications for Proterozoic supercontinents

Authors:

D’Agrella-Filho et al

Abstract:

The Aguapei paleomagnetic pole obtained for mafic sills and dykes from Salto do Céu region at the western margin of the Amazonian Craton constrains its links with Baltica and Laurentia in Rodinian reconstructions. A new U-Pb age on baddeleyites at 1439 ± 4 Ma for the intrusives, constrasts strongly with a previous Ar-Ar age at 981 ± 2 Ma, with important consequences for paleogeographic reconstructions. We report new paleomagnetic and magnetic anisotropy results for sills from the Salto do Céu region and reassess the paleomagnetic data in view of the new geochronological age. A total of 155 samples were collected from thirteen new paleomagnetic sampling sites, five of them corresponding to sedimentary rocks located at the borders of the sills in an attempt to perform baked contact tests. After thermal and alternating field demagnetization, the sills provided a characteristic magnetic component at Dm = 208.2°, Im = 68.5° (N = 8, α95 = 6.4°), with a corresponding paleomagnetic pole at 46.4°S; 277.0°E (A95 = 10.2°). Directions obtained in this study are similar to those reported previously for other mafic sills and dykes in the same region. A pole integrating the new results for the sills with those of the previous Aguapeí pole is situated at 56.0°S; 278.5°E (A95 = 7.9°). This new combined Salto do Céu pole supersedes the previous Aguapeí pole. Magnetic mineralogy studies, optical and electronic microscopy indicates PSD magnetite as the main magnetic carrier in these rocks. The baked contact tests were inconclusive, but the similarity between Salto do Céu pole and other high-quality poles with ages around 1420-1430 Ma suggests they carry a primary thermoremanence of that age. The Salto do Céu and other coeval poles are compatible with a connection between Amazonia and Baltica at the Mesoproterozoic in a paleogeographic configuration slightly different from SAMBA (South America-Baltica). At the same time, the new geochronological and paleomagnetic data imply that all paleogeographic interpretations for the position of Amazonia in Rodinia based on the previously published Aguapeí pole (and now renamed as Salto do Céu pole) must be revised.

Lessons From Venus: Impact Origin of Archean Cratons

Impact origin of Archean cratons

Author:

Hansen

Abstract:

Archean cratons consist of crustal granite-greenstone terrains (GGTs) coupled to roots of strong, buoyant cratonic lithospheric mantle (CLM). Although this association is unique to the Archean and formed from ca. 4.0 to 2.5 Ga, the origins of terrestrial cratons are debated. I propose that crustal plateaus, quasi-circular craton-like features (∼1400−2400 km diameter, 0.5−4 km high), on Earth’s sister planet Venus might serve as analogs for Archean cratons. Crustal plateaus, which are isostatically supported by a compositionally controlled low-density root, host a distinctive surface called ribbon-tessera terrain. Ribbon-tessera also occurs as arcuate-shaped inliers in the Venus lowlands, widely interpreted as remnants of rootless crustal plateaus. Within each crustal plateau, surface ribbon-tessera terrain comprises a vast igneous province analogous to terrestrial GGTs, and the plateau root is analogous to CLM. Crustal plateaus and ribbon-tessera terrain collectively represent Venus’ oldest preserved features and surfaces, and they formed during an ancient period of globally thin lithosphere. To explain the linked features of crustal plateaus, a bolide impact hypothesis has been proposed in which a large bolide pierces ancient thin lithosphere, leading to massive partial melting in the sublithospheric mantle. In this model, melt escapes to the surface, forming an enormous lava pond, which evolves to form ribbon-tessera terrain; mantle melt residue forms a strong, resilient buoyant root, leading to plateau support and long-term stability of an individual crustal plateau. Building on the similarity of GGT−CLM and Venus crustal plateaus, I propose an exogenic hypothesis for Archean craton formation in which a large bolide pierces thin Archean lithosphere, causing localized high-temperature, high-fraction partial melting in the sublithospheric mantle; melt rises, forming an igneous province that evolves as a GGT, and melt residue develops a complementary CLM. By this mechanism, Archean cratons may have formed in a spatially and temporally punctuated fashion at a time when large bolides showered Archean Earth.

Short PR.

A Heterodox Position for Continent Atlantica During the Orosirian PaleoProterozoic

Paleomagnetic study on mid-Paleoproterozoic rocks from the Rio de la Plata craton: Implications for Atlantica

Authors:

Rapalini et al

Abstract:

The first successful paleomagnetic study on middle Paleoproterozoic rocks from the Rio de la Plata craton is reported. Samples collected from the Soca and Isla Mala granitic bodies, located in southern Uruguay, provided characteristic remanences that were used to compute the first paleomagnetic poles for the craton for ca. 2.05–2.02 Ga. The poles were complemented by a virtual geomagnetic pole from the slightly older Marincho and Mahoma complexes. The paleomagnetic results suggest fast apparent polar wander at high paleolatitudes for the Rio de la Plata craton. Comparison with coeval poles from the Guiana, Congo–São Francisco and West African cratons indicates that a configuration of Atlantica that resembles their Western Gondwana fit is not supported by paleomagnetic data. The geologic similarities in these four cratons are supportive of a major crustal forming event between 2.2–2.0 Ga. A modified configuration for Atlantica is proposed that is consistent with our new (and older) paleomagnetic data. Atlantica was assembled at 2.1–2.05 Ga at polar latitudes and drifted towards the equator soon afterwards.

Pilbara Craton Controversy: Were There or Were There NOT Hadean Continents?

Hf isotopes in detrital and inherited zircons of the Pilbara Craton provide no evidence for Hadean continents

Authors:

Kemp et al

Abstract:

Predictions of large volumes of stabilized continental crust by the early Archaean stand in stark contrast to the actual amount of pre-3.5 Ga rocks presently exposed on Earth's surface. The Pilbara Craton of Western Australia, one of the best preserved Paleoarchean crustal blocks on Earth, is believed to have developed on a cryptic, possibly ≥3.8 Ga continental basement. If substantiated, this could support the notion of a widespread and enduring Hadean (ca. 4.5–4.0 Ga) felsic–intermediate crust. To test this, and to elucidate the earliest evolution of the Pilbara Craton, we report Hf isotope data from previously dated detrital zircon grains, and inherited zircon crystals hosted by granitic gneisses, the crystallization ages (3.80–3.55 Ga) of which substantially exceed those of the oldest exposed igneous rocks of the craton (∼3.52 Ga). The Hf isotope compositions of the ancient zircons analyzed in this study are consistent with most of the earliest components of the Pilbara Craton being extracted from near chondritic mantle between ∼3.7 and 3.6 Ga, with little or no input from significantly older crust. These new data suggest either that the Pilbara Craton developed remote from the isotopic influence of the putative Eoarchean to Hadean continental masses, or that the stabilized volumes of the earliest continents have been overestimated. The latter scenario would be consistent with the extreme scarcity of greater than 3.9 Ga rocks and minerals, and the dominantly chondritic Hf isotope composition of the oldest continental rocks in Earth's most ancient Archaean cratons.

Paleo-MesoProterozoic Mafic Dykes From Bastar Craton may be Related to the Columbia Supercontinent

Geochemistry and petrogenesis of Paleo-Mesoproterozoic mafic dyke swarms from northern Bastar craton, central India: Geodynamic implications in reference to Columbia supercontinent

Authors:

Srivastava et al

Abstract:

Field setting, petrography, geochemistry and available radiometric ages of Proterozoic mafic dykes from the northern Bastar craton have helped to identify four sets of mafic dykes; two Paleoproterozoic [viz. NW-SE North Bastar dykes (NBD) and ENE-WSW Dongergarh-Chhura dykes (DCD)] and two Mesoproterozoic [viz. 1.42 Ga ENE-WSW Bandalimal dykes (BDD) and 1.44 Ga N-S Lakhna dykes (LKD)]. Their petrographic and geochemical characteristics are very distinct and suggest their derivation from different mantle melts. Chemistry of all the four sets suggests different petrogenetic histories and samples of each distinct set are co-genetic nature. The NBD, the DCD and the BDD samples are sub-alkaline tholeiitic in nature, whereas the LKD samples show alkaline nature. Very distinct REE patterns are observed for all the four sets again suggesting their different petrogenetic histories. Geochemical comparison between the studied samples and mafic dyke samples of southern and central parts of the Bastar craton suggest very different picture for the northern Bastar craton. Only one set of northern Bastar dykes, i.e. the NBD, match with BD1 dykes; no other dyke set match with any of the dyke swarms identified in southern and central Bastar craton. Geochemically it is not straightforward to confirm crustal contamination, however, on the other hand, possibility of crustal contamination cannot be ruled out completely. A petrogenetic model based on trace element data suggests that all the four sets are derived from different mantle melts. The NBD and the DCD are probably generated within spinel stability field, whereas the BDD and the LKD may be derived from melts generated within garnet stability field. Available geological and geochemical data support emplacement of studied dykes in a stable continental rift tectonic setting, however earlier intrusions have chemistry similar to N-MORB. The available geological, geochemical and geochronological data on the four indentified sets of mafic dykes from the northern Bastar craton indicate their relation to the assembly and break-up of Columbia supercontinent.

Paleosol Evidence of Ediacaran NeoProterozoic Gaskiers and/or Fauquier Glaciations From PaleoContinent

Petrology, mineralogy and geochemical climofunctions of the Neoproterozoic Baltic paleosol

Authors:

Liivamägi et al

Abstract:

The Oxisolic Baltic paleosol is a well preserved Neoproterozoic weathering sequence located in the Russian Platform of the East-European Craton, former Baltica continent. The Baltic paleosol is developed on different hostrock types ranging from amphibolites and metagabbros to gneissic rocks and is characterized by up to several meters of thick weathered uppermost residuum composed of kaolinite (greater than 60%), Fe-oxyhydroxides and residual quartz. The mineral index of alteration (MIA) and molecular weathering indices (CIA, CIA-K, PIA) all indicate strong and deep weathering most likely under warm and humid climate. Geochemical climofunctions, although not directly applicable to the Precambrian Baltic paleosol, agree with this interpretation suggesting high mean annual temperatures (greater than 17 °C) and precipitation (1300-1800 mm yr−1) similar to modern day tropical conditions. The age of the weathering is between 560-600 Ma. During that time interval the Baltica continent was not drifting at tropical latitudes, but it is plausible that the Baltic paleosol was formed as a consequence of increased weathering rates during greenhouse event(s) possibly related to the termination of the Ediacaran Gaskiers and/or Fauquier glaciations, or in the relation to the Shuram-Wonoka isotope excursion. The Baltic paleosol is most likely one of the few well preserved examples of paleosol formation in the latest Neoproterozoic because it developed in a tectonically stable interior of the Baltica paleocontinent while physical denudation prevailed on other continental blocks due to the build-up of Gondwana that erased weathering profiles from the geological record.

Evidence of Minas Accretionary Orogeny From Siderian PaleoProterozoic

A juvenile accretion episode (2.35-2.32 Ga) in the Mineiro belt and its role to the Minas accretionary orogeny: zircon U-Pb-Hf and geochemical evidences

Authors:

Teixeira et al

Abstract:

Zircon U-Pb-Hf and geochemical data provide new clues for a juvenile TTG segment in the Mineiro belt, outlined by published information about the nearby 2.35 Ga Lagoa Dourada suite. The Resende Costa Orthogneiss and coeval rocks yield U-Pb crystallization ages from 2351 ± 48 to 2317 ± 16 Ma and coherent Sm-Nd TDM ages between 2.3 and 2.5 Ga. These rocks show ɛNd(2.35Ga) and ɛSr(2.35Ga) values that suggest derivation from a short-lived, slightly depleted source akin to products of island arcs, whereas the geochemistry is consistent with a tholeiitic source with minor crustal assimilation. The positive to negative zircon ɛHf(2.35Ga) suggest the subordinate involvement of crustal components during the hypothesized c. 2.35 Ga slab subduction event. In a similar manner, metabasalts (lower unit) of the adjoining Congonhas-Itaverava belt show Nd-Sr isotopic and chemical constraints indicating derivation from an early Paleoproterozoic tholeiitic source subjected to crustal assimilation. A coeval metagraywacke from the upper unit yields U-Pb zircon detrital ages as young as 2349 + 14 Ma suggesting the maximum depositional age for the precursor basin (foreland setting?) whilst most of the grains indicate contribution from nearby Archean sources. The Resende Costa-Lagoa Dourada rocks characterize the precocious arc magmatism of the Minas accretionary orogeny. This composite orogeny created the Mineiro belt and the adjoining Mantiqueira and Juiz de Fora belts in the 2.35-2.00 Ga interval. Due to such a polycyclic framework the Resende Costa Orthogneiss and coeval rocks show multiple metamorphic overprints dated at 2140 and 2050 Ma. The new and compiled data and regional geologic correlations provide evidence for a growing Paleoproterozoic landmass in the South America continent.

Paleoproterozoic to Mesoproterozoic Intracratonic Basin Cycles

Age constraints on crystal-tuff from the Espinhaço Supergroup — Insight into the Paleoproterozoic to Mesoproterozoic intracratonic basin cycles of the Congo–São Francisco Craton

Authors:

Guadagnin et al

Abstract:

The U–Pb data from the volcanic and detrital zircon grains of the Tombador Formation in Chapada Diamantina, Bahia, provide the depositional age of the top of this unit and define the main sedimentary sources of the Archean, Paleoproterozoic and Mesoproterozoic Eras. The lithofacies, petrographic and geochemical data from crystal-tuff samples indicated that a volcanic source was located to the east of the Chapada Diamantina region. The available zircon data defined three first-order intracratonic basinal cycles, including the Lower (Statherian), Middle (Calymmian) and Upper (Stenian–early Tonian) Espinhaço basins. These sequences are well exposed at the Espinhaço Supergroup type-section in the Chapada Diamantina region. The zircon Hf isotope data from the 1.4 Ga crystal-tuff in the Middle Espinhaço and the 1.2 Ga tuffaceous rock in the Upper Espinhaço suggested different sources, with model ages of 2.1 to 1.9 Ga and less than 1.9 Ga, respectively. The tectonic development of these Paleoproterozoic to Mesoproterozoic intracratonic basins can be explained by the far-field stress at the paleoplate margins, which has been shown in other Phanerozoic intracratonic basins. The data presented here support a tectonic scenario in which the Congo–São Francisco paleoplate interior recorded the main global events related to the geodynamic processes after the Columbia supercontinent collage and the formation and fragmentation of the Rodinia supercontinent.

Volcanic-hosted Massive Sulfide Mineralization in Archean Yigarn Craton, Australia

A review of volcanic-hosted massive sulfide (VHMS) mineralization in the Archean Yilgarn Craton, Western Australia: Tectonic, stratigraphic and geochemical associations

Authors:

Hollis et al

Abstract:

The Archean Yilgarn Craton of Western Australia represents a world-class metallogenic province, hosting considerable resources of Au, Ni-sulfides and iron ore. Despite close geological similarities with the volcanic-hosted massive sulfide (VHMS)-rich Superior Province of Canada, there is a strong disparity in the number of discovered VHMS deposits between the two areas. This paper brings together recently published U-Pb zircon geochronology and stratigraphic constraints from across the Yilgarn Craton, with a large number of existing whole rock geochemical datasets (881 samples from ∼125 localities). Recognized VHMS occurrences are placed in a detailed tectonic and stratigraphic framework. Temporal and geochemical associations to mineralization are discussed. Areas of VHMS mineralization in the Yilgarn Craton are preferentially associated with areas of thinned, juvenile crust as revealed through regional (Nd, Pb) isotope variations. The characteristics identified here for prospective host sequences are: largely bimodal volcanic complexes, synvolcanic faults, a spatial and temporal association to HFSE-enriched synvolcanic intrusions, and the following geochemical signatures of felsic rocks. VHMS-bearing felsic rocks in the Youanmi Terrane and Eastern Goldfields are characterized by low Zr/Y, La/YbCN and Th/Yb ratios, high Sc/TiO2, Sc/V, HFSE and HREE contents, and flat HREE profiles similar to those of Abitibi greenstone belt, Canada, and the Pilbara Craton of Western Australia. Chondrite-normalized REE profiles for felsic rocks overlying 2.82-2.80 Ga plume-related basalts and large igneous complexes of the Youanmi Terrane are flat. Other VHMS-bearing felsic rocks are characterized by slight LREE enrichment (La/SmCN less than 3) and flattish HREE profiles. In the Youanmi Terrane four distinct periods of economic mineralization can be recognized: (i) greater than 2.9 Ga (e.g. Golden Grove, Ravensthorpe), associated with early bimodal-mafic greenstone belts subjected to extension; (ii) c. 2815 Ma (e.g. Austin, Just Desserts, Youanmi), following a major plume event and coeval with the emplacement of large igneous complexes across the northern Youanmi Terrane at shallow levels in the crust (Meeline and Boodanoo suites); (iii) 2760-2745 Ma (e.g. Hollandaire, Mt. Mulcahy) in areas of rift-related magmatism during the deposition of the Greensleeves Formation; (iv) c. 2725 Ma, in the Gum Creek greenstone belt associated with a second major plume event, broadly coeval with the emplacement of high-level sills of the Yalgowra Suite. In the Eastern Goldfields Superterrane, VHMS mineralization formed between c. 2700 and 2680 Ma (e.g. Teutonic Bore camp, Anaconda, Nimbus, Erayinia). All episodes of VHMS mineralization in the Yilgarn show strong temporal and spatial associations to suites of HFSE-enriched granitic intrusions (e.g. Eelya suite, Mt. Kenneth suite, Kookynie supersuite).

The Superior Craton Breakup & Reassembled in the NeoArchean

Neoarchean disaggregation and reassembly of the Superior craton

Authors:

Bédard et al

Abstract:

The southern and western Superior craton of Canada (SWSC) is widely considered to be a tectonic collage accreted from north to south by multiple coeval subduction zones. We propose an alternative non-plate tectonic scenario where SWSC continental fragments are not exotic but derived by partial disaggregation of a heterogeneous older (Superior I) craton in response to a mantle overturn event that started at ca. 2780 Ma. During overturn, radial mantle outflow stretched and disaggregated the lithosphere to create the concentric Neoarchean fabric of the Superior craton, the southern part of which (SWSC) broke up into ribbon continents separated by oceanic tracts. Neoarchean calc-alkaline magmas record interaction between plume-related magma and older crust. A change in the mantle flow field at ca. 2720 Ma caused southward drift of the Northern Superior cratonic block as a result of mantle traction on its lithospheric keel, and SWSC terranes accreted to its leading edge.

West African Sedimentation From the Orosirian PaleoProterozoic From an Enigmatic Source

New insights on Proterozoic tectonics and sedimentation along the peri-Gondwanan West African margin based on zircon U-Pb SHRIMP geochronology

Authors:

De Waele et al

Abstract:

New mapping and age data from rocks of the West African Craton and its western margin, as well as a review of published age data for the region indicate that the southwestern margin of the craton is composed of an Archaean basement of TTG gneisses and granitoid rocks that formed over a long period of time between 3.54 and 2.64 Ga. The age data refute the previously held notion of two craton-forming events, the ∼3.0 Ga Leonian and ∼2.7 Ga Liberian cycles, but instead indicate a pulse at ∼3.4 Ga, followed by near-continuous crustal growth between ∼3.05 and 2.64 Ga. Age data for the Kenema Assemblage, a strip of Archaean rocks separated from the main Archaean exposure further east by Neoproterozoic cover, suggest that this represents a tectonic window.

To the west of the Archaean craton, the Rokel-Kasila Belt occurs, which incorporates two tectonic terranes: the Palaeoproterozoic Kasila terrane, accreted to the Archaean craton and comprised of granulite-facies paragneiss units of the Kasila Group, and the Meso- to Neoproterozoic Marampa terrane, thrust on top of the Archaean basement, and comprised of greenschist-facies schists and metabasites of the Marampa Group. A metavolcanic unit in the Kasila Group provides an age of 1941 ± 4 Ma interpreted to date the emplacement of the Kasila succession. Xenocrystic zircon in the sample suggest the presence of cryptic older crust with components of 2.7-2.6 Ga and 2.2 Ga, and the Kasila Terrane is interpreted to represent a Ganderian-type peri-Gondwanan terrane left attached to the West African Craton. Detrital age data on two schist units of the Marampa Group suggest a maximum age of deposition of between 1076-1030 Ma. The detrital age peaks indicate source terranes in part consistent with a West African affinity, but also comprising significant sources of between 2.0 and 1.0 Ga for which no suitable source terranes are known in West Africa. We suggest that at the time the Marampa Group was deposited along the West African margin at ∼1.05 Ga, a source terrane with significant matching components of Palaeoproterozoic and Mesoproterozoic source rocks, was present to provide the sedimentary input, possibly the Amazonian Craton.

Subduction-Driven Plate Tectonics MAY Date From MesoArchean

Petrogenesis of Neoarchean TTG rocks in the Yangtze Craton and its implication for the formation of Archean TTGs

Authors:

Wu et al

Abstract:

The Yangtze Craton is one of the most important cratonic blocks in eastern Asia. However, its early evolution has not been well constrained yet, because of the poor exposure of Archean basement rocks. In this contribution, a combined study of zircon U-Pb age, Hf-O isotope compositions, as well as whole rock element compositions was carried out for gray gneiss rocks from the Douling complex in the northwestern part of the Yangtze Craton. Zircon U-Pb dating for two gneiss samples yielded weighted mean ages of 2509 ± 21 and 2496 ± 15 Ma. The gray gneisses have high SiO2 (63.64-73.74%), Na2O (3.66-6.21%), but low K2O (0.76-2.93%) contents, and belong to the trondhjemite, tonalite and granodiorite series rocks. They are characterized by enrichments of LILEs and LREEs, but notable depletions in Nb–Ta and Ti, as well as heavy REEs and Y. All the samples plot into the fields of typical Archean TTG rocks in the (La/Yb)N vs. (Yb) N and Sr/Y vs. Y diagrams. Therefore, the gray gneisses in the Douling complex are Neoarchean TTG rocks. The relatively high Nb/Ta and Zr/Sm ratios of 19.6-46.0 and 14.5-143.4 suggest their formation under eclogite-facies conditions. They have low ɛHf(t) values of -3.21 and -3.39 and Hf model ages of 3194 ± 26 and 3205 ± 24 Ma, indicating their derivation from ca. 3.2 Ga juvenile mafic rocks. Their δ18O values of 6.41 ± 0.12 and 5.44 ± 0.12 ‰ suggest that surface rocks were differently recycled into their sources. Collectively, the TTG rocks in the Douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary. A compilation of available zircon U-Pb and Hf isotope compositions of the Yangtze Craton reveals that there are quite differences of Hf isotope compositions in Archean. The ca. 3.8-3.4 Ga zircons exhibit ɛHf(t) values of -5.8 to 0.0. These chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra-crustal reworking of ancient protoliths without juvenile input. By contrast, zircons with ages younger than ca. 3.2 Ga register both negative and positive ɛHf(t) values up to the depleted mantle, providing clear evidence for the addition of juvenile materials. These imply that the subduction-driven plate tectonics might have been operated since ca. 3.2 Ga in the Yangtze Craton.

Evidence of Continental Rifting in the Columbia Supercontinent

Constraints on the Statherian evolution of the intraplate rifting in a Paleo-Mesoproterozoic paleocontinent: New stratigraphic and geochronology record from the eastern São Francisco craton

Authors:

Fiho et al

Abstract:

An integrated approach of stratigraphic analysis and U-Pb age dating reveals some information on the tectono-sedimentary evolution of the Statherian cover of the São Francisco craton in the so-called Espinhaço basin (Atlantic shield in eastern Brazil). Here, continental sedimentation patterns, such as alluvial fan, braided-plain and lacustrine facies associations, with associated volcanic rocks are documented in two superposed basin fill-successions, which are defined as the Algodão and Sapiranga Synthems and grouped in the Botuporã Supersynthem. Both studied units consist mainly of conglomerates and cross-bedded sandstones and minor amounts of mudstones, sedimentary breccias, volcanic lava beds and volcaniclastic rocks, which were deposited in a rift basin – the Botuporã rift – during two syn-rifting phases. The Algodão Synthem represents the first rifting phase. The basal synsedimentary conglomerates of this unit were deposited mainly by subaerial debris flows, most likely along and near a rift border fault. The framework of this rock consists of only crystalline rock clasts from the basement and no fragments of volcanic rocks. Detrital zircons grains that were extracted from this facies show ages older than 2.05 Ga. The remainder of the section is dominated by fluvial sandy lithofacies with minor conglomerate lenses and sandstone-mudstone heterolithic lithofacies, which represent distal, waning-flood deposits in a lacustrine environment. The upper section also contains hummocky cross-stratified sandstone lithofacies, which are related to a storm-influenced deposition. On top of the Algodão succession, the volcanic rocks were dated at 1775 ± 7 Ma, which was interpreted as the near final age of the first rift-phase. Representing the second rift-phase, the Sapiranga Synthem shows similar sedimentation patterns to the Algodão Synthem. The Sapiranga Synthem rests directly on the volcanic rocks of the Algodão Synthem, and its basal conglomerates (which are most likely also related to a master fault) contain voluminous clasts of volcanic rocks, sandstones and crystalline rocks. The detrital zircons grains that were extracted from this facies show ages of 1741 ± 14 Ma. The volcanic rocks on the upper succession of the Sapiranga Synthem record ages of 1740 ± 10 Ma, which finalized the Botuporã rift evolution. A preliminary geochemical study of volcanic rocks from the Botuporã Supersynthem showed that these rocks are alkaline rocks with high K2O/Na2O ratios, which belong to an ultrapotassic suite. The low concentrations of MgO wt% suggest a felsic ultrapotassic character. The Botuporã Supersynthem is unconformably covered by a volcano-sedimentary rift-succession of EoCalymmian age – the Pajeú Synthem, which represents the second rifting stage of the Espinhaço basin. Several Statherian-related volcano-sedimentary sequences and anorogenic granitoids were dispersed in the São Francisco block, which requires a regional geologic model to explain the extensional and magmatism process during this time. We used the last Columbia paleocontinental reconstruction to constrain these processes by relating them to far-field continental extensional and magmatic record as part of a silicic LIP, which was triggered by the convection-driven tectonic-plate motion on the western border of the Atlantica block inside of the Columbia supercontinent.

Evidence of Dharwar Craton Being at the Continental Margin in the NeoArchean

Neoarchaean felsic volcanic rocks from the Shimoga greenstone belt, Dharwar Craton, India: Geochemical fingerprints of crustal growth at an active continental margin

Authors:

Manikyamba et al

Abstract:

The felsic volcanic rocks of Neoarchaean Shimoga greenstone terrane of western Dharwar Craton in India are dominantly represented by rhyolites occurring at stratigraphically upper horizons. The Shimoga rhyolites are associated with conglomerates, quartzites, argillites, limestones, cherts, basalts and intermediate volcanic rocks clearly suggesting an accretionary package. The rhyolites of Daginkatte and Shikaripura areas are potassic, with porphyritic alkali feldspar and quartz as essential minerals and chlorite, biotite and opaques as accessory phases. Geochemically, the rocks show enrichment in LILE and depletion in HFSE relative to primitive mantle values, with negative Nb-Ta, Zr-Hf anomalies and positive Th anomalies. These features of the Shimoga rhyolites compare well with the geochemical characteristics of magmas generated in subduction-related tectonic settings. Their alkaline compositions, intermediate to low HFSE abundances, moderate to high Zr/Y values (1.5–8.3), with La/Ybn (2–28), pronounced negative Eu anomalies, and variable LREE/HREE fractionation trends resemble the FI and FII rhyolites of Wabigoon and Uchi belts of Superior Province, Canada. The Shimoga rhyolites are interpreted to be products of melting of thick basaltic crust metamorphosed to amphibolite/eclogite grade, with garnet- and amphibole-bearing mantle residue. The rhyolites show prominent negative Eu and Ti anomalies, moderate to strong LREE fractionation, flat to mildly fractionated HREE patterns and are geochemically analogous to Type 1 and Type 3 rhyolites of Superior Province, Canada suggesting their derivation from intracrustal melting and fractional crystallization of basaltic liquids with prominent contribution from mantle wedge and slab components. Our data suggest the contribution of Neoarchaean active continental margin processes for the growth and evolution of continental crust in the western Dharwar Craton.

Evidence of the Neoarchean in the North China Craton

Zircon U–Pb–Hf isotopes and geochemistry of Neoarchean dioritic-trondhjemitic gneisses, Eastern Hebei, North China Craton: Constraints on petrogenesis and tectonic implications

Authors:

Bai et al

Abstract:

Dioritic-trondhjemitic gneisses of Neoarchean age are widely distributed throughout the Gualanyu–Shangying area in Eastern Hebei, North China Craton. These orthogneisses consist of dioritic, trondhjemitic, and quartz-rich trondhjemitic gneisses, which were subjected to amphibolite- to granulite-facies metamorphism. LA-ICP-MS zircon U–Pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535 ± 23 to 2513 ± 8 Ma, reflecting an important Neoarchean dioritic-trondhjemitic magmatic event followed by sequential Proterozoic tectonothermal events between 2472 ± 60 and 2307 ± 50 Ma.

Dioritic and trondhjemitic gneisses exhibit similar zircon ɛHf (t) values of +1.5 to +5.2 and +2.9 to +5.3, respectively, indicating their depleted mantle affinity. Geochemical analysis shows that the dioritic gneisses exhibit low SiO2 contents (55.4%–63.0%), various MgO contents (2.07%–4.78%) and Mg# (36.07–63.56), and high (La/Yb)N ratios (5.45–15.19). In contrast, the trondhjemitic gneisses exhibit higher SiO2 contents (67.4%–73.8%) and (La/Yb)N ratios (12.86–76.73), and lower MgO contents (0.66%–2.67%), although their Mg# (36.29–61.41) are similar to those of dioritic gneisses. These geochemical and zircon Lu–Hf isotopic features, together with geochemical modeling, suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab-derived melts or fluids, and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas. Quartz-rich trondhjemitic gneisses exhibit extremely high SiO2 contents (77.4%–84.6%) with zircon ɛHf (t) values ranging from −3.4 to +4.1, suggesting that their precursors formed after the partial melting of a mixed magmatic source. It is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids, and a felsic member derived from the partial melting of ancient crustal material. These new data, combined with previous studies of contemporaneous metavolcanic rocks in Gualanyu-Shangying area and surrounding areas, suggest that the magmatic precursors of these Neoarchean dioritic-trondhjemitic gneisses formed in a subduction-related tectonic environment at an E-W strike convergent plate margin. Furthermore, the Eastern Block of the North China Craton had a major lateral accretion of crustal growth in the north margin in the Neoarchean, whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the Eastern Block.

Investigating the Crustal Root of the Eastern Dharwar Craton in India

Crustal root of the Eastern Dharwar Craton: Zircon U–Pb age and Lu–Hf isotopic evolution of the East Salem Block, southeast India

Authors:

Glorie et al

Abstract:

The Salem Block represents the northern part of the Southern Granulite Terrane in SE India, adjacent to the southern Dharwar Craton. New zircon U–Pb and Lu–Hf results on charnockites, granitoids and migmatites from the eastern Salem Block indicate that it mainly consists of bimodal juvenile Neoarchaean – early Palaeoproterozoic (∼2.75–2.65 Ga and ∼2.55–2.48 Ga) crust that underwent peak metamorphism at ∼2.53 Ga. These U–Pb ages and Lu–Hf isotopic signatures characterize the Salem Block as part of the lower crust of the Eastern Dharwar Craton. The oldest (∼2.75–2.65 Ga) granulites can be traced along important shear zones (Salem-Attur and Gangavalli Shear Zones), which may thus expose deeper levels of Eastern Dharwar lower crust, in a way similar to that proposed for the Nilgiri granulites. An additional (minor) contribution of recycled Western Dharwar Craton in the Salem Block is suggested by the presence of limited ∼3.65–3.35 Ga recycled crust and ∼2.9–2.6 Ga detritus.

Younger intrusions within the Salem Block include the Cryogenian (∼820–800 Ma) Yelagiri alkaline complex and the Ediacaran (559.1 ± 3.5 Ma) Sankari-Tiruchengode granitoid complex. These were sourced from recycled Neoarchaean protoliths of the Salem Block. Cryogenian-Tonian age magmatism is widespread within the Southern Granulite Terrane and is possibly related to initial subduction of the Mozambique Ocean. The Ediacaran A-type granitoids are thought to have formed during the Malagasy Orogeny that is related with the final consumption of the Mozambique Ocean and the collision of India with Azania-East Africa.

Was the Grawler Craton a Continental Margin During the NeoArchean Through PaleoProterozoic

SHRIMP U-Pb zircon age constraints on the tectonics of the Neoarchean to early Paleoproterozoic transition within the Mulgathing Complex, Gawler Craton, South Australia

Authors:

Reid et al

Abstract:

The Mulgathing Complex within the Gawler Craton, South Australia, preserves evidence for magmatism, sedimentation and metamorphism spanning the transition between the Neoarchean and Paleoproterozoic (ca. 2555–2410 Ma). Prior to this study, limited data were available to constrain the timing of these tectonothermal events. We report SHRIMP zircon U-Pb dating of metamorphosed sedimentary and magmatic rocks from the Mulgathing Complex that shows metasedimentary gneisses (Christie Gneiss) have maximum depositional ages ca. 2480 Ma, in contrast to previous studies that have suggested deposition had occurred ca. 2510 Ma. The oldest metamorphic zircons in our data are ca. 2465 Ma, thus indicating there was a time interval of less than 15 Myr between the cessation of sedimentation and the occurrence of high-grade metamorphism. Metamorphic zircons have a range of ages, from ca. 2465 to ca. 2415 Ma, consistent with a period of ∼50 Myr during which high-grade metamorphism occurred. Mafic and felsic intrusions have ages that range from ca. 2520 Ma to 2460 Ma, indicating magmatism occurred during sedimentation and continued during the early stages of metamorphism and deformation of these rocks. The presence of bimodal magmatism with continental arc affinity (Devils Playground Volcanics) together with mafic intrusions showing temporal overlap with sedimentation within the Mulgathing Complex suggests that the overall tectonic regime prior to deformation likely involved lithospheric extension, possibly related to a continental magmatic arc. The Mulgathing Complex shows similarities in the nature and timing of these tectonothermal processes with several other Neoarchean - Paleoproterozoic terranes, in particular the Terra Adelie Craton in Antarctica, the Sask Craton, Canada, and regions within the North China Craton and the North Australian Craton. It is possible that similarities in process could imply along-strike relationships between these terranes, for example as laterally continuous terranes at the margin of a former continental domain (or domains). Basement rocks with ages of ca. 3250–3150 Ma, ca. 2950–2800 Ma and 2600–2550 Ma within the Gawler Craton, as inferred from magmatic and inherited zircons, are temporally equivalent to some of the main basement units within the Pilbara Craton, and basement rocks in both the Gawler and Pilbara cratons are overlain by sedimentary rocks at ca. 2480 Ma that include banded iron formations. It is possible to envisage the Gawler Craton as representing an extended continental margin adjacent to a larger, more intact continental domain, such as the Pilbara.

Evidence From Orosirian PaleoProterozoic Hongcheon, Korea Links to Orogeny in North China Craton

Paleoproterozoic magmatic and metamorphic events in the Hongcheon area, southern margin of the Northern Gyeonggi Massif in the Korean Peninsula, and their links to the Paleoproterozoic orogeny in the North China Craton

Authors:

Lee et al

Abstract:

The Hongcheon area is located at the northern part of Gyeonggi Massif (NGM) in the Korean Peninsula. The metamorphic rocks of the NGM at the Hongcheon area are composed of the Paleoproterozoic Yongduri Gneiss Complex (YGC), Euiam Group (EG) and the Euiam Gneiss Complex (EGC). The quartz-feldspathic gneisses in the northeastern part of the YGC record peak metamorphic temperature–pressure condition of 790–840 °C and 7.2–8.9 kbar, whereas the granitic and garnet gneisses in the western part of the YGC record peak metamorphic temperature–pressure conditions of 690–720 °C and 6.1–6.9 kbar, and 640–660 °C and 5.0–5.4 kbar, respectively. The peak metamorphic conditions represent a regional low-P/T metamorphic event (M2) in which metamorphic temperature and pressures increased towards the east. SHRIMP zircon U–Pb age dating indicates that the M2 metamorphic stage occurred at ca. 1886–1870 Ma. The presence of relict kyanite in the gneisses within the YGC suggests that the intermediate-P/T metamorphic stage (M1) occurred in ca. 1925 Ma. Whole-rock geochemistry of the metagranitoid in the EGC indicates that they are originally post-collision granitoid. SHRIMP zircon U–Pb age dating indicates that the metagranitoid formed at ca. 1885–1869 Ma. Considering the similarity in age between the Paleoproterozoic intrusion and the M2 low-P/T metamorphism, the M2 metamorphism also occurred in a post-collision tectonic setting and the M1 intermediate-P/T metamorphism with kyanite represents collision event. Further, we suggest that the Paleoproterozoic orogeny in the Hongcheon area is closely linked to the Paleoproterozoic orogeny in North Korea and the Jiao-Liao-Ji belt in the North China Craton.