Metavolcano-sedimentary Sequence Research Articles

Source and age of the Ngaye banded iron formations, Adamawa Yadé Domain, Central Cameroon: Constraints from whole-rock geochemistry and U-Pb zircon geochronology

The Ngaye inlier of the Adamawa Yadé Domain is located in the Central African Fold Belt, adjacent to the Nyong Complex at the northern margin of the Congo craton, in central Cameroon. This area consists of metamorphosed granite-greenstone associations comprising amphibole- and pyroxene-rich banded iron formations (BIFs), amphibolites, and migmatitic gneisses. This study reports detailed petrographic, whole-rock geochemical, and LA-ICP-MS U-Pb zircon data for the Ngaye BIFs and associated amphibolites to better constrain their source, tectonic setting, and age. Amphibole-rich BIFs exhibit high REE-Y content and uncommon LREE-enriched patterns. In contrast, pyroxene-rich BIFs display combined low REE-Y content, seawater-like patterns, and positive Eu anomalies, suggesting a mixture of seawater and hydrothermal fluids during their deposition. The geochemical data of associated amphibolites suggest a back-arc setting for the Ngaye metavolcanosedimentary sequence, similar to that of the Nyong Complex. LA-ICP-MS U-Pb zircon dating of amphibole-rich BIFs indicate a maximum depositional age of ca. 2186 Ma and subsequent metamorphism at ca. 2038 Ma, overlapping with that of metavolcanosedimentary sequences from the Nyong Complex. Neoproterozoic age of ca. 598 Ma obtained for these BIFs is interpreted as the Pan-African metamorphic/hydrothermal imprint. This finding suggests that the post-depositional fluid overprint was probably related to the regional Pan-African tectono-thermal event.

Combined geochronological U-Pb, Hf isotopes and trace element zircon studies from Piedra Alta Terrane, Rio de la Plata Craton (West Uruguay, South America): A geodynamic model

This work delves into the intricate Paleoproterozoic geological setting of Uruguay, focusing on the Rio de la Plata Craton (RPC) and its prominent constituent Piedra Alta Terrane (PAT). The PAT constitutes the core of the RPC, characterized as a complex geological entity that encompasses a granite-migmatite basement, metamorphic belts, granite intrusions, and Proterozoic magmatism. To the west, it interfaces with the Transbrasiliano-Kandi tectonic corridor, bounded by the Late-Neoproterozoic Pampia Terrane. The PAT s southeastern boundary extends to Sierra de Tandil and perhaps to Sierra de La Ventana, demarcating its limits with Phanerozoic rocks of the North Patagonian Massif. The Sarandí del Yí shear zone defines the eastern limit of the PAT. Despite varied interpretations of the Piedra Alta Terrane’s significance, it is important to note that consensus prevails regarding its four major associations: a granite-gneiss-migmatitic basement complex, three volcanosedimentary belts of low to medium metamorphic grade, diverse late igneous intrusions, and extensional Statherian magmatism (1.79 Ga), represented by tholeiitic gabbroic dikes. In this study, a novel tectonic model for the PAT evolution emerges, supported by an extensive zircon dataset comprising new U-Pb ages, Hf isotopes, and trace element geochemistry. The analytical core involves U-Pb analyses on zircon for dating igneous rocks and establishing the provenance and maximum age of deposition of metavolcanosedimentary sequences. Lu-Hf isotopic analyses on the same zircon crystals provide petrogenetic insights, revealing episodes of crustal growth and minor recycling. Trace element geochemistry and zircon εHf(t) data further accentuate these findings, hinting at mantle source enrichment due to subduction within an intraoceanic magmatic-arc arc setting.

Structural analysis applied to the tectonostratigraphic reconstruction of the Abadia dos Dourados Metavolcano-sedimentary Sequence, southern Brasília Orogen: Implications for the West Gondwana assemblage

Structural analysis is a valuable tool for organizing the stratigraphy of metamorphic terranes. It allows the reconstruction of pre-to sin-orogenic geological scenarios by unraveling the main transformations, such as distortion, rotation and translation, associated with the collisional process in orogenic belts. The Brasília Orogen, for example, encompasses a complex spectrum of lithological associations and structural patterns that are not well understood because this orogenic belt still has a limited coverage of detailed structural analysis work. The implications of this are problematic for the reconstruction of pre- and sin-collisional processes related to the evolution of western Gondwana. Therefore, this contribution focuses on the Abadia dos Dourados Metavolcano-sedimentary Sequence located between the states of Minas Gerais and Goiás, Brazil. The study includes a field-based structural investigation, including geologic mapping and structural analysis at various scales of observation. It was recognized that the Abadia dos Dourados Metavolcano-sedimentary Sequence comprises a bimodal volcanic association with metarhyolite, metabasalt, banded iron formation and tourmalinite, as well as metapelites and metapsamites from base to top. The data provide additional evidence for a rift-related or back-arc basin-related model for the origin of the Meso-to Neoproterozoic volcano-sedimentary sequences in the Brasília orogen. The structural framework indicates a northeast-southwest crustal shortening event associated with the collision of the Paranapanema and São Francisco-Congo crustal blocks. The event initially produced a large (about 10 km) northeast-verging recumbent fold nappe in the area, which caused a stratigraphic inversion of the metavolcanic-sedimentary sequence. The structure underwent progressive deformation, resulting in superposition patterns between folds, foliations, and intersection lineations. In addition, the structural framework suggests that the West Gondwana collage may be related to a progressive rather than a multiphase structural evolution in the southern Brasília Orogen.

Biogenic signatures in the Cryogenian Brusque Metamorphic Complex, Brazil: Geochronological and paleoclimatological implications

The Cryogenian was a period of drastic environmental changes across the entire planet, with glacial erosion limiting the preservation of life forms in most of the geological record. Therefore, to explore more of the biogenic evidence and its implications for this period, here we investigate some metavolcano-sedimentary sequences from the Brusque Metamorphic Complex in Brazil, that record a mid-latitude basin so far unexplored from a paleontological point of view. On the one hand, field discoveries reveal the occurrence of Conophyton-type stromatolites preserved in dolomitic marbles that are isotopically incompatible with cap carbonates. On the other hand, 13C-depleted graphitic schists interspersed in these marbles indicate an accumulation from exceptional volumes of organic matter. Raman spectroscopy and transmission electron microscopy also highlight disordered structures, heterogeneous morphologies, and structures similar to ultra-small microorganisms in this graphite, in compatibility with a biogenic origin. Combined with geochronological data, these records reflect formation during the late Cryogenian greenhouse, between the Sturtian and Marinoan Snowball Earth glaciations. From this, we suggest deposition under conditions of warmer and brighter waters without ice cover, favoring the growth of some stromatolitic barriers. Sturtian nutrient-rich meltwater pulses may also have led to exceptional phytoplankton blooms followed by mass mortality, promoting the exceptional deposition of organic-rich horizons during the transition window to a Precambrian algae-dominated world.

Geology and geochronology of Cerros San Juan formation (department of Colonia, Uruguay), Río de la Plata Craton

The Cerros San Juan Formation is located in the southwest of the Colonia Department, in Uruguay. It is represented by a metavolcanosedimentary sequence in green schist facies, locally reaching amphibolite facies. This formation outcrops poorly into two separated blocks by a dextral N30°W fault. These blocks, named East and West, present N30° - 50°E and N50° - 70°E general trends, respectively. The unit is largely covered by Cenozoic deposits and soil cover and its main exposure occurs in the San Juan Hill. The metasedimentary sequence is represented by marbles with tremolite, schists with talc, schists with chlorite, schists with quartz and epidote, metaconglomerates, quartzites rich in hornblende and quartzites sensu stricto. The metavolcanic sequence comprises metatuffs, metarhyolites, metarhyodacites, metaandesites and metabasalts. Associated granophyric rocks occur locally. The whole rock chemical analysis of the metavolcanic unit shows that the acidic metalavas have calc-alkaline and peraluminous affinity, while the basic metalavas have tholeiitic to transitional and metaluminous affinity. The metalavas of the San José Belt, developed to the east of Cerros San Juan Formation, show similar geochemical behavior in general terms. The isotopic data obtained by zircon U–Pb SHRIMP (ca. 2.1 Ga), together with the petrological and geochemical data, suggest that the sequence could be related to a convergent or orogenic environment.

Basement inliers of the Araçuaí-West Congo orogen: key pieces for understanding the evolution of the São Francisco-Congo paleocontinent

The Araçuaí-West Congo orogen is a confined orogen, formed by the inversion of a gulf of the São Francisco-Congo paleocontinent during the Neoproterozoic assembly of Gondwana. In this convergent process, fragments of the gulf's continental margin were tectonically dismembered and involved in the growing orogenic pile, and these domains are now exposed by the erosion as basement inliers within the orogen. These basement assemblages (i.e. rocks older than 1.8 Ga) occupy almost 30% of the orogen and occur in various domains along its external zone , amid Meso-to Neoproterozoic metaigneous and metasedimentary rocks. The main recognized basement domains are named Itarantim, Gavião, Porteirinha, Guanhães, Gouveia, Quadrilátero Ferrífero, Mantiqueira and Juiz de Fora (South America), and Kimezian (Africa). They comprise Archean gneisses, granitoids and metavolcanosedimentary sequences and various Rhyacian-Orosirian plutonics and supracrustal units, whose history trace back to the Minas-Bahia orogeny, event responsible for the assembly of the São Francisco-Congo paleocontinent. In this paper we present a detailed and unprecedented review of literature that aims to condensate the state-of-the-art of the geological knowledge about each of these basement inliers and, based on that, to discuss and propose paleogeographic scenarios for the Minas-Bahia orogeny. Due to the confined nature of the Araçuaí orogen, all their basement inliers correspond to fragments of the São Francisco-Congo paleocontinent reworked during the Neoproterozoic Brasiliano orogeny. In the context of the Rhyacian-Orosirian Minas-Bahia orogeny, Guanhães, Gouveia, Porteirinha and Gavião domains are understood as parts of the continents involved in the collision, while the Mantiqueira, Kimezian, Juiz de Fora and Itarantim domains mostly record magmatic arcs, formed in various environments. This review revealed that the density of geological information available for each basement inlier is very uneven, and that there are many gaps for further investigation in all the domains.

The El Cortijo Formation: Ocean Plate Stratigraphy in the Tandilia Belt Paleoproterozoic basement (Argentina). Insights into subduction-accretion processes in the southern sector of the Río de la Plata Craton

An Ocean Plate Stratigraphy (OPS) sequence is a record of the oceanic crust geological evolution which allows the identification of paleo-oceans preserved in suture zones of orogenic belts. In the Paleoproterozoic basement of the Tandilia Belt (Argentina), Río de la Plata Craton, the El Cortijo Formation (ECF) crops out in a large suture zone with an E-W orientation and vertical to subvertical inclination. It includes a low-grade metavolcano-sedimentary sequence constituted by metacherts, metabasites, metacarbonate, metawacke, and phyllite. In order to provide genetic interpretations of the ECF, based on the OPS model, petrographic, geochemical, and geochronological studies of some of these rocks were performed. Mineral assemblages identified in metabasites, metawacke, and phyllite indicate a transition from prehnite-pumpellyite to greenschist facies conditions. Metabasites, classified as metavolcanite of intermediate to basic composition, chlorite schist and tremolite-actinolite schist, are interbedded between metacherts and metacarbonate bands. Geochemical analyses revealed particular chemical fingerprints which indicate that the protoliths of these metabasites would have been part of a volcanic arc rock association consistent with adakites, Nb-enriched basalts, high-Mg andesites, and “normal” tholeiitic to calc-alkaline basalts and andesites. The metawacke and the phyllite seem to derive from wackes and semi-pelites, respectively, whose main source is consistent with igneous-metamorphic rocks of acidic to intermediate composition linked to a continental arc system. The first LA-ICP-MS U–Pb data obtained for detrital zircons of the phyllite from the ECF yielded a weighted average age of 2138 ± 9 Ma and a youngest individual 207Pb/206Pb spot age of 2047 ± 22 Ma, which locates the maximum sedimentation age in the early Orosirian, during the maximum tectono-magmatic activity of the Tandilia Belt. According to Lu–Hf data, phyllite zircons derived from crustal sources and yielded Archean Hf model ages. Moreover, εHf(t) data of this rock exhibit a more subchondritic signal than the corresponding for some plutons of the Tandilia Belt, which could be indicating that zircons were not derived from plutons but are instead representative of the host rocks of such plutons. In this sense, the information indicates that the top of the sequence, constituted by the metawacke and the phyllite, represents the arrival of the oceanic plate to the trench zone. The study of the ECF has revealed conclusive evidence to postulate that the whole unit corresponds to the upper part of an OPS sequence of convergent type linked to intra-oceanic arc and continental fore-arc trench tectonic environments. The presence of this oceanic sequence, now preserved in a subduction-accretion complex, allows the Tandilia Belt to be classified as a Paleoproterozoic Pacific-type orogenic belt.

Unravelling major magmatic episodes from metamorphic sequences of the Dom Feliciano Belt central sector, southernmost Brazil – A comparative study of geochronology, elemental geochemistry, and Sr-Nd data

The geochemical and isotopic signatures of magmatic events preserved in deformed metavolcano-sedimentary sequences may inform about geological setting and original basin stratigraphy. This paper discusses elemental and isotopic geochemical literature data on metavolcanic rocks interleaved with highly-deformed metasedimentary rocks from the Dom Feliciano Belt central sector, southernmost Brazil: the high-grade Várzea do Capivarita, and the lower greenschist to amphibolite facies Porongos and Passo Feio complexes. We also provide new geochemical data for seven samples and two U-Pb zircon determinations. REE patterns, together with geochronological data, permit to sort the dataset into three main groups: i) Group 1 - LaN/YbN ≈ 10, Eu/Eu* = 0.38 to 0.86; ii) Group 2 - LaN/YbN = 12 to 44, no Eu anomaly, and iii) Group 3 - negative Eu anomaly (Eu/Eu* ≈ 0.70), but higher LREEs absolute values (LaN/YbN ≈ 22), and rare basic metavolcaniclastic rocks (Bmvc). New U-Pb zircon analyses of two Passo Feio acidic metavolcanic rocks yield a crystallization age of 580 ± 2 Ma (2σ). Group 1 samples account for a magmatic event at 800–770 Ma in the Várzea do Capivarita and Porongos (Cerro da Árvore sequence) complexes. They represent a magmatic association produced from subduction-related sources, with crustal contribution shown by 87Sr/86Sr(790 Ma) > 0.715 and ƐNd(790 Ma) from −5 and −11 (up to –22). Groups 2 and 3 account for a magmatic event at ca. 600 to 580 Ma in the Passo Feio and western Porongos (Capané sequence) complexes. Groups 2 and 3 rocks are interpreted to have been sourced from subduction-related enriched mantle without major crustal contamination, with 87Sr/86Sr(580 Ma) = 0.7035–0.7050 and ƐNd(580 Ma) > -10. For the basic metavolcanoclastic sample (Bmvc), the data suggest mantle-derived origin without significant contribution of subduction-related sources, possibly indicating multiple sources for this newly-described and not yet fully understood Ediacaran magmatism. Our comparative study leads to the novel conclusion of a shared tectonic evolution between the Passo Feio Complex and the western Porongos Complex at ca. 600–580 Ma. They also corroborate a shared tectonic evolution between the Porongos Complex and Várzea do Capivarita Complex at ca. 800–770 Ma, as reported by previous authors.

Thermal evolution of a Variscan syn-orogenic intracontinental basin (Servoz basin, Western Alps): RSCM geothermometry and geochronology

The Variscan basement of the Aiguilles-Rouges massif (Western Alps) exposes the Servoz syncline which consists of a metavolcano-sedimentary sequence composed of (i) a volcanic unit of unknown age and origin, (ii) Early Carboniferous sedimentary series affected by the Variscan orogeny and intruded by the Montées-Pélissier pluton, and (iii) a Late Carboniferous late-orogenic sedimentary sequence. We combined field investigations, Raman Spectroscopy on Carbonaceous Material geothermometry, and LA-ICPMS U-Th-Pb geochronology on zircon in order to reappraise the sedimentary sequence of the Servoz syncline. Our results allow us to identify three distinct sedimentary formations (F1, F2 and F3). The F1 formation is composed of metagreywackes, bimodal volcanic and magmatic rocks formed during basin opening at an early rifting stage (370–350 Ma) within a back-arc geodynamic setting. This extensional regime was responsible for a high thermal event recorded by a ca. 115 °C/km apparent geothermal gradient. Local anatexis of the basement rocks under the basin is dated at 351 ± 5 Ma. Basin inversion occurred between 350 and 330 Ma in response to oblique collision, with the development of large-scale dextral shear zones and syn-kinematic 340–330 Ma granite intrusions. Subsequent dextral transtension was responsible for the opening of a pull-apart basin between ca. 330 and 310 Ma with the deposition of the F2 phyllite formation that was later deformed by the ongoing dextral transcurrent Variscan tectonics at temperatures between 200 and 350 °C. Finally, the F3 terrigenous sedimentary rocks deposited at ca. 310–290 Ma in a late-orogenic extensional basin. The Alpine-related tectonic event overprinted all the temperatures below 350 °C. Although similar basins have been recognized in other External Crystalline Massifs of the Alps, the Servoz syncline is the first example that allows a major part of the polyphase tectonic evolution, since the early stages of the Devonian, to be recognized. Comparison with similar back-arc basins from the French Central massif, the Vosges massif and the Bohemian massif suggests that the External Crystalline Massifs initially belonged to the Moldanubian hinterlands of the Variscan belt.

Geochronological and geochemical constraints for the metavolcanosedimentary succession of the Nyong Complex, northwestern margin of the Congo craton: Implications for depositional age and tectonic setting of associated banded iron formations

The Mewongo area of the Nyong Complex is located in the northwestern part of the Congo craton in southwestern Cameroon. The metavolcanosedimentary succession in this area comprises banded iron formations (BIFs), interlayered with amphibolites and gneisses and intruded by metamorphosed dolerite dykes. Within the framework of detailed field investigation, we report whole-rock geochemical data for the BIFs, metadolerites, and gneisses to constrain their provenance and tectonic setting, and present LA-ICP-MS U-Pb-Hf isotopic data for zircons from amphibolite and gneiss to assess the depositional age of BIFs from the Nyong Complex and understand the crustal evolution at the northwestern margin of the Congo craton. The geochemical composition of the Mewongo BIFs indicates a contribution of hydrothermal fluids, derived via submarine alteration of volcanics, and seawater. Geochemical and LA-ICP-MS U-Pb detrital zircon data for the gneisses reveal felsic protolith with ages between 2.97 and 2.65 Ga. Our zircon U-Pb-Hf isotope data suggest that a significant growth of juvenile crust occurred at ca. 2.7–2.2 Ga, followed by a reworking event at ca. 2.1 Ga with only minor juvenile crust addition. The metadolerites, cutting the metavolcanosedimentary succession, range in composition between tholeiitic and calc-alkaline basalts. They can be subdivided into two groups based on their trace element signature. The group I metadolerites have OIB-like geochemical composition (e.g., enriched LILE, HFSE, and LREE), while the group II metadolerites exhibit MORB-like geochemical signature (e.g., flat HFSE and HREE patterns). Metadolerites belong to two different so far undated magmatic events on the Congo craton that were previously recognized on the adjacent Ntem Complex.LA-ICP-MS U-Pb zircon dating of amphibolites, combined with chemical composition of metadolerites and gneisses, indicate deposition of the metavolcanosedimentary sequence at ca. 2.1 Ga in a pro-foreland basin sandwiched between the West and East Gabonian blocks, at the time when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny. The Nyong Complex BIFs were therefore deposited in a deep, open-marine setting during the ca. 2.1–2.0 Ga time interval.

Late Tonian explosive volcanism and hyaloclastites in northern Paraguay Belt, Central Brazil: A record of Rodinia break-up in western Gondwana

Explosive volcanism and oceanic crust generation are key aspects to understanding the break-up history of the Rodinia. Its impact on the extreme glaciations of the Cryogenian was marked by ocean chemistry fluctuations in these times, which are still poorly known and preserved in western Gondwana. Here we describe in detail and provide a series of U-Pb ages for the Nova Xavantina Metavolcanosedimentary Sequence, where extrusive rocks, such as hyaloclastite and metabasalts, as well as mafic and ultramafic intrusive rocks and ignimbrites, are associated with diamictites, limestones, and iron formations in the Paraguay belt, Central Brazil. Two styles of pyroclastic eruptions were identified in the studied area. Basic-ultrabasic Surtseyan eruption was enhanced by magma-water interaction and is represented by hyaloclastites and incorporated bombs of scoria. Felsic Plinian eruption, in its turn, was formed by violent explosions in a subaerial environment and is represented by widely exposed acidic tuffs and ignimbrites. Shallow subaqueous environments, such as lakes, salt lakes, and possibly proto-oceans, developed in this precursor basin concomitant with bimodal volcanism. High resolution U-Pb geochronology results indicate they were formed in at least three magmatic events, starting with the formation of ca. 745 Ma seafloor metabasalts and felsic volcanics dated at ca. 735 Ma, and intrusive gabbros at ca. 714 Ma. Alkaline ultramafic rocks of ca. 577 Ma intrude the region and may be associated with similar rocks that crop out regionally as the Planalto da Serra Alkaline Complex. Our results from Nova Xavantina volcanism explain the main Neoproterozoic source until now unrecognized and previous recorded in detrital zircons from the Paraguay belt sediment provenance.

Geology of the Neoarchean-Rhyacian Supracrustals of the Northern Intracontinental Sector of the Araçuaí Orogen: Evidence for Overlapping Basins

The northern region of the intracontinental sector of the Araçuaí Orogen preserves geological records related to overlapping basins and tectonic reactivations between the Mesoarchean to the Cryogenian. The representatives are the metavolcanosedimentary sequences (i) Leandrinho (LMVS), here defined, that intercalates aluminous schists (metapelite), metagreywackes, acid metatuffs, and metalapillites, with a crystallization age of 2550 ± 2 Ma and; (ii) Rio São João (RSJMVS), with metagreywackes and metarkoses, metapelites, metarhytmites, itabirites, hematite-schists, and schistified mafic-ultramafic metavolcanics, presenting maximum depositional ages of 2280 ± 46 and 1971 ± 50 Ma for the Ibiassucê and Água Branca units, respectively. The metavolcanics of the LMVS are mainly subalkaline, sodic, magnesian, metaluminous to peraluminous, and of continental arc setting, with chemistry similar to TTG correlates. One sample has a chemical composition similar to that of sanukitoids. The model age (TDM) varies between 3003 and 3074 Ma and ƐNd between −5,06 and −5,84. In the SMRSJ, schistified metamafics, metagreywackes and metarkoses, and metapelites present chemical signature and geological context associated with a continental arc setting. These siliciclastic rocks have a model age (TDM) between 2618 and 3069 Ma, with ƐNd between −2,70 and −9,21. The entire sequence of the Araçuaí-West Congo intracontinental sector was deformed and metamorphosed during the Ediacaran, generating a prominent gneissic banding that embeds the sequences. The main point of the paper is the description of the previously unrecognized Leandrinho metavolcanosedimentary sequence (LMVS). Subordinately, to demonstrate the superposition of two basin-forming events, at the end of the Neoarchaean and during the Rhycian/Orosirian.

Deciphering the geochemistry of two key Paleoproterozoic siliciclastic sequences of the Piedra Alta Terrane (Río de la Plata Craton, Uruguay)

The geochemistry of two metavolcano-sedimentary sequences deposited in the Piedra Alta Terrane is compared, and their geotectonic evolution is discussed. The Ojosmín Unit (OU) comprises MORB-like basic rocks at the base and a fine-grained siliciclastic sequence interpreted as marine turbidites towards the top. The succession was later obducted during the Orosirian. Whole-rock geochemistry of the metasedimentary rocks of the OU indicates the lack of source rock alteration during deposition (Chemical Index of Alteration, CIA ca. 40-53), implying ice-house climatic conditions in correlation with the worldwide Rhyacian glaciations. Recycling of zircon fractionates the Rare Earth Elements (REE) increasing the amount of HREE, Y, and Hf. Variation ranges of Th/Sc (0.4-4.9), Zr/Sc (30-410), Th/U (2.5-4.3), and of the Eu/Eu * negative anomaly (0.4-0.7) approximate Upper Continental Crust (UCC) values. A new clastic metasedimentary, gently folded unit, the Cerro de la Figurita Formation (CFFm), is erected. The CFFm clastic sedimentation (3,000 m in thickness) represents a deepening upward sequence, starting with polymictic conglomerates deposited in an alluvial fan-dominated environment that evolves to marine turbidites. The CFFm is probably related to a foreland geotectonic setting developed during the Orosirian. The geochemistry of the CFFm reveals similarities to unrecycled UCC, and weathering of the source rocks increases up section (CIA 45-92). Low ratios of Th/Sc (0.3-1.5), Zr/Sc (6-20), Th/U (3-6), high Cr/V (1.1-12.2), and a less pronounced Eu/Eu* negative anomaly of certain samples (0.5-0.9) suggest a contribution from mafic source rocks (probably ophiolitic).

Gravity surveys and 3D integrated model with magnetic and geological data of the granite-greenstone terrane in Pitangui Synclinorium, NW of the Quadrilátero Ferrífero (MG, Brazil)

The Pitangui Greenstone Belt (PGB) is a Meso to Neoarchean metavolcano-sedimentary sequence, inserted in the Pitangui Synclinorium, located in the northwest portion of the Archean-Paleoproterozoic metallogenic province of the Quadrilátero Ferrífero, southern of the São Francisco craton (Brazil). The PGB holds important gold mines like the Turmalina Complex and recently discovered deposits, as São Sebastião. The PGB is mapped in detail scale, but there is still little information about its subsurface configuration. This paper presents gravity surveys, constrained by geological, structural, boreholes, aeromagnetic and seismic data, aiming to provide a better understanding of the 3D geometry of this granite-greenstone terrane. The treatment of the available data resulted in Bouguer anomaly maps, 2D gravity modelling and a 3D geological-geophysical integrated model. The gravity Bouguer anomalies of the region present high amplitudes associated with the PGB, alternating with low zones related to the surrounding granite-gneiss rocks. The profiles and the model suggest that the maximum depth of the greenstone belt is approximately 5 km in its central portion, being strangled by granitic intrusions in its southernmost region. The PGB is about 100 km long, but most of it is covered by Neoproterozoic sediments of the Bambuí Group and Quaternary deposits, which significantly increase its metallogenic potential, especially in regions with few mineral exploration works. The 3D model contributes to the understanding of the tectonic framework and evolution of the PGB, showing the spatial relation with the surrounding granite-gneiss basement and intrusions, and younger sedimentary covers, providing valuable insights about their depth, vertical and lateral geometry. Additionally, this new data can become a valuable tool for developing strategies to select deep mineral targets, expanding the exploratory potential for this mineral province.

New geochronologic and geochemical constraints for the Pedro Pereira metavolcanosedimentary sequence: Evidence for a 2.77 Ga oxygen oasis record in the São Francisco-Congo paleocontinent

Exposed within the reworked basement of the Araçuaí orogen, eastern Brazil, the Pedro Pereira metavolcanosedimentary sequence (PPMS) comprises a lower unit dominated by metaultramafic/mafic rocks, an intermediate unit dominated by metarhyolites and a predominant upper unit of acid metavolcanites associated with chemical and clastic metasedimentary rocks. This sequence has been defined and investigated in the ’80s when it was considered as a Mesoarchean greenstone belt. Since then, few studies have addressed these rocks, and due to the lack of reliable geochronological data, a Paleoproterozoic age has also been suggested for the sequence. To reevaluate the age and evolution of the unit, new whole-rock geochemical and zircon U–Pb (LA-ICPMS) geochronologic data were obtained for the iron formation and acid metavolcanic rocks, respectively. When normalized to post-Archean Australian Shale standard, BIF samples can be divided into three groups. Group 1 BIF samples are marked by significant enrichment of the LREE, very high ∑REE + Y, positive La anomaly, extreme negative Ce anomaly, negative Eu anomaly, and subchondritic Y/Ho ratio. In contrast, Group 2 BIFs are marked by the enrichment of HREE, low ∑REE + Y, positive La anomaly, negative Eu anomaly, slight negative Ce anomaly, and predominantly super- and subchondritic Y/Ho ratios. The massive iron formation (MIF) group shows very high middle-to-heavy REE + Y enrichment, strong negative Ce anomaly, negative Eu anomaly, very high ∑REE + Y, and variable Y/Ho ratios. The modification of the iron formation by post-depositional hydrothermal alteration might have disturbed the original basin water signature. However, Group 2 is more compatible with pure chemical precipitates and its slight negative anomalies suggest an oxic environment. A PPMS metarhyolite from the upper portion of the sequence interlayered with the BIFs yielded the Concordia Age of 2771 ± 8.4 Ma. This age rules out a Paleoproterozoic origin for the PPMS, as suggested/considered by recent papers. The results imply that PPMS possibly represents one of the oldest records of oxygenation previous to the GOE in the geologic archives and could provide information to understand the early evolution of localized pockets of oxygen before the Great Oxidation Event.

Petrogenesis of the Tampanchi Ultramafic–Mafic Complex (Ecuador): Geodynamic implications for the northwestern margin of South America during the late Cretaceous

The Tampanchi Ultramafic–Mafic Complex (TUMC), located in the central segment of the Cordillera Real (Ecuador), is an oval-shaped intrusive body of approximately 18 km2 emplaced within a Cretaceous metavolcano-sedimentary sequence. Field investigations, zircon geochronology, mineral and whole-rock elemental and isotopic compositions constrain the nature of the parental magma as well as the physical conditions of emplacement, age of crystallization and tectonic setting of formation of the TUMC. The Complex consist of wehrlite and olivine-hornblende clinopyroxenite crosscut by hornblende gabbros and minor dikes/veins of diorite and granite. Hornblendites formed mainly at the contact between the olivine-hornblende clinopyroxenite and the intrusive hornblende gabbros by reaction-replacement processes. Geochemical data for both the wehrlites and pyroxenites define a trend with dominantly olivine and clinopyroxene accumulation, whereas a second trend is formed by hornblende gabbros that differentiated to leucocratic rocks, with amphibole as the dominant fractionating mafic phase. The trace elements show enrichment in large-ion lithophile elements, depletion in high field strength elements (Nb, Zr, Ti) and P, and enrichment in Pb, Sr and Ba relative to primitive mantle, indicating subduction-related parental liquids. The Nd and Sr isotopic composition suggests a uniform mantle source metasomatized by subducted crustal components. Thermobarometric estimations constrain emplacement and crystallization at intermediate to shallow continental crust levels (∼12.5 km depth) of hydrated basaltic melt under oxygen fugacity conditions above the nickel-nickel oxide buffer (ΔNNO = 0 to 2). Zircon SHRIMP U-Th-Pb age data constrain crystallization at 75.1–76.0 Ma, with scarce inherited zircons that record limited recycling of the Cordillera Real basement. These results confirm magmatic arc activity at the continental margin of South America during the late Cretaceous as a result of an E-dipping subduction zone prior to the accretion of the Ecuadorian-Colombian-Caribbean oceanic plateau, and support simultaneous double subduction at the northwest margin of South America during the Cretaceous.

GEOLOGIA, GEOQUIMICA E QUIMICA MINERAL DOS STOCKS GRANÍTICOS RAPAKIVI CAMPINA DO VEADO E SANTA BLANDINA, SUDOESTE DO ESTADO DE SÃO PAULO

Os stocks graníticos Campina do Veado e Santa Blandina localizam-se na região sudoeste do estado de São Paulo, no município de Nova Campina, intrusivos em rochas da sequência metavulcanossedimentares do Grupo Itaiacoca. Os corpos encontram-se relacionados à evolução da granitogênese rapakivi neoproterozoica da parte norte do Terreno Apiaí e da Província Mantiqueira Central. As rochas do Campina do Veado, abrangem uma área de aproximadamente 20 km² e as suas rochas ocorrem no leito do rio Taquari Guaçu, na interface dos contrafortes erosivos da Unidade Geomorfológica da Depressão Periférica, a partir de rochas dos grupos Furnas e Itararé da sequência sedimentar da Bacia do Paraná. O corpo de Santa Blandina de aproximadamente 4 km2 é identificado por pequenas ocorrências isoladas de rochas graníticas. As rochas são representadas por variedades graníticas de coloração rósea a vermelha, rara ocorrência cinza, isotrópicos e porfiríticas a equi- a inequigranulares, com variedades viborgíticas e apresentando como mineral máfico preservado somente a biotita. As rochas podem ser distintas por composições monzograníticas, sienograníticas e álcali granitos, além das rochas das fases tardias aplíticas e pegmatíticas. O magmatismo dos stocks graníticos demonstra uma forte relação de semelhanças entre as suas rochas, sugerindo constituir em um mesmo corpo. Os granitos rapakivi são peraluminosos e podem ser correlacionados à séries cálcio-alcalina alto potássio a shoshonítica, do Tipo I Caledonianos, pós-colisional a anorogênico de intraplaca, altamente diferenciado e da fase final da construção do Arco Magmático Ribeira ou associado aos granitos peraluminosos alcalinos do tipo A. Estão associados às estruturas transtensivas das zonas de cisalhamento do final do evento colisional da Orogênese Ribeira e reflete o arranjo final geotectônico de estabilização do Terreno Apiaí.

Remote sensing data for Geological mapping and gold prospecting of Inteet area, northern Sudan

The mineral exploration has improved in the last decades, today there are different methods of remote exploration, that are applied in mining industry worldwide, helping to discover ore minerals deposits in zones that have not access to them. The study area of this article is located about 400 km from Khartoum, characterized by low reliefs covered by sand and gravely sand. The study wants to carry out the geological mapping and the prospective zones using remote sensing and GIS techniques. Sudan is characterized by geological formations over inaccessible areas, in that way the remote sensing technique has a great value in these conditions, saving time and money. The used methodology has been divided into three phases: Pre-field office work, field work and post-field work. The processing of the satellite images includes color band composites, in order to obtain the lithological and geological features, the different types of rocks were defined by a different color. Obtaining the following rock types: High-grade gneisses and migmatites described to comprise the basement complex, ophiolitic mafic-ultramafic rocks that appear in the northern and southern sides of the area, metasediments that cover most of the study area, syn-orogenic intrusions that cut the older mafic-ultramafic units and the metavolcanosedimentary sequences, post-orogenic intrusions and gold mineralization. Based on the image analysis results of Landsat 8 OLI, 32 ore samples were collected to analysis gold and pathfinder elements obtaining high anomalies results for Au, Ag, Cu, Zn, Pb and Fe.

Petro-Structural Study of the Paleoproterozoic Formations of the Faboula Gold Deposit (Bougouni-Kékoro Basin, Leo-Man Shield)

Recent petro-structural investigations on the Faboula gold deposit located in the Bougouni-Kékoro basin, in southern Mali, north-west of the Leo-Man Shield, have provided new data on the nature and spatial organization of the lithostratigraphic units as well as their deformation style. The deposit is covered by a thick lateritic layer and is hosted by a metavolcano-sedimentary sequence of Paleoproterozoic age intersected by intrusive bodies and filled fractures of various shapes and types. The lithostratigraphic units consist of metagreywackes, metasiltstones, meta-argillites, slates and schists. Metagreywackes and metasiltstones are generally feldspathic, both may contain biotite and locally amphibole, just as slates may contain andalusite which is locally stretched. Plutonic units most often occur as stocks or as dikes on the drill core, up to 1 m. The metavolcano-sedimentary rocks are schistose and deformed under greenschist facies conditions, and locally they reach the epidote-amphibolite facies. The structural study revealed that the deposit is affected by several stages of deformation evolving from a ductile type to a brittle type via a ductile-brittle type. The dominant ductile and brittle-ductile deformations show a combination of isoclinal folding and strike-slip faults. Both the isoclinal folding and the strike-slip faults whose sigmoidal en-echelon tension gashes indicate a dextral movement in the NNE-SSW direction are the result of the same ENE-WSW regional shortening. Consequently, they highlight a transpressive deformation. This deformation noted here D2Fb, could be equivalent to the regional D2 or D3 deformations identified at the scale of the Leo-Man Shield if we refer to the style of deformation. There is an abundance of quartz veins networks. Their relation within the structural features indicates that the mineralization is structurally controlled during a hydrothermal event linked certainly to the circulations of fluids during the transpressive event D2Fb.

The Archean Rio das Velhas greenstone belt revisited: new insights into the stratigraphy

The Neoarchean Rio das Velhas greenstone belt (RVGB), southeast Brazil, contains major tectonic and stratigraphic discontinuities that allow its division into three fault-bound tectonostratigraphic blocks named Santa Barbara, Nova Lima-Caete and Sao Bartolomeu. Detailed 1:25,000 scale mapping of the RVGB northern part revealed that each block contains important particularities and were subdivided into different lithostratigraphic formal units. The Santa Barbara block comprises the basal ultrabasic rocks of the Quebra Osso Group, followed by metavolcano-sedimentary successions of the Nova Lima Group (Vigario da Vara, Santa Quiteria, and Corrego do Sitio formations), and metasedimentary rocks of the Maquine Group (Palmital and Casa Forte formations). The Nova Lima-Caete block comprises the basal ultrabasic/basic volcanism (Ouro Fino and Morro Vermelho formations), metavolcano-sedimentary sequences (Mestre Caetano and Ribeirao do Brumado formations) of the Nova Lima Group, and the upper clastic sedimentary rocks of the Maquine Group (Palmital and Casa Forte formations). The Sao Bartolomeu block has a distinct rock assemblage given by metasedimentary rocks of the Nova Lima Group (Fazenda Velha, Corrego da Paina, and Pau D'Oleo formations), and clastic sedimentary rocks of the Maquine Group (Andaimes Formation). Despite some remarkable differences between these blocks, it is possible to recognize stratigraphic correlations and similar depositional evolution. The authors suggest volcanic architecture and sedimentary depositional models for the studied area, which includes: i) fissure eruption of komatiite lavas of the Quebra Osso Group in the SB block with subordinate clasto-chemical sedimentation; ii) ultrabasic/basic volcanism related to multiple volcanic centers in a subaqueous environment, characteristic from the basal portion of the Nova Lima Group in the SB and NLC blocks; iii) intermediate to acid volcanic and volcaniclastic rocks from the Nova Lima Group, present only in the NLC block, related to explosive eruptions; iv) clasto-chemical sedimentation, well recorded in the SB block; v) thick sequence of clastic sediments recorded in the three blocks with a predominance of greywacke-argillite cycles, deposited by turbidity currents in tectonically active submarine basins; vi) metasediments formed in alluvial-fluvial, coastal and shallow marine environments of the Maquine Group.