Paleoproterozoic Domain Research Articles

An oblique transpressive soft-collision records the amalgamation of the Siderian-Orosirian Mineiro belt (Brazil) and the São Francisco paleo-craton (Kenorland Supercontinent): Insights from the Bom Sucesso metasedimentary sequence

The southernmost São Francisco craton carries a long and polycyclic tectonic evolution from the Archean to the Proterozoic. The worldwide characterizations of rocks formed between these Eons, such as those of the Bom Sucesso metasedimentary sequence, is key to understand significant changes on Earth's geodynamic and atmosphere. This sequence is located between two important geotectonic domains in Brazil: the Archean São Francisco paleo-craton (a piece of the Kenorland Supercontinent) and the Siderian-Orosirian Mineiro belt. Although the Bom Sucesso metasedimentary sequence presents a strategic position to investigate the docking between different tectonic domains, longtime tectono-metamorphic episodes challenge the full understanding of the broad tectonic scenario. This work was performed aiming to unravel the links between the tectonic evolution of the Bom Sucesso metasedimentary sequence and their adjoining domains, represented by the Archean São Francisco paleo-craton and the Paleoproterozoic Mineiro belt. The original sedimentation of the Lagoa da Prata unit at the base of the Bom Sucesso metasedimentary sequence, the Bom Sucesso Banded Iron Formation and the Tabuãozinho unit developed in a passive margin setting with different Archean sources associated with nearby orthogneisses and metagranites of the São Francisco paleo-craton. This sequence yields an MDA of 2615 ± 4 Ma and is chronocorrelated to the Moeda Fm. at the base of the Minas Supergroup. After the deposition of the Bom Sucesso metasedimentary sequence in a passive margin setting, the amalgamation of the Paleoproterozoic Mineiro belt to the São Francisco paleo-craton between ca. 2.07 and 2.00 Ga by a soft-collision process produced two distinct compressional events. The first with N–S direction (σ1 = 181/16) originated 2 km-scale thrust faults, associated with a transcurrent movement between the Bom Sucesso metasedimentary sequence and both the adjoining Archean and Paleoproterozoic domains. The same compressional event also developed tectonic foliations under amphibolite facies conditions (2.8–6.3 kbar; 550–632 °C) and imprints a sillimanite-andalusite isograd along the Ibituruna and Bom Sucesso ridges. The second event has a NW-SE direction (σ1 = 333/18) and produced crosscutting structures under a ductile-brittle regime at lower temperatures been considered as retrogressive. A thermal influence of the Brasiliano orogeny in the region is suggested by new LA-ICPMS zircon Concordia age of 623 ± 4 Ma obtained in a garnet schist of the Lagoa da Prata unit, which is in agreement with Neoproterozoic Concordia Lower intercepts (probably Pb-loss) reported in zircon grains, and Ar–Ar ages (biotite) obtained along most of the Mineiro Belt area. This age may imply in a superposition of temporally distinct metamorphic events (Paleoproterozoic and Neoproterozoic) on the rocks of the Bom Sucesso metasedimentary sequence. The structural, metamorphic, and geochronologic data presented here brings new insights on structural and P-T-X conditions of the Paleoproterozoic geotectonic evolution of the southernmost São Francisco craton.

Zircon U-Pb-Hf isotope evidence for juvenile Penokean sources (ca. 1.9 Ga) in the SW Grenville Province

The Proterozoic evolution of Laurentia involved intermittent orogens and arc-related crustal growth along a modern-style convergent margin, termed the Great Proterozoic Accretionary Orogen (GPAO). The major lithotectonic provinces of the GPAO in the United States are exposed as telescoped equivalents within the thrust stack of the Grenville Province in Canada. We report zircon UPb and LuHf isotope analyses for two orthogneisses and one quartzite from Archean and Paleoproterozoic domains in the gneiss belt of the SW Grenville Province, Canada. The data constrains Paleoproterozoic crustal growth and sedimentation for this part of the Laurentian margin. The orthogneisses have similar zircon UPb and LuHf isotope systematics that reflect igneous reworking (ca. 1700–1660 Ma) of a Penokean aged (ca. 1.9 Ga) depleted mantle derived source, along with metamorphic reactivation at both ca. 1.45 Ga and during the Grenville Orogeny (1.2–1.0 Ga). The quartzite has a bimodal Paleoproterozoic (2.1–1.8 Ga) and Archean (> 2.5 Ga) detrital zircon age spectrum. The Paleoproterozoic detrital zircon grains have a wide range of Hf(t) compositions (εHf(t) = −12.5 to 10) reflecting co-eval interaction of Archean and depleted Paleoproterozoic sources, contrasted by a more restricted range for the Archean population (εHf(t) = −7 to 4). The new data support the presence of reworked ca. 1.9 Ga depleted mantle sources in the Paleoproterozoic Parautochthon, which extend further east than previously thought. They also identify correlations amongst detrital zircon age spectra and Hf isotope compositions in quartzites from different locations of the Archean Parautochthon.

Brittle tectonic evolution of Gondwana: implications for shale-gas and groundwater exploration

Abstract Brittle structures are crucial for enabling several key natural processes in the Earth's upper crust. In addition, understanding the 3D characteristics and geological evolution of these features is equally important to support various developmental objectives, such as those, inter alia , linked to natural gas, groundwater, hydrothermal minerals and seismicity. In this study, we map various fractures of Gondwana based on the available geological information, satellite imagery and digital elevation data. The lengths and orientations of more than 10 000 fractures in their present-day position reveal four clearly defined patterns, with those striking NW being predominant. Archean–Paleoproterozoic domains are defined by fractures oriented north and NE, whereas the Mesoproterozoic has dominant NNW-striking fractures. In contrast, the Neoproterozoic has mostly NE-striking fractures and the Phanerozoic sequences are defined by a predominant NW and a subordinate west fracture pattern. The style and geometry of these structures can be linked to major geodynamic events that led to the formation of Gondwana building blocks during the Eburnean ( c. 2.2–1.8 Ga), Kibaran ( c. 1.4–1.0 Ga) and Pan African–Brasiliano ( c. 800–550 Ma) orogens, and amalgamation of Pangaea ( c. 350–250 Ma). Many structures were reactivated and new faults formed during opening of the Atlantic and Indian oceans ( c. 180–120 Ma), the India–Asia collision and rifting across East Africa since about 40 Ma. Although the changes in palaeogeography remain difficult to model with accuracy, major structural orientations are corroborated by the occurrence of major mineral deposits and seismicity. The spatial distribution of mapped patterns across the different continents also correlates well with large shale gas prospects and increased groundwater yields. Thus, Gondwana fractures need to be considered in more detail for informing future development related to water and energy use, especially across regions of Africa.

New lithostructural map of the Doropo region, northeast Côte d'Ivoire: Insight from structural and aeromagnetic data

This study, performed in the Doropo region (northeastern Côte d'Ivoire) in the Paleoproterozoic domain (southern part of the WAC), aims to produce a detailed lithostructural map of this area. The geology of this region is less known due to the thick lateritic overburden and the scarcity of outcrops. Recent airborne geophysical data (aeromagnetic) integrated with field structural and lithological observations allowed to distinguish several lithologies including biotite granite, gneissic and migmatitic granite, granodiorite, tonalite, gabbro, amphibolite, rhyolite and dolerite. Filtering techniques of derivatives, upward continuation, tilt, analytical signal, spectral analysis and 3D Euler deconvolution showed the presence of faults, shear zones and intrusions. The structural analysis revealed four deformation events (D1-D4): (i) D1 is a N–S compression marked by E-W foliation (S1), conjugate NE-SW and NW-SE sinistral and dextral shears, respectively, as well as F1 folds with E-W axial planes; (ii) D2 is a major transpressive phase characterized by an intense NE-SW foliation (S2), E-W dextral shear zones, a weakly dipping stretching L2 lineation plunging N-E, and NE-SW faults; (iii) D3 is a NE-SW compressional phase generated NW-SE foliation (S3) with ∼E-W shears, and NW-SE axial plane F3 folds; (iv) D4 is a late, rather brittle phase characterized by brittle structures (faults, fractures, etc.) and the emplacement of NE-SW and NW-SE dolerite dykes. This tectonic evolution demonstrates that the Doropo region was subject to ductile and then brittle deformations. Furthermore, we suggest that the geodynamic model which could have prevailed in the Doropo region occurred in an oceanic arc collisional tectonic context.

Geochemical Signatures of Séguéla Peridotites in the West African Craton

The present study deals with peridotites found in the paleo-proterozoic domain of the Leo Man Shield, Séguéla region, west-central of Côte d’Ivoire. Results show that Séguéla peridotites are composed of lherzolites, dunites and harzburgites. However, iherzolites are the most abundant. The phenocrysts in these rocks are olivine most often serpentinised, and pyroxenes, represented by diopside and enstatite. Amphiboles are divided into two groups: magmatic amphiboles and those from the destabilization of clinopyroxene. Spinels have CrO3 content which varies between 28% and 37%, Al2O3 between 33% - 41% and MgO is equal to 18%; they are poor in TiO2 and do not contain zinc oxide. Séguéla peridotites are characterised by a negative anomaly in Nb-Ta, associated with an enrichment of lithophilic elements and light rare earth elements with a fractionation rate (La/Yb) which varies between 46.22 and 150.72 and heavy rare earth elements, and HFSE depletion, which may suggest that were formed in an art context, that is, a mantle enriched by fluids from a subduction zone. However, another hypothesis could be considered, that of the interaction between the mantle and magmas in a subduction zone context.

U–Pb geochronology, Nd–Hf isotopes, and geochemistry of Rhyacian granitoids from the Paleoproterozoic Lourenço domain (Brazil), southeastern Guiana Shield

The southern sector of the Paleoproterozoic Lourenço domain in the central-eastern region of the Amapá state, northern Brazil, consists mainly of Rhyacian granite-greenstone terranes with some Archean fragments. It is located at the transition with the Archean Amapá block, a Meso-Neoarchean continental landmass (2.85–2.60 Ga), strongly reworked during the Transamazonian orogeny (2.26–1.95 Ga). Geochemical and petrographic data together with combined zircon U–Pb and Lu–Hf (LA-ICP-MS) as well as whole-rock Sm–Nd (TIMS) dating, establish the chronology and petrogenesis of the granitoids and record the crustal growth and reworking at the transition between the Archean and Paleoproterozoic domains in the southeastern Guiana Shield. Two episodes, dated at ~2.18–2.16 Ga and 2.14–2.13 Ga, respectively, include medium-to high-K calc-alkaline and metaluminous to peraluminous rocks of the Flexal and Papa-Vento intrusive suites, emplaced in a magmatic arc environment. A third episode, dated at ~2.08 Ga, encompasses mostly peraluminous granites with a medium-to high-K calc-alkaline signature, of the syn-to late-collisional Vila Bom Jesus Suite. The zircon ƐHf (−3.5 to −4.3) and whole-rock ƐNd (−2.7 to −4.3) signatures with respective Hf-TDMC (2.9–3.1 Ga) and Nd-TDM (2.6–2.7 Ga) model ages indicate a mixture of Rhyacian juvenile material with a Mesoarchean crustal component for the magma sources of the Flexal and Papa-Vento intrusive suites. The geochemical characteristics, zircon ƐHf (−6.6 to −12.5) and whole rock ƐNd (−3.7 to −5.4) signatures, and Hf-TDMC (2.9–3.6 Ga) and Nd-TDM (2.7–2.9 Ga) model ages, together with the presence of Rhyacian (2.19 Ga) and Mesoarchean (2.82 Ga) inherited zircon crystals in the Vila Bom Jesus Suite, indicate dominant crustal reworking of Rhyacian magmatic arc rocks and Archean rocks from the adjacent Amapá Block during the arc-continent collision. The geochemical and isotopic signatures of the magmatic arc granitoids (Flexal and Papa-Vento intrusive suites) may be attributed to the incorporation in the juvenile magmatic sources of Archean continental sediments or hidden crustal fragments from the Amapá Block during the subduction processes. Geochemical affinities with continental magmatic arc and subchrondritic Hf–Nd isotopic signatures are consistent with a scenario of a long-lived Mesorhyacian (~2.20–2.13 Ga), Cordilleran-type magmatic arc setting, which evolved to a collisional setting with the amalgamation of the Archean Amapá block and the Rhyacian Lourenço domain during the Neorhyacian (2.11–2.08 Ga).

Crustal growth and reworking of Archean crust within the Rhyacian domains of the southeastern Guiana Shield, Brazil: Evidence from zircon U–Pb–Hf and whole-rock Sm–Nd geochronology

The southeastern Guiana Shield, northern Amazonian Craton, is part of a Paleoproterozoic orogenic belt that was built up during the Transamazonian orogenic cycle (2.26–1.95 Ga). This cycle includes large segments of Rhyacian juvenile crust and some reworked Archean terranes. The geology in this region consists mainly of Paleoproterozoic granulitic-migmatitic-gneissic complexes, deformed and metamorphosed metavolcanic and metasedimentary rocks, and granitoids (granitic and TTG magmatism). Three tectonic domains are distinguished in the Brazilian territory of the southeastern Guiana Shield. They are known as the Amapá Block, Lourenço Domain, and Carecuru Domain. The Amapá Block is a Meso–Neoarchean continental block that was intensely reworked during the Transamazonian orogeny. The other two domains represent Rhyacian landmasses, the evolution of which involved several stages of subduction of oceanic lithosphere in magmatic arc environments. There are also relics of reworked Archean continental crust, the formation of which was followed by a collisional stage of tectonic accretion of the magmatic arcs. Whole-rock Sm–Nd and U–Pb zircon geochronology have confirmed the juvenile character of much of this Transamazonian orogenic belt. However, for the Lourenço and Carecuru domains, Nd isotopic signatures indicate the participation of Meso–Neoarchean crustal material in the sources of the magmatic rocks. Combined zircon U–Pb and Lu–Hf isotopic analyses by LA-ICP-MS were performed on eleven Rhyacian granitoids and orthogneisses from the Lourenço and Carecuru domains. The aim was to verify the extension of the influence of the Archean continental crust in the adjacent Paleoproterozoic domains. The main magmatic episodes were identified in the Lourenço Domain (~2.17–2.18, 2.14 and 2.12–2.09 Ga) and Carecuru Domain (2.14 Ga) by U–Pb zircon geochronology. The Lu–Hf isotope data point to the predominance of crustal reworking processes (ƐHf(2.2–2.1 Ga) < 0; 67% of zircon crystals) during the formation of Lourenço and Carecuru domains. Hf model ages were found to be mostly Archean (98.4%), even for zircon grains that have positive ƐHf(2.2–2.1 Ga) values. For the terrane at the border of the Lourenço and Carecuru domains with the Amapá Block, assimilation of Archean crust of different ages and proportions in a magmatic arc environment accounts for the Hf–Nd isotopic signatures and Hf model ages of Rhyacian magmatism. In the northwestern part of the Lourenço Domain, more than 100 km north of the Amapá Block, the Hf–Nd isotopic signatures and Hf model ages indicate the participation of Archean crustal material, either as continental fragments and/or through incorporation of continental sediments in island arc environments, similar to what has been recorded for some Birimian terranes of the West African Craton in Ghana.

Chemical and boron isotope composition of tourmaline from the Kiaka orogenic gold deposit (Burkina Faso, West African Craton) as a proxy for ore-forming processes

The Kiaka orogenic gold deposit (Burkina Faso), located in the Paleoproterozoic domain of the West African Craton, is characterized by a two-stage gold mineralization hosted in volcano-sedimentary metamorphic rocks that was formed during the Eoeburnean (2.20–2.13 Ga) and Eburnean (2.13–2.05 Ga) orogenic cycles. These two stages include an early disseminated low-grade gold mineralization and a late vein-hosted high-grade gold mineralization. Paragenetic studies indicate that the first gold stage was coeval with the deposition of hydrothermal tourmaline. The aim of this paper is twofold: (i) to determine the processes responsible for deposition of the economic disseminated gold mineralization and (ii) to identify the source of the mineralizing fluids. For this purpose, we performed an in situ study on tourmaline by combining electron probe microanalysis and secondary ion mass spectrometry measurements of boron isotopes. Hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes has a dravite composition but shows different δ11B values falling within the two intervals of − 25.1 to − 22.0‰ and − 19.6 to − 15.1‰, respectively. Our results suggest that tourmaline formed from a distal, high-temperature (ca. 400 °C), reduced, and low-salinity hydrothermal fluid that interacted with the local host rocks. Based on the modeling of tourmaline–fluid boron isotope fractionation, we propose a metamorphic fluid origin derived from devolatilization of deeply buried muscovite schists during the regional prograde to peak metamorphism prior ca. 2.13 Ga. This metamorphic fluid–rock interaction model may possibly extend to other orogenic gold deposits in the West African Craton.

Syn-kinematic ferroan high-K I-type granites from Dschang in southwestern Cameroon: U–Pb age, geochemistry and implications for crustal growth in the late Pan-African orogeny

Abstract The Dschang granites, SW Cameroon, are post-Panafrican granitoids, hosted in Paleoproterozoic orthogneiss in the Western domain of the Central African fold belt. They include coarse-grained K-feldspar biotite granite and fine-grained magnetite granite. Magnetite forms nodules surrounded by a quartzo-feldsparthic halo. Major and trace element data of both granites show high SiO 2 , Al 2 O 3 , total alkalis and Ba and low TiO 2 , MgO, P 2 O 5 and Nb contents, conferring a fractionated I-type high-K alkali-calcic and weakly peraluminous character. Both granites are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and show a rare earth element pattern with Eu/Eu* = 0.3 to 0.9. Primitive mantle-normalized spidergrams display negative Nb, Ta and Ti anomalies, typical of their derivation from subduction-related post-collisional melt. Laser inductively coupled plasma mass spectrometry U–Pb data on zircons yielded crystallization ages of 578 +7/−11 Ma and 563+8/−3 Ma for the biotite and magnetite granites respectively. Inherited zircon (1991–1442 Ma), whole-rock Nd model ages in the range 2.1–1.9 Ga, highly negative ε Ndt (−16 to −18) and initial 87 Sr/ 86 Sr ratios in the range 0.7075–0.7114, support the remobilization of a Paleoproterozoic crust. The felsic magmas were probably produced by partial melting of Paleoproterozoic orthogneiss host rocks, under oxidizing conditions. Magnetite granite is also known from the Nkambé region. Such granites from both the Dschang and Nkambé regions suggest the presence of a Paleoproterozoic domain rich in iron mineralization. This domain potentially represents a Cu, Pb, Zn and Mo metallogenic province, heavily reworked during the Pan-African orogeny.

Dating post-Archean lithospheric mantle: Insights from Re-Os and Lu-Hf isotopic systematics of the Cameroon Volcanic Line peridotites

Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9–91.2) and low spinel Cr# (8.4–19.3), together with moderate to high whole-rock Al2O3 contents (2.0–3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01 ± 0.18 Ga (1σ), and when olivine and spinel are considered, is 1.82 ± 0.14 Ga (1σ). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2–2.4 Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0 Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ∼2.0 Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ∼2.0 Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.

Structures, Deformation Mechanisms and Tectonic Phases, Recorded in Paleoproterozoic Granitoids of West African Craton, Southern Part: Example of Kan’s Complex (Central of Côte d’Ivoire)

The granito-gneissic complex of Kan is located in the central part of the Paleoproterozoic domain of Cote d’Ivoire. It consists essentially of migmatitic and mylonitic gneisses with basic intrusions and xenoliths. This Proterozoic domain belongs to the Man Leo shield, southern part of West African craton (WAC). The present study, essentially based on a structural analysis at outcrop scale, aims to identify deformation mechanisms and tectonic phasesrecorded in the granito-gneissic complex of Kan. Deformation mechanisms include: (1) flattening, (2) constriction, (3) simple shear (4), rotation (5), brittle shear, and (6) extension. The Kan complex deformation occurred during four major tectonic phases named D1, D2, D3 and D4. D1 corresponds to WNW-ESE compression. It led to the formation of NS to NNE-SSW foliation, of stretching lineation, and of folds with sub-horizontal axes. It is accompanied by N170° and N10° sinistral shear zones, which constitute globally a NS major transcurrent shear zone in the central part of Cote d’Ivoire. D1 is also marked by N90° dextral shear zones. Tectonic phase D2 is associated with EW compression. It is marked by N50° dextral and N110° sinistral transcurents shear zones. D3 corresponds to NNE-SSW compressive phase and is responsible of N110° crenulation cleavage formation. (D4) constitutes a brittle deformation phase. It correspondsto posteburnean deformation in the Proterozoic crust of Cote d'Ivoire. Generally, these deformation phases are similar to D2 and D3 reported in the Man-Leo shield and that are part of regional collisional phase referred to as Eburnean orogeny in the WAC.

Archean and paleoproterozoic crust generation events, Amparo complex and Serra Negra orthogneiss in southern Brasília Orogen, SE Brazil

The Amparo Complex and Serra Negra orthogneisses consists largely of Archean and Paleoproterozoic domains, which represent the older tectonizied basement of Ribeira Belt, southeastern portion of the Brasília Neoproterozoic Orogenesis. This is the subject of UPb SHRIMP and Nd and Sr isotopic and geochemical investigations aiming to increase the knowledge of these juvenile crustal domains. In this region, older Meso and Neoarchean rocks occur in physical proximity to younger Paleoproterozoic rocks of Serra Negra Orthogneiss. Mesoarchean gneisses with UPb zircon ages of 2956 Ma have a composition of TTG suites with TDM of 3176 Ma and a ƐNd(t) of −0.14, suggesting derivation from the partial fusion of older mafic rocks. Mafic (amphibolite) and ultramafic rocks are associated with these domains. Neoarchean porphyritic granites are associated with the former rocks, occurring as intrusive high-K granites. They have a TDM value of 3404 Ma and a ƐNd (2.7Ga) of −4.29, which demonstrates their derivation from older Archean crust. Paleoproterozoic units occur throughout the belt represented by granodioritic or tonalitic orthogneiss with intercalations of amphibolites and ultramafic rocks. In the extension to the west the units crop out in the Campinas region, and to the east are denominated Pouso Alegre Complex. They constitute a TTG-type granite-tonalite suite, as verified by their geochemical signatures, REE and incompatible elements, high Na/Ca ratios, and negative anomalies of Ta, Nb, and Ti. SHRIMP UPb zircon ages are between 2.09 and 2.16 Ga (Rhyacian) with TDM between 2.11 and 2.36 Ga and ƐNd(2.1 Ga) between +0.88 and + 3.18, indicating juvenile nature. Low 87Sr/86Sri ratios disclose a possible contribution of mafic precursors in their origin. Neoproterozoic Ediacaran ages of Ribeira Belt event are seen in zircon crystals of all samples analyzed for this study. The effects of this Neoproterozoic orogenic event on basement rocks resulted in anatexis, banding and folding of the older rocks. As suggested for the Paleoproterozoic Pouso Alegre Complex, it is possibly an important period of juvenile continental growth that, in association with the Archean gneisses, may be related to the southern end of the São Francisco Craton.

New insights into the crustal growth of the Paleoproterozoic margin of the Archean Kéména-Man domain, West African craton (Guinea): Implications for gold mineral system

The Baoulé-Mossi Paleoproterozoic domain on the West African Craton is known for its world-class orogenic gold deposits formed during the Eburnean orogeny between ca. 2.15 and 2.07Ga. New results of U-Pb zircon dating in combination with Hf-isotope geochemical data indicate that a suite of mafic and felsic potassic plutonites were emplaced in Guinea (in vicinity to the Archean Kéména-Man domain, KMD) between ca. 2.10 and 2.08Ga. All plutonites are characterized by (i) enrichment in LILE and LREE, (ii) strong positive Pb spikes and (iii) depletion in Ta-Nb-Ti, but based on zircon U-Pb-Hf isotope data two groups can be distinguished, each comprising Qtz-monzodiorites and Bt-granites. Bt-granites of group 1, sampled away from the craton are xenocryst-free despite similar or higher Zr-saturation temperatures (∼820°C) compared to group 2 rocks (∼780°C), and reveal mostly superchondritic ɛHf(2.1 Ga) up to +3.7, (TDM=2.8–2.4Ga). Rocks of group 2, sampled in close vicinity to the KMD, show highly negative ɛHf(2.1 Ga) of −16 to −10 (TDM=3.5–3.2Ga) and contains abundant zircon xenocrysts with ages between 3.71 and 2.78Ga (TDM=4.2–3.4), overlapping with zircon U-Pb-Hf data obtained from three granodiorites of the KMD, dated at ca. 3.09, 3.08 and 2.87Ga. This difference indicates that the potassic plutonites resulted from partial melting of different sources within the crust and mantle during the same magmatic event at ca. 2.10–2.08Ga. The Qtz-monzodiorites derived from metasomatized mantle sources enriched by subducted sediments that in case of group 1 were derived from relatively juvenile Paleoproterozoic island arcs, and in case of group 2 from the Archean KMD. Bt-granites of group 1 might result either from reworking of relatively juvenile Paleoproterozoic crust, or from hybridization between (fractionated) monzodiorites and hot crustal melts, whereas Bt-granites of group 2 can be explained by in situ reworking of Archean TTG crust. Contemporaneous partial melting of the different mantle and crustal domains perhaps happened in response to lithospheric delamination following the collision of the juvenile Paleoproterozoic domain with the KMD. We further propose that this process also caused remelting of residual Au-rich cumulates left in the deep lithosphere by previous arc magmatism or Au-enriched metasomatic veins in lithospheric mantle.

Characterization of the Paleoproterozoic Hottah terrane, Wopmay Orogen using multi-isotopic (U-Pb, Hf and O) detrital zircon analyses: An evaluation of linkages to northwest Laurentian Paleoproterozoic domains

The Paleoproterozoic Wopmay orogen is modeled as a simple arc-continent collision involving the Archean Slave craton and an enigmatic continental arc terrane termed the Hottah terrane. Hottah is a composite terrane that includes a juvenile 2.6-2.1 Ga basement and a younger 1.93-1.90 Ga continental arc. The extent and crustal age structure of the terrane is poorly known which limits interpretations as to its origin, and eventual role in the development of the orogen. Multi-isotopic (U-Pb, Hf, O) analyses of detrital zircon from rocks of the Hottah terrane and its cover sequence are used to indirectly evaluate its age and crustal characteristics. The data confirm that ca 2.6-2.1 Ga crust is an important component of the terrane but Hf isotopic data indicate mostly non-juvenile compositions consistent with recycled Neoarchean crust sources. Ages of detrital zircon are distinct from those expected of the Slave craton but are similar to adjacent 2.0-1.90 Ga Paleoproterozoic terranes of northwestern Laurentia. The similarity to terranes such as the Ksituan/Buffalo Head and Taltson-Thelon magmatic zones, south and east of the Wopmay Orogen indicate that Hottah could represent a terrane that initially evolved adjacent to the earliest assembled parts of the Nuna continent between 2.0 and 1.9 Ga, and was rifted prior to the collision with the Slave margin that produced the Calderian orogeny.

Compilation of West African mineral deposits: Spatial distribution and mineral endowment

The West African Craton is highly endowed in minerals, and their spatial and temporal distribution varies from single to multi-phase mineralization events. They are broadly related to three major tectono-metallogenic elements and formed during distinct mineral epochs: (1) In both Archean Shields (Kénéma-Man and Reguibat) and Paleoproterozoic domains (Baoulé-Mossi, Eglab). These are characterized by giant iron ore deposits that formed between ca. 2.5–2.3Ga, nearly all gold, porphyry copper, lead–zinc and sedimentary manganese ore that developed between 2.2 and 2.1Ga, and primary diamonds that formed between two intervals at ca. 2.2–2.0Ga and in the Mesozoic. (2) Across Pan-African and Variscan belts. These are distinguished by major Precambrian IOCG's, copper–gold that formed at ca. 2.1Ga and approximately 680Ma, and Neoproterozoic sedimentary iron ore and phosphate deposits. (3) Within intracratonic and coastal basins. These include the development of Cenozoic lateritic bauxites over Mesozoic dolerites, Tertiary/Quaternary mineral sands deposits, oolitic iron ore and sedimentary phosphate deposits. Geological, spatial and temporal correlations using the multi-commodity West African Mineral Deposit Database highlight that gold and non-gold commodities formed in multiple phases. This commenced in the Liberian Orogeny (2.9–2.8Ga) with the enrichment of iron ore, nickel sulphides, diamonds and gold in the earth's crust. The pre-Eburnean or Tangaean–EoEburnean–Eburnean I Event yielded gold, and the major Eburnean Orogeny yielded gold, iron ore, manganese, diamonds, magmatic nickel sulphides, copper–gold, lead–zinc, and REE minerals. Throughout the Pan-African event sedimentary manganese deposits, lead–zinc, REE minerals, sedimentary phosphates, and again gold were formed. Primary diamonds and magmatic nickel sulphides are related to the break-up of Gondwana, followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin.

Paleoproterozoic granitoids from the northern limit of the Archean Amapá block (Brazil), southeastern Guyana Shield: Pb–Pb evaporation in zircons and Sm–Nd geochronology

Central Amapá, northern Brazil is located at the boundary between: (a) a northern Paleoproterozoic domain, consisting mainly of granite-greenstones terrains and (b) a southern Archean continental block (Amapá block), including an Archean basement reworked during the Transamazonian orogeny (2.26–1.95 Ga). Field investigations, Pb–Pb zircon and Sm–Nd whole rock geochronology supported by geochemical data on granitoids brought further constraints on Paleoproterozoic crustal growth in the southeastern Guyana Shield. A first magmatic episode, dated at 2.26 Ga, is marked by the crystallization of metaluminous low-K tholeiitic tonalites and quartz-diorites, which geochemical affinity with volcanic arc and association with T-MORB amphibolites suggest that they formed in a back-arc basin – island arc system. This event is coeval to the oceanic stage registered in French Guyana during the Eorhyacian (2.26–2.02 Ga). A second magmatic episode is represented by peraluminous, medium- to high-K calc-alkaline tonalite and granodiorite, which revealed some similarities with Mesorhyacian TTG rocks of French Guyana. For granitoids of both episodes, TDM and εNd values indicate the contribution of some Archean crustal component, probably by assimilation or contamination. This second magmatic episode occurred at 2.10 Ga, indicating that the period of successive calc-alkaline magmatic arcs formation may have extended until the Neorhyacian. Meanwhile, during this time, tectonic accretion by collision of the newly formed continental landmass was the prevailing process in French Guyana. The latter magmatic episode, even though poorly constrained, was registered around 2.08–2.02 Ga in central Amapá. It corresponds to the emplacement and solidification of high-K collisional granitoids, produced by partial melting of the Archean continental crust, as testified by the Archean TDM, inherited Pb–Pb zircon ages and strongly negative εNd values. Our results point toward the existence of a protracted episode of crustal growth during the Neorhyacian in the southeastern Guyana Shield. This episode has been predominantly driven by magmatic arc accretion during, at least, 160 My, along the period of 2.26–2.10 Ga. This cycle ended with diachronic closure of the oceanic basins and arc–continent collision.

The metacarbonate rocks of Itatuba (Paraíba): A record of sedimentary recycling in a Paleoproterozoic collision zone of the Borborema province, NE Brazil

The Alto Moxotó Terrane (AMT) is a Paleoproterozoic domain structured within the Neoproterozoic collage that forms the Borborema Province in Northeastern Brazil. In the Itatuba region (state of Paraíba) the dominant rocks consist of banded granodioritic–monzogranitic gneisses, augen syenogranitic gneisses and migmatites with limited occurrences of metasedimentary rocks. A metaplutonic–metacarbonate complex appears as concordant intrusive bodies whose emplacement was controlled by contractional shear zones. This complex includes metamafic–ultramafic rocks, leucotonalitic orthogneisses and metacarbonate rocks cut by alkaline orthogneisses emplaced along late transcurrent shear zones. The metamafic–ultramafic complex presents patterns of a tholeiitic magmatic series and exhibits symplectite textures, suggesting that they have reached a high-pressure (eclogite?) metamorphic peak. Uncommon associated rocks are represented by massive and brecciated metacarbonates, which contain abundant inclusions of the country rocks. The structure of this brecciated facies varies from plastic clastomylonitic to a true melting state, confirming the existence of intrusive metacarbonates. Field and petrographic evidence, such as the occurrence of rare rounded pyroxenite inclusions and the association with lamprophyre, suggests the involvement of the lower crust, or even the mantle in their formation. However, the geochemical (rare earth elements (REE) and incompatible elements) and isotopic (C–O) patterns preclude a mantle source for these rocks, confirming that they are remobilized sedimentary carbonates. U–Pb SHRIMP and TIMS zircon studies have been performed on the country and intrusive orthogneisses. In the host orthogneiss, zircon cores yield an age of 2,308±22.5Ma, whereas zircon rims exhibit a cluster with a mean age of 2012±17Ma. For the leucotonalitic orthogneisses that are frequently associated with the rocks of the mafic–ultramafic complex, zircon cores exhibit a spread of ages that may be attributed to the formation of the mafic to felsic complex, and the majority of these ages are centred approximately 2,180Ma. The rims exhibit two clusters of 2,086±19Ma and 1,953±25Ma. The TDM Nd model age of this rock is 2,067Ma; consequently, it is possible that this age interval represents another rock-forming episode associated with the probable eclogite episode. It is therefore likely that the metamafic–ultramafics, leucotonalitic orthogneisses and the metacarbonate rocks represent an Orosirian collisional complex of the AMT. Sm–Nd and U–Pb data from the late alkaline Neoproterozoic transcurrent granites (549, 587, and 621Ma) show reworking of the Paleoproterozoic crust.

Mafic granulite xenoliths from the East Indian Shield: Evidence for recycled continental crust in the Archean mantle

Massif-type charnockites of the Eastern Ghats granulite terrain, India, abound in mafic enclaves, which are generally hornblende mafic granulites with relatively minor occurrences of pyroxenite enclaves in the marginal segments only. The mafic granulite enclaves may be interpreted as earlier mafic melts within plutonic charnockite, where prograde heating in the hornblende in these mafic granulite enclaves was probably due to the host charnockite crystallization. Pyroxenite enclaves, on the other hand, are likely to be cumulates from an episode of mafic magmatism. The trace-element characteristics of hornblende–mafic granulite xenoliths are akin to arc-derived basalt, indicating a tectonic setting of subduction and slab melting. Further, low values of primitive mantle–normalized Nb/U ratios and enriched radiogenic isotopic compositions in the mafic xenoliths clearly indicate recycled continental crust in the mantle source region. While most of the internal segments of the Eastern Ghats mobile belt are Paleoproterozoic domains, the marginal (cratonic) segments in the north and west are Archean domains. The average Nd model age of ca. 2.5 Ga for the protoliths of hornblende–mafic granulite xenoliths for the Paleoproterozoic domains may be interpreted as the age of arc magmatism. For the marginal segments, the average Nd model age of ca. 3.3 Ga probably represents earlier Archean arc magmatism. Initial 87 Sr/ 86 Sr ratios calculated at these periods of mafic magmatism are high, which, together with negative e Nd values calculated for 2.5 and 3.3 Ga mafic magmatism, indicate recycled continental crust in their mantle source region. However, juvenile crustal addition seems to have been significant at 2.5 Ga, as is evident from the positive e Nd values for the majority of samples representing 2.5 Ga magmatism.

Evolution of a Paleoproterozoic “weak type” orogeny in the West African Craton (Ivory Coast)

The Paleoproterozoic domain of the Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of “domes and basins” geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of long-lived Paleoproterozoic “weak type” orogen.

Crustal growth of the central-eastern Paleoproterozoic domain, SW Amazonian craton: Juvenile accretion vs. reworking

The Trans-Amazonian cycle was an important rock-forming event in South America, generating voluminous juvenile and reworked fractions of continental crust. The Bacajá domain, in the southern sector of the Maroni-Itacaiúnas Province in the Amazonian craton, is an example of the Trans-Amazonian terranes adjacent to the Archean Carajás block. Zircon Pb-evaporation and whole-rock Sm–Nd analyses were carried out on representative samples of six lithological units, and allowed the proposal of a comprehensive tectonic-magmatic evolutionary sequence for the central and eastern parts of this domain, from the Neoarchean to the Rhyacian. Gneisses with ages of ca. 2.67 and 2.44 Ga are the oldest rocks recorded in the region, and probably represent remnants of island and continental arcs. The Três Palmeiras succession, emplaced between 2.36 and 2.34 Ga, hosts gold deposits and represents the first record of Siderian supracrustal rocks in the Amazonian craton. It was probably part of an island arc/ocean floor accreted to a craton margin. Rhyacian granitogenesis lasted for ca. 140 My (2.22–2.08 Ga), marking different stages of the Trans-Amazonian cycle. The first stage is represented by continental arc granitoids formed by melting of Archean crust at 2.22–2.18 Ga. The second is characterized by the production of juvenile material between 2.16 and 2.13 Ga. The third and final stage at ca. 2.08 Ga is represented by a large volume of granitoids originated from either juvenile material or reworked crust during compressive stresses. Nd isotopes reveal that juvenile rocks dominated in the northern part of the domain, whereas those formed from reworked crust predominate in the south. The present-day configuration of the Bacajá domain results from collision against the Archean Carajás block at the end of the Trans-Amazonian cycle.