Metavolcano-sedimentary Rocks Research Articles

Neoproterozoic pre-collisional events recorded in the Sergipano belt, Southern Borborema Province, West Gondwana

ABSTRACT The Canindé Domain is crucial for understanding the pre-collisional events in Sergipano Belt, Borborema Province, especially during the Late Stenian/Early Tonian (740–680 Ma). We present thermobarometric, geochronology, and isotopic studies of the metavolcanosedimentary rocks of the Novo Gosto unit. The calculated metamorphic conditions resulted in a range of T: 600–740°C and P: 3.5–10.0 kbar for non-mylonitic amphibolites and values of T: 636–808°C and P: 7.0–12.0 kbar for mylonites, both of amphibolite facies. The minimum metamorphic conditions imprint in these rocks are T: 390°C and P: 2.5 kbar, indicating retrometamorphism in greenschist facies. The probable age of the peak metamorphic event was obtained in zircon rims from a quartzite at 692 ± 8.5 Ma. U-Pb detrital zircon and Nd model ages of the metasedimentary rocks point out Early Tonian as primary source rocks. Subordinate contributions of Mesoproterozoic, Paleoproterozoic, and Neoarchean sources were also recognized. These primary sources are related to Cariris Velhos event rocks, whereas the subordinate sources are from the Pernambuco-Alagoas superterrane. Early Tonian and Late Stenian ages were recognized for the first time in the Canindé Domain, with U-Pb zircon ages of 1005 ± 3 and 989 ± 6 Ma in amphibolites and mylonitized granites, mostly outcropping at the southwestern limit of the area. New ages were also obtained in the Novo Gosto surrounding intrusions: Canindé Layered Gabbroic Intrusion (~718 Ma), Garrote (~715 Ma) and Curralinho/Boa Esperança (~708 Ma) metagranites, and Gentileza metavolcanic rock (~700), which integrate the early pre-collisional magmatism of the Sergipano Belt (~740-680 Ma). Finally, U-Pb zircon ages were obtained in the Poço Redondo Domain, potential source area of the Novo Gosto metasedimentary rocks, resulting in ages of 957 ± 11 and 988 ± 15 Ma. The new data bring perspective to future correlations with adjacent belts and the understanding of the structuration of West Gondwana during the Brasiliano-Pan-African Orogeny.

Geochemical, geochronological, and isotopic constraints for the Archean metamorphic rocks of the westernmost part of the Carajás Mineral Province, Amazonian Craton, Brazil

The Archean nucleus of the Amazonian Craton, represented by the Carajás Mineral Province (northern Brazil), crops out as a broad exposition of igneous, sedimentary, and metamorphic rocks showing a complex polyphase geologic history. A succession of orthogneisses (metagranitoids), amphibolites, and metavolcano-sedimentary rocks that show greenstone belt characteristics occur in Tancredo Neves Village district, northeast of São Félix do Xingu city (state of Pará), is the focus of this study. Detailed field, petrographic, and stratigraphic data reveal that these metamorphic rocks underwent two different tectono-metamorphic events: (i) the regional metamorphism (>2.8 Ga), which occurred under greenschist-to mid-upper amphibolite-facies conditions; and (ii) the dynamic metamorphism (>2.6 Ga) which reached amphibolite-facies conditions and overprinted previous metamorphic textures. Associated regional strike-slip NW–SE to EW-trending ductile shear zones generated mylonitic, ultra-mylonitic, and migmatitic zones. Whole-rock geochemistry discriminates four groups of metagranitoid protoliths. G1 group is composed of peraluminous trondhjemitic rocks samples, calc-alkaline, weakly fractionated, and enriched in HREE. G2 group comprises metaluminous and alkaline monzogranite samples with high Zr, Y, Nd, HREE, and intermediate fractionation. U–Pb (LA-MC-ICP-MS) dating of G2 zircons yields a crystallization age of 2745.6 ± 5.0 Ma. Groups G3 and G4 encompass peraluminous, calc-alkaline to alkali-calcic, anorogenic, fractionated monzogranitic and granodioritic rocks. G3 and G4 groups are further distinguished by high and low La/Yb(N) ratios, respectively. The results obtained for Tancredo Neves Village district very much resemble the Canaã dos Carajás (Carajás Mineral Province) area characteristics, comprising intense magmatism (2.7–3.0 Ga) and reworking related to at least two distinct metamorphic events in the Archean. As Canaã dos Carajás, Tancredo Neves Village can be a promising area for mining exploration.

Archean–Ediacaran evolution of the Campos Gerais Domain — A reworked margin of the São Francisco paleocontinent (SE Brazil): Constraints from metamafic–ultramafic rocks

The Campos Gerais Domain (CGD) in southeastern Brazil is an approximately 180 km × 35 km area of Archean–Proterozoic rocks located southwest of the São Francisco Craton (SFC). The Archean–Paleoproterozoic evolution of the CGD — alongside its potential correlation with the SFC or other cratonic blocks in the region — is currently poorly-constrained. We present the results of systematic petrography, bulk-rock geochemistry, mineral chemistry and geochronology for a suite of scarcely studied mafic–ultramafic rocks from the CGD. We also provide a compilation of previously reported bulk-rock geochemical and spinel group mineral chemical data for mafic–ultramafic rocks throughout the CGD, and geochronological information for various lithotypes in the region. The CGD records a protracted Mesoarchean to Statherian (3.1–1.7 Ga) crustal evolution, which we interpret to share a common history with the southern SFC and their related reworked segments, suggesting that it is a westward extension of this cratonic terrain. The metavolcano-sedimentary rocks of the Fortaleza de Minas and Alpinópolis segments represent a Mesoarchean greenstone belt that is stratigraphically and chemically comparable to Archean greenstone belts worldwide, and that is broadly coeval with a local suite of tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites. U-Pb SHRIMP zircon data from a subalkaline metagabbro yielded a concordia age of ca. 2.96 Ga, revealing a previously unrecognized phase of Archean magmatism in the CGD that can be chrono-correlated with metakomatiite and TTG generation elsewhere in the São Francisco paleocontinent. Our data contradict a hypothesis whereby the metavolcano-sedimentary rocks of the Jacuí-Bom Jesus da Penha and Petúnia segments represent an ophiolite, as previously suggested, instead presenting features that point to formation in association with a continental arc. Coupled with a U-Pb (SHRIMP) crystallization age of ca. 2.13 Ga recorded by zircon grains from a metaultramafic rock, these data highlight that a magmatic event was chrono-correlated with the main accretionary phase of the Minas Orogeny, and with the Pouso Alegre/Amparo and São Vicente complexes. Finally, a U-Pb (SHRIMP) concordia age of ca. 590 Ma — obtained from metamorphic-textured zircon grains from a metaultramafic rock — points to a late metamorphic overprint related to upper amphibolite conditions, brittle fault activation and the juxtaposition of crustal blocks in association with the latest stages of western Gondwana’s assembly in the southern SFC, with later retrogression to greenschist-facies.

Origin of highly fractionated peraluminous granites in South China: Implications for crustal anatexis and evolution

Voluminous Mesozoic intra-continental granitic rocks occur in South China, which provide insights into the processes and associated P-T conditions of crustal anatexis and evolution in an intra-continental setting. Most granitic plutons contain multiple phase intrusions, commonly with an early-stage main-phase granite and a late-stage minor-phase highly fractionated granite, and the latter is generally thought to result from extreme differentiation of the main-phase granite. We challenge this model based on our case study on the Tianmenshan composite pluton that is composed of an early-stage biotite granite and a late-stage two-mica granite. Zircon U-Pb dating yields similar emplacement ages of 157.3 ± 3.1 Ma for the biotite granite and 158.5 ± 3.4 Ma for the two-mica granite. The two rock types are peraluminous with A/CNK = 1.07 to 1.17 and show comparable Nd isotopic compositions, with εNd(t) = −9.4 to −11.5 for the biotite granite and −10 to −11.2 for the two-mica granite. The two-mica granite is characterized by high silica (SiO2 = 75.2–77.5%), low abundances of Sr (2.3–42 ppm) and Ba (10–114 ppm), large negative Eu anomalies in the chondrite-normalized REE patterns and typical REE tetrad effect, which, along with its high Ga/Al ratios, Fe-index and F contents, suggesting its affinity to highly fractionated granite. Nevertheless, the petrographic observations, zircon REE concentrations and biotite compositions consistently demonstrate that the late-stage two-mica granite formed under more water-deficient and reduced conditions than the early-stage biotite granite. This, along with a modeling study of trace elements (Rb, Sr, Ba,Y and Nb) and the lack of continuum in terms of whole-rock chemistry and biotite compositions between the two rock units, suggests that the two-mica granite cannot be formed through differentiation of the main-phase biotite granite. We suggest that the parental magma to the late-stage two-mica granite could have been derived from partial melting of a granulitic residue after extraction of granitic melts to form the biotite granite from the Neoproterozoic metavolcano-sedimentary rocks, followed by a significant feldspar-dominated fractionation during magma ascent. This model may apply to most other composite granite plutons in South China, and is of significance to better understand the process of crustal anatexis and evolution in an intra-continental setting.

Neoproterozoic (740-680 Ma) arc-back-arc magmatism in the Sergipano Belt, southern Borborema Province, Brazil

The Novo Gosto amphibolites represent the oldest unit of the Canindé domain, a Neoproterozoic sequence located in the NW Sergipano Belt, southern Borborema Province, Brazil. The domain comprises an association of metavolcano-sedimentary rocks, gabbroic intrusions, bimodal magmatic rocks, and syn-to post-tectonic granitoids. Due to the lithostratigraphic and geochemical complexity of this domain, several controversial interpretations of the tectonic setting have been proposed over the years. The protholits of the Novo Gosto amphibolites are here classified as high Fe tholeiitic basalt type rocks. Three geochemical groups were identified: Group 1 is more depleted in incompatible elements and has a pattern similar to island arc basalts with HFSE depletion (Nb–Ta, Zr–Hf, weak Ti); Group 2 displays a pattern similar to Group 1, but with more evident anomalies (Nb–Ta, Ti) and varied enrichment in incompatible elements; Group 3 is more enriched in incompatible elements than Group 1 rocks, with the pattern being flatter with milder HFSE depletion (Nb–Ta, Zr–Hf, weak Ti, Y) and resembling a geochemical pattern slightly more enriched than E-MORB. Our results suggest an environment similar to the Ryukyu-Okinawa (Japan) and Kamchatka arc-back-arc (Russia) setting. LA-ICP-MS U–Pb zircon dating of one amphibolite sample resulted in a date of 743 ± 3 Ma and confirms the geological field relationships that indicate this unit to be the oldest in the domain. The new petrographic, geochemical, and isotopic data for the Novo Gosto amphibolites lead to the recognition of an ancient Neoproterozoic arc-back-arc system, a rare setting in the earlier phases of the Brasiliano Orogeny within the Borborema Province, which ended with the closure of the restricted Canindé sea and the associated Sergipano ocean at around 630 Ma.

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.

High-K granites between the Archean Gavião and Jequié blocks, São Francisco Craton, Brazil: Implications for cratonization and amalgamation of the Rhyacian Atlantica continent

Precambrian terrane boundaries are sites of diverse rock associations and structural complexity. Motions along the boundary zones are commonly related to the construction of ancient large continental masses. This seems to be the case in the São Francisco Craton, Brazil, where three major blocks (Gavião, Jequié and Serrinha) collided in the Paleoproterozoic to form the northern part of the craton. This manuscript presents zircon U-Pb ages and whole-rock geochemistry for rock associations on the western border of the Jequié Block, which is defined by the north-trending left-lateral 600-km long Contendas-Jacobina fault zone. The studied rocks are granulite to amphibolite facies Neoarchean orthopyroxene-hornblende granite gneisses of the Jequié block, which also occur tectonically imbricated within Paleoproterozoic metavolcanosedimentary rocks of the Contendas-Mirante belt. The rocks are 2.65-2.63 Ga high-K ferroan, metaluminous to slightly peraluminous syeno-monzogranite in composition, with negative ƐNd(2650) and Nd model ages of 3.05-2.88 Ga. They are depleted in Sr and Ti, enriched in Th, Ba and Pb enrichment, and have marked negative Eu/Eu*. This geochemical signature coupled with the Nd-isotope data indicate that the parental granitic magma was generated by crustal reworking of felsic Mesoarchean crust distinct from the western-sitting >3.3 Ga Gavião Block. The investigated granitic gneisses are similar to other potassic granitoids that occur in the São Francisco Craton, which mark the thermal stabilization of the Archean lithosphere. The westward thrusting of the Neoarchean hornblende granitic gneiss and associated high-Mg tremolitite (meta-basalts) onto Paleoproterozoic volcano-sedimentary sequences establishes field evidence of Paleoproterozoic nappe stacking before lateral displacement during block collision. The crustal amalgamation is related to the assembly of the São Francisco-Congo Craton, a major landmass established before the Columbia supercontinent.

Petrology and U–Pb geochronology of high-grade metavolcano-sedimentary rocks from central Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is a high-grade metamorphic belt exposed along the southern margin of the North American plate in Mexico. Its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels, which culminated in the Paleocene. On the basis of field and petrographic work, this study integrates a petrological modelling approach with LA-ICP-MS U–Pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt. The study area in central XC includes a suite of alternating migmatitic paragneiss and garnet-bearing mafic schist. Petrological evidence and phase equilibria calculations demonstrate that a staurolite–kyanite grade metamorphism occurred at ~640–670 °C and 8–9 kbar before extensive migmatization took place. Differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration-melting reactions, and continued during peak metamorphism at granulite-facies conditions of ~800–820 °C and ~5–7 kbar. According to Ti-in-zircon thermometry, melt generation/mobilization progressed during cooling until crystallize at ~700 °C. The age of prograde metamorphism was likely recorded by monazite cores that give mid- to Late Cretaceous ages. Cooling and melt crystallization could be constrained by leucosome zircon dated at 61.8±0.6 Ma. Both zircon and low-Y monazite from whole rock and melanosome portions, respectively, are consistent with an episode of growth around 60.9±0.5 Ma. Monazite, however, would have continued its crystallization process for a period of ca. 10 Ma. Thin zircon and high-Y monazite rims show ages between ~34–25 Ma and are interpreted to reflect a stage of reheating by magmatic advection related to large Oligocene plutonism in the region. Age and nature of central XC basement show good correlation with other Early-Cretaceous volcano-sedimentary basins of central and southern Mexico. A period of crustal thickening would have buried the sequence at depths of ~30 km, causing Barrovian-type metamorphism during mid- to Late Cretaceous time and eventually, anatexis that evolved at granulite-facies conditions during orogenic collapse in the Paleocene (≥62 Ma). The time span from leucosome crystallization to late-stage reheating in central XC would imply a protracted high-temperature evolution indicating a middle- to upper-crustal residence time of at least ca. 30 Ma.

Structural and hydrothermal evolution of the shear-zone-related iron-oxide enrichment in metavolcano-sedimentary rocks of the intracontinental araçuaí orogen: The case of the espírito santo iron deposit

The Rio São João metavolcano-sedimentary sequence is located in the eastern border of the northern Espinhaço fold thrust belt, in the intracontinental Araçuaí-West Congo Orogen. Mafic schist, quartz-, amphibole- and carbonate itabirites, as well as a carbonate–silicate rock, occur in the study area. These contain mineral assemblages compatible with the amphibolite-facies metamorphic conditions. Magnetite- and hematite-rich itabirites form a series of iron deposits in the region near Igaporã-Caetité, Bahia state, Brazil. The Espírito Santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol% of the rock volume; siliciclastic and mafic rocks also host iron-rich domains, variably enriched in magnetite and hematite. The deposit is hosted in compressional duplexes with structural top towards the SW. These structures have an Sn-1//Sn foliation represented by compositional banding and parallel schistosity that is observed at all scales. Other common features are S/C/C, boudins, pinch-and-swell structures, as well as sheath, syn-tectonic folds, and a mineral stretching lineation (Lxn). Hydrothermal alteration was contemporaneous to the development of duplex and the Sn-1//Sn foliation. A distal alteration zone is dominated by a potassic (muscovite and-or biotite) ± chlorite assemblage and the intermediate zone by carbonate development. The proximal alteration zone constitutes the iron oxide (magnetite and hematite) domains; sulfide minerals as well as quartz veins (silicification) in association with other phases also formed. Hydrothermal magnetite is associated with C’ structures, which grows either including silicates and skeletal carbonates, or forming corrosion borders in these minerals. Lamellar hematite forms polycrystalline aggregates, characterizing the S/C structure in the Sn-1//Sn foliation, as well as the axial planar foliation (Sn) in intrafolial folds and the mineral stretching line Lxn. Data obtained by the LA-ICPMS technique show that, in general, in quartz-rich and amphibole itabirites hydrothermal magnetite is richer in REE than an early-stage magnetite. Hydrothermal magnetite REE pattern is close to that of the country rock of the mineralization. The formation of hematite- and magnetite-rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates, quartz and the carbonates of all host rocks, and precipitated new hydrothermal phases taking advantage of the Ediacaran shear structures.

Strain Analysis and Microstructural Investigation of the Jabal Tays Ophiolite Complex, Eastern Arabian Shield, Saudi Arabia

The present study deals with the Jabal Tays ophiolite area, which was affected by Al Amar-Idsas fault. It is one of the most important tectonic features in the Eastern Arabian Shield. This study aims to accomplish through field investigation, finite strain and microstructural analysis for understanding the nature of tectonic evolution and the deformation in the Jabal Tays area. Field and microscopic investigations indicate that mafic-ultramafic rocks at Jabal Tays include the large amount of serpentinite, subordinate amounts of gabbro intruded by mafic dikes, melange, serpentinite schist, and listwaenite. The serpentinite makes up the bulk of Jabal Tays, is variably sheared and typically consists of relatively massive serpentinite cut by shear zones marked by serpentinite schist. Based on finite strain and microstructural analysis, mafic-ultramafic rocks in the Jabal Tays area are generally confined to brittle-ductile shear zones, which they are sub-parallel to the prevailing NW or NNW structural trends. It is confined mainly to the metamorphic terrains, especially to zones intruded by mafic volcanic rocks. It is concluded that the contacts between the Abt schist and metavolcano-sedimentary rocks were formed during progressive thrusting under brittle to semi-ductile deformation conditions by simple shear that also involved a component of vertical shortening, causing subhorizontal foliation in the Jabal Tays area.

Geology, petrography and mineral chemistry of iron oxide-apatite occurrences (IOA type), western sector of the neoproterozoic Santa Quiteria magmatic arc, Ceará northeast, Brazil

Iron oxide-apatite (IOA) occurrences have recently been identified in the western sector of the Neoproterozoic Santa Quitéria magmatic arc (Ceará state, northeast Brazil), close to the boundary with the Paleozoic Parnaíba Basin. The IOA mineralization is hosted by albitized metadiorites and metavolcano-sedimentary rocks, which are crosscut by a late- to post-Brasiliano/Pan-African biotite granite intrusion with a U-Pb age approximately 548 ± 4 Ma. Within the metavolcano-sedimentary sequence, metavolcanic rocks are bimodal and consist of albitized metabasalt-andesite and metarhyolites with a U-Pb age of 554 ± 6 Ma, whereas the metasedimentary units are represented by calc-silicate, marble, and pelitic gneisses. The iron oxide-apatite mineralization occurs as (i) banded–stratabound lenses/layers composed of magnetite (50–70%), with or without ilmenite exsolution; apatite (≤8%); monazite; and locally subordinate copper sulfides (~2%); (ii) massive magnetite-(specular hematite)-apatite bodies; (iii) disseminated and vein magnetite-apatite within albitized metadiorite; (iv) massive rhombohedral hematite bodies; and (v) garnet-magnetite type. The host rocks also display marialite, diopside, albite, and epidote (sodic-calcic alteration), biotite and K feldspar (potassic alteration), chlorite and epidote in lower-T hydrothermal alteration assemblages. Mineral chemistry data reveal that (i) except for vein types, apatite in all the other occurrences is F rich (2.4 ≤ F ≤ 4.4%) and low in Cl (<0.5%), and (ii) magnetite contains variable concentrations of Ti, V, Cr and Ni attributed to both igneous and hydrothermal environments. In addition, the magnetite of the stratabound occurrence is low in TiO2 (≤1.6%); however, its oxy-exsolutions are rich in TiO2 (14–52%), indicating a titanium-rich original iron oxide. The low sulfide content and high concentration of apatite indicate that the investigated Fe-P occurrences are of the IOA type, similar to those of IOA provinces elsewhere, such as Kiruna, El Laco, Gushan, Pea Ridge, Pilot Knob, and Bafq. Hence, the discovery of these occurrences in the Neoproterozoic Santa Quitéria magmatic arc has a twofold metallogenic significance: (i) these are the first records of IOA-type deposits in Brazil; and (ii) open a favorability potential for the exploration of IOCG-type deposits in this tectonic domain.

Geology and structural control of the Cutia Prospect, Serra Leste, Carajás Mineral Province, Pará, Brazil / Geologia e controle estrutural do Garimpo Cutia, Serra Leste, Província Mineral de Carajás, Pará, Brasil

The Cutia Prospect is an occurrence of gold located at the Serra Leste, Carajás Mineral Province. This occurrence is related to magnetite/hematite breccia zones, hosted by hydrothermally altered metavolcano-sedimentary rocks in contact wich felsic ignous rock. The contact has a structural control and hosts oxidized quartz veins and mineralized siliceous breccias. This structure has features of a shear zone with characteristics of a brittle-ductile environment, showing a high degree of maturity, represented by breccias and faults, anastomosed foliation and porphyroclast deformation with the formation of kinematic indicators. Structures such as ruptured and rotated layers, tension gashes, well developed foliation and high angle are evidence of a transcurrent shear zone. Exploration activities in the area allowed to verify that the gold was removed from a transition zone of a gossanic sequence where the oxidized zone of the gossan comprises a superficial ferruginous crust.

Isotopic and geochemical characterization of the metavolcano-sedimentary rocks of the Jirau do Ponciano Dome: A structural window to a Paleoproterozoic continental arc root within the Southern Borborema Province, Northeast Brazil

The Sergipano Fold Belt is a Neoproterozoic orogen exposed in the southern Borborema Province (Northeast Brazil), which is interpreted as the result of the oblique convergence process between the São Francisco-Congo block and smaller crustal fragments. The Nicolau-Campo Grande Complex is the metavolcanic-sedimentary portion of the Jirau do Ponciano Dome, an important basement inlier within this collisional belt. Whole-rock-geochemical major and trace-element data of major and trace elements from the metavolcanic rocks of the Nicolau-Campo Grande Complex indicate basaltic-andesite, andesitic, rhyodacitic, and rhyolitic compositions. Rare earth element and trace elements show the typical signature of magmatic arc rocks with negative Nb, Ta, P, and Ti anomalies. New U-Pb zircon data from amphibolite and hornblende-biotite paragneiss samples from the complex confirm the existence of a Paleoproterozoic zircon population of a Paleoproterozoic age of approximately ca. 2054 Ma with Archean inheritance ranging in age between 2779 and 3324 Ma. A maximum sedimentation age of 2028 Ma is constrained by the youngest zircon grain in the hornblende-biotite paragneiss. Zircon U-Pb data from the metarhyolite and rhyodacites indicate Paleoproterozoic ages of ca. 2061 and 2074 Ma, respectively, interpreted as their crystallization age. Nd isotopic data in the metarhyolite and rhyodacites indicate Nd TDM ages between 2.54 and 3.07 Ga and 87Sr/86Sr (2061Ma) values from 0.71396 to 0.72351. Based on these data, we suggest that the probable source area of the metavolcanic-sedimentary complex is a Paleoproterozoic magmatic arc that was exhumed and covered by the paleobasins of the Macururé Domain in the northeast portion of the Sergipano Fold Belt.

An Ediacaran back-arc basin preserved in the Transversal Zone of the Borborema Province: Evidence from geochemistry, geochronology and isotope systematics of the Ipueirinha Group, NE Brazil

The Ipueirinha Group is a ca. 75 km-long by 13 km-wide succession of metavolcano-sedimentary rocks situated in the westernmost Transversal Zone, Borborema Province, NE Brazil. It is mostly composed of marine terrigenous sedimentary rocks represented by immature metarhythmites and quartzites, local metaultramafites, rhyolitic metavolcaniclastic rocks and rare lenses of metamarl. The metavolcaniclastic rocks show highly fractionated chondrite-normalized REE patterns (LaN/YbN = 3.68–8.94) with slight to moderately negative Eu anomalies (Eu/Eu* = 0.72–0.94), incompatible trace element signatures similar to those of volcanic arc-related rocks, including a Nb-Ta trough, and negative ƐNd(t) ranging from −3.4 to −12.2. Metarhythmites show LREE enrichment (LaN/YbN = 4.83–15.32), moderate Eu anomalies (Eu/Eu* = 0.68–0.81), and incompatible element signatures consistent with provenance from a continental arc. Nd isotope data indicate important detrital contribution from juvenile sources for these metarhythmites, as reflected by the ƐNd(t) values between −3.6 and 0.3. Detrital zircon LA-ICP-MS U-Pb data reveal that the main sources of detritus for the metarhythmites are Neoproterozoic rocks (maximum age peak at ca. 845 Ma and younger zircons at ca. 636 Ma). Hf isotope data obtained from detrital zircons show evidence for an important period of juvenile magmatism in the Borborema Province from 895 to 834 Ma (ƐHf(t) = 2.0–5.7), and suggest that the Neoproterozoic crustal evolution of this province involved mixing between older sources with negative ƐHf(t) and younger juvenile sources. Metaultramafites associated with the Ipueirinha Group show ƐNd(t) ranging from −1.3 to −8.5 and TDM = 1.33–1.64 Ga. The whole unit was submitted to four deformation phases and to heterogeneous metamorphic conditions from lower to upper greenschist facies. Additional geochronological data obtained for a granitic sill (578 ± 13 Ma; zircon U-Pb) along with monazite U-Pb data from a chloritite sample (595 ± 29 Ma) indicate that strike-slip deformation, metamorphism and syn-orogenic melt generation coexisted in the Ipueirinha Group during the Late Ediacaran. We suggest that the Ipueirinha Group comprises a Cryogenian/Ediacaran back-arc basin to the Tamboril-Santa Quitéria magmatic arc, which is one of the few records of initial convergence tectonics in the West Gondwana margin of the Borborema Province.

Geophysical prospection in tin mineral occurrence associated to greisen in granite São Sepé (RS)

Abstract The discovery of new mineral resources involves various research techniques through direct and indirect studies. The geophysical methods are important tools in the detection of deep deposits because they use the contrast between the physical properties of the hosting rocks and the mineralized zone. In this sense, electrical geophysical methods are widely used in the prospecting of sulfide deposits. This work presents and discusses the results of the application of the Induced Polarization geophysical method in the investigation of a tin mineral occurrence associated with greisen, located on the western border of Sao Sepe Granite. The study area is located in the central portion of the Rio Grande do Sul State, in the municipality of Sao Sepe (RS), housed in the metavolcano-sedimentary rocks of the Vacacai Metamorphic Complex. Six lines of electrical tomography were parallel arranged in the N30W direction. The 2D inversion models allowed to identify areas of high chargeability (above 5.0 mV/V), possibly attributed to areas with sulfide accumulation. The areas with low values (below 1.9 mV/V) area associated with soil and rocks absent of mineralization. Through the 3D visualization models, it was possible to identify that the mineralized zone apparently has continuity for more than 36 m. These models also allowed identifying the morphology and the lateral continuity of the sulfide zone. Thus, the results of this work demonstrate the possibility of using Induced Polarization in the research of cassiterite deposits with associated sulfides due to the contrast of physical properties obtained from the high polarizability of the sulfides.

Aspectos tectônicos do emplacement da borda leste do Complexo Granítico Três Córregos, Cerro Azul (PR)

Grandes quantidades de rochas graníticas foram geradas durante a evolução do Ciclo Brasiliano, destacando-se o Complexo Granítico Três Córregos (CGTC), um importante complexo ígneo do Neoproterozoico, intrudido nas rochas metassedimentares mesoproterozoicas da Formação Água Clara. No âmbito da Formação Água Clara reconheceram-se duas associações litológicas: uma constituída por rochas calciossilicáticas e outra composta por mármores calcíticos intercalados a carbonato xistos e filitos. O padrão estrutural observado nessas rochas é caracterizado por duas foliações, onde S1 configura uma clivagem ardosiana que evolui para uma xistosidade contínua, subparalela ao bandamento composicional, observada nos micrólitons de foliação S2, esta desenvolvida em um sistema de transposição parcial a total de S1 em uma tectônica de cavalgamento, resultando em paralelismo entre as duas estruturas. As fácies graníticas Arrieiros-Cerro Azul e São Sebastião reconhecidas na borda leste do CGTC apresentam foliação de fluxo magmático definida pela orientação de máficos da matriz e/ou orientação de fenocristais de feldspato potássico, com atitudes concordantes com os contatos e com as foliações das rochas metassedimentares. Foram reconhecidas paragêneses das fácies xisto verde e anfibolito superior, com gradiente expresso por uma distribuição consistente de minerais índices de temperaturas relativamente mais elevadas em toda a área de estudo. As paragêneses compatíveis com a zona da sillimanita em rochas pelito-aluminosas e calciossilicáticas, a relação dos aspectos petrotectônicos dos granitos com suas encaixantes, bem como os padrões estruturais observados nos granitos e rochas metassedimentares concordantes podem ser explicados pela sobreposição, ao menos parcial, entre os eventos de instalação do CGTC e do desenvolvimento da foliação S2, indicando que os processos iniciais de abertura de espaço para a colocação do CGTC estão relacionados à tectônica de baixo ângulo.

Geologia e geofísica do greenstone belt Vila Nova, porção NE do Cráton Amazônico, Amapá, Brasil

Poucos afloramentos e um acentuado intemperismo caracterizam a região do greenstone belt Vila Nova, no sudeste do Amapá. Este artigo vale-se de dados aerogeofísicos na interpretação geológica e estrutural durante o mapeamento geológico, objetivando aumentar o entendimento geológico e tectônico dessa porção do Cráton Amazônico. A interpretação qualitativa das imagens magnetométricas e gama-espectrométricas geradas para o projeto foi efetuada em ambiente de Sistema de Informações Geográficas (SIG). O reconhecimento de unidades magnetométricas e gamaespectrométricas presentes na área de estudo teve como base a classificação hierárquica dos polígonos delineados pela interpretação visual. A partir dessa análise foram delimitados os principais domínios geológicos e definidos os traçados geológico-estruturais, em conjunto com dados geológicos de campo, dados geofísicos e da análise petrográfica. Essa integração permitiu delimitar as rochas das áreas do embasamento arqueano composto por ortognaisses e granitóides do Complexo Tumucumaque, as rochas metavulcano‑sedimentares do Complexo Vila Nova e os maciços graníticos intrusivos de idade paleoproterozoica. A integração entre os dados geofísicos e de campo resultou no aumento da definição da cartografia geológica, ressaltando-se a importância dessa metodologia para o reconhecimento de tramas estruturais e litológicas complexas em áreas de pouca exposição e dificuldade de acesso.

Zircon geochronology and Hf isotopes of the Dwalile Supracrustal Suite, Ancient Gneiss Complex, Swaziland: Insights into the diversity of Palaeoarchaean source rocks, depositional and metamorphic ages

We report the results of combined U-Pb and Lu-Hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano-sedimentary rocks of the Dwalile Supracrustal Suite (DSS) from the ca. 3.66 to 3.20Ga Ancient Gneiss Complex (AGC) in Swaziland. The results indicate that the DSS is made up by Palaeo- and Mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the Neoarchaean. The oldest Dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca. 3.46Ga, and are derived from the same magma source as the Tsawela Gneisses. Based on the combined U-Pb-Hf isotope data, two groups of metasedimentary rocks can be distinguished: an older Dwalile Group I that is similar in age to the Hooggenoeg and Kromberg Formations (3.46–3.40Ga), but with a different source area. A younger Dwalile Group II is coeval with the Fig Tree and possibly Moodies Groups (<3.23Ga) and the data suggest there was a similar provenance.The U-Pb-Hf isotope data and petrological observations demonstrate that the DSS was affected by three stages of metamorphic overprint: an event at 3.23Ga, pervasive amphibolite-facies metamorphism at 3.15Ga, and low-P granulite-facies metamorphism at 2.99Ga. Metamorphic zircon in the amphibolite-facies rocks resulted from new growth in aqueous fluids at 3.15Ga, whereas in the granulite-facies rocks new zircon was formed in the presence of melt. The 3.23Ga event coincides with the last significant metamorphic overprint documented in the Barberton Greenstone Belt (BGB), whereas as two younger events are restricted to the AGC.The data support the model suggesting amalgamation between the AGC and BGB terranes at ca. 3.23Ga. In addition, they require crustal thickening in the AGC terrane at ca. 3.15Ga, causing Barrovian-type metamorphism in the DSS and granulite-facies metamorphism in the Luboya-Kubuta Terrane. Low-P granulite-facies metamorphism at ca. 2.99Ga is most likely related to crustal extension during the onset of Pongola Basin rifting, causing mantle upwelling, mafic magmatism, and heat transfer from the mantle into the thinned crust.

Indications of HP events in the volcanosedimentary succession of the Orlica–Śnieżnik Dome, NE Bohemian Massif: data from a marble-amphibolite interface

A volcanosedimentary succession of the Młynowiec-Stronie Group (MSG) in the Orlica–Śnieżnik Dome (OSD), the Sudetes, NE Bohemian Massif underwent multiple folding and shearing during the Variscan Orogeny. In the sheared domains, there are less deformed pods in which rocks preserve better records of metamorphic events prior to the regional temperature peak. In one such pod, near Gniewoszów on the western limb of the dome, marbles enclosed by massive amphibolites occur. In these rocks, zoned plagioclase with actinolite and epidote inclusions and zoned amphibole grains allowed recognition of three mineral assemblages and three P-T stages at: (1) 310°C/3–4 kbar, (2) 480–500°C/10.5 kbar, (3) 500–530°C/6–6.5 kbar, based on isopleth intersections and checked against conventional thermobarometry. These define a steep clockwise P-T path and a geothermal gradient of 17°C/km before peak conditions were attained, which suggests subduction of the metavolcano-sedimentary rocks (Stronie Formation of the MSG) on the western limb of the OSD, with a transient yet discrete higher pressure episode. Mineral relicts capable of demonstrating a higher pressure event are scarce in the supracrustal rocks of the dome, mainly because they became more thoroughly equilibrated and obliterated during the temperature peak at mid-amphibolite facies conditions and the subsequent ubiquitous greenschist facies overprint.

P–T–time-isotopic evolution of coesite-bearing eclogites: Implications for exhumation processes in SW Tianshan

The Chinese Southwestern Tianshan high- to ultra-high pressure low temperature (HP–UHP/LT) metamorphic belt exhibits well-preserved mafic layers, tectonic blocks/slices and boudins of different sizes and lithology embedded within dominant meta-volcanosedimentary rocks. Despite a wealth of previous studies on UHP relicts, P–T path estimates and age constraints for metamorphism, controversies still exist on P–T–t assessments and regional exhumation patterns (i.e., tectonic mélange versus internally coherent “sub-belt” model). This study focuses on a group of coesite-bearing eclogite samples from a thick (~5m) layered metabasalt outcrop in order to unravel its detailed tectono-metamorphic evolution through space and time (both prograde, peak and exhumation). Using SIMS zircon U–Pb and oxygen isotope analyses, TIMS Sm–Nd multi-point isochron dating, in situ laser-ICP-MS trace-element analyses, classical thermobarometry and thermodynamic modeling, we link the multistage zircon growth to garnet growth and reconstruct a detailed P–T–time-isotopic evolution history for this UHP tectonic slice: from UHP peak burial ~2.95±0.2GPa, 510±20°C around 318.0±2.3Ma to HP peak metamorphism ~2.45±0.2GPa, 540±20°C at 316.8±0.8Ma, then, with eclogite-facies deformation ~2.0±0.15GPa, 525±25°C at 312±2.5Ma, exhumed to near surface within ca. 303 to ca. 280Ma. Our P–T-time-isotopic results combined with the compilation of regional radiometric data and P–T estimates notably point to the existence of a short-lived period of rock detachment and exhumation (<10Ma, i.e. at ca. 315±5Ma) with respect to subduction duration.