Mauro Cesar Geraldes

Proterozoic geologic evolution of the SW part of the Amazonian Craton in Mato Grosso state, Brazil

Abstract This paper presents new geochronologic, isotopic, and geochemical data bearing on the evolution of Proterozoic crust in SW Mato Grosso state, Brazil, which is at the southern end of the ≈1.6–1.8 Ga Rio Negro–Juruena orogenic belt of the Amazonian Craton (Amazonia). Our data define three major crustal events: (i) the Alto Jauru terrane occurs in the eastern part of the region and is comprised of island arc-related rocks with U/Pb ages from 1.79 to 1.74 Ga. These rocks have e Nd ( t ) values range from +2.8 to +2.0 with crustal residence ages ( T DM ) from 1.93 to 1.78 Ga, indicating a mainly juvenile signature at 1.8 Ga; (ii) the Cachoeirinha suite occurs in the central part of the region and consists of calc-alkaline plutons emplaced into Alto Jauru terrane host rocks. These plutons have U/Pb ages from 1.56 to 1.54 Ga and e Nd ( t ) values ranging from +1.0 to −0.8, with crustal residence ages ( T DM ) from 1.88 to 1.75 Ga. We interpret this orogen as the roots of a continental margin arc built upon basement comprised of the Alto Jauru terrane; (iii) the Santa Helena batholith occurs in the western part of the region and is a large, elongate body of calc-alkaline rocks ranging from granodiorite to highly evolved granite. These units yield U/Pb ages from 1.45 Ma to 1.42 Ga, with e Nd ( t ) values ranging from +4.1 to +2.6 and crustal residence ages ( T DM ) from 1.70 to 1.50 Ga. The Rio Alegre domain occurs west of the Santa Helena batholith and includes juvenile 1.52 to 1.47 Ga volcanic and mafic plutonic rocks. Regional geologic relationships suggest that these rocks are part of the crust into which the Santa Helena batholith was emplaced. We interpret this batholith as the magmatic core of a juvenile arc accreted to the edge of the Alto Jauru terrane, with incorporation of some older crust (Alto Jauru terrane) in the east and derivation from mainly juvenile crust (Rio Alegre domain) in the west. The Rio Branco suite occurs to the east of exposed Alto Jauru terrane rocks as large hills protruding through younger Aquapei Group sedimentary rocks. It consists of gabbro and granophyric rocks with U/Pb ages of 1.47 Ga (gabbro) and 1.43 Ga (granophyre) and inherited older Nd, suggesting hinterland derivation from Alto Jauru terrane basement during development of the Santa Helena batholith. Several undeformed 1.5–1.4 Ga granitic plutons occur within the Alto Jauru terrane; these are also regarded as inboard manifestations of subduction related magmatism associated with accretion of the Rio Alegre domain and formation of the Santa Helena batholith. The siliciclastic Aguapei Group was deposited sometime between 1.0 and 1.4 Ga; it overlies Jauru terrane basement, rocks of the Santa. Helena batholith, and the Rio Alegre domain. In the east, it is flat-lying and undeformed, but in the west it is deformed and metamorphosed in the NNW trending Aguapei thrust belt. Farther west the Aguapei Group is horizontal, undeformed, and overlies the Paleoproterozoic Paragua block in Bolivia, where it has been correlated with the Sunsas Group. The Aguapei thrust belt has K/Ar cooling ages of about 930 Ma and is apparently a foreland fold and thrust belt formed by reactivation of an older rift basin during the 1.0 Ga Sunsas orogeny, which occurs west of the Paragua block in Bolivia.

Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture

New 40 Ar/ 39 Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- topic age resetting at 1.18-1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilandia belt between these two cratons, new 40 Ar/ 39 Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40 Ar/ 39 Ar, and Rb/Sr thermochronologic data are used to establish post- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15-30 km) exposed in North America to shallower levels (5- 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- al thickness. This interpretation explains the widespread preservation of both pre- Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- genic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed.

Middle Proterozoic vein-hosted gold deposits in the Pontes e Lacerda region, southwestern Amazonian craton, Brazil

Structural, geochemical, and isotope studies were carried out on the gold deposits of the Pontes e Lacerda region (Mato Grosso state, Brazil), where rocks of the Aguapei and Rondoniano mobile belts (southwestern Amazonian craton) occur. The orebodies are hosted in metavolcanic, gneiss-granite, quartzite, tonalite, and granite units. Tectonics involve oblique overthrusting (from northeast to southwest), which led to the formation of recumbent folds and thrusts (pathways for the mineralizing fluids), upright folds, and faults with dominant strike-slip component. These unconformities represent potential sites for mineralization. During geological mapping, it was observed that the orebodies consist of quartz, pyrite, and gold, and that the hydrothermal alteration zone contains quartz, sericite, pyrite (altered to limonite), and magnetite (altered to hematite). Chalcopyrite, galena, and sphalerite occur only in the Onca deposit. Chemical analysis of sulfides indicates high contents of Bi, Se, and Te in sulfide...

The 1590-1520 Ma Cachoeirinha magmatic arc and its tectonic implications for the Mesoproterozoic SW Amazonian craton crustal evolution

Isotopic and chemical data of rocks from the Cachoeirinha suite provide new insights on the Proterozoic evolution of the Rio Negro/Juruena Province in SW Amazonian craton. Six U-Pb and Sm-Nd analyses in granitoid rocks of the Cachoeirinha suite yielded ages of 1587-1522 Ma and TDM model ages of 1.88-1.75 Ga (EpsilonNd values of –0.8 to +1.0). In addition, three post-tectonic plutonic rocks yielded U-Pb ages from 1485-1389 Ma (TDM of 1.77-1.74 Ga and EpsilonNd values from –1.3 to +1.7). Variations in major and trace elements of the Cachoeirinha suite rocks indicate fractional crystallization process and magmatic arc geologic setting. These results suggest the following interpretations: (1) The interval of 1590-1520 Ma represents an important magmatic activity in SW Amazonian craton. (2) TDM and arc-related chemical affinity support the hypothesis that the rocks are genetically associated with an east-dipping subduction zone under the older (1.79-1.74 Ga) continental margin. (3) The 1590-1520 Ma age of intrusive rocks adjacent to an older crust represents similar geological framework along the southern margin of Baltica, corroborating the hypothesis of tectonic relationship at that time.

Geoquímica e Assinaturas Nd-Sr do Complexo Granitoide Pensamiento, Província Rondoniana-San Ignacio, Pré-Cambriano de Bolívia Oriental: Caracterização Petrogenética de um Arco Magmático no Mesoproterozoico

O Complexo Granitoide Pensamiento (CGP) ocorre na porcao norte do Pre-Cambriano Boliviano, estando tectonicamente associado a evolucao da provincia Rondoniana-San Ignacio (1.55 - 1.30 Ga) do Craton Amazonico, constituido por uma provincia central de idade arqueana e provincias proterozoicas marginais. A evolucao proterozoica resulta do desenvolvimento de cinturoes acrescionarios sucessivamente mais jovens para sudoeste, a exemplo das orogenias Rondoniana-San Ignacio (1.37 - 1.32 Ga) e Sunsas (1.20 - 1.00 Ga). O CGP ocorre na parte setentrional do Pre-Cambriano Boliviano, ao norte do cinturao Sunsas, sendo constituido por granitos e termos subvulcânicos. Subordinadamente ocorrem sienitos, granodioritos, tonalitos, trondjemitos e dioritos. Em termos tectonicos, essas rochas sao classificadas em dois conjuntos: plutons sin a tardicinematicos e tardi a pos-cinematicos. Treze analises quimicas em rocha total para elementos maiores, tracos e ETR foram realizadas em rochas granitoides orogenicas do CGP. Diagramas de correlacao geoquimica indicam tendencia negativa entre os conteudos de MgO, Al2O3 and CaO em funcao do aumento de SiO2, sugerindo processos de cristalizacao fracionada na petrogenese das rochas investigadas. Em adicao os dados indicam uma composicao principalmente peraluminosa, subalcalina de alto K, compativel com ambiente de arco magmatico, para a geracao dos plutons estudados, corroborado pelo padrao de fracionamento dos ETRL/ETRP. Datacoes SHRIMP em zircao dos plutons La Junta e San Martin (sin a tardicinematicos; 1347 ± 21 e 1373 ± 20 Ma, respectivamente) em conjunto com idades modelo T DM entre 1,9 e 2,0 Ga e valores de eNd(1330) entre +1,8 e -4,3 sao semelhantes a resultados publicados em outros corpos coevos. Em adicao, os plutons Porvenir, San Cristobal e Piso Firme (tardi a pos-cinematicos) tem idades T DM modelo entre 1,6 e 1,7 Ga e valores de eNd(1330) positivos entre +2,7 e +1,5, o que sugere uma origem em arco magmatico intraoceânico. O pluton Diamantina (tardi a pos-cinematico; idade SHRIMP em zircao de 1340 ± 20 Ma) tem idades T DM modelo entre 1,6 e 1,9 Ga com valores de eNd(1330) entre +0,4 e -1,2. Isto corrobora a hipotese de significativa contribuicao de material juvenil mesoproterozoico durante a sua genese. Os resultados aqui obtidos interpretados em conjunto com os dados geologicos de unidades contemporâneas na contraparte brasileira reforcam a existencia de um arco magmatico juvenil mesoproterozoico que finalizou a evolucao acrescionaria da provincia Rondoniana-San Ignacio

Isotopic Evidence for the Late Brasiliano (500-550 Ma) Ore-Forming Mineralization of the Araés Gold Deposit, Brazil

The Araés gold deposit, located in eastern Mato Grosso State, central Brazil, is hosted in Neoproterozoic volcanosedimentary rocks of the Paraguay belt, which formed during collision of the Amazonian craton and the Rio Apa block. 40Ar/39Ar geochronology and Pb and S isotopic analyses constrain the timing and sources of mineralization. Three biotite flakes from two samples of metavolcanic host rock yield 40Ar/39Ar plateau ages between 541 and 531 Ma, interpreted as cooling ages following regional metamorphism. Clay minerals from a hydrothermal alteration zone yield an 40Ar/39Ar integrated age of 503 ± 3 Ma. Galena grains from ore-bearing veins yield values of 206Pb/204Pb from 17.952 to 18.383, 207Pb/204Pb from 15.156 to 15.811, and 208Pb/204Pb from 38.072 to 39.681. Pyrite grains from ore-bearing veins yield values of 206Pb/204Pb from 18.037 to 18.202, 207Pb/204Pb from 15.744 to 15.901, and 208Pb/204Pb from 38.338 to 38.800. Pb isotope variations may be explained in terms of mixing a less radiogenic lead component (μ ~ 8.4) from mafic and ultramafic basement host-rocks (Nova Xavantina metavolcanosedimentary rocks) and a more radiogenic lead component (μ ~ 9.2) probably derived from supracrustal rocks (Cuiabá sedimentary groups). Sulfur isotope compositions are homogeneous, with δ34S values ranging from -1.1‰ to 0.9‰ (galena) and -0.7‰ to 0.9‰ (pyrite), suggesting a mantle-derived reservoir for the mineralizing solutions. Based on the Ar, Pb, and S isotope data, we suggest that the precious metals were remobilized from metavolcanic host rocks by hydrothermal solutions during Brasilide-Panafrican regional metamorphism. The Araés gold deposit probably formed during a late stage of the orogeny, coeval with other mineralization events in the Paraguay Belt.

Widespread ca. 1.4 Ga intraplate magmatism and tectonics in a growing Amazonia

Abstract High-quality U–Pb (Isotope Dilution-Thermal Ionisation Mass Spectrometry; ID-TIMS) baddeleyite ages were obtained for the Salto do Céu (SC) gabbroic sill (1439 ± 4 Ma) and Nova Lacerda mafic dyke swarm (1387 ± 17 Ma) located ca. 150 km apart in the Jauru terrane (Paleo- to Mesoproterozoic) – SW Amazonia. From a geodynamic perspective, the new ages mark widespread Mesoproterozoic extensional tectonics and associated magmatism (e.g. dolerite dykes and rapakivi suites) in a growing continental margin. The SC sill is coeval with the nearby Rio Branco anorogenic rapakivi granite. Intermittent extensional tectonics (1.44 and 1.39 Ga) is widespread in the central and northern portions of the Amazonian Craton, given by co-magmatic charnockites and rapakivi granites, and mafic–ultramafic complexes. On a global scale, this activity may be coeval with a major intra-continental-related igneous event in Laurentia and Fennoscandia, as well as with mafic dykes in NW West African Craton. This suggests large igneous province (LIP)-scale magmatism. A further aspect is that the 1.4 Ga magmatism is age-equivalent with convergent-margin processes (Alto Guaporé orogen and the related Rio Alegre oceanic remnant) that evolved outboard the active margin of the proto-Amazonian Craton. This may represent a coincidence in time between intraplate rifting, LIP magmatism (plume related) and subduction. Other causal mechanisms are also considered such as pulses of back-arc extension behind the accreting arcs. Our data provide new clues as to the longevity of the Columbia (or Nuna) supercontinent, and are consistent with previously published paleomagnetic poles from Mesoproterozoic intraplate magmatism in Amazonia, Laurentia and Baltica (South America-Baltica model).

Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism

Abstract This work integrates the available geological information and geochronology data for the Cretaceous–Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the mid-ocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian–Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian–Albian ages are observed in a few coastal igneous centres. In the SE Brazilian margin, an east–west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous–Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (Early Tertiary–Recent). Volcanic islands such as Ascencion (1.0–0.65 Ma), Tristão da Cunha (2.5–0.13 Ma) and the St Helena islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading centre. Within the South America platform and deep oceanic regions, the following volcanic islands are observed: the Rio Grande Rise (88–86 Ma), Abrolhos (54–44 Ma), the Vitória–Trindade Chain (no age), Trindade (2.8–1.2 Ma) and Fernando de Noronha (12–1.5 Ma). There are several volcanic features along the NW–SE-trending Cruzeiro do Sul Lineament from Cabo Frio to the Rio Grande Rise, but they have not been dated. The only known occurrence of serpentinized mantle rocks in the South Atlantic margin is associated with the Saint Peter and Saint Paul Rocks located along the São Paulo Fracture Zone. The Cameroon Volcanic Line in NW Africa is related to the magmatism that started in the Late Cretaceous and shows local manifestations up to the Present. The compilation of all available magmatic ages suggests an asymmetrical evolution between the African and South America platforms with more pre-break-up and post-break-up magmatism observed in the Brazilian margin. This is most likely to have resulted from the different geological processes operating during the South Atlantic Ocean opening, shifts in the spreading centre, and, possibly, the rising and waning of mantle plumes. Supplementary material: A complete table with radiometric dates that have been obtained by universities, government agencies and research groups is available at: www.geolsoc.org.uk/SUP18596

The pyroclastic dyke and welded crystal tuff of the Morro dos Gatos alkaline intrusive complex, State of Rio de Janeiro, Brazil

The present article reports the geologic, lithologic, and petrographic characteristics of the pyroclastic dyke and phenocryst-rich welded crystal tuff of the Morro dos Gatos alkaline intrusive body, State of Rio de Janeiro, Brazil. The intrusive complex has an extension of 1.2 x 0.8 km occupying an area of 0.5 km2. It is constituted mainly by monzonite and trachyte. At the south flank of the massif, there exits a pyroclastic dyke and at the northwest flank, an intrusive welded crystal tuff. The pyroclastic dyke is 40 to 80 cm wide and intrudes into the basement porphyritic granite. This rock is constituted by welded tuff breccia with large amount of lapilli-size angular lithic fragments of trachyte. Along the contact plane of this dyke, there is an intercalation of the trachyte. The crystal tuff is characterised by abundant alkaline feldspar phenocrysts and mineral fragments of alkaline feldspar, quartz, biotite, and amphibole, showing a crystal matrix-supported texture. Large phenocrysts are aggregations of idiomorphic alkaline feldspar crystals with a general size of 2 mm and a maximum one of 6 mm. The matrix is composed mainly of mineral fragments of less than 1 mm in general size. The size-frequency diagram for the mineral fragments suggests that they have been originated from an explosive eruption. The matrix is strongly welded and completely devitrified showing a cryptocrystalline texture. The crystals were concentrated by sorting with the help of the volcanic gas ascending in the volcanic conduit during a continuous and effusive phase of the eruption. The limited special distribution of the pyroclastic rocks and small outcrop size indicate that they are not constituent of pyroclastic flow deposits, but of subvolcanic conduit and fissure.

Petrografia, geoquímica e geocronologia das rochas do orógeno Rio Alegre, Mato Grosso: um registro de Crosta oceânica mesoproterozóica no SW do Cráton Amazônico

The volcano-sedimentary rocks of the Rio Alegre terrane occur in the SW part of the Amazonian Craton and comprise mafic and ultramafic volcanic rocks, chemical sedimentary rocks, and mafic to felsic intrusive rocks metamorphosed at greenschist to low amphibolite facies. Petrographic, chemical and isotopic studies allowed the subdivision of the complex into three subunits: the basal Minouro formation comprises abundant basic to ultrabasic volcanic rocks and subvolcanic intrusive rocks associated with cherts and banded iron formations. Chemical data indicate an ocean-floor tectonic setting for these rocks; the middle Santa Isabel formation comprises intermediate and acid lavas and pyroclastic rocks. Chemical results indicate an island arc setting for them, which is corroborated by the U-Pb zircon ages of 1509 - 1503 Ma and T DM ages of 1.54 Ga and 1.48 Ga and µNd(t) values from +3.7 to +4.1 for the intermediate rocks, suggesting a mantle origin for the magmas. The uppermost Sao Fabiano formation is composed of metasedimentary rocks with a chemical signature similar to that of the volcanic rocks, suggesting that the latter are the source rocks. The basic to ultrabasic intrusive rocks are differentiated gabbroic rocks and serpentinites. Chemical data indicate the influence of fractional crystallization in the evolution of these rocks. U-Pb zircon dating of these rocks yielded ages from 1509 Ma to 1494 Ma, T DM of 1.67 to 1.48 Ga, and µNd(t) values of +4.5 to +2.5 suggesting mantle-derivation. U-Pb zircon analyses carried out on associated intrusive basic and felsic rocks yielded ages of 1.48 - 1.46 Ga. T DM ages vary from 1.53 Ga to 1.50 Ga and µNd(t) values from +3.7 to +4.1 suggesting that they were generated in a magmatic arc environment. The lithologic association, chemical and isotopic evidence suggest that volcanic and subvolcanic basic and ultrabasic rocks originated in a back-arc basin or ocean ridge; and that the basic to intermediate rocks were formed in an island arc setting at 1.51 - 1.50 Ga. These rocks were later intruded at 1.48 - 1.46 Ga by basic and felsic rocks formed in a magmatic arc setting. Metamorphism under green-schist to lower amphibolite facies (biotite to garnet-kyanite zone), and mylonitization producing a N20W foliation were associated with collision of the arc against the Amazonian Craton during Mesoproterozoic time.