Araguaia Belt Research Articles

The electrical structure of the lithosphere in the southeastern margin of the Amazonian craton: Contribution to West Gondwana tectonics

A high electrical conductivity belt near the southeastern margin of the Amazonian craton, known as Paraguay-Araguaia Conductivity Anomaly (PACA), has been discovered and mapped over a distance of more than 1,000 km by a large array of magnetometer measurements. To precisely determine the location and structure of the conductive anomaly, we conducted a 200-km-long magnetotelluric (MT) profile with typical site spacing of 8 km over the northern part of PACA and crossing the Goiás magmatic arc and Araguaia belt in Central Brazil. Three-dimensional resistivity models derived from this data indicate that the PACA is not a single conductor but comprises at least two contiguous bodies at the upper to middle crust. The enhanced conductivity of the northern part of PACA is likely due to sulfide minerals deposited in an inter-arc or back-arc basin that were deformed and subsequently emplaced tectonically beneath the western Goiás magmatic arc. The PACA is truncated to the west by a prominent sub-vertical resistor, attributed to the Transbrasiliano lineament (TBL), a continental-scale structural feature interpreted here as a mylonitic shear zone developed late in the Neoproterozoic to Cambrian tectonic history involving the collision between Amazonian craton and proto-West Gondwana. We cannot confirm the prevailing view of an east-dip subduction of oceanic lithosphere from the Amazonian plate under the Goiás arc terranes. Instead, our results probably represent the record of an oblique continental collision. Regardless of its origin, the PACA may serve as a proxy for the paleo border of the Amazonian plate. Therefore, additional geophysical mapping, especially in areas covered by thick Phanerozoic sediments, is essential to constrain the tectonic evolution of West Gondwana.

Scapolite metagabbros of the Xambica Suite: A Tonian OIB magmatism in the crustal evolution of the Araguaia Belt

The geological evolution of the Araguaia Belt (AB), an important Neoproterozoic orogen located in central-north Brazil, is marked by different stages in its tectonic evolution. These stages are characterized by various magmatic events represented by felsic alkaline plutons, ophiolitic suites, mafic-ultramafic intrusions, S-type granites, diabase dykes and tholeiitic basalts distributed along its 1200 km N–S long. One of these events was identified in the Xambioá region, in the NW of Tocantins/SE of Pará state, in which dozens of bodies of scapolite metagabbros and amphibolites were identified through mapping and fieldwork, which were gathered in the Xambica Gabbroic Suite (XGS). Field relations and structural observations reveal that they were intrusive bodies in the sedimentary successions of the Estrondo Group before metamorphism. We present complementary studies involving detailed petrographic, lithochemical and geochronological analyses to characterize these rocks and contextualize this magmatic event in AB evolution. The petrographic study allowed to classify the rocks of the XGS into metagabbros with plagioclase and clinopyroxenes relicts, scapolite-hornblende metagabbro, garnet-scapolite metagabbro, amphibolites, and rarely listwanite and steatite. Geochronological dating of zircon crystals from this suite of rocks was performed using two different techniques. Single zircon Pb-evaporation dating of a metagabbro (XL-02) yielded the age of 819 ± 5 Ma. U–Pb zircon dating by LA-MC-ICP-MS of a metagabbro (BV-01) and an amphibolite (XL-09) gave 822 ± 2 Ma and 826 ± 26 Ma, respectively. Two other metagabbro samples (XL-02 and XB-08) yielded concordant ages of 861 ± 21 Ma and 873 ± 15 Ma. The younger concordant ages are interpreted as the crystallization of the gabbroic protoliths in the Tonian, while the older concordant ages are related to the presence of inherited zircon. The geochemical signature of metagabbros and amphibolites is similar to that of ocean island basalts (OIB) related to an intraplate tectonic environment, characterizing an extensional tectonic phase in AB evolution, which differs from other mafic events already recognized in AB of a MORB or tholeiitic signatures. Similar Tonian age are reported in basic to intermediate volcanic secession in the Goiás Magmatic Arc representing a contemporary collisional tectonic event, which reveals a link in the evolution between these two geotectonic units.

The Neoproterozoic basement of the Parnaíba Basin (NE Brazil) from combined geophysical-geological analysis: A missing piece of the western Gondwana puzzle

A regional Pre-Silurian unconformity marks the erosive planar base of the Phanerozoic Parnaíba Basin (PB), and indistinctly truncates key tectonic units for the understanding of the Neoproterozoic evolution of western Gondwana in northeastern Brazil. We present a comparative analysis of geophysical and geological datasets in the PB and propose a new tectonic configuration for its pre-Silurian basement, composed of different terranes amalgamated during the Brasiliano orogeny. Using integrated seismic interpretation and gravity modelling, constrained by an updated crustal thickness map, well data and a compilation of recent geophysical studies, we have identified two main blocks in the center of the basin. They represent pre-Brasiliano inliers, surrounded by Brasiliano mobile belts, and assigned to two major crustal building blocks of western Gondwana: the Amazonian-West Africa (AWB; to the west/north) and the Central African blocks (CAB; to the east/northeast). The Grajaú block belongs to the AWB and is characterized by low gravity anomaly, thicker crust (41–45 km), transparent seismic character of the basement and a high velocity lower crust. The Teresina block belongs to the CAB and is characterized by slightly thinner crust (39–41 km), higher values of gravity and magnetic anomalies and by the presence of mid-crustal reflectors (MCR), observed in seven seismic lines and here interpreted as a remnant of a paleosuture zone between the two blocks. Along a NE-SW 500-km seismic and gravity profile, we interpreted the MCR as composed of crustal-scale thrust faults verging westwards and defining the Barra do Corda belt, which was formed by the closure of the Goiás-Pharusian ocean along the Transbrasiliano-Kandi corridor and subsequent collision of the Grajaú and Teresina blocks. This belt deforms the eastern margin of the Ediacaran Riachão foreland basin (RB), observed in seismic and well data beneath the SW portion of PB and overlying the southern Grajaú block. The western margin of RB is bounded by eastwards verging thrust faults, interpreted as a zone of back-thrusts and thinned crust (∼36 km) in the eastern prolongation of the Araguaia belt beneath PB. To the east, the limit between the Teresina block and the Borborema Province is marked by the NE-SW Transbrasiliano Shear Zone, along which Neoproterozoic mylonites were recovered from wellbores, and a system of narrow pull-apart basins is interpreted in the seismic data, triggered by Early Paleozoic transtensional reactivations of the TBSZ, prior to the deposition of the basal Ordovician-Silurian sequence of PB. This complex collisional tectonic setting of PB basement contradicts the idea of a stable cratonic block beneath it, inciting new formation models that account for the observed crustal and lithospheric heterogeneities.

A Holocene record of floodplain development in the northernmost portion of the Araguaia Belt, southeastern Amazonia

This work aimed to understand the drivers of landscape evolution and floodplain formation in a Precambrian Shield area of southeastern Amazonia. This area evolved during the Quaternary into a denudational system with strong structural control, which produced valley side slopes between the Araguaia Belt (AB) and the topmost Parnaíba basin and narrow floodplains in the lower Vermelho River. These floodplains developed at approximately 11215–11410 cal yr BP with the deposition of muds on the weathered basement rocks of the Couto Magalhães Formation in the Ab (CMF) long before sea-level stabilization on the Amazon coast. The chondrite- and upper continental crust-normalized patterns, along with Al/Ti and La/Th ratios, clearly indicate that muds were derived from the CMF-AB. The strong positive correlation of most of the trace elements and rare earth elements (REEs) with Al and Ti indicates their control by aluminous clay phases and a similar transport process. Overbank flows of the Vermelho River resulted in the deposition and vertical accretion of sands from the fluvial channel over the floodplain, forming discontinuous marginal levees after 4520–4710 cal yr BP. The accreted sands with the predominance of Si, Zr, and Hf and the depletion of most of the major and trace elements are likely associated with quartz enrichment and a strong sorting of heavy minerals, which is also supported by Al2O3/SiO2 vs Fe2O3/SiO2. The tributaries of the lower Vermelho River were active until 3890–4095 cal yr BP, and subsequently, the channel was abandoned as a result of the filling process. At approximately 3440–3560 cal yr BP, the floodplain was influenced by prolonged and severe droughts that fully exposed the muddy deposits and caused the enrichment of Fe-Mnoxyoxide phases. The affinity of most of the potentially toxic elements with Al signifies that these floodplain deposits are a natural record for geochemical background studies.

Intracontinental basaltic magmatism in the sedimentary succession of the Tucuruí Group: An important record of the final evolution of the Araguaia Belt

The region of Tucuruí, northeastern Pará, is located in the transition between the Amazon Craton and the Araguaia Belt. In this region, there is an extensive graben filled by a volcano-sedimentary succession, 35 km wide that extends over 100 km in the NNW-SSE direction and which is represented by the Tucuruí Group. This group consists of a succession of basaltic flows and diabase sills interbedded with sub-arkose and siltstones. Together with the metasedimentary rocks of the Tocantins Group, these rocks represent the latest records of the Araguaia Orogen in the extreme North of Brazil. The volcanic deposits consist of basalt flows and volcanic breccias associated with diabase sills. The flows are predominantly composed of massive and locally amygdaloidal basalts and basaltic breccias. The basalts are aphanitic, fine-grained or glassy, holo-to hypocrystalline, and are made up of labradorite, augite, Fe–Ti minerals oxides, sulfides, and apatite. The massive basalts have intergranular or intersertal texture, whereas the amygdaloidal basalts are characterized by fibro-radial zeolites, epidote, and chlorite. The basaltic breccias are composed of elongated, hypovitreous basalt fragments encompassed by a volcanoclastic matrix with siltstone films and anastomosed siliceous-zeolitic materials. The basalts are subalkaline with tholeiitic Ocean Island Basalts affinity, intraplate, and are interpreted as products of fissural volcanism in a subaerial environment. Deformation is predominantly ruptile, and fluid remobilization occurs along faults and fractures, generating veins of quartz, carbonate, chlorite, and epidote by hydrothermal processes at low temperatures (anki-metamorphism). The sedimentary succession of the Tucuruí Group represents a paleo-segment formed in a shallow marine environment influenced by storm waves. The evolution of the succession is related to the installation and fill of a rift basin during the final stage of evolution of the Araguaia Belt at the Neoproterozoic/Paleozoic transition. This event is related to the extensional tectonics that marked the transition from a ductile to ruptile regime in the late stage of evolution of the Araguaia Orogen, and it represents a rare record of the end of the Brasiliano Cycle on the Brazilian Platform.

Zircon U-Pb geochronology and Lu-Hf isotope systematics of the Araguaia Belt basement Rocks: Evidence of links with the southeastern Amazonian Craton, Brazil

Archean and Paleoproterozoic basement rocks are exposed in the Araguaia Belt, a Neoproterozoic orogen bordering the eastern portion of the Amazonian Craton. Zircon LA-MC-ICPMS U-Pb and Lu-Hf isotopic systematics allowed a more accurate dating of the basement rocks and investigating the evolution of this large Archean/Paleoproterozoic crustal segment and discuss the relationship with the southeastern Amazonian Craton. Archean TTG orthogneisses (2.93 to 2.87 Ga) prevail in the northern belt while distinct magmatic events generated their protoliths in approximately 60 Ma. Zircon Hf model ages (~3.0 to 3.4 Ga) and ƐHf(2.87-2.93) from −4.3 to +5.3 suggest crustal reworking of Mesoarchean juvenile sources, with the subordinated contribution of older crust. A Mesoarchean source is also indicated by the Hf model ages (2.93–3.25 Ga) and ƐHf(−1.86 Ga) from −4.8 to −1.6 of a 1.86 Ga granitic gneiss intrusive in the Archean TTG. An orthogneiss and an intrusive granitoid from southern segment of the belt were dated 2.06 Ga and 1.87 Ga, respectively. The Hf model ages (2.65–2.86 Ga) determined for these rocks, the ƐHf(-2.06 Ga) (−4.8 to −1.6) for the orthogneiss and the ƐHf(1.87 Ga) (−15.8 to −9.7) found for the granitoid reveal a dominant crustal source for the southern basement rocks. The Archean and Paleoproterozoic basement constituted a single crustal segment by the Orosirian, since both segments present intrusive granites of this geological period. The Archean orthogneisses of the Araguaia Belt and the granitoids of the Carajás Province Mesoarchean domains in southeastern Amazonian Craton share some similarities, such as the TTG affinity magmatism formed in a relatively similar time interval, the Orosirian intrusive granites, and the Mesoarchean juvenile source. Therefore, the basement rocks cropping out in the northern segment of the Araguaia Belt may represent portions of the adjacent Amazonian Craton.

Shortening history of the Neoproterozoic oroclinal bending in Paraguay belt, Central Brazil, based on structural interpretation of field work and high resolution aerogeophysical data

Different phases of the orogen's evolutionary history can be geometrically restored by identifying associated tectonic features in the thrust-and-fold belts in the foreland basin. The geometry and fold styles allow understanding the rate of crustal shortening of the Paraguay Belt Neoproterozoic oroclinal. We suggest that high resolution airborne geophysical data provides critical information to understand the tectonic history of Brasiliano fold belts. Geophysical studies integrated to structural analysis in field mapping may unveil the crustal architecture as well as the sequence of events of rocks exposed in two structural windows of the basement in the region of Planalto da Serra and Nova Xavantina, in the state of Mato Grosso, northern segment of the Paraguay Belt, Central Brazil.The evolutive history of rocks exposed southwards of the Amazonian craton margin represents extensional tectonics followed by the development of a passive margin. It is estimated that the original width of the depositional space was ~350 km during the formation of the hyperextended platform and, after the intense shortening during the Brasiliano orogenesis, the oroclinal was restricted to ~130 km wide, in the post-deformation phase. Planalto da Serra region is characterized by asymmetrical, harmonic folds with thickened hinge zones. They show variable styles between tight and isoclinal and form pairs of EW and NE anticlines and synclines. In Nova Xavantina structural window, the magnetic lineaments mark important lithospheric discontinuities represented by the Campinápolis (CAL) and General Carneiro (GCL) lineaments, which seem to be the deep basement faults that controlled the extension and where the rift axial zone developed. The Transbrasiliano Lineament (TBL) is a major shear zone that controls the secondary lineaments propagation, forming a network of folds and drag faults, with lateral escapes and vertical movement. Between the two main lineaments occurred the deposition of sediments and volcanic successions related to Nova Xavantina Metavolcanosedimentary Sequence representing a marginal ocean basin. The transition from EW trending features of the Paraguay Belt to NE trend Araguaia Belt lies within Nova Xavantina region and is marked by the direct influence of TBL. TBL is evident in the magnetic images, forming a mosaic of different crustal blocks and corresponding to various curvilinear shear zones with unidirectional trend N30-45E. It is composed of mega-sigmoids interlaced with mainly dextral kinematics, within shallow and deep levels.During the Neoproterozoic orogeny, a foreland basin system was formed in response to the flexural subsidence due to the tectonic inversion of the basin. Several depocenters were formed and served as accumulation sites for the deposition of sediments of the Alto Paraguay Group (Raizama, Sepotuba and Diamantino formations). Gravimetric data allowed individualization of the foreland basin system, which is characterized by four discrete depozones (back-bulge, forebulge, foredeep and wedge-top), between the Amazonian Craton and the inner structural zone.

Structural evolution and U/Pb zircon age of the Xambioá gneiss dome, contributions to the Araguaia fold belt tectonic history

The Araguaia belt in central Brazil is an important key to re-access ancient geological settings, as it records the Gondwana agglutination and the final stages of an oceanic basin closure. Small basement inliers occur along transversal position in the Araguaia belt and could compose the extension of Amazonian craton. The structural analysis of the Xambioá gneiss dome region shows that three main deformational events were responsible for the actual scenario. The first deformational event was a NW-SE non-coaxial compression event (Dn), which led to the formation of ductile structures, such as open and slightly open folds (Fn), from a previous Sn-1 structures, axial plane foliation and spaced foliation (Sn), in the mantling metasedimentary sequence, and the mylonitic foliation (Sn) in dome rocks. This event was associated with a thrust system developed under regional greenschist to local upper amphibolite facies. The second deformational event Dn+1 was characterized by refolding with Sn transposition, followed by asymmetric Fn+1 folds and transposed foliation (Sn+1), and local stretching patterns forming boudins and Ln+1 lineations. The third deformational event Dn+2 is represented by WNW-ESE sinistral transcurrent faults (Sn+2) which sectioned the Dn structures and formed shear zones of different scales. U–Pb zircon ages in gneiss and in a migmatite of the Xambioá dome indicate Mesoarchean (~2.93 Ga) crystallization ages. U–Pb zircon provenance ages from metasedimentary rocks bordering the Xambioá dome show significant contribution of Archean to Neoproterozoic sources. The consistent loss of Pb in the dome rocks evidences tectono-metamorphic events presumably connected to the lithosphere ocean closure and continental collision responsible for the Araguaia Belt edification during Neoproterozoic-Cambrian period.

U–Pb SHRIMP zircon ages of Ediacaran-Cambrian granitic bodies in central Brazil: Implications for the tectonic evolution of the Araguaia belt

The small granitic plutons of the Ramal do Lontra, Presidente Kennedy, Barrolândia and Santa Luzia are emplaced into the Estrondo Group metasedimentary rocks of the Neoproterozoic Araguaia Belt. They have similar petrographic and compositional characteristics and are coeval with the medium-grade regional metamorphism of the belt. New U–Pb SHRIMP zircon data yielded quite robust ages. These results fall within the experimental error, and the integrated Concordia age of 542 ± 3 Ma was chosen as representative of their magmatic crystallization. A relevant number of inherited grains are present, showing overgrowth rims with high-U content, formed during the later stage of magmatic crystallization. LA-ICP-MS U–Pb dating on detrital zircon grains from a metasedimentary rock of the Estrondo Group yielded several Paleoproterozoic ages and a few Neoarchean ages, but the major peak in the histogram was between 1000 and 1200 Ma. The youngest grains indicate an Ediacaran age near 600 Ma, which is considered a reasonable upper limit for the deposition of the Estrondo Group. The Araguaia Belt started as an intracontinental system, segmenting the Amazonian Craton, formed by rifting during the breakup of Rodinia and was after occupied by a sedimentary basin which evolved to form an oceanic lithosphere. In the Ediacaran, the huge continental collisions that formed the West Gondwana Orogen most likely caused its remote activation, producing tectonic inversion with vergence to the west. Granitic plutons were emplaced in the last stages of the evolution of the Araguaia Belt, during the Brasiliano-PanAfrican orogenesis, at the Ediacaran-Cambrian boundary.

Monte Santo suite, an example of Ediacaran-Cambrian deformed alkaline rocks in the Araguaia Belt, Central Brazil. Implications for Western Gondwana evolution

Deformed alkaline rocks and carbonatites (DARCs) can be important tectonic indicators of regions that have experienced two separate events of rifting and collision, but can also represent alkaline magmas that were synkinematically emplaced along major structures and were therefore generated and deformed in the same geologic event. The Monte Santo Suite consists of two foliated and variably deformed massifs (Estrela and Monte Santo) that crop out in the Araguaia Belt and are characterized by igneous and subsolidus microstructures. In this study, we use the chemistry, mineralogy and age of the Monte Santo Suite to understand its genesis and the relations between its emplacement, the Brasiliano-Pan-African orogeny and the final evolution of Western Gondwana. The intrusions are predominantly nepheline syenite and alkali feldspar syenite. Pyroxene varies in composition from omphacite to aegirine-augite to aegirine and has mixed igneous and metamorphic characteristics. Biotite varies from phlogopite to siderophyllite to annite and presents primary, reequilibrated and neoformed trends. Amphibole varies from hastingsite to taramite. Nepheline occurs as both high-temperature and low-temperature varieties. The REE and trace element patterns display parallel trends with significant bulk content variations, positive anomalies for Nb, Ta, Zr and Hf, and negative anomalies for Ba, Sr, P and Ti in multielement diagrams, The Zr/Hf ratios for all the rocks of the massifs are similar and whole-rock εNd values range from −10.1 to −3.0. Trace element and isotopic data suggest that these magmas experienced extensive fractional crystallization, crustal contamination, and evolved from the same source, probably an enriched mantle. The Monte Santo Suite crystallization age is dated to 545 Ma, when these magmas were generated and deformed synkinematically with shearing along major regional faults related to the Transbrasiliano Lineament. Thus, this suite is an example of alkaline rocks generated and deformed in the same geological event throughout Western Gondwana.

Geology and petrology of the mafic dike swarms of the Araguaia Belt: Evidence for distinct events of intracontinental magmatism in central-northern Brazil and correlations with large igneous provinces

Extensive N–S and NNW-SEE-trending subparallel mafic dike swarms are intruded into metasedimentary rocks of the Tocantins Group, Araguaia Belt, Central-North of Brazil. They are under-examined, and there is little to no information about their origin and mantellic sources, as well as uncertainty about whether they are part of two different episodes of magmatism. Representative mafic dikes from the Santa Maria das Barreiras-Conceição do Araguaia region, at the boundary between the states of Pará and Tocantins, were studied in order to address these problems. It was possible to separate the dikes into two groups: one consisting of metadiabases affected by the Neoproterozoic regional metamorphism of the Araguaia Belt with varying degrees of transformations and mineral deformation; and the other consisting of unmetamorphosed and undeformed diabases and leucodiabases. The studied dikes are compositionally classified as subalkaline basalts with tholeiitic affinity and were interpreted to be originated in an intracontinental setting with the aid of Ti–V, Zr–Zr/Y, and Zr–Ti discrimination diagrams. However, metadiabases present an arc-like geochemical signature characterized by a pronounced Nb–Ta negative anomaly, whereas leucodiabases and diabases lack a negative Nb–Ta anomaly and show a LREE-enriched pattern, which resembles the signatures of plume-generated basaltic rocks. Also, there is evidence of an important contribution of enriched (EN) mantle components in the source of metadiabases and significant contribution of primitive mantle (PM) in the source of both leucodiabases and diabases. It was suggested, then, that the metadiabases represent the exposed plumbing system of arc-like intracontinental basalts which precede the regional Neoproterozoic metamorphism of the belt, and that the leucodiabases and diabases represent the exposed conduits of intracontinental basalts whose magmatism succeed the metamorphic event. Geochemical models were elaborated to demonstrate that fractional crystallization processes could account for the differentiation of the metadiabases and for the genesis of the leucodiabases via fractionation of the melt from which the diabases formed. The rocks from the earlier event share several similarities with Neoproterozoic mafic rocks from the eastern domain of the Araguaia Belt and nearby Tonian rocks of the 1100 Ma Rincón del Tigre-Huanchaca LIP event, while the rocks from the later event are remarkably similar to nearby CAMP basalts and diabase dikes.

Serra do Tapa Ophiolite Suite - Araguaia Belt: Geological characterization and Neoproterozoic evolution (central-northern Brazil)

The Araguaia Belt is a Neoproterozoic collisional orogen located in the northern Tocantins tectonic province, North-central Brazil, being one of the most expressive orogens of West Gondwana. The Serra do Tapa Ophiolite Suite is the most important representative of the ophiolitic bodies of the Araguaia Belt, exposed along the low-grade metamorphic domain of the Belt and extending over 80 km in the N-S direction. It constitutes a set of N-S aligned bodies that represent a disrupted ophiolite special sequence comprising serpentinized peridotites and dunites, massive and pillow basaltic lavas, volcanosedimentary rocks, cherts, jaspilites, and their metamorphic products. The serpentinized rocks have protoliths of mantelic harzburgites and dunites, with relict original protogranular texture partially replaced by pseudomorphic texture after olivine and orthopyroxene, and non-pseudomorphic (replacements). Submarine basaltic flows represent the volcanic succession, overlying serpentinized peridotites. The volcanic succession occurs as homogeneous massive basalts or pillow basalts, and locally volcanoclastic breccias (hyaloclastite). The pillow structure shows zoning, where the core consists of homogeneous massive metabasalt with preserved igneous features whereas pillow outer zone consists of hyalobasalt with quenching textures whose pillow surface is transformed into chlorite-rich rocks that appear fine-grained and related to the metasomatic transformations due to interactions with seawater. Geochemically, the massive metabasalts display a subalkaline-tholeiitic affinity, compatible with Middle Oceanic Ridge Basalts types (MORB). The suite evolution is a testimony of an oceanic stage during the Neoproterozoic of the Araguaia Basin that generated an oceanic lithosphere followed by volcanic activity, the interaction between lava flows and pelitic sediments, and deposition of chemical sedimentary rocks (jaspilites and cherts). Subsequently, the detachment of the oceanic lithosphere and the compressional tectonic evolution of the Araguaia Orogen inverted the sequence leading to the obduction of ophiolite bodies along overthrust and transcurrent faults projecting/thrusting a mass toward the Amazonian Craton. The tectonics was accompanied by regional metamorphism from anchymetamorphism to low greenschist facies conditions. All these processes are related to the evolution of the Brasiliano/Pan-African Cycle of the Araguaia Belt. The Serra do Tapa Ophiolite constitutes a mega-slice of an oceanic crust and marks a phase of lithosphere construction and the early opening of the Araguaia Basin related to the crustal evolution of the Araguaia Belt in the Neoproterozoic (transition Tonian-Ediacaran).

Does the metavolcanic-sedimentary Rio do Coco Group, Araguaia Belt, Brazil, represent a continuity of the Quatipuru ophiolitic complex? – Constraints from U-Pb and Sm-Nd isotope data

Serpentinized peridotites and basaltic volcanic rocks from the Quatipuru, Serra do Tapa, and Morro do Agostinho complexes are part of dismembered ophiolite bodies hosted by metasedimentary units of the Neoproterozoic Araguaia Belt, Tocantins Province, central Brazil. Approximately 10 lenticular ultramafic bodies, which contain primitive (serpentinite) and more evolved (actinolite schist, actinolite-talc schist, actinolite-serpentinite schist, talc schist, talc-serpentine schist, actinolitite, cloritite) rocks, were identified in the Rio do Coco Group (RCG), a metavolcanic-sedimentary sequence in the southern portion of the belt. These bodies are intercalated with metasedimentary rocks. All lithologies are metamorphosed to upper greenschist and lower amphibolite facies. The sequence was initially characterized as a classic Archean greenstone belt, but the present study suggests that it should be considered an ophiolite suite. On the basis of the ƐNd(t) values the RCG rocks can be subdivided into two groups: Group 1, with positive ƐNd(t) values, neodymium model ages (TDM) of ca. 1.0 to 1.1 Ga, with ocean-island-basalt (OIB) affinity, and being less contaminated with metasedimentary rocks of the Baixo Araguaia Supergroup; and Group 2, with negative ƐNd(t) values, TDM ranging from 1.2 to 1.7 Ga, with a variable ocean island basalt (OIB), plume or enriched mid-ocean-ridge basalt (P- and E-MORB) affinity, and being more contaminated with metasedimentary rocks. Two samples were analyzed by the U-Pb method on zircon (LA-MC-ICPMS). The weighted average age from a serpentinite (715 ± 26 Ma) and an actinolite schist (702 ± 24) indicates a maximum emplacement age for this sequence at 708 ± 35 Ma, which is similar to the previously published age for the Quatipuru Complex (Sm-Nd isochron age of 757 ± 49 Ma). This may suggest that the metavolcanic-sedimentary sequence of the Rio do Coco Group and the Quatipuru ophiolite system could be part of the same dismembered oceanic lithosphere.

Petrologia, geoquímica e geocronologia do Granodiorito Presidente Kennedy: contextualização na evolução do Cinturão Araguaia

Diversos corpos graníticos têm sido identificados no extremo leste do Cinturão Araguaia (CA) em seu domínio de mais alto grau metamórfico (Grupo Estrondo). Tais granitos têm sido interpretados como de posicionamento sin a tardi-cinemático em relação à tectônica principal e metamorfismo regional da evolução do CA, no final do Neoproterozóico. Neste contexto o Granodiorito Presidente Kennedy (GPK) localizado a 6 km a noroeste da cidade de Presidente Kennedy (TO) compõe a forma de um stock elíptico, que abrange uma área aflorante cujo diâmetro maior é de aproximadamente 10 km, e o menor de 5 km, encaixado em micaxistos do Grupo Estrondo e parcialmente coberto por rochas sedimentares da Formação Pimenteiras. O estudo petrográfico identificou rochas hololeucocráticas (M<6) granodioríticas e monzograníticas equigranulares, de cor cinza levemente rosada, e com textura granular hipidiomórfica e alotriomórfica. Dados litoquímicos indicam que o GPK tem caráter levemente peraluminoso, com elevados valores SiO2, Al2O3 e álcalis e baixos valores de MgO, Fe2O3t e TiO2. O estudo geocronológico de evaporação de Pb em zircão forneceu idade 539 ± 5 Ma, interpretada como idade mínima de cristalização do GPK. Os estudos petrográficos, geoquímicos e geocronológicos do GPK, indicam semelhanças com os demais granitos ocorrentes no CA. Os estudos isotópicos pelo método Sm-Nd definiram idades modelo (TDM) de 2,13 Ga e 2,17 Ga para o GPK e valores εNd negativos indicando que essa rocha tem proveniência de fonte crustal, gerada da fusão de crosta antiga Paleoproterozóica. Os estudos comparativos indicam semelhanças entre este e os demais granitos do CA, possivelmente cogenéticos relacionado à fase principal do metamorfismo do CA, no limete Neoproterozóico-Paleozóico.

Mafic dyke swarms at 1882, 535 and 200 Ma in the Carajás region, Amazonian Craton: Sr[sbnd]Nd isotopy, trace element geochemistry and inferences on their origin and geological settings

The Carajás-Rio Maria region, together with the Rio Maria domain of the Central Amazonian province, comprises the eastern margin of the Amazonian Craton with the Neoproterozoic Araguaia belt. This region hosts several basaltic dyke swarms whose UPb baddeleyite ages highlighted three intrusive events at 1882, 535 and 200 Ma. New geochemical and SrNd isotopic data were obtained for the different groups of the Carajás dykes allowing new insights on i) the mantle source composition beneath the Carajás region through time and ii) the geodynamic setting of the intrusive events. The 1882 Ma swarm is coeval to the Uatumã SLIP event which is one of the oldest intraplate events of the proto-Amazonian craton. Trace elements and isotopic values suggest that the dyke parent melt for those dykes have a crustal component derived from a sedimentary source similar to GLOSS (GLObal Subducting Sediment compositions). This is consistent with the emplacement of the dykes in a supra-subduction setting or in a post-collisional setting. Trace and isotopic values of the 535 Ma dyke swarm are consistent with an enriched mantle source from EMII component. These geochemical features suggest an enrichment of the mantle from an oceanic lithosphere poor in sediments, different to that of the 1882 Ma source. The age of this swarm matches magmatic activity during a post-collisional extensive-transtensive event recorded in the marginal Araguaia belt after the amalgamation of the Amazonian Craton to the Western Gondwana during Neoproterozoic. The 200 Ma dyke swarm which is related to the CAMP (Central Atlantic Magmatic Province) and opening of the Atlantic Ocean shows trace element composition similar to Atlantic E-MORB. The coupled isotopic values are consistent with an enriched mantle source with EMII component. These particular geochemical features suggest that the plume activity responsible for the CAMP near the rifting zone has not affected the mantle beneath the Carajás region.

The Cambrian peraluminous Santa Luzia granite suite in the Araguaia Belt, central Brazil: Evidence for closure of the Clymene Ocean based on zircon and monazite U[sbnd]Pb data

The Araguaia Belt (AB) in northcentral Brazil formed as the result of the amalgamation of the Amazonian and São Francisco-Congo cratons along with the Parnaíba Block during late Neoproterozoic times. Ediacaran/Cambrian late- to post-orogenic magmatism took place at ∼540 Ma following this continental collision at 550-540 Ma. Such magmatism is represented in the southern portion of the Araguaia Belt by the peraluminous Santa Luzia and Barrolândia granites, as well as by the Presidente Kennedy Granodiorite in the central portion of the AB. Inherited zircon cores with distinct rims are abundant in the Santa Luzia Granite. This pluton yields UPb MC-LA-ICP-MS zircon and monazite ages of 538 ± 1 Ma and 534 ± 5 Ma, respectively. In addition, UPb zircon and monazite ages of 536 ± 6 Ma and 524 ± 6 Ma, respectively, are obtained for the Barrolândia Granite. The data presented in this work and in the literature allow to group the studied Santa Luzia and Barrolândia granites, together with Presidente Kennedy Granodiorite, in a single suite, named Santa Luzia Suite, maintaining the Santa Luzia pluton as the type-area. Rocks related to this Cambrian magmatism are essentially peraluminous, magnesian to slightly ferroan, and calc-alkaline in character. Such rocks probably originated from magmas formed by partial melting of meta-igneous sources that were subsequently contaminated via assimilation of metasedimentary rocks. In the AB, both the collision related to the 550-540 Ma Brasiliano/Pan-African orogeny, and the intimately associated ∼540 Ma late- to post-orogenic Santa Luzia Suite magmatism record the closure of the Clymene Ocean as well as the late stages of compressional tectonics related to west Gondwana amalgamation. The latter was followed by exhumation related to collapse of the Araguaia orogen.

A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up: The Araguaia Belt, central Brazil

The Araguaia Belt encloses a poorly constrained Pan-African (Brasiliano Cycle) continental suture marked by a series of (~750 Ma) ophiolitic units which, when properly characterized, could provide important informations on its geological history, closely linked with the Rodinia demise and further western Gondwana amalgamation. We present new bulk-rock and mineral major and trace element compositions for these ultramafic and mafic units. They mainly consist in fully serpentinized harzburgite, scarce dunite lenses and chromite pods, tectonically overlain by basaltic pillow lavas. Low Al2O3/SiO2 ratios (0.01 to 0.06), rather high MgO concentrations (42.28 to 45.29 wt%) and spinels' Cr# and Mg# ratios comprised between 0.36 and 0.51 and 0.59 and 0.72, respectively, indicate a depleted oceanic-like protolith. MORB-peridotite interactions are evidenced both by pyroxenite, olivine gabbro and diabase occurrences in the serpentinites and by high TiO2 (up to 0.42 wt%) contents in spinels from some Serra do Quatipuru serpentinites. These observations support that the Araguaia Belt ophiolitic bodies are the remnants of the upper mantle section of a MOR or subcontinental lithosphere. The serpentinites whole-rock REE content can be modeled as resulting from a dry partial melting involving 14 to 24% of melt extraction, coupled with refertilization by fertile melts, generated deeper in the mantle. Such an oceanic-like setting is also supported by the N-MORB signature of Serra do Tapa and Morro do Agostinho pillow lavas basalts. All together, these results tend to infirm the supra-subduction zone (SSZ) setting previously proposed for these ophiolitic units. Important LILE, B and Li enrichments in the serpentinites likely result from a metasomatic event involving sediments-derived fluids that occurred during the obduction of the units on the Amazonian Craton. Our results combined with (1) the apparent scarcity of igneous crustal rocks, (2) the proximal nature of the metasedimentary rocks hosting the ophiolitic units, and (3) the occurrences of Amazonian Craton fragments eastward of the ophiolitic bodies, allow us to propose that the Araguaia Belt comprises a fossil ocean-continent transition (OCT) accreted on the eastern border of the Amazonian Craton.

Geochronology and thermochronology of the gneisses of the Brasiliano/Pan-African Araguaia Belt: Records of exhumation of West Gondwana and Pangea break up

The 1200 km long Araguaia Belt, central-north Brazil, was formed as a result of the Neoproterozoic (Ediacaran) to Early Cambrian collage of West Gondwana and has several dome structures in whose nuclei crop out basement inliers of Archean and Paleoproterozoic orthogneisses. Thus, the orthogneisses from Lontra, Cocalândia, Colméia and Cantão dome structures were selected for fission-track thermochronology and U-Pb double dating. Three Archean magmatic episodes ranging from 2830.9 ± 6.9 Ma (Colméia dome) to 2905 ± 5.1 Ma (Lontra dome) and one Paleoproterozoic magmatic event at 1835 ± 31 Ma (Cantão dome) are recognized by U-Pb LA-MC-ICP-MS zircon dating. Fission-track thermochronology in zircon permitted to identify three well-defined age populations, which correspond to major regional geological events. The older population (489 ± 15 Ma to 498±8Ma) records the exhumation process and the orogenetic collapse of the Araguaia Belt. The intermediate population (331 ± 8 Ma to 345 ± 13 Ma) is related to the Gondwanides orogeny which affected the western margin of Gondwana and therefore the interior of South American. Finally, the younger fission track ages (197 ± 3 Ma, 197 ± 4 Ma, 198 ± 2 Ma and 208 ± 10 Ma) reflect crustal heating around 200 Ma, which is associated with the Lower Jurassic Mosquito tholeiitic magmatism and extended the influence of the Central Atlantic Magmatic Province to the Araguaia Belt. This mafic magmatism has been recognized in different regions of the South American and was an important source of heat for hydrocarbon generation in some Paleozoic intracratonic basins.

Geochemistry, geochronology and Nd isotopes of the Gogó da Onça Granite: A new Paleoproterozoic A-type granite of Carajás Province, Brazil

The Gogó da Onça Granite (GOG) comprise a stock located in the Carajás Province in the southeastern part of Amazonian Craton near its border with the Araguaia Belt. Three facies were identified in the pluton: biotite-amphibole granodiorite, biotite-amphibole monzogranite and amphibole-biotite syenogranite. The GGO crosscut discordantly the Archean country rocks and are not foliated. All Gogó da Onça Granite varieties are metaluminous, ferroan A2-subtype granites with reduced character. The major and trace element behavior suggests that its different facies are related by fractional crystallization. Zircon and titanite U–Pb SHRIMP ages show that the pluton crystallized at ∼1880–1870 Ma and is related to the remarkable Paleoproterozoic magmatic event identified in the Carajás Province. Whole-rock Nd isotope data (TDM ages 2.78 to 2.81, εNd values of −9.07 to −9.48) indicate that the GOG magmas derived from an Archaean source compatible with that of some other Paleoproterozoic suites from Carajás Province. The GOG show significant contrasts with the Jamon and Velho Guilherme Paleoproterozoic suites from Carajás Province and the inclusion of the Gogó da Onça granite in any of these suites is not justified. The GOG is more akin to the Serra dos Carajás Suite and to the Seringa and São João granites of Carajás and to the Mesoproterozoic Sherman granite of USA and the Paleoproterozoic Suomenniemi Batholith of Finland. This study puts in evidence the relevance of precise geochronological data and estimation of magma oxidation state in the characterization and correlation of A-type granites.

Análise estrutural do sistema de grabens do Rio Tocantins, borda oeste da Bacia do Parnaíba (estados do Tocantins, Pará e Maranhão - Brasil)

A borda oeste da Bacia do Parnaíba apresenta estruturas que a diferenciam do contexto em outras regiões na bacia. Destacam‑se duas estruturas do tipo graben (Araguaína e Muricizal), que compõem o sistema de grabens em estudo, a primeira já no interior da bacia, a segunda implantada diretamente sobre o embasamento cristalino (Faixa Araguaia). Nas bordas do graben principal (Araguaína), as formações Motuca e Pedra de Fogo são balizadas por falhas normais ou normais oblíquas sinistrais. Mais a sul, na região de Palmas-Miracema do Tocantins, o Grupo Serra Grande, a Formação Pimenteiras e o embasamento cristalino estão em contato por falhas normais. O segundo graben, localizado a oeste do principal, também acomoda a Formação Motuca e unidades sotopostas, possuindo a mesma orientação. A análise de imagens SRTM auxiliou na cartografia dos fotolineamentos, dos grabens e, associada aos dados de campo, permitiu a proposição de três eventos deformacionais (D1, D2 e D3). A orientação N-S das falhas envolve uma variação de 20º-30º para E ou W, controlada pelas anisotropias do embasamento cristalino nesta borda da bacia, relacionadas à Faixa Araguaia. Falhas de regime normal e normal sinistral evidenciam uma distensão NE durante a formação dos grabens (evento D1). A idade mínima para D1 é estimada no intervalo Permo-Triássico, baseada nas relações estruturais e estratigráficas. Na região norte do mesmo, fotolineamentos E-W/ENE controlam o alojamento de basaltos e diabásios eojurássicos da Suíte Mosquito (evento D2), que interceptam as estruturas do graben principal. A distensão NNE na Suíte Mosquito e falhas normais ou transcorrentes exibindo distensão NW (evento D3) são tentativamente correlacionadas aos eventos de abertura do Atlântico Central e Sul, respectivamente.