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In the Parnaíba basin, northeastern Brazil, some records of basic tholeiitic magmatism have been recognized as part of large igneous provinces (LIP's). They are represented by the Mosquito basaltic flows (Triassic–Jurassic; ca. 200 Ma) that occurs in the west portion of the basin as well as the Sardinha diabases (Cretaceous; ca. 124 Ma) eastwards. The Mosquito and Sardinha magmatic rocks of the Parnaíba basin were related to the processes of distension and rupture of Pangea and Gondwana, respectively. This paper presents the petrogenesis of diabase sills that occur in the eastern portion of the basin, probably associated with the Sardinha formation. The studied rocks are mostly equigranular, medium-grained diabases. They are tholeiitic andesite-basalts and stricto sensu basalts, comprising a single Low-TiO2 (TiO2 < 2 wt%) magmatic suite here denominated as Batalha Suite. They show relative enrichment in the broad spectrum of LILE (large ion lithophile elements), HFSE (high field strength elements) and REE (rare earth elements) when compared to MORB, chondrite and primitive mantle, similar to continental flood basalts (CFB) elsewhere. This suite evolved by fractional crystallization as attested by whole-rock geochemistry, mineral chemistry, geochemical modeling and Sr–Nd isotope data. The studied Low-TiO2 basalts derived from an enriched mantle source likely to be represented by the subcontinental lithospheric mantle (SCLM). The diabases are related with 16% of partial melting of either a less enriched residual garnet harzburgitic source or a more enriched residual spinel-harzburgite. [ABSTRACT FROM AUTHOR]

This study reports the first geochemical and Pb isotopic data for mantle xenoliths from beneath the Neoproterozoic Ribeira Belt, southeastern Brazil. The cm-sized spinel peridotite xenoliths are hosted by a Cretaceous lamprophyre dike that intruded high-grade metamorphic rocks. Major- and trace-element compositions of the main minerals indicate that the xenoliths derive from a shallow fertile mantle that has undergone a low degree of melt extraction (2–9% partial melting). On the basis of modeled isochemical phase diagrams for lherzolites, pressure and temperature conditions are inferred to vary from 1300–1350 °C and 17–19 kbar (fertile composition) to 1330–1430 °C and 17–23 kbar (relatively depleted composition), which correspond to high geothermal gradients of 65–80 mW/m2. Temperatures of last equilibration calculated based on the average REE content of pyroxenes for the same lherzolite samples vary from 1233 ± 56 °C to 1085 ± 42 °C, while conventional thermometry (TBKN) yields average values of 807 and 755 °C, indicating re-equilibration at lower temperatures. Pb isotope ratios of clinopyroxene define a mixing line that intercepts the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 200 Ma. Linear regressions yield two errorchrons of 56 ± 75 Ma and 571 ± 99 Ma (95% confidence level). These results combined with the ages and tectonic settings of host rocks are suggestive of an overprint of a younger tectono-thermal event, most likely related to the opening of the South Atlantic Ocean, over a mantle previously equilibrated during the Precambrian development of the Ribeira Belt. [ABSTRACT FROM AUTHOR]

Coquinas are a sedimentary rock composed mainly of shells and their fragments with a carbonate or siliciclastic matrix. These rocks exhibit a high range of porosity and permeability values that are related to depositional and diagenetic processes. A series of coquina reservoirs were discovered in the Campos and Santos Basins in the eastern margin of Brazil and the understanding of the petrophysical properties of these rocks is essential for reservoir production. In this work, samples from the Morro do Chaves Formation (Sergipe–Alagoas Basin) and the Itapema Formation (Santos Basin) were selected to identify the porosity vs permeability relationship according to the biofabric characteristics and porosity control. The biofabric textures where classified into four groups according to taphonomic patterns and diagenetic modification: (i) well-sorted and preserved biofabric, (ii) well-sorted and altered biofabric, (iii) unsorted and preserved biofabric, and (iv) unsorted and altered biofabric. The porosity control varies in all groups from depositional to diagenetic with distinct pore types, pore sizes, and pore connectivity that are located in different global hydraulic elements. The analysis of the pore space was carried out through laboratorial measurements (gas injection) and by images from high-resolution computed tomography from plug samples, resulting in a pore network model where values of pore radius, pore throat radius, and coordination number were obtained. The best values of porosity and permeability are related to well-sorted altered biofabrics, whereas the lowest values, either with unsorted biofabrics, with low pore connectivity, or biofabrics that had the porosity reduced by compaction and cementation. [ABSTRACT FROM AUTHOR]

Gneiss–migmatitic rocks and granites between the Cubatão and Itariri shear zones characterize the Mongaguá Complex-Coastal Terrane. Three major units are defined: (1) the Itariri Suite (IS) composed of monzogranites, granodiorites, tonalites, and locally gneiss–migmatitic rocks; (2) the Areado pluton composed of monzogranite and locally tonalite; and (3) the Ribeirão do Óleo Pluton mainly composed of monzogranites, which is the subject of this article. Combined field observations, petrography, Sr–Nd–Hf isotopes, U–Pb zircon data, and preliminary geochemical studies are presented for the Ribeirão do Óleo Pluton (ROP), intrusive in granite-gneisses of the IS. It is a plutonic association of weakly peraluminous, medium-to-high-K biotite monzogranites that surround microgranular intermediate enclaves, which are products of magma mingling. Different from the most examples of magma mingling described in the literature, the ROP mingling did not result from the intrusion of hot mafic magma into colder host felsic magma. Conversely, the emplacement of the monzogranite started most likely during the medium-to-final stage of crystallization of the quartz microdiorite. A detailed study of LA-ICP-MS U–Pb zircon ages, including zircon morphology and internal structure, was useful to determine more precisely the emplacement sequence and the relation between the units of the ROP. Intermediate and acid facies have Paleoproterozoic and Neoproterozoic inheritances, moreover a mixture of xenocrysts, antecrysts, and autocrysts, indicative of a complex evolution of the ROP. Several magma crystallization pulses are recorded at 655–624 Ma, 627–601 Ma, and 613–586 Ma. The negative εHf(t) ranging from − 17.26 to − 12.51 in monzogranites, εNd(t) values of − 10.66 to − 7.83 at t = 600 Ma in both facies, 87Sr/86Sr of ca. 0.708 for the monzogranite and 0.706 for the quartz microdiorite indicate a strong crustal contribution in the genesis of intermediate and acid magmas of this pluton. [ABSTRACT FROM AUTHOR]

A comprehensive analysis of thermal gradient and heat flow data was carried out for sedimentary basins situated in the continental margin of Brazil (CMB). The results point to the existence of a narrow belt within CMB, where temperature gradients are higher than 30 °C/km and the heat flow is in excess of 70 mW/m. This anomalous geothermal belt is confined between zones of relatively low to normal heat flow in the adjacent continental and oceanic regions. The width of the belt is somewhat variable, but most of it falls within the range of 100-300 km. The spatial extent is relatively large in the southern (in the basins of Pelotas, Santos and Campos) and northern (in the basins of Potiguar and Ceará) parts, when compared with those in the central parts (in the basins of South Bahia, Sergipe and Alagoas). The characteristics of heat flow anomalies appear to be compatible with those produced by thermal sources at depths in the lower crust. Hence, magma emplacement at the transition zone between lower crust and upper mantle is considered the likely mechanism producing such anomalies. Seismicity within the belt is relatively weak, with focal depths less than 10 km for most of the events. Such observations imply that 'tectonic bonding' between continental and oceanic segments, at the transition zone of CMB, is relatively weak. Hence, it is proposed that passive margins like CMB be considered as constituting a type of plate boundary that is aseismic at sub-crustal levels, but allows for escape of significant amounts of earth's internal heat at shallow depths. [ABSTRACT FROM AUTHOR]

The Southern Brasília Orogen is a Neoproterozoic belt that occurs along the southernmost border of the São Francisco Craton where the Andrelândia Nappe System represents the subducted sedimentary domain and is divided into three allochthonous groups, of which the ages and P-T conditions of metamorphism are studied here. The basal unit, the Andrelândia Nappe, exhibits an inverted metamorphic pattern. The base of the structure, composed of staurolite, garnet, biotite, kyanite, quartz, and muscovite, marks the metamorphic peak, whereas at the top, the association of the metamorphic peak does not contain staurolite. The Liberdade Nappe, the middle unit, presents a normal metamorphic pattern; its base, close to the Andrelândia Nappe, shows paragneiss with evidence of in situ partial melting, and towards the top, coarse-grained staurolite schist is found. The staurolite-out and melt-in isograds are coincident and parallel to the main foliation. Thus, the shear zone that limits the nappes is syn-metamorphic, reheating the underlying Andrelândia Nappe and influencing the establishment of metamorphic inversion. This suggestion is supported by the monazite chemical ages, which indicates that the Andrelândia Nappe metamorphic peak (586 ± 15 Ma) is younger than that of the Liberdade Nappe (622.3 ± 7.6 Ma). The upper unit, the Serra da Natureza Klippe, bears a typical high-pressure granulite mineral assemblage that is composed of kyanite, garnet, K-feldspar, rutile, and leucosome, as well as a metamorphic peak at 604.5 ± 6.1 Ma. This tectonic assembly, with inverted and non-inverted metamorphic patterns and generation of klippen structures, is consistent with exhumation models and a strong indentor located in the lower continental crust. [ABSTRACT FROM AUTHOR]

The Borborema Province is the western part of a major Brasiliano belt that extends from Brazil through NW Africa in pre-drift reconstructions. This province resulted from convergence and collision among the West African, Congo-São Francisco, and Amazonian cratons about 600 Ma. This study focuses on the Pernambuco-Alagoas (PEAL) domain, which is a complex of magmatic, migmatitic, and metamorphic rocks, located in the southern part of the Borborema Province. U-Pb geochronology and Sm-Nd data for metasedimentary sequences (Rio Una, Inhapi) of the PEAL domain and a sample from a sequence of the Transversal Zone domain suggest that their deposition occurred during a Cryogenian extensional event, within the interval 850-631 Ma (or slightly younger). This extensional event occurred in the PEAL, Transversal Zone, and Sergipano domains before the onset of the Brasiliano collision and was followed by syn- and post-collisional magmatism. The Rio Una sequence and the sequence from Transversal Zone domain were deposited over a Rhyacian (ca. 2.0-2.2) basement having a juvenile Palaeoproterozoic Nd signature, whereas the Inhapi sequence was deposited over an Early Neoproterozoic (Tonian) basement. The deposition of the studied sequences is coeval with metasedimentary sequences to the north and south in other domains of the Borborema Province. However, differences in Nd isotopic signatures between the sequences from PEAL, Transversal Zone, and Sergipano domains suggest that they were formed in distinct basins. Metasedimentary sequences from the PEAL domain have Meso- and Palaeoproterozoic T model ages. These data suggest that the orogens where the metasedimentary sequences are located have a strong ensialic component. T model ages of ca. 1.0 Ga and εNd (0.6 Ga) values around zero recorded in granites from the southern part of the PEAL, suggesting that juvenile material was accreted to the southern part of the PEAL domain crust during the Tonian. The migmatites from the PEAL domain have a wide range of T ages along with a wide range of εNd (0.6 Ga) values. The transition between the northern and southern parts of the PEAL domain denotes the approximate northern margin of a Late Mesoproterozoic to Early Neoproterozoic rift. [ABSTRACT FROM AUTHOR]

Zircon (ZFT) and apatite (AFT) fission-track low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the breakup of southwestern Gondwana. The thermochronological data obtained from samples of both the Precambrian basement and the Paleogene to Neogene sedimentary rocks from the continental rift of southeastern Brazil provided ZFT ages between 148 (15) and 64 (6) Ma, and AFT ages of 81 (8)-29 (3) Ma. These data clearly indicate syn- and post-rift reactivations during the Early Cretaceous, with great emphasis on Paleogene to Neogene times. Integrating the results of older thermochronological studies, the reactivation of the southeastern Brazilian margin can be described in three main phases related to the rift to post-rift evolution of SE Brazil. In general, ZFT and AFT data yield spread values that become younger as samples are closer to the reactivated Neoproterozoic shear zones and might reflect source area exhumation. The analysis of ZFT and AFT data allowed interpretations regarding the main phases that occurred in the study area related to the thermotectonic and tectono-stratigraphic evolution in southeastern Brazil. [ABSTRACT FROM AUTHOR]

The Apiaí gabbro-norite is a massive fine-grained Neoproterozoic intrusion emplaced in a core of synformal structure that deforms low-grade marine metasedimentary rocks of the Ribeira Belt of south-eastern Brazil. The lack of visible magmatic layering or any internal fabric has been a major limitation in deciding whether the emplacement occurred before or after the regional folding. To assist in the tectonic interpretations, we combine low-field anisotropy of magnetic susceptibility (AMS) and silicate shape preferred orientation (SPO) to reveal the internal structure of the mafic intrusion. Magnetic data indicate a mean susceptibility of about 10 SI and a mean anisotropy degree ( P) of about 1.08, essentially yielded by titanomagnetite. The magnetic and silicate foliations for P ≥ 1.10 are parallel to each other, while the lineations tend to scatter on the foliation plane, in agreement with the dominant oblate symmetry of the AMS and SPO ellipsoids. For lower P values, the magnetic and silicate fabrics vary from coaxial to oblique, and for P ≤ 1.05, their shapes and orientations can be quite distinct. The crystal size distribution (CSD) of plagioclase for P > 1.05 is log linear, in agreement with a bulk simple crystallisation history. These results combined show that for a strong SPO, corresponding to a magnetic anisotropy above 1.10, AMS is a reliable indicator of the magmatic fabric. They indicate that the Apiaí gabbro-norite consists of sill-like body that was inclined gently to the north by the regional folding. [ABSTRACT FROM AUTHOR]

Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of São Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area. [ABSTRACT FROM AUTHOR]

Dissolution cavities in weathered pebbly quartzite of the ~2.5-Ga Moeda Formation at Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil, are decorated with suspended filaments of opaline silica. The filaments sustain xenotime-hematite aggregates in the open space. Xenotime occurs as inclusions in buds and botryoidal aggregates of hematite. The filamentous structures consist of strand-forming buds, hypha-like extensions, and thin strands that compose mat-like arrangements. They resemble microbial filaments that were replaced by opaline silica and fossilized. The occurrence of spherical hematite as protuberances on hematite-free opaline hyphae is interpreted as accretion of dissolved iron onto extracellular polymers. Phosphate sites in polymeric substances expelled from the microbial filaments might have adsorbed yttrium and heavy rare-earth elements from groundwater to the iron-accreting polymers. These would have resulted in botryoidal aggregates of hematite with xenotime inclusions. The presence of authigenic xenotime in the weathering zone opens a new possibility to constrain the evolution of lateritic profiles by xenotime geochronology. [ABSTRACT FROM AUTHOR]

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300 million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The São Gabriel block in the west is characterized by c.760-690 Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880 Ma old igneous rocks. Both igneous and metasedimentary rocks have positive ε Nd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the São Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D) yield an U-Pb zircon age (LA-ICP-MS) of 701 ± 10 Ma giving the approximate minimum age of the São Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789 ± 7 Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234 ± 28 Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100-2,000 Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800-700 Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and São Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880 Ma (Passinho event) and (ii) accretion of the 760-700 Ma Cambaí/Vila Nova magmatic arc (São Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW-NE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590 Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530 Ma and can be assigned to final stages of the amalgamation of West Gondwana. [ABSTRACT FROM AUTHOR]

review of the lithostratigraphic units in the Río de la Plata Craton and of new and previously published geochronological, isotopic and geophysical data is presented. Sm-Nd T model ages between 2.6 and 2.2 Ga characterize the Piedra Alta Terrane of this craton. Crystallization ages between 2.2 and 2.1 Ga for the metamorphic protoliths and 2.1-2.0 Ga for the post-orogenic granitoids indicate juvenile crust, followed by a short period of crustal recycling. Cratonization of this terrane occurred during the late Paleoproterozoic. Younger overprinting is not observed, suggesting it had a thick and strong lithosphere in the Neoproterozoic. A similar scenario is indicated for the Tandilia Belt of Argentina. Sm-Nd T model ages for the Nico Pérez Terrane show two main events of crustal growth (3.0-2.6 and 2.3-1.6 Ga). The crystallization ages on zircon ranges between 3.1 and 0.57 Ga, which is evidence for long-lived crustal reworking. The age for cratonization is still uncertain. In the Taquarembó Block, which is considered the prolongation of the Nico Pérez Terrane in southern Brazil, a similar scenario can be observed. These differences together with contrasting geophysical signatures support the redefinition of the Río de la Plata Craton comprising only the Piedra Alta Terrane and the Tandilia Belt. The Sarandí del Yí Shear Zone is regarded as the eastern margin of this Craton. [ABSTRACT FROM AUTHOR]

The Mantiqueira Province represents a series of supracrustal segments of the South-American counterpart formed during the Gondwana Supercontinent agglutination. In this crustal domain, the process of escape tectonics played a conspicuous role, generating important NE-N-S-trending lineaments. The oblique component of the motions of the colliding tectonic blocks defined the transpressional character of the main suture zones: Lancinha-Itariri, Cubatão-Arcádia-Areal, Serrinha-Rio Palmital in the Ribeira Belt and Sierra Ballena-Major Gercino in the Dom Feliciano Belt. The process as a whole lasted for ca. 60 Ma, since the initial collision phase until the lateral escape phase predominantly marked by dextral and subordinate sinistral transpressional shear zones. In the Dom Feliciano Belt, southern Brazil and Uruguay, transpressional event at 630-600 Ma is recognized and in the Ribeira Belt, despite less coevally, the transpressional event occurred between 590 and 560 Ma in its northern-central portion and between ca. 625 and 595 Ma in its central-southern portion. The kinematics of several shear zones with simultaneous movement in opposite directions at their terminations is explained by the sinuosity of these lineaments in relation to a predominantly continuous westward compression. [ABSTRACT FROM AUTHOR]