Mario da Costa Campos Neto

Terrane accretion and upward extrusion of high‐pressure granulites in the Neoproterozoic nappes of southeast Brazil: Petrologic and structural constraints

The high-grade crystalline nappes exposed southeast of the Sao Francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of Neoproterozoic deep continental crust: the Socorro-Guaxupe nappe above, derived from an arc terrane, and the Tres Pontas-Varginha nappe below. Metamorphism in the Tres Pontas-Varginha nappe is characterized by the exceptional preservation of kyanite granulites (700–750°C, 15 kbar), and followed by limited retrogression. Maximum temperatures around 900–950°C were reached toward the base of the overlying Socorro-Guaxupe nappe during the intrusion of charnockitic-mangeritic magmas. Lower-pressure metamorphism, accompanied by anatexis, prevailed at shallower crustal levels. Our petrological results document an inverted thermal structure with isobaric heating of the top of the high-pressure granulite nappe. Both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units, both metamorphosed to high-pressure but lower-temperature conditions. Significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes. The proposed geodynamic scenario considers that continental subduction took place westward underneath Neoproterozoic oceanic lithosphere. The two granulite units crystallised at ∼ 45 km depths under distinct paleogeotherms within this subduction zone around 630 Ma. The kyanite granulites were rapidely exhumed through the mechanism of low-angle “forced” extrusion, whereas syncollisional collapse affected the soft, anatectic middle crust of the overlying arc terrane. The final emplacement of the thinned nappe pile onto the cold Sao Francisco craton and its platform cover, with at most, anchizonal to greenschist-facies metamorphism, occurred around 600 Ma.

Migração de orógenos e superposição de orogêneses: um esboço da colagem brasiliana no Sul do Cráton do São Francisco, SE - Brasil

The Meridional Tocantins Orogen represents a collision nappe pile involving three different tectonic environments. A magmatic arc domain developed in the Paranapanema continental active margin (Socorro-Guaxupe Nappe); a subducted continental domain (Andrelândia Terrane); and a passive margin domain and/or related to the Sanfranciscana plate (Carrancas and Lima Duarte Nappe systems). The magmatic activity related to the subduction started at 670 Ma and its metamorphism took place around 625Ma (active margin geothermal pattern modified in the subducted domain). The magmatic arc was active during flysch-type sedimentation in the Andrelândia terrane. Deformation and metamorphism related to the collisional process migrated from E to NE between 620 and 580 Ma. In the western internal nappe the tectonic process was very short-lived, less than 7 Ma, with some 612 Ma post-tectonic plutonism. On the other hand, in the eastern nappe the same process lasted ca. 20 Ma. Exhumation in the subducted terrane was very fast (ca. 2,2 mm/year). Superposed magmatic activities controlled by extensional regimes (Itu Granite Belt) followed orogen exhumation and preceded installation of continental-marine successor basins at 570 Ma. The Aracuai Orogen produced by convergence between the eastern passive margin of the Sanfranciscana plate and the Juiz de Fora Terrane (a microplate in an active margin environment) exhibits a metamorphic peak in the internal domain at around 560 Ma. The migration of the Aracuai nappes towards the cratonic domain (thermal metamorphic event ca. 530 Ma) reflects the superposition of the Mantiqueira Orogenic System. In the Cambrian the basement of the cratonic border was regenerated and included in the Aracuai orogen which involved the foreland in a thin-skinned thrust system well documented in the Quadrilatero Ferrifero. Oblique collision between the Mantiqueira Orogenic system and the proto-continent formed during the Meridional Tocantins orogeny was responsible for greenschist metamorphism at about 550 Ma in the successor basins. After this, the collisional process migrated northwards until 500 Ma. In the southern part of the Sao Francisco Craton the Brasiliano orogenic collage represents the interaction of several short orogenic steps in a long, convergent tectonic process involving plate consumption with eastward polarity and westward orogenic migration that lasted until the Cambro-Ordovician limit. The latest documented pre-collage metamorphism observed in the cratonic basement occurred between 2.03 and 2.06 Ga, probably under extensional conditions.

Polycyclic evolution of Camboriú Complex migmatites, Santa Catarina, Southern Brazil: integrated Hf isotopic and U-Pb age zircon evidence of episodic reworking of a Mesoarchean juvenile crust

The Camboriu Complex is the only gneissic-migmatitic inlier within the Neoproterozoic Brusque Group supracrustal rocks, in the northernmost part of the Dom Feliciano Belt, southern Brazil. It comprises the Morro do Boi migmatites and the diatexitic Ponta do Cabeco Granite. Zircon U-Pb dating of migmatites and associated granitic neosomes shows that crustal evolution started in the Paleo- Mesoarchean (3.3-3.0 Ga), continued with events through the Neoarchean and Paleoproterozoic and ended in the Neoproterozoic (0.64-0.61 Ga). Integration of zircon Hf isotopic data and U-Pb ages indicate that juvenile crustal accretion was restricted to the Archean and that afterwards intracrustal reworking predominated. The exception to this is the ca. 1.56 Ga xenoliths (basic dike remnants?), whose magmatic zircons have juvenile Hf isotopic signatures. This basic magmatism marks extension of the earlier Precambrian complex. Although the Camboriu Complex is dominated by early Precambrian crustal additions, it was so strongly reworked in the Neoproterozoic that melts derived from it intruded the adjacent Neoproterozoic Brusque Group supracrustal rocks. Because of this strong overprint, we regard the Camboriu Complex as a Neoproterozoic (Ediacaran) geotectonic unit. In terms of its history, the Camboriu Complex most closely matches the Atuba Complex, the basement of the Curitiba Microplate that occurs further to the north, close to the Ribeira Belt, another Neoproterozoic orogen of southern Brazil.

Geocronologia e geologia isotópica dos terrenos pré-cambrianos da porção sul-oriental do Estado de São Paulo

In the southeastern region of Sao Paulo State four major tectonic domains were defined. The Embu Domain, north of the Cubatao Shear Zone (CSZ), is composed of metasedimentary rocks, and peraluminous granites (ca. 600 Ma), whose intrusion was controlled by E-NE shear zones, and which have model Nd Tdm age around 2.0 Ga. Gneiss-migmatite rocks (612 Ma) and related granites (580 Ma) predominate in the Mongagua Domain, which is limited by the Cubatao and Itariri shear zones. These rocks have different model Nd Tdm ages between 1.7 and 2.2 Ga. The Registro Domain, between Cubatao - Itariri Shear System (CISS) and the Serrinha Shear Zone (SSZ), is formed by metasediments and granitic rocks with migmatitic features, and represents a paleoproterozoic domain (1.9 - 2.2 Ga) intensely affected in Neoproterozoic times (750 - 580 Ma). Rocks of the Iguape Domain, limited to the north by the SSZ, include granites (ca. 600 Ma) and low grade metasediments. It is probable that all these tectonic blocks were juxtaposed during a short time interval at the end of Neoproterozoic. The joining of the Registro to the Embu Domain occurred at about 596 Ma along an E-W shear zone. The 598 Ma peraluminous granites of the Embu Domain may register the arrival of the Mongagua Domain against the newly formed Registro-Embu Domain. The welding of the Iguape and Registro domains probably occurred at 575 Ma, as suggested by the U-Pb (monazite) age of the protomylonitic granites of the SSZ.

The inventory of geological heritage of the state of São Paulo, Brazil: Methodological basis, results and perspectives

An inventory of geological sites based on solid and clear criteria is a first step for any geoconservation strategy. This paper describes the method used in the geoheritage inventory of the State of São Paulo, Brazil, and presents its main results. This inventory developed by the geoscientific community aimed to identify geosites with scientific value in the whole state, using a systematic approach. All 142 geosites representative of 11 geological frameworks were characterised and quantitatively evaluated according to their scientific value and risk of degradation, in order to establish priorities for their future management. An online database of the inventory is under construction, which will be available to be easily consulted and updated by the geoscientific community. All data were made available to the State Geological Institute as the backbone for the implementation of a future state geoconservation strategy.

Oxygen isotope composition and geothermometry of granulite to greenschist facies metamorphic rocks: a study from the Neoproterozoic collision-related nappe system, south of Sao Francisco Craton, SE Brazil

Abstract Oxygen isotope studies were carried out across units of a Neoproterozoic nappe system, south of Sao Francisco Craton. A temperature decrease toward the base of the system is found, consistent with a previously recognized inverted metamorphic pattern. The tectonic contact of the basal unit and the reworked southern Sao Francisco craton show a steep temperature gradient, suggesting that low temperature thrusting acted as the dominant tectonic process. The contrasts between the δ18O values of the Tres Pontas-Varginha and Carmo da Cachoeira nappes and the differences among the samples and minerals are consistent with the preservation of sedimentary isotopic composition during metamorphism. The small differences in the δ18O values between the undeformed and the deformed calc-silicate samples (∼1.6‰) suggest that the δ18O value of mylonitization fluids was close to that which equilibrated with the metamorphic assemblage. The distinct δ18O values of metapelitic and calc-silicate samples and the great temperature difference from one type to the other indicate that no large-scale fluid interaction processes occurred during metamorphism. Oxygen isotopic estimations of both Tres Pontas-Varginha undeformed rocks and Carmo da Cachoeira unaltered equivalents indicate δ18O values of up to 18‰. Comparison between these values and those from the ‘basement’ orthogneisses (8.3–8.5‰) indicates the latter are not sources for the metapelites.

Reconstrução e comparação de trajetórias P-T no Sistema de Nappes Andrelândia, sul da Faixa Brasília, MG

The Andrelândia Nappe System consists of three main nappes, from bottom to top: the lower Andrelândia nappe, the middle Liberdade nappe and the upper Tres Pontas-Varginha nappe and associated klippen, Pouso Alto, Aiuruoca, Carvalhos and Serra da Natureza. In the Andrelândia Nappe System, metamorphism increases from north to south and east to west, with the highest temperatures and pressures recorded in rocks of the Tres Pontas-Varginha nappe and associated klippen. Samples of pelitic and mafic rocks were selected from the three nappes to determine the conditions of metamorphism using the program THERMOCALC. In this study, peak metamorphic conditions were calculated for the following samples: one sample of the Andrelândia Nappe (688 ± 35 °C and 5.63 ± 0.9 kbar), one sample of the Liberdade Nappe (648 ± 23°C and 7.41 ± 1 kbar), and three samples of the Carvalhos Klippe (845 ± 53oC and 15.7 ± 5.2 kbar, and 847 ± 45oC and 13.6 ± 5.8 kbar for two samples of pelitic granulites respectively, and 854 ± 71oC and 15 ± 1.4 kbar for one sample of mafic granulite). P-T paths inferred on the basis of the observed textures are clockwise and typical of collisional belts.

The Tectonic History of the Southern Adamastor Ocean Based on a Correlation of the Kaoko and Dom Feliciano Belts

Closure of the southern Adamastor Ocean led to the development of the Neoproterozoic-Early Cambrian orogens on both margins of the present South Atlantic Ocean, which preserve a rich record of West Gondwana assembly and large-scale crustal evolution. Here we describe the distinct stages and tectonic regimes related to the evolution of the southern Adamastor Ocean as reconstructed from the geological record in southeastern Brazil, Uruguay and southwestern Africa. The welding of the Rio de la Plata/Paranapanema and African cratons was the result of a long history that generated the Dom Feliciano Belt in southeastern Brasil and Uruguay and its African counterparts, the Kaoko, Gariep and Saldania belts. Recent ideas and previous hypotheses are discussed and integrated into a tectonic model for the evolution of the southern Adamastor Ocean, largely based on comparison of the Neoproterozoic Kaoko and Dom Feliciano belts. The history of the southern Adamastor Ocean spans between 900 and 590 Ma, from the earliest records of magmatism related to Rodinia break-up (980–780 Ma), through the climax of the extensional phase that led to the opening of a vast ocean (780–640 Ma). Tectonic inversion led to subduction towards the east (in today’s coordinates), which generated an extensive magmatic arc on the western margin of the Congo and Kalahari cratons (640–600 Ma) and eventually the collision between the Congo and Rio de La Plata Cratons (600–590 Ma) juxtaposing the magmatic arc and the western South American schist belts. At around 530 Ma, still under the influence of plate convergence, a positive flower structure was generated causing the extrusion of the Granite Belt, the eroded remnant of the magmatic arc, and reactivation of the doubly verging thrusts that affected the supracrustal units of the Kaoko and Dom Feliciano belts. Only at that time did deformation reach the Itajai and Nama foreland basins.