Luís Alberto D'Ávila Fernandes

Deformation patterns in the southern Brazilian branch of the Dom Feliciano Belt: A reappraisal

Abstract Structural mapping of key areas linked by reconnaissance investigations in the southern Brazilian portion of the Brasiliano/Pan-African Dom Feliciano Belt has shown the importance of large-scale flat-lying and strike-slip shear zones in the tectonic evolution of this belt. These zones of intense deformation form km-thick mylonitic belts in Brasiliano granites and supracrustal rocks as well as their Transamazonian/Eburnian basement. Structures developed within flat-lying shear zones under metamorphic conditions of amphibolite facies are interpreted as having formed during oblique collision between crustal blocks — the Kalahari Craton and the Magmatic Arc I — in which the direction of tectonic transport was NW-SE. Further collision between this assemblage and the Rio de La Plata Craton gave rise to a transpressive regime whose tectonic transport direction is indicated by regionally consistent NE/SW-oriented stretching lineations, parallel to the belts length.

The Neoproterozoic Dorsal de Canguçú strike-slip shear zone: its nature and role in the tectonic evolution of southern Brazil

Abstract The Dorsal de Cangucu Shear Zone (DCSZ) is part of a strike-slip fault system showing trends parallel to the Neoproterozoic Dom Feliciano Belt in southern Brazil. As an attempt to assess the role played by this fault system in the tectonic evolution of the continental crust in southern Brazil, a re-evaluation of the main structural, magmatic and geochronological characteristics of the best known shear zone of this system was conducted. Magmatism syntectonic to the strike-slip shear zone is represented by mantle-derived granodioritic magmas emplaced into transtensional segments. These were followed by crustal melts, represented by successively younger peraluminous granites. The porphyritic granodiorites have a mixed origin involving a parental dioritic magma that suffered fractional crystallisation and assimilation of crustal rocks. They present a well-developed subvertical magmatic fabric with northeast to north-south trending foliation and low plunging lineations defined by dimensional orientation of K-feldspar megacrysts. Partial melting of the country rocks is the most likely petrogenetic process for the origin of the peraluminous granites. Microstructures produced by solid-state deformation under lower amphibolite- to greenschist-facies metamorphic conditions exhibit ubiquitous kinematic indicators of sinistral displacement. The contribution of the transcurrent fault zones to crustal growth was limited to the emplacement of relatively small volumes of mantle-derived dioritic magma during their early stages of development. Large-scale tectonic control of the overall strain field responsible for the nucleation and sinistral displacement of these faults is likely to be a far-field effect of convergence between the Kalahari and Zaire Cratons during the final stages of Neoproterozoic amalgamation of West Gondwana.

SHRIMP U–Pb age for the emplacement of the Santana Granite and reactivation of the Porto Alegre Suture, southern Brazil

SHRIMP U–Pb zircon ages are reported from a sample of the Santana Granite that represents the beginning of alkaline magmatic activity in the Neoproterozoic of the Sul-Rio-Grandense Shield, Brazil. This granite outcrops in the city of Porto Alegre and has a NE–SW elongated shape (20 km long and 5 km wide). It is a medium- to coarse-grained syenogranitic rock with subvertical magmatic foliation and subhorizontal lineation with NE–SW trends, overprinted by a weak solid-state fabric. Zircons of this rock with 206Pb/238U ages between ca. 575 and ca. 617 Ma are interpreted as having formed during magmatic crystallisation; the best fit for this age using five analyses provided a mean 206Pb/238U age of 600±10 Ma. This age is also interpreted as the age of the reactivation of the Porto Alegre Suture, a crustal discontinuity along which the Santana Granite was emplaced. Younger isotopic data (<575 Ma) for zircons from this sample are interpreted as due to Pb loss from metamictization. The 600-Ma age is consistent with previous results and marks a magmatic episode, probably related to an extensional tectonic episode that affected the NE of the Sul-Rio-Grandense Shield at the end of the Brasiliano Cycle.

Thermobarometry, Sm/Nd Ages and Geophysical Evidence for the Location of the Suture Zone Between Cuyania and the Western Proto-Andean Margin of Gondwana

Abstract Basement rocks comprising ortho- and paragneisses and schists whose tectono-metamorphic evolution is poorly known, are exposed in the Sierras de Umango, Maz-Espinal and Las Ramaditas, in the northwest of the La Rioja Province, Argentina. These units were included in the Maz, El Taco, El Zaino complexes, as well as the Tambillos Metamorphics that would be part of the northern end of the Cuyania terrane, a microcontinent derived from Laurentia that collided with Western Gondwana during the early Paleozoic, or belong to the active margin of the continent. To recognize rocks belonging to each of these tectonic units and to understand the history and physical conditions of accretion were some of the main goals of the multidisciplinary investigation whose preliminary results are presented here. Geochemical studies, trace and REE elements and Sm-Nd model ages allowed the recognition of several episodes of crustal accretion in these rocks. The oldest one occurred at ca. 2.2 Ga in an arc/back-arc environment along the eastern segment of the Sierra de Maz, and was possibly coeval with development of a early Proterozoic continental crust that acted as source to sediments of Maz Complex. The following episode of crustal accretion that formed rocks in this region was at ca. 1.4 Ga and is registered by tonalites emplaced in an extensional environment cropping out in the western flank of the Sierra del Espinal. In the Sierra de Umango, an arc/back-arc sequence registered an episode of crustal accretion during the Grenvillian Cycle (ca. 1.3 Ga). The last episode of crustal accretion detected in this area (800 Ma) is represented by an old alkaline volcanism in the Sierra de Umango. This episode could be representing the first stage of break-up of the Rodinia supercontinent during the Neoproterozoic. The metamorphic grade reached by these rocks is mostly represented by fabrics with mineral assemblages of intermediate to high pressures and high temperatures, typical of collisional environments. The oldest rock-forming fabrics tectono-metamorphic episode recognized is of middle Proterozoic age (ca. 1.04 to 0.969 Ga, garnet-whole-rock Sm/Nd age) being registered by metapelites from Maz Complex that attained temperatures of 650°C-6.3 kbar. A younger metamorphic event (463 Ma, garnet - whole-rock Sm/Nd age) is verified in metatonalites intrusive in these metapelites. Another metamorphic event at ca. 301 Ma (garnet-WR Sm/Nd age) was recognized in metasediments from El Taco Complex. Peak metamorphic conditions of this event, probably registering the last major tectonic episode that affected rocks of this area was established in 868°C-9.8 kbar. It is impossible to distinguish fabrics belonging to totally different tectonic episodes based on structural or metamorphic data. Therefore, distinction between major tectono-thermal events of totally different ages such as the high-T middle Proterozoic deformation and with N-NWestwards tectonic transport direction registered in the Sierras de Maz-Espinal and Umango from the youngest one (ca. 301 Ma) that attained the highest-P/T conditions, recognized in the Sierra de Las Ramaditas, had to be done on the basis of Sm/Nd ages. Geophysical evidence indicates the presence of extensive WNW-oriented lineaments that separate basements blocks of different magnetic and gravimetric signatures that are thought to represent ancient Grenvillian age suture zones. On the other hand, the northern segment of the Valle Fertil lineament that runs between Sierras de Umango and Maz-Espinal is at present interpreted as marking the eastern boundary of the Cuyania terrane. This is supported by isotopic data as well as the contrasting history of tectono-metamorphic events as determined for both of these segments of the NW Sierras Pampeanas.

The Cerro Olivo Complex: a pre-collisional Neoproterozoic magmatic arc in Eastern Uruguay

The Cerro Olivo Complex is one of the few occurrences of the basement rocks in the Dom Feliciano Belt. It contains migmatitic paragneisses and orthogneisses that host granites of ca. 600–540 Ma Aiguá Batholith. The main orthogneisses are rich in orthopyroxene + Ca-plagioclase (Cerro Bori unit), but K-feldspar augen gneisses are also common (Centinela-Punta del Este unit). The paragneisses (Chafalote unit) are semi-pelitic migmatites that contain restites of metapelites, quartzites, amphibolites, and calc-silicate rocks. A clockwise pressure–temperature–time (P–T-t) path and two deformational events affected the Cerro Olivo Complex rocks. Granulitic high-pressure (HP)–high-temperature (HT) peak conditions were followed by low pressure (LP)–HT decompression. The first deformation (K1) developed an E–W gneissic foliation and westward-stretching lineations, whereas the second (K2) produced NS to NE–SW low-temperature mylonitic foliation and southward-stretching lineations. New SHRIMP U–Pb data from zircon cores in magmatic textural domains yield an intrusive age of 782 ± 7 million years for the Cerro Bori unit. The zircon rims have an age of 657 ± 7 million years, reflecting a younger partial melting event. Inherited ages in zircon xenocrysts span from 2655 to 768 million years, but are mostly ca. 1.0–1.2 thousand million years old. Bulk-rock geochemistry indicates a magmatic arc setting for the source rocks. The Cerro Bori unit represents calk-alkaline tonalitic and granodioritic rocks mixed with minor gabbros; in contrast, the Centinela unit consists of post-orogenic granites. A continental magmatic arc developed between ca. 800 and 770 Ma attending convergence of the Kalahari and Rio de la Plata palaeocontinents, but prior to their collision.