Edinei Koester

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.

Evolution of heterogeneous mantle in the Acampamento Velho and Rodeio Velho volcanic events, Camaqua Basin, southern Brazil

Abstract The Camaqua Basin, developed during the last phases of the Brazilian/Pan-African Orogeny and was filled with a thick volcano-sedimentary succesion, in which two volcanic events of alkaline affinity are represented by the Acampamento Velho Alloformation and the Rodeio Velho Member. The Acampamento Velho Alloformation records a bimodal event with a lower association of mafic flows and an upper association of felsic pyroclastic rocks and flows. It was formed during extension, after the subduction of the Adamastor oceanic plate beneath the Rio de La Plata continental plate at the end of the Neoproterozoic III. The second event, the Rodeio Velho Member, represented by mafic flows, intrusions and piroclasts, took place during overall extensional tectonism, probably in the middle Ordovician. Rb, Sr, Sm, and Nd isotopic measurements were carried out on samples from both units. Regardless the event they represent, all the samples display negative values for epsilon Nd, ranging from 2.97 to 10.31 for the Acampamento Velho Alloformation and from 8.39 to 13.92 for the Rodeio Velho Member. The initial 87 Sr/ 86 Sr ratios vary from 0.706 to 0.707 and from 0.704 to 0.707 for the Acampamento Velho Alloformation and Rodeio Velho Member, respectively. Mafic flow deposits in both units show a preferential enrichment in Ba relative to Th. Flow samples from the Rodeio Velho Member also display a distinctive enrichment in the Ba/Th ratio, without a change in the initial Sr, compared to the mafic flow deposits from the Acampamento Velho Alloformation, which show a slight enrichment in those ratios. As for the Acampamento Velho Alloformation, the mafic lavas could be a mixture of depleted mantle-derived basalts plus 20% to 30% of crustal contamination by sediment (probably Neoproterozoic arkosic quartzites). The formation of a magmatic chamber and the separation of the magma into two fractions gave rise initially to the mafic rocks at the base of the Acampamento Velho Alloformation The other magma fraction underwent a significant enrichment in crustal component before the felsic rocks of this Alloformation were formed. The flows from the Rodeio Velho Member originated in a distinct magma chamber, with EM I characteristics that was much more enriched in incompatible elements and depleted in radiogenic Sr.

Provenance of dust to Antarctica: A lead isotopic perspective

Antarctic ice preserves an ~800 kyr record of dust activity in the Southern Hemisphere. Major efforts have been dedicated to elucidate the origin of this material in order to gain greater insight into the atmospheric dust cycle. On the basis of Pb isotopes in Antarctic dust samples and potential sources, this contribution demonstrates for the first time that Patagonia is the main contributor of dust to Antarctica during interglacial periods as well as glacials, although the potential importance of Tierra del Fuego remains unclear because of its geochemical similarities to Patagonia. An important new finding is that the Puna-Altiplano sector of the continent is a second important dust source to eastern Antarctica during both glacials and interglacials, being more prominent during interglacials. The data indicate South America is the primary dust source to Antarctica during both glacials and interglacials.

Lithogeochemistry of the meta-igneous units from Arroio Grande Ophiolitic Complex, southernmost Brazil

Ophiolites are defined as slices of genetically-related upper mantle serpentinized peridotites and oceanic crustal rocks, tectonically displaced from its primary igneous origin of formation by plate convergence and associated (meta) sedimentary rocks of marine origin. From this premise, a meta-ultramafic-mafic-sedimentary complex (Cr-rich magnesian schists - upper mantle or crustal ultramafic cumulate candidates; epidote amphibolites, metadiorites and metagabbros - oceanic crust candidates; metasedimentary schists, quartzites and marbles - marine sedimentary rocks candidates), located in southeastern Dom Feliciano Belt (southernmost Brazil), started to be interpreted as possible slices of an ophiolitic complex related to the closure of a paleo-ocean during Brasiliano/Pan-African orogenic cycle and was called Arroio Grande Ophiolitic Complex. The present research fills the lack of geochemical data from previous studies and tests the hypothesis of an oceanic setting for the meta-igneous units of this complex from a lithogeochemistry point of view. The meta-ultramafics were interpreted as peridotites (mantle or crustal cumulates) that were subsequently serpentinized (probably in the ocean floor) and posteriorly metasomatized (probably in a continental setting). The meta-mafics were interpreted as oceanic gabbros/basalts formed in a back-arc basin. The results, together with field relationships, rock associations and petrographic evidences, support an oceanic origin for the protoliths of the meta-igneous units. The hypothesis that these rocks represent metamorphosed slices of an ophiolitic complex is still the most reasonable one. This work updates the geologic knowledge of the area and supports discussions about the evolution of Dom Feliciano Belt and Western Gondwana paleocontinent.

Alojamento do granito Lavras e a mineralização aurífera durante evolução de centro vulcano-plutônico pós-colisional, oeste do Escudo Sul-riograndense: dados geofísicos e estruturais

Based on the integration of original and available geological, geophysical and structural data, we assess the construction of the Lavras granite in an evolving volcanoplutonic center. This center encompasses the Lavras do Sul intrusive complex and the Hilario trachyandesitic sequence (604 - 590 Ma), both formed in the western foreland setting during the post-collisional period of the Dom Feliciano Orogeny, Sul-riograndense Shield. Brittle structures and magnetic lineaments indicate the volcanic activity starting near the orogenic collapse, since it was initially controlled by NW-SE and WNW-ESE-striking dextral transtensive systems that inverted to sinistral during tectonic relaxation. The intrusive complex was formed from the north to the south, with emplacement of the Tapera monzonite and the Lavras granite respectively, following slip change along the N70-75°W fault zone cutting it across. The granite has moderate dimensions (325 km³) and a tabular shape (length - L: thickness - T ≈ 3:1) tapering to the south, as deduced from gravity data. AMS petrofabric (anisotropy of magnetic susceptibility) reinforces two compositional-structural domains in the center and in the border of this granite body, represented by the magnesian terms (granodiorite and monzogranite) and the ferroan ones (syenogranite and alkali feldspar granite) respectively. These data together with brittle structures point to a multistage pluton constructed in two resurgent episodes involving: (1) the central laccolith through the emplacement of granodiorite beneath the monzogranite sill; and (2) the annular intrusions of ferroan granites, induced by expansion of the high-level reservoir due to recharge with lamprophyric mafic-ultramafic magmas. Its structural control, spatial distribution and association with lamprophyric dykes attest the link of gold mineralization with the last resurgent episode in a mature volcanoplutonic center.

Petrologia do Granito Chasqueiro, região de Arroio Grande, sudeste do Escudo Sul-Rio-Grandense

Chasqueiro Granite comprises an 400 km2 N50oE- trend elongated body located at the southernmost portion of the eastern Sul-Rio-Grandense Shield. It consists of monzo to syenogranite, light gray and leucocratic porphyritic texture with megacrystals K-feldspar in a coarse equigranular hypidiomorphic matrix, composed of quartz, K-feldspar, plagioclase, biotite, hornblende, opaque minerals and accessories. The modal megacrystals reach proportions between 30.0 and 60.0%, and range from 4 to 8 cm, with mafic minerals of the matrix showing a subvertical magmatic foliation and a tectonic foliation prominent in the borders marked by intense ductile deformation and protomylonite generation. Another important aspect of the Granite Chasqueiro is the obiquitous presence of microgranular mafic enclaves of dioritic composition in the granite, having different shapes and sizes. The Chasqueiro Granite is high-K calc alkaline, metaluminous to slightly peraluminous and characterized by a narrow SiO2 variation, with characteristic signature of granites generated in post-collisional tectonic setting. Geochronological analyzes obtained by the U-Pb method (LA-ICP-MS) and isotope geochemistry of Lu-Hf zircon data (LA-ICP-MS) presenting, respectively, crystallization age 574 ± 3 Ma and negative eHf, which suggest a granite relationship with deformational event D2 and supporting a magmatic crustal sources with subordinate mantle component.

Pb isotope geochemistry and reappraisal of Sr-Nd isotopes of the Cerro Morado basic magmatism (Ischigualasto-Villa Union Triassic basin, NW Argentina): Implications for the mantle sources

Manuscript ID: 20170106. Received in: 09/08/2017. Approved in: 12/10/2017. ABSTRACT: Sr-Nd-Pb isotopic signatures were determined for silica-undersaturated hypabyssal alkaline basic rocks of the Los Baldecitos Formation in the Cerro Morado area, situated in the Ischigualasto — Vale de la Luna Provincial Park (northernmost San Juan Province, Argentina). The basic rocks show slight variations of the Sr-Nd-Pb isotopes and similar behavior of whole-rock geochemistry, which suggest a single source for the Cerro Morado volcanic rocks. The present-day Pb isotopic data show moderately radiogenic Pb compositions (206Pb/204Pb = 18.31 – 18.35), in addition to low values of 87Sr/86Sr initial ratios of 0.70314 – 0.70386 and highly radiogenic Nd initial isotopic compositions (+7.1 < εNd (228 Ma) < +9.3), which point to a PREMA mantle reservoir signature for the alkaline basalts with little evidence of crustal contamination. The involvement of a HIMU component in the source, previously suggested for the Cerro Morado volcanics, should be disregarded in the light of the Pb isotopic signature. The Sr-Nd signature together with Nd-TDM model ages not older than 295 Ma does not support the involvement of an ancient crust.

Estudo da suscetibilidade à inundação com base em análise geomorfológica, bacia hidrográfica do Arroio Santa Isabel, região costeira do Rio Grande do Sul, Brasil

Floods cause damage worldwide, affecting mainly the population who lives in areas near the rivers. Considering the diversity of flood conditions, this study aims to understand the interrelationship between geomorphological, climatological, and anthropic factors to map and classify flood susceptibility in the Arroio Santa Isabel basin, a tributary of the Camaqua River, south of Rio Grande do Sul, a region which is frequently stricken by floods. The analysis of geological, geomorphological, hypsometric, clinographic, vegetation and land use maps was based on historical series of rainfall data (46 years) and on images related to periods of flooding captured by the Landsat 5 sensor, making it possible to identify five levels of susceptibility to flooding in this basin. The determining factors for this classification were: amplitude of the relief, slope of the floodplain and its immediate surroundings, and direct and indirect anthropogenic interventions in this lowland, with emphasis on removal of vegetation and changes in the channels (dam construction). The influence exerted by the Camaqua River on the downstream part of the basin was also considered. In this basin, 75% of the fluvial system has moderate to very high susceptibility to flooding. The methodology uses an integrated approach of easy-access data and was successful in analyzing flooding, providing a foundation for territorial ordering.