Maria Helena B.M. Hollanda

Fluoride in the Serra Geral Aquifer System: Source Evaluation Using Stable Isotopes and Principal Component Analysis

Groundwater with anomalous fluoride content and water mixture patterns were studied in the fractured Serra Geral Aquifer System, a basaltic to rhyolitic geological unit, using a principal component analysis interpretation of groundwater chemical data from 309 deep wells distributed in the Rio Grande do Sul State, Southern Brazil. A four-component model that explains 81% of the total variance in the Principal Component Analysis is suggested. Six hydrochemical groups were identified. δ18O and δ2H were analyzed in 28 Serra Geral Aquifer System samples in order to identify stable isotopes patterns and make comparisons with data from the Guarani Aquifer System and meteoric waters. The results demonstrated a complex water mixture between the Serra Geral Aquifer System and the Guarani Aquifer System, with meteoric recharge and ascending water infiltration through an intensive tectonic fracturing.

Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview

Atlantic margin in the northeast Brazil (South America) is characterized by a modest magmatic activity compared with the Southern Atlantic margin, where the ParanáEtendeka Province is accounted for eruption of millions of cubic kilometers of mafic and acid magmas (flow basalts, dyke swarms). The most significant magmatic activity in the Equatorial Atlantic margin is the Rio Ceará Mirim Dyke Swarm (CMDS), which marks the early stages of rifting in the Cretaceous. The CMDS consists of an arcuate, nearly 800 km-long mafic dykes that crosscut rocks and structures of the Precambrian basement. The swarm can be divided into two laterally continuous segments, one of E-W strike that is parallel to the southern border of the Mesozoic Potiguar basin, and one barely studied NE-SW segment which, after recent high-resolution aeromagnetic anomalies, can be traced for 300 km along the east border of the Paleozoic Parnaíba basin up to the northern boundary of the São Francisco craton. The E-W-trending segment encompasses essentially tholeiitic basalts including plagioclase, clinopyroxene (±olivine), Fe-Ti oxides and pigeonite in their groundmass. The dykes show oneto 150-meters in width and of up to one kilometer in length; a few dykes exceeding 100 kilometers have been inferred from the aeromagnetic anomalies. The tholeiites have been subdivided into three groups: high-Ti olivine tholeiites, evolved high-Ti tholeiites (TiO2≥1.5 wt.%; Ti/Y >360), and low-Ti tholeiites (TiO2≤1.5 wt%; Ti/Y≤360), with all exhibiting distinct degrees of enrichment in incompatible elements relative to Primitive Mantle. Negative Pb anomalies are found in all three groups, while Nb-Ta abundances similar to those of OIB-type magmas are found in the olivine tholeiites, with moderate to high depletions being observed, respectively, in the evolved high-Ti and low-Ti tholeiites. The latter exhibit some contamination with crustal (felsic) materials. The initial isotopic compositions of the olivine tholeiites show uniform and unradiogenic 87Sr/86Sr (~0.7035–0.7039) combined with (in part) radiogenic 143Nd/144Nd and 206Pb/204Pb (>19.1) ratios, which together reveal a contribution of FOZO (FOcalZOne) component in their genesis. The other tholeiite groups show quite variable Sr-Nd initial ratios with relatively consistent 206Pb/204Pb ratios clustering toward an isotopically enriched mantle (EM1) component. Taken in conjunction with the Nb-Ta anomalies, this enriched signature reflects the involvement of a subduction-modified lithospheric mantle in the source of the evolved high-Ti and low-Ti tholeiites. Hence, FOZO and EMI components might to be coexisted and contributed in varying extents to the generation of the CMDS primary melts. Plagioclase dating of one evolved high-Ti tholeiite dyke provided two plateau ages of 127.1 ± 0.2 Ma and 128.2 ± 1.3 Ma, with an integrated mean age of 127.7 ± 0.1 Ma. Plagioclase multigrain fractions from one low-Ti tholeiite dyke provided two plateau ages of 131.6 ±0.7 Ma and 131.0 ± 0.4 Ma, with mean age of 131.2 ± 0.1 Ma. These 40Ar/39Ar ages clearly reveal that the low-Ti and high-Ti magmas encompassing the E-W segment of the CMDS were emplaced as two pulses during the Early Cretaceous. AMS investigations of the E-W-trending segment evidenced (at least) two main feeder zones located at the intersection of the dykes with Cenozoic N-trending volcanic centers referred in NE Brazil as the Macau magmatism. Such feeder zones were characterized by vertical magnetic fabrics (via steep-plunging magnetic Maria Helena B. M. HOLLANDA, Carlos J. ARCHANJO, Paul R. RENNE, Donald E. NGONGE, David L. CASTRO, Diógenes C. OLIVEIRA and Antomat A. MACÊDO FILHO, 2016. Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview. Acta Geologica Sinica (English Edition), 90(supp. 1): 109-110.

Geologia e caracterização química do magmatismo peralcalino ultrapotássico do enxame de diques Manaíra-Princesa Isabel, Província Borborema

The Manaira-Princesa Isabel dike swarm forms one of the most expressive examples of Neoproterozoic (c. 600 Ma) peralkaline magmatism in the Borborema Province (NE Brazil). It consists of about a hundred NE-trending bodies intrusive in older, Neoproterozoic porphyritic granites (Princesa Isabel and Tavares plutons), and orthogneisses and low-grade metasediments of the Eo-neoproterozoic Riacho Gravata complex. The dike swarm includes mostly silica-saturated syenites, with potassic to ultrapotassic, peralkaline affinity, containing microcline and sodic amphibole ± pyroxene as the main mineral assemblage. Amphibole is dominantly Mg-riebeckite (Manaira, Princesa Isabel and Tavares sub-swarms), whereas pyroxene is mainly aegirine-augite (Manaira and Tavares sub-swarms). A minor set of dikes from this swarm is slightly metaluminous with Mg-biotite as the major mafic phase. Geochemical and isotopic signatures indicate strong enrichment in incompatible elements (Rb, Ba, K, Th, U), in association with a negative Nb anomaly, and strongly radiogenic 87Sr/86Sr and 143Nd/144Nd (= negative eNd values) initial ratios. These features suggest a common source, which was probably an enriched lithospheric mantle reservoir, chemically modified by an ancient subduction component inferred to be Paleoproterozoic from T DM model ages. Conversely, Pb isotopic ratios lower than average crustal values indicate the influence of a non-radiogenic component interacting with the enriched mantle source. A SHRIMP U-Pb age of c. 600 Ma obtained for the Manaira-Princesa Isabel dike swarm defines an important crustal exhumation period which was relatively synchronous with intensive tectonomagmatic activity related to the Brasiliano orogeny, which occurred in distinct structural domains within the Transverse Zone of the Borborema Province.

Petrografia, geoquímica e geocronologia dos granitos patrimônio Santo Antônio e São Domingos (Suíte Cunhaporanga, Paraná, Sudeste do Brasil)

Os Granitos Patrimonio Santo Antonio e Sao Domingos localizam-se na porcao noroeste da Faixa Itaiacoca constituinte do Terreno (ou dominio tectonico) Apiai segmento do Cinturao Ribeira Meridional da Provincia Mantiqueira localizada no sudoeste do Brasil. Ambos estao relacionados com a Suite Granitica Cunhaporanga e sao intrusivos em rochas metassedimentares do Grupo Itaiacoca. Sao constituidos por ocorrencias esparsas de sienogranitos e majoritariamente por monzogranitos porfiriticos de carater metaluminoso a peraluminoso, alcali-calcico de alto K e comportamento ferroso a magnesiano. O comportamento dos elementos tracos, por vezes afetado por alteracao hidrotermal, e sugestivo de magmatismo pos a tardi-colisional, com assinaturas geoquimicas que sugerem origem a partir de manto enriquecido pela adicao de elementos incompativeis remobilizados de crosta oceânica durante subduccao. A idade geocronologica obtida para o Granito Patrimonio Santo Antonio (589 ± 6 Ma, zircoes, U-Pb, SHRIMP) situa o magmatismo destes plutons entre os estagios finais de evolucao da Suite Granitica Cunhaporanga (ca. 650 a 590 Ma) e da colocacao dos plutons pos-orogenicos e anorogenicos do Dominio Apiai (ca. 590 a 570 Ma).