Gamma-spectrometric Surveys in Differentiated Granites. II: the Joaquim Murtinho Granite in the Cunhaporanga Granitic Complex, Paraná, SE Brazil

Mapeamento detalhado na região de Joaquim Murtinho, no Complexo Granítico Cunhaporanga (CGC), Neoproterozoico, no estado do Paraná, SE do Brasil, permitiu redefinir o Granito Joaquim Murtinho (GJM), uma intrusão tardia no CGC, composta inteiramente por alasquitos, álcali-feldspato leucogranitos, com área de exposição em torno de 10 km². A unidade está em contato tectônico para W com o Grupo Castro, Neoproterozoico a Eocambriano, com rochas sedimentares e vulcânicas, e é intrusivo em unidades menos evoluídas do CGC, mais para E. Petrograficamente, o GJM é composto por mesopertita e quartzo, com micas alteradas subordinadas e minerais acessórios, principalmente zircão; os granitos constituintes, equi a inequigranulares, estão em geral deformados (com texturas cataclásticas), frequentemente brechados, e podem mostrar estruturas miarolíticas. Atividades hidrotermal e deutérica são geradoras de minerais tardios como albita, sericita, carbonatos e hematita. Um estudo gamaespectrométrico terrestre (canais de contagem total, K, eU, eTh; 231 estações) coletou dados para a construção de vários mapas (CT-Ueq, K%, eU-ppm, eTh-ppm, e derivados), o GJM mostrando anomalias nos respectivos canais de CT e dos elementos K, eTh e eU (menos evidentes em vários mapas derivados), em comparação com outras unidades vizinhas do CGC, por estar enriquecido nestes três elementos. O comportamento geoquímico do K, do U e do Th é usado como base para avaliação dos mapas. Forte intemperismo (clima subtropical, temperaturas médias moderadas a elevadas, fortes precipitações) afeta em especial os feldspatos e biotita, lixiviando deles o K, e pode desestabilizar também fases acessórias portadoras de U e Th. O Th, pouco solúvel como cátion, deve acumular-se no solo residual retido em minerais restíticos, enquanto parte do U pode migrar (em ambiente oxidante como íon uranila). O feldspato potássico, em particular, converte-se nesta situação completamente em argilominerais sem K (caulinita). Métodos gamaespectrométricos são claramente eficientes para identificar fácies em rochas granitoides, em especial quando aplicados ao estudo daqueles enriquecidos em K, Th e U.Detailed mapping at the NW corner of the large Neoproterozoic Cunhaporanga Granitic Complex (CGC), Paraná state, SE Brazil, redefined the Joaquim Murtinho Granite (JMG), a late intrusion in CGC with an exposed area of about 10 km², made up mainly by evolved"alaskites" (alkali-feldspar leucogranites). This unit is in tectonic contact with the Neoproterozoic-Eocambrian volcano-sedimentary Castro Group, to the W, and is intrusive into other less evolved granitic units of the CGC to the E. Petrographically, JMG shows mainly mesoperthite and quartz, with subordinate amounts of altered micas and some accessory phases, mainly zircon. The equi to inequigranular granites are usually deformed with cataclastic textures, are often brecciated, and may have miarolitic structures. Formation of late albite, sericite, carbonate and hematite was caused by deuteric and hydrothermal alteration. A gamma-ray spectrometric survey at 231 stations which measured total counts (TC), Ueq K%, eU ppm and eTh ppm was used to construct several direct and derived maps. Compared to neighboring units the JMG has significant anomalies, especially in the TC, %K, eTh and eU maps, although the differences are less obvious in some derived maps. These evolved granites are enriched in these three elements. Geochemical behavior of K, Th and U is used to analyse the results observed in maps. Enhanced weathering under a subtropical climate with moderate to high average temperatures and heavy rainfall affects mainly feldspars and biotite, and may also destabilize most U and Th-bearing accessory phases. Th is most likely retained in restite minerals in soils, being relatively immobile, while part of U may migrate as uranyl ion in oxidizing media. K is especially affected by feldspar alteration to K-free clays (mainly kaolinite), and may be completely leached. Gamma-ray spectrometric methods are valid tools to study facies in granitic rocks, especially in those that are enriched in K, Th and U

Levantamentos Gamaespectrométricos em Granitos Diferenciados. I: Revisão da Metodologia e do Comportamento Geoquímico dos Elementos K, Th e U

Este trabalho é parte do projeto de estudo detalhado do Complexo Granítico Cunhaporanga (CGC), neoproterozoico, aflorante na região do Arco de Ponta Grossa (estado do Paraná, SE do Brazil). Inicialmente, foram utilizados os arquivos dos dados gamaespectrométricos do Projeto Aerogeofísico Serra do Mar Sul, da década de 70, em levantamento realizado para a CPRM. A seguir, foi realizado levantamento gamaespetrométrico terrestre, concentrado nas áreas do Granito Joaquim Murtinho (GJM), à NW, e do Granito Serra do Carambeí, à SW. Os estudos geofísicos sobre o GJM são apresentados como duas contribuições: nesta primeira, são discutidas a metodologia e a representação dos dados gamaespectrométricos em mapas color-scale, indicando que as respostas em granitos dependem fortemente de um fator climático, devido à mobilidade do K durante intemperismo em ambientes de clima subtropical com fortes chuvas, fator que também controla a migração mais acentuada do U. São citados os minerais portadores de U e Th, documentados em granitoides, e inventariados os processos que controlam a mobilidade de K, U e Th em solos. Nestes climas, sinais fortes do K sobre granitoides indicam preservação de rocha fresca e/ou alteração hidrotermal, enquanto que sinais diminuídos ou nulos evidenciam sua lixiviação por intemperismo. U e Th devem ficar retidos no solo residual, em minerais resistentes, com possibilidade de enriquecimento seletivo, com possível transporte coluvial para níveis topográficos inferiores. A solubilidade maior de U (como íon uranila) facilita a sua liberação, com migração, limitada pela retenção total ou parcial em fases minerais e orgânicas pedogênicas. O Th deve ficar retido na sua quase totalidade nas fases de resistatos e, quando liberado, incorporado nas substâncias pedogênicas orgânicas e inorgânicasThis contribution is part of a research project on the Neoproterozoic Cunhaporanga Granitic Complex (CGC), cropping out in the Ponta Grossa Arch (Paraná state, SE Brazil). An initial study used the gamma-spectrometric data of the Serra do Mar Sul Aerogeophysical Project, performed during the 70s for CPRM. Later, terrestrial gamma-spectrometric surveys focused on the study of the differentiated Joaquim Murtinho Granite (JMG) in the NW corner of CGC, and the Serra do Carambeí Granite, to the SW. In this paper, the results obtained for JMG are presented in two parts. The first deals with methodology and the presentation of several gamma-spectrometric color-scale maps, indicating that results obtained in granites depend strongly on a climatic factor, given the mobility of K during weathering in subtropical climates with strong rainfalls, also favoring a greater mobility of U. Minerals that are U and Th hosts, documented in granites, are reviewed, together with the weathering processes that control the mobility of K, U and Th in soils. Strong K signals in granitic areas submitted to these climates document the presence of fresh rock and/or effects of hydrothermal alteration, while weak or nil signals are evidence of strong leaching of K during weathering. U and Th will be retained in the residual soils, in part leading to their selective enrichment, also coupled with soil migration to lower topographic levels by colluvial transport. The larger solubility of U (as uranyl ion) allows its liberation under oxidizing conditions, and its migration, limited by the possibility of absorption in newly formed mineral and organic soil phases. Th should be retained almost totally in resistant phases and, when liberated in solution, will mostly be fixed in organic and inorganic soil substance

Estudo geofísico da transição continente-oceano e das anomalias-J na parte distal do sistema rifte ibéria-newfoundland : paleorreconstrução e implicações para a evolução tectônica

Orientador: Prof. Dr. Francisco José Fonseca FerreiraCoorientadores: Prof. Dr. Gianreto Manatschal (UNISTRA) e Profa. Dra. Monica Pereira da Costa Lavalle Heilbron (UERJ)Tese (doutorado) - Universidade Federal do Paraná, Setor de Ciências da Terra, Programa de Pós-Graduação em Geologia. Defesa : Curitiba, 05/03/2021Inclui referências: p. 97-108Resumo: O estudo da região oceânica distal apresenta a dificuldade do acesso à dados de alta resolução espacial. Dados de poços e sísmicos estão disponíveis, mas são espacialmente limitados em comparação com a dimensão da área a ser estudada. Por isso, os dados magnéticos são muito utilizados na interpretação geológica, por exemplo no mapeamento estrutural, para estudos de eventos magmáticos e paleorreconstrução. Para a transição continente-oceano pobre em magma, a magnetometria aparentemente tem vantagem sobre a gravimetria no delineamento de estruturas e de crosta, pois é presumível que não haja contraste lateral de densidade suficiente nos litotipos (peridotito versus rochas magmáticas oceânicas). No sistema de rifteamento pobre em magma Ibéria-Newfoundland, as Anomalias-Js surgem como sinal inquestionável de processos magmáticos distais. No entanto, estas anomalias são pós-rifte, e não coincidem com o início da crosta oceânica. Com a limitada discussão na literatura sobre a natureza e origem da Anomalia-J, suas fontes permanecem enigmáticas. A hipótese deste trabalho é a de que a fábrica preexistente do manto condicionou grande parte da geometria e padrão de anomalias magnéticas distais da margem pobre em magma, incluindo as Anomalias- J. Com isso, o objetivo da investigação é interpretar possíveis heranças tectônicas do manto superior e da crosta distal, relacionando-as com o conhecimento existente. Para tal, a metodologia utilizada foi a filtragem de dados magnéticos (amplitude do sinal analítico e integral vertical), identificação do padrão de lineamentos e a paleorreconstrução com base em dados magnéticos. Como resultado, tais mapas revelaram estruturas retilíneas na porção distal a ultra-distal das margens, e possíveis limites bem marcados para as Anomalias-J. A reconstrução paleogeográfica demonstrou a afinidade dos lineamentos magnéticos com a rede de fraturamento do embasamento pré-rifte. O filtro VIAS exprime a presença de fontes profundas magnetizadas na porção oceânica ibérica, o que pode ser interpretado como intrusões gabróicas no peridotito. As principais conclusões oriundas destas observações são: 1) que o manto em exumação encerra uma trama pré-existente, possivelmente herdada de ciclos orogênicos anteriores; 2) que essa litosfera anisotrópica parece ter exercido um controle estrutural sobre o manto em exumação e influenciado a migração e trapeamento de magma; 3) que as fontes das Anomalias-J estão alojadas no manto; 4) que a orientação principal da Anomalia-J reconstruída segue a trama regional NE-SW do embasamento pré-rifte. Com isso, a interpretação dos dados magnéticos corrobora a hipótese de uma fábrica litosférica preexistente que exerceu controle estrutural no padrão de alojamento do magma. Palavras-chave: Magnetometria. Herança do embasamento. Margem pobre em magma. Terra Sólida.Abstract: Distal oceanic regions have a lack of data with high spatial resolution. Drill and seismic data are available, but they are spatially limited given the broad studied area. Therefore, magnetic data are extensively used on geological interpretations, as for structural mapping, magmatic processes and plate restorations. For ocean-continent transition, magnetometry works better than gravity method for outlining structures and crustal features, since the presumable low or absent density contrast in the lithotypes (peridotite versus magmatic rocks). Regarding the Iberia-Newfoundland magma-poor system, J-anomalies appear as a result of unquestionable evidence of magma addition on both margins. However, J-anomalies come after their hosting basement, and also it did not match the landward limit of the oceanic crust. Nature and origin of J-Anomalies are little debated on literature, and then their sources remain enigmatic. The hypothesis is that magma-poor geological context and a preexistent mantle fabric ruled most of geometries and textural pattern on magnetic data, including J-anomaly. Therefore, this research aims to interpret possible tectonic inheritance within distal crust and upper mantle, in line with previous literature. For this purpose, methods include filtering of the magnetic data (analytic signal and vertical integration), mapping magnetic lineaments and paleo-reconstruction. As a result, filtered magnetic maps indicate linear structures along the distal margins, and possible limits for J-anomaly. Paleo-reconstructions revealed similarities between magnetic lineaments and the pre-rift tectonic framework. VIAS filter reveals the existence of deep mantle magnetic sources on the Iberian oceanic area, which can be interpreted as gabbroic intrusions on peridotite. The main conclusions are: 1) the exhuming mantle has a preexistent fabric, possibly inherited from previous orogenic cycles; 2) this anisotropic lithosphere may be ruled some structural features within the exhuming mantle, conditioning migration and entrapment of magma; 3) Janomaly has mantle sources; 4) the main orientation of the reconstructed J-anomaly matches the NE-SW pre-rift basement fabric. Therefore, magnetic data interpretation corroborates the hypothesis of a preexistent lithospheric fabric controlling the hosted magmatic pattern. Key-words: Magnetometry. Basement inheritance. Solid Earth. Magma-poor margin

EVIDENCE OF MANTLE INHERITANCE ON THE ULTRA-DISTAL WESTERN IBERIAN MARGIN FROM TRANSFORMED TOTAL MAGNETIC ANOMALY

International audienc

Architectural elements, geometry, and magmatism of the Sub-Saharan Western African passive margin basins: Seismic and gravity data integration

International audienceThe Western African Sub-Saharan continental margin includes a large region from Cameroon to South Africa comprising distinct margin types - magma-rich, magma-poor and transform margins. The integrated study of geological, seismic and potential field data allows the mapping of a structural framework subdivided in proximal, necking, distal, outer and oceanic domains, characterized by a set of comparable architectural elements and magmatism. These key domains display a strong and consistent continuity along the continental margin. Longitudinal transitional regions occur in a gradual way, but interactions between structural domains along transversal section may also occur, displaying spatial and temporal overprints. Because of this complexity, the outer domain that is the key feature in the magma-rich margin, tends to disappear northwards where the distal and proximal domains of the magma-poor segments become the most important domains of the margin. In the north of Gabon Basin, it is observed both the reappearance of the outer domain and the reworking of the distal domain by fracture zones. The basement of the distal domain within the magma-poor margin is interpreted as made by a hyperextended continental crust but with magmatic additions. Finally, the new proposed continent-ocean boundary (COB) in the region of the Namibe Basin (transitional zone between magma-rich and magma-poor margins) achieves a best fit in a reconstruction model with the limits recognized at the South America side. The results will imply in a drastic reduction on the gap proposed by other recent reconstruction models and also minimize the intraplate deformation required by some other models

Gamma-spectrometric Surveys in Differentiated Granites. I: a Review of the Method, and of the Geochemical Behavior of K, Th and U

This contribution is part of a research project on the Neoproterozoic Cunhaporanga Granitic Complex (CGC), cropping out in thePonta Grossa Arch (Paraná state, SE Brazil). An initial study used the gamma-spectrometric data of the Serra do Mar Sul AerogeophysicalProject, performed during the 70’s for CPRM. Later, terrestrial gamma-spectrometric surveys focused on the study of the differentiatedJoaquim Murtinho Granite (JMG) in the NW corner of CGC, and the Serra do Carambeí Granite, to the SW. In this paper,the results obtained for JMG are presented in two parts. The fi rst deals with methodology and the presentation of several gamma-spectrometric“color-scale” maps, indicating that results obtained in granites depend strongly on a climatic factor, given the mobility of Kduring weathering in subtropical climates with strong rainfalls, also favoring a greater mobility of U. Minerals that are U and Th hosts,documented in granites, are reviewed, together with the weathering processes that control the mobility of K, U and Th in soils. StrongK signals in granitic areas submitted to these climates document the presence of fresh rock and/or effects of hydrothermal alteration,while weak or nil signals are evidence of strong leaching of K during weathering. U and Th will be retained in the residual soils, in partleading to their selective enrichment, also coupled with soil migration to lower topographic levels by colluvial transport. The larger solubilityof U (as uranyl ion) allows its liberation under oxidizing conditions, and its migration, limited by the possibility of absorptionin newly formed mineral and organic soil phases. Th should be retained almost totally in resistant phases and, when liberated in solution,will mostly be fi xed in organic and inorganic soil substances

Geophysical evidence for lithospheric scale asymmetry and inherited mantle in the SE Brazilian-Angola and Newfoundland-Iberia rifted margins

International audienceRift systems in the central South Atlantic and the southern Northern Atlantic margins exhibit anomalous wide ocean-continent transitions, extreme thinning of the continental crust, and scarce volcanism indicative of low magmatic budgets during rifting and breakup, characteristic of magma-poor rifted margins. While numerous studies investigated the crustal architecture at these magma-poor rifted margins, the nature of the underlying mantle lithosphere remains virtually unknown. In this work, we have interpreted seismic tomography and electromagnetic data with the aim of comparing large-scale aspects and mantle geophysical anomalies at these conjugate rifted margins. The data exhibits low electrical conductivity and high P-wave velocity in the mantle underlying the Iberia and the Angolan margins, while the Newfoundland and SE-Brazil margins show a lack of high-velocity mantle and exhibit higher electrical conductivity. The P-wave tomography values at middle lithospheric depths are suggestive of an oceanward lateral continuity of rigid lithospheric mantle at both the Iberian and Angola margins that may correspond to an anomalous mantle. Rifting-induced delamination at the onset of Atlantic opening is supported by previous geochemical studies. In addition, low electrical conductivity areas may indicate dry mantle conditions, which can be caused by inherited subcontinental mantle, and/or pre-to syn-rift magmatic extraction. A lack of high-velocity and high electrical conductivity in the mantle under the Newfoundland and Santos-Campos margins suggest a relatively weaker and newly formed mantle, in agreement with the removing and/or initial absence of a strong middle-to-lower inherited lithosphere beneath those margins. Therefore, the geophysical observations support different types of the nature of the middle mantle lithospheres flooring the two conjugate, magma-poor rifted margins