The compositional evolution of the phyllosilicates in the lateritic profile of the Catalão I Ultramafic Alkaline-carbonatitic Complex (GO)

A evolução composicional dos filossilicatos no perfil laterítico do complexo ultramáfico alcalino-carbonatítico de Catalão I (GO) é proposta neste trabalho por meio de estudos petrográficos, de difratometria de raios X e química mineral (WDS) por microssonda eletrônica. Os minerais micáceos encontrados no nível rocha sã são classificados como flogopita (associada a diferentes paragêneses) e tetraferriflogopita (produto de metassomatismo). Nos níveis Rocha Alterada e Saprólito Isoalterítico, os filossilicatos derivados de micas são vermiculita e interestratificado regular flogopita-vermiculita (formação e características similares aos da hidrobiotita), caracterizando um processo principal de vermiculitização neste perfil. Subordinadamente, em porções superiores e pontuais do nível isoalterítico, a alteração da flogopita vermiculitizada gera produtos esmectíticos, caracterizados principalmente pela ocorrência de interestratificado regular expansível e mesmo pela ocorrência de esmectita. No topo do nível saprolítico e no nível aloterítico, há ocorrência de caolinita. A sequência evolutiva dos filossilicatos neste complexo apresenta a seguinte ordem de formação: flogopita, interestratificado flogopita-vermiculita, vermiculita, produtos esmectitizados (interestratificados expansíveis de ocorrência pontual), esmectita e caolinita, sendo os dois últimos provavelmente associados à contribuição supérgena. Tal sequência é compatível com a das proposições mais recentes para a formação de argilominerais em solos. As principais diferenças entre as flogopitas e seus produtos da alteração vermiculítica dizem respeito aos teores de K+ que diminuem significativamente, e à oxidação do Fe2+, mudanças estas próprias do processo de vermiculitização. No caso específico das vermiculitas, a posição interfoliar é preenchida predominantemente por Mg2+. Além disso, a presença de minerais interestratificados (produtos intermediários de alteração) é indicativa principalmente de perfis pouco alterados e relativamente recentes.The compositional evolution of the phyllosilicates in the lateritic profile of the Catalão I ultramafic alkaline-carbonatitic complex (GO) is proposed in this work based on petrographic and X-ray diffraction analysis, as well as, chemical composition obtained by WDS electron microprobe. The micaceous minerals found in unweathered rocks are classified as phlogopites (with different parageneses) and tetraferriphlogopites (metasomatic products). In the levels of altered rocks and isalteritic saprolite, the phyllosilicates derived from micas are vermiculite and regular interstratified phlogopite-vermiculite (which formation and characteristics are similar to hydrobiotite), characterizing a main vermiculitization process in this profile. Subordinately, in the upper portions and punctual areas of isalteritic saprolite, the alteration of the vermiculitized phlogopite generates smectitic products, which are characterized mainly by the occurrence of expansible regular interstratified mineral and even smectite. Kaolinite occurs on the top of the isalteritic saprolite and on the alloteritic level. The evolution sequence of the phyllosilicates in the weathering profile of the Catalão I complex presents the following formation order: phlogopite, interstratified phlogopite-vermiculite, vermiculite, smectitic products (expansible interstratified minerals of local occurrence), smectite and kaolinite, of which, the last two are probably associated to supergene processes. This sequence is compatible with the most recent theories for the formation of clay minerals in soils. The main differences between the phlogopites and their products of vermiculitic alteration are related to a significant decrease of K+ and oxidation of Fe2+, characteristic changes of the vermiculitization process. In the specific case of vermiculites, the interlayer site is filled mainly by Mg2+. In addition to that, the presence of interstratified minerals (intermediate alteration products) is a key indicator of relatively recent and low-altered weathering profiles

Geologia e Geocronologia do Maciço Alcalino Máfico-Ultramáfico Ponte Nova (SP-MG)

The Ponte Nova alkaline mafic-ultramafic massif lies in the northern sector of the Serra do Mar Province, close to the eastern part of the Mantiqueira mountain range, SE Brazil. The massif is exposed in two different areas separated by basement rocks: the larger (~5.5 km²) is elliptical in shape and has a wide variety of rock types; the smaller (~1 km²) is irregular, petrographically less complex, and occurs south of the former. It is mainly composed of an alkaline gabbro association generated by successive magmatic pulses. The more abundant rocks are melagabbros and ultramafic cumulates (e.g., olivine clinopyroxenites and olivine-bearing melagabbros) found in the lower parts of the massif, together with porphyritic, equigranular and banded gabbros and monzogabbros in the upper partions. A magmatic breccia and a suite of dykes (lamprophyres to tephriphonolites) cut the intrusion. K/Ar geochronological data for the different rock types yielded an average Late Cretaceous age of 87.6 Ma which is comparable to the age of other alkaline intrusions present in the northern sector of the referred province. The difference in ages makes it difficult to sustain the hypothesis of a genetical association with the Trindade mantle plumeO maciço alcalino máfico-ultramáfico Ponte Nova, localizado no setor norte da Província Serra do Mar, junto à porção oriental da Serra da Mantiqueira, na região Sudeste do Brasil, encontra-se encaixado em granitoides e gnaisses migmatíticos Pré-Cambrianos. O maciço apresenta duas áreas distintas de exposição, estando ambas separadas por rochas do embasamento: uma maior (~5,5 km²), de forma elíptica e com grande variedade de litotipos; outra menor (~1 km²), de forma irregular e petrograficamente menos complexa, e situada a sul da primeira. É predominantemente composto por uma associação gabroide alcalina, gerada por sucessivos pulsos magmáticos. As rochas mais abundantes são cumulatos máficos e ultramáficos (e.g., olivina clinopiroxenitos e melagabros com olivina) presentes nos níveis inferiores do maciço, ao lado de rochas gábricas e monzogábricas porfiríticas, equigranulares e bandadas nos níveis superiores, todos representativos da evolução magmática dos principais pulsos. Um corpo de brecha magmática e uma suíte de diques (lamprófiros a tefrifonólitos) interceptam todas as rochas intrusivas. Análises geocronológicas por K/Ar para diferentes rochas indicam uma idade média de 87,6 Ma (Cretáceo Superior), que é similar às obtidas para as demais ocorrências do setor norte da referida província. A incompatibilidade das idades torna difícil sustentar a hipótese de sua associação genética com a pluma mantélica de Trindad

Cretaceous carbonatites of the southeastern Brazilian Platform: a review

ABSTRACT: This paper reviews general aspects of alkaline-carbonatitic rocks of Brazilian, Paraguayan and Bolivian terrains. Although 30 such occurrences are known in literature, only the major ones have been thoroughly investigated. The carbonatites are of Cretaceous age, with two well-defined Lower Cretaceous and Upper Cretaceous generation episodes. A clear tectonic control by ancient structural features such as archs, lineaments and faults characterizes most cases. The rocks exhibit a large compositional variation, in decreasing orders of abundance from calciocarbonatites to magnesiocarbonatites to ferrocarbonatites. In some complexes, they form multistage intrusions. C-O isotopes indicate that, in general, the carbonatites were affected by post-magmatic processes associated with the topographic level of emplacement and low-temperature H 2 O-CO 2 rich fluids responsible for the increased amount of heavy carbon and oxygen. Sr-Nd isotopic compositions similar to those of coeval alkaline silicate rocks, ranging from depleted to enriched mantle sources, have been influenced by two distinct metasomatic events in Proterozoic at 2,0-1.4 Ga and 1.0-0.5 Ga. Sr-Nd-Pb-Os data seem related to an isotopically enriched source, their chemical heterogeneities reflecting a depleted mantle that was metasomatized by small-volume melts and by fluids rich in incompatible elements. Fractional crystallization and liquid immiscibility are believed to be the most effective processes in the formation of the Cretaceous carbonatites, with minor contribution of crustal contamination. Pb isotopic ratios yield evidence that HIMU and EM I mantle components played an important role in the genesis of the carbonatitic magma

Contact relationships between mafic and syenitic alkaline rocks in Jabaquara Beach, northern sector of São Sebastião Island, SP

Contact relationships between syenitic rocks of the Serraria stock and plutonic to hypoabisal mafic rocks of the northern sector of the alkaline suite of São Sebastião Island are described here for the first time, allowing the indication of temporal relationships between different magmatic batches. The outcrops show a hololeucocratic syenitic variety hosting mega-, macro- and microxenoliths from different rocks forming agmatites. Five main rock units were described: alkali feldspar syenites, melasyenites, diabases, mafic cumulates (melagabbro/clinopyroxenite), and heterogeneous gabbros. Geochemically, the bimodal alkaline magmatism found on the island is well represented by the studied rocks. The sequence of magmatic events is as follows: emplacement and crystallization of basic alkaline magma batches in a magmatic chamber, forming a varieties of gabbroic cumulates, mainly olivine-bearing melagabbros and clinopyroxenites, and breccias with gabbroic matrix with evidences of textural reequilibrium by thermal action; new intrusions of alkaline basic magma batches, represented by diabase fragments, which also present textural reequilibrium (predominant granoblastic texture); syenitic magma intrusion in the mafic chamber, which fragmented the already partially or fully crystallized gabbro, carrying a large proportion of xenocrysts and xenoliths, and forming melasyenite; new syenitic magma intrusion, which fragmented the whole system forming an agmatite with megaxenoliths and microxenoliths of cumulative gabbro, diabase, agmatite with gabbroic matrix (heterogeneous gabbros) and melasyenite; pegmatite veins and dykes of quartz syenites crosscutting all lithotypes. All types sampled as xenoliths have some degree of recrystallization and chemical reaction with the final syenitic host magma. Relações de contato entre as rochas sieníticas do stock de Serraria e as rochas máficas plutônicas e hipoabissais do setor norte da suíte alcalina da Ilha de São Sebastião (Ilhabela) são descritas pela primeira vez, permitindo a indicação de relações temporais entre diferentes pulsos magmáticos. Os afloramentos apresentam variedade sienítica hololeucocrática hospedando mega-, macro- e microxenólitos de diferentes rochas, formando agmatitos. Cinco unidades principais são descritas: álcali feldspato sienitos, melassienitos, diabásios, cumulatos máficos (melagabros/clinopiroxenitos) e gabros heterogêneos. Geoquimicamente,o magmatismo alcalino bimodal encontrado na ilha é bem representado pelas rochas dos afloramentos estudados. A sequência de eventos magmáticos consiste em: colocação e cristalização de pulsos de magmas básicos alcalinos, em ambiente de câmara magmática, gerando as variedades gabroicas cumuláticas, principalmente melagabros e clinopiroxenitos com olivina, e brechas de matriz gábrica com evidências de reequilíbrio textural por ação de temperatura; intrusão de novos pulsos de magma básico alcalino, representados pelos fragmentos de diabásios, que apresentam também, reequilíbrio textural (textura granoblástica predominante); intrusão de pulso sienítico na câmara máfica que fragmentou o gabro já parcial ou totalmente cristalizado, transportando ampla proporção de xenocristais e xenólitos, o que gerou rochas classificadas como melassienitos; nova intrusão de pulso sienítico, que fragmenta todo o sistema formando um agmatito com megaxenólitos e microxenólitos de gabro cumuláticos, diabásios, agmatitos de matriz gabroica (gabros heterogêneos) e melassienitos; veios e diques de pegmatito e aplito de sienitos com quartzo cortam todos os litotipos. Todos os tipos amostrados como xenólitos apresentam algum grau de recristalização e de reação química com o magma sienítico final que os hospeda

A Disciplina Geologia Geral - Sistema Terra, Introdutória ao Curso de Geologia da USP: Conteúdo e Reflexão

The discipline General Geology - The Earth System was introduced in the Geology course curriculum with the objective of providing first year students an ample and integrated view of physical, chemical and biological processes that affect Earth's geosphere, and of their complex interactions. Through a familiarization with basic theoretical and field geological concepts the discipline aims at allowing students to be able to interpret geological phenomena, thus stimulating them to enlarge their knowledge of the Geosciences throughout the rest of the curriculum. More general topics treated include a view of the Earth as part of the Solar system, of areas of professional activities of geologists and the important contribution they can make to Earth's environment sustainability. The discipline begins in the first semester with lectures on the evolution of the geological thought, origin of the Universe and the solar system. These are followed by talks on Earth's internal dynamics, in the context of global tectonics, and rock generation processes. Lab classes include recognition of most important mineral and rock types, and use of the Brunton compass. Topics of external dynamics follow, including weathering processes and origin, transport and deposition of sediments. At the end of the semester a series of lectures are given on glacial, eolian, fluvial and marine processes as external dynamic agents. The second semester initiates with lectures on geological time from the paleontological, stratigraphic and geochronological view points. Themain geological structures are then dealt with in connection with the use of the Brunton compass in their three-dimensional reconstruction. Examination of topographic and geological maps serves as a basis for understanding the geological evolution of an area. The final part of the program is devoted to lectures on Earth's geological resources, such as water, fossil fuels and minerals, as well as on the importance of Geology to Engineering. These are complemented by talks by invited, well-known, professional geologists from different specialties. During the two semesters, classroom and laboratory classes are intercalated with short (one day) field activities. A broad familiarization with the geology of State of São Paulo and Brazil completes the class-room activities of the discipline. A first experience with geological mapping is then offered, during a two-day field trip at the end of the semester, for which the presentation of reports is required. This is followed by presentation of seminars by the students on themes treated in classroom. As a general rule the teaching team of ST tries to restrict the use of specialized technical terms as a way to facilitate the transfer of knowledge from one topic to another by the students.A disciplina Geologia Geral - Sistema Terra, foi concebida com o objetivo de proporcionar, a alunos do curso de Geologia, o entendimento amplo e integrado, embora introdutório, dos processos atuantes na geosfera terrestre e suas complexas interações. Por meio da familiarização com conceitos e técnicas geológicas básicas, a disciplina visa capacitar os estudantes a interpretar fenômenos geológicos, motivando-os a ampliar seus conhecimentos das Geociências. Tópicos mais gerais tratados incluem a visão da Terra como parte do Sistema Solar, as áreas de atuação profissional do geólogo e sua responsabilidade frente à sustentabilidade da Terra. A disciplina tem iní­cio, no primeiro semestre, com apresentação da evolução do pensamento geológico e, em seguida, com apresentações sobre a origem do Universo e do Sistema Solar. Prossegue com os conceitos básicos da dinãmica interna do planeta, com ênfase na Tectônica Global e nos processos formadores de rocha. Aulas práticas incluem o reconhecimento de minerais e rochas mais importantes, visualização de mapas topográficos e geológicos básicos e introdução ao uso de bússola Brunton. Discute-se, a seguir, a dinãmica externa, com aulas sobre intemperismo e origem, transporte e deposição dos sedimentos. Encerra-se o semestre com aulas sobre ação geológica do gelo, rios, ventos e mares. Inicia-se o segundo semestre com a apresentação dos conceitos do tempo geológico, fundamentado com noções sobre Paleontologia, Estratigrafia e Geocronologia. As principais estruturas geológicas são apresentadas, destacando-se o uso da bússola Brunton para medição de atitudes de planos inclinados. A partir do exame de mapas geológicos, é enfatizada a importância de adquirir visões bi e tridimensionais dos terrenos representados, assim como das diferentes etapas da evolução geológica. A parte final da disciplina é dedicada à discussão da importância econômica dos recursos hí­dricos, energéticos e minerais, ainda acrescida de uma aula sobre a contribuição da Geologia nas obras de engenharia e ocupação urbana. Ao transcorrer da disciplina, as aulas são complementadas com palestras ministradas por profissionais das diferentes áreas de atuação profissional. Ao final, `´é apresentado um esboço sobre a geologia do Brasil e do Estado de São Paulo, com introdução sobre técnicas de mapeamento geológico. A primeira experiência de mapeamento é desenvolvida durante atividade de campo de dois dias, para a qual é exigido relatório. Também no final do segundo semestre, os alunos elaboram e apresentam seminários sobre temas tratados na disciplina. A equipe que ministra a disciplina vem buscando restringir ao má­nimo o uso de nomenclatura técnica e aprimorar a concatenação entre os conceitos abordados entre uma aula e outra para, dessa forma, ser atingido o objetivo de apresentar o Planeta Terra de forma sistámica

Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

ABSTRACT: The Monte de Trigo alkaline suite is a SiO2-undersaturated syenite-gabbroid association from the Serra do Mar alkaline province. Eudialyte-group minerals (EGMs) occur in one nepheline microsyenite dyke, associated with aegirine-augite, wöhlerite, låvenite, magnetite, zircon, titanite, britholite, and pyrochlore. Major compositional variations include Si (25.09- 25.57 apfu ), Nb (0.31- 0.76 apfu ), Fe (1.40-2.13 apfu ), and Mn (1.36- 2.08 apfu ). The EGMs also contain relatively high contents of Ca (6.13- 7.10 apfu ), moderate enrichment of rare earth elements (0.38-0.67 apfu ), and a relatively low Na content (11.02-12.28 apfu ), which can be correlated with their transitional agpaitic assemblage. EGM compositions indicate a complex solid solution that includes eudialyte, kentbrooksite, feklichevite, zirsilite-(Ce), georgbarsanovite, and manganoeudialyte components. EGM trace element analyses show low Sr and Ba contents and a negative Eu/Eu* anomaly, which are interpreted as characteristic of the parental magma due to the previous fractionation of plagioclase and/or alkali feldspar. The EGMs from the dyke border have higher contents of Fe, Sr (2,161-2,699 ppm), Mg (1,179-3,582 ppm), and Zn (732- 852 ppm) than those at the dyke center. These differences are related to the incorporation of xenoliths and xenocrysts of melatheralitic host rock into the nepheline-syenitic magma followed by crystal-melt diffusive exchange

Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

ABSTRACT: The Monte de Trigo alkaline suite is a SiO2-undersaturated syenite-gabbroid association from the Serra do Mar alkaline province. Eudialyte-group minerals (EGMs) occur in one nepheline microsyenite dyke, associated with aegirine-augite, wöhlerite, låvenite, magnetite, zircon, titanite, britholite, and pyrochlore. Major compositional variations include Si (25.09- 25.57 apfu ), Nb (0.31- 0.76 apfu ), Fe (1.40-2.13 apfu ), and Mn (1.36- 2.08 apfu ). The EGMs also contain relatively high contents of Ca (6.13- 7.10 apfu ), moderate enrichment of rare earth elements (0.38-0.67 apfu ), and a relatively low Na content (11.02-12.28 apfu ), which can be correlated with their transitional agpaitic assemblage. EGM compositions indicate a complex solid solution that includes eudialyte, kentbrooksite, feklichevite, zirsilite-(Ce), georgbarsanovite, and manganoeudialyte components. EGM trace element analyses show low Sr and Ba contents and a negative Eu/Eu* anomaly, which are interpreted as characteristic of the parental magma due to the previous fractionation of plagioclase and/or alkali feldspar. The EGMs from the dyke border have higher contents of Fe, Sr (2,161-2,699 ppm), Mg (1,179-3,582 ppm), and Zn (732- 852 ppm) than those at the dyke center. These differences are related to the incorporation of xenoliths and xenocrysts of melatheralitic host rock into the nepheline-syenitic magma followed by crystal-melt diffusive exchange

The Hidden Magmatic Chamber from the Ponte Nova Mafic–Ultramafic Alkaline Massif, SE Brazil: Clues from Clinopyroxene and Olivine Antecrysts

Clinopyroxene and olivine primocrysts in the intrusions of the Ponte Nova mafic–ultramafic alkaline massif (SE Brazil) present several textures and zoning that indicate open-system processes. Important compositional differences were found in the clinopyroxene. Diopside relict cores (mostly partially corroded) present higher Mg, Cr and Ni and lower Ti, Na, Al, REE and Sr than Ti-augite mantling and rims. Subordinately, two types of olivine crystals were recognized, one related to very zoned crystals with high Mg (Fo up to 86 mol.%) and Ni cores (mostly with corroded rims), and other almost without clear zonation and with lower Mg contents. Relict cores of high-Mg clinopyroxene and olivine crystals are representative of antecrysts formed in deeper chamber environments. Temperature and pressure estimates based on clinopyroxene-liquid geothermobarometers indicate crystallization of the antecrysts at ~1171 ± 10 °C and ~5.7 ± 0.3 kbar, pointing to a deeper hidden magmatic chamber, whereas mantling and rim compositions indicate a shallow chamber environment. Clinopyroxenes of this hidden chamber have progressive enrichments of incompatible elements with the Mg# decrement and inflection points in Sr and REE due to the starting of co-precipitation of apatite. The evolution trend of clinopyroxene antecrysts indicates that the main intrusions in the Ponte Nova shallow chamber were fed by a single deeper hidden chamber mainly controlled by typical fractional crystallization processes. These antecrysts indicate the presence of a complex plumbing system, which is also supported by similar antecrysts found in the lamprophyre and alkali basalt dikes of this region. The preferred petrological model for the Ponte Nova massif could be summarized as repeated influxes of antecryst-laden basanite magmas that deposited most of their suspended crystals on the floor of the upper-crust magma chamber

Agpaitic Alkaline Rocks in Southern Brazilian Platform: A Review

General information is presented on ten agpaitic occurrences located in southern Brazil and at the border between Brazil and Paraguay. All the Brazilian agpaitic rocks are Late Cretaceous in age, whereas the Paraguayan ones are older than Early Triassic. The most significant occurrence is Poços de Caldas, the largest alkaline massif in South America. In general, these agpaitic rocks contain mineral assemblages that indicate presence of typical halogen-bearing Na–Ca–HFSE phases, eudialyte-, rinkite- and wöhlerite-group minerals being the most frequent ones. However, these associations are indeed more complex in terms of composition, with accessory phases in some cases consisting of various minerals, including U–Th oxides/silicates, Nb oxides, REE–Sr–Ba bearing carbonates–fluorocarbonates–phosphates–silicates and Zr–Na rich silicates. They usually form late magmatic stage to hydrothermal/deuteric assemblages linked with coarse and fine-grained, mainly silica-undersaturated evolved rocks. Data also indicate significant differences in type, amount and composition of agpaitic minerals in all investigated occurrences