Uma síntese sobre o magmatismo alcalino do Paraguai Oriental

Alkaline magmatism occurs in six distinct areas of Paraguay and forms bodies of variable size, shape, composition and age. The oldest rocks are found in the north and correspond to the Permo-Triassic Alto Paraguay Province (241 Ma). Four Early Cretaceous events can be distinguished in Eastern Paraguay: the Rio Apa and Amambay Provinces (139 Ma), both predating the tholeiites of the Serra Geral Formation, are located in the northern and northeastern regions, respectively; and the Central (126 Ma) and Misiones Provinces (118 Ma) in the central-eastern and southern regions, respectively. The youngest alkaline rocks are volcanic rocks in Asunción of Tertiary age (59 Ma). Excluding the Alto Paraguay rocks influenced by the Amazonian craton, the emplacement of alkaline bodies is clearly controlled by a tectonic extensional regime that generated NW-SE-trending faults and grabens. Geochemically, Paraguayan alkaline rocks are predominantly miaskitic, with a potassic or sodic affinity, the former being characterized by strongly fractionated rare earth-elements, negative Ta-Nb-Ti anomalies, and high Sr and low Nd radiogenic isotopes. Sodic rocks have slightly positive Ta and Nb anomalies and are less enriched in Sr, approaching bulk earth values. Carbonatites behave similarly to the associated pre-tholeiites potassic rocks. The Sr-Nd-Pb isotope ratios suggest that two main mantle components were involved in the genesis of the Paraguayan rocks: an enriched mantle I component dominated the Early Cretaceous potassic magmatism, and an high 238U/204Pb ou high U/Pb component that was important for the late Early Cretaceous and Tertiary sodic magmatism. The close association of potassic and sodic suites, such as in the Asunción-Sapucai-Villarrica graben, indicates that their parental magmas were derived from a heterogeneous subcontinental mantle, enriched with incompatible elements.Magmatismo alcalino ocorre em seis áreas distintas do Paraguai e forma corpos variáveis quanto ao tamanho, forma, composição e idade. As rochas mais antigas são encontradas no Norte e correspondem à Província Permo-Triássica Alto Paraguai (241 Ma). Quatro eventos do Cretáceo Inferior são reconhecidos no Paraguai Oriental: as Províncias Rio Apa e Amambay (139 Ma), ambas predatando os toleítos da Formação Serra Geral, estão situadas, respectivamente, nas regiões norte e nordeste; e as Províncias Central (126 Ma) e Misiones (118 Ma), respectivamente, nas suas regiões centro-oriental e sudeste. As rochas alcalinas mais novas são as vulcânicas de Assunção, de idade Terciária (59 Ma). À exceção das rochas do Alto Paraguai influenciadas pelo cráton amazônico, a colocação dos outros centros alcalinos é claramente controlada por um regime tectônico extensional que gerou falhas e grábens de orientação NW. Geoquimicamente, as rochas alcalinas paraguaias são em sua quase totalidade miaskíticas, de afinidade potássica ou sódica, com as primeiras caracterizadas por intenso fracionamento dos elementos terras raras, anomalias negativas de Ta-Nb-Ti, e teores altos e baixos, respectivamente, de Sr e Nd radiogênico. Rochas sódicas apresentam anomalias positivas de Ta e Nb e são menos enriquecidas em Sr, com valores próximos ao da ­Terra global. Carbonatitos comportam-se similarmente às rochas potássicas ­pré-toleítos. Isótopos de Sr-Nd-Pb sugerem que dois componentes mantélicos estiveram envolvidos na gênese das rochas paraguaias: um componente EMI dominou o magmatismo potássico do Cretáceo Inferior, e um componente HIMU foi importante para o magmatismo sódico do final do Cretáceo Inferior e Terciário. A estreita associação entre as suítes potássica e sódica, como na região do gráben Assunção-Sapucai-Villarrica, indica que seus magmas parentais foram derivados de um manto subcontinental heterogêneo, enriquecido em elementos incompatíveis.FAPESPNational Research Council and Ministry of University and Researc

A Ocorrência Alcalina de Cananeia, Litoral Sul do Estado de São Paulo: Química Mineral

A ocorrência alcalina de Cananeia, de idade Cretáceo Superior (Ar-Ar, 83,6 Ma), localiza-se no Vale do Ribeira e é representada por dois pequenos corpos: Morro de São João (1,8 km²), na Ilha de Cananeia, e Morrete (0,4 km²), na vizinha Ilha Comprida. Ela é coberta por sedimentos quaternários e constituída dominantemente por rochas intrusivas de natureza sienítica, reunidas em dois grupos principais: álcali feldspato sienitos e quartzo-álcali feldspato sienitos. Microssienitos com textura variada ocorrem subordinadamente. Quanto à mineralogia, as rochas apresentam grande riqueza em feldspatos, sendo mesopertita a fase mais abundante, e plagioclásio, na forma de cristais isolados, de presença tão-somente ocasional. Os máficos consistem dominantemente em clinopiroxênios e anfibólios, enquadrados nos grupos cálcico, cálcico-sádico e sádico, mostrando variações químicas expressivas, que se traduzem principalmente no aumento da relação Fe/Mg e do teor em Na, e da diminuição da concentração em Ca, em função do grau de evolução das rochas. Relações texturais indicam a transformação clinopiroxênio → anfibólio. Biotitas são comuns, sobretudo em associação com anfibólio e opacos, quando ocupam as porções periféricas dos grãos do primeiro mineral e se concentram ao longo de suas fraturas e planos de clivagem. Olivinas, de natureza faialítica, estão raramente preservadas. Os principais acessários incluem opacos, essencialmente magnetita com lamelas exsolvidas de ilmenita, apatita, titanita e zircão. Processos de alteração tardi a pás-magmática podem explicar as texturas de substituição encontradas nos principais minerais primários, bem como as estruturas de exsolução presentes nos feldspatos e opacos.The Cananeia alkaline occurrence of Late Cretaceous age (Ar-Ar, 83.6 Ma) lies in the Ribeira Valley and it is represented by two small bodies: Morro de São João (1.8 km²) in Cananeia Island, and Morrete (0.4 km²), in the neighboring Comprida Island. It is covered by Quaternary sediments and mostly constituted by intrusive rocks of syenitic composition, assembled into two principal groups: alkali feldspar syenites and quartz-alkali feldspar syenites. Microsyenites of variable texture are subordinate. Regarding the mineralogy, the rocks are very rich in feldspars, mesoperthite being the most abundant phase, and plagioclase is only occasionally found as isolated crystals. Mafic minerals consist dominantly of clinopyroxenes and amphiboles, both belonging to the calcic, calcic-sodic and sodic groups and showing significant chemical variations, such as the increase in the Fe/Mg ratio and Na content, and the decrease in Ca, as a function of the degree of evolution of the rocks. Textural relationships show that clinopyroxene reacts to form amphibole. Biotite is common, mainly in association with amphibole and opaques. Biotite and opaques together are found at the borders of amphibole grains or concentrated along their fractures and cleavage planes. Fayalitic olivine is rarely preserved. Most common accessories include opaques, mainly magnetite with exsolved ilmenite lamellae, apatite, titanite and zircon. Late to post-magmatic alteration processes can explain the replacement textures shown by the principal primary minerals and the exsolution structures found in feldspars and opaques

A ocorrência alcalina de cananéia, litoral Sul do Estado de São Paulo: geologia e geocronologia

Novas idades (Ar-Ar e Rb-Sr) são fornecidas para as rochas alcalinas do maciço de Cananéia, no Vale do Ribeira, representado por dois pequenos corpos: Morro de São João (1,8 km²) e Morrete (0,4 km²). São rochas de natureza sienítica, dominantemente intrusivas, cobertas por sedimentos arenosos mais recentes, e têm como encaixante o embasamento pré-Cambriano do Grupo Açungui. Do ponto de vista tectônico, estão associadas ao Alinhamento Guapiara, no limite norte do Arco de Ponta Grossa, além de incluídas na província homônima, a qual reúne grande número de centros alcalinos. Datações radiométricas Ar-Ar em biotita indicam uma idade média de 83,6 Ma para os sienitos, não muito diversa daquela obtida a partir de isócrona de referência Rb-Sr em rocha total para diferentes litologias, apresentando valor de 85,6 ± 2,7 Ma e razão inicial 87Sr/86Sr de 0,70667 ± 0,00014 (MSWD = 1,14). Esses dados confirmam a inclusão do maciço de Cananéia no cronogrupo formado pelas diversas ocorrências alcalinas do Cretáceo Superior.New radiometric ages (Ar-Ar and Rb-Sr) are available for the alkaline rocks from the Cananéia massif in the so-called Ribeira Valley, which is represented by two small bodies (Morro de São João, 1.8 km², and Morrete, 0.4 km²). The rocks are syenitic in composition, dominantly intrusive, covered by recent sediments and emplaced into the Precambrian basement of the Açungui Group. From the tectonic point of view, they are associated with the Guapiara Alignment at the northern limit of the Ponta Grossa Arch and intruded into the homonimous province that includes a great number of alkaline centers. Ar-Ar radiometric determinations in biotite indicate an average age of 83.6 Ma for the syenites, not much different of that given by a Rb-Sr reference isochron on whole rock for different lithologies which yielded an age of 85.6 ± 2.7 Ma, and an initial 87Sr/86Sr ratio of 0.70667 ± 0.00014 (MSWD = 1.14). The data confirm the inclusion of the Cananéia massif within the age group of the Late Cretaceous alkaline occurrences

Os diques na encosta ocidental da Serra de Ybytyruzú, região centro‑oriental do Paraguai

The Cordillera del Ybytyruzú region of central‑eastern Paraguay is predominantly formed by tholeiitic lava flows from the Early Cretaceous age (130–134 Ma) cut by K‑alkaline dykes and intrusive bodies aged between 125 and 127 Ma. Petrochemical studies focusing on 14 samples of dykes from the region show an apparent affinity to Roman Province Type rocks and total consistency with the overall petrographic association of the Asunción‑Sapucai‑Villarrica (ASV) graben. Two main potassic suites are distinguished: B‑P (basanite‑tephrite‑phonotephrite‑phonolite) and AB‑T (alkali basalt‑trachybasalt‑trachyandesite‑trachyphonolite/trachyte).The occurrence of lamproitic rocks in the area has so far not been confirmed.A Cordilheira do Ybytytyruzú na região centro‑oriental do Paraguai é formada dominantemente de derrames de lavas toleíticas de idade Cretáceo Inferior (130–134 Ma) cortados por diques alcalinos potássicos e corpos intrusivos com idades entre 125 e 127 Ma. Estudos petroquímicos focalizando 14 amostras de diques da região mostram uma aparente afinidade com as rochas da Província do Tipo Romano e total consistência com a associação petrográfica global do gráben Assunção‑Sapucai‑Villarrica (ASV). Duas suítes potássicas principais são distinguidas: B‑P (basanito‑tefrito‑fonotefrito‑fonolito) e AB‑T (álcali basalto‑traquibasalto‑traquiandesito‑traquifonolito/traquito). A ocorrência de rochas lamproíticas na área não foi até então confirmada

Carbonatites from the southern brazilian platform. A review. II. Isotopic evidences

Early and Late Cretaceous alkaline and alkaline– carbonatitic complexes from southern Brazil are located along the main tectonic lineaments of the South America Platform. Calcium-, magnesium-, and ferrocarbonatites are well represented and frequently associated even in the same complex. Primary carbonates present significant variations in C–O isotopic compositions, which are mainly due to isotope exchange with H2O–CO2-rich hydrothermal fluids, whereas fractional crystallization or liquid immiscibility probably affects the δ18O and δ13C values by no more than 2δ‰. Our isotope exchange model implies that the most significant isotopic variations took place in a hydrothermal environment, e.g., in the range 400–80°C, involving fluids with the CO2/H2O ratio ranging from 0.8 to 1. Sr–Nd–Pb isotope systematics highlight heterogeneous mixtures between HIMU and EMI mantle components, similar to the associated alkaline rocks and the flood tholeiites from southern Brazil. In spite of the strong variation shown by C–O isotopes, Sr-Nd–Pb–Os isotopic systematics could be related to an isotopically enriched source where the chemical heterogeneities reflect a depleted mantle “metasomatized” by small-volume melts and fluids rich in incompatible elements. These fluids are expected to have promoted crystallization of K-rich phases in the mantle, which produced a veined network variously enriched in LILE and LREE. The newly formed veins (enriched component) and peridotite matrix (depleted component) underwent a different isotopic evolution with time as reflected by the carbonatites. These conclusions may be extended to the whole Paraná–Etendeka system, where isotopically distinct parent magmas were generated following two main enrichment events of the subcontinental lithospheric mantle at 2.0–1.4 and 1.0–0.5 Ga, respectively, as also supported by Re–Os systematics. The mantle sources preserved the isotopic heterogeneities over a long time, suggesting a non convective lithospheric mantle beneath different cratons or intercratonic regions. Overall, the data indicate that the alkaline–carbonatitic magmatism originated from a locally heterogeneous subcontinental mantle

Carbonatites from the Southern Brazilian platform: I

We present a comprehensive overview of the geochemical characteristics and evolution of the carbo- natites from the southern Brazilian Platform (Paraná Basin). The carbonatites from dierent complexes dis- play large compositional variability in terms of abun- dances of incompatible and rare earth elements. This is in agreement with an origin from heterogeneous litho- spheric sources, as conrmed by isotopic data (see Speziale et al., this issue). The characteristic major and trace element abundances of these carbonatites present compelling evidence for invoking liquid unmixing as the main mechanism of their formation and evolution albeit few exceptions. We propose an evolutionary trend for the Brazilian carbonatites, which can be summarized as following: exsolution of the primary Ca- or Mg-carbona- titic liquids systematically takes place at the phonolite- peralkaline phonolite stage of magma dierentiation; this is followed by progressive Fe-enrichment and by nal emplacement of uorocarbonatites associated with hydrothermal uids

O maciço alcalino de Tunas, PR: novos dados geocronológicos

The NW-SE elongated Tunas intrusion, occupying an area of 22 km², crops out about 80 km from the city of Curitiba, and it is emplaced into Precambrian metavolcanic-sedimentary rocks of the Açungui Group. Five sub-circular volcanic structures are recognized. Petrographically, the massif mainly consists of syenites and alkali syenites with subordinate alkali gabbros, essexites, syenogabbros and syenodiorites; small late syenitic dykes and magmatic breccias are also found. Previous K-Ar data give a Late Cretaceous mean age of 82.2 Ma for Tunas rocks. In this article we present new U-Pb age determinations on zircon crystals from syenitic rocks (ID-TIMS, 82.7 ± 0.7 Ma; SHRIMP, 84.7 ± 1.2 Ma) which confirm that age, placing the massif into the Late Cretaceous group of alkaline intrusions tectonically associated with the São Jerônimo-Curiúva Lineament, a NW-SE-trending branch of the Ponta Grossa Arch in southern Brazil.A intrusão de Tunas, de forma alongada para NW-SE e ocupando área de 22 km², ocorre a aproximadamente 80 km da cidade de Curitiba e está encaixada em terrenos metavulcanossedimentares pré-cambrianos do Grupo Açungui. Cinco estruturas vulcânicas de contorno subcircular são reconhecidas. Do ponto de vista petrográfico, o maciço consiste principalmente em sienitos e álcali sienitos, com presença subordinada de gabros alcalinos, essexitos, sienogabros e sienodioritos; pequenos diques, de formação tardia e natureza sienítica, e brechas magmáticas estão também presentes. Idades K-Ar obtidas anteriormente (valor médio de 82,2 Ma) indicam que as rochas de Tunas são do Cretáceo Superior. Neste trabalho são apresentadas novas determinações,utilizando-se do método U-Pb em cristais de zircão de rochas sieníticas (ID-TIMS, 82,7 ± 0,7 Ma; SHRIMP, 84,7 ± 1,2 Ma),que confirmam essa idade e permitem colocar Tunas no grupo de intrusões alcalinas do Cretáceo Superior associadas tectonicamente com o Lineamento São Jerônimo-Curiúva, uma feição estrutural ligada ao Arco de Ponta Grossa, de orientação NW-SE, na região Sudeste do Brasil

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