The Early Triassic magmatism of the Alto ParaguayProvince, Central South America: Paleomagneticand ASM data

Abstract A paleomagnetic studystudywork was carriedout on the Alto Paraguay Province (APP), a belt of alkalinecomplexes that parallel the Paraguay river for morethan 40 km at the border of Brazil and Paraguay. Theprovince is well dated by 40Ar/39Ar method giving ages inthe range 240–250 Ma with a preferred age of 241 Ma. Intrusiverocks are predominant but the stocks may be toppedby lava flows and ignimbrites. Paleomagnetic work onstocks, dikes and flows of the APP identified normal andreversed magnetic components which are carried mainlyby titanomagnetites. The calculated paleomagnetic polelocated at 319ºE 78ºS (α95 = 6º; k = 23) is in agreementwith other South American poles of Permo-Triassic age.Most of the sampling sites showed large variations in rockmagnetization, but similar patterns in the variation of thewithin-site magnetizations, mainly in dikes, suggest geomagneticpolarity transition records. The magnetizationdata along with the anisotropy of magnetic susceptibilitydeterminations suggested that the South and North areasof the province have different evolution characteristics

Special topics on magmatic petrology

The technological development of the last decades allowed great advances in analytical processes, both in variety and speed of results. Sophisticated instruments, highly-developed chemical processes and the use of computers have enabled quantizing elements and isotopes that are concentrated in minerals based on the magmatic processes involved. In 2010, the lead author of this text taught a course at the Geosciences Institute of the University of Sao Paulo on the most recent subjects in petrology, complemented with computer programs for specific applications. The success of that course suggested the need to collect his lecture notes in an organized format, so as to make them available to the scientific community. This text, organized in twenty separate topics, is designed to complement the basic knowledge in the area of petrology.O desenvolvimento tecnológico das últimas décadas permitiu um grande avanço dos processos analíticos, tanto quanto à variedade como à rapidez na obtenção dos resultados. Instrumentos sofisticados, processos químicos refinados e o uso de computadores possibilitaram quantizar elementos e isótopos concentrados nos minerais de acordo com os processos magmáticos envolvidos. Em 2010, o primeiro autor deste texto ministrou um curso no Instituto de Geociências da Universidade de São Paulo sobre os temas mais recentes de interesse petrológico, complementado por programas de cálculo de interesse específico. O sucesso do curso levou à ideia de reunir as anotações das aulas ministradas e elaboradas pelo autor para colocar tal conteúdo à disposição da comunidade científica. Em vinte tópicos, o texto visa apenas complementar os conhecimentos básicos já adquiridos

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

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

Late-stage magmatic to deuteric/metasomatic accessory minerals from the Cerro Boggiani agpaitic complex (Alto Paraguay Alkaline Province)

This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr Na rich silicates. They form a late-stage magmatic to deuteric/ metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteratio

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

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

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