Valdecir de Assis Janasi

Late Proterozoic granitoid magmatism in the state of São Paulo, southeastern Brazil

Abstract The Precambrian basement in the state of Sao Paulo is made up of a series of metamorphic units exposed at various erosional levels and in part separated by large faults. A strong overprint of the Brasiliano cycle (700-500 Ma) is evident from isotopic data over most of the region; nevertheless, a large part of the metamorphic belts belong to older (Archaean to Middle Proterozoic) reworked sequences. About 30% of the total basement area consists of intrusive granitoids of Late Proterozoic age. These can be grouped into six major groups, on the basis of petrography, geochemistry, rock association, associated manifestations and (whenever possible) age relationships. Gneissic mangerite-charnockite-hastingsite granite associations appear as folded tabular bodies within high-grade regions. The Sao Jose do Rio Pardo suite is chemically comparable to the “aluminous A-type” granitoid series, and its emplacement possibly signals a period of extensional tectonics prior to the onset of the Brasiliano cycle (∼1000 Ma?). A series of elongated batholiths (∼650 Ma) with predominant porphyritic hornblende-biotite granitoids is present mainly in high-grade areas. The compositions are generally granitic to granodioritic, but intermediate (e.g. monzodioritic) rocks are locally common. These high-K calc-alkaline suites are in some aspects comparable to the Cordilleran I-type granitoids, and their origin might be related to a subductional environment. Migmatitic granitoids constitute a peculiar association, occurring as a series of small bodies within high-grade allochthonous domains. These polyphase, heterogeneous, often nebulitic granitoids are source-related products of crustal anatexis. Syn- to late-orogenic Brasiliano granitoids in the low- to medium-grade domains are dominantly porphyritic calc-alkaline biotite granites and granodiorites, reminiscent of Caledonian I-types. In contrast, a peraluminous association (porphyritic biotite granitoid to equigranular two-mica granite) predominates in high-grade domains, which possibly reflects a different tectonic evolution for the two regions. Late- to post-orogenic Brasiliano granitoids intruded after the major orogenic development of the region (∼610-590 Ma). I-type calc-alkaline and aluminous A-type granitoids are geographically associated in the Itu belt, and appear to show some temporal and geochemical (?) overlapping. Wiborgitic facies and some Sn-W mineralization are locally present in the A-type suites. A post-orogenic extensional period is marked in the south of the area by the emplacement of shallow A-type granitoids in the Serra do Mar belt (

Adaptation of ICP–OES routine determination techniques for the analysis of rare earth elements by chromatographic separation in geologic materials: tests with reference materials and granitic rocks

Abstract The present contribution shows the results of tests on geological materials for determination of the rare-earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, as well as Y and Sc. The samples were prepared using lithium borate fusion of powdered rock sample followed by dissolution in hydrochloric acid. A separation was performed in cationic exchange columns with resin by hydrochloric and nitric acid gradient concentration and subsequent analysis using inductively coupled plasma atomic emission spectrometry (ICP–OES) and an ultrasonic nebulizer. All potentially interfering elements, except Ba and Zr, were eliminated using this procedure. Spectral interferences generated by Zr and the REE among themselves (Ce on Gd, Nd on Sm–Ce–Er, Gd on Sm, Pr on La–Ce–Nd–Ho–Tb, Tb on Ho, Sm on Tb) were identified and corrected quantitatively. The whole procedure was tested with international geological standard reference materials AC-E and JG-3 (granitic rocks), JB-1a (basalt), JA-2 (andesite), JR-2 (rhyolite) and JGb-1 (gabbro). A Student’s statistical test showed that the obtained analytical results were mostly satisfactory. The methodology, now performed on a routine basis, was applied for the determination of the REE patterns of assorted granitic rocks occurring in the state of Sao Paulo, southeastern Brazil.

Geology, petrography and geochemistry of the acid volcanism of the Paraná Magmatic Province in the Piraju-Ourinhos region, SE Brazil

A succession of trachydacitic lava flows over 150 m thick constitutes the lowermost portion of the Serra Geral Fm. along the Paranapanema River valley in the Piraju-Ourinhos region, State of São Paulo. The flows have a well-defined zonality: the central portion is made up of grey porphyritic dacites with low glass and vesicle proportion which grade towards the contacts to strongly vesiculated varieties with glassy groundmass which, when in contact with the Botucatu Fm. basal sandstones or with larger bodies of inter-flow sandstone, show a typical chocolate-brown color. The observed compositional variation is small, and in part a reflection of secondary alteration. Independent of texture, mode of occurrence and stratigraphic position, the rocks show similar contents of SiO2 (65-67 wt%) and all other major oxides, and a ferrous (mg#= 20-27) and metaluminous A/CNK= 0.85-0.95) character; alteration associated to vesicle infilling and color change to chocolate-brown results in lower mg# and higher A/CNK. Mineral and whole rock chemical data indicate high crystallization temperatures around 1000oC (pyroxene thermometry; apatite saturation), consistent with the predominant mode of occurrence as flows.

Volcanic stratigraphy of intermediate to acidic rocks in southern Paraná Magmatic Province, Brazil

Este artigo apresenta o primeiro mapa em escala de detalhe de uma area coberta pelas rochas vulcânicas tipo Palmas, na borda sul da Provincia Magmatica Parana, de idade eocretacica. O estudo das caracteristicas estruturais, juntamente com a petrografia e geoquimica, permitiu separar as rochas em tres sequencias vulcânicas principais, reconhecendo a sua estratigrafia. As rochas da primeira sequencia, denominada Caxias do Sul, recobrem fluxos basalticos de baixo Ti (tipo Gramado), e correspondem a um empilhamento de derrames lobados, fluxos laminares e lava domos, emitidos por erupcoes continuas. O final da atividade vulcânica e marcado por erupcoes mais intermitentes, permitindo assim a intercalacao de delgados depositos de arenitos. Estas rochas tem composicao dacitica (~ 68 %, em peso, SiO2) com microfenocristais de plagioclasio, piroxenios e Ti- magnetita imersos em matriz vitrea ou desvitrificada. A segunda sequencia, denominada Barros Cassal, e composta por diversos fluxos de lavas de andesito basaltico, andesito e dacito (~ 54; ~ 57 e ~ 63%, em peso, SiO2, respectivamente), com microfenocristais de plagioclasio, piroxenios e Ti-magnetita. A frequente intercalacao de arenitos entre os fluxos atesta o comportamento intermitente deste evento. A sequencia superior, Santa Maria, e composta pelas rochas mais ricas em silica (~ 70%, em peso, SiO2) que ocorrem como fluxos laminares, fluxos lobados e lava-domos. Estas rochas sao de composicao riolitica, com microfenocristais de plagioclasio e Ti-magnetita imersos em uma matriz vitrea ou desvitrificada com microlitos. As estruturas e texturas apresentadas pelas rochas das tres sequencias acidas indicam que elas foram geradas por erupcoes de carater predominantemente efusivo, refletindo as altas temperaturas do magma (~ 1.000 oC).

Magmatismo há ca. 660 - 640 Ma no Domínio Socorro: registros de convergência pré-colisional na aglutinação do Gondwana Ocidental

Uranium-lead zircon ages between 660 and 640 Ma, obtained from a series of calc-alkaline orthogneisses and plutons in southeast Brazils Central Mantiqueira Province, suggest that a significant period of magmatism occurred in this region prior to the collisional assembly of West Gondwana (presently constrained in the region between ca. 625 and 580 Ma). While the nature of this earlier magmatism is presently unclear, some preliminary Sm-Nd data suggest that these magmas were not solely derived from the Paleoproterozoic lithosphere, but appear to represent hybrid products of Paleoproterozoic and Neoproterozoic sources. As such hybrid mixtures have been most commonly observed in continental arc settings, it is possible that the 660 to 640 Ma magmatism represents arc magmatism that resulted from subduction of Neoproterozoic oceanic crust during early precollisional convergence and closure of a branch of either the Adamastor or Goianides oceans.

Rare earth element and yttrium geochemistry applied to the genetic study of cryolite ore at the Pitinga Mine (Amazon, Brazil)

ABSTRACT This work aims at the geochemical study of Pitinga cryolite mineralization through REE and Y analyses in dissem-inated and massive cryolite ore deposits, as well as in fluorite occurrences. REE signatures in fluorite and cryoliteare similar to those in the Madeira albite granite. The highest 6REE values are found in magmatic cryolite (677 to1345 ppm); 6REE is lower in massive cryolite. Average values for the different cryolite types are 10.3 ppm, 6.66 ppmand 8.38 ppm (for nucleated, caramel and white types, respectively). Disseminated fluorite displays higher6REE val-ues(1708and1526ppm)thanfluoriteinlateveins(34.81ppm). Yttriumconcentrationishigherindisseminatedfluoriteand in magmatic cryolite. The evolution of several parameters (REE total , LREE/HREE, Y) was followed throughoutsuccessive stages of evolution in albite granites and associated mineralization. At the end of the process, late cryolitewas formed with low REE total content. REE data indicate that the MCD was formed by, and the disseminated oreenriched by (additional formation of hydrothermal disseminated cryolite), hydrothermal fluids, residual from albitegranite. The presence of tetrads is poorly defined, although nucleated, caramel and white cryolite types show evidencefor tetrad effect.

The inventory of geological heritage of the state of São Paulo, Brazil: Methodological basis, results and perspectives

An inventory of geological sites based on solid and clear criteria is a first step for any geoconservation strategy. This paper describes the method used in the geoheritage inventory of the State of São Paulo, Brazil, and presents its main results. This inventory developed by the geoscientific community aimed to identify geosites with scientific value in the whole state, using a systematic approach. All 142 geosites representative of 11 geological frameworks were characterised and quantitatively evaluated according to their scientific value and risk of degradation, in order to establish priorities for their future management. An online database of the inventory is under construction, which will be available to be easily consulted and updated by the geoscientific community. All data were made available to the State Geological Institute as the backbone for the implementation of a future state geoconservation strategy.

Remoção de Pb2+ e Cr3+ em solução por zeólitas naturais associadas a rochas eruptivas da formação serra geral, bacia sedimentar do Paraná

The capacity of natural zeolites and its host rock (dacite) to remove Pb2+ and Cr3+ from aqueous solutions has been investigated. Results showed that both samples prefer to remove Pb2+ instead of Cr3+. Almost 100% of Pb2+ was removed from solutions with concentration until 50 mg L-1 and 100 mg L-1 of this metal, respectively by dacite and zeolite. The equilibrium of metals adsorption process was reached during the first 30 min by both materials. Na+ can be used to recover Pb2+, but not to remove Cr3+ from the treated samples. The Sips model showed a good fit for experimental data of this study.

Enhancing Maficity of Granitic Magma during Anatexis: Entrainment of Infertile Mafic Lithologies

Most studies of migmatites examine how anatexis occurred in the most fertile units and what happened to that melt, whereas the associated minor lithologies are typically ignored. The Kinawa migmatite in the southern S~ ao Francisco Craton of Brazil is the product of water-fluxed melting of a leucogranodiorite that contained dykes of amphibolite. The migmatite consists mostly of pink diatexites, metatexites and leucosomes, but it also contains schollen of amphibolite. This study examines the behaviour of these minor mafic rocks during anatexis to determine what role they play in the formation of migmatites and development of granitic magmas in their source region.The amphibolites are massive or banded HblþPl, and rarely HblþPlþCpx, schollen in the diatexite migmatite. The amphibolite schollen melted very little, and show a complex morphology suggesting mechanical and chemical interaction with the enclosing leucocratic pink diatexite migmatite. Diatexites and leucosomes immediately adjacent to the schollen have a considerably higher proportion of amphibole (up to 12%) and/or biotite (up to 10%) compared with the diatexite a few tens of centimeters farther away. Six stages of disaggregation and interaction of mafic schollen with the enclosing diatexite magma are recognized: (1) amphibolite layers break up to form schollen, but are mineralogically and texturally unchanged; (2) melt infiltrates into fractures and foliation in the schollen; (3) schollen disaggregate into swarms of single amphibole crystals within the diatexites; (4) amphibole is partially replaced by biotite; (5) flow of the enclosing diatexite magma arranges the detached amphibole crystals into schlieren and aggregates of biotite; (6) detached crystals are completely replaced by biotite and dispersed by magmatic flow to produce a mesocratic to melanocratic homogeneous diatexite. Geochemical modelling indicates that the composition of the diatexites and leucosomes is changed by the wholesale entrainment of the disaggregated mafic schollen or in some cases by the preferential entrainment of detached hornblende or plagioclase crystals. This contamination increases the maficity of initially felsic, leucodiatexite magma, by the addition of FeOþMgO, CaO and TiO2 (which results in a concomitant decrease in SiO2), to become a mesocratic to melanocratic diatexite magma that is comparable with typical I-type granites found around the world. Entrainment of mafic material and hornblende in particular strongly influences the behaviour of the rare earth elements, lowering LaN/YbN ratios. Thus, non-protolith mafic lithologies within migmatites represent a source of contamination for anatectic melts that results in a significant increase in maficity.

Mathiasite-loveringite and priderite in mantle xenoliths from the Alto Paranaíba Igneous Province, Brazil: genesis and constraints on mantle metasomatism

Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55–60 wt.% TiO2; 5.2–6.7 wt.% ZrO2) occurs in peridotite xenoliths rimming chromite (∼50 wt.% Cr2O3) and subordinate ilmenite (12–13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4–9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events.