Delia del Pilar Montecinos de Almeida

The Acampamento Velho Formation, a Lower Cambrian Bimodal Volcanic Package: Geochemical and Stratigraphic Studies from the Cerro Do Bugio, Perau and Serra De Santa Bárbara (Caçapava Do Sul, Rio Grande Do Sul, RS - Brazil)

Abstract During the late stages of the Brasiliano orogenic cycle (Lower Cambrian), the Camaqua Basin was gradually filled by the alkaline-trending, bimodal volcanic rocks of the Acampamento Velho Alloformation. This volcanic package consists of two facies associations: the lower one composed of andesites and basaltic andesites, and the upper one of rhyolitic rocks. The rhyolitic association comprises alternating pyroclastic rocks (lapilli, tuffs and welded tuffs) in the middle section and flows at the top. Geochemical evidence, especially trace elements and REE, confirmed the stratigraphic succession proposed herein for the volcanic rocks, as well as their co-genetic relationships and the fractional crystallization of the felsic sequence. The Acampamento Velho Formation seems to have been generated in an extensional regime preceding the collision of the Rio de la Plata and Kalahari continental plates. This extensional regime probably occurred during subduction of the Adamastor oceanic plate beneath the Rio de la Plata plate in a retroarc setting.

Evolution of heterogeneous mantle in the Acampamento Velho and Rodeio Velho volcanic events, Camaqua Basin, southern Brazil

Abstract The Camaqua Basin, developed during the last phases of the Brazilian/Pan-African Orogeny and was filled with a thick volcano-sedimentary succesion, in which two volcanic events of alkaline affinity are represented by the Acampamento Velho Alloformation and the Rodeio Velho Member. The Acampamento Velho Alloformation records a bimodal event with a lower association of mafic flows and an upper association of felsic pyroclastic rocks and flows. It was formed during extension, after the subduction of the Adamastor oceanic plate beneath the Rio de La Plata continental plate at the end of the Neoproterozoic III. The second event, the Rodeio Velho Member, represented by mafic flows, intrusions and piroclasts, took place during overall extensional tectonism, probably in the middle Ordovician. Rb, Sr, Sm, and Nd isotopic measurements were carried out on samples from both units. Regardless the event they represent, all the samples display negative values for epsilon Nd, ranging from 2.97 to 10.31 for the Acampamento Velho Alloformation and from 8.39 to 13.92 for the Rodeio Velho Member. The initial 87 Sr/ 86 Sr ratios vary from 0.706 to 0.707 and from 0.704 to 0.707 for the Acampamento Velho Alloformation and Rodeio Velho Member, respectively. Mafic flow deposits in both units show a preferential enrichment in Ba relative to Th. Flow samples from the Rodeio Velho Member also display a distinctive enrichment in the Ba/Th ratio, without a change in the initial Sr, compared to the mafic flow deposits from the Acampamento Velho Alloformation, which show a slight enrichment in those ratios. As for the Acampamento Velho Alloformation, the mafic lavas could be a mixture of depleted mantle-derived basalts plus 20% to 30% of crustal contamination by sediment (probably Neoproterozoic arkosic quartzites). The formation of a magmatic chamber and the separation of the magma into two fractions gave rise initially to the mafic rocks at the base of the Acampamento Velho Alloformation The other magma fraction underwent a significant enrichment in crustal component before the felsic rocks of this Alloformation were formed. The flows from the Rodeio Velho Member originated in a distinct magma chamber, with EM I characteristics that was much more enriched in incompatible elements and depleted in radiogenic Sr.

Geochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores

Volcanic rocks that make up Faial Island, Central Azores, consist of four volcano-stratigraphic units, with ages between 730 ka and the present. Lavas range from alkali basalts to trachyandesites and belong to the alkaline-sodic series. The oldest unit is the Ribeirinha Volcanic Complex, generally characterized by low MgO contents. The Cedros Volcanic Complex is composed of basalts to benmoreites with low MgO contents. The Almoxarife Formation represents fissure flows, containing MgO contents similar to to slightly higher than those of the underlying Cedros Volcanic Complex. The youngest unit, the Capelo Formation, consists of mafic rocks with MgO values higher than those of the other units. Bulk-rock major and trace element trends suggest that differentiation of the three earliest units were dominated by fractional crystallization of plagioclase ± clinopyroxene ± olivine ± titanomagnetite. Capelo bulk-rock compositions are the most primitive, and are related to a period when volcanic activity was fed by deep magmatic chambers, and melts ascended more rapidly. Comparison among geochemical patterns of the trace elements suggests a strong similarity between the lavas from Faial and Pico islands. Corvo Island volcanism contrasts with the geochemistry of Faial and Pico lavas, reflecting its strong K and Rb depletion, and Th, U, Ta, Nb, La, and Ce enrichment. Absence of the Daly gap in the Faial volcanics is attributed to early crystallization of Ti-Fe oxides. The probable source of the Faial magma coincides with the MORB-FOZO array, which implies the presence of ancient recycled oceanic crust in the mantle source. Ratios of incompatible trace elements suggest the similarity of Corvo volcanic rocks with magmas derived from HIMU sources, whereas the Faial and Pico volcanic rocks could have been produced from sources very close to EMII-type OIB.

Mineral Chemistry of Volcanic Rocks of South Shetland Archipelago, Antarctica

This study presents the results of mineral chemistry data of Meso-Cenozoic volcanic rocks of Livingston, Robert and Ardley islands (South Shetland Arc, Antarctica). These rocks include basalts and basaltic andesites that exhibit pilotaxitic, intergranular, and intersertal textures. Glomeroporphyritic clusters consist of phenocrysts of plagioclase, augite, and Ti-magnetite or only plagioclase. The rocks are composed of plagioclase, augite, olivine, Ti-magnetite, and ilmenite phenocrysts. Geochemistry studies indicate that these rocks are rich in Al2O3, Rb, Ba, Sr, and show enrichment in LREE relative to HREE. The Ni, Cr, Co, and MgO contents are lower than primary compositions, and demonstrate that these magmas are evolved. Mineral chemistry indicates that the volcanic rocks belong to the calc-alkaline series typical of island arcs. Plagioclase crystals are mostly bytownite and labradorite in Livingston and Robert islands. Nevertheless, some Robert Island phenocrysts are of anorthite composition. The plagioclase of volcanic rocks of Ardley Island is only labradorite. Augite is the main mafic phase (Wo23-48, En40-58, Fs9-21) in all studied islands. Olivine shows a chrysolite composition. Oxide phases are represented by Ti-magnetite and ilmenite. The evaluation of whole-rock geochemistry and mineral chemistry supports the hypothesis of fractional crystallization for the evolution of the rocks of Livingston, Robert, and Ardley islands. Geothermometry for olivine-clinopyroxene pairs suggests a crystallization temperature of 962°C ± 10°C (1 bar) and of 965°C ± 10°C (500 bars).

Late sodic metasomatism evidences in bimodal volcanic rocks of the Acampamento Velho Alloformation, Neoproterozoic III, southern Brazil

A mineralogical study was carried out in mafic and felsic volcanic rocks of the Acampamento Velho Alloformation at Cerro do Bugio, Perau and Serra de Santa Barbara areas (Camaqua Basin) in southern Brazil. The Acampamento Velho bimodal event consists of two associations: lower mafic at the base and upper felsic at the top. Plagioclase and alkali-feldspar were studied using an electronic microprobe, and magnetite, ilmenite, rutile, illite and alkali-feldspar were investigated through scanning electron microscopy. The rocks were affected by a process of late sodic autometasomatism. In mafic rocks, Ca-plagioclase was transformed to albite and pyroxenes were altered. In felsic rocks, sanidine was partially pseudomorphosed, generating heterogeneous alkali-feldspar. In this association, unstable Ti-rich magnetite was replaced by rutile and ilmenite. In mafic rocks, the crystallization sequence was: (1) Ti-rich magnetite (?), (2) pyroxene and Ca-plagioclase, (3) albite (alteration to Ca-plagioclase), (4) sericite, chlorite and calcite (alteration to pyroxene), and kaolinite (alteration to plagioclase/albite). In felsic rocks: (1) zircon, (2) Ti-rich magnetite, (3) sanidine, (4) quartz. The introduction of late Na-rich fluids, generated the formation of (5) heterogeneous alkali-feldspar, (6) ilmenite and rutile from the Ti-rich magnetite, (7) albite in the spherulites. Finally, alteration of sanidine, vitroclasts and pumice to (8) illite.

Utilização de critérios geoestatísticos para comparação de malha de sondagem visando à maximização da quantidade de recursos

Mineral deposit assessment is based on estimates of the attributes within the ore body, where the sampling of these attributes is usually obtained through cores from diamond drill holes. Due to the high costs associated with drilling and sampling, the amount of drill holes is limited and their locations must be correctly defined to simultaneously maximize the uncertainty reduction related to the estimates and the confidence of the resource and reserve inventories. Having posed the problem, this article presents a methodology for defining the ideal drill-hole grids for optimizing the distribution of resources according to their measured, indicated, and inferred classes. The method combines the estimation error and geological continuity through an indicator kriging approach. This methodology is currently used as a benchmark in all the ferrous resource classification done at the Vale Ferrous mine planning department, and a comprehensive case study was conducted in two different iron ore deposits demonstrating its robustness and applicability in any mineral inventory assessment.

O vulcanismo Serra Geral em Torres, Rio Grande do Sul, Brasil: empilhamento estratigráfico local e feições de interação vulcano-sedimentar

No Municipio de Torres, nos morros do Farol, das Furnas, das Cabras e Torre Sul, foram identificadas tres associacoes de facies: ( i ) basaltos, ( ii ) arenitos e ( iii ) feicoes de interacao vulcano-sedimentar. A partir de descricoes de campo e petrograficas das feicoes de interacao vulcanosedimentares, e possivel sugerir seus mecanismos geneticos. As estrias no topo dos arenitos sao produto do fluxo de lava sobre areia inconsolidada. Os diques clasticos sao o preenchimento das disjuncoes do basalto, ja resfriado, pela areia que migra sobre ele, ou injecoes de areia para dentro do derrame, durante seu fluxo. Este mecanismo de injecao, quando encontra lava em estado plastico, da origem a peperitos, que tambem se formam por tracao e fragmentacao da base e frente do derrame. E possivel identificar em Torres, tres derrames individuais, cada um deles definindo um nivel estratigrafico. Estes niveis sao compostos por um ciclo de eventos, iniciado pelo derrame, que da origem aos peperitos, recobertos, entao, pela deposicao eolica de areia. Com a definicao de tais niveis estratigraficos torna-se evidente que o arenito aflorante no Morro das Cabras (Parque da Guarita) e uma lente de arenito intertrapico da Formacao Serra Geral, e nao pertence a Formacao Botucatu, subjacente. Palavras-chave: vulcanismo Serra Geral, estratigrafia, interacoes vulcano-sedimentares, peperito.