The Coral Island Xenolithic Dike (CIXD) is a composite intrusion with mafic borders and apophyses (Pulse 1) and a felsic centre with a high amount of granitic xenoliths and xenocrysts (Pulse 2). The ...xenoliths, most abundant in the dike core, are granitic and vary from millimetres up to 1.5 m in size. The xenolithic composite dike is low-TiO2 Gramado-type. The xenoliths are concentrated in the dacitic core, partially molten and accompanied by relict feldspar and partly resorbed quartz xenocrysts. Field, petrographic and geochemical data attest to contamination by assimilation processes in the central part. Flowage differentiation (Bagnold Effect) has concentrated the large xenoliths in the dacitic core. Evidence of xenolith partial melting comprises spherulitic K-feldspar/albite intergrowths and polycrystalline quartz aggregates. Coronas around the pyroxenes in the dacite and the absence of mesostasis in the dike magmas attest to supercooling of the system at shallow-level conditions. The ascent of the central part of the dike (dacite + xenoliths + relict xenocrysts) was only possible due to the proximity of the magma reservoir to the emplacement site and chamber recharging with hot basic magma input (Pulse 1) that prevents the system from cooling and crystallising.
•Composite dyke with mafic borders and apophysis and xenolith-rich dacitic centre.•Field, petrographic and geochemical data point to assimilation on the dacitic centre.•Melting and supercooling textures attest crystallisation in shallow level crust.•Shallow level reservoirs are central to contamination by xenolith assimilation.•Upper crustal assimilation on the evolution of the Gramado magma type.
The Cambirela Silicic Volcanic Sequence is the only Neoproterozoic volcanic manifestation in the Florianópolis Batholith. The sequence is associated with the late stages of the post-collisional ...magmatism emplaced in the Dom Feliciano Belt during the Brazilian/Pan-African Orogenic Cycle that was responsible for the amalgamation of the West Gondwana supercontinent. Distinct from other ignimbrite occurrence, that are generally associated with caldera and sedimentary sequences, the Cambirela Silicic Volcanic Sequence is associated with shallow granitic bodies in a plutono-volcanic system. The sequence is arranged along a high geomorphological feature with NNE-SSW orientation, distributed in a total area of 88 km2, and is constituted by effusive and pyroclastic units. The pyroclastic unit is dominant in the sequence and characterized by ignimbrites with 15–50% of quartz, K-feldspar and plagioclase crystals, crystal and lithic fragments immersed in a tuffaceous matrix, presenting incipient eutaxitic to parataxitic structure. Due to its geotectonic structure and the absence of associated caldera and sedimentary sequences, these ignimbrites were classified as fissure ignimbrites. The effusive unit is composed by rhyolites with 3% of quartz, K-feldspar and plagioclase phenocrysts immersed in an equigranular fine matrix. These units were divided into five lithofaceis: porphyritic rhyolite (pR), lapilli-tuff with low-crystal content and eutaxitic structure (lceLT), lapilli-tuff with high-crystal content and eutaxitic structure (hceLT), lapilli-tuff with parataxitic structure (pLT) and crystal-rich lapilli-tuff (crLT). Based on geochemical data, these were classified as high-silica rhyolites, related to the high-K calc-alkaline series, close to the shoshonitic series. The sequence has a slightly metaluminous to peraluminous character and high contents of alkalis, FeOt/(FeOt + MgO) and agpaitic indexes. These characteristics are associated to the A-type magmatism in post-collisional settings in southern Brazil. Regarding to the rheology, the sequence present glass transition temperatures ranging from 731 to 781 °C, liquidus temperatures from 940 to 968 °C and viscosities from 7.72 to 8.90 log Pa.s. These values are similar to the chronocorrelated Ilha Granite, but slightly different from the Ana Dias Rhyolite. The rheological and geochronological similarities between the Cambirela Silicic Volcanic Sequence and the Ilha Granite, together with the contact relationships and interaction observed in the field between these units, indicate that these melts probably belonged to the same magma chamber constituting a magma mush located in the shallow portion of the crust.
•Ediacaran rhyolitic volcanic and subvolcanic systems occur in southernmost Brazil.•A portion of this volcanism is associated with the Florianópolis Batholith in the Dom Feliciano Belt.•The high-grade ignimbrites are crystal-rich and associated to fissural systems.•High-silica magmatism is related to subalkaline series.•They are associated to a post-collisional setting.
The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport ...through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.
•AMS and geochemical data attest to sill and lava dome along the volcanic sequence.•LTi, Esmeralda sill emplaced between Palmas and Urubici lavas attest to magmatic recurrence.•AMS data show normal or intermediate fabrics and NE to SE propagation for the sill.•Flow directions are S for mafic lava flow; SSE, NNE or vertical for silicic lavas.
The porosity of volcanic rocks can be strongly affected by secondary processes such as the percolation of water and hydrothermal fluids, thus promoting crystallization of secondary minerals in the ...pores. With an aim to evaluate the impact of secondary mineral fillings, this paper shows the segmentation of secondary and filled pores based on the micro-CT technique. The mineralogy of the sample was analyzed with optical microscopy. Multiscale analysis was performed to determine the total porosity before and after the processes. The results show the influence on the permeability parameter caused by crystallization.
•Primary permeability predictions for a volcanic rock that suffered pore filling processes.•Multi-scale porosity determination by means of a mathematical model applied to microCT images.•Multi-scaled pore size distribution of a volcanic rock sample.•Analysis of the influence of pore-filling events by minerals in the permeability of a sample.
In the Rio Grande do Sul State, southern Brazil, the volcanic sequence of the Paraná-Etendeka Igneous Province consists of pahoehoe and rubbly pahoehoe lava flows with basaltic and basaltic andesitic ...composition respectively, overlaid by acidic volcanic rocks. The acidic volcanic rocks of the Paraná-Etendeka Igneous Province exhibit textures and structures that can be related to effusive and/or explosive eruptions generating predominantly rheoignimbrites. The huge lava volume related to the emplacement of large igneous provinces implicates on efficient feeder systems that are more commonly observed in continental environments. In the Paraná-Etendeka Igneous Province, feeders of basaltic rocks are exposed in several dyke swarms (Ponta Grossa NW trending, Florianópolis/Skeleton Coast (NW Namibia) N-S trending, Serra do Mar NE trending and Henties Bay/Outjo NE trending). In contrast, the only feeder system proposed to the acidic rocks of the Paraná-Etendeka Igneous Province is the Messum complex in Namibia (Milner et al. 1995). In the study area, the opening of three quarries for the extraction of dimension stones has exposed impressive structures/textures that show the effusive emplacement and the ductile to fragile-ductile magma transition along the acidic feeder dykes. Besides that, magma mixing/mingling processes between two acidic magmas are observed along the dykes. Here we describe new occurrences of acidic feeder dykes, correlate the dykes with acidic flows and discuss their importance to understand the emplacement of the Palmas type acid units in southern Brazil.
•We describe acidic feeder dykes correlated with acidic flows.•We identified the effusive acid flows of the Paraná-Etendeka Igneous Province.•We describe the feeder systems of the acidic lava flows in 3D.
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.
Neoproterozoic magmatism in southern Brazil is associated with translithospheric shear belts and strike-slip basins in a post-collisional setting related to the last stages of the Brasiliano-Pan ...African orogenic cycle. It evolved from an association of high-K calc-alkaline, leucocratic peraluminous and continental tholeiitic magmas, to shoshonitic, and eventually to sodic mildly alkaline series. Bimodal volcanism in the last group was coeval with subaerial siliciclastic sedimentation in all post-collisional basins preserved in the region. Three magmatic associations were identified in the bimodal volcanism: (1) low-Ti basalts and rhyolites; (2) high-Ti basalts and rhyolites; and (3) high-Nb rhyolites and basalts. Basic parental magmas were produced from sources related to EM1-type mantle previously modified by Brasiliano subduction, whereas silicic high- and low-Ti rocks were probably produced through fractional crystallization. Significant crustal contamination was recognized only in some slightly peraluminous rhyolites with low contents of HFS elements. High-Nb rhyolites, the youngest volcanic rocks, probably reflect the participation of asthenospheric components in the source, and mark the last magmatic event related to the Brasiliano-Pan African orogenic cycle in southern Brazil.
In the São Marcos (RS) and Antonio Prado (RS), the Serra Geral Formation exposes at the base basalts of pahoehoe type, coveredby basalts of the ´a´ā type. The first succession was generated by a low ...rate of eruption in a closed flow system allowed the flow toreach distances > 100 km of the source.T he ´a´ā lava flow types were generated by higher rates of eruption andt ransported in openchannels where rapid cooling prevented long distances from the source to be reached. The two types of basalts are low-TiO2 tholeiiticand the morphology of flows is not related to variations in SiO2 and MgO contents. Above these rock types outcrop acidic volcanicrocks geochemically of Caxias Group (Palmas Subgroup). Dimension stones extraction exposed the inner portions of the acidicfeeder dikes with vertical magmatic foliations. The lava domes have exogenous characteristics and horizontal foliations. We proposea model for the generation of domes involving the diapirically rise of acids magmas that become vesicular and more viscous, thatstop near the surface. New magmatic pulses extracted “pieces” of the vesicular fraction generating autobreccias in the conduit andvertical structures that extend laterally toward the surface organizing the lava domes with vitrophyres in the base and in the top, witha thin massive phaneritic core. Magmatic textures of the domes are typical of effusive units and the identification of the feeder dykesin the area allows the understanding of the emplacement process of acidic flows in the Serra Geral Formation.
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