Geological Relationships of Basalts, Andesites and Sand Injectites at the Base of the Paraná Volcanic Province, Torres, Brazil
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Journal of Volcanology and Geothermal Research 237-238 (2012) 97–111 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Geological relationships of basalts, andesites and sand injectites at the base of the Paraná volcanic province, Torres, Brazil L.A. Hartmann ⁎, K.R. Arena, S.K. Duarte Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500, 91501‐970 Porto Alegre, Rio Grande do Sul, Brazil article info abstract Article history: The Cretaceous interaction of the heated, giant Guarani aquifer with lavas, dikes and sills of the Paraná vol- Received 7 December 2011 canic province is here evaluated based on the description of the large exposures in Torres, southernmost Bra- Accepted 24 May 2012 zil. Chalcedony silicified sandstone dikes, sills and breccias containing volcanic clasts in a sandstone matrix Available online 2 June 2012 are common in the Paraná volcanic province. Sandstone layers mark either the contact between lava flows or the base of the vertically-jointed, massive core. In Torres, one basaltic andesite and one andesite lava Keywords: flow of the low-Ti Gramado type, and basalt and basaltic andesite sills and dikes, are correlated over 10 km Paraná volcanic province Torres with rocks in the Graxaim quarry. Scintillometric emission rates vary systematically with contents of K, Th Sand injectites and U and are characteristic of each lava or intrusive rock. Major and trace elements are also characteristic Intrusive breccias of each lava. Three large outcrops of breccias display angular, amygdaloidal volcanic clasts immersed in a si- Lava stratigraphy licified sandstone matrix. No evidence was found of high temperature (1150 °C) interaction of the lava with Lava correlation the loose, erg of the Botucatu Formation, such as fluidal volcanic clasts, quenched rims or thermal recrystal- lization of the sand; only a thin (5 mm) layer of thermally modified sand is present on top of a paleodune in Torres. In Torres, the interaction of andesite with wet sand occurred after degassing of the lava, because the breccia clasts are amygdaloidal. All evidence points to hydrothermal processes for the formation of the breccias, such as lozenge fractures filled with silicified sandstone, preserved detrital characteristics of the sandstone grains and cold (below 130 °C) contact between the volcanic rock and the sand. Most significant is the timing of sand intrusion, because thin (1 mm) dikes of silicified sandstone cut fractures in the rock and the zeolite- filled amygdales. The sandstone dikes, sills and breccias are interpreted as a result of the ascent of over- pressured heated water and its vapor from the huge, underlying Guarani aquifer, carrying fluidized sand and continuously silicifying the intruded sand. The three breccias are sills intruded into cold lava flows. The description and correlation of the magmatic bodies and the field relationships of the silicified sandstones establish the stratigraphy of the Paraná volcanic province in Torres and elucidate the processes responsible for the injection and effusion of sand in the Cretaceous. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The discovery of the geological controls for sand mobilization and injection in volcanic rocks (Hartmann et al., 2012)inthe The Paraná volcanic province is a giant, bimodal basalt–rhyodacite Cretaceous Paraná volcanic province (~135 Ma; Pinto et al., 2011a; sequence that comprises the Serra Geral Group located in southeast- Janasi et al., 2011) of South America indicates that water and ern South America. The province evolution poses major questions its vapor from the huge Guarani aquifer attained 130°C during that require investigation, including the sequence and correlation of lozenge fracturing of the lava flows and sand fluidization with magmas generated in the beginning of magmatism and the interac- accompanying hydrothermal breccia intrusion. Sand dikes are com- tion of a large number of small-volume sand injectites and extrudites mon in this volcanic province and in the underlying sedimentary (nomenclature of Hurst et al., 2011) along the entire volcanic stratig- Paraná basin (e.g., Arioli et al., 2008; Perinotto et al., 2008; raphy. Magmatic processes are not evaluated here; they are related Machado et al., 2009). to variable extents of melting of lithospheric and asthenospheric Comparable continental flood basalt provinces in the world are mantle, followed by crustal contamination and magma fractionation underlain by weak aquifers and only display minor sand mobilization, (e.g., Hartmann et al., 2010a and references therein). e.g., Columbia River (USA), Karoo (South Africa) and Deccan (India). The Paraná volcanic province offers a unique opportunity to study the tectonic and hydrothermal interaction of two large geological ⁎ Corresponding author. Tel.:+55 51 33087202; fax:+55 51 33087203. structures, the Guarani aquifer and the Serra Geral Group of basalts E-mail address: [email protected] (L.A. Hartmann). and rhyodacites. 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2012.05.017 98 LA. Hartmann et al. / Journal of Volcanology and Geothermal Research 237-238 (2012) 97–111 The dynamic interaction of loose sand with overlying seal layers is a common process in offshore petroleum basins, resulting in the in- jection of large volumes of fluidized sand as dikes and sills into the overlying claystones and extrudites at the sea bottom (e.g., Huuse et al., 2010; Vétel and Cartwright, 2010; Hurst et al., 2011). It is also a significant process in continental basins where sand is mobilized by water. For instance, large-scale erg sand injection into overlying Jurassic units in Idaho included the ascent of meter-sized clasts of sedimentary rocks (Huuse et al., 2005). The entire set of sand bodies can be designated “sand injectites” and includes sand dikes, sills, lopoliths, cups, volcanoes, flows and breccias (Hurst et al., 2011). The fluid in the offshore basins is commonly at 20–70 °C and the injected sand bodies remain highly porous; e.g., Schwab et al. (2011) observed that “the injected sands are an excellent reservoir with consistently high porosity and permeability” in offshore Norway basins. A mudstone is the most common caprock because this abun- dant rock type constitutes the impermeable seal that is required for the development of overpressure in the fluid (commonly water or oil). In the Paraná volcanic province, sand dikes, sills and breccias are common features over the entire extent of the province and along most of the stratigraphic column of the Serra Geral Group (1800 m at depocenter, 300 km to the northwest of the studied area), which has ~120 lava flows (e.g., Pinto and Hartmann, 2011; Pinto et al., 2011b; Hartmann et al., 2012). In the central region of the province, Waichel et al. (2007) registered the occurrence of numerous breccias containing basalt clasts in a sandstone matrix and considered the rocks as peperites formed by the interaction of hot lava with wet sand. In the presently studied region, sand dikes were described by Petry et al. (2007), whereas Arioli et al. (2008) described numerous sand dikes and layers in the state of Paraná, located 600 km to the Fig. 1. Geological map displaying the area of occurrence of the Serra Geral Group and northwest of Torres. The breccias in the Torres hills were considered sedimentary cover of Paraná basin (Peate et al., 1992). The Torres town, where the by Petry et al. (2007) as dry peperites. It is also novel that the cap- studied region is located, is indicated by an arrow. rocks for sand intrusion in this continental province are basalt lava flows (also andesite, rhyodacite) and not mudstones as in offshore The detailed study of the exposures during field work was inte- basins (e.g., Schwab et al., 2011) and in most environments that dis- grated with scintillometric (gamma-spectrometric) measurements play sand intrusion. It is also significant that the sand bodies in the and rock geochemistry to reach a better understanding of the unique Paraná volcanic province are intensely silicified and have very low relationships of hot water and its vapor with loose, desert dune sand porosity, in strong contrast with the sand injection complexes of off- buried by volcanic caprocks. A portable gamma ray scintillometer shore petroleum basins. model GR0110G, by SAIC Exploranium, was used for the emission Numerous sandstone dikes, sills and breccias are well-exposed at rates measurements. Measurements (n=10–15, cps = counts per the sea cliffs of Torres, Brazil (Petry et al., 2007), intrusive into lava second) were registered in all visited outcrops. This is the total flows of the Serra Geral Group (Fig. 1). gamma-ray rate of emission from radioactive elements, mostly K, U We propose a model of sand injection by low-temperature hydro- and Th. thermal processes into basaltic rocks from Torres, opposed to a previ- Selected rock samples from the four studied hills and from the ous hypothesis (Petry et al., 2007) that proposed high-temperature Graxaim quarry were analyzed for major and trace elements in lava–sediment interaction for the same outcrops. Field observations the ACME laboratory, Canada, using analytical procedures named are the basis of this novel hypothesis, supported by gamma-ray by ACME as “Group 4A and Group 4B”. Group 4A analyses used scintillometry and geochemical analyses of rocks. We also establish LiBO2 fusion/dilute nitric digestion then ICP-ES for major oxides the stratigraphy and correlation of lava flows, sills and dikes in this plus minor elements; LOI is weight loss on ignition. Group 4B anal- basal portion of the Paraná volcanic province. yses of rare earths and refractory elements used ICP-MS. All data This is a case-study in Torres of the general process registered over were integrated and interpreted with the support of computers at the entire ~1,000,000 km2 area covered by the volcanic sequence of Universidade Federal do Rio Grande do Sul, using Excel and GCDKit the Serra Geral Group in Brazil, Paraguay, Argentina and Uruguay.