Volcano-Stratigraphic and Structural Evolution of Brava Island (Cape Verde) Based on 40Ar/39Ar, U–Th and Field Constraints
Total Page：16
File Type：pdf, Size：1020Kb
Journal of Volcanology and Geothermal Research 196 (2010) 219–235 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Volcano-stratigraphic and structural evolution of Brava Island (Cape Verde) based on 40Ar/39Ar, U–Th and field constraints José Madeira a,b,c,⁎, João Mata a,d, Cyntia Mourão a,d, António Brum da Silveira a,b,c,Sofia Martins a,d, Ricardo Ramalho b,e, Dirk L. Hoffmann f,1 a Faculdade de Ciências da Universidade de Lisboa, Departamento de Geologia (GeoFCUL), Campo Grande, Edifício C6, 1749-016 Lisboa, Portugal b LATTEX, Laboratório de Tectonofísica e Tectónica Experimental, Lisboa, Portugal c Instituto Dom Luiz, Laboratório Associado (IDL–LA), Lisboa, Portugal d Centro de Geologia da Universidade de Lisboa (CeGUL), Lisboa, Portugal e Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, UK f School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, UK article info abstract Article history: Three volcano-stratigraphic units were identified at Brava Island in the Cape Verde Archipelago on the basis Received 2 February 2010 of field relationships, geologic mapping and 40Ar/39Ar and U–Th ages. The Lower Unit comprises a 2-to-3 Ma- Accepted 18 July 2010 old submarine volcanic sequence that represents the seamount stage. It is composed of nephelinitic/ Available online 24 July 2010 ankaramitic hyaloclastites and pillow lavas, which are cut by abundant co-genetic dikes. Plutonic rocks of an alkaline–carbonatite complex, which intruded the submarine sequence 1.8 to 1.3 Ma ago, constitute the Keywords: Middle Unit. A major erosional surface developed between 1.3 and ~0.25 Ma. The post-erosional volcanism Cape Verde Brava Island recorded in the Upper Unit started 0.25 Ma ago and is dominated by phonolitic magmatism. This phase is 40Ar/39Ar dating characterised by explosive phreato-magmatic and magmatic activity that produced block and ash flow, volcano-stratigraphy surge, and pyroclastic fall deposits and numerous phreato-magmatic craters. Effusive events are represented uplift by lava domes and coulées. One peculiarity of Brava is the occurrence of carbonatites in both the plutonic complex and the post-erosional phase as extrusive volcanics. The intrusive carbonatites are younger than those occurring on Fogo, Santiago and Maio islands. Young (Upper Pleistocene to Holocene) extrusive carbonatites occurring in the late stages of volcanism are unknown in other Cape Verde islands. The occurrence of pillow lavas and hyaloclastites above the present sea level (up to 400 m) and raised Upper Pleistocene beaches indicates continuous uplift of Brava since the seamount stage. By dating raised marine markers, uplift rates were estimated at between 0.2 and 0.4 mm/a. The evolution of Brava was controlled by faults with directions similar to those described for Fogo, suggesting a common stress field. A detailed geological map (1/25,000) of Brava is presented. © 2010 Elsevier B.V. All rights reserved. 1. Introduction processes on a specific island, dating and determining the rates of lithospheric vertical movements, and studying the geochemical Cape Verde has long been considered the result of lithosphere temporal evolution of magmatism at either a specific island or the impingement by a mantle plume (e.g. Crough, 1978). The role of a archipelago scale. mantle plume in causing intra-plate magmatism can be inferred from Geochronology of the Cape Verde magmatism has improved the spatial age distribution, which relies on high-quality age significantly with the publication of numerous 40Ar/39Ar ages for databases. Age data are also essential for understanding the building Santo Antão (Plesner et al., 2002), São Vicente, São Nicolau and Santiago (Bosse et al., 2007), São Nicolau (Duprat et al., 2007), Santiago, Sal and São Vicente (Holm et al., 2008), Santiago and São ⁎ Nicolau (Ramalho et al., 2010a) and Boavista (Dyhr and Holm, 2010). Corresponding author. Faculdade de Ciências da Universidade de Lisboa, Departa- 40 39 mento de Geologia (GeoFCUL), Campo Grande, Edifício C6, 1749-016 Lisboa, Portugal. For Maio, Ar/ Ar ages were previously presented along with K/Ar Tel.: +351 21750342; fax: +351 217500064. ages (Mitchell et al., 1983). In addition, K/Ar ages were published for E-mail addresses: [email protected] (J. Madeira), [email protected] (J. Mata), Fogo (Lancelot and Allègre, 1974), Santiago, Brava and Maio (Bernard- [email protected] (C. Mourão), [email protected] (A. Brum da Silveira), Griffiths et al., 1975), Maio (Grunau et al., 1975), Sal (Torres et al., [email protected] (S. Martins), [email protected] (R. Ramalho), 2002) and Fogo (Madeira et al., 2005). Cosmogenic 3He exposure [email protected] (D.L. Hoffmann). 1 Present address: Geochronology Research Group, CENIEH, Paseo Sierra de dating was performed on pre- and post-caldera collapse lavas of Fogo Atapuerca s/n, 09002 Burgos, Spain. (Foeken et al., 2009). 0377-0273/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2010.07.010 220 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 Nevertheless, the geochronological data for Brava were scarce several islands, which reach 450 mapsl (above present sea level) on and inconsistent with field evidence. Bernard-Griffiths et al. (1975) Santiago Island (Serralheiro, 1976; Holm et al., 2008; Ramalho et al., in presented K/Ar ages for a nephelinite (2.4±0.2 Ma) and a phonolite press). (5.9±0.1 Ma) that disagree with their relative volcano-stratigraphic Cape Verde has been renowned for its abundant carbonatites, position. The only other age known to date was presented by Hoernle which occur on at least 6 of the 10 islands (including Brava; e.g., et al. (2002) for an intrusive granular calciocarbonatite (2.1 Ma by K/Ar). Assunção et al., 1965; Allègre et al., 1971; Silva et al., 1981; Turbeville We present fourteen 40Ar/39Ar age determinations from rocks et al., 1987; Hoernle et al., 2002; Mata et al., 2010; Mourão et al., covering the entire exposed Brava sequence. Laser ablation U–Th 2010). Previously, “calcareous dikes” and “calcareous masses of disequilibrium geochronology was used on three corals to date a volcanic origin” were mentioned by Bebiano (1932). Carbonatitic Quaternary marine deposit. Field observations and geochronological melts, which act as mantle metasomatic agents, are believed to data allowed reconstruction of the volcano-stratigraphic evolution of influence the geochemistry of some of the Cape Verde silicate magmas Brava. The tectonic structure of Brava is presented and uplift rates for (Martins et al., 2010). the island are inferred. This information is synthesised on a detailed Brava (64 km2) is the westernmost island of the NE–SW-aligned geological map of the island at a 1/25,000 scale (see Supplementary chain along with Maio, Santiago and Fogo. It is located 18 km west of data — Appendix A), which we consider a fundamental tool for future Fogo, from which it is separated by depths in excess of 1400 m. research on the island. Fig. 1, a reduced version of this map, represents Multibeam bathymetry and backscatter studies revealed a submarine major volcano-stratigraphic units and volcanic and volcano-tectonic field of volcanic cones in the area between these islands (Masson structures. et al., 2008; Grevemeyer et al., 2009). Geochemistry is beyond the scope of this paper; it will be the main The earliest geological study of Brava was conducted by Bebiano focus of a forthcoming paper. However, some data are presented here (1932),butthefirst serious effort to establish a stratigraphic to characterise the magmatic products building Brava Island. succession for the island was made thirty-five years later (Machado et al., 1968). These authors briefly described the petrography of the 2. Geological setting main lithotypes, including intrusive carbonatites, that were reported earlier by Assunção et al. (1965) and Machado et al. (1967). More The Cape Verde Archipelago (15–17°N, 23–26°W) is composed of recently, the petrology and geochemistry of carbonatites and 10 islands and various islets that roughly form a westward-facing occasionally their associated silica-undersaturated rocks have been horseshoe. They are located 600 to 900 km west of the African coast, discussed in papers by Kogarko (1993), Hoernle et al. (2002), Mourão on the southwestern part of the Cape Verde Rise, a swell ≈2.2 km et al. (2010) and Mata et al. (2010). high and 1400–1600 km wide that is considered the largest oceanic Although no historical eruptions have occurred on Brava, the intra-plate bathymetric anomaly (e.g., Lodge and Helffrich, 2006). The island is seismically active. In contrast, Fogo, which is located just archipelago stands on old (120–140 Ma; Williams et al., 1990; Müller 18 km to the East, has experienced at least 27 historical eruptions but et al., 2008) and thick (≈85 km; Cazenave et al., 1988) oceanic much less seismicity (Bebiano, 1932; Heleno da Silva and Fonseca, lithosphere. Crustal thickness is anomalously high (up to 22 km) 1999). Recent data show that seismic activity originates offshore and beneath the islands, but normal (≈7 km) between them (Ali et al., is likely related to either the submarine volcanic field situated be- 2003; Lodge and Helffrich, 2006; Pim et al., 2008). tween Fogo and Brava or the Cadamosto seamount, a growing 3-km- The archipelago is also associated with important residual geoid, tall volcano located southwest of Brava (Heleno da Silva et al., 2006; gravimetric and heat flow anomalies (e.g. Dash et al., 1976; Courtney Le Bas et al., 2007; Masson et al., 2008; Grevemeyer et al., 2009). and White, 1986); elsewhere, these features are believed to result from mantle plumes (e.g.