Volcano-Stratigraphic and Structural Evolution of Brava Island (Cape Verde) Based on 40Ar/39Ar, U–Th and Field Constraints

Journal of Volcanology and Geothermal Research 196 (2010) 219–235

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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).

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. Sleep, 1990). The genesis of Cape Verde 3. Methods from a deeply anchored mantle plume seems to be supported by seismic tomography studies (Montelli et al., 2006; Zhao, 2007) and by 3.1. 40Ar/39Ar geochronology unradiogenic He isotope signatures obtained both from silicate and carbonatitic rocks (R/Ra up to 15.7), which point to the contribution of Ages were determined for fourteen samples using the 40Ar/39Ar a high 3He/4He reservoir that could plausibly exist in the deepest parts method at the Noble Gas Mass Spectrometry Laboratory, Oregon State of the lower mantle (cf. Christensen et al., 2001; Doucelance et al., University (USA). Samples were selected based primarily on their 2003; Mourão et al., 2007; Mata et al., 2010). Notably, however, a stratigraphic position as well as their lithological and geochemical recent analysis of P-to-S receiver functions showed that the time characteristics (see Supplementary data — Appendix B). The materials separation between the 410 and 660 km discontinuities is not to be analysed within each sample were selected according to specific modified beneath Cape Verde (Helffrich et al., 2010). mineralogical constraints and the granularity and homogeneity of the Currently exposed volcanic sequences in the archipelago range in samples. age from the Miocene to the present. The oldest published date for the Whole-rock material was collected by a diamond-tipped drill bite Cape Verde hotspot (≈26 Ma) was obtained for a submarine basalt which produced a 5-mm-diameter core that was divided into disks of from the basal complex of the Island of Sal (Torres et al., 2002). The 100–300 mg. To recover groundmass and mineral separates, the most recent eruption at Fogo Island occurred in 1995, when 0.054 to samples were crushed and sieved (fraction 250–500 μm), and 0.068 km3 of basanitic lava was emitted in a 55-day-long eruption phenocrysts were removed by magnetic methods. The samples were

(Madeira et al., 1997; Torres et al., 1997). The mean rate of melt then cleaned using a mild acid treatment (5% HNO3), washed in an crustal emplacement for the archipelago (0.026 km3a−1) corresponds ultrasonic bath, rinsed with de-ionized water and dried in an oven. to ≈9% of the flux calculated for Hawaii (Holm et al., 2008). The long Finally, samples were hand-picked under a binocular microscope, magmatic history and the semi-stationary position of the lithosphere washed with acetone and de-ionized water and oven-dried (Duncan (Pollitz, 1991; Holm et al., 2008) explain how such a weak plume and Keller, 2004). produced the world largest oceanic swell (e.g. Mata et al., 2010). In the TRIGA reactor at Oregon State University, samples were The region has been subjected to important vertical movements, as irradiated with neutrons at 1 MW power for 6 h along with the FCT-3 demonstrated by the occurrence of MORB at Maio and Santiago (De biotite standard (28.03±0.01 Ma age) to monitor the neutron flux. Paepe et al., 1974; Gerlach et al., 1988; Millet et al., 2008), seafloor A MAP 215-50 rare gas mass spectrometer was used to sediments of probable Lower Cretaceous age at Maio (Azéma et al., perform40Ar/39Ar analyses. Ar extraction by incremental step heating 1990; Holm et al., 2008) and uplifted submarine alkaline lavas on was achieved for whole-rock or groundmass/mineral separates with a .Mdiae l ora fVlaooyadGohra eerh16(00 219 (2010) 196 Research Geothermal and Volcanology of Journal / al. et Madeira J. – 235

fi Fig. 1. Simplified geologic map of Brava. The inset on the upper right corner shows the geographic setting of Brava in the context of the Cape Verde archipelago. Geological mapping is based on 1:25,000-scale topographic maps (Serviço Cartográ co do 221 Exército, Portugal, 1979) and stereoscopic black and white aerial photos at the 1:30,000 scale (Centro de Geografia do Ultramar, Portugal, 1957). A larger version of the map, at the 1:25,000 scale, is available as Supplementary data (Appendix A). 222 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235

Heine low-blank double vacuum resistance furnace and a Merchantek which was determined by FUS-ICP. Duplicate measurements of the

10-Watt continuous fire CO2 laser, respectively. Samples were lowest-concentration sample for each type of technique give an degassed during 10 to 16 temperature steps, depending on individual estimate of the total reproducibility of the analyses. For whole-rock sample characteristics, from 400 °C to 1400 °C. Prior to age calcula- samples, the reproducibility is: (i) on the order of 1% for major tion, all data were corrected for system blanks, mass fractionation and element contents, (ii) better than 3% for Rare Earth Elements (REE, interfering argon isotopes generated by Ca, K and Cl during except for Gd which gives 6.5%), (iii) around 5% for High Field irradiation, and the decay and J-value were calculated. Strength Elements (HFSE) and (iv) better than 2% for elements that Sample40Ar/39Ar ages were calculated using the ArArCALC v2.2 are generally highly incompatible in the oceanic context (Rb, Ba, U, software package (Koppers, 2002). The decay constant used through- and Th). Detailed information on the analytical methods can be out the step age calculation was λ=(5.530±0.097)×10−10 a−1, the found at http://www.actlabs.com. corrected value of Steiger and Jäger (1977) reported by Min et al. (2000). The initial Ar composition was assumed to be atmospheric 4. Volcano-stratigraphy of Brava Island (40Ar/36Ar=295.5) for plateau calculations. The data are presented as 39 “ ” step ages plotted against cumulative Ar released ( plateau Brava is characterised by an irregular plateau between 300 and 39 40 36 40 diagrams) or as isotope correlation diagrams ( Ar/ Ar vs. Ar/ Ar, 976 m above sea level, which is bounded by steep coastal cliffs and cut inverse isochron diagrams), in which the slope of the collinear step by fluvial incision in a generally radial drainage pattern (Fig. 2). The 40 36 fi compositions is equivalent to age and the Ar/ Ar intercept speci es plateau presents some aligned, hemi-spherical hills formed by the initial Ar composition of the analysed material (Duncan and Keller, phonolite lava domes, a number of closed depressions that corre- 2004). spond to recent phreato-magmatic craters, and several NNW–SSE to NW–SE fault scarps that define a 2.85-to-2.3-km wide graben – 3.2. U Th disequilibrium geochronology structure in the southern part of the island (the Cachaço Graben; Figs. 1, Figure 2). In addition to constructive volcanic and tectonic – Three fossil coral samples (CY-213a c) from a Quaternary terrace processes, marine and fluvial erosion and mass wasting processes were dated by laser ablation (LA) multi-collector (MC)-inductively contributed to the present morphology of the island (Madeira et al., – coupled mass spectrometry (ICP-MS) U Th disequilibrium techni- 2008). ques, following the method outlined by Hoffmann et al. (2009). All Field observations revealed the presence of an older basement MC-ICP-MS measurements were performed at the Bristol Isotope composed of a submarine volcanic sequence and an intrusive complex Group (BIG) laboratory using a ThermoFinnigan Neptune coupled that is unconformably covered by younger sub-aerial volcanic with a New Wave Research UP193HE ArF Excimer laser system. deposits. These sequences allowed the definition of three major Samples were cut, polished, cleaned in an ultrasonic bath for 5 min volcano-stratigraphic units designated Lower, Middle and Upper, as and dried. Next, they were placed in a laser sample cell together with a described below. secular equilibrium calcite ‘standard’ for correction of the instrumen- tal biases of LA U–Th isotope measurements on CaCO3. Potential matrix effects due to differences between aragonite and calcite are 4.1. Lower Unit: submarine volcanism negligible within the uncertainties achieved for U–Th isotope measurements using the LA technique (Hoffmann et al., 2009). The northwestern littoral region of Brava (between Portete and Ablation was done with He as the carrier gas, which was mixed Sorno bays) hosts a sequence of alternating hyaloclastites, pillow breccias, and pillow lava piles of nephelinitic/ankaramitic composi- with the Ar sample gas and N2 in a quartz mixing cell before injection into the Ar plasma. Typical laser power density was 5 J/cm² at 70% tion (Fig. 3a). A small outcrop occurs on the NW coast, near Vinagre. power output. For the U–Th isotope LA measurements, a repetition The submarine sequence is cut by two major families of dikes trending – – fi rate of 7 Hz and a spot size of 250 μm were used. Material was ablated N S and E W(Fig. 4) that, based on eld evidence and petrographic/ from a 0.5-mm-long track in 6 passes by moving the laser spot at a geochemical considerations, are also integrated in this unit. Some speed of 20 μm/s. A standard sample–standard bracketing procedure dikes present deformation structures that are interpreted as the result was applied, and data collection and corrections were done according of small-scale submarine slides that were more or less contempora- to Hoffmann et al. (2009). neous with dike intrusion. These movements testify to the gravita- tional instability of the steep volcanic pile. The submarine volcanic 3.3. Major and trace element analyses sequences plunge to the northwest in the Fajã d'Água area and to the north in the Sorno region. The dips vary significantly but are generally During geological mapping, samples from all volcano-stratigraphic steep (30 to 40°), indicating that volcanism occurred on abrupt units were collected for petrographic and geochemical studies. Samples submarine slopes. This explains the abundance of pillow breccia were selected for bulk geochemistry analysis on the basis of their accumulations, which result from pillow detachment and fragmenta- petrographic specificities and freshness. Samples were crushed by tion. Locally, where submarine slopes were gentler, pillows were able hydraulic press to remove all visible signs of alteration, and then to accumulate and form densely packed piles that were almost devoid fi reduced in size by a jaw crusher and powdered in an agate swing mill. of sediment- or hyaloclastite- lled spaces. Although limited to a small Major and trace element whole rock analyses were performed coastal area, the outcrops suggest an outward (radial) dipping at Activation Laboratories (Canada) according to Code 4Lithore- structure of the submarine volcano. They are now raised to heights search+Code 4BINAA analytical packages (plus Code 4F for of up to 400 mapsl; the highest known outcrop is located in the carbonatites). Alkaline dissolution with lithium metaborate/tetra- headwall of the Fajã de Água valley. borate, followed by nitric acid dissolution, was performed for all Both the extrusive products and the dikes are truncated by an analyses, except for the determination of Cd, Cu, Ni and Zn, which irregular erosive surface and covered, in angular unconformity, by were achieved after acid digestion. Major element concentrations products of primarily sub-aerial, recent volcanism (Upper Unit). were determined using Fusion-Inductively-Coupled Plasma (FUS- ICP) (except for fluorine in carbonatites, which was determined by 4.2. Middle Unit: intrusive complex Fusion-Ion-Selective-Electrode, FUS-ISE). Trace element concentrations were determined using Inductively Coupled Plasma-Mass The southern and eastern littoral areas of Brava are mainly composed Spectrometry (ICP-MS), except for zirconium in carbonatites, of plutonic rocks that form an alkaline–carbonatite complex. J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 223

Fig. 2. Digital terrain model of Brava showing island morphology and the most important toponyms mentioned in the text. Main streams (ribeiras): 1. Ribeira da Furna, 2. Ribeira de Rasque, 3. Ribeira Funda, 4. Ribeira de Aguadinha, 5. Ribeira de Aguada, 6. Ribeira dos Moinhos, 7. Ribeira dos Ferreiros, 8. Ribeira da Lomba de Peixe Vermelho, 9. Ribeira do Morro, 10. Ribeira da Fajã de Água, and 11. Ribeira do Sorno.

Intrusive rocks are highly variable in composition and include plutonic rocks to be intrusive into the submarine volcano; this clinopyroxenites, ijolites–melteigites–urtites, nepheline syenites and interpretation is confirmed by geochronological ages (see Section 6.1; carbonatites. In many cases, the different lithotypes occur in close Table 1). association in the field but have extremely complex outcrop relations. For instance, at Chão de Ouro, pyroxenite bodies are associated with 4.3. Upper Unit: post-erosional volcanism diffuse zones of ijolites (s.l.) and nepheline syenites and are crosscut by metric masses and thin dikes of carbonatitic composition (Fig. 3b). Post-erosional volcanism is dominantly phonolitic but also Nepheline syenites, however, do not always occur in association with includes small volumes of mafic and carbonatitic extrusions. mafic and ultramafic rocks. At Porto de Ferreiros, for instance, they are isolated from other silicate intrusives and associated only with dikes 4.3.1. Phonolite volcanism and masses of carbonatitic composition. The close field relationship The phonolitic sequence was produced by phreato-magmatic and between mafic and ultramafic rocks suggests that these units are co- magmatic volcanism. Phreato-magmatic pyroclastic deposits are genetic and that they result from cumulation/differentiation process- abundant; they include surges and lithic-dominated fall layers that es affecting magmas of ijolitic composition. In all, the intrusive suite of frequently contain accretionary lapilli. Local, thick, massive layers of rocks can be interpreted as a set of shallow magma chambers and consolidated ash deposits supporting dispersed lithic blocks are magma pockets related to a magmatic phase for which the volcanic interpreted as muddy ash flows related to phreato-magmatic activity counterparts are no longer preserved. (Fig. 3e, f). Thin pumice lapilli fall layers correspond to short-lived The intrusive rocks crop out between sea level and 700 mapsl and magmatic phases during phreato-magmatic activity. A voluminous are cut by the same erosional surface described for the submarine pumice fall deposit, present in the southern area of the Campo Baixo sequence. The present altitude of these rocks suggests a deep level of crater, represents an eruption with plinian to sub-plinian character- erosion and significant uplift. The geometric (stratigraphic) relation istics. A pumice and ash flow (ignimbrite) is exposed at the Vigia fault between the Middle Unit and the submarine sequence (Lower Unit) is scarp (see Section 5, Fig. 2). The eruptive centres responsible for the not observed because the cliffs where the contact could be seen are large volume of phonolite pyroclasts include around thirty phreato- covered by younger volcanic deposits. However, we consider the magmatic craters with variable dimensions (Fig. 3c; morphometry 224 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 225

Fig. 4. Stereographic plot of all measured dikes from the Lower Unit showing dominant E–W and N–S dike families: a) β diagram; b) rose diagram. Schmidt net, lower hemisphere. Stereonet for Windows V1.2. (Allmendinger, 2003).

Table 1 40Ar/39Ar age summary of samples from Brava Island.

Sample Unit Lithotype Material used Plateau characteristics Isochron characteristics

Plateau age nº steps used 39Ar K/Ca MSWD p Isochron age 40Ar/36Ar MSWD p (Ma±2σ) (n of total) (% of total) (± 2σ) (F) (Ma±2σ) intercept (F)

CY-193 Upper Phonolite Groundmass 0.034±0.021 10 (10) 100.0 1.44±0.45 0.76 0.64 0.007±0.008 296±1 0.44 0.91 CY-106 Upper Phonolite Feldspar 0.240±0.010 8 (10) 92.9 38.14±3.38 0.07 0.99 0.238±0.032 295±5 0.08 0.99 CY-27 Upper Phonolite Feldspar 0.253±0.005 11 (11) 100.0 5.85±7.57 0.80 0.63 0.248±0.009 297±3 0.77 0.64 CY-226 Upper Carbonatite (extrusive) Biotite 0.50±0.29 12 (12) 100.0 3.38±1.27 0.11 0.99 0.52±0.73 295±1 0.12 0.99 CY-154 Middle Nepheline Syenite Feldspar 1.36±0.02 10 (13) 73.6 15.86±2.35 0.55 0.84 1.30±0.11 303±11 0.45 0.89 CY-103 Middle Nepheline Syenite Feldspar ––––––1.46±0.05 434±14 0.77 0.51 CY-116 Middle Carbonatite (intrusive) Biotite 1.55±0.01 10 (16) 68.3 4.89±1.02 0.30 0.97 1.55±0.11 296±26 0.35 0.95 CY-55 Middle Pyroxenite Amphibole 1.58±0.03 6 (12) 71.0 0.15±0.004 0.30 0.91 1.55±0.14 308±59 0.33 0.86 CY-118 Middle Nepheline Syenite Biotite 1.77±0.03 6 (14) 71.7 11.30±4.74 0.84 0.52 1.68±0.13 319±33 0.49 0.74 CY-245 Middle Pyroxenite Amphibole ––––––1.95±0.38 473±37 0.00 1.00 CY-8 Lower Melilite nephelinite Groundmass 1.99±0.09 9 (10) 90.5 0.019±0.018 0.23 0.98 1.98±0.11 296±6 0.26 0.97 CY-9 Lower Melilite nephelinite Whole-rock 2.17±0.03 5 (10) 72.9 0.54±0.12 1.23 0.30 2.16±0.04 296±4 1.52 0.21 CY-165 Lower Foidite Amphibole ––––––2.28±0.13 295±2 0.22 0.95 CY-166 Lower Basanite Amphibole 2.92±0.11 5 (11) 73.1 0.047±0.017 0.13 0.97 2.90±0.36 297±30 0.17 0.92

Preferred ages in bold; underlined values are considered “non-accepted” ages; F = goodness of ﬁt parameter (MSWD); p = probability of occurrence. Analyses reported for minerals were performed on grain aggregates of 30 to 70 mg (see Supplementary data/Appendix B).

data in Supplementary data, Appendix C). Many small craters are same trends as the craters. A fault zone, which divides the outcrops of nested inside larger ones. The largest crater, Campo Baixo (~2.8 km2), the two basement units, apparently also separates the cluster of may correspond to a small caldera. Two unconformities in the craters from the southern set of domes. This distribution is probably pyroclastic sequence are exposed inside and south of the crater (in related to the presence or absence of significant aquifers on fault- the Ribeira dos Ferreiros valley) and separate probable pre-, syn- and bound blocks, which is in turn controlled by lithological and structural post-caldera deposits. The conduits of another twenty two volcanic differences between the two basement units. The presence of aquifers centres are sealed by phonolite domes (Fig. 3d; morphometry data in to the northwest of the fault zone may explain the frequency of Supplementary data, Appendix C). phreato-magmatic activity; to the southeast groundwater–magma The geographical distribution of the volcanic centres is character- interaction was less common. Where the slopes were steep, some ised by a cluster of craters in the centre–north area of the island. domes generated thick phonolite flows (coulées). The Morro das Several volcanic alignments indicate structural control by NE–SW to Pedras dome, which presents a pristine volcanic morphology (Fig. 3d), ENE–WSW and NNW–SSE faults and fractures (Figs. 1, 5). Effusive represents one of the youngest phonolitic events. The dome grew volcanism is represented by thick coulées and domes. Domes are inside a fluvial valley cut into pyroxenites and ijolites (s.l.), south of mostly located in the southern half of Brava or in the northern littoral Cachaço village. This extrusion is almost untouched by erosion and and may reach 1.4 km in diameter and 330 m in height; the most presents no vegetation, indicating a very young, probably Holocene, voluminous dome exceeds 0.2 km3. The domes are aligned along the age.

Fig. 3. Geologic features of Brava Island: a) pillow lavas overlying hyaloclastites, cut by a dike, from the Lower Unit in the Fajã Grande area. Note the steepness of the contact, b) pyroxenites/ijolites cut by abundant carbonatite dikes (Middle Unit), unconformably covered by a stratiﬁed sequence of pyroclastic deposits from the Upper Unit (Chão d'Ouro area). c) Phreato-magmatic crater of Cova Joana (Upper Unit). The formerly closed depression is now open towards the west as a result of headward erosion from Ribeira da Fajã de Água. d) Morro da Pedras, the youngest phonolite lava dome (Upper Unit). e) Exposed surge deposits from the Upper Unit, displaying beautiful climbing ripples (road to Fajã de Água). f) Ash ﬂow deposit from the Upper Unit, containing accretionary lapilli indicative of phreato-magmatic activity (road to Fajã de Água). g) The Vigia fault and fault scarp, cut by the incision of the Ribeira dos Ferreiros valley. The fault scarp is 70 m tall at this location. h) Slickensides displaying dominant strike–slip at the Minhoto fault zone. 226 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235

Fig. 5. Geological sketch map of Brava showing the tectonic and volcano-tectonic structures superposed on the major stratigraphic units. G — Monte Gâmbia fault; S — Sorno fault zone; CG — Cutelo Gregório fault; M — Minhoto fault zone; C — Cachaço fault; V — Vigia fault; CO — Chão d'Ouro fault zone. Streographic plots of the main fault systems: a — Plot of 5 faults and their slickensides from the Sorno Bay fault zone. Slickensides are partially coherent with riedel and anti-riedel fractures associated with a NW–SE left lateral shear; however, some planes bear superposed slickensides that are incompatible with a single stress ﬁeld. b — Stereographic plot of 21 measurements of striated fault surfaces on the Vigia fault, obtained on the Boca de Porco and Monte Gratão areas, indicating dominant dip–slip. c — Stereographic plot of all faults (n=32) measured in the ﬁeld; note that the plot is biased by the high number of measurements obtained on the Vigia fault. d — Rose diagram representing the direction of all measured faults; concentric circles correspond to 5% intervals of the total number of fault measurements (5, 10, 15 and 20%) and the same note applies. Schmidt net, lower hemisphere. Stereonet for Windows V1.2. (Allmendinger, 2003).

4.3.2. Mafic volcanism lithic fragments of nepheline syenite, pyroxenite and carbonatite. The Mafic volcanism is rare in the Upper Unit. Examples include the proximal deposits contain abundant juvenile mafic spatter. In the strombolian cone of Alcatraz, the phreato-magmatic crater of Achada Santa Bárbara–Furna area, several small-volume mafic lava flows de Chão de Ouro and related deposits, and a few small lava flows that occur at stratigraphically high positions in the volcanic sequence. are mostly located on the northeast slopes of Brava along with Although the Upper Unit is characterised by essentially the same extrusive carbonatites. Of these, the Alcatraz eruption was the most mafic lithologies found in Lower Unit, it also includes more evolved important event. It produced a major cinder cone and a lapilli fall. compositions like tephrites and phonotephrites (Fig. 6). Mafic lapilli fall layers, probably related to the Alcatraz event, occur on the summit of Monte Miranda dome, in the walls of the Achada de 4.3.3. Carbonatite volcanism Chão de Ouro crater and at the Vigia fault scarp. The Achada do Chão The occurrence of extrusive carbonatites was briefly mentioned by de Ouro event post-dates the Alcatraz eruption and produced a Turbeville et al. (1987), Peterson et al. (1989) and Hoernle et al. phreato-magmatic crater cut into syenites. This event covered the (2002). More recently, Mourão et al. (2010) identified and mapped surrounding area with a pyroclastic deposit that is rich in angular twenty outcrops of extrusive carbonatites in the northwestern area of J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 227

crustaceans (balanus, crabs), and briozoarians; the other is a 60-cm- thick accumulation of rhodoliths, Patella shells, and coral heads in a bioclastic sand matrix. Three of these coral fragments were dated by U–Th disequilibrium geochronology (see Section 6.2). Accumulations of breccia, which are related to important mass movement events, occur at littoral cliffs, the slopes of deeply incised ﬂuvial valleys, and crater walls. The most voluminous deposits, ranging from 7×106 to 3×108 m3, are located in the slopes above Sorno bay, in the sea cliffs of Fajã de Água, inside the ﬂuvial valleys of Fajã de Água, Ferreiros and Portete creeks, and in Achada Figueirinha crater (Madeira et al., 2008).

5. Faults and volcano-tectonic lineaments

Most volcanic centres (craters and domes) of the Upper Unit are aligned along or displaced by faults (Figs. 1, 5), indicating tectonic control of volcanism. The most prominent morpho-tectonic features are the Vigia (Fig. 3g) and Cachaço fault scarps, which form a NNW– SSE-trending graben in the southern part of the island. Fig. 6. Total alkali–silica (TAS) diagram (Le Maitre et al., 2002). The thick dashed line is a The main tectonic directions are NW–SE to NNW–SSE, NE–SW, N–S compositional divider between alkaline and subalkaline volcanics (MacDonald, 1968). and roughly E–W(Fig. 5C, D). These directions are similar to those According to Le Bas (1989) the normative content of albite and nepheline was used to correctly classify samples plotting in the “foidites” and “basanites/tephrites” fields: previously observed for the neighbouring island of Fogo (Brum da Silveira nephelinites have N20% normative Ne and melanephelinites and basanites have b20%. et al., 1997a,b; Madeira and Brum da Silveira, 2005), suggesting that the Melanephelinites have b5% normative Ab and basanites have N5% Ab. fracture systems have regional significance. The NW–SE to NNW–SSE- trending Vigia Fault presents dip–slip striations that indicate normal fault the island around Nova Sintra, in the southwest near Campo Baixo, kinematics (Fig. 5B). Another major fault system, the Minhoto Fault Zone, and in the south around Cachaço and Morro das Pedras. Stratigraphi- crosses the island in a NE–SW direction. It is marked by fault breccia, cally, the deposits are among the most recent of the Upper Unit. Most slickensides, fault scarps, and linear fluvial valleys in the Minhoto–Baleia of the extrusive carbonatites are pyroclastic; they comprise magmatic area (east slope) and by crater alignments on the plateau area. The Ribeira and/or phreato-magmatic ash and lapilli fall deposits, one pyroclastic dos Ferreiros valley may also be controlled by this fault zone, which plays a flow, and a probable lava flow. Pyroclastic deposits are mostly major role in the structure of the island. Indeed, the geographical formed by fine to very fine ash (see Mourão et al., 2010, for detailed distribution of the basement units defines a rectilinear contact zone that is descriptions). interpreted as a fault contact along the Minhoto fault zone. This structure was responsible for significant vertical offset and contributed to the 4.4. Non-volcanic deposits increased uplift of the southeastern half of Brava, where plutonic rocks crop out, relative to the northwestern region of the island, where Sedimentary formations include marine sediments, mass wasting submarine volcanic products occur. Still, observed slickensides indicate deposits, current beach sand and gravel, talus deposits, and alluvia. dominant dextral strike–slip (Fig. 3h). At Sorno Bay a WNW–ESE to Several outcrops of marine sediments, which are usually fossilif- WSW–ENE-trending fault zone presents weakly dipping slickensides that erous sand and/or conglomerate, mark the presence of raised beaches indicate dominant strike–slip. The reactivation of fault planes is indicated at different altitudes. In littoral cliffs in Sorno, Fajã de Água and Furna by slickensides superposed on grooves with different pitches. These faults areas the remains of marine sediments can be found at elevations of are compatible with roughly E–W maximum horizontal compression, about 125 m, 90–100 m, 20–30 m, 10–15 m and 2–5 m. Some of these which corresponds to a WNW–ESE left lateral shear zone with synthetic outcrops correspond to sediments preserved in notches in sea cliffs, E–W riedel (R) and antithetic WSW–ENE anti-riedel (R') shears (Fig. 5A). while other deposits stand in narrow, wave-cut platforms. Along the However, this stress field is incompatible with NW–SE to NNW–SSE road to Fajã de Água, at an altitude of 100 mapsl, sediments from a normal faulting (eg., the Vigia and Cachaço Faults), suggesting the raised beach are intercalated in a phonolitic phreato-magmatic influence of different stress fields in the region. sequence, near the base of the Upper Volcanic Unit, just above the erosional surface cut on the hyaloclastites of the Lower Unit. Two 6. Geochronology results notches cut into the hyaloclastites at elevations of 98 m and 95 m (2 and 5 m below the erosional surface) contain marine sand and gravel. 6.1. 40Ar/39Ar incremental heating ages Therefore, these beach deposits are interpreted as contemporaneous with the beginning of the younger volcanism. The fourteen samples selected for dating by 40Ar/39Ar incremental The lower marine event is marked by a narrow and discontinuous heating cover a large range of lithotypes (phonolites, nepheline wave-cut platform and by notches cut into sea cliffs at approximately 2 to syenites, carbonatites, pyroxenites and basalts s.l.). Four samples 5 mapsl. Although these platforms are usually erosive surfaces that are belong to the Lower Unit, six samples belong to the Middle Unit, and devoid of deposits, they locally support small patches of conglomerate four samples belong to the Upper Unit. containing shell fragments. At least in two areas, burrows excavated by Detailed 40Ar/39Ar step-heating data for all of the analysed samples sea-urchins, which mark the top of the inter-tidal zone, are now exposed are available as Supplementary data (Appendix B). Dating results are above sea level. South of the village of Furna, marine sediments are summarised in Table 1 and Fig. 7. Plateaux are defined as sections of preserved at elevations of 4 to 5 mapsl in notches in the Ribeira de Rasque the age spectra carrying 50% or more of the total released 39Ar gas valley, a few meters from the stream mouth. These are interpreted as contained in at least three consecutive heating steps with overlapping correlative with the 2-to-5-mapsl wave-cut platform and notches. There ages; uncertainty is defined at the 95% confidence level (Fleck et al., are two outcrops: one is formed by gravel in a 40-to-80-cm-thick, 1977; Baksi, 2003). bioclastic sand matrix and, contains a marine macrofauna of gastropods The Mean Square Weighted Deviate (MSWD=F), also reported in (e.g. Patella, Fissurella), bivalves, echinoderms (sea urchin spines), Table 1,isanF-statistic that measures the scatter of the individual step 228 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 ages. Although values below 2.5 are commonly considered acceptable, Brava's silicate rocks are diverse, ranging from ultrabasic to this value is dependent on the degrees of freedom of the data and so intermediate compositions (SiO2 =35.91% to 57.84%, on an anhy- further statistical tests should therefore be performed (Baksi, 2003; drous basis; Fig. 6). Despite the cumulative character of most of the Norman et al., 2010). To ensure the statistical reliability of the age intrusive silicate rocks of the Middle Unit, these rocks were also data, a chi-squared test was used. A chi-squared value is obtained by plotted on the TAS diagram, to facilitate the preliminary comparison multiplying the MSWD by the corresponding degrees of freedom of with extrusive rocks from the other units. the data set (number of steps minus 1 or minus 2 for plateau or The majority of the submarine lavas and dikes forming the Lower isochron ages, respectively). The result can be compared with relevant Unit are ultrabasic foidites. Their highly silica-undersaturated char- chi-squared tables to assess whether the data belongs to the same acter is evidenced by normative compositions that typically present normal distribution (Baksi, 2003, 2006; Ivanov et al., 2009; Table 1)at nepheline, leucite and larnite (dicalcium silicate). Given the appre- a 95% confidence level (probability of occurrence, P, above 0.05). ciable amount of normative larnite (up to 8.6%) in most rocks of this Eight of the analysed samples produced indisputable plateau ages unit, the predominant lithotype is melilite melanephelinite; basanite based on the above-mentioned requirements and do not need further is the second most abundant rock type. Some dikes are extremely comments. The remaining six results are discussed below. porphyritic with more abundant clinopyroxene than olivine pheno- Samples CY-193 and CY-226 show large errors due to limited chrysts, conferring an ankaramitic character to these rocks. radiogenic 40Ar coupled with elevated atmospheric 40Ar. These The Middle Unit is dominated by intrusive rocks that present uncertainties make the obtained age results “non-accepted”. textural evidence of cumulative processes. This is particularly true for A reliable plateau was not reached in sample CY-103, for which excess pyroxenites, which are interpreted here as having formed at the base of 40Ar is evident from the 40Ar/36Ar intercept of 435±15. However, a magma chambers by pyroxene accumulation from ijolitic (s.l.) magmas. significant isochron (F=0.77; p=0.51) was attained with the first five Different degrees of fractionation/accumulation of such magmas explain consecutive heating steps producing an age of 1.46±0.05 Ma. the common association of pyroxenites, melteigites (clinopyroxe- Despite evidence of excess 40Ar, sample CY-118 has produced a neNnepheline), ijolites and urtites (nephelineNclinopyroxene) within reliable plateau age (1.77±0.03 Ma) based on statistical parameters the same intrusions. Silicate rocks are often intruded by carbonatite (F=0.84; p=0.52). rocks, forming alkaline–carbonatitic associations similar to those The plateau for sample CY-245 presents the common “U”-shaped described in continental (e.g., Le Bas, 1977; Wooley, 1979; Ray, 2009) pattern of excess Ar (40Ar/36Ar intercept of 473.7) and did not produce and oceanic areas (Muñoz et al., 2005; De Ignacio et al., 2006). a reliable age determination. The isochron age (1.95±0.38 Ma), Carbonatites are also associated with hyper-solvus nepheline syenites. obtained with the last six consecutive heating steps is therefore the In some places, carbonatites caused fenitisation of the associated ijolitic probable crystallisation age of the sample. (s.l.) and nepheline syenitic rocks, mainly producing clusters of large A seven steps isochron age of 2.28±0.13 Ma (F=0.22; p=0.95) biotite flakes. was preferred for sample CY-165 owing to the irregular step-heating Volcanic silicate rocks from the Upper Unit are bimodal. The spectrum. melanocratic pole is dominated by nephelinites (normative nepheline For all of the obtained ages, P values (probability of occurrence) up to 27.4%; normative olivine up to 24.2%) but also includes tephrites are between 0.2 and 1.0, well above the 0.05 threshold. (normative olivineb10%) and basanites (olivineN10%). These rocks tend to be more silica-rich than melanocratic rocks from the Lower 6.2. U–Th disequilibrium ages Unit. Based on the TAS diagram, mesocratic/leucocratic rocks can be subdivided into nephelinites (phonolitic nephelinites in the sense of Isotope ratios and the resulting ages for samples CY-213a–care Cox et al., 1979) and phonolites. These nephelinites are readily summarised in Table 2. All three samples are within the U–Th dating distinguished from those forming the melanocratic pole of the Upper range and have limited variability indicating small or negligible diagenetic Unit by their lack of normative olivine. Hauyne–nosean is the alteration of the corals. This is also confirmed by the initial 234U/238U dominant phenocryst phase of the studied phonolites. activity ratios of the coral samples, which (within 2 sigma uncertainty) Mafic rocks from the Upper and Lower Units are also distinct in are in the range of modern corals (1.146±0.002; Edwards et al., 2003). terms of trace elements. Indeed, while lavas and dikes from the Lower The initial 234U/238U activity ratios of corals should remain within 10 to Unit are characterised by well-marked negative K anomalies and 20‰ of the modern value for the last several hundred thousand years significant Nb and Ta enrichment relative to both light rare earth (Edwards et al., 2003). Thus, the deposit is accurately attributed to Marine elements (e.g., La and Ce) and large ion lithophile elements (e.g., Ba Isotope Stage 5c (MIS5c), at around 0.105 Ma. and Rb), their counterparts of the Upper Unit are typically Ba- enriched and have less well-defined negative K anomalies (Supple- — 7. Geochemistry of magmatic rocks mentary data Appendix D; Fig. 8). Considering that incompatible trace element ratios tend to be unaffected by partial melting and A detailed petrological and geochemical study of the Brava igneous fractional crystallisation processes (e.g., Allègre et al., 1977; Minster rocks is beyond the scope of this work. However, given the lack of and Allègre, 1978), these differences strongly suggest that melano- information about the island, the main lithotypes from Brava are cratic rocks from these two units are derived from distinct types of briefly characterised from a geochemical point of view. Petrographic magmas/mantle sources. descriptions of these rocks can be found in Machado et al. (1967, 1968) and Mourão et al. (2010). 7.2. Carbonatitic rocks

7.1. Silicate rocks Carbonatitic rocks occur in the Middle and Upper Units as intrusive and extrusive facies, respectively. Previous work on carbonatitic Despite the abundance of pyroclastic rocks, we focus on intrusive rocks (Assunção et al., 1965; Machado et al., 1967; Hoernle et al., and effusive rocks as they preserve better the chemical characteristics 2002) has only described the occurrence of calciocarbonatite varieties. of magmas because of their higher resistance to alteration. However, our data indicate two groups of carbonatites (Fig. 9): (1)

Fig. 7. Plateau and isochron ages for Brava Island. a — Step-heating 40Ar/39Ar apparent age spectrum. Reported errors are 2σ. Filled steps were included in the plateau age calculation. Goodness of ﬁt parameter (F) and probability (p) are indicated. b — Inverse isochron diagrams for samples for which a plateau age was not attained (also shown). Filled squares indicate the steps included in isochron calculation. Solid lines represent the calculated isochron. Goodness of ﬁt parameter (F), probability (p) and 40Ar/36Ar intercept are indicated. J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 229 230 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235

Fig. 7 (continued). J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 231

Table 2 Laser ablation U–Th disequilibrium geochronology results.

230 238 234 238 234 238 Sample ( Th/ U)A ( U/ U)A U–Th age (ka) ( U/ U)A0 CY-213a 0.686±0.019 1.092±0.008 106.1±5.0 1.124±0.011 CY-213b 0.689±0.016 1.105±0.008 104.4±4.2 1.141±0.010 CY-213c 0.686±0.015 1.100±0.007 104.5±3.9 1.134±0.009

230 238 234 238 234 238 All errors 2σ; subscripts: A — Th/ U and U/ U activity ratio; A0 — U/ U initial activity ratio. WGS84 geographical coordinates of sampling site: 14°53′03″ N; 24°40′45″ W. calciocarbonatite rocks (sövites and alvikites) with high CaO (N37 wt.%) and low MgO (b6 wt.%) and (2) magnesiocarbonatite rocks with lower CaO (b32 wt.%) and higher MgO (N15 wt.%). All extrusive samples are classiﬁed as calciocarbonatite (CaON44.88 wt.% and MgOb1.52 wt.%). Recently Mourão et al. (2010) showed that extrusive calciocarbonatites are isotopically more Sr-radiogenic and Nd-unradiogenic than the intrusive calciocarbonatites from the Middle Unit. This clearly shows that these two groups of carbonatites were not generated from sources of similar composition. These authors also demonstrated that the extrusive carbonatites are the product of a nephelinite–carbonatite immiscibility process, which is supported, among other argu- Fig. 9. Plot of the intrusive and extrusive carbonatites from Brava in the diagram ments, by the comparable Sr and Nd isotopic ratios for the coeval proposed by Woolley and Kempe (1989). Oxides expressed in wt.%. silicate and carbonatitic rocks.

8. Discussion and conclusions intruded by shallow magma chambers that formed an alkaline– carbonatite complex (Middle Unit). These magma bodies may have fed 8.1. Volcano-stratigraphic evolution eruptions and eventually contributed to the sub-aerial development of the island. However, no evidence of such volcanism is currently We present the first 40Ar/39Ar ages for magmatic rocks from Brava preserved because a major erosive phase deeply truncated the island. that cover the whole exposed stratigraphic sequence. The age range The erosive event, which lasted for around 1 million years (from 1.3 to (2.9 to 0.24 Ma) is more restricted than could be inferred from the 0.3 Ma ago), exposed the plutonic rocks. This time lag is compatible 5.9 Ma K/Ar age obtained by Bernard-Griffiths et al. (1975); the with the considerable amount of erosion and uplift pre-dating the phonolite from which this age was obtained, belonging to the now- younger volcanism. During the last 300 ka, a sub-aerial, post-erosional defined Upper Unit, is stratigraphically above a 2.4-Ma-dated volcanic phase covered the erosion surface with phonolite lava flows/ nephelinite. These relations demonstrate that these K/Ar ages can domes and pyroclasts and sparse mafic and carbonatitic products. no longer be considered when discussing the stratigraphy of Brava. Some of these events present a very fresh morphology, indicating a Three volcano-stratigraphic units were identified (Figs. 10, Fig. 11). probable Holocene age. The Lower Unit represents the Brava seamount stage. It is composed of The ages obtained for the Brava basement (Lower and Middle Units) alternating pillow lavas, hyaloclastites, and pillow breccia accumula- are consistent with the 2.1 Ma K/Ar age reported by Hoernle et al. (2002) tions, which are crossed by frequent co-genetic dikes. The depth of for an intrusive carbonatite. We emphasise that these carbonatites are formation for these submarine volcanics cannot be inferred from these younger than those occurring at the neighbouring island of Fogo, for deposits, and a significant amount of the edifice is currently missing. which ages are usually older than 3.5 Ma (Lancelot and Allegre, 1974; Isotopic ages obtained for pillow lavas and their associated dikes Hoernle et al., 2002; Madeira et al., 2005; Foeken et al., 2009). indicate that the submarine deposits cropping out above present sea Bathymetric data (e.g., Masson et al., 2008) show that Fogo and Brava level represent at least 1 million years of growth (from ~3 to 2 Ma are two distinct volcanoes separated by depths in excess of 1400 m. In ago). More recently, from about 1.9 to 1.3 Ma ago, the volcano was conjunction with the above-mentioned carbonatite geochronological

Fig. 8. Multielemental, primitive mantle normalised diagrams for maﬁc rocks from Lower and Upper Unit from Brava Island. Normalising values are from Palme and O'Neill (2003). 232 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235

age uncertainty interval (18 m at 2.055 Ma; Miller et al., 2005), a vertical displacement in excess of 382 m is necessary to explain the present position of the unit. Beach deposits at about 100 mapsl provide additional evidence for significant uplift. Those sediments are intercalated at the base of the Upper Unit; the onset of Upper Unit volcanism may have occurred at around 0.253±0.005 Ma. Using this value as an approximate age for the deposit, a vertical displacement of ~177 m is inferred; the average sea level for the age uncertainty interval corresponds to −77 m (Miller et al., 2005). However, marine sediments deposition during MIS9 (11 mapsl at ~0.325 Ma; Miller et al., 2005) would correspond to a more conservative vertical displacement of 89 m. Finally, samples from the Ribeira de Rasque marine deposit yielded a consistent age around 0.105 Ma, which corresponds to MIS 5c. The sediments presently crop out at 4–5 mapsl and represent beach deposits based on their fossil content and sedimentary features. The deposits seem to be inter-tidal, indicating that the contemporaneous sea level stood at this approximate elevation (~5 mapsl). Considering that sea level during MIS 5c was around −39 m (Miller et al., 2005), a vertical displacement of ~45 m is inferred since the deposition of these sediments. The reported observations suggest that Brava experienced a Fig. 10. Distribution of 40Ar/39Ar ages obtained for the three major volcano- minimum overall uplift rate of approximately 0.21 mm/a in the last stratigraphic units in Brava. Grey rectangles represent age intervals for each unit as 2 Ma. The younger deposits indicate that uplift rates were higher in 40 39 indicated by the new Ar/ Ar analysis (error bars are 2σ age uncertainty). Most recent times, reaching as much as approximately 0.4 mm/a in the last probable ages are tentatively indicated by dashed rectangles. 0.105 Ma. The uplift seems to have been differential. An important NE–SW-trending fault zone may have contributed to increased uplift data, these observations strongly argue against previous interpretations of the southeastern half of Brava, where plutonic rocks outcrop, (Assunção et al., 1965; Day et al., 1999; Madeira et al., 2005), which relative to the northwestern region of the island, where the proposed that carbonatites on both islands were part of an older common submarine volcanic products occur. basement upon which the sub-aerial parts of both islands grew. The uplift rates determined for Brava are among the highest values obtained for the Cape Verde islands (Ramalho et al., in review) and 8.2. Quantifying the rate of vertical movements show that the island was subjected to a significant uplift trend throughout the majority of its eruptive life despite the surface loading Brava endured significant vertical movements that resulted in produced by the growth of the edifice and the neighbouring Fogo uplift (this study; Ramalho et al., 2010a). These movements were volcano. Uplift cannot be explained by the cumulative far-field effects quantified by inferring the relative sea level position during the of multiple surface loading because the island still lies on the flexural evolutionary stages of the edifice and comparing it with eustatic sea moat of the younger volcano of Fogo and because the amount of uplift level from a chosen curve (e.g., Miller et al., 2005). is too large to be flexural in nature (see Ramalho et al., 2010b, in The Lower Unit is exclusively submarine and crops out up to review). Because Brava's uplift is significantly higher than that 400 mapsl. Although the top of the unit is eroded, it nevertheless reported for many of the Cape Verde islands, the mechanism for indicates a minimum relative sea level at this height. Considering the Brava's uplift is most likely local and associated with magmatic younger age bound for the Lower Unit (at 1.99±0.09 Ma), the intrusions at several crustal levels (Ramalho et al., 2010a, in review). approximate eustatic level can be estimated as the average value for In fact, it has been demonstrated that ascending magmas tend to the age uncertainty interval. According to the eustatic curve of Miller stagnate and pond beneath oceanic volcanoes; this is regarded as a et al. (2005), the average sea level for the defined interval corresponds cause of significant uplift (e.g., Staudigel and Giannerini, 1986; Klügel to −23 m, suggesting a vertical displacement in excess of 423 m et al., 2005). At the moment, data about the crustal thickness beneath (Fig. 12). Even considering the value of the highest sea stand for the Brava, which would facilitate the assessment of the importance of

Fig. 11. Geological section showing the stratigraphic and tectonic relationships among the three main volcano-stratigraphic units deﬁned in Brava. J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235 233

Fig. 12. Age versus elevation of inferred relative sea level markers. The time-dependent increasing vertical displacement for older markers denotes continuous uplift. magmatic additions at the crustal level, do not exist. However, the still preserve their high-relief, volcanic, constructive, morphology. In island's plutonic core, the profusion of phenocryst-rich lavas and dikes contrast, the morphologies of S. Vicente, Santa Luzia, Sal, Boavista and in the Lower Unit and the abundance of erupted phonolitic materials Maio are dominated by erosion: low-lying wave-cut platforms (which require large volumes of shallow magmatic chambers to form) surround residual relief and are occasionally punctuated by a few make this hypothesis plausible. A small regional component yielding recent volcanic cones. Brava, in contrast, presents a high relief that is additional uplift cannot be discounted; this component is probably mainly the result of uplift. associated with a regional pulse in the Cape Verde swell (Ramalho et al., in review). Acknowledgements

8.3. Geological and volcanological peculiarities This work is a contribution from FCT/FEDER through the PLINT project (POCTI/CTA/45802/2002) and a PhD scholarship from FCT From the volcanological point of view, Brava presents several (SFRH/BD/39493/2007) which was co-financed by FEDER for CM. The features that clearly differ from other Cape Verde islands. One of these work of DLH was supported by the Leverhulme Trust. L.C. Silva features is the remarkable abundance of phreato-magmatic activity. contributed with continuous encouragement and fruitful discussions Phreato-magmatic craters are very rare in Cape Verde (e.g., Cova and on Cape Verde geology and petrology. The authors appreciate the Lagoa in Santo Antão; Pedra Lume in Sal), but approximately 62% of analytical work and data clarification by John Huard (College of the phonolitic volcanic centres from Brava's Upper Unit are phreato- Oceanic and Atmospheric Sciences, Oregon State University). We also magmatic craters, indicating the availability of abundant ground acknowledge constructive reviews by Andreas Klügel and an water. Another peculiarity is the relative importance of phonolite anonymous reviewer and editorial handling of the manuscript by fi volcanism versus ma c events in the Upper Unit. At Brava, about 85% the editor, Joan Martí. of eruptive centres are phonolitic while only 15% are mafic and carbonatitic; phonolitic volcanic rocks cover more than 90% of the island (see also Bebiano, 1932). Brava has by far the most abundant Appendix A. Supplementary data evolved rocks of the Cape Verde Islands (cf. Holm et al., 2008; Dyhr and Holm, 2010), indicating the development of magma chambers Supplementary data associated with this article can be found, in with significant volumes during the post-erosional volcanic phase. the online version, at doi:10.1016/j.jvolgeores.2010.07.010. Brava is the only island in the Cape Verde archipelago where carbonatites occur in two distinct volcano-stratigraphic units. The References occurrence of Quaternary carbonatitic eruptions associated with a late Ali, M.Y., Watts, A.B., Hill, I., 2003. A seismic reflection profile study of lithospheric phase of island development (Mourão et al., 2010) is also exceptional flexure in the vicinity of the Cape Verde Islands. J. Geophys. Res. 108, 2239–2262. in the context of the archipelago; at all other Cape Verde Islands, these Allègre, C.J., Pineau, F., Bernat, M., Javoy, M., 1971. Evidence for the occurrence of rocks are restricted to the “basal” complexes (e.g., Serralheiro, 1976; carbonatites on Cape Verde and Canary Islands. Nat. Phys. Sci. 233, 103–104. Alves et al., 1979; Silva et al., 1981; Jørgensen and Holm, 2002; Allègre, C.J., Treuil, M., Minster, J.F., Minster, B., Albarède, F., 1977. Systematic use of trace element in igneous processes. Part I: fractional crystallization processes in Madeira et al., 2005) and the only reported extrusive carbonatites volcanic suites. Contrib. Mineral. Petrol. 60, 57–75. (Silva et al., 1981) occur in the Ancient Complex (N5 Ma; Holm et al., Allmendinger, R.W., 2003. Stereonet for Windows v.1.2. WWW Page, http://gcmd.nasa. 2008) of Santiago Island. A future carbonatitic eruption, which is gov/records/StereoNet.html. Alves, M.C.A., Macedo, J.R., Silva, L.C., Serralheiro, A., Peixoto Faria, A.F., 1979. Estudo possible in Brava, would make this island one of the few active geológico, petrológico e vulcanológico da ilha de Santiago (Cabo Verde). Garcia de carbonatitic volcanoes in the world. Orta 3 (1–2), 47–74. Brava also differs significantly from all other islands in the Assunção, C.F.T., Machado, F., Gomes, R.A.D., 1965. On the occurrence of carbonatites in the Cape Verde Islands. Bol. Soc. Geol. Portugal 16, 179–188. archipelago in terms of geomorphology. The islands of Santo Antão, Azéma, J., Fourcade, E., De Wever, P., 1990. Découverte de Valanginien inférieur à S. Nicolau, Santiago, and Fogo present significant fluvial incision but Calpionelles à Maio (République du Cap Vert): discussion de l'age des sédiments 234 J. Madeira et al. / Journal of Volcanology and Geothermal Research 196 (2010) 219–235

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