The Geological History of Maio, Cape Verde Islands

J. geol. Soc. London, Vol. 139, 1982, pp. 347-361, 4 figs, 1 table. Printed in Northern Ireland. The geological history of Maio, Cape Verde Islands

C. J. Stillman, H. Furnes, M. J. LeBas, A. H. F. Robertson & J. Zielonka

SUMMARY: The oldest igneous rocks on Maio are pillow lavas of Mid-Ocean Ridge pillow basalts character which have been tilted and uplifted about4 km from the ocean floor to outcrop as a partial ring, dipping steeplyaway from a central plutonic complex made upof pyroxenites, essexites, syenites and carbonatites. The ocean floor volcanic rocks are overlain conformablyby a stratigraphically continuous pelagic carbonate succession which demonstrates a shallowing depositional environment from the Upper Jurassic to Upper Cretaceous times,when tuffaceous bedsindicate renewed volcanism. The tuffs areassociated with rudites demonstrating the emergence of theisland and amongst the clasts are plutonics indicating Upper Cretaceous magmatismand the unroofing of thevolcano to a substantial depth. Deformation under compressive stress resulted in the folding and local repetition by thrusting of this sedimentary cover, which, together with the plutonic core, had been intensively injected by major sills. The Mesozoic succession has been planed off and overlain with marked unconformity by a largelyNeogene sequence of volcanicand terrestrial sedimentary rocks. There is a hiatus throughout the Palaeogene, and constructional activity appearsto recommence with ankaramitic hyaloclastite and lava deltas and subaerial ankaramitic flows. These are overlain by fluvial sediments and tuffs. Stratigraphically above these is an extensive plateau of silica-undersaturated lavas, olivine- melilitites and nephelinites, which rest on a planed and locally lateritized surface. At topographically higher levels in the eastern part of the island there are thick ankaramitic lavas and pyroclasts which evidently flowed eastward through valleys cut down into the Mesozoic strata, and appear to be of Pliocene age. The subsequent history of the island appears to be non-volcanic.

The island of Maio, which is one of the smaller islands complement and extend the data recovered from east- of the Cape Verde Archipelago(see Fig. 1) situated in ern Atlantic drilling sites by the DSDP. the Atlantic Ocean some450 km W of Dakar, came to Inorder to place the material collected forthe the attention of geologists because of abundant expos- geochemical,petrological and much of the sedimen- ures of Mesozoic fossiliferous marine sediments, which tologicalstudies in correctperspective, it proved are extremely rare on Atlantic volcanic islands. These necessary tore-map the island andthis account is rocks attracted repeated visits from stratigraphers and intended to accompany the map. Micropalaeontologi- palaeontologists, the conclusions of which were cal data by Rigassi (1972),radiometric age data by clearlysummarized by Mitchell-ThomC (1976),who Bernard-Griffiths et al. (1975)and Grunau et al. also gavepetrographic and chemical datafor the (1975),and new K-Ar dating by J. Mitchell (pers. igneous rocks, both volcanic and plutonic, exposed on comm.) have all beenused to place the mappedsucces- the island,Subsequently, detailed field mapping re- sion in a geological time-scale. sulted in the publication of a coloured geological map and accompanying memoir by Serralheiro (1970). Geological setting and Then, in 1974, De Paepe er al. published surprising evidence that also exposed on the island was a frag- stratigraphy ment of the original Mesozoic ocean floor with mid- The only existing stratigraphic scheme for Maio was ocean ridge pillow basalts (MORB). The island thus produced by Serralheiro (1968, 1970). Inherent in this provided the opportunity for the examination of the was his belief that the Mesozoic limestones, the ‘light- evolution of an Atlantic island from the ocean floor coloured compact limestones intercalated with flints’, upwards. overlie a complex unit of mafic lava flows and intru- The present study developed from a parallel inves- sivesintercalated with highly disruptedshales and tigation of the Mesozoic and Tertiary history of Fuer- limestonestermed the ‘Shale-Limestone-Eruptive teventura in the Canary Islands (Stillman er al. 1975; Complex’. By contrast, the current study shows that Robertson & Stillman 1979a,b). The two islands pro- almost all the associated mafic rocks are intrusive and vide the only known comprehensive evidence of the so post-date the Mesozoic sediments. Serralheiro also processes of volcanic island initiation and early build- subdividedhis ‘Eocretaceous’ part of the succession up in the middle Atlantic, as well as data on ocean into twounits, namely a lower‘Shales, marls and island magmatic activity fromthe Mesozoic tothe marly limestones’ and an upper ‘Light-coloured com- present. The extensivesedimentary sequences also pact limestone intercalated with flints’. Present work

0016-7649/82/0500-0347$02.00 @ 1982 The Geological Society

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FIG. 1. Locationmap with bathymetry and magneticanomalies of theCape Verde- Canariesregion of theAtlantic. Magnetic anomalies of theM-series after Hayes & Rabinowitz (1975). Numbered triangles: D.S.D.P. sites.

now indicates that the upper unit is, in fact, a tectonic ceeded by thelimestones of theMorro Formation. repetition of part of Serralheiro’s underlying ‘Upper From these earliest limestonesRigassi (1972) recorded Jurassic’ limestone unit. nannoplankton which, although too poorly preserved As a result of therecent detailed mapping and for accurate determination, he thought to be Lower measurement of sedimentary logs, it is now possible to Jurassic to early Upper Jurassic in age. In overlying erect a generalstratigraphic column and to define finely micritic beds he obtained a radiolarian micro- units in linewith currently accepted international fauna containing Crassccollaria breuis, Saccocoma sp. usage (Holland er al. 1978). and Protoglobigerina, which indicate an Upper Jurassic The succession is divided into a Mesozoic ‘Basement age. He believed the succession tobe750m Complex’overlain unconformably by a number of thick and consequently suggestea thatit represents the Tertiary and Quaternary formations. The oldest unit, whole upper Jurassic, i.e. Oxfordian, Kimmeridgian the volcanic Batalha Formation, is conformably suc- and Tithonian. It is now known that this sequence of

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beds is not a continuous succession but is tectonically plutonic outcrops (Grunau et al. 1975) and new clasts repeated by thrusting;the wholesequence is much from these localites suggest that thesedates may be thinner than Rigassi supposed. too old. However, all ages so far determined by K-Ar Theupper boundary of theMorro Formation is methods must be treated with caution since there has takenwhere the lithologychanges tothe more apparently been severe overprinting by thermal events heterogeneousshales and thin-bedded limestones, at around 10-12 Ma. The effects of this may be seen in namedthe Carqueijo Formation. from which beds the Pillow Lavas, which are known from stratigraphic Rigassi (1972) reported an Albian/Cenomanian fauna. evidence to be as old as the Jurassic/Cretaceous Morro The incoming of rudites and tuffs is used to define the Formation limestones, yet K-Ar dates range from 40.1 base of the succeeding Coruja Formation, beds in the to62.1 Ma (J. Mitchell & H. Furnes, pers. cornm.). upper part of which are, according to Rigassi (1972), This overprinting appears to derive from a heat source of Turonian/Senonian age. below the pile, and less affected ages give evidence of The sequence of Morro, Carqueijo and Coruja for- intrusionat aroundat least 80-90 Ma or earlier(J. mationshas been grouped into the Monte Branco Mitchell & K. Storetevedt, pers. cornrn.). Such intrusions Group, and represents a continuous period of marine may have been the source of the clasts in the Coruja sedimentationlasting from the Upper Jurassic to at Formation and may well have had eruptive expression least the Upper Cretaceous. Its ultimate upwardex- as lavas and tuffs which have been entirely removed tension is not known, since the upper limit is a pro- during the long erosional period which followed the nounced erosional unconformity, overlain by an upper Basement Complex. Tertiarysequence of volcaniceruptives and mainly non-marine sediments. The earliest of theTertiary formations comprises The Basement Complex ankaramitic volcanics of the Casas Velhas Formation, which erupted mainly into shallow marine or littoral The Batalha Formation environments,but in places built upto subaerial structures.These are followed by thediachronous The oldest igneous rocks observed on the island are Pedro Vaz Formation, made up largely of conglomer- pillow lavas and associated hyaloclastites on which the ates derived in part by the contemporaneous erosion uppermostJurassic limestones of theMorro Forma- of Casa Velhas lavas, and with intercalated lava flows tion have been deposited. This passage from volcanic of similarcomposition. Foraminifera in siltstones in rocks to sediments is best keen on the north-western this formation indicate that part at least was deposited flank of MonteBranco, where pelagic calcilutites in a marine environment in mid-Miocene times. Suc- occur between and above pillows, the highestof which ceeding units repeat periods of volcanic activity alter- are embedded in the limestone. nating with periods of erosion, and give evidence also The pillow basaltshave in generalbeen intensely of repeated uplift and subsidence. disrupted by the injection of sills and dykes of basic The geologicalhistory andstratigraphic column is rocks of quite different, more alkaline chemistry, but summarized in Table 1. in several areas, such as at Monte Branco, the coast S Though the formations of the Monte Branco Group and SE of D. Joao, Monte Esgrovere, Chao do Monte have been defined by measured logs in type sections, and Monte Batalha, the pillow lavas constitute a high neverthelessprecise details as to thicknessand bed- proportion of the outcrop. This is particularly the case formremain unclear because the succession is in- inthe Monte Batalha area, where the lavascan be tensely intruded by sheet intrusions, mainly sills, and followedalong strike moreor less continuouslyfor by irregular plutonic bodies. The present distribution about 3 km. Since the base of the pillow lava pile is of the Basement Complex is apparently controlled by nowhere seen, and accurate estimation of the propor- a domal structure cored by a Central Igneous Com- tions of sheetintrusions and volcanics is extremely plex. The bedded sequences all dip steeply away from difficult, only an approximate figure for the exposed this structure, the roof of which has been stripped to lavathickness, of theorder of 200-400m, can be leaveonly a few roof-pendants of coverrocks. The proposed.Where bedding defined by pillowscan be intensity of sheet intrusion diminishes generally away measured,the strike roughly parallels the coastline from the Central Igneous Complex, but in its central swinging round the southern part of the island and the regions achieves intensities commonly of up to 100%. seaward dip demonstrates the domal structure. Radiometricdating of the intrusiveevents is being The major part of the formation consists of pillow undertakenand appears to indicate the presence of lava. The pillows are non-vesicularand many show several phases, emplaced over a long time span. Clasts variolitic texture indicative of eruption at water depth of plutonic rocks found in Coruja rudites demonstrate probablygreater than 1000-1500m (Moore 1965; the existenceand indeed unroofing of plutonics by Moore & Schilling 1973; Furnes 1973). The varioles Upper Cretaceous times, whilst ages as young as 21 & characteristically occur as abundant, small (c. 1 mm), 6 Ma and14*Ma have been obtained fromsome isolated, spherical bodies in the chilled margins of the

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pillows which, when traced inwards, coalesce to define pillow lavas of theBatalha Formation, indicate a a homogeneous phase some 2-3 cm from the margin. strong similaritywith theMORB, whereas all later The size of the pillows varies considerably and the igneous activity (onMaio and on theother Cape biggest may reach a diameter of more than 1m, the VerdeIslands) is distinctly different, beingcharac- average being around 30-40 cm. The pillows of the terized by strongly silica-undersaturated alkaline rocks well-exposed MonteBranco sequence appear to be (De Paepe et al. 1974; Gunn & Watkins 1976: Klerkx generallysmaller than those of the other areas. The & De Paepe1976). This led Klerkx & DePaepe shapes of the pillows are alsovariable; some are (1976) to the conclusion that the pillow lavas repres- nearly spherical with buds protruding in different di- ent anuplifted segment of the MesozoicAtlantic rections,others, less common,form elongate, tube- ocean floor, whichwas thusunrelated to the later like bodies. The former indicate slow effusion rate on magmatism associated with the build up of the oceanic a flat bottomsurface whereas the latter are more island. Fieldobservations made during the present characteristic of pillows formed on a relatively steep investigation concur with this interpretation. However, slope (Moore et al. 1971). Other evidence of topog- the assumption that the pillow lavas as a whole repres- raphic irregularities on the sea floor during the pillow ent MORB must be treated with reserve until detailed lava eruption is seenat Monte Branco, where the petrological and geochemical studies have been com- pillow lava sequence defines planar structures that in pleted by H. Furnes & C. J. Stillman. some places make a rather high angle (up to 40-50") Evidence of the episodic nature of the final stages of with thebedding of overlappinglimestone, yet Batalha Formation pillow lava eruption is given by the elsewhere are conformable with it. The deposition of intercalation of sediments in theupper part of the bottom sediment on a hilly surface of pillow lava is a succession. In the Monte Branco area, interlava sedi- commonfeature of theocean floor (Van Andel & ment is restricted to small volumes of grey interstitial Ballard 1979). limestone which is in places recrystallized by contact Broken pillow breccia is commonly found,appar- metamorphism.Metalliferous, commonly ferruginous entlyat various stratigraphic levels. Inthe Monte interlava sediments are seen in the Monte Batalha and Batalha area such breccias, up to 45 m thick, consist Monte Esgrovere areas. In the former a thin (40cm) predominantly of angularfragments ranging from but extensive ochre horizon is traceable along strike around 1-60 cm across, set in a fine-grained, unbed- for about 1 km, at a level some 500 cm below the top ded matrix of completely palagonitized hyaloclastite. of the sequence. Probably the best exposuresof metal- The shapeof many of the fragments, and theglassy rim liferousinterlava sediments are 0.5 km E of Monte of the convex side of some, clearly indicate their origin Esgrovere where the upper 50-60m of the lava pile asbroken pillows. Occasionally whole pillows also contain laterally continuousintercalations up to 60- occur. Within this thick breccia unit a 5-m thick pillow 70 cm thick of finely-laminated, red, ferruginous silt- lava occurs in which the various stages in the forma- stone,mudstone, siliceous limestone and vitreous tion of the breccia can be seen, from the initial break- chert. These sediments, while highly ferruginous, are up of the pillowskins to advancedstages showing deficient in manganese. The volcanogenicsiltstone completedetachment of pillowybodies from their clearly involved derivationby sea-floor weathering and feeder. demonstrates a significant time interval between erup- Based on these field relations, the pillow lavas can tive phases. Chemical data are needed to decide the be interpreted asrelatively deepwater extrusions origin of the iron; the initial interpretation suggests a above the carbonate compensation depth. The evenly- hydrothermal origin. The chalcedonic cherts are likely pillowed lava records relatively quiet lava eruption. By to have originated by diagenetic silicification of silice- contrast,the lavabreccias and hyaloclastites signify ous pelagic ooze mixed with ferruginous hydrothermal eruptionon relativelysteeply-sloping sea floor, the precipitates. breccias and hyaloclastiteforming as the pillow sur- faces spalled off whilst tumbling down slopes. Interca- The Morro Formation lation of brecciatedand non-brecciated pillow lava flows may indicate active sea-floor faulting during lava Basal fucies extrusion and in this context a small screen of broken Inmost places partly recrystallized, grey, pelagic pillow brecciafound ata low stratigraphic level on calcilutites directly overlie the Batalha lavas with no MonteBatalha may be significant. Thiscontains a obvious trace of metal enrichment. Locally, however, narrow zone about 1m across in which the fragments in theMonte Esgrovere area, the basal facies are in the breccia have been strongly deformed. The de- highlyferruginous. In azone traceable laterally for formation pre-dates the main phase of sheet intrusion some 100m the basal sediments around 2-3m thick and indicates a penecontemporaneous NW-SE shear are brightly coloured. This colour fades upwards from zone. brilliant orange through buff and red to grey. Small- Geochemicaldata, in particularthe REE patterns scale sedimentary structures such as parallel lamina- andthe Sr isotope compositionfrom certain of the tion, cross lamination and minute scours are present.

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Such a basal ferruginous facies, like that in the lavas etc.) which might signify lengthy breaks in deposition. beneath, must have been precipitated within a hydro- The scatteredchert nodules are regarded as typical thermal field, activeduring and for some time after products of replacement of pelagic carbonate by silica the final stages of volcanism in this area. The metal derived from siliceous fossils (radiolaria and diatoms). supplyseems tohave gradually waned, hence the The primary ‘gel’ origin invoked by Serralheiro (1968) upward transition to normal pelagic carbonates. Dur- is not confirmed. The systematically thicker bed-size in ingdeposition, bottom currents were sufficiently the extreme SE of the island (Ribeira de Tras sequ- strong to produce the small-scale scouring, lamination ence)implies faster sedimentation rates here than and interclasts not seen in the overlying limestones. In elsewhere.Possibly this was depressiona on the generalthese metal-rich facies are comparable with sea floor into which pelagiccarbonate was continuously the basal facies known widely from the ocean floors, ponded.This period of quiet pelagicsedimentation (e.g.Dymond etal. 1974)and fromophiolites (e.g. came to an endsome time in the Albian(mid- Robertson & Fleet1976). However, unlike most of Cretaceous). these, the Maio metal sediments appear to be manganese deficient. The Carqueijo Formation This consists of a heterogeneous assemblageof thin- Pelagic limestones bedded siliceous pelagic limestones, cherts and many black shales ranging up to c. 90 m in thickness. Typi- Apart from the basal facies, the Morro Formation cally it is much the most fissile and softest of the three comprises180-350 m of remarkablyhomogeneous, Mesozoicsedimentary formations, and perhaps be- grey calcilutites, which makeup the bulk of the cause of this has undergone extensive sheet intrusion, Mesozoic sedimentary rocks exposed on Maio, depo- often approaching loo%, which much dilates the for- sitedaccording to Rigassi (1972) throughout part of mation. Its principal outcrop areas arein the E and SE the Upper Jurassic and the whole of the Lower Cre- of the island but it is also found locally in the S near taceous. Long intact sequences are seen in the type Lagoa, and in the W in the Ribeira do Morro. In the section in the SW of the island, andon the eastern type section, on the northeastern flank of Monto Car- slopes of Monte Branco and in valleys running to the queijo, the thin-bedded transitional limestone unit at easterncoast. The individualbeds range from 20- the top of the Morro Formation passes up into thinly- 60 cm in thickness and are laterally continuous. The bedded, fissile, siliceouslimestones with numerous lowest horizons are strongly burrowed and there are smallred ferruginous concretions each up to 2 cm concentrations of finely divided shell fragmentson diameter. Above this is a distinctive unit of orange- some bedding planes. Although an extensive macro- weathering, red siliceous calcilutite about 2.5 m thick fauna has been described (Mitchell-ThomC 1976), well which has been noted near the base of the formation preservedmacrofossils arerare. Higher parts of the in all areas exposed. sequencetypically alternatebetween slightly thicker Thin-bedded, finely-laminated,soft-weathering, and thinner beds in units up to 10m thick. Nodules of chalcedonicreplacement chert (Serralheiro 1968, pale grey to whitemudstones, found abundantly in 1970) are present throughout, but never coalesce to theformation are interpreted asbleached black form continuous chert beds. The even-bedded limes- shales. Otherinterbeds of siliceouslimestone show tones invariably terminate with 1.8-2.5 m of thin but grading and parallel lamination diagnosticof turbidites. evenly-bedded grey chalky calcilutites with few chert Theheterogeneous Carqueijo sediments were de- nodules. positedrelatively rapidly, probably during After cessation of localizedhydrothermal activity, Albian/Cenomanian times, in distinct contrast to the the pelagicoozes were deposited slowly on anex- slowly settled uniform Morro pelagic limestones. The tremly stable, deep water sea floor for the long period thin-bedded, chalky limestones at the topof the Morro from late Jurassic to mid-Cretaceous. Throughout, the Formationmark the end of stableconditions and sea floor remained above the carbonate compensation thoughpelagic limestones continue tobe deposited depth, isolatedfrom any significant supply of fine- they are more thinly-bedded and siliceous and some grainedterrigenous material. Where the basal sedi- weredeposited byturbidity currents. The non- mentslocally show ‘swirly’ laminationand patchy calcareous lithologies suggest a terriginous source on cementation, this is attributed to growth of carbonate thecontinental rise establishment of topographic concretions prior to compaction of soft sediment. The differentiation. The red iron staining probably records carbonate oozes were only subjected to gentle bottom adepositional hiatus. The completeassemblage is current activity producing the fine winnowed partings. taken to indicate rapid shallowing. Sedimentationwas apparently even and continuous; The Coruja Formation individualbeds can often be traced laterally over several hundred metres, and there is an absence of any The base of this formation is definedas the first appearance of coarser ‘tuffs’ of areniteand rudite

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grain size; microfauna in the upper part of the unit Fieldwork has demonstratedthat rather more of suggests anUpper Cretaceous (Turonian/Senonian) thecentre of the island of Maio is occupied by age(Rigassi 1972,1975). The formation outcrops plutonics than was hithertosupposed and a major extensively aroundthe eastern and northern peri- problem is to ascertain whether these rocks comprise a phery of the Central Intrusive Complex. single large pluton or several smaller ones of differing The typesection begins with the appearance of ages.Some marginal relationships arerevealed by well-stratified, yellow and green, soft-weathering tuffs detailedmapping of the hmba da Vigia-Figueira showing small-scale channelling,wavy bedding, rippling Capada regionjust S of MonteBranco, where the and local burrowing. Most significantlythere is a later- basicplutonic complex has a well exposedintrusive ally discontinuous c. 8 mthick horizon of clast- marginagainst the Mesozoiclimestones which are supported rudite. Theclasts, up to 40 cm diameter, are stronglymetamorphosed with local development of well-rounded,water-worn, and include spilitized skarns. Numerous large rafts and xenoliths of limes- ocean floor basalt, ankaramites, mugearites and granu- tone and chert, some muchbrecciated, occur within lar plutonics including microsyenites similar to those the gabbroic rocks not only near the margins but also of the central intrusive complex. The matrix is lime- sometimes up to a kilometre away from the exposed stone, mostly recrystallized. The ruditepasses up with- margin. Thelatter are probably roof pendants. The outbreak into alternations of medium- to coarse- sameregion demonstrates marginala zone about grained massive tuffs in beds up to 70 cm thick. NW of 250 m wide of medium-grained essexite with an abun- Monte Coruja around to Monte Vermelho the forma- dance of mafics, mainlypyroxene and amphibole. tion is moreheterogeneous, containing interbeds of Olivine is rare and the feldspar is alkali feldspar, not limestone,often highly burrowed, alternating with plagioclase. These micro-essexites evidently represent tuffs and volcanicconglomerates, sometimes chilledmargins of a pluton; similarmedium-grained channelled. rockshave been observed elsewhere within the Inaddition to spilitic basalts, theseconglomerates plutonic mass, reinforcing the view that there are sev- containbasaltic pumice clasts possiblyindicative of eral intrusive phases in the complex, though none has subaerialeruption, biotite-bearing olivine basalts, asyet been mapped out asprecisely as near Monte melilitites, and some fragments of calcite and ankerite Branco.Though distinct plutonshave not been de- which could have been derived from a carbonatite. ciphered, and all attempts to determineages radiomet- The formation is dominated bytuffs. Inthe type rically have indicated that only Neogene outcrops have section the basal tuffs display ripple structures indica- beensampled, a variety of ages isgiven by their tive of subaqueous deposition. Clasts in the overlying relationships to members of thebedded succession. ruditeswere faceted in high-energyshallow water The earliest evidence of plutonism comes from derived (coastal or fluvial) andthe occurrence of plutonic blocks of gabbroic character occurring in the Coruja rocks in addition to basaltand ankaramite lava or Formation tuffs, indicating the existence of plutonic sheet intrusion clasts implies that extensive intrusion, rocks by mid-Cretaceous times. This phase of pluton- uplift and erosion had already taken place. The pres- ism mightpossibly be responsible forthe Mesozoic ence of the melilitite clasts is particularly significant, growth and uplift of the island to near sea level, in a demonstrating a phase of extremely alkaline magmat- manner analogous to that proposed for the island of ism in the Mesozoic. The coarser tuffs higher in the Fuerteventura in theCanary Islands(Robertson & typesequence have sedimentary structures such as Stillman 1979a). Another pierced the Mesozoic sedi- largewavelength, low amplitudesymmetrical ripples mentsand lavas and was apparentlyresponsible for indicative of subaerial deposition. Some ‘agglomerate’ the uplift which caused the domal structure and defor- is probablya cinder deposit. By contrast,the bur- mation of thesedimentary cover. A thirdand later rowed limestone interbeds of the sequences to the N phaseapparently produced dykes which cut thrust and W imply deposition in shallowseas, and locally planes which repeat the sedimentary succession [one matrix-supportedrudite with limestone clasts gives suchdyke has been dated at 10Ma by Bernard- evidence of deposition by mass-flow on a sloping sea Griffiths et al. (1975)]and probably the essexites, floor. Despite all this evidence of large scale intrusion dated by Grunau et al. (1975) at 14-21Ma old but anduplift, there is littlesign as yet of tilting on a now believed to be as young as 8.2Ma (J. Mitchell, major scale, since theCoruja Formation rocks are pers. comm.), which may well be plutonic equivalents virtually conformable with those beneath. of the Neogene ankaramatic volcanics. It is still not clear whether the gabbroic rocks OC- cupy the whole of the central area, as much of that The Central Igneous Complex area is occupied by dvkelsill swarms of such intensity The centraligneous intrusion complex comprises that the host rocks frequently cannot be recognized. coarse-grained pyroxenites, essexites and syenites all When the host rock is seen it is commonlystrongly cut by dense swarms of sills and dykes which vary in brecciated.Sometimes the breccia fragments are composition from basanite to ankaramite and trachyte. merelythose of earlierdykes and sills showing the

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fragmentedremnants of chilledmargins. In some Latersheet intrusions cutting the deformed bedded other cases,blocks of coarse-grainedbasic igneous succession are also common; these are predominantly rocks carrying abundant large crystals of pyroxene and dykes but have a similar range of lithologies. veined by syenite can be identified, leaving little doubt By contrast,a number of late syeniteintrusions thatthe host was here pyroxenite/essexite/syenite. strongly affect the basic plutonics, often cutting across Such blocks appear to beeasily brecciated and takeon intrusive contacts and developing both discrete intru- a pseudo-ankaramitic appearance, and sometimes the sive bodies such as plugs and dykes and also complex only relics that can be recognized are scattered pyrox- vein networks. Some of the bodies are distinctly silica- ene crystals sitting in a dark, chloritized and shattered undersaturated;for example, the nepheline-syenite matrix. plug S of Monte Vermelho. It is possible that some, if It appears that in general the internal age relation- notmost, of the stronglysilica-undersaturated ships of the pyroxenites, essexites and syenites are that plutonics relate to the later Tertiary volcanism. the pyroxenites occuras patches, probablyof cumulate Carbonatites are a late and rather rare feature of origin,within the essexites,and thesyenites form the Central Igneous Complex, occurring principally as segregation pods, veins and sometimes larger bodies a few dykesup to ametre across. Some of these up to somehundred metres acrossenclosed in the penetrateor brecciate Mesozoiclimestones, making essexites, but clearly later than them. The three types their identification difficult. However,others cut the appear to represent asimple crystallization differentia- mafic rocks, particularly the pillow lavas and sill/dyke tion sequence, presumably repeated in each plutonic complexes, or the Coruja tuffs. No undoubted exam- phase. The pyroxenite is alkalineand comprises pleshave been observed in theplutonic complex. mainly titaniferous pyroxene and opaque oxides; it is Microscopic examination indicates that these are true not,however, ajacupirangite belonging to aper- carbonatites;the pure calcite typescommonly carry alkaline ijolitic sequence,since it containsminor apatite,others are ankeritic or dolomiticand have feldsparas well assome feldspathoid. The gabbroid igneous textures reminiscent of sovites, alvikites and rock which makes up the majority of the outcrops is ferrocarbonatites.Their intrusive relations are un- correctlytermed essexite, since it contains alkali doubted. Intwo small, ovoid, pipe-like bodies 15- feldsparas well asplagioclase, and commonly some 30 m across piercing the limestones near the summit of feldspathoid. Some examples are rich in Ti-augite and Monte Branco, a zonal structure was observed com- grade modally intopyroxenite. Others are rich in prising a marginal brecciated zone, an inner zone of alkali feldspar and grade into shonkinite. Yet others pale brown, apatite-bearing carbonatite and a central, grade into more leucocratic rocks, ultimately syenites red-brown ferrocarbonatite which alsocontains apa- with or without feldspathoids. These rock types span tite. A few veins of late-stage alvikitic carbonatite smoothly across the Streckeisen igneous classification criss-cross the main and central carbonatites and could fields 13, 8 and 7, andmay be thought of as the correspondto the C4 late-stagecarbonatites recog- plutonic equivalents of the ankaramites, tephrites and nized in African carbonatite complexes (LeBas 1977). phonolitic tephrites and trachytes. An ijolite with Ti- Late stage silicification is also seen. augite occurs W of Mt Coruja. Carbonatitic breccia dykes of a pale orange-brown The sheet intrusions are at their most intense in the colour also cut sill/dyke swarms and Batalha Forma- central and southern part of the complex where intru- tion lavas. The matrix to the breccia fragments is all sionintensity may reach 100% and all hostrock is calcite or dolomite, and the prismatic texture of some obliterated.It seems that the maximumintrusion is of the carbonate suggests rapid cooling. The fragments into the pyroxenite-essexite plutonics, the surrounding have very diffuse margins but clearly were originally Batalha Formation pillow lavasand the thin-bedded angular; they are composed of interlocking laths and Carqueijo Formation. The more massive Morro lime- sheaves of albite with little or no twinning, also some stones in places contain many sills but are not totally laths of biotite,needles of apatite,haematite obscured,and it is only by tracingsheet intrusions pseudomorphing a cubic opaque mineral, patches of outward from theBatalha Formation to the Morro analcime and a little chlorite, all very finely crystalline. Formation, where bedding reference planes canstill be An original sub-ophitic texture can be faintly recog- seen, that the predominance of sills over dykes can be nized between crossed polars. The fragments appear demonstrated. Since themajority of the sills in the to have been derived from the adjacent pillow basalts Monte Branco Group sediments are foldedwith the andto havebeen albitized by thecarbonatite now sedimentsand tuffs in adeformation phase possibly occupying the matrix. This is the only sign of fenitiza- associatedwith thesecond phase of plutonism, it tion by carbonatites on Maio. seemspossible that many of the basalticand an- The proximity of carbonatites cutting the limestones karamitic sills representtapping of this magmaand of Monte Branco to the intrusive margins of a pluton injection of the roof zone ahead of the uprising second which has clearly displaced limestone material,promp- phase.The existence of sills related tothe earlier ted an early view that the carbonate in the dyke-like pre-Coruja plutonism cannot be proved or discounted. ramifying veinlets was derived fromthe limestone, and

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wasnot igneous and carbonatitic. This view was The mostsurprising feature of thestructure, and further encouraged by the absence of the typical ijoli- one which hasnot been reported previously, is the tic igneous rocks which normally accompany carbona- presence of majorthrusts which repeat the stratig- lites. The ijolites which occur as a separate intrusion raphy along the eastern margin of the Central Com- W of Mt Coruja are melanocratic and carry Ti-augite plex(see Fig. 2). A wedge of BatalhaFormation as their main pyroxene, unlike the ijolites and syenites pillow lavas is present above the main thrust on the typically associated with carbonatites, and seen in this northern slopes of Monte Branco, but in general the association on other Cape Verde islands. Such ijolitic plane of detachment of the upper or eastern sheet is rockscarry sodic pyroxenes andamphiboles and somewherewithin the Morro limestones. The thrust characteristicaccessory minerals such as melanite, cuts up-section, at least locally, on the eastern flank of perovskite and/or abundant apatite and sphene, while Monte Branco. Minor thrusting also occurs within the the Maio ijolites carry Ti-augite and no melanite. The lower sheet, itself truncatedsouthward by asecond ijolites,like the carbonatite dykes, cut the Coruja major thrust, which is, however, obscured to the Wby Formationand are thus post-Turonian. It is consi- intense dyke intrusion. It is noticeable that throughout dered likely thatthe ijolite and carbonatite are a the sediments encircling the Central Igneous Complex, genetically linked pair, such as occur at Jacapiranga, folding and thrusting appear to bemutually exclusive. Brazil. Anotherfeature of thearc in which thrusting is observed, is the style and frequency of faulting. The thrustsheets are repeatedly dislocated by dip-slip The structure of the Basement faults distributed radially to the central complex and the thrust planes themselves are laterally terminated Complex by such faults. Such faulting is much less apparent in the areas not affected by the thrusting. TheNeogene succession on Maio is deposited on Aroundthe entire circumference of theCentral eroded and truncated Basement Complex units which Igneous Complex the bedded strata, whether folded, areoften steeply dipping and in placestectonically thrust or merely steeply inclined, dip away from the repeated acrossthrust planes. Estimates of the style centre of the complex. However, whilst the pattern of and degree of deformation in this intensively intruded dipclearly demonstratesa pronounced domalstruc- sequenceare feasible only in thebedded rocks in ture geometricallycoincident with thecentre of the which reference planes are available to give an indica- complex, there are a number of features which suggest tion of original attitude. Such data are not to be found that the local tight folding and thrusting may not be in the Central Intrusive Complex, whereit is often not related to the doming. possible even to distinguish between sills and dykes. In Firstly, an analysis of the directed yet non-cleavage- the Batalha Formation pillow lavas a certain amount forming folds shows that those on opposite flanks of of informationregarding way-up and dip may be the dome demonstrate a consistent vergence suggest- obtained;however, moststructural data comefrom ing that the dome was imposed on already deformed the sediments. Dips vary from the vertical near Bar- strata. Secondly, the style of deformation, in particular reiro to as little as 30" in the Monte Branco area and the thrusting, implies a compressive stress regime, yet nearMonte Esgrovere and Monte Batalha. In most the doming appears to relate to dilation and diapirism areasthe widelyexposed Morro limestones dip ra- associatedwith high level plutonicmagma emplace- dially away from the Central Igneous Complex, often ment, which would produce a tensional stress. maintainingtheir steepness of dipacross the whole If this is the case, then it seemspossible that the width of outcrop. Some regions demonstrate moderate compressive phase may be related to an entirely sepa- tostrong folding, though significantly, cleavage is rate cause, a candidate for which must be movement everywhereabsent. The Morro limestones along the on the E-W oceanic fracture zone which is known just southeastern coast S of Ribeirade Tras have nearly to the N of Maio (see Fig. 1). upright angular folds with amplitudes up to 3 m and The assigning of doming to magmatic diapirism and wavelengths of tens of metres. Similar folds near Bar- dilation itself presents problems. The rapid shallowing reiro, in the SW of the island, deform the majority of in mid-Cretaceous times during which the Carqueijo sill intrusionswhich here form up to 80% of the Formation was deposited, may well have been due to outcrop. This is again the situation in folded Coruja localizeduprise of the pluton whichitself ultimately Formation beds S of Monte Vermelho, where sills are contributed to the Coruja Formation rudites (see Fig. less abundant. The most spectacular fold train is pres- 3). Yet this uplift was apparently not responsible for entat the base of theCarqueijo Formation in the any deformation, compressive or tensional, since there Ribeira do Morro,where deformation is takenup is no significant angulardisconformity between the almost entirely in the highly incompetent thin-bedded Coruja Formation and the preceding strata. Further- facieswhilst theunderlying more competent Morro more,examination of roof pendantsand marginal limestones are virtually unaffected. relationships of theCentral Igneous Complex show

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SW NE I Monte Penoso I

E Mt Bronco I

3 3'

W E 8r

W I IE

FIG.2. Geological cross-sectionsof part of the island of Maio. Lines of cross-section (numbered from 1 to 4) shown on the geological map. Symbols as for the maps (Fig. 4). Vertical exaggeration X2. Thrust planes labelled t.

that subsequent injection of sills and plutonics into the Tertiary volcanism and Mesozoic sedimentary sequence, whilst clearly causing considerable dilation, also pre-dates the folding and the sedimentation doming. Nevertheless, because of geometric consider- ations and in the absence of other evidence it seems that the doming can most reasonably be explained by The Casas Velhas Formation a Palaeogene phase of high-level diapiric pluton emp- This formation is preserved mainly in river sections lacement which resulted in a more localized and rapid in the SW of the island, where it lies unconformably uplift of the plutonic core of the island. In view of the on the Basement Complex. It is essentially composed known propensity of peralkaline magmas to generate of ankaramites with abundant clinopyroxene pheno- uncommonly substantial uplift (LeBas 1980) it is pos- crysts(Ti-rich salite to Ca-augite) and occasional sible that the phase of diapirism with sufficient uplift olivine in agroundmass of clinopyroxene, olivine, to tilt and dome the overlying strata may have been opaques,apatiteand brown glass. These produced by the peralkaline magmatic phase responsi- rocks have been variously describedby previous workers ble for the carbonatites. as limburgites (Klerkx & De Paepe 1976) and augitites

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GENERALISED CROSS SECTION ACROSS MAID VE= 8 SCALE

LOO Metres L STEEPSTEEP SUB SlOPE SUB SLDPE ON WEST ON EAST Km VERTICAL EXAGGERATION x0 WEST m

PENOSAANKARAMITE LAVAS

+++++++++ ++C++++++ +++++++++++c+++++++ ++++++++++ ++++++++++ ++++++++++ ++++++++++ ++++++++++ ++++++++++

FIG. 3. Schematiccross-section E-W across the island of Maio and the adjacent ocean floor showing the principal structures envisaged. Vertical exaggeration x8.

(Serralheiro 1970). The lava flowsin the Morro and veloped in thenorth-eastern part of the island, be- Casas Velhas river sections give evidence of the prox- tween Montes Penoso and St. Antonio, where it com- imity of the Palaeogene coast line, as lava delta de- prisesseriesa of ruditesinterbedded with fine posits with associated hyaloclastites containing isolated tuffs, andsome ankaramite lava flows. The rudites pillows are common. Foreset dip measurements sug- consist of conglomeratic units, 3-5 m thick, often gesta complex flow regimeresulting from subaerial wedging out rapidly, separated by thinhorizons of eruptions whichproduced interdigitating flows on a fine-beddedarenite. The conglomeratesare matrix- dissected landscape flowing into the sea,pillowing and supported, with rounded to sub-rounded ankaramite brecciating on contact withthe seawater. Owingto sub- clasts and boulders, ranging in size from a few cen- sequent uplift and erosion, preservation of the topog- timetres to c. 40 cm, and often show crude sorting into raphicallyhigher subaerial parts of these flowsis units containing similar clast-size distributions. Imbri- ratherlimited, the mostextensive forming a coastal cation in the Pedro Vaz type-area suggests a complex strip S of Lagoa. Theabundant dykes and sills of changing derivation, initially from the E and N, with ankaramiteand alkaline basaltwhich intrude these iaterinput from the S. Thisrapidly changing flow- eruptivessuggest that they are only remnants of a regime and the general character of the deposit indi- much thicker original pile. cate that it may represent a series of immature fluvial New K-Ar datingindicates that the Casas Velhas fanglomerates. The petrography of the clasts suggests Formation is Neogene and probably in part Pliocene, thatthe source material may havebeen the Casas as an age of c. 9.8*4 Ma has been obtained from one Velhas Formation, and in the SW, in the Ribeira do of the ankaramite flows (J. Mitchell, pus. comm.). Morroand S of MonteEsgrovere, Pedro Vaz conglomerates rest unconformably on these lavas. An age of Middle Miocene is indicated by microfossils from The Pedro Vaz Formation the Pedro Vaz Formation (Rigassi 1972). Lying unconformably on the Casas Velhas eruptives Rigassi’s microfauna, obtained from siltstones in the andcontaining material derived from them is the Pedro Vaz area, are foraminifera which, he suggested, diachronousPedro Vaz Formation. This is well de- indicate an open sea environment.The intercalation of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/3/347/4887475/gsjgs.139.3.0347.pdf by guest on 02 October 2021 358 C. J. Stillman et al. theserocks with fluvial conglomeratesand clearly calcite, in combinationwith zeolitic material, is subaeriallavas poses a problem in interpretingthe characteristicthroughout the compositional range. environment of theformation, the answer to which Disaggregated, altered, peridotite xenoliths are occa- probably lies in rapid vertical movements and proxim- sionally present in the olivine-melilitite, together with ity to a steeply shelving shoreline. The coarser clastic syenitic xenoliths, which occur throughout the forma- components of the Pedro Vaz Formation clearly indi- tion. cate a period of uplift, active erosion being associated The lavas appear uniform in the field and in hand with andthen following the fluvial deposition,and specimen, and no definable flow or cooling units have there is evidencethat planation achieved astable been distinguished, though restricted areas of colum- plateau level in some part of the island, for remnants narjointing, truncated flow bandingand occasional of lateritic cover of altered Basement Complex rocks intercalations of tuff suggest that the pile represents arepreserved in placesbeneath the succeeding the products of more than one eruption. It is, how- Neogene lava flows. The formation is intruded mainly ever, generally free fromdykes and sills, and no by ankaramite dykes, many of which may have acted possible feederzone has been recognized,with the as minor feeders to the ankaramite eruptions of the exception of adyke-like minor intrusion (40-50m Penoso Formation. wideand 100 mlong) of olivinenephelinite, about 1.5 km W of Monte Coruja, petrologically similar to the olivine-nephelinite lava. Malhada Pedra Formation

Following the Pedro Vazsedimentation, extensive The Penoso Formation volcanism re-commenced with a series of highly silica- undersaturatedalkaline eruptions producing large In its final phase, Neogene activity reverted to an- quantities of subaerial plateau lavas, encompassing the karamites and was responsiblefor the eruption of a wholerange of olivine-melilitite, olivine-melilite substantial stratovolcano, remnants of which are pre- nepheliniteand olivine-nephelinites. Part (1950) de- served in the Monte Penoso and Monte St. Antonio scribedthese as ankaratrites. The geometry of the massifs which form the topographically highest ground plateau appears to be that of an extensive, composite on the island. The ankaramites extend continuously to sheet, inclined theSEto (dippingat 2- theeastern and north-eastern coasts and appear to 3"), thickest at its north-western margins (with a max- havebeen erupted onto a well-dissectedtopography imum remaining thicknessof c. 100 m at Monte Batalha)formed after the olivine melilitite/olivine-nephelinite and extending to the S coastaround Vila do Porto eruptions.Typically the base is eithera pyroclastic Ingl&s.The sub-volcanic surface consistsof topographi- breccia,containing fragments of BasementComplex cally smooth, eroded Basement Complex, lateritic in sedimentary rocks as at Lomba Vermelho, or a bed- places. ded clinopyroxene tuff as at Monte Penoso and Monte The lowestand volumetrically most important St. Antonio, which lies unconformably on Basement member of theFormation is olivine-melilitite lava, Complex and Pedro Vaz Formation lithologies. forming the Ribeira Preta section and the greater part The ankaramites, previously described as basanites of the Malhada Pedra plateau type-area. The olivine- (Serralheiro1970), or analcimebasanites (Klerkx & melilitite consists of abundantskeletal, embayed De Paepe 1976) are petrographcally uniform but con- olivinephenocrysts (average Foso-s,J and melilite sist of numerousthin (up to 10-15 m)flows often laths (often showing sector zoning, median cracks, peg withcolumnar centres and scoriaceous vesiculated structureand alteration tocebollite), in well-a tops. crystallized groundmass of Ti-augite, melilite, opaque Theyconsist of abundant largezoned phenocrysts oxides, apatite, perovskite, rare phlogopite and occa- (up to 20mm long) of clinopyroxene (titaniferous au- sionalaltered nepheline. The lavas appearto grade gite) and occasionalolivine, in groundmassa of modallyupwards through olivine-melilite-nephelinite plagioclase, clinopyroxene, analcime (and other zeol- to olivine-nephelinite, exposed in the stratigraphically ites), opaque oxides, biotite and patches of green and highest parts of the plateau at Monte Batalha and in a brown glass. series of scattered outliers to SWthe of Monte Forte and Interbedded tuffs weatherout to providecharac- to the S of Lomba da Vigia. Olivine-nephelinite con- teristic step (trap) topography. The strongly porphyri- sists of abundant olivinephenocrysts, occasional Ti- tic flows occasionallypass up to less-phyricmore augite micro-phenocrysts and euhedral nephelines, all differentiated flow-banded units. The geometry of the in a groundmass of clinopyroxene, nepheline, opaque flows, withsurface dips of 5-16" in directionsfrom oxides,perovskite, rare phlogopite and patches of ENEto ESE, and a suspicion of deltoid flow over interstitial glass. Olivine-melilite-nephelinite is petrog- existing topography, suggest derivation from a central raphically similar, but with the addition of some modal type stratovolcano to the W. Almost the whole of the melilite, both as a micro-phenocryst and groundmass NW of the island is covered in Holocene alluvial and phase.Nepheline alteration to analcime, zeolite and wind-blown dune deposits and no traceof the possible

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/3/347/4887475/gsjgs.139.3.0347.pdf by guest on 02 October 2021 materi al, is ;itional range. liths are occa , togeth er with GEOLOGI CA L MAP OF MAIO , CA PE )Ut the fo rma- VERDE ISLANDS d and in hand ling units have Colco renites, be ach, dune, scree reas of colum Quaternary . ~ Undif f erent iated cove r and alluvial deposits 91 md occasional 80 '!. Sill I d y ke inte n s ity )il e represents m. It is, how- A n koromite Mt Penoso Fm. Suboeriol lavas and brecc ias Dip and st r ike of bedd ing sills, and no ized, with the Thrust ;ion (40-50 m Melo nephelinite ~ Molhodo Pedro Fm. 90 1elinite, abo ut Suboeriol lavas Fault all y similar to ~ L ( L = Loterite ) L ithologicol Boundary Pedro Vaz Fm. N eo9ene Fluvioti le conglomerates Ankoromite lavas ( ma inly submarine >- Casas Velhos Fm . hyoloc lostites and pi llow breccias 89 ~ ver t e d to an :: a: ..

•SO and Monte I o n Basement ologies. Mid Corujo Fm. t ufts congl omerates j as basani tes u Cretaceous , iCr~H; ,,~"; es (Klerkx & Corqueijo Fm. limestones, tufts 0 Basement 86 form but con N 0 Complex /~~/" . \70 ~'· •• •'.'1 . ·:.:: :. :: : :: :• :: :: \- > , ~ '.'~~ .,. [ ~ ~ : :. :::: :i ) fl ows often

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vent region has yet been found, though it is possible Maiowith theMorro Formation pelagiclimestones that the Monte Vermelho plutonmay be related to the though the relative scarcity of organic material, when sub-volcano chamber. compared with the comparable ocean floor succession The subsequent geological history of the island in- seen at DSDP site 367, about 350km to the SE, might volved no further volcanic activity, thoughrepeated suggest that the area wastopographically somewhat vertical movementinvolving uplift of manymetres elevated.Throughout the deposition of theMorro suggests that the island and the crust beneath may not Formation the ocean floor remained above the local yet have cooled down and become totally quiescent. carbonatecompensation depth, did not receive any The PenosoFormation was planed off, thensubsi- significant amount of terrestrialsediment and re- dence resulted in the deposition of marine calcarenites mained tectonically undisturbed. By Albian times the around the coasts and sand dunesinland. Recent uplift incoming Carqueijo Formation shales and calciturbidites has expanded the island area and these marine beds give evidence of considerableshallowing and the are now inland and partly covered by recent scree and CorujaFormation rudites and tuffs demonstrate alluvium. the onset of volcanism, emergence, exposure and erosion of a substantial volcanicedifice. That this contained high-level plutonics emplaced in the lower Cretaceous Summary of the Mesozoic which were unroofed in the Cenomanian might sug- and Tertiary history of Maio gest that the shallowing and emergence were due to and its significance to the history uplift associatedwith theemplacement of plutons, presumably fed by a local magma source. of the Cape Verde Rise Acomparison with ocean floor history shown by DSDP Sites 367 and 368 of Leg 41 is relevant. Site Maio is located about 80 km W of the magnetic quiet 367 drilled into the Cape VerdeBasin SE of Maio, bot- zone.Hayes & Rabinowitz(1975) showed the Cape tomed in low-K tholeiitic basalt [which gave a K-Ar Verde Islands to be situated within the M-sequence minimum age of 88-92 Ma and an incremental heating (Eastern Keathley’s) magnetic lineations which mark 4oAr-39Ar age closeto 122 Ma (Lancelot et al. 1977)] :?.e end of the ‘Graham’ positive polarity quiet zone which is overlain by Oxfordian to Kimmeridgian red and indicate an Atlantic ocean crust generated at the argillaceous limestone, in turn overlain by Tithonian Mid-AtlanticRidge between 153 and 107Ma. Al- andLower Cretaceous limestones which pass up to though the magnetic lineations do not appear to be late Aptian to early Albian black shales. The deposi- traceabledirectly through the archipelago, perhaps tion of these shales was apparently associated with the because of an apron of volcaniclastic debris surround- incoming of terrigenousmaterial (Lancelot et al. ing the islands, it seems probable that the crust be- 1977). The succession, as far as the Aptian, resembles neaththe islandswas generatedbetween anomalies that on Maio but subsequently the histories diverge; M2 and M16. Specifically, the crust beneath Maio was whilst carbonatesedimentation on Maiocontinues probably generated between M11 and M16. The time with a shallowing sequence to eventual emergence by scale for the M-series based on isochrons of 113 Ma Albian times,the seafloor at Site 367 remained at depth, and 148 Ma for anomalies M2 and M22 respectively and possiblyeven sank below thecarbonate com- (Larson & Hilde 1975), wouldsuggest that the sub- pensation depth. Indeed, the sequence is remarkably Maiocrust may thushave formed between 125 and similar to that in the Western Atlantic (Lancelot et al. 135 Ma. 1977) controlled largely by gradual subsidence of the Such dating implies formation on or near the mid- sea floor consequent upon cooling of the crust away oceanridge and there is evidence in the available from the Mid-Atlantic Ridge. chemical and field data that part at leastof the Batalha The Cape Verde Archipelago is located on an ele- Formation pillow lava sequence formed on or close to vated region of the present ocean floor which forms a spreading axis, a conclusion supported by the nature part of theCape Verde Rise. In the vicinity of the of the metal-rich sediments formed on the surface of islands this feature is a dome about 400 km across and the lava. Definition of the precise date, however, pres- its origin is generallybelieved to beassociated with ents some problems. volcanism,the islands representing local volcanic Microfaunalevidence for the age of theearliest edifices on thecrest of thedome (Lancelot et al. pelagic sediment,as reported byRigassi (1972) is 1977). Presumably a dome of this dimension repres- somewhatimprecise but undoubtedly indicates that ents a major feature in the mantle, possibly related to sedimentation on mafic pillow lavas took place in the extensivedecompression and partial melting (see upperJurassic. Taking both palaeomagnetic and LeBas 1980), which would obviously provide a source palaeontological evidence into account, it seems prob- for the magmas erupted in the island volcanoes. How- able that at least by the Tithonian stage ocean floor ever evidence, for example from Site 368 situated on volcanism had ceased and sedimentation begun. the rise to the NE of the archipelago, indicates that Normalocean floor sedimentation continued on the rise did not begin to produce a positve feature on

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the ocean floor until Albian times, as the Turonian to CasasVelhas ankaramites which were to acertain Aptian period is represented by black shales contain- extent sub-aqueous. Lava deltas in the SW show that ing terrigenous material similar to those seen in Site the shoreline then was close but somewhat inland of 367.The first evidence of intrusiveigneous activity its present position. In the N of the island the bulk of here is the injection of basaltic sills into late Aptian to the Casas Velhas extrusives were uplifted and rapidly early Albian sediments. The exact height in the crust eroded to produce the Pedro Vaz Formation. In gen- at whichthese sills intruded waspresumably deter- eralthis comprises ankaramitic rudites deposited as mined by mechanical and hydrostatic controls, but the fluvial fanglomerates,interbedded with tuffs, intrusioncannot have been earlier that late Aptian. minorsubaerial flows, andankaramitic lava. Locally There is some evidence that the rise did not become in the Ribeira do Morro, the Casas Velhas Formation elevated above the level of influx of terrestrially de- deltasand hyaloclastites are overlainunconformably rived sediment until the end of the Palaeogene (Lan- by the Pedro Vaz rudites. celot et al. 1977). It must, therefore, be inferred that Continued uplift led to the subaerial erosion of the the Lower Cretaceous commencement of igneous ac- Pedro Vazrocks, in placesstripping down to the tivity of Maio must have been a very local affair and Basement Complex. Lateritized rocks are preserved in the elucidation of its magmatic relationships with the variousplaces beneath the succeeding late Neogene succeedingmuch wider-spread Neogene activity is a lavas of the Monte Batalha area. The Neogenevolcan- majorobjective of thecurrent study. An important ism tookplace in atleast two phases. The earlier point may be the recognition of melilitite clasts in the producedthethick silica-undersaturated olivine- mid-Cretaceous rudite, which suggests that there may melilitite/olivine-nephelinite plateau lavas seen in the be repeated cycles of magmatic activity in Maio, each SE. There was then further strong uplift and erosion, containing mafic alkaline to strongly alkaline composi- at least in the N of the island, to produce an uneven tions. topography across which the youngest lavas-the an- Whilst thedeposition of tuffs continuedinto the karamites of the Penoso Formation-flowed from an Upper Cretaceous, Palaeogene sediments areunknown as yet unlocated stratovolcano conduit somewhere in on Maio, either because of erosion or non-deposition. the NW. Sometime prior tothe eruption of theNeogene Since that time the island has been eroded, partly ankaramites of the Casas Velhas Formation and the peneplanedand overlain by severalgenerations of deposition of thePedro Vaz fluvial sediments,the marineand non-marine beach and coastal deposits, Mesozoic island wasintruded by several generations of scree and alluvium (Serralheiro 1970). sills anddykes. The sills rose to alevel where they were concentrated in the pillow lavas and the thinner ACKNOWLEDGMENTS.This study was made possible by NERC beddedshales, tuffs andcherts of theCarqueijo grant GR 3/3955and a NorwegianCouncil for Science Formation in a restricted zone, presumably above the grant. Thanks are especially due to the Cape Verde Govern- uprising pluton. At some time during or shortly after ment for their permission to carry out the research and for this intrusive phase, thrusts and folds developed, after their active interest and help with thework. Particular thanks which thecentre of the island was drastically up- must go to Engr H. Soares of the Ministerio do Desenvol- domed.This magmatismwas basic and alkaline, as vimento Rural, Praia, and to Sr M. Monteiro of the Minis- distinct to at least part of the ocean floor pillow lavas terio Coordinacao Economica, Praia and to the staff of these which were of MORB type. Probably as a result of the ministries, especially Sr M. Gomes of Praia and Srs Martins updoming, active erosion peneplaned theisland, which and Fortes ofVila do Maio. We also owe thanks to other members of the expedition-in particular to Sr L. C. Silva then sank somewhat. It is possible that the unusually withoutwhose inspiration, help and encouragement the strong updoming which tilted the cover sequence was whole study would never have been made. Lastly wemust produced by asecond extremely alkaline phase of thankDr J. Mitchell of theUniversity of Newcastlefor magmatism, perhaps related to the carbonatites. permissionto refer to some of his current work on K-AI By the Neogene, volcanism was re-initiated with the dating, which is as yet unpublished.

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Received 1 June 1981; revised typescript received 6 November 1981. C. J. STILLMAN, Departmentof Geology, Trinity College, Dublin 2, Ireland. H. FURNES, Geological Institute, Allegt. 41, 5041 Bergen, Norway. M. J. LEBAS & J. ZIELONKA, Departmentof Geology, University of Leicester, Leicester LE1 7RU. A. H. F. ROBERTSON, Grant Institute of Geology, West Mains Road, Edinburgh EH9 3JW.

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