The Geochemistry and Petrology of an Alkaline Lamprophyre Sheet Intrusion Complex on Maio, Cape Verde Republic

Journal ofthe Geological Sociery, London, Vol. 144, 1987, pp. 227-241, 9 figs. 5 tables. Printed in Northern Ireland

The geochemistry and petrology of an alkaline lamprophyre sheet intrusion complex on Maio, Cape Verde Republic

H.FURNES' & C. J. STILLMAN* 1 Geologisk Znstitutt avd.A, Allegt. 41, 5041 Bergen, Norway 2 Department of Geology, Trinity College, Dublin 2, Ireland

Abstra& The Atlantic island of Maio in the Cape Verde archipelago has a Basement Complex of Mesozoic and Tertiary age which is invaded by an intense swarm of sheet intrusions ranging in age from c. 16 to 8 Ma. These cut an alkaline ultramafic, mafic and felsic plutonic complex (Central Igneous Complex) and its host, a segment of Jurassic ocean floor with pillow lavas and abyssal sediments. The Basement Complex is dissected and unconformably overlain by a sequence of lavas of basanitic and nephelinitic composition. The sheet intrusions comprise sills and dykesof alkaline lamprophyre, many of which are aphyric or sparsely porphyritic. Phenocrysts are predominantly clinopyroxene, withsmaller amounts of olivine and Ti-magnetite. Microphenocrysts are kaersutite, apatiteand biotite. The groundmass is dominated by potassium feldspar and analcime. Calcic plagioclase is extremely rare. Chemically the intrusions are basanitic, with major and minor element compositions directly comparable with the Neogene and Pleistocene basanitic lavas which cap the island. Calculations based on observed phenocryst phases suggest that the liquids involved inthe crystallization of the sheet intrusionsmay have been derived by crystal fractionation from a range of basanitic parents. These were similar to those from which the later lavas but by water-rock enhanced were Igenerated. modified to some extent variable interactions by trapped volatiles.

The island of Maioin theCape Verde archipelago, is preferential emplacement of dykes in the arch region during situated in the Atlantic ocean some 450 km W of Dakar, or subsequent to its formation, and thirdly the possibility Senegal (Fig. 1). The island exposes Mesozoica and that the sheets increase in frequency downwards. Tertiary Basement Complex containing remnantsof Jurassic There is field evidencethat the bedded rocks were ocean floor with pillow lavas and sediments, intruded by a affected by thrusting prior to doming, and many of the sheet Central Igneous Complex of pyroxenites, essexites, syenites intrusions pre-date and were affected by the thrusts. Other and carbonatites. The Basement Complex is invaded bya minorintrusions, mainly dykes,appear to berather later large number of sheet intrusions, both sills and dykes, which than the thrusting. in many areasattain a frequency of almost100%. The A major account of the geochronology of Maio is given Basement Complex is deformed, truncated by erosion and byMitchell et al. (1983), from which the following details covered with markedunconformity by Neogeneand are taken. Conventional whole-rock K-Ar ages for the sheet Pleistocene volcanics andterrestrial sedimentary rocks intrusionsrange from 15.4f0.3Ma to 8.1 f 0.6Ma. A (Stillman et al. 1982). majorperiod of sill anddyke intrusion isrecognized at The main purpose of this study of the sheet intrusions about 11 Ma, but sheet intrusions were probably emplaced has been to determine whether they belong to a coherent from the Lower Miocene onward (Mitchell et al. 1983). A group, derived from a single magma source, or whether they dyketruncated bya thrusthas yieldedan age of could be assigned to a number of distinct magmas whose 9.0 f 0.2 Ma indicating that the thrusting may postdate the kinds are represented by other rocks on the island. 11 Ma phase of intrusion but must predate the lavas of the MalhadaPedra Formation (see Stillman et al. 1982), the oldest date from which is 7.3 f 0.2 Ma. Geological summary Many of the dykesand sillshave closea spatial The distribution of lithologies in the Basement Complex is relationship to the largerplutons of the CentralIgneous controlled by a domal structure which appears to centre on Complex, and indeed the intensity of sheet intrusions in the theCentral Igneous Complex, and has produced steep central area rises to almost 100% over much of the area. outwarddips in the pillowlavas andsedimentary cover The oldest K-Arages recorded in the plutonic rocks are sequence,as well asin the sills of thesheet complex close to 20 Ma and plutons were emplaced until about 8 Ma emplaced in the bedded rocks. However, there is evidence (Mitchell et al. 1983). It may be presumed that the plutons fromthe distribution of thesheets (see Fig. 1)that they were derived from parental magmas which also contributed might havebeen injected initially along a linearzone to the sheet intrusions at various times. However, it has not trending NW, a directionsomewhat oblique tothe proved possible in the intenselyinjected region of the approximately N-S magneticstripes of the surrounding Central Igneous Complex, to distinguish sheet intrusions of oceanfloor. Theapparent focusing of the swarm onthe varying ages, nor to correlate any particular minor intrusion plutoniccore seems to haveresulted from three separate to a specific plutonic phase, though thereis clear evidence of factors: firstly the localization of sills prior to or at an early several such phases (Stillman et al. 1982). stagein the uprise of domestructure, secondly in the The earliest of the Neogene volcanics (essentially 227

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intrusion Coruia Formotion tuffs and Of sedimenh 10- SW0 aOcean flow pillow ~ovas Mand sediments 51 -80% Central Igneous Complex plutonics > 804h

ATLANTIC OCEAN

CAPE VERDE 0 ~

Fig. 1. Geological map of south Maio (simplified after Stillmanet al. 1982) with specimen localities and intensities of sheet intrusion. The left inset map shows the geographical locationof the Cape Verde Islands, and the right inset figure shows a summary of the igneous development of Maio (after Mitchellet al. 1983, and Robertson 1984).

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ankaramites) is the Casas Velhas Formation (Fig. 1) which epidote assemblages indicatingmoderate temperatures, is cut by abundant dykes and sills. These are indistinguish- presumably due to proximity to hot igneous bodies such as ablefrom those cutting the BasementComplex, both in laterdykes. It is noticeablethat the higher-temperature terms of petrography and chemistry. These dykes are dated assemblages of chlorite and epidote are found in intrusions to 9.4 f 0.4Ma by Mitchell et al. (1983). The subsequent emplaced in ocean-floor rocks, and seldom in the intrusions lavastheof MalhadaPedra Formation are olivine cutting the plutonics of the Central Igneous Complex. On nephelinites,compositiona not found in thesheet theother handalteration in thelatter usually involves intrusions. The succeeding andfinal stratovolcano composed considerablereplacement by biotiteandtor phlogopite,a of basanites of theMonte PenosaFormation dated at form of alteration which is rare in thesheet intrusions around 7 Ma, again have compositions very close to those of outside the plutons. the sheet intrusions. Mineralogy Petrography Analytical methods Thesheet intrusions are commonly aphyricor sparsely porphyritic with phenocrysts of clinopyroxene,Fe-Ti The electron microprobe analysesof minerals were carried out with oxides, olivine and apatite, microphenocrysts of amphibole an ARL SEMQ housed at the Geological Institute, University of Bergen,employing standard wavelength dispersive techniques andbiotite, in groundmassa of K-feldspar,analcime, (Reed 1975), an accelerating voltage of 15 KV, and a beam current clinopyroxene,Ti-magnetite, amphibole, biotite, sodic of 10nA. Well-characterizedminerals, synthetic oxides and pure plagioclase,calcite and (occasionally) zeolites. Ocelli of metals were employed as standards. Net peak intensities, corrected analcime and calcite are sometimes present. Olivine, where for dead-time effects and beam-current drift as monitored from the present, is commonly corroded and the amphibole and mica objective aperture, were reduced by MAGIC IV (Colby 1968). are usually microphenocrystsor groundmass phases only. Detailed microprobeanalyses of unalteredsamples representative ofthe sheet intrusions have been undertaken, to The phenocrysts aresmall, almost alwaysless thanone determine the compositions of phenocryst and groundmass phases. centimetre in length,and are usually individualcrystals, Theseresults are summarized in Table 1, whichindicates the though in one or two instances glomeroporphyritic clusters mineral content of representativeporphyritic sills and dykes. havebeen noted. No xenolithshave been observed. The Complete modal analysis by point counting has not been attempted groundmass is usually holocrystallineand quench textures because of thedifficulty in distinguishing between the various are not uncommon. Deuteric and post-magmatic alteration fine-grainedgroundmass minerals such asfeldspars, analcime, zeolite. Representativeanalyses of thevarious phenocryst and iswidely developedbut is notubiquitous nor intense. It groundmassphases are given in Table 2. Completedata are appearstobe largelyhydrous, and has resulted in available upon request or via Supplementary Publication No. SUP low-temperatureassemblages of smectitesandsub- 18045from the British Library Document Supply Centre, Boston microscopic carbonates,through zeolites, to chlorite and Spa, W Yorkshire, UK, or the Society Library.

Table 1. Mineral content of probed specimens

Phenocrysts Groundmass

Sample 01 Cpx Am Bi Fe/Ti-OxK-fsp An PI Cpx Am K-fspBi K/Na-fspNa-fsp Fe/Ti-Ox An Cc Ap G1 Ze

Silk FM 24 X X X xx X FM35 X 30 8 €Q2 FM42 X X X xx X FM108 X X X X X 35 12 12 20 752223 FM 110 24 4 3484 4 20 1 l FM113 X X X X xxx X xx FM 118 xxxx X X X X FM 158 X X X X X X FM 193 X X X X xx X FM 197 2 FM 25 xx 5 38 X 20 55 FM 198 xxx X 24 12 28 202 4 10 FM 202 4 X 2 X 10 4 5 10 50 15 FM 206 X X xx X xx X FM 229 40840 4 44 Dykes FM 20 xx xx X X X X FM 123 xx X xx X X X X FM 133 xx X X X X X FM 142 xxx xxxx X X X FM 149 xxx 8 20 60 4 44 FM 151 X xx X X X X FM 168 xx X X xx X FM 169 X 10 60 5 15 SM 110 X X X 8 50 20 4412 2

Key: 01, olivine; CpX, clinopyroxene;Am, amphibole; Bi, biotite; Ox, oxide; fsp, feldspar;An, analcime; p1, plagic-clase; Cc, calcite; Ap, apatite; 131, glass; Ze, zeolite; X , mineral is present; 20, modal percentage from ‘point counting’ using electron microprobe (see text for details).

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Table 2. Representative mineral analyses of the lamprophyres Clinopyroxene FM 133 Kaersutite Ti-MagnetiteKaersutite 133 FMClinopyroxene Biotite FM 149 FM118 FM148 FM142 FM158 A1k.f. Analc. A1k.f. 01FM158 FM142 FM148 FM118 FM149 FM 42(c) (c)FM42(c) (c) (c) (r) (g) (g)(8) (c) (r) FM42(g)FM42(g)

SiO, 39.20 SiO, 47.51 43.51 44.18 38.19 37.35 36.24 34.80 - - 62.86 52.35 TiO, - 2.57 3.98 4.55 5.66 7.16 4.34 7.36 21.66 22.73 - - A1203 - 5.65 8.59 9.28 13.64 13.70 14.40 14.87 6.26 6.56 19.44 22.65 FeO' 16.96 6.44 7.13 6.44 16.96 FeO' 0.12 0.30 59.42 61.46 14.20 17.75 8.0410.75 11.07 MnO 0.26 0.19 0.09 0.15 0.09 0.19 0.26 MnO 0.49 0.60 0.290.09 0.39 0.12 - - MgO 42.93 13.67 12.06 10.48 12.72 11.91 5.26 13.32 7.15 6.30 7.1513.32 5.26 11.91 12.72 10.48 12.06 13.67 42.93 MgO - - CaO 0.0211.0012.16 12.21 21.83 22.67 22.82 0.26 - 1.61- 0.07 0.49 0.51 1.30 2.23 1.57 2.55 0.57 2.55 Na,O1.57 2.23 1.30 - 0.51 0.49 - - 10.86 0.71 KZ0 - - 8.81- 2.48 - 1.63 1.64 - - 2.65 15.60 Total 99.64 99.34 98.54 99.81 97.45 96.17 94.41 94.26 97.13 95.50 98.98 90.2498.98 95.50 97.13 94.26 94.41 96.17 97.45 99.81 98.54 99.34 99.64 Total

0 = 4 0=6 0=4 0 = 23 0=11 0=32 0=8 0=7

0.785 1.663 1.668 5.72 5.65 5.82 2.64 5.82Si 5.65 5.72 1.6680.997 1.663 0.785 - - 2.30 2.94 Ti 0.63- 0.128 0.115 0.073 5.20 4.910.81 0.41 0.52 - - 0.250 0.388 0.413 2.41 2.44 2.72 1.33 2.23 2.35 1.07 1.18 1.07 2.35 2.23A1 1.33 2.72 2.44 2.41-0.413 0.388 0.250 Fe 0.360 0.203 0.228 0.253 1.39 1.34 2.38 1.34 1.39 0.253 0.228 0.203 0.360 Fe 0.900.01 15.11 15.47 - Mn 0.001 0.001 0.001 Mn 0.02 0.05 0.01- 0.01 0.001 0.16 0.12 - - 1.627 0.765 0.688 0.590 2.84 2.68 1.26 1.51 3.20 2.85 3.20 1.51 1.26 2.68 Mg 2.84 0.590 0.688 0.765 1.627 - - 0.918 0.928 0.883 1.95 1.97 1.89 0.01Ca 1.89 1.97 1.95- 0.883 0.928 0.918 - - - 0.08 Na 0.48 0.65- 0.095 0.038 0.035 0.80 0.09 - - 0.06 0.93 K - 0.86 - 0.51 0.32 0.32 - 0.15- 0.93 Fe) 0.819 0.67 0.67 0.34 0.67 0.67 0.819Mg/(Mg + Fe) Mg/Fe 1.67 2.00 2.00 2.00 X 2.00 2.00 2.03 2.05 2.03WXY 2.05 2.03 40.58 37.31 34.20En 37.31 40.58 Fe 10.74 12.35 14.64 12.35 10.74 Fe 48.68 50.34 51.16WO 50.34 48.68

Key: 01, olivine; Alk.f., alkali feldspar; Analc, analcime; (c), core of phenocryst; (r), rim of phenocryst; (g), groundmass.

Olivine frequentlyzoned. From cores to rims there is agradual Olivine is present only as a phenocryst phase, never in the reduction in silica, increase in alumina and a concomitant groundmass. It is sparse, and has been noted only in a small increase in Fe/Mg ratios (Fig. 2a,d). The trends of variation number(less than 20%) of the intrusions. The unzoned of various crystals (see Fig.2c) are remarkably consistent crystals are rounded and marginally resorbed. Compositions fromintrusion to intrusion, indicating thesubstitution rangefrom FoBhto FO,~. One sample(FM34), however, Ti + 2A1= (FeMg) + 2Si (Scott 1976). The groundmass and contains highly cleavedand clouded olivines of more quench pyroxenes are consistently more TiO, and AI,-rich magnesian composition (FoB8); these may be xenocrysts. It than the phenocryst rims, variation from rim to groundmass would seem that while the liquids have experienced a period almost exactly back-tracking the core-rim trends. Whilst the of olivine crystallization, few of the crystals are present at partitioning of alumina and titania in quench pyroxenes and theexposed level of the intrusions.However, the groundmass is known beto unpredictabledue to experimentalresults on the olivine-liquid equilibria by disequilibrium (Gamble & Taylor 1980), nevertheless in Ford et al. (1983), show that the least to most fractionated theserocks there isa notable clustering of groundmass samples,represented bySM33 and SM 122, respectively compositionsinto two discrete fields (see Fig. 2b). (Table 3) would be in equilibrium witholivine having a Furthermore different quench crystals in the same sample Fo-content of 85.5 and 78.7 at temperatures of 1137 "C and may fall intoeither field. Thisargues for someform of 1103 "C. These calculated olivine compositions fit well with non-uniformity in the liquidcompositions atthe time of thoseanalysed, except for the mostforsterite-rich (FoB8) solidification-possibly function a of rapidly reduced olivine in FM 34, which would indicate equilibrium with a diffusion ratesasundercooling increased, resulting in moreMgO-rich liquid thanrepresented by thesampled inhomogeneitiesbetween domains adjacent to anddistant sheet intrusions. from growingphenocrysts. The variation plots used here illustrate the distribution of silica, alumina, and titania; the Clinopyroxene last two will clearly be affected by the appearance of biotite The most commonphenocryst phase clinopyroxeneis during quenching as well as near-ubiquitous Fe-Tioxides. (rangingfrom salite to ferrosalite to Ca-rich ferrosalite), Nevertheless the variation in Fe/Mg ratio demonstrates that occurringas subhedral, usually equant crystals which are the apparentlygreater degree of fractionation of the rim

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Clinopyroxene Phenocrysts

151 Q ,P"

Clinopyroxene X Cores Phenocrysts e Rims

Clinopyroxene

M( X Phermcrpt cores

A Groundmoss (ReverseZoned Rims)

The line connecting core / rim /groundmass in zoned examples (one only of each type (normal, reversed) shown) 5 L !2 TiOz Fig. 2. Composition of clinopyroxene. (a) and (b). SiO, vs. A1,0, for phenocrysts and groundmass, respectively. In (a), dots represent core compositions and open circles represent rims. Cores and rims of reversely-zoned phenocysts(RZ) are not included in the respective enclosed areas. The tholeiitic, alkalineand peralkaline fields are after Le Bas (1962). (c) Al, vs. TiO, for phenocrysts and groundmass. (a) Cores and rims of phenocrysts expressed in termsof the atomic ratio Ca: Mg: Fe+ Mn.

compared with the groundmass is not an artifact produced groundmass or quench phases are consistently more Ti-rich by the appearance of such phases. thanthe phenocrysts and are somewhatmore variable in Certain other rocks have unzoned phenocrysts or ones composition,but arenot significantly different in stoichi- which show reversed zoning. Whilst it might be argued that ometry. The groundmassamphibole of FM 108, however, thesehave begun to grow at a late stagein the evolved differs significantly from the others in being more Ti0,- and liquids, it may also be that the microprobe beammay not be MgO-poor, Fe0'-rich, and having a very low Mg/(Mg + Fe) sampling the true cores of these crystals. ratio of 0.34 (Table 2). According to Leake (1978) it is a ferro-kaersutite. Amphibole Amphibole occurs occasionally as unzoned phenocrysts, but Biotite is much more common as a microphenocryst or quenchBiotite very rarelyforms phenocrysts, but is verycommon phase. It is a kaersutite according to hake (1978) as a quench or groundmassphase. In many of the intrusions (Mg/(Mg + Fe): 0.58-0.71; Si: 5.44-5.85; Ti: 0.63-0.92) the it appearsas microphenocrysts, but in those cutting the

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Table 3. Representative analyses of the aphyric lamprophyres SM33 FM162 FM41FM121SM121FM151FM107FM160SM15 17 FM

SiO, 40.63 38.37 38.56 40.41 42.23 43.66 39.87 45.16 40.90 41.96 TiO, 3.77 4.41 3.80 3.92 4.21 3.70 4.40 4.13 3.80 3.48 A203 12.89 11.20 13.15 13.43 13.38 15.18 12.74 14.88 14.53 15.20 5.93 8.17 7.23 6.10 7.67 5.15 5.67 4.63 6.04 5.22 Fe0 5.04 3.50 3.64 5.03 3.88 4.86 5.17 5.04 3.40 4.35 MnO 0.18 0.25 0.26 0.24 0.13 0.20 0.75 0.15 0.28 0.28 M@ 5.86 5.11 5.05 4.92 4.90 4.74 4.69 4.59 4.51 4.46 CaO 11.60 11.37 9.36 10.85 8.64 9.60 9.65 8.78 11.22 9.98 Na,O 1.00 2.03 2.60 3.44 2.75 2.38 3.40 2.80 1.os 3.22 KZ0 3.35 2.45 4.46 2.92 2.78 3.34 1.36 4.05 3.05 2.31 P205 1.01 0.60 0.97 1.01 0.72 1.29 0.76 1.16 1.13 0.92 L01 7.75 8.99 8.82 6.03 7.08 5.99 8.85 3.92 7.92 6.61 Total 99.01 96.45 97.90 98.30 98.47 100.09 96.81 99.29 98.83 97.99 v 442 470 442 408 452 419 501 396 379 358 Cr 324 29 28 24 27 36 30 29 19 24 CO 32.2 - - 38.3 36.9 28.4 36.4 25.0 24.0 31.0 Ni 61 29 50 23 30 11 29 5 10 21 Zn 72 87 56 88 88 126 96 65 101 102 Rb 61 51 53 85 55 74 24 84 16 39 Sr 1441 703 1211 1162 953 1752 1085 1241 3380 1150 Y 31 30 36 26 33 35 30 39 34 33 Zr 345 307 425 426 334 392 430 397 380 347 Nb 127 67 111 113 83 144 116 104 134 112 Hf 7.4 - - 8.3 7.8 8.3 9.8 10.9 9.9 8.4 Th 7.42 - - 10.8 6.35 12.02 8.18 7.86 15.79 11.69 Ta 3.20 - - - 4.89 3.90 8.04 3.68 8.88 - La 89.3 - - 81.6 63.1 99.6 81.9 95.0 127.6 101.7 ce 152.8 - - 142.4 118.5 192.7 154.0 189.7 234.8 174.7 Nd ------Sm 12.8 - - 11.6 9.0 11.4 11.9 13.5 15.8 12.7 m .I Eu 3.1 - - 3.5 3.05 3.8 3.46 4.4 4.3 3.4 Gd 9.6 - - 9.6 8.2 9.1 11.2 12.7 14.5 11.1 Tb ------1.6 - - Tm - - - 0.50 - 0.42 - - - - Yb 1.8 - - - - 2.5 - 2.4 3.1 - Lu 0.23 - - 0.29 0.24 0.32 0.28 0.36 0.35 0.34 Key: LOI, Loss on ignition.

plutonicsit often replaces other minerals.It is extremely Occasional larger crystals apparently in equilibrium with the richin TiO, (7-10 wt.%) andshould betermed a liquid, suggest that this has been a crystallizing phase for titaniferous iron-rich biotite (see Table 2). Even though the some time, but not in any great quantity. Mg/Fe ratio varies from 1.49 to 3.37, SiO, and Alz03 are remarkablyconstant (33.16-34.95 and 14.87-16.26 wt.%, Plagioclase respectively). Ti02 varies to some extent (7.29-10.01 wt.%) butunsystematically relative tothe Mg/Fe ratio, as also Calcic plagioclasehave been recorded intwo intrusions shown by Rock (1982 a). only, and range in composition from andesine (FM 142) to oligoclase (FM 206). Fe- Ti oxides Otherminerals which occur as phenocrysts are Fe-Ti Other groundmass minerals oxides, commonly titaniferousmagnetite (Table 2), less The groundmass is almostentirely holocrystalline and is oftenilmenite. The composition of thetitanomagnetite dominated by potassium feldspar and analcime (Table 2), shown pronouncedvariations with respectA1203to which, together with clinopyroxene and an Fe-Ti oxide, are (3.96-7.90 wt.%),FeO' (59.42-75.21 wt.%) and MgO amost ubiquitous. The CaO content,as well as that of Na,O (0.11-7.33 wt.%) but without any clear relationship to the and K20,of the clear interstitial analcime is highly variable whole-rock composition. Within individual grains, however, (0.16-2.14,9.27-12.76 and 0.00-2.65 wt.% respectively). rims are slightly higher in TiO, and A1203 and poorer in As CaO decreases, Na,O decreases, and the analysis lowest FeO', MgO and CrzO3 than the cores (Table2). in CaO is also relatively low in A1203 andhigh in SiO,. This is similar tothe trends reported by Henderson & Gibb Apatite (1983), who believe the CaO-richvariety to represent Apatite crystals are commonly found but are usually small, originalanalcime, whereas those poor in CaOto have and mainly appear to be late stage, or even quench crystals. recrystallizedat lower temperatures. Table 1 gives modal

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Table 3. Continued

FM 168 FM115 FM161 FM204 FM157 FM104FM157FM204FM161FM115FM168 FM2 SM122SM21FM169

SiO, 43.03 41.27 43.73 41.16 45.15 49.08 46.87 45.60 46.98 49.05 Ti02 4.11 3.59 3.96 3.66 3.45 2.81 2.95 3.14 1.88 2.74 A1203 14.14 13.35 14.63 14.23 15.69 16.02 17.17 16.76 17.39 17.97 Fe203 5.58 7.64 5.73 5.83 2.95 6.51 3.68 4.51 3.22 2.91 Fe0 5.07 3.06 4.22 3.67 5.35 1.63 4.35 3.64 3.17 4.14 MnO 0.28 0.24 0.26 0.28 0.27 0.24 0.27 0.26 0.18 0.15 MgO 4.35 4.28 4.10 4.03 3.61 3.32 3.14 2.82 2.65 2.43 CaO 9.28 9.56 8.06 9.47 8.07 5.73 7.71 7.19 4.77 6.00 Na,O 1.91 3.31 1.94 1.93 2.27 3.60 3.96 4.22 5.14 2.24 KZ0 4.24 2.30 4.73 4.73 4.36 4.07 2.65 4.01 4.81 6.86 p205 0.77 0.58 0.89 0.88 0.88 0.83 0.77 0.88 0.58 0.62 L01 4.35 9.66 5.74 7.51 6.56 5.95 5.21 5.48 5.98 4.10 Total 97.11 98.84 97.99 98.60 99.79 98.73 98.51 96.75 99.21 V 394 400 368 389 304 293 280 285 201 248 Cr 21 29 21 21 21 25 16 17 23 23 CO 34.9 29.5 27.1 31.3 20.59 - 19.8 18.7 11.6 - Ni 23 29 10 18 10 17 5 9 6 5 Zn 85 82 100 87 88 88 108 108 97 289 Rb 70 32 83 101 58 61 33 94 76 136 Sr 1015 655 1384 1251 1401 764 1743 1053 1376 1437 Y 35 31 36 31 33 32 31 35 30 37 Zr 360 302 379 337 414 380 378 429 397 431 Nb 87 70 96 109 109 117 143 118 156 122 Hf 9.5 6.5 8.8 7.7 8.7 - 8.6 9.8 8.2 - Th - - 7.92 12.51 8.81 - 18.89 13.0 17.10 - Ta 6.14 4.00 6.20 7.09 7.25 - 9.79 8.17 4.42 - La 79.8 57.3 79.1 81.8 84.9 - 118.8 97.3 125.8 - Ce 145.8 100.1 142.3 154.2 138.5 - 200.7 179.8 213.0 - Nd ------81.2 - Sm 11.7 8.5 11.7 11.6 11.0 - 12.2 12.5 11.0 - Eu 3.6 2.6 3.5 3.4 3.3 - 3.9 3.6 3.2 - Gd 10.1 7.3 9.5 9.1 8.1 - - 11.9 - - Tb ------1.6 - Tm 0.45 - 0.43 ------Yb 2.2 - 2.4 2.24 - - 3.05 - - - Lu 0.32 0.31 0.32 0.29 0.28 - 0.37 0.40 0.38 -

analyses forthe groundmass of anumber of intrusions, virtually identical to that of the Monte Penoso Formation where it has been possible to make such analysis using the basaniticlavas (Fig. 8). However,thepresence of electron microprobe in a point-counting mode. For each of phenocrysts in certain of the intrusions clearly indicates a the point-counted thin sections (Table l),the electron beam needfor caution in ascribingliquid compositions, and was moved in a grid pattern to determine the groundmass according to the IUGS Subcommission on the Systematics of minerals at 50 points. Each point wasvisually checked to Igneous Rocks (Streckeisen 1979) the mineral phases must make sure that the beam never resided at grain boundaries. be taken into account in the nomenclature. On this basis, as A distinctivefeature is thevirtual absence of calcic theyform minor intrusions,these rocks must be termed plagioclase as a distinguishable crystalline phase, evenin the lamprophyres. Further support for this is given in the major groundmass of plagioclase-phyric samples.Although the review by Rock (1977), who also suggests thatthe anorthite molecule appears in the CIPW norms calculated lamprophyresshould be distinguished from basanites and fromthe whole-rock analyses (in amounts ranging from basalts by their essentially primary amphibole and/or mica. 12-20%),it would seemthat the plagioclase components Whilst not all Maio sheetintrusions display amphibole have in factbeen accommodated in the highly aluminous phenocrysts, it is very common as a microphenocryst or a clinopyroxene and amphibole lattices. groundmass phase. Lamprophyre thus seems an appropriate general name and by reference to Rock’s (1977) Table 111, theMaio intrusions fall intothe Alkaline Lamprophyre Nomenclature class. Characteristicallythey are associated with alkaline eruptives(basanites and nephelinites), agabbro-syenite If the compositions of the aphyric rocks (see Table 3) may complex, and carbonatites. be taken as equivalent to magmatic liquids then they are Despiteassociation with anessexite gabbro-syenite basanites,and indeed their trace-element geochemistry is plutonic mass, a notable feature of the Maio lamprophyre

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intrusions is the near-total absence of plagioclase as a recommended values. The RE elements together with Hf, Th, Ta phenocryst phase. Anorthite is however normative, and high and CO were determined by instrumental neutron activation, using Pco2, (Rock 1976), has probably resulted in the enrichment internationalstandards for calibration. Methods are described in Brunfelt & Steinnes (1971) and Baedecker et al. (1977). Fifty-eight of alumina in the pyroxene and amphibole at the expense of of the samples are from aphyric intrusions (31 sills, 27 dykes) the the anorthite molecule. remainderare porphyritic samples whose phenocrysts have been analysedby electron microprobe (see Table 2). Analysesof representative samples are reported in Table 3. Complete data are Geochemistry availableupon request as Supplementary Publication NO. SUP 18045 andcan also be retrieved on line from the Igneous Analytical methoh GeochemicalData Bank (IGBA) via the NGDB Data Bank Ninety of the Maio sheet intrusions have been analysed for major Manager, British Geological Survey, Keyworth, Nottingham, UK. and trace element (V, Cr, Ni, Zn, Rb, Sr, Y, Zr, Nb) using Phillips PW 1450 automaticX-ray fluorescent spectrometer thein GeologicalInstitute, University Bergen.of The glass-bead Major element geochemistry technique ofPadfield & Gray (1971) wasused for the major elements and pressed powder pellets for the trace elements using The major-element analyses are shown in Fig. 3 in a series international basalt standards for calibration and Flanagan's (1973) of Harker variation diagrams. The analyses of the aphyric

Ti02 2Y203 c-. I" I

Si02 10 38 40 42 44 42 40 38 46 48 50

MgO 0

CaO Na,O 12-

0 10- *O

6- Si02 Si02 SiO, 0, 4 0 40 42 44 46 48 4638 44 42 40 50 38 4442 40 46 48 50 38 40 42 44 46 46 501

* FM 41 Aphyricsills

X Aphyric dykes Si02 Oi I I 1 l l 3844 42 40 46 48 50

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samples define rather dispersed trends where AI2O3, Na,O largescatter, whereas for Y-Zr and Nb-Zr positive and K20 increase, and TiO,, FeO' MgO and CaO decrease correlationsexist (Fig. 4). Figure 5 shows pairs of highly with increasingSiOz. Sills anddykes are chemically compatibleand incompatible elements suchas Ni-Zr and indistinguishable and the aphyric samples do not define a Cr-Zr, defining broadbands within whichNi and Cr trend different to that of the slightly porphyritic varieties, decreaserapidly with only slight increase in Zr.The though the former show a wider compositional scatter. relationships Ti0,-Zr and V-Zr give examples of pairs of Taking the group overall, the high alkali content (and in elements one of which remains 'incompatible' (Zr) and the particular the variable but generally highK,O/Na,O ratio others (TiO, and V) vary from incompatible to compatible (0.18-3.35, averaging 1.17) with a SiOz range from38-49%, (Fig. 5), which is demonstrated by the change in patterns as well as the C.I.P.W. normative mineralogy (average Or frompositive to strongly negative Ti0,-Zr and V-Zr 18.12, Ab 10.70, An 15.77, Ne 6.99, Di 21.25, 01 2.25, Mt correlations. 4.80, I1 7.23, Hm 2.95, Ap 2.43), is consistent with a Figure 6 shows a series of multi-element (Rb, Sr, Nb, P, basanitic chemistry for the entire group of intrusions. Whilst Zr,Ti, Y, and Mg) diagrams,normalized toa internal the majority of the samples define a small silica variation comparator FM41 chosen as one of the most primitive (between 40 and 47%), indicatinglittle fractionation members of the series (see Fig. 3), and compiled from the towards more salic members, the inter-element variation is complete list of analysedaphyric intrusions, divided into consistent with early-stage fractionation, presumably under threegroups according to their MgO contents. MgOwas hydrous conditions, of abasanitic magma at moderate to chosen as adifferentiation index so thatliquids at an low pressure, involving a steady fall in MgO, FeO', TiO, approximately equal evolutionary stage might be compared and CaO with a concomitant rise in A1203,K,O and Na,O. directly. Within each group samples defining similar curves The wide scatter of P,05 values is to be expected in rocks in in the multi-element diagram, have been grouped together. which apatite appears as microphenocrysts. Pronounced variations exist in element ratios, and as will be If the scatter illustrated in Fig. 3 can be attributed to the discussed later, can hardly be accounted for by fractional primary compositions, then the data provide little evidence crystallization alone. forevolution from a single body of magma by fractional The REE, Hf, Th and Ta dataof selected representative crystallization or even that the intrusions were all derived samples, are presented in Table 3. Their REE patterns as fromthe same source. Radiometric data (Mitchell et al. shown in Fig. 7 are characterized by being highly enriched in 1983) indicatethat they were emplaced over a lengthy LREE and depleted in HREE, and are very similar to the period of time (>7 Ma), yet despitelack of evidence of Monchiquealkaline lamprophyres described by Rock consanguinity there is equally no evidence that the parental (1978). magmas changed significantly throughout the period. Indeed To classify the source liquids, the trace elements have the closest equivalents to the minor intrusions amongst the first beennormalized to average modern MORB (Pearce Maio effusives arethe basanitic lavas of the Mte Penosa 1980) and plotted in Fig. 8 where they are compared with Formation (see Fig. 8) which were erupted some 2 Ma later. boththe MORB of theocean crust exposed on Maio, (Stillman et al. 1982) and with the basanitic lavas of the Mte Trace element geochemistry Penoso Formation. The range in composition is of the same order as that seen in the basanitic lava sequence, which is Figure 4 shows the relationships between Sr, Rb, Y, Nb, believed to bederived from a single source magma (J. and Zr. the Sr-Zr and Rb-Zr diagrams, the data show In Zielonka,pers. comm. 1983), thoughvariation in certain elements, notably Rb, Sr, Ti, is greater, and the Mg content 150 Rb Aphyrlc sdls X AphyrbC dykes is in general slightly lower than in the basanites. 0 Phyric 51s and dykes The differences observed between the basanitic lavas of l theMte Penso Formation and the intrusives might be ascribedto the latter's finalsolidification as a hypabyssal ratherthan an effusive body,where vapour pressure and oxygen fugacity would bemaintained at a higher level throughout the crystallization process. The lower values for Ti in the sheet intrusions may relate to magnetite and/or ilmenitecrystallization under such conditions. Other variations may also be ascribed to raisedPHZO: minor Zr intrusions which are emplaced in ocean-floor sediments and 2000 S, 20 ,,..,..,.,..,.I 2w 3w 400 500 3012 3380 pillow lavas show some degree of alteration to epidote and Fm5d Fml51i. chlorite whilst those intruding the essexites and syenites do 0 not, and it is the former which give the most erratic values i l Nb for Rb and Sr. A similar effect has also been noted in the carbonatite intrusions of Maio (M. J. Le Bas, pers. comm. 1983).

1 0 -0- 00 1 Petrogenesis Asthe sheet intrusions were emplaced over an extended period of time and show some petrographic and chemical Fig. 4. Rb, Sr, Y and Nb vs Zr for aphyric and phyricsills and variability,a principal objective of thestudy has been to dykes. ascertain the nature of the source magma or magmas and to

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300 Ni

X Aphyric dykes 1) Dcr= 8 0 Phyric sills and dykes 2) Dcr =l5 0 DZr = 0.1 F* 100 - 0 I l O\ 15% :o

l 0 '0" Zo 2.0 0 X Zr 1.5 200 500

20%

10-

0 .. . l 30%

4- Zr I !I 10200 Zr 500150, 200I Zr I 21 500 500

Fig. 5. Ni, Cr, TiO, and V vs Zrfor aphyric and phyric sills and dykes.Vectors indicate Rayleigh fractionation trends and percentage fractionation at given bulk partition coefficients.

determine whether the observed variations may be ascribed interpreted as fractionationtrends, the following may be to suggested: The Ni-Zr diagram would indicate some 30% (a)auniform parent which has undergone varying fractionation(Rayleigh) of minerala assemblage with degrees of fractional crystallization; or DNi 8 and D,, = 0.1. It will be argued later that the main (b)auniform parent which hasbeen subject to a fractionatingminerals that have modified theliquids are number of differentprocesses such as fractional clinopyroxeneand Ti-magnetite and probably kaersutite. crystallization, magma contamination mixing, etc. ; However,the partition coefficient for Ni betweenthe or above-mentioned minerals and liquid are all <8 (Table 4), (c) a series of parents. and it is therefore suggested that fractionation of olivine, whose KNI>> 8 (Table 4), also may have been involved. The Fractional crystallization models trend of the Cr-Zrdiagram changes slightly to become Both the mineral and whole-rock chemistries suggest that steeperat a Cr content of around 70 ppm(Fig. 5). This fractional crystallization has taken place.The evidence given indicates a change in the bulk distribution coefficient, and a by the phenocryst phases is that during emplacement of the fractionation (c. 30%) model, where the Dcr changes from liquids there was in situ crystallization of olivine, clinopyro- 8 to 15 at DZr= 0.1, gives trends parallel to those defined by xene, and Fe-Ti oxide and in some cases an amphibole was the data. This may also suggest that olivine, whose Gris also onthe liquidus. As the liquids cooled, amphibole, biotite, low (Table 4) was a fractionating mineral at the early stage and apatite crystallized as microphenocrysts before the final but that clinopyroxene and perhaps kaersutite, whose Kcr solidification of the groundmass. The compositions of all are high (Table 4), became the most important fractionating these phases are known and it is thus possible to assess their minerals at a later stage. significance. Ti is an incompatible element with respect to olivine and If the trend of the broad bands definedby the data in the clinopyroxene,whereas it is stronglyaccommodated in Ni-Zr,Cr-Zr, TiO,-Zr and V-Zr diagrams(Fig. 5) are magnetite, and also to some extent in kaersutite (Table 4).

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1 GROUP A (MgO 5.5-4.5%)1

(3) 51, , , , , , , Fib Sr Nb P Zr Ti V MQ (1)

i' ifi) I\ .-.U

-3 l

Rb Sr Nb P Zr Ti Y MQ Rb dr Nb P Zr TI V M4 (VII) Fig. 6. Whole-rock trace element variation, normalized to sample FM 41.

Thetrend of theTi0,-Zr diagram, and also the V-Zr To make more quantitative calculations, a least-squares (though partition coefficients for V are lacking), would also computer mixing programme, based on the original method show the progressivelystronger effect of magnetite given by Wright & Doherty (1970) has been applied. The fractionation. choice of twelve possible parent-daughtersamples was An alternative explanation of the progressively steepen- made on the basis of the Harker diagrams of Fig. 3. ing trends of theNi-Zr, Cr-Zr, Ti02-Zr and V-Zr If we in the first place consider only the major oxide diagramsduring the later stages of fractionation(Fig. 5) compositions of therocks, ignoring their trace-element would be increasing DZrwith fractionation, and which would concentrations, acceptable models fromthe approximately imply zircon fractionation. least to most fractionated samples, giving small residues and For the major elements, Fig. 9 shows the compositional realisticextracts, can beachieved. Models forfour range of the whole group of intrusives in a series of Harker parent-daughter pairs are shown in Table 5, and as already diagramsin which the compositions of thephenocryst indicated by examination of Fig. 9, the best model is given phases are also plotted. Inspection of Fig. 9 indicates that by fractionation of clinopyroxene and magnetite (model 2), the effect of particularmineral extracts would notbe whereasmoreata fractionated of clinopyroxene + uniform throughout the compositional range of the group. magnetite f kaersutite give satisfactoryresults (model 4). Amphibole and/or biotite could have little effect on those The inclusion of olivine in the mixesgive generally richer insilica (Fig. 9). The compositionalvariation is higher residuals, or olivine has to be added to the parent, consistent with theextraction of the observedphenocryst which is an unacceptable result, thoughin model 3 (Table 5) phases, but it would be necessary to remove a substantial removal of a small quantity of olivine gives an acceptable amount of all phasesobtainto the full range of result. It is also evident from Table 5 that clinopyroxene is compositions. Petrographic observation indicates, however, the main fractionatingmineral at anearly stage, whereas thatamphibole and biotite both occur predominantly as magnetitebecomes moreimportant at later stages during microphenocrysts, may not have been long on the liquidus fractionation. by the time of intrusionand quenching and could hardly By comparing the calculated incompatible trace-elements havebeen extracted insufficient quantity, by normal concentrations of the calculated parents with those of the fractional crystallization, to produce the more fractionated measuredparents the number of acceptableparent- of the aphyric intrusives. daughtermodels is furtherreduced. The calculated

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X FM 157 . FM 121 A FM 107 FM 151 X SM 121 o A FM 17 FM 169 0 FM l61 + FM 188 100- 0 FM 204 -1 00 A FM 160 (C 1 400-

300- - 50 a, r L U- C 0 r .0 X 0 0 100- 100- a-

-10

- 0 SM 33 X FM 115

10 - 10-

6J I 6 ,, la de Nd Sm Eu Gd Tb Yb Tm Lu La Ce Nd Sm TbEu Gd Tm Yb Lu Fig. 7. REE patterns of the Maio alkaline lamprophyres. The shaded area in lower partof (a) shows the field occupiedby the samplesof the upper part of the diagram, and the shaded partof (c) shows the field occupied by the samples shown(a). in Rb Sr Nb P Zr Ti Y Mg trace-element concentrations, in this case by applying the ' I 50 Rayleigh fractionation model to the mineral extractsgiven by 40 theleast-squares mass balancemodels, willof course depend on the partition coefficients used, and those chosen

Penoso Formation for this study are given in Table 4. A large number of mixes __ lavas weretried, but only a few gave reasonable fits between observed and calculated major and trace elements. The best models,spanning most of thecompositional range as

Table 4. Minerallmelt partition coefficients uredin modelling

X Mt 01Am CPX a Rb 0.01' O.Olb 0.045b - Sr 0.016' 0.12" 0.5Sa - Zr O.Old O.ld OSd O.ld Y O.Old OSd l.Od 0.2d Nb O.Old O.ld O.Sd 0.4d Ti O.0Zd 0.3d 1.5d 7Sd Cr 1.S 20' 128 1.2h Ni 23' 3' 2.6' 6.2'

.5 ' , I 1 I I I I Key: Partitioncoefficient taken from: Rb Sr Nb P Zr Ti Y Mg (a) Nagasawa(1973); (b) Arth (1976); (c) Frey Fig. 8. Whole-rock trace element variation. Rb, Sr, Nb, P, Zr, Ti, et al. (1978); (d)Pearce & Norry (1979); Y and Mg normalizedto MORB (average values after Pearce 1980). (e)Irving (1978); (f) Lindstdm(1976); (g) Cox For comparison the fields occupied by the Mte Penosa Formation et al. (1979); (h) Schock(1979); (i) Duncan & lavas (valuesfrom J. Zielonka, pen. comm. 1980), and the Maio Taylor(1968); -: Partitioncoefficient taken as MORB (unpublished data from the authors) are indicated. zero.

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'Ol

A Cllnopyroxene m Olivine + Amphlbole

X Biotlte 0 Magnetlte

0 I 0 10 20 30 40 50 60 Si02

25- 50-

20- 40-

30-

k 10- 20-

X X *:A 4 4% Q 10 0 I

o+T--T7Z=700 Si02 Fig. 9. Variation diagrams for major element oxides vs. SiO, for whole-rock compositions (shaded areas), together with compositions of phenocryst phases, plottedto show the effect of crystal extract on residual liquid composition. Arrows show the direction of fractionation trend in whole-rock composition.

regards both major and trace elements, are shown in Table of the elements, but vary considerably in Rb content. The 5. The incompatible trace elements that generally show the other intrusions in Group A show a variety of different poorest fits are Rb and Sr. From thesemixing models it can patternsfor which explanations must besought. The be seen that clinopyroxene and Ti-magnetite are the most majority of elements concerned here (Nb, P, Zr, Ti, Y) are importantfractionating minerals in driving the liquid widely regarded as relatively immobile under conditions of compositions towards a more evolved state, with kaersutite alterationand metamorphism, thus the cause of variation and apatite as necessary constituents in some cases. must be soughtelsewhere, either in the process of Inorder to rationalize the trace-elementvariations fractionation or in thenature of thesource liquid. within thegroup of aphyricintrusions the analyseshave Fractionation involves the crystallization of phenocryst been normalized to one of their number chosen on the basis phases, known from petrographic observations to include (in of MgO and SiO, content as the most primitive, i.e. least apparentorder of appearance) olivine, clinopyroxene, evolved composition. As mentioned above they have then magnetite and/or ilmenite, amphibole, biotite and apatite, been assembled in sets containing samples of closely similar and using published partition coefficients (Table 4) it can be Mg values (Fig. 6). If the intrusions so groupedwere all seen that olivine and clinopyroxene crystallization have very derived from the same parental magma by the same process little effect onthe relativeconcentrations of the trace of crystal fractionation,then the trace element patterns elements in the residue.Magnetite fractionation and by should be parallel and vary only by increments determined inference ilmenite even more so has a dramatic effect on Ti by the degree of fractionation. This clearly is not the case, concentrationalone. Zirconium clearly behaves as a truly even in Group A, the set withMg content closest to the incompatibleelement throughout. The same is clearly not normalizing rock FM 41. The concept of parallel evolution truefor Rb, which,with K becomes compatibleon the of trends reflecting derivation from one parental magma will crystallization of biotite, and would then be depleted in the of course only apply if the fractionating assemblage remains liquid if the bulk partition coefficient became greater than mineralogically and chemically constant, or if the fractionat- unity. The trace-element patterns of some of the diagrams ing phasesappearing or disappearing with evolutionhave of Fig. 6 could beexplained if biotitehad indeed been the same partition coefficients. Within each of the groups of removed as afractionating phase; however, petrographic Fig. 6 themajor element chemistry and mineralogical observation as well as the least-squares mass balance models variations are minor, and hencebulk partition coefficients indicate that this isunlikely as biotite does not become a are not likely to vary to any significant extent. A substantial liquidus phase until very late in the solidification. A possible percentage, almost 33%, have flat parallel patterns for most explanation for the depletion of Rb (and K,O) can be found

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Table 5. Least squares mass balance models for the compositional change from parent to derivative by fractional crystallization

M odel 2 Model 3 Model 4 Model 3 ModelModel 1 2 Model

Calc. Obs. Calc. Obs. Calc. Obs. Calc.Calc.Obs. Obs. Calc. Calc.Calc. Obs. Calc.

Deriv. Par. Par.Par.Deriv. Par.Deriv.Par. Par. Deriv.Par. Par. Par. Deriv.Par.Par.Par. FM24FM33 1 FM161 2168FM 1 FM161122SM 1 FM169FM133 2 1 2

SiO, 45.70 44.90 44.13 44.31 47.70 46.67 46.81 51.73 47.70 47.59 47.50 49.25 48.35 48.05 48.15 TiO, 3.63 4.47 3.88 3.81 4.32 4.46 4.42 2.89 4.32 3.56 3.69 3.39 3.61 3.63 3.59 AW3 15.7414.6614.3114.2115.9615.3315.1518.9515.9616.4816.6418.1016.7717.2417.19 FeO' 10.57 12.11 12.01 12.08 10.23 10.94 10.95 7.13 10.23 10.36 10.32 8.32 9.52 9.49 9.53 MnO 0.29 0.30 0.28 0.28 0.28 0.30 0.28 0.16 0.28 0.19 0.18 0.28 0.26 0.28 0.28 6.14 6.00 4.47 4.72 5.05 2.56 4.47 4.64 4.53 3.05 4.18 3.81 3.703.81 5.004.18 MgO 3.05 4.53 4.64 4.47 2.56 5.05 4.72 5.644.47 6.00 6.14 CaO 9.88 11.26 11.05 11.08 8.79 10.06 9.83 6.33 8.79 9.05 9.13 7.77 7.85 8.39 8.42 Na,O 3.35 2.98 2.94 2.93 2.12 2.07 1.97 2.36 2.12 1.93 1.98 4.56 4.21 4.25 4.25 KZ0 4.77 2.66 4.09 4.14 5.16 4.60 4.66 7.23 5.16 5.52 5.47 4.33 4.37 4.00 4.01 P*O5 1.06 1.13 1.17 1.18 0.97 0.84 0.88 0.65 0.97 0.68 0.56 0.95 0.83 0.87 0.88 Percent derivative 85.68 87.48 90.13 75.91 90.13 87.48 85.68 derivative Percent 92.52 91.74 74.26 Olivine - - - 1.17 - - - 9.65 8.31 17.06 16.22 17.06Clinopyroxene 8.31 8.46 9.65 5.384.43 K aersutite 3.55 Kaersutite - - 2.17 0.90 5.11 - M agnetite 3.19 3.45 1.41 4.38 4.23 1.89 2.09 1.89 4.23 4.38 1.41 3.45 3.19 Magnetite A patite 0.69 0.65 0.69 Apatite - 0.44 0.19 - - 2.89 2.94 0.22 1.21 1.31 0.78 1.05 0.78squares 1.31 of 1.21 Sum 0.22 (r') 2.94 2.89 Calculated trace element abundances 87 45 75 76 83 70 75 136 83 103 101 94 72 86 87 86 72 94 101 103 83 136 75 70 83 76 75Rb 45 87 1559 1234 1378 1380 1384 1015 1261 1437 1384 1124 1130 1053 1183 984 980 9841183 1053 1130 1124 1384 1437 1261 1015 Sr1384 1380 1378 1234 1559 33 28 31 30 36 35 34 37 36 32 32 35 34 34 33 34 34 35 32 32 36 37 34 35 36 30 31 Y 28 33 339 359 360 379 360 345 431 379 337 338 429 352 429 338 337 379 431 345 360Zr 379 360406 359 339 400 400 N b 124 97 112 97 124 Nb 111 112 105 111 87118 9699 97 8896 122

Key: Minerals used in mixes. Model 1. Cpx, Mean of cpx from FM 42 and FM 108; kaersutite from FM 149; magnetite from FM 206; apatite from Deeret al. (1970)(anal. 1, Table 50). Model 2. Cpx from FM 168; kaersutite, magnetite and apatite as above. Model 3. Cpx from FM 149; olivine from FM 42; kaersutite, magnetite and apatite as above. Model 4. Cpx from FM 133; Kaersutite, magnetite and apatite as above. in the mobility of hydrous fluids during the final stages of Velhas and Monte Penosa Formations on Maio. Whilst in crystallization of the groundmassphases potassic feldspar broad terms some of the intra-lamprophyre variations may and analcime, under the hypabyssal conditions of elevated be explained by variable extents of crystal fractionation (up P,, whichmay seriously affect the distribution of the aboutto 25%) of predominantly clinopyroxene and alkalies. The Maio rocks may be considered as crystallizing magnetite and minor olivine, kaersutite and apatite,it seems from a basanitic liquid in which volatiles, including CO, inevitable from the evidence of trace elements that several havebeen retained until crystallization. Presumably the parental liquids were involved. retention may be connected with the hypabyssal emplace- Since the sheet intrusions make up almost 50% of the ment of these rocks in narrow intrusions which solidified exposedvolume of the BasementComplex and probably before diffusion into the wall rock permitted the escape of something of the orderof 25-30% of the total volume of the the volatiles. Eruptive equivalents may have degassed prior island of Maio,it is clearthat these parental liquids to the arrival of amphibole and biotite onto the liquidus, represent a major product of the island's magma source. In and hence crystallized as basanites. Such a conclusion would view of the fact that the 7 Ma Penoso lavas, which bring to a concur with the ideas of Rock (1976; 1982), who suggests a close the magmatic history of the island, have an almost derivation of alkali lamprophyres from a primary basanitic identical chemistry, it seems likely that this generative magma whichevolved withvolatile retention to a stage processwas inoperation at this precise sitefor at least where primary amphibole, then biotite could crystallize. 8Ma,-possibly as much as 13 Ma if,as Mitchell et al. (1983) suggest, some of the sheet intrusions are as old as 20 Ma. It is quite possible that the liquids may have been held as Conclusions distinct batches in separate magma chambers, but at least The alkaline lamprophyres of Maio are probably derived by some of the chemical diversity (particularly in Na, K, Rb, crystal fractionation from parental basanites. Such lavas are andSr) and variability of groundmass mineralogyis the abundantly represented in the Neogene lavas of the Casas result of variable effects of water-rock interactions,

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enhanced by theentrapment of volatilesdue to the LEAKE,B. E. 1978. Nomenclature of amphiboles. Mineralogical Magazine, hypabyssal mode of emplacement. 42, 533-63. LE BAS, M. J. 1962. The role of aluminium in igneous clinopyroxenes with relation to their parentage. American Journal of Science, 260, 267-88. This work forms part of the Cape Verde Magmatism International LINDmt0M, D. J. 1976. Experimental smdy of the partitioning of the transition

Research~ ~~ Proiect. Financial assistance from NERC and the Nansen metab benoeen dinopyroxene and coexisting silicate liquids. PhD thesis, Fund for the field study is gratefully acknowledged. Interlaboratory University of Oregon. co-operation was made possible by grants from NATO and NTNF. MITCHELL,J. G., LE BAS,M. J. ZIELONKA,J. & FURNES,H. 1983. On dating the magmatism of Maio, Cape Verde Island. Earth and Planetary Science Thanks are also due to the Cape Verde Government, particularly Letters, 64, 61-76. theMinisteiro do Desenvolvimento Rural and the Ministeiro NAGASAWA,H. 1973. Rare-earth distribution in alkali rocks from Oki-Dogo CoordinacaoEconomica, for their permission to undertakethe Island, Japan. Contributions to Mineralogy and Petrology, 39, 310-8. study and help in carrying out the field work. We thank N.M.S. PADFIELD,T. & GRAY,A. 1971. Major element rock analyses byx-ray Rock,B. Robins, S. Maal@e,and M. J. Le Basfor constructive fluorescence-asimple fiuion method. N. V. 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Received 29 May 1986; revised typescript accepted 19 June 1986.

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