GEOCHEMISTRY OF THE VOLCANIC ISLAND OF ...

Chemical Geology, 35 (1982) 115--128 Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

115

GEOCHEMISTRY OF THE VOLCANIC ISLAND OF ANNOBON, GULF OF GUINEA

J.M. LIOTARD 1, C. DUPUY 1, J. DOSTAL 2 and G. CORNEN 3

Centre G$ologique et G~ophysique, UniversitJ des Sciences et Techniques du Languedoc (U.S. T.L. ), 34060 Montpellier (France) 2 Department of Geology, Saint Mary's University, Halifax, N.S. B3H 3C3 (Canada) 3Laboratoire de P~trologie, Universit~ de Nantes, 44072 Nantes (France) (Received March 26, 1981; accepted for publication August 26, 1981)

ABSTRACT Liotard, J.M., Dupuy, C., Dostal, J. and Cornen, G., 1982. Geochemistry of the volcanic island of Annobon, Gulf of Guinea. Chem. Geol., 35: 115--128. Annobon Island in the Gulf of Guinea is composed predominantly of volcanic rocks of alkali character ranging from basanites to trachytes. The most primitive rocks, basanites, were probably formed by variable degrees of melting of a single upper-mantle source, previously enriched in incompatible trace elements. The more differentiated rocks could have been derived from basanitic magma by fractional crystallization mainly involving kaersutite, clinopyroxen e and plagioclase. It seems that, overall, there are no obvious differences in alkali basalt sequences from oceanic and continental environments.

INTRODUCTION

Annobon Island (1°26'S, 5°37'E) is located in the Gulf of Guinea (Atlantic Ocean) at the southern end of a series of volcanic islands which extend toward Africa in a NE direction. The other islands include S~o Tom~, Pr~ncipe and Macias Nguema Biyogo. The islands are made up predominantly of volcanic rocks of alkali affinities [alkali basalts, basanites and related more evolved rocks (Fuster Casas, 1954; Assunqao, 1957; Fitton and Hughes, 1977; Cornen and Maury, 1980)] and have been related either to hot-spot activities (Gorini and Bryan, 1976) or to an intraplate extension fracture system (Cornen and Maury, 1980). Annobon is the exposed tip of a 5000 m high stratovolcano built on oceanic crust (Gorini and Bryan, 1976). The geology of the island has been described by Schultze (1913), Tyrell (1934), Fuster Casas (1954) and recently by Cornen and Maury (1980) who also give detailed petrographic descriptions of the rocks. The paleomagnetic data on the lavas have been reported by Piper and Richardson (1972). According to Cornen and Maury (1980), three successive volcanic stages 0009-2541/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

116 led to the formation of Annobon Island. During the earliest stage, palagonitic breccias resulting from submarine volcanism were formed and were subsequently intruded by basaltic dikes (basanites with phenocrysts of pyroxene and olivine). The second stage is represented by layered basanitic flows which were intersected and fed by dikes dated at 5.35 Ma (Cornen and Maury, 1980). The third stage is characterized by the emplacement of tristanitic and trachytic plugs and by an ankaramitic basanite flow. The plugs were dated at 3.9 Ma (Cornen and Maury, 1980) while the flow is ~2.6 Ma old (Piper and Richardson, 1972). The purpose of this paper is to present some geochemical data on a volcanic suite from Annobon Island and to discuss the implications of the data. The petrography of the analyzed samples together with the major-element compositions of both the whole-rocks and constituent mineral phases were reported by Cornen and Maury (1980). PETROGRAPHY The Annobon lavas include basanite, potassic hawaiite and differentiated lavas (tristanite and oversaturated trachyte). The suite displays a typical "Daly gap" with rocks of intermediate SiO2 contents missing. The basanites are porphyritic, composed of phenocrysts of olivine and clinopyroxene set in a groundmass containing olivine, clinopyroxene, plagioclase, magnetite, ilmenite and glass. Compared to basanites, potassic hawaiites also contain phenocrysts of amphibole (kaersutite), magnetite and ilmenite with alkali feldspar in the groundmass. Tristanites (potassic benmoreites) consist of phenocrysts of amphibole (kaersutite), plagioclase, rare magnetite, apatite and sphene. The groundmass contains clinopyroxene, plagioclase, magnetite, ilmenite and alkali feldspar. Two types of trachytes have been identified (Cornen and Maury, 1980). One type (sample A19) is mineralogically similar to tristanite with the addition of phlogopitic mica phenocrysts. The second type (samples A1 7 and A1 ) contains phenocrysts of mica, magnetite, sphene, apatite and abundant alkali feldspar in a groundmass of magnetite, ilmenite, alkali feldspar and tridymite (sample A1). ANALYTICAL NOTES Twenty six trace elements have been determined in a suite of representative samples (Table I). Li, Rb, St, Ba, V, Cr, Co, Ni, Cu and Zn were analyzed by atomic absorption whereas Y, Zr and Nb were determined by X-ray fluorescence. An instrumental neutron activation technique was used for the analysis of REE, Sc, Hf, Cs, Th, U and Ta. The precision and accuracy of the methods were given by Dupuy et al. (1979) and Dostal et al. (1980).

117 GEOCHEMISTRY

The studied rocks show regular variations of the major elements. SiO2, A1203, Na20 and K20 increase whereas CaO and MgO decrease with differentiation; TiO2, FeOtot and P2Os decrease after an initial increase. On the basis of the major elements, Cornen and Maury (1980) have suggested that the suite (basanite--hawaiite--tristanite--trachyte) was fonned as a result of fractional crystallization. The occurrence of oversaturated lavas has been at~ tributed to amphibole fractionation. Trace elements

In general, all transition elements decrease with increasing differentiation although there are some subtle differences in their behaviour (e.g., Fig. 1). In basanites, Cr, Co and Ni decrease sharply, suggesting the early fractionat-ion of olivine and pyroxene while other elements such as Zn remain relatively constant. Like Ti and Fe, V increases slightly and then decreases. Similar trends have been observed in Nuku Hiva alkali basalt series (Maury et al., 1978), suggesting, in agreement with mineralogical studies (Cornen and Maury, 1980), an increase of oxygen fugacity during differentiation. The Ti/V ratios of the basanites, which range between 56 and 82, are typV ppm 20[

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118

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ical of alkali basalts and also resemble amphibole-rich upper-mantle xenoliths (Wass, 1980). Except in sample A10A, the abundances of Cu and the Cu/Zn ratio are low and characteristic of alkali basatts, This depletion of Cu can be partly attributed to the equilibration of alkali magma with residual sulphide during partial melting (Andriambololona and Dupuy, 1978; Frey et al., 1978). The high abundances of Ni and Cr in samples A I O A and A 1 3 as well as their high MMF ratio [100 MgO/(MgO + FeO)] suggest some accumulation of olivine and/or clinopyroxene. Li, Rb, Cs, Th, U and Nb show a regular increase toward the most differentiated rocks. Ba, Y, Zr, Hf and Ta display similar trends except in sample A1 where these elements are depleted. The decrease of Ba suggests KT A B L E II Trace-element a b u n d a n c e s and partition c o e f f i c i e n t s o f kaersutite

Abundances Partition coefficient, K

Ti

Sc

V

Cr

Co

Ni

Cu

Zn

La

3.51

23.4

417

112

52

78

12

267

55.4

4.03

5.33

5.27

9.33

5.20

6.0

1.50

2.64

0.63

The host rock o f kaersutite is b e n m o r e i t e A 1 2 (Table I); partition c o e f f i c i e n t = c o n c e n t r a t i o n in a m p h i b o l e / c o n c e n t r a t i o n in rock; the a b u n d a n c e s are in p p m e x c e p t Ti w h i c h is in %.

121

feldspar fractionation while the depletion of Zr and Hf implies crystallization of zircon. Trachytes are lower in Sr than less differentiated rocks, indicating that the transition from benmoreite to trachyte was accompanied by the crystallization of plagioclase. Chondrite-normalized REE patterns are shown in Fig. 2A for basaltic lavas and in Fig. 2B for more differentiated rocks. Basanites and hawaiite display sut~parallel REE patterns marked by an enrichment in light REE (LREE) and fractionated heavy REE (HREE). Their La/Yb ratios range between 18 and 30. Such features are typical of some alkali basalts (Schilling and Winchester, 1969; Zielinski and Frey, 1970; Frey et al., 1978) in both oceanic and continental environments. Most of the differentiated rocks (Fig. 2B) have similar HREE content and patterns as hawaiite AIOC b u t they have higher L R E E contents and a higher La/Yb ratio (>30). Trachyte A1 differs from the other samples by its strong depletion in HREE. This depletion is probably due to the fractionation of zircon and sphene (Nagasawa, 1970; Hellman and Green, 1979), which both occur in these rocks.

Differentiation of the Annobon alkali sequence The abundances and variations of the trace elements in the studied suite (basanite-trachyte) resemble those of several alkali sequences for which a fractional crystallization process has been suggested (Price and ChappeU, 1975; Sun and Hanson, 1976; White et al., 1979; Price and Taylor, 1980). The several trace-element variations suggest that the initial stage of differentiation was dominated by the crystallization of olivine and clinopyroxene while Fe-Ti-oxides, plagioclase and clinopyroxene were important in the intermediate stage. The role of K-feldspar and zircon was limited to the final stage of differentiation. Amphibole first appears in hawaiite and is present in other more differentiated rocks, although its influence on the chemical composition is masked by other mineral phases (REE b y clinopyroxene and V, Sc and Ti b y Fe--Ti-oxides). In order to evaluate the role of amphibole, one kaersutite from the benmoreite sample A12 has been ana-

Ce

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120 16.7 0.73 2.07

Eu

Tb

Yb

Lu

Hf

Ta

Th

U

5.1 2.0 3.4 0.5 9.0 5.9 2.1 0.5 Abundances Partition 1.61 1.72 1.30 1.19 0.65 0.86 0.11 0.30 coefficient, K

122

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Fig. 3. Chondrite-normalized REE composition of a liquid (dashed line) produced by fractional crystallization of a magma having the composition of sample A10C. Benmoreite A12 is shown for comparison.

lyzed for several trace elements (Table II). The partition coefficients (K) are also given on the assumption that amphibole was in equilibrium with a liquid having the composition of its host-rock (Fig. 3). These partition coefficients are similar to values reported for amphiboles (Irvilg, 1978; Nicholls and Harris, 1980) thus justifying the assumption of equilibrium conditions. It is noteworthy that the partition coefficient for Ta is relatively high, a feature which has also been observed by Villemant et al. (1980). In order to further evaluate the role of kaersutite during fractional crystallization, the abundances of several trace elements from the studied rocks are plotted together with the calculated compositions of residual liquids produced by the fractionation of kaersutite, clinopyroxene and plagioclase (Fig. 4). The increase of the La/Yb (Fig. 4A) and Zr/Y ratios (Fig. 4C) without an increase in Yb and Y content from hawaiite to benmoreite suggests the fractionation of kaersutite. Similarly, the increase of the Th/Ta ratio from 1.5 in hawaiite to 2.8 in benmoreite agrees with the relatively high KTa in kaersutite. The model calculations of REE abundances confirm that benmoreite may be derived from hawaiite by fractional crystallization involving mainly kaersutite, clinopyroxene and plagioclase in the proportion 6 7 : 2 1 : 1 2 as previously suggested by Cornen and Maury (1980) from major-element composition (Fig. 3). The higher content of SiO2 in benmoreite as compared to hawai ite is also consistent with the fractionation of kaersutite which dominates the transition from hawaiite to benmoreite. In a later stage of differentiation, the role of kaersutite is very limited while that of pyroxene and plagioclase increases. This is apparent in Fig. 4 where the Zr/Y and Rb/Ba ratios are similar in benmoreite and" trachyte.

123 LalYb

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124 to those of oceanic tholeiites and chondrites. Even the ratios of very mobile elements (e.g. Rb/Cs, Cs/U) fall within the range of the published values for the upper-mantle rocks and oceanic tholeiites (Hertogen et al., 1980). However, a major difference between the two types of basanite parental magmas becomes apparent when-comparing ratios involving elements with different degrees of incompatibility. In the alkali basalts and related rocks, the high and variable La/Yb ratios and the fractionation of Ht~EE are usually explained b o t h b y the low degree of partial melting of a chondritic upper-mantle source and by the presence of garnet in the residuum (Gast, 1968; Shimizu and Arculus, 1975). However, such a model cannot account for the high Ti/V (56--74) and Ba/Sr (0.60) ratios of the studied samples since garnet does not significantly affect these ratios. Furthermore several other ratios involving less and more incompatible elements are high and characteristic of alkali basalts. They suggest that a relative enrichment of these elements increases with increasing incompatibility of the elements (Sun and Nesbitt, 1978). All these factors indicate that the basanites were derived from an enriched upper-mantle source. This observation is consistent with the data on ultramafic xenoliths from the sub-oceanic and sub-continental mantle (Menzies and Murthy, 1980). Likewise Carter et al. (1978) have invoked mantle enrichment events as precursors to the genesis of alkali basalts. Model calculations were used to estimate the composition of the source rock for basanites A 1 6 and A7 which are considered to be the most primitive samples of the two types of rocks. The parental material was assumed Lo be a hydrous upper mantle which, after partial melting, left a residuum composed of olivine (75%), orthopyroxene (20%) and clinopyroxene (5%). The degree of anatexis (F) was then calculated using the equilibrium melting equation of Shaw (1970) and assuming the initial parental concentration for Yb (C0Yb ) is 0.25 ppm. The resulting F is 7.2% for sample A7 and 5.5% for sample A16. These degrees of partial melting are closely comparable to those obtained using P2Os and a pyrolite model (Frey et al., 1978). The initial parental concentrations for the other elements were calculated using this established F. The results are reported graphically normalized to chondrites in Fig. 5 with the elements arrange~l according to their degree of incompatibility (Sun and Nesbitt, 1978). The different compositions of the source rocks for samples A7 and A 1 6 fall within the range of analytical precision for most elements. The exceptions are Ti, V, Sc and Ta, the different concentrations of which may be explained by the presence of 1% of amphibole in the residuum left after extraction of a liquid with the c omposition of sample A16. Comparable calculations for sample A6 gives a sourcerock composition similar to that of sample A7 except for La, Ce and Sm abundances. For these elements the differences can be attributed to the presence of 0.1% of apatite in the residuum. The normalized abundances of trace elements increase in the calculated source rocks (Fig. 5), according to their degree of incompatibility. The exceptions are the most mobile elements Cs, Rb, K and U, and perhaps Th,

125 l

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U Th Nb Ta Ba La Ce Sr Sm EuZr Ti Tb Y Yb Sc

i,10 Z CD Z

c.~

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o

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1 0.6 C$ Rb

Fig. 5. Normalized abundances of incompatible elements in the calculated mantle source of basanites A7 and A16. The equation of equilibrium partial melting (Shaw, 1970) was used with degree of partial melting of 7.2% for sample A7 and 5.5% for A16. Residuum was assumed to be 75% olivine, 20% orthopyroxene and 5% clinopyroxene. Partition coefficients for REE and V were obtained from Schilling et al. (1978); for Nb and Ta from Wood et al. (1979); and for Rb, K, U, Th, Ba and Sc from Sun and Hanson (1975) and Irving (1978). The normalizing values were taken from Sun and Nesbitt (1978). • = sample A7; a = sample A16.

which seem to be relatively depleted. There are several possible explanations of this depletion. Frey et al. (1978) have shown that K and Rb do n o t correlate with the other incompatible elements in some basaltic rocks and suggested that the alkali abundances have been affected by the volatile-rich fluids. Alternatively, this depletion could reflect the presence of phlogopite in the mantle residuum which was in equilibrium with alkali magma. Another possibility is that the depletion of alkalies m a y reflect the composition of the upper mantle before the metasomatic event. A partial melting model involving garnet in the residuum (Frey et al., 1978) can also explain the observed differences among basanites provided that the mantle source has 2.5--3 times chondritic abundances of Yb. A similar enrichment is required for the other least incompatible elements (Sc and V). The former model is preferable for the studied basanites since it assumes nearly chondritic abundances in the parental source for Yb, Sc and V. CONCLUSION

The studied sequence displays several features c o m m o n to many oceanic alkali series and also to some continental basanites such as those of southeastern Australia (Frey et al., 1978). This would indicate that overall there are no obvious differences in alkali basaltic sequences from oceanic and continental environments. In the A n n o b o n alkali series, the differentiated rocks m a y be the result of fractional crystallization involving kaersutite, clinopyroxene and plagioclase in variable proportions according to the stage of differentiation. In partic-

126 ular, t h e b e n m o r e i t e is p r o d u c e d f r o m t h e h a w a i i t e m a i n l y b y a d o m i n a n t k a e r s u t i t e f r a c t i o n a t i o n . T h e b a s a n i t e s m a y be g e n e r a t e d f r o m t h e s a m e m a n t l e s o u r c e w h i c h was p r e v i o u s l y e n r i c h e d in i n c o m p a t i b l e t r a c e e l e m e n t s . T h e d i f f e r e n t c o m p o s i t i o n o f b a s a n i t e s m a y be e x p l a i n e d b y a v a r i a b l e d e g r e e o f p a r t i a l m e l t i n g leaving a v e r y small a m o u n t o f a p a t i t e or amp h i b o l e in t h e r e s i d u e . ACKNOWLEDGEMENTS F u n d i n g f o r this p r o j e c t was p r o v i d e d b y C . N . E . X . O . , F r a n c e a n d b y t h e Natural Sciences and Engineering Research Council Canada (operating grant A 3782).

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127 Higuchi, H. and Nagasawa, H., 1969. Partition of trace elements between rock forming minerals and the host volcanic rocks. Earth Planet. Sci. Lett., 7: 281--287. Irving, A.J., 1978. A review of experimental studies of crystal/liquid trace element partitioning. Geochim. Cosmochim. Acta, 42: 743--770. Maury, R.C., Andriambololona, R. and Dupuy, C., 1978. Evolution compar~e de deux s~ries alcalines du Pacifique Central -- RSle de la fugacit~ d'oxyg~ne et de la pression d'eau. Bull. Volcanol., 41(2): 1--22. Menzies, M. and Murthy, V.R., 1980. Mantle metasomatism as a precursor to the genesis of alkaline magmas -- isotopic evidence. Am. J. Sci., 280A: 622--638. Nagasawa, H., 1970. Rare earth concentrations in zircons and apatites and their host dacites and granites. Earth Planet. Sci. Lett., 9: 359--364. Nagasawa, H., 1973. Rare earth distribution in alkali rocks from Oki-Dogo Island, Japan. Contrib. Mineral. Petrol., 39: 301--318. Nicholls, I.A. and Harris, K.L., 1980. Experimental rare earth element partition coefficients for garnet, clinopyroxene and amphibole coexisting with andesitic and basaltic liquids. Geochim. Cosmochim. Acta, 44: 287--308. Pearce, J.A. and Norry, M.J., 1979. Petrogenetic implications of Ti, Zr, Y and Nb variations in volcanic rocks. Contrib. Mineral. Petrol., 69: 33--47. Piper, J.D.A. and Richardson, A., 1972. The palaeomagnetism of the Gulf of Guinea volcanic province, West Africa. Geophys. J., 29: 147--171. Price, R.C. and Chappell, B.W., 1975. Fractional crystallization and the petrology of Dunedin Volcano. Contrib. Mineral. Petrol., 53: 157--182. Price, R.C. and Taylor, S.R., 1980. Petrology and geochemistry of the Banks peninsula volcanoes, South Island, New Zealand. Contrib. Mineral. Petrol., 72: 1--18. Schilling, J.G. and Winchester, J.W., 1969. Rare earth contribution to the origin of Hawaiian lavas. Contrib. Mineral. Petrol., 23: 27--37. Schilling, J.G., Sigurdsson, H. and Kingsley, R.H., 1978. Skagi and western neovolcanic zones in Iceland -- Geochemical variations. J. Geophys. Res., 83: 3983--4002. Schultze, A., 1913. Die Insel A n n o b o n im Golf yon Guinea. Petermann's Geogr. Mitt. Jahrb., 59: 131--133. Shaw, D.M., 1970. Trace element fractionation during anatexis. Geochim. Cosmochim. Acta, 34: 237--243. Shimizu, N. and Arculus, R.J., 1975. Rare earth element concentrations in a suite of basanitoids and alkali olivine basalts from Grenada, Lesser Antilles. Contrib. Mineral. Petrol., 50: 231--240. Sun, S.S. and Hanson, G.N., 1975. Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites. Contrib. Mineral. Petrol., 52: 77--106. Sun, S.S. and Hanson, G.N., 1976. Rare earth element evidence for differentiation of McMurdo volcanics, Ross Island, Antarctica. Contrib. Mineral. Petrol., 54: 139--155. Sun, S.S. and Nesbitt, R.W., 1978. Petrogenesis of Archaean ultrabasic and basic volcanics and mantle evolution -- Evidence from rare earth elements. Contrib. Mineral. Petrol., 65: 301--325. Tyrrel, G.W., 1934. Petrographical notes on rocks from the Gulf of Guinea. Geol. Mag., 71: 16. Villemant, B., Joron, J.L., Jaffrezic, H., Treuil, M., Maury, R. and Brousse, R., 1980. Cristallisation fractionn~e d ' u n magma basaltique alcalin: la s~rie de la cha~ne des Puys (Massif Central, France). Bull. Mineral., 103: 267--286. Wass, S.Y., 1980. Geochemistry and origin of xenolith-bearing and related alkali-basaltic rocks from the Southern Highlands, New South Wales, Australia. Am. J. Sci., 280-A: 639--666. White, W.M., Tapia, M.D.M. and Schilling, J.G., 1979. The petrology and geochemistry of the Azores Islands. Contrib. Mineral. Petrol., 69: 201--213.

128 Wood, H.A., Tarney, J., Varet, J., Saunders, A.D., Bougault, H., Joron, J.L., Treuil, M. and Cann, J.R., 1979." Geochemistry of basalt drilled in the North Atlantic by IPOD leg 49: Implications for mantle heterogeneity. Earth Planet. Sci. Lett., 42: 77--97. Zielinski, R.A., 1975. Trace element evaluation of a suite of rocks from Reunion Island, Indian Ocean. Geochim. Cosmochim. Acta, 39: 713--734. Zielinski, R.A. and Frey, F.A., 1970. Gough Island: Evaluation of a fractional crystalliza tion model. Contrib. Mineral. Petrol., 29: 242--254.