Existence of Quaternary ankaramites among …Existence of Quaternary ankaramites among Tertiary flood basalts at Koutaba (Bamoun Plateau, Western Cameroon): petrology and isotope data

115

ÑÏÈÑÀÍÈÅ ÍÀ ÁÚËÃÀÐÑÊÎÒÎ ÃÅÎËÎÃÈ×ÅÑÊÎ ÄÐÓÆÅÑÒÂÎ, ãîä. 70, êí. 1—3, 2009, ñ. 115—124

REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 70, part 1—3, 2009, p. 115—124

Existence of Quaternary ankaramites among Tertiary floodbasalts at Koutaba (Bamoun Plateau, Western Cameroon):petrology and isotope data

Amidou Moundi1, Pierre Wandji2, Richard Ghogomu Tanwi1,Jacques-Marie Bardintzeff3,4, Isaac Njilah Konfor1, Issiaka Foumboure1,Benjamin Ntieche1

1 Department of Earth Sciences, Faculty of Science, University of Yaounde I, PO Box 812 Yaounde, Cameroon;E-mail: [email protected] Laboratoire de Géologie, Ecole Normale Supérieure, Université de Yaoundé I, B.P. 47 Yaoundé, Cameroun;E-mail: [email protected] Laboratoire de Pétrographie-Volcanologie, UMR CNRS IDES 8148, Bât. 504, Université Paris Sud, 91405Orsay, France; E-mail: [email protected] IUFM, Université de Cergy-Pontoise, 95000 Cergy-Pontoise, France

Êâàòåðíåðíè àíêàðàìèòè âñðåä òåðöèåðíèòå áàçàëòîâèïîêðîâè íà Êóòàáà (ïëàòîòî Áàìóí, Çàïàäåí Êàìåðóí):ïåòðîëîãèÿ è èçîòîïíè äàííè

Àìèäó Ìóíäè1, Ïèåð Óàíäæè2, Ðèøàð Ãîãîìó Òàíóè1, Æàê-Ìàðè Áàðäèíöåô3, 4,Èñààê Íæèëàõ Êîíôîð1, Èñèàêà Ôóìáóðå1, Áåíæàìåí Íòèåøå1

1 Äåïàðòàìåíò „Íàóêè çà Çåìÿòà“, Óíèâåðñèòåò ßóíäå I, Ï.Ê. 812 ßóíäå, Êàìåðóí2 Ëàáîðàòîðèÿ ïî ãåîëîãèÿ, Åêîë Íîðìàë Ñóïåðèîð, Óíèâåðñèòåò ßóíäå I, Ï.Ê. 47 ßóíäå, Êàìåðóí3 Ëàáîðàòîðèÿ ïî ïåòðîãðàôèÿ è âóëêàíîëîãèÿ, UMR CNRS IDES 8148, áë. 504, ÓíèâåðñèòåòÏàðèæ Þã, 91405 Îðñå, Ôðàíöèÿ4 IUFM, Óíèâåðñèòåò Ñåðæè-Ïîíòîàç, 95000 Ñåðæè-Ïîíòîàç, Ôðàíöèÿ

Ðåçþìå. Ïëàòîòî Áàìóí å ÷àñò îò Êàìåðóíñêàòà âóëêàíñêà ëèíèÿ (CVL). Ïîäîáíî íà äðóãèòå âóëêàíñêè ïëà-òà (Áàìèëåêå, Àäàìàóà) òî å ôîðìèðàíî ñëåäñòâèå íà åôóçèâíà äåéíîñò. Èçãðàäåíî å îò ïóêíàòèííè áàçàëòè(51,8—46,7 Ma), ñâúðçàíè ñ öåíòðàëíè âóëêàíè ñ âúçðàñò 45,5—44 Ma è ñ êâàòåðíåðíè ìîíîãåííè âóëêàíè. Ïîñëåä-íèòå ñà èçãðàäåíè êàêòî îò åêñïëîçèâíè, òàêà è îò åôóçèâíè ïðîäóêòè, âñðåä êîèòî ñà è àíêàðàìèòîâèòå ïîòîöè(0,82 Ma) ñ ïðèçìàòè÷íà íàïóêàíîñò â ðàéîíà íà Êóòàáà. Òîâà ñà ìàôè÷íè ñêàëè (42,8 < SiO2 < 43,6 òåãë.%), íå-íàñèòåíè ïî îòíîøåíèå íà SiO2 (12,9 < CIPW normative Ne < 14,9 òåãë.%). REE îòíîøåíèÿ (30 < LaN/YbN < 31 è19 < CeN/YbN < 20) è èçîòîïíèòå äàííè (0,703263 < 87Sr/86Sr < 0,703285; 0,518670 < 143Nd/144Nd < 0,512877) ñà ïî-äîáíè íà òåçè â ïîâå÷åòî ëàâè íà CVL. Àíêàðàìèòèòå íà Êóòàáà ïðîèçëèçàò îò àñòåíîñôåðåí ìàíòèåí èçòî÷íèêîò HIMU òèï.

Êëþ÷îâè äóìè: àíêàðàìèòè, àëêàëåí âóëêàíèçúì, Òåðöèåð, Êâàòåðíåð, K/Ar âúçðàñò, èçîòîïè (Sr, Nd),ìàíòèåí ðåçåðâîàð, ïëàòî Áàìóí, Êàìåðóíñêà âóëêàíñêà ëèíèÿ.

Abstract. The Bamoun Plateau is one of the main plateaus of the Cameroon Volcanic Line (CVL). Like the other pla-teaus (Bamileke, Adamawa), it was emplaced by an effusive volcanism. This produced fissural basalts (51.8—46.7 Ma)extruded by central volcanoes (45.5 to 44 Ma) and recent (Quaternary) monogenic volcanoes. These recent volcanoes havebeen built by both explosive and effusive volcanisms from which ankaramite flows (0.82 Ma) were emplaced. These anka-ramites outcrop in the form of prisms at Koutaba. They are mafic (42.8 < SiO2 < 43.6 wt.%) and silica undersaturated(12.9 < CIPW normative Ne < 14.9 wt.%). REE ratios (30 < LaN/YbN < 31 and 19 < CeN/YbN < 20) and isotope data(0.703263 < 87Sr/86Sr < 0.703285; 0.518670 < 143Nd/144Nd < 0.512877) are similar to main lavas of the CVL. The Koutabaankaramites originated from an asthenospheric mantle source of HIMU type.

Key words: ankaramite, alkaline volcanism, Tertiary, Quaternary, K/Ar ages, isotopes (Sr, Nd), mantle reservoir,Bamoun Plateau, Cameroon Volcanic Line.

116

Résumé. Les principaux plateaux de la Ligne Volcanique du Cameroun (LVC), à l’exemple du plateau Bamoun, sont con-struits par un volcanisme effusif à l’origine des basaltes fissuraux (51,8–46,7 Ma), eux-mêmes extrudés par des volcans cen-traux (45,5 à 44 Ma) et des volcans monogéniques récents (Quaternaire). Ceux-ci se sont édifiés à la suite d’épisodes volca-niques explosifs ou effusifs dont des coulées d’ankaramites datées de 0,82 Ma. Elles se sont parfois solidifiées en prismescomme à Koutaba. Ces laves basiques (42,8 < SiO2 < 43,6%) et sous-saturées en silice (12,9 < Ne normative < 14,9%) ont lescaractéristiques géochimiques (30 < LaN/YbN < 31 et 19 < CeN/YbN < 20) et isotopiques (0,703263 < 87Sr/86Sr < 0,703285 et0,518670 < 143Nd/144 Nd < 0,512877) typiques de la LVC. Les ankaramites quaternaires de Koutaba proviennent d’une sourcemantellique asthénosphérique de type HIMU.

Mots clés: ankaramite, volcanisme alcalin, Tertiaire, Quaternaire, âges K/Ar, isotopes (Sr, Nd), réservoir mantellique,Plateau Bamoun, Ligne Volcanique du Cameroun.

Introductionand geological setting

The Bamoun Plateau is one of the main units ofthe Western Cameroon highlands, an importantmember of the Cameroon Volcanic Line (CVL),which is still active (Fig. 1). The CVL has beendescribed as a succession of horsts and grabens,which stretches from the Pagalu Island in the Gulfof Guinea to Lake Chad (Déruelle et al., 1991;Fig. 2). Most of the horsts are represented by pla-teaus, the most prominent of which are the Ad-amawa Plateau (altitude 1.400 m) and the WestCameroon Plateau comprising the Ndu Plateau

(1.600 m), the Bamileke Plateau (1.400 m) and theBamoun Plateaus (1.200 m). In each plateau, floodbasalts outcrop for more than half of surface areaand their emplacement was favoured by reactiva-tions of Pan-African structures, marked by N30“en echelon” mega tension joints, a major charac-ter of the CVL (Moreau et al., 1987). Plateaus ofbasaltic lavas dated between 51 and 11 Ma (Fit-ton, Dunlop, 1985; Fitton, 1987; Lee et al., 1994;Marzoli et al., 2000; Ménard et al., 2002; Moundi,2004; Fosso et al., 2005; Tchokona Seuwi et al.,2006; Moundi et al., 2007; Wandji et al., 2008),bear the signatures of old volcanic activities asshown by their significant weathering that hasformed deep ferrallitic soils.

Fig. 1. Active lava flow during the Mount Cameroon April 1999 eruption (photo J.-M. Bardintzeff)

Ôèã. 1. Ëàâîâ ïîòîê ïî âðåìå íà åðóïöèÿòà íà Ìàóíò Êàìåðóí ïðåç àïðèë 1999 ã. (ôîòî Æ.-Ì. Áàðäèíöåô)

117

Along the CVL, the Bamoun Plateau lies within lon-gitudes 10°40'—10°56' E and latitudes 5°36'—6°00' N,covering an area of about 900 km2. The flood basaltsare essentially transitional basalts that outcrop in thewestern part, mainly near Bangourain and Foum-ban, while alkaline basalts outcrop in the south-east-ern part, around Foumban and Koutaba (Moundi etal., 1996; Moundi, 2004; Moundi et al., 2007). TheBamoun flood basalts (BFB) represent the oldestdated lavas from the CVL (40K/40Ar ages of 51 and46 Ma, Ménard et al., 2002; Moundi, 2004; Moundiet al., 2007). The Precambrian granito-gneissic bed-rock mainly outcrops in the north of the BamounPlateau.

This paper presents new geochemical data of theKoutaba ankaramites, witness of a recent volcanismwithin the Bamoun flood basalts (BFB).

Analytical methods

Whole rock geochemical analyses

Whole rock geochemical analyses were prepared atEidgenössische Technische Hochschule (ETH), Zu-rich, Switzerland. The major elements were deter-mined by X-Ray Fluorescence (XRF) on glass. Thepowders, melted after ignition and Li2B4O7 addition(in the ratio of 1/5) in platinized gold drying ovensat 1150 °C, were analyzed with the Philips (PW 1404)automatic sequential spectrometer at “EidgenössischeMaterialprungsanstalt” (EMPA), Dubendorf, Switzer-land. The results were then corrected for deviation,background and material effect. Loss of ignition(LOI) was obtained by calorimetry after ignition ofthe powders and of FeO. CIPW norms were calcu-lated on an anhydrous basis according to Middlem-ost (1989). Trace element concentrations were deter-mined by XRF using batteries, and 10 grams of therock powder of the samples were analyzed using the“Background” synthetic method in which the majorelement concentrations are known. The detection lim-it is 3—10 ppm. The REE were analyzed by ICP-MS(Inductively Coupled Plasma Mass Spectrometry)with ELAN5000 Perkim Elmer Sciex at the EMPA,calibrated with pure standard solutions, with a de-tection limit of 1—10 ppb. For all these analyses, theUSGS reference samples were used for calibration.

Mineralogical analyses

The mineralogical analyses of the rocks were doneat ETH, Zurich. The minerals (polished thin sec-tions) were analyzed with the CAMECA SX50 mi-croprobe equipped with 5 crystal spectrometers. Theincident ray was fixed at 12 kV and 20 nA for a totaltime of measurement up to 96 s. Eleven elements wereanalysed with variable peaks of 10 s (Si, Al, Ca, Mg,Na, K) or 24 s (Fe, Cr, Ti, Mn, Ni) while a PAP cal-culation automatically corrected the raw data of thequantitative mineral analyses; this is a modification

of the ZAF correction procedure. The Super ProbeJEOL 8600 was used to obtain a regrouping of allthe images of the deviated electrons. Mineral stand-ardization calculations were carried out as follows:clinopyroxene 6 cations and 12 charges; plagioclase5 cations and 16 charges; titaniferous oxides 2 cati-ons and 6 charges; olivine 3 cations and 8 charges.

Isotope analyses (Sr and Nd)

Isotopic analyses were carried out at the Centre deRecherche Pétrographique et Géochimique (CRPG)of Nancy, France. The samples were first dissolvedin a mixture of Hf, HNO3 and HCl and isotopic com-positions were measured using the Finnigan CHECH-MATE 262-RPQ apparatus.

Geochronological analyses (K-Ar)

Geochronological analyses (K-Ar) were done at theUniversité de Bretagne Occidentale, Brest, France.The age determinations were done on a “whole rock”fraction of the samples: groups of grains (diameter0.30 to 0.15 mm) were taken for the measurement ofmass spectrometry, the isotopic composition of ar-gon and the concentration of radiogenic argon (40Ar).The “whole rock” concentration of potassium wasdetermined by atomic absorption spectrometry andfound to be 0.77%. The ages were calculated usingthe constants recommended by Steiger and Jager(1977), and the uncertainties by standard deviationcalculations using the Mahood and Drake (1982)equations.

Petrography and mineralogy

The ankaramites are found within the Koutaba fis-sural alkaline basalts, which is a part of the Bamounflood basalts (BFB). Good outcrops are abundantaround the Mamevouo region. They form prisms (10to 30 cm in diameter as true and false colonnades ina sector of 4 by 2 km (Fig. 2). Here, the direction ofinclination of the false colonnades and their exten-sion show that the lava flows originated from jointsof strike N90. These outcrops are generally of flatrelief but may form locally volcanic cones with gen-tle sloping sides (about 20°).

The Koutaba ankaramites are dark grey in colourand their textures are porphyritic microlitic. The samemineral phases are found as phenocrysts and as mi-crolites: clinopyroxene constitutes 55 vol.% of thewhole rock, coexisting with olivine (15 vol.%), plagi-oclase (10 vol.%) and titaniferous oxides (6 vol.%).Moreover glass (14 vol.%) coexists with microliths inthe groundmass. This mineralogical composition isclearly different from that of the BFB, which con-tain more than 50 vol.% of plagioclase, less devel-oped clinopyroxene, and little or no olivine. Thechemical analyses of the main mineral phases of the

118

Fig. 2. Geological map of the Bamoun Plateau situated on the LVC (after Moundi et al., 2007), showing the Koutaba area1, ankaramites; 2, Nkogam massif; 3, Mbam massif; 4, alkaline basalts of the BFB (Bamoun flood basalt); 5, transitional basaltsof the BFB; 6, leucogabbros; 7, mylonites; 8, orthogneiss; 9, basement granitesThe encircled numbers on the map indicate analysed samples (chemical analyses, Sr and Nd isotopes, K-Ar dating)

Ôèã. 2. Ãåîëîæêà êàðòà (ïî Moundi et al., 2007) íà ïëàòîòî Áàìóí îò âóëêàíñêàòà ëèíèÿ Êàìåðóí (LVC) ñ îáîçíà÷åíèåíà ðàéîíà íà Êóòàáà1 — àíêàðàìèòè; 2 — ìàñèâ Íêîãàì; 3 — ìàñèâ Ìáàì; 4 — àëêàëíîáàçàëòîâè ïîêðîâè íà Áàìóí (BFB); 5 — ïðåõîäíîàëêàë-íè áàçàëòè íà Áàìóí; 6 — ëåâêîãàáðî; 7 — ìèëîíèòè; 8 — îðòîãíàéñè; 9 — ãðàíèòè íà öîêúëàÖèôðèòå â êðúã÷å îáîçíà÷àâàò íîìåðà íà àíàëèçèðàíèÿ îáðàçåö (õèìè÷åñêè àíàëèçè, Sr è Nd èçîòîïè, K-Ar äàòèðàíå)

ankaramites, determined using the microprobe mi-cro-analyser, are given in Table 1.

Clinopyroxene is euhedral, well developed and isfound as phenocrysts (2×4 mm) and microcrysts(0.5×4 mm). The phenocrysts are more abundant(40 vol.% of the rock). Some phenocrysts and all mi-crocrysts (15 vol.%) are characterized by a concen-tric zonation, marked by the transition from a lightbrown or reddish core to a dark brown or dark greyrim. The cores of clinopyroxene phenocrysts arericher than their rims in Al2O3 (5 against 3 wt.%),CaO (23 against 20 wt.%) and MgO (13 against 20wt.%). Reversely they are poorer in FeO (6.5 against6.7 wt.%), MnO (0.13 against 0.15 wt.%), TiO2 (1.75against 1.80 wt.%) and SiO2 (47.4 against 48.8 wt.%).That suggests a low activity of silica at the beginningof crystallization and an evolution of the rock con-trolled by oxygen fugacity. The chemical composi-tions of the clinopyroxene (Wo42—39En39—37Fs23—20) showthe coexistence of augite, diopside and hedenbergite(according to Morimoto et al., 1988; Fig. 3, Table 1).For all the clinopyroxenes, Ca + Na values are high-er (0.98 to 0.99) than for those of the flood basalts

Fig. 3. Distribution of clinopyroxenes of ankaramites of Kouta-ba (full squares) in the Wo-En-Fs (wollastonite-enstatite-ferro-silite) molecular proportion diagram after Morimoto et al. (1988).

Empty squares, alkaline flood basalts of the Bamoun Plateau;open triangles, transitional flood basalts of the Bamoum Pla-teau, plotted for comparison

Ôèã. 3. Ñúñòàâ íà ïèðîêñåíèòå íà àíêàðàìèòèòå îò Êóòàáà(çàïúëíåíèòå êâàäðàò÷åòà), íàíåñåí íà Wo-En-Fs (âîëàñòî-íèò-åíñòàòèò-ôåðîñèëèò) äèàãðàìà íà Morimoto et al. (1988).

Ïðàçíè êâàäðàò÷åòà — ïîêðîâíèòå àëêàëíè áàçàëòè; ïðàç-íèòå òðèúãúëíè÷åòà — ïîêðîâíèòå ïðåõîäíîàëêàëíè áà-çàëòè íà ïëàòîòî Áàìóí (çà ñðàâíåíèå)

119

Tab

le 1

Sele

cted

che

mic

al a

naly

ses

of the

mai

n m

iner

al p

hase

s of

the

ank

aram

ites

of K

outa

ba

Òàáë

èöà

1È

çáðà

íè õ

èìè÷

åñêè

àíà

ëèçè

íà

ãëàâ

íèò

å ì

èíåð

àëíè

ôàç

è íà

àíê

àðàì

èòèò

å îò

Êóò

àáà

120

(1996). They would roughly correspond to those ofthe beginning of crystallization (1027 °C) and oflater re-equilibrium (996 °C). They are little lowerthan those obtained for equivalent rocks of theTombel Plain (1225 °C; Nkouathio et al., 2002), MtRoumpi (1030 °C; Nkoumbou, 1990), Nganha mas-sif (1025 °C; Nono et al., 1994), the Kapsiki Pla-teaus (1035 °C; Ngounouno et al., 2001) and theMount Cameroon (1150—1200 °C; Suh et al., 2008).

Indeed, the Koutaba ankaramites show major dif-ferences with the Bamoun flood basalts, which theycrosscut. They are fresher, with olivine always present,clinopyroxenes well developed (≥ 50 vol.%), and lowplagioclase content (≤ 10 vol.%).

Geochemistry

Compared to the representative Bamoun flood ba-salts (EF transitional and P4 alkaline), the Koutabaankaramites have relatively low SiO2 contents (42.8—43.6 wt.%), but relatively high P2O5 (1.1—1.3 wt.%)and alkalis (5.1—5.5 wt.%) contents, with Na2O (3.7—3.9 wt.%) higher than 2K2O (Table 2). In the sameway, they have high contents in Ba (938—1083 ppm),Sr (1116—1217 ppm) and Nb (101—107 ppm) andlow Y/Nb ratio (0.31—0.33). They are mainly under-saturated as witnessed by the presence of high pro-portions of normative nepheline (12.9—14.9 wt.%).

High contents in CaO (11.9—12.7 wt.%) and inMgO (7.1—7.4 wt.%), and middle contents in Ni (74—98 ppm), Cr (80—00 ppm) and V (108—132 ppm)can be correlated with the presence of high propor-tions of modal clinopyroxenes (45 vol.%) and nor-mative olivine (12—15 vol.%). These concentrationsare characteristic of alkaline and sodic mafic rocksand are quite different from those of the BFB, whichare transitional (EF, Table 2) or fairly alkaline (P4)(Moundi, 2004; Moundi et al., 2007).

The ankaramites show high light REE enrichments(102—108 ppm of La, 170—179 ppm of Ce). These en-richments are well evidenced by the profiles normal-ized to chondrites (Fig. 5). These patterns also evi-dence the alkaline affinities of these rocks, which aredifferent from the transitional BFB (sample EF, Fig. 5).Their LaN values are a little higher (32.7) than thoseresulting from the partial fusion of an enriched man-tle or C3 type chondrite according to Grossman et al.(1977). Moreover, the Sm/Nd (0.12—0.13) ratios arelower than the average of that of the crust, which is0.19 (Davies et al., 1985). The values of the LaN/YbN(30—32) and CeN/YbN (19—20) ratios are similar to thoseof the alkaline basalts of the CVL (Déruelle et al.,1991; Wandji, 1985, 1995; Wandji, Tchoua, 1989;Ngounouno et al., 2001; Ngounouno et al., 2002;Ngounouno et al., 2008). But they are higher than thoseof the transitional BFB whose average values are 10and 8 respectively (Moundi, 2004; Moundi et al., 2007).

The values of 87Sr/87Sr (0.703263—0.703285) and143Nd/144Nd ratios (0.512877—0.512867) obtained fromtwo Koutaba ankaramites (Table 2, Fig. 6) could beconsidered as those of the initial ratios as ages are

Fig. 4. Distribution of clinopyroxene of ankaramites of Koutaba(full squares) compared with those of the alkaline flood basalts(empty squares) and transitional flood basalts (open triangles)of Bamoun Plateau in the diagram of Leterrier et al. (1982)

Field of clinopyroxenes of alkaline basalts from the LVC (Déruelleet al., 2000; Wandji et al., 2000) is indicated.

Ôèã. 4. Äèàãðàìà íà Leterrier et al. (1982) ñ íàíåñåíè ñúñòà-âèòå íà ïèðîêñåíèòå íà àíêàðàìèòèòå îò Êóòàáà (çàïúëíå-íèòå êâàäðàò÷åòà), ñðàâíåíè ñ òåçè íà ïîêðîâíèòå àëêàë-íè áàçàëòè (ïðàçíèòå êâàäðàò÷åòà) è ïîêðîâíèòå ïðå-õîäíîàëêàëíè áàçàëòè (ïðàçíèòå òðèúãúëíè÷åòà) íà ïëà-òîòî Áàìóí

Ïîëåòî íà êëèíîïèðîêñåíèòå íà àëêàëíèòå áàçàëòè îò LVCå ïî Déruelle et al. (2000) è Wandji et al. (2000)

(0.7 to 0.8), whereas Si remains low (1.75 to 1.81against 1.82 to 1.97 for the floods). According toLeterrier et al. (1982), these suggest the alkaline na-ture of the ankaramites (Fig. 4). The presence ofmolecules of CaAl2SiO6 (0.034 to 0.078) and ofCaTiAl2O6 (0.04 to 0.08) with AlIV (0.19 to 0.29) clearlyhigher than 2Ti (0.10 to 0.11) suggest that coupledsubstitutions accompanied the crystallization of thesepyroxenes.

Olivine is found as regularly cracked microcrysts(0.5—2 mm) and as inclusions in the clinopyroxenesand plagioclases. It has various contents in MgO(33—41 wt.%) and CaO (0.3—0.6 wt.%). These arehyalosiderites with Fo contents varying from 77 atthe core to 66 at the rim of the microcrysts. TheirKd (0.33) well correspond to those of the primitivemantle rocks (0.3 < Kd < 0.4) as defined by Roederand Emslie (1970).

Plagioclase exists in the form of microcrysts andmicrolites (up to 0.3 mm). They are mainly labra-dorites (An59—62Ab36—39Or2—5) with some andesines(An47—48Ab41—44Or9—11). Variations in the chemical com-position of the plagioclase microcrysts are observedon the well developed sections, indicating an evolu-tion and a normal zoning, less calcic from the core(An60) to the rim (An55).

Titanomagnetite is found as microcrysts or micro-grains included in the other minerals, representingearly formed phases.

Ankaramites are emplaced between 996 and1027 °C. These temperatures were obtained fromthe clinopyroxene-olivine geothermometer of Loucks

121

Tab

le 2

Che

mic

al c

ompo

sitio

n of

the

ank

aram

ites

of K

outa

ba (

I15

to I

24), B

amou

n floo

d ba

salts

and

Mba

m M

assif

gabb

ro (

G1)

for

com

pariso

n

Òàáë

èöà

2Õèì

è÷åñ

êè ñ

úñò

àâ í

à àí

êàðà

ìèò

èòå

îò Ê

óòàá

à (I

15 to

I24)

, áà

çàëò

îâèò

å ïî

êðîâ

è íà

Áàì

óí è

çà

ñðàâ

íåíè

å ãà

áðî

îò ì

àñèâ

à Ì

áàì

(G1)

EF,

tra

nsiti

onal

fro

m M

bam

Mas

sif;

B14

, tr

ansi

tiona

l fr

om F

oum

ban

Pla

teau

; P

4, a

lkal

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122

Fig. 5. REE spectra for representative samples ankaramites of Koutaba (I17) and of Bamoun flood basalts (EF transitional andP4 alkaline) normalised to chondrites according to Sun and McDonough (1989)

Ôèã. 5. REE õîíäðèò-íîðìèðàíè (õîíäðèòè ïî Sun, McDonough, 1989) êðèâè íà ïðåäñòàâèòåëíè àíàëèçè íà àíêàðà-ìèòè îò Êóòàáà (I17) è íà ïîêðîâíèòå áàçàëòè íà Áàìóí (EF-ïðåõîäíîàëêàëíè è Ð4-àëêàëíè)

Fig. 6. Nd and Sr isotopic diagramAnalyses: ankaramite (I17); alkaline flood basalt of the Bamoun Plateau (P4); transitional flood basalts of the Bamoun Plateau(EF and B14); gabbro (G1)Field of other lavas from LVC (Mount Cameroun and Mount Manengouba, Halliday et al., 1990), fields of Fangataufa, FrenchPolynesia (Bardintzeff et al., 1994); of tholeiite and of alkaline lavas of Hoggar (Taharaq, Algeria) (Aït-Hamou et al., 2000), ofEthiopia (Pik et al., 1999), of Kerguelen (Bardintzeff et al., 1994; Gautier et al., 1990); MORB and mantle compositions DMM(depleted), EM1 (enriched), HIMU, following Zindler and Hart (1986)

Ôèã. 6. Nd è Sr èçîòîïíà äèàãðàìàÀíàëèçè: I17 — àíêàðàìèò; Ð4 — ïîêðîâåí àëêàëåí áàçàëò; EF è B14 — ïîêðîâíè ïðåõîäíîàëêàëíè áàçàëòè íà ïëàòîòîÁàìóí; G1 — ãàáðîÏîëåòî íà äðóãèòå ëàâè íà LVC (Ìàóíò Êàìåðóí è Ìàóíò Ìàíåíãóáà) å ïî Halliday et al. (1990), íà Ôàíãàòîôà, ÔðåíñêàÏîëèíåçèÿ — ïî Bardintzeff et al. (1994); íà òîëåèòèòå è àëêàëíèòå ëàâè íà Õîãàð (Òàõàðàê, Àëæèð) — ïî Aït-Hamou et al.(2000), íà Åòèîïèÿ — ïî Pik et al. (1999) è íà î-â Êàðãåëåí (Èíäèéñêè îêåàí) — ïî Bardintzeff et al. (1994) è Gautier et al.(1990); MORB è ìàíòèéíèòå ñúñòàâè DMM (îáåäíåí), EMI (îáîãàòåí), HIMU ñà ïî Zindler, Hart (1986)

123

rather young. They are similar to those obtained onalkaline basalts of the CVL (including alkaline BFB),which group around 0.703325 and 0.51289 respec-tively (Halliday et al., 1988; Nono et al., 1994; Rank-enburg et al., 2005). They are typical of the HIMUreservoir (high µ, where µ is 238U/204Pb). They are clear-ly different from those of the transitional BFB (sam-ples EF and B14) with average values 0.7045 and0.5126 respectively that evidence the role of EM (En-riched Mantle) source.

The ankaramites of Koutaba have been emplacedduring Quaternary (40K/40Ar datation of 0.82 ± 0.05Ma, Moundi et al., 2007; Table 2). The BFB, whichare crosscut by these ankaramites are clearly older,dated of 51—46 Ma (Eocene) (Moundi, 2004; Moun-di et al., 2007).

Discussion and conclusion

The Koutaba ankaramites, which crosscut basalts ofthe Bamoun Plateau in its south-eastern part, aremafic (42.8 ≤ SiO2 ≤ 43.6 wt.%) and under-saturated(12.9 ≤ normative nepheline ≤ 14.9 wt.%). The crys-tallization of all the mineral phases has been proba-bly influenced by a low silica activity and low oxy-gen fugacity. The temperature of the beginning ofthe crystallization is 1027 °C, with exsolution of ti-taniferous oxides at 966 °C.

Ce/Yb (71—74) and Sm/Yb (5.38—5.70) ratios wit-ness that the ankaramites originate from a rather deep

source (≥ 100 km) according to Nakamura (1974). Thevalues of the Rb/Nb (0.42—0.50), Zr/Nb (3.02—3.20),Ba/Nb (9.22—10.65) and La/Nb (0.96—1.09) ratios aswell as their low variation imply a single mantle sourceof the HIMU type for these rocks. According to thehigh Zr/Y (8.59—10.06), LaN/YbN (30—32) and CeN/YbN(19—20) values, the parental magmas of these ankara-mites would have resulted from a relatively low degreepartial melting from an asthenospheric mantle.

The low value of the 87Sr/86Sr ratio (0.703263—0.703285) and high initial value of 143Nd/144Nd ratio(0.512867—0.512877) are similar to those of alkalinebasalts of the continental and oceanic domain ofthe CVL. They suggest only little or no crustal influ-ence in the genesis of the magmas at the origin ofthe Koutaba ankaramites. According to their Qua-ternary age (0.82 Ma, 40K/40Ar) the Koutaba ankara-mites represent the evidence of a recent magmaticepisode from an asthenospheric mantle source ofHIMU type. They strongly differ from the Eocene(51—46 Ma) transitional and alkaline flood basaltsof the Bamoun Plateau.

Acknowledgements. The authors sincerely ac-knowledge E. Reusser (IMP, ETH, Zurich), L. Reis-berg (CRPG, Nancy) and H. Bellon (Université deBretagne Occidentale, Brest) for assisting in theprocurement of the microprobe, isotope and geo-chronological analyses respectively. B. Bonin isthanked for useful remarks. L. Daumas is thankedfor figures.

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(Ïîñòúïèëà íà 01.12.2009 ã., ïðèåòà çà ïå÷àò íà 23.03.2010 ã.)Îòãîâîðåí ðåäàêòîð Éîöî ßíåâ