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Some Geochemical Considerations in the Petrogenesis of Madagali Granitoids, Northeastern Nigeria

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Some Geochemical Considerations in the Petrogenesis of Madagali Granitoids, Northeastern Nigeria GLOBAL JOURNAL OF GEOLOGICAL SCIENCES VOL, 9, NO.2, 2011 253-263 253 COPYRIGHT© BACHUDO SCIENCE CO. LTD PRINTED IN NIGERIA. ISSN 1596-6798 www.globaljournalseries.com , Email: [email protected] SOME GEOCHEMICAL CONSIDERATIONS IN THE PETROGENESIS OF MADAGALI GRANITOIDS, NORTHEASTERN NIGERIA SAIDU BABA (Received 30 November 2010; Revision Accepted 10 February 2011) ABSTRACT Major and trace element geochemical data on granitoids of Madagali area of northeastern Nigeria show a series that ranges from K-calcalkaline, through granodiorite-monzogranite to granite. They are characterized as mildly peraluminous to metaluminous and I-type based on alumina saturation index (ASI). Trace element contents preclude the possibility of entire mantle origin and indicate significant crustal involvement in the evolution of the granitoids. The enrichment in LREE ( La, Ce and, Nd), depletion in HREE (Ho, Er, Yb and Lu) and a pronounced negative Eu anomaly with (La/Yb) N suggest that the granitoids must have evolved by partial fractionation or partial melting of the pre-existing crust in which plagioclase feldspar was retained. Furthermore, the enrichments in some large-ion- lithophile elements (LILE) such as Rb, Ba, Th and K and depletion in some high field strength (HFS) elements such as Nb, P, and Ti as well as some transitional elements Cr and Ni are supportive of crustal involvement in their genesis. KEY WORDS : Madagali, Nigeria, Granitoids, Geochemical considerations, Petrogenesis. INTRODUCTION (1986) concluded that the Pan-African granitoids in The study area is the northeastern tip of the central Nigeria were derived by partial melting of the Nigerian Basement complex, separated from the country rocks (schists and gneisses), while Ezepue and southern part (Oban-Obudu Massif) by the Upper Benue Odigi (1993) showed that the various components of Trough. It forms the western terminal of the Western the granites are not co-genetic and that they were Cameroon Domain (WCD) (Ferre et al. 1996, 2002; derived from upper mantle and crustal materials. Ekwueme and Kroner, 1997, 1998); (Fig.1). The WCD is Ukaegbu and Beka (2007) deduced that the enderbite- part of the remobilized Pan-African terrain formed in the charnockite association in the Pan-African Obudu Neoproterozoic (approx. 600Ma) by continental collision plateau are peraluminous (S-type) formed at lower crust. between the converging West African Craton, Congo Ferre et al (1998), using Sr initial ratios suggested that Craton and East Saharan block. It consists of an the source of the granitic magmas of the Pan-African amalgamation of Precambrian terrains (Caby, 1989; rocks of eastern Nigeria is the lower crust characterized Castaing, et al. 1994; Black and Liegeois, 1993; Ferre et as ferro-potassic with trans-alkaline affinity. Dada (2008) al. 1996, 2002) that show long and complex crustal in his broad multi disciplinary synthesis of Nigeria- evolution with strong imprints of Neoproterozoic events Boborema province opined for a mixing model between (Toteu et al. 2004), which include: Neoproterozoic juvenile Pan-African material and the Archean basement medium-high grade schists and gneisses (Ngako, 1986; with a prominent involvement of the later in the genesis Ngel, 1986; Toteu, 1990) ; Pan-African pre-,syn-, and of the Pan-African granitoids. late tectonic calc-alkaline granitoids (Toteu, et al. 1987, In their study of the Pan-African granitoids of the 2001); and Post-tectonic alkaline granitoids (felsic and extreme northeastern tip of the Nigeria, north of the mafic dykes,e.g. the Burashika Group). Benue Trough, Islam and Baba (1992) described the Madagali area is dominantly underlain by the rocks as alkaline and of mixed origin. Baba et al. (1996) Neoproterozoic medium to high-grade gneisses intruded described the Liga Hills granites as being products of by large volumes of Pan-African granitoids and many of anatexis of crustal materials. On the petrogenesis of the these Pan-African granitoids in Nigeria have been Pan-African granitoids from Gwoza area Baba, et al variously discussed in terms of nomenclature, (2006) reasoned that they were calc-alkaline and I-type. classification and origin (Baba et al, 1996). For example, Following the suggestion in Baba et al (2006), this paper Umeji (1991) described them as calc-alkaline presents some trace and rare earth elements data on monzogranites with charnockitic affinity in Jato-Aka area the granitoids and discusses their significance in the of southeastern Nigeria and concluded that they were petrogenesis of the rocks in addition to providing an formed from mantle-derived magmas. Onyeagocha updated geological map of the area. Saidu Baba , Department of Geology, University of Maiduguri, P.M.B 1069, Maiduguri, Borno State 254 I.M. AKAEGBOBI, G.O. OGUNGBESAN AND J.E. OGALA Fig. 1: Sketch diagram showing the location of the Study Area within the Pan African Belt 1-: Post Pan-African Cover; 2- Pan-African Belt; 3- Pre-Mesozoic Series; 4- Archean to Paleoproterozoic Crateos; 5 - Craton Limits; 6- Faults; 7- State boundaries. WCD - Western Cameroun Domain (modified after Toteu et al. 2004) FIELD RELATIONS AND PETROGRAPHY sphene, zircon and apatite make up the accessories. Neoproterozoic migmatitic gneisses, minor The average modal compositions give quartz (32%), schists (unmappable) and quartzites intruded by large plagioclase (27%) K-feldspar (26%) and biotite (13%). volumes of Pan-African granitoids (Fig.2) underlie the Microcline occurs in anhedral to subhedral form study area. The migmatitic gneisses and their characterized by crosshatch twining but often-developed associated dismembered amphibolites that form the perthitic texture, which obliterates the twining. Quartz basement sensu stricto (Dada et al. 1995) are cross cut occurs in two generations, the first generation made up by felsic veins (Baba et al, 2006). N-S to NNE-SSW of large anhedral, often cracked grains with sutured foliation trends with near vertical dips are predominant. margins and undulose extinction while the second The schists and the quartzites are relatively minor and generation grains are smaller in size, subhedral to exposed only at foothills and stream channels. euhedral and often polygonal with triple junction. In The granitoids form large bodies in form of batholiths some sections, the second-generation quartz fill-up the concordant with the foliation trends in the gneisses and cracks in the first generation grains. Plagioclase crystals sometimes oval-shaped plutons making up the main are subhedral prismatic, generally cloudy and in some Mandara range. The dominant hill-forming granitoids, sections, the lamellae are deformed and sometimes which are the rocks of interest, are mineralogically contain sericites. The undeformed and unaltered uniform but variable in texture ranging from fine grained crystals give extinction angles of 7 0-10 0 (oligoclase). through medium to coarse and even porphyritic. Biotite flakes are sometimes bent and show slight Contacts between them are often sharp and sometimes alteration to iron oxides along their cleavage planes, lobate especially amongst the late-phase (post-tectonic) often containing pleochroic haloes resulting from the members, suggesting contemporaneous emplacement. radioactive nature of the zircon inclusions. Iron oxides They also carry xenoliths of gneisses and schists to as opaques are the dominant accessory minerals suggest possible anatexis (Baba et al. 2006). occurring as anhedral to subhedral grains intimately Minerals identified in thin-section include K-feldspar associated with biotite and sphene. Sphene occurs in its (mainly microcline), quartz, plagioclase and biotite with characteristic rhombic form, zircons appear dominantly hornblende in some sections. Iron oxides (as opaque), as inclusions in biotite while apatite occurs as discrete CARB ONATE MICROFACIES AND MAJOR ELEMENT CONTENT OF THE PALEOCENE – EOCENE SECTIONS 255 grains as well as inclusions in sphene. Using the modal Madagali granitoids as quartz monzonite-granite. Q-A-P Streckeisen diagram, Baba et al (2006) showed LEGEND N O 11 15’ # P ulka Linea m ents K ira wa # Majo r _R o ad Min o r _R o ad A sh ig as hia G w oza # River # RHYOLITES N gos he # SYENITE MYLONITE Lim ank ara # CATACLASITE PEGMATITE Ma dag ali FINE GRAINED GRANITE madagali# CAMEROON MEDIUM GRAINED GRANITE G ulak COARSE GRAINED GRANITE # GRANODIORITE 0 10 Km G um c hi # O 10 40’ N O 13 15’ E 14 O E GEOCHEMISTRY (Fig.3) when Q l-Anor (CIPW-Norm values) diagram of Analytical Methods: 15 representative samples of the Streickeisen and Le Maitre (1979) was used in this granitoids were separately crushed and pulverized to study, as given in Bowden et al (1984), In the SiO 2 vs. about 106 microns. Each sample was then K2O diagram of Rickwood (1989), all the samples except homogenized and later quartered and sent for analysis one (sample 7) plot in the field of high K calc-alkaline in the Activation Laboratories, Ontario, Canada where (HKCA) (Fig.4). major, trace and rare earth elements were determined The average content of Al 2O3 (13.86%) in the rocks is by Inductively Coupled Plasma (ICP) Spectrometry. considered moderate and in all the samples however, Analytical precision in general is better than ± 2% for the alumina values are only slightly greater than total major and ± 5% in trace elements. alkalis plus lime (Al 2O3 > Na 2O +K 2O + CaO). The molecular Al 2O3/ (CaO + Na 2O + K2O) or Alumina Major Elements Saturation Index (ASI) A/CNK ratio in almost all the Table 1 shows
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