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Vol. 8(2), pp. 13-27, July, 2016 DOI: 10.5897/JGMR2015.0230 Article Number: 9FB684759542 Journal of Geology and Mining ISSN 2006-9766 Copyright © 2016 Research Author(s) retain the copyright of this article http://www.academicjournals.org/JGMR Full Length Research Paper Highlighting the root of a paleoproterozoic oceanic arc in Liptako, Niger, West Africa Soumaila Amadou1, Garba Zibo1,3, Moussa Issaka Abdoulkader2*, Nouhou Halitt1 and Sebag David4,5,6 1Department of Geology, University of Abdou Moumouni, Niamey, Niger. 2Department of Geological and Environmental Sciences, University of Zinder, Niger. 3University of Dosso, Niger. 4University of Rouen, CNRS, Mont-Saint-Aignan, France. 5HydroSciences of Montpellier, IRD, France. 6Université de Ngaoundéré, Cameroun. Received 5 July, 2015; Accepted 25 May, 2016 In the southwest branch of the Diagorou-Darbani greenstone belt, mafic to ultramafic plutonic rocks outcrop in Pogwa area. Field relations show that these rocks are clearly intrusive in flattened pillowed- metabasaltes. They are made up of gabbros and pyroxenites with ultramafic rocks cropping out as panels or scattered lenses. Petrographic characters are suggestive of metamorphic grade varying from greenschist to amphibolite facies. The geochemical characters revealed that most of the gabbroic rocks were generated by fractional crystallization; however, variations in Ni and Cr contents in some samples are attributed to variable degrees of partial melting. The rare earth elements REE and spider diagrams distinguished two kinds of patterns: 1) most rocks with light rare earth elements LREE enriched patterns, heavy REE HREE depleted patterns, large ion lithophile elements LILE enrichment and negative anomalies in high field strength elements HFSE (Nb, Ta, Zr and Hf). These features are suggestive of an oceanic arc emplacement, corroborated by Zr-Nb-Y and Cr-Y discrimination diagrams. 2) The other (Ultramafic rocks and metatonalite) with tholeiitic character with REE patterns depleted in LREE and enriched in HREE. In fact, the tonalite rock may represent a highly evolved term of an ultramafic-mafic suite of oceanic crust. The mafic and ultramafic rocks of Pogwa constituted the out cropping root of a paleoproterozoic oceanic-arc in the birimian formations in Liptako, Niger. Key words: Liptako, Pogwa, palaeaoproterozoic, oceanic arc, root. INTRODUCTION Precambrian rocks of the West African craton outcrop in western parts of these shields, while the paleoproterozoic two shields: the Reguibat shield in the northern part and ones (birimians rocks) constitute the basement of the the Man or Baoulé-Mossi shield in the southern part eastern zones. The Birimian formations are characterized (Figure 1A). The Archean rocks are exposed in the by an alternating granitoids and linear or curved have *Corresponding author. E-mail: [email protected]. Tel: (00227)99322286. Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License 14 J. Geol. Min. Res. 2°E a 2 5°N b 4 D arb ani B b 3 TB Tilla béry 2 c 15° N Liptako Téra 1 4°N d 1 Diago rou Da rgo l a 300Km Niam ey 5°N A A (Fig1.C) 10° W 5°W 0 ° 5° E e f 13° N 5 0K m 1°E 13 1°E 1°E30’ 12 C Darbani 11 10 14°N30’ Tillabéry 9 8 0°E30’ Ouama 7 14°N Téra 14°N 6 Diagorou Dargol 1°E30’ 5 Bandio 4 3 13°E30’ 2 0 5 10Km 1 0°E30’ 1°E FigureFigure 1.1 A): (GeographicalA) Geogica lmapma ofp theof tWesthe W Africanest A fCratonrican andCra PanAfricanton and P reworkedanafric azones.n rew a)o rManked shield,zone sb). areguibat: Man shieldshield, ;TB:b: rTouaregéguiba tshield,shield ;1:T ArcheanB: Tou arocks;reg s h2:ie lBirimiand; 1: A rrocks;chean 3:R oPrecambriancks; 2: B irbasins;imian R4:o reworkedcks; 3: P basement;recambri a5:n reworkedBasins; 4zones;: Rew 6:or kPhanerozoiced basem ebasins.nt; 5: RB)e wGeographicalorked Zone maps; 6 :ofP hLiptakoanero z(Dupusoic ba setin al.,s. 1990). 1: Greenstone salts; 2: granitold(B): Ge plutons;ogical 3:m sedimentaryap of Lip tcover;ako ( 4:D utectonicpuis econtact;t al., 1 a:9 9Goroul0). 1 :greenstoneGreensto belt;ne bb:e Teralts; 2pluton;: gra nc:it oDiagorouïd plut-oDarbanins; 3: greenstoneSediemen belt;tary B:C oOccver ;Westren4: Tec tbranch;onic C B.on tOrac tEastern; a: G oBranch;rouol gd:re Dargolensto npluton;e bel t;e:b Sirba: Té ragreenstonePluton; cbelt;: D if:a gTorodiorou- Pluton.Darba nC)i Geologicalgreenston mape b eoflt ;theB .DiagorouOcc: W-eDarbanstern i bgreenstoneranch; B .belt.Or :1:E Metabaslats;astern Bra n2:c Talcschistsh; d: Dar gandol PTalcluto-chloritoschist,n; e: Sirba 3: Metasediments and volcano-sediments; 4: Amphibphites with locally associated micaschists; 5: Granitoids; 6: Gabbros; 7:gr eassemblagesenstone be oflt; basicf: To randodi ultrabasicPluton. rocks; 8: Quartz Metadiorites; 9: Calc-alcalines volcanic rocks; 10: Late granits; 11:(C) Syenithic: Geolo grock;ical 12:ma Uncertainp of the shearzone;Diagorou 13:-D aShearrban zone.i gre enstone belt 1: Metabasalts; 2: Talcschists and Talc-chloritoschists; 3: Metasediments and volcano-sediments; 4: Amphibplites with locally associated micaschists; 5: Granitoïds; 6: Gabbros; 7: assemblages of basic and ultrabasic rocks; 8: Quartz Metadiorites; 9: calc-alcalines volcanic rocks; 10: Late Granits; witnessed11 : aS yperiodenitic roofck ; major12: un ccrustalertain s haccretionear zone ; 1from3: S hearand/zoneor. volcano-sedimentary rocks. They are generally greenstone belts. In West Africa, they are considered to metamorphosed in low greenschist locally, amphibolit 2.1 Ga (Abouchami et al., 1990; Boher et al., 1992) to 2.3 facies conditions, especially in greenstone belts - Ga (Lahondère et al., 2002; Gasquet et al., 2003; granitoid contacts (Dupuis et al., 1991; Pons et al., 1995; Soumaila et al., 2008). The greenstone belts are made Soumaila et al., 2006). up of mafic rocks (volcanic and plutonic), sedimentary The mafic rocks are of tholeiitic composition, Soumaila et al. 15 suggesting that they are emplaced in geodynamic context volcanic rocks, cross cut the latter. The sector of Pogwa similar to that of oceanic plateau in Ivory Cost is the southwestern most part of the Diagorou-Darbani (Abouchami et al., 1990), or to that of oceanic arc context greenstone belt, where mafic to ultramafic plutonites are in Niger (Ama-Salah et al., 1996; Pouclet et al., 1996). intrusive in metabasaltes and talcshistes. More complex geodynamic environments, with interaction between mantle plumes and subduction zones with associated back-arc basins have been suggested MATERIALS AND METHODS (Soumaila et al., 2004). The compilation of chemical data from greenstone belts Aerial photographs have been selected to cover the sector of Pogwa in order to draw a detailed geological map. Tree Cross- of the whole West African craton suggested that large sections (A, B, and C) are carried out to establish chronologic volcanic basic rocks were produced by a mantle plume in succession of formations. Two field missions, with sampling oceanic domain. This volcanic event was followed by a surveys, have been undertaken. About fifty samples were collected, subsidence related to plateau basalts weight. Partial and seventeen samples were selected for thin sections to perform melting took place at the bottom of the mafic pile, with petrographic studies. Thin sections were prepared in the laboratory at the Department of Geology, University of Abdou Moumouni. production and rising of granitoid melts (Lompo, 2009). Geochemical analyses on the total of the rocks were performed by Locally, in Niger and Burkina Faso, assemblages of the Rocks and Material Analysis Service of Petrographic and ultramafic to mafic plutonites crop out in the greenstone Geochemical Research Center Nancy, France. Major elements (Si, belts. In Burkina Faso, these assemblages have been Al, Ti, Mg, Fe, Mn, Ca, Na, K, P) were measured by atomic suggested to be relics of magmatic chamber at the base emission spectrometer (inductively coupled plasma Atomic of oceanic arc (Beziat et al., 2000). In Niger, such Emission Spectrometer-ICPAES). Samples were crushed and brought to fusion with Li2BO2 and dissolved in 2% HNO3.The ultramafic to mafic plutonic rocks out crop in the Pogwa sample solution is introduced into the emission spectrometer sector in Diagorou-Darbani greenstone belt (Machens, (Thermo Fischer ICap 6500). It becomes atomized with argon gas 1973), but they are poorly studied. This paper aims at into hot plasma. The outer-shell electrons of the elements in the studying the petrographic and geochemical features of sample are excited, emitting light wavelengths which is an energy these rocks in order to highlight their character and characteristic of its elements. A mirror reflects the light through the geodynamic emplacement. entrance of the spectrometer which separates particular element wavelengths and directing each of them to a specific photomultiplier tube detector. The more intense this light is, the more concentrated the element. A computer converts the electronic signal from the Regional geology photomultiplier tubes into concentrations. The limits of detection are of 0 2% for SiO2, Al2O3 0.1%, Fe2O3, MgO. The mass spectrometer In Niger, birimian rocks crop out in the Liptako area, the (ICP-MS) Agilent 7700X was used to measure trace elements concentrations. The sample solution is introduced into ionizing northeastern most part of Man shield. The geological where it is atomized with argon gas into hot plasma. Four feature of this area is greenstone belts alternating with electrically charged rods (the quadruple mass filter) separate granitoid plutons (Figure 1A). From northwest to appropriate ions according to the ratio m/z (m = mass of element southeast greenstone belts are of Gorouol, Diagourou- and z = charge). These ions arrive at the detector for counting. Darbani and Sirba. The granitoid plutons are of Tera, Electrical signals received at the detector are amplified, processed Dargol and Torodi (Figure 1B).