Accepted Manuscript A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints Asobo N.E. Asaah , Tetsuya Yokoyama , Festus T. Aka , Tomohiro Usui , Mengnjo J. Wirmvem , Boris Chako Tchamabe , Takeshi Ohba , Gregory Tanyileke , J.V. Hell PII: S1674-9871(14)00087-5 DOI: 10.1016/j.gsf.2014.04.012 Reference: GSF 305 To appear in: Geoscience Frontiers Received Date: 24 September 2013 Revised Date: 8 April 2014 Accepted Date: 17 April 2014 Please cite this article as: Asaah, A.N.E., Yokoyama, T., Aka, F.T., Usui, T., Wirmvem, M.J., Tchamabe, B.C., Ohba, T., Tanyileke, G., Hell, J.V., A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints, Geoscience Frontiers (2014), doi: 10.1016/ j.gsf.2014.04.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT MANUSCRIPT ACCEPTED ACCEPTED MANUSCRIPT 1 A comparative review of petrogenetic processes 2 beneath the Cameroon Volcanic Line: Geochemical 3 constraints 4 5 6 Asobo N.E. Asaah a, b, *, Tetsuya Yokoyama a, Festus T. Aka c, Tomohiro Usui a, Mengnjo J. 7 Wirmvem d, Boris Chako Tchamabe d, Takeshi Ohba d, Gregory Tanyileke c, J.V. Hell c 8 9 a Department of Earth & Planetary Sciences, Tokyo Institute of Technology, 2-12-1, 10 Ookayama, Meguro-ku, Tokyo, 152-8551, Japan 11 b Department of Mines, Ministry of Mines, Industry and technological Development, Yaoundé, 12 Cameroon 13 c Institute of Mining & Geological Research, P.O. Box 4110, Yaoundé, Cameroon 14 d Department of Chemistry, School of Science, Tokai University, Hiratsuka, 259-1211, Japan 15 MANUSCRIPT 16 *Corresponding author: Tel : +81-3-5734-3539; Fax : +81-3-5734-3538; E-mail address : 17 [email protected] 18 19 20 21 22 23 24 ACCEPTED 25 26 27 28 ACCEPTED MANUSCRIPT 29 Abstract 30 The origin and petrogenesis of the Cameroon Volcanic line (CVL), composed of 31 volcanoes that form on both the ocean floor and continental crust, are difficult to understand 32 because of the diversity, heterogeneity, and nature of available data. Major and trace elements, 33 and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been 34 compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, 35 basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a 36 compositional gap between SiO 2 58–64 wt.%. Similarities in geochemical characteristics, 37 modeled results for two component mixing, and the existence of mantle xenoliths in most 38 mafic rocks argue against significant crustal contamination. Major and trace element 39 evidences indicate that the melting of mantle rocks to generate CVL magma occurred 40 dominantly in the garnet lherzolite stability field. Melting models indicate small degree 41 (<3%) partial melting of mantle bearing (6–10%)MANUSCRIPT garnet for Mt. Etinde, Ngaoundere Plateau 42 and Biu Plateau, and <5% of garnet for the oceanic sector of the CVL, Mt. Cameroon, Mt. 43 Bambouto, Mt. Manengouba and Oku Volcanic Group. The Sr-Nd-Pb isotope systematics 44 suggest that mixing in various proportions of Depleted MORB Mantle (DMM) with enriched 45 mantle 1 and 2 (EM1 and EM2) could account for the complex isotopic characteristics of 46 CVL lavas. Low Mg number (Mg # = 100×MgO/(MgO+FeO)) and Ni, Cr and Co contents of 47 CVL mafic lavas reveal their crystallization from fractionated melts. The absence of 48 systematic variation in Nb/Ta and Zr/Hf ratios, and Sr-Nd isotope compositions between the 49 mafic and felsicACCEPTED lavas indicates progressive evolution of magmas by fractional crystallization. 50 Trace element ratios and their plots corroborate mantle heterogeneity and reveal distinct 51 geochemical signatures for individual CVL volcanoes. 52 ACCEPTED MANUSCRIPT 53 Key words: Petrogenesis; Cameroon Volcanic Line; Depleted MORB mantle; Enriched 54 mantle; Mantle heterogeneity 55 56 57 58 59 60 61 62 63 64 65 66 MANUSCRIPT 67 68 69 70 71 72 73 ACCEPTED 74 75 76 ACCEPTED MANUSCRIPT 77 1. Introduction 78 In recent years, volcanic rocks of continental rifts have received considerable attention; 79 however, the diversity in their compositions means they are not explainable by a single 80 process (Jung and Masberg, 1998). Although continental and oceanic intraplate magmas 81 show considerable variation in trace element abundances and ratios, they are typically more 82 enriched in incompatible trace elements than basalts erupted at subduction zones and mid- 83 ocean ridges (MORs). As a result, trace element modelling for petrogenesis of intraplate 84 magmas invariably requires their derivation from a mantle source that is chemically distinct 85 from the upper-mantle magma source that produces the MORBs (Weaver, 1991). Continental 86 intraplate rift magmatism is geochemically similar to ocean island basalts (OIB) (Weaver, 87 1991) and is thought to be generated from a convectively upwelling asthenosphere during 88 continental extension associated with the formation of rift systems (Zou et al., 2000). The 89 upper mantle beneath intraplate rift-related CenozoMANUSCRIPTic volcanoes is thought to have 90 experienced metasomatism in the past (Wilson and Downes, 1991). These processes result in 91 the production of hydrous minerals (amphibole and phlogopite), a range of accessory mineral 92 phases, and enrichment in incompatible elements (Wilson and Downes, 1991; Wedepohl et 93 al., 1994). 94 Unique to other intraplate rift related volcanoes, the Cameroon Volcanic Line (CVL) in 95 West Africa, consists of a chain of Cenozoic volcanoes developed on both the Atlantic 96 oceanic floor and the continental crust of the African plate (Fig. 1). There is no evidence of 97 systematic age migrationACCEPTED from one volcano to another (Lee et al., 1994; Marzoli et al., 2000; 98 Aka et al., 2004) as observed in a typical hot spot settings such as the Hawaii-emperor chain. 99 These complex characteristics (clear volcano alignment but lack of a consistent time-space 100 migration) of the CVL have generated some debate concerning its origin. Geological (Grunau 101 et al., 1975; Gorini and Bryan, 1976; Freeth, 1978; Sibuet and Mascle, 1978; Fitton, 1980; ACCEPTED MANUSCRIPT 102 Morgan, 1983; Burke, 2001), geochemical (Fitton and Dunlop, 1985; Deruelle et al., 1991; 103 Lee et al., 1994; Njonfang et al., 2011), structural (Moreau et al., 1987; King and Anderson, 104 1995), geophysical (Fairhead, 1988; Fairhead and Binks, 1991; Meyers et al., 1998; King and 105 Ritsema, 2000; Reusch et al., 2010; Milelli et al., 2012) and geochronological (Dunlop and 106 Fitton, 1979; Aka et al., 2004) data have been used to develop complementary models to 107 explain the origin of the CVL. Overall, mafic rocks of the CVL show chemical features 108 consistent with plume activity outlined by their OIB major and trace elements and radiogenic 109 isotope characteristics (Mbassa et al., 2012). 110 Geochemical data and its interpretation are challenging for the CVL system, because of 111 large variation. Lavas from the CVL range from mafic end-members e.g., nephelinite, 112 basanite and alkali basalt to felsic end-members e.g., phonolites, trachytes and rhyolites. 113 Available data show that some volcanoes (Mt. Etindé, Mt. Cameroon and Bui Plateau, Fig. 1) 114 are composed only of mafic lavas while others (MounMANUSCRIPTts Manengouba, Bambouto, the Oku 115 Volcanic Group or OVG and the Ngaounderé Plateau) are bimodal, with the presence of a 116 Daly gap. The degree of differentiation of lavas varies from one volcano to another with no 117 well-defined trend. There is heterogeneity even at a local scale within each central volcano 118 (Nkouandou and Temdjim, 2011) which is thought to be due to a different degree and depth 119 of partial melting (Marzoli et al., 2000; et al., 2007; Kamgang et al., 2013). Cameroon 120 Volcanic line magmatism is characterized by melting in the garnet lherzolite stability fields 121 (Marzoli et al., 2000; Yokoyama et al., 2007; Kamgang et al., 2013), although melting in the 122 spinel lherzoliteACCEPTED stability fields have been reported in the Ngaoundere Plateau region (e.g., 123 Lee et al., 1996; Nkouandou and Temdjim, 2011) where the lithosphere is significantly thin 124 (Poudjom Djomani et al., 1992, 1997). In addition, a mixing of both garnet and spinel melting 125 fields has been reported in Mt. Cameroon (Tsafack et al., 2009). Both mafic and felsic rocks ACCEPTED MANUSCRIPT 126 show chemical features consistent with plume activity outlined by their OIB characters, and 127 their isotopic ratios (Mbassa et al., 2012). 128 Information based on trace element abundances and isotope systematics of CVL lavas 129 shows that far from being a simple alignment of individual volcanoes, the CVL is divided 130 into three distinct sectors – the oceanic, ocean-continent boundary and the continental sectors 131 (Halliday et al., 1990; Lee et al., 1994, 1996). These data can be complemented by 132 compatible trace elements with similar mineral/melt partition coefficients, as these are less 133 sensitive to variations in partial melting of the source mantle and mineral crystallization in 134 the magma plumbing system. The geochemical complexity and tectonic setting of the CVL 135 makes it an ideal environment to investigate characteristics of source magmas in an intraplate 136 setting, and their petrogenetic evolution. In addition, the parallelism (Fig. 1) between the 137 CVL alignments and the central African shear zone strike (Ngako et al., 2006) could provide 138 insights into the potentials of mineral deposits asMANUSCRIPTsociated with these structures (Njonfang et 139 al., 2011).