Acta Geochim (2019) 38(1):40–67 https://doi.org/10.1007/s11631-018-0272-9 ORIGINAL ARTICLE Baossi–Warack monogenetic volcanoes, Adamawa Plateau, Cameroon: petrography, mineralogy and geochemistry 1,2 2 3 Anicet Feudjio Tiabou • Robert Temdjim • Pierre Wandji • 4 1,5 6 Jacques-Marie Bardintzeff • Vivian Bih Che • Edith Ekatah Bate Tibang • 7 7 Caroline Neh Ngwa • Franc¸ois Xavier Onana Mebara Received: 19 December 2017 / Revised: 2 April 2018 / Accepted: 13 April 2018 / Published online: 23 April 2018 Ó Science Press, Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Three monogenetic cones in the Baossi–Warack include titano-magnetite and titano-hematite, nepheline, area, Ngaounde´re´, Adamawa Plateau forming part of the apatite and amphibole xenocrysts. Sanidine occurs in some Cameroon Volcanic Line (CVL) are documented in this samples and sodi-potassic albite in others. Some olivines and study. Basaltic lavas (\ 1km3) scattered around these vents clinopyroxenes exhibit resorbed margins and thin reaction and restricted volcaniclastic deposits were emplaced by rims while plagioclase displays oscillatory zoning, and Hawaiian and mild strombolian style eruptions. The lavas are sieved textures as a result of magma mixing. Whole-rock porphyritic, mainly composed of olivine (chrysolite) and geochemistry data indicates that the lavas are silica-under- clinopyroxene (diopside and augite) phenocrysts and pla- saturated, composed of basanites and basalts, showing little gioclase (andesine) microphenocrysts. Accessory minerals compositional variations (SiO2: 39.20 wt.%–48.01 wt.%, MgO: 5.29 wt.%–9.70 wt.%). Trace elements patterns of these lavas suggest they are enriched in LILE including Pb, Pierre Wandji: Deceased. probably due to crustal contamination. REE patterns suggest cogenetic magmas below Baossi 1 and Baossi 2 volcanoes, Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11631-018-0272-9) contains supple- and distinct sources below Warack volcano and nearby lavas. mentary material, which is available to authorized users. The lavas studied show affinity to high-l (HIMU), enriched type I (EM1) and Oceanic Island Basalt (OIB)-like mantle & Anicet Feudjio Tiabou [email protected]; [email protected] signatures and thus indicate a heterogeneous mantle source underneath the vents as noted at other monogenetic and 1 Remote Sensing Unit, Faculty of Science, Department of polygenetic volcanoes along the CVL. Primary melts Geology, University of Buea, P.O. Box 63, Buea, derived from low degrees of partial melting (0.5%–2%) and South West Region, Cameroon encountered low rates of fractionation, and crustal contam- 2 Department of Earth Sciences, Faculty of Science, University ination coupled with magma mixing. These melts evolved of Yaounde´ 1, P.O. Box 812, Yaounde´, Cameroon independently through structural weaknesses in the 3 Laboratoire de Ge´ologie, Ecole Normale Supe´rieure de basement. Yaounde´, P.O. Box 47, Yaounde´, Cameroon 4 Univ. Paris-Sud, Sciences de la Terre, Volcanologie, Keywords Cameroon volcanic line Á Adamawa Plateau Á ´ ˆ ´ Planetologie, UMR CNRS 8148 GEOPS, Bat. 504, Universite Volcanic field Monogenetic volcano Magma mixing Paris-Saclay, 91405 Orsay, France Á Á Á Petrogenesis 5 Higher Teacher Training College, Department of Geology, University of Bamenda, P.O. Box 39, Bambili, Bamenda, Cameroon 1 Introduction 6 Institute for Geological and Mining Research (IRGM), P.O. Box 333, Garoua, Cameroon Small-scale volcanic systems are the most widespread type 7 Institute for Geological and Mining Research (IRGM), Branch of Volcanologic and Geophysics Research (ARGV), of volcanism on Earth and occur in diverse tectonic settings P.O. Box 370, Buea, Cameroon (Can˜o´n-Tapia 2016; Smith and Ne´meth 2017). They are the 123 Acta Geochim (2019) 38(1):40–67 41 smallest in terms of the volume of magma that erupts Fig. 1 Structural disposition of the Cameroon Volcanic Line (CVL, c (McGee and Smith 2016). In such systems, the volcanic in black) with ages of the principal volcanic centers (after Marzoli et al. 1999) indicated. The study area with the Baossi–Warack cones edifices are usually basaltic in nature with restricted vol- is part of the Adamawa complex around Ngaounde´re´ umes and their eruptions are inferred to be short-lived events (Valentine and Gregg 2008; Ne´meth 2010; Bar- dintzeff 2016). Monogenetic volcanoes have long been regarded as being geochemically and volcanologically around Ngaounde´re´ on the Adamawa Plateau and form the simple compared to long-lived polygenetic volcanoes central part of the CVL, but they have not yet been studied. where greater degrees of magma evolution are expected This work focuses on volcanic cones within the Baossi– (Brenna et al. 2010). However, field data at a number of ´ ´ monogenetic volcanoes have identified eruption products Warack area in Ngaoundere northeast. We present new petrographic, mineralogical and whole-rock geochemical such as lava flows, pyroclastics of strombolian style erup- data for a suite of samples dispersed over the entire Baossi– tions as well as phreatomagmatic deposits pointing to a range of eruption styles and chemical compositions of Warack complex in a bid to contribute to our understanding of monogenetic style eruptions and their compositional erupting magma at these edifices (Ne´meth et al. 2003; variation. Martin and Ne´meth 2006; Valentine et al. 2007; Genareau et al. 2010; Kereszturi et al. 2011; Bardintzeff et al. 2012; McGee et al. 2012, 2013). For example, compositional discontinuities resulting from successive partial melting of 2 Geological setting distinct, but contiguous source components with differing The CVL has been extensively discussed in the literature melting characteristics in a heterogeneous source were (see Fitton and Dunlop 1985; Sato et al. 1990, 1991; reported in the Auckland Volcanic Field (McGee et al. 2012, 2013). The sequential eruption of two magma bat- Wandji et al. 2000; Aka et al. 2004; Temdjim et al. 2004; De´ruelle et al. 2007; Suh et al. 2008; Nkouathio et al. 2008; ches has also been reported at Parı`cutin in the Michoaca´n- Kamgang et al. 2013; Marzoli et al. 2015; Bate Tibang Guanajuato volcanic field (Erlund et al. 2010) and at Mt. ` Gambier in Australia (Van Otterloo et al. 2014). Thereby, et al. 2017; Ngwa et al. 2017; Njombie et al. 2018; Ziem a Bidias et al. 2018). Along this megastructure, the volcanic it is noticed that monogenetic volcanoes can exhibit great edifices are simultaneously developed into both oceanic complexities in their compositional features and petroge- nesis (e.g. Van Otterloo et al. 2015) but the wide spectrum and continental domains. The oceanic sector is composed of Pagalu, Sa˜oTome´, Principe, and Bioko islands and the of compositional changes accompanying the magmatic continental part of Mts Etinde, Cameroon, Manenguba, activities at these volcanoes are not fully understood. This ´ ´ can be investigated through detailed stratigraphic analysis Bambouto, Oku, Mandaras and Ngaoundere Plateau, which are built upon horsts that alternate with grabens towards the alongside systematic mineralogical and geochemical continental southern end of the chain, where monogenetic sampling. ´ Prominent among numerous intraplate volcanic fields cones predominate (Deruelle et al. 2007). The earlier work on monogenetic volcanoes along the CVL includes a worldwide (e.g. Cook et al. 2005; Brenna et al. 2010, 2012; comparative study of basalts between polygenetic and McGee et al. 2012, 2013; Kereszturi et al. 2013; Jordan et al. 2015) is the Cameroon Volcanic Line (CVL), an monogenetic volcanoes (see Sato et al. 1990). This study suggests that the major sources of magma feeding poly- active large intraplate volcanic province that comprises genetic volcanoes along the CVL is the upwelling several eruptive centers separated by uplifted and eroded plutons (De´ruelle et al. 2007). The CVL is a * 1700 km asthenosphere underneath the African plate, while those feeding monogenetic volcanoes result from partial melting long chain of numerous volcanic edifices that trends in the lowest portion of the subcontinental lithosphere. N030°E from Pagalu Island in the Atlantic Ocean to Lake Chad on the African plate, and spans 70 million years of Other researchers working on the Barombi Koto volcanic field located northeast of Mt. Cameroon reported the activity, with Mt. Cameroon still active today (see sum- presence of ten cinder cones and two maars (see Nkouathio mary in Njome and de Wit 2014; Fig. 1). Numerous studies ´ along this chain are concentrated on the large volcanic et al. 2002; Tamen et al. 2007; Tchamabe et al. 2013). In this volcanic field, the reconstruction of the stratigraphic edifices. Small volume volcanoes have so far received little sequence at Barombi Mbo maar highlighted the polycyclic attention with previous works focused on maars and cinder cones scattered in the continental southern end of the CVL nature of this maar, demonstrating the complexities of such volcanoes (Tchamabe´ et al. 2015, 2016). Ngwa et al. (e.g. Sato et al. 1990; Nkouathio et al. 2002; Tamen et al. (2017) also investigated the origin and evolution of prim- 2007; Ngwa et al. 2010, 2017; Tchamabe´ et al. 2014, 2015, 2016). These types of volcanoes also exist itive melts from the Debunscha maar, a monogenetic 123 42 Acta Geochim (2019) 38(1):40–67 123 Acta Geochim (2019) 38(1):40–67 43 volcano forming part of the Mt. Cameroon volcanic field, (REE) analyses of 20 lava samples were carried out by composed of over a hundred unstudied pyroclastic cones inductively coupled plasma mass spectrometry (ICP-MS) and craters scattered around it (Suh et al. 2003). In the at Activation Laboratories Ltd. (Actlabs). Sample solutions Debunscha area, there is evidence for repeatedly mixing of were produced from * 30 mg of sample powder. Here, compositionally diverse melts in a magma chamber at the powders were mixed with HF–HNO3–HCl in Teflon upper mantle depths prior to eruption (Ngwa et al. 2017). vials and placed on a hot plate at 150 °C for * 48 h.