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Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon volcanic line: Constraints from the anisotropy of magnetic susceptibility (AMS)
Journal Pre-proof Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon volcanic line: Constraints from the anisotropy of magnetic susceptibility (AMS) T. Njanko, M. Gountié Dedzo, J. Tamen, E.B. Bella Nke, O.S. Kadji Kouémo, E.M. Fozing, G. Piankeu Doumsab, J. Tchakounté PII:
S1464-343X(19)30383-8
DOI:
https://doi.org/10.1016/j.jafrearsci.2019.103728
Reference:
AES 103728
To appear in:
Journal of African Earth Sciences
Received Date: 11 April 2019 Revised Date:
29 November 2019
Accepted Date: 2 December 2019
Please cite this article as: Njanko, T., Gountié Dedzo, M., Tamen, J., Bella Nke, E.B., Kadji Kouémo, O.S., Fozing, E.M., Piankeu Doumsab, G., Tchakounté, J., Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon volcanic line: Constraints from the anisotropy of magnetic susceptibility (AMS), Journal of African Earth Sciences (2020), doi: https://doi.org/10.1016/ j.jafrearsci.2019.103728. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
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Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon
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Volcanic Line: constraints from the anisotropy of magnetic susceptibility (AMS)
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Njanko T. a,b*, Gountié Dedzo M. c, Tamen J. a, Bella Nke E.B. d, Kadji Kouémo O.S. a,
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Fozing E.M. a, Piankeu Doumsab G. a, Tchakounté J. e
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a
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Dschang – Cameroon;
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b
Ministry of Scientific Research and Innovation, DPSP/CCAR, P.O. Box 1457, Yaoundé – Cameroon;
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c
Laboratory of Geology, High Teacher Training School of Maroua; P.O. Box 46, Maroua – Cameroon;
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d
Department of Earth Sciences, Faculty of Sciences, University of Maroua; P.O. Box 46, Maroua – Cameroon;
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e
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Cameroon.
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* Corresponding author. E-mail address: [email protected] (Njanko T.)
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Abstract
Laboratory of Environmental Geology, Department of Earth Sciences, University of Dschang, P.O. Box 67,
Department of Earth Sciences, Faculty of Sciences, University of Yaoundé 1, P.O. Box 812, Yaoundé -
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The Zindeng phonolitic flow forms a NW-SE elongated body located on the SE flank of
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Mount Bambouto (West-Cameroon), along the Cameroon Volcanic Line. It covers basalt
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emplaced on top of the Dschang biotite megacrystic granite. The magnitude of the magnetic
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susceptibility (Km) in the phonolitic lava flow varies between 196 µSI and 15769 µSI,
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indicating a dominantly ferromagnetic sensus lato behavior (Km > 1000 µSI; 89% of the
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stations) due to the presence of pseudo-single to multi-domain magnetite grains as deduced
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from hysteresis data. The low values of the degree of magnetic anisotropy, the dominantly
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oblate shape of the AMS ellipsoids and the concordance between lava plane and magnetic
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foliation point to flow-related magnetic fabrics. The pattern of the magnetic foliations and
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lineations indicate that the SE part of the phonolite was emplaced as a dome, whereas the NW
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part corresponds to a flow domain sensus stricto. It is suggested that the viscous phonolitic
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magma rose from a feeding conduit below the southeastern part of the lava flow and
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propagated laterally towards the NW.
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Keywords: AMS; Phonolite; Lava flow; Zindeng; Mount Bambouto; Cameroon.
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1. Introduction
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The emplacement of volcanic rocks and the development of the relevant magmatic
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pathways may control the growth of the volcanoes along with magma composition and the
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eruptive styles (Sparks, 2003). The understanding of magma flow in magmatic rocks is
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imperative for our deep knowledge of crustal dilatation and volcanism (e.g. Hrouda et al.,
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2002; Buck et al., 2006; Philpotts and Philpotts, 2007; Paquet et al., 2007). In rock samples,
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the variation in magnetic susceptibility with direction is recognized as anisotropy of magnetic
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susceptibility (AMS). AMS is one of the perfect methods for petrofabric analysis in igneous
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rocks, even in visually isotropic rocks without well-developed field foliations and lineations
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(e.g. Bouchez, 2000). It has been demonstrated that the magnetic fabric is representative of
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the whole mineral fabric in plutonic rocks (Archanjo et al., 1995; Grégoire et al., 1998), as
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well as in volcanic rocks, either of basaltic, trachytic or rhyolitic composition (Knight and
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Walker, 1988; Gountié Dedzo et al., 2011; Bella Nke et al., 2014). In volcanic rocks, AMS
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has been a very useful tool, used during the past three decades, to study the internal fabric and
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commonly, to access the ill-defined fabric of pyroclastic flows, tabular bodies such as such as
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sills, volcanic dykes or domes (Ernst and Baragar, 1992; Tarling and Hrouda, 1993; Raposo
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and Ernesto, 1995; Callot and Guichet, 2003; Borradaile and Jackson, 2004; Silva et al., 2004;
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Egydio-Silva et al., 2005; Gil-Imaz et al., 2006; Borradaile and Gauthier, 2006; Craddock et
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al., 2008; Cañón-Tapia et al., 2009; Creixell et al., 2009; Bolle et al., 2010; Eriksson et al.,
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2011; Yamato et al., 2012; Bella Nke et al., 2014; among others). It provides important
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informations about their flow mechanism and can also help in locating feeder magma
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chambers (Ray et al., 2007; Silva et al., 2010; Gountié Dedzo et al., 2011; Bella Nke et al.,
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2014; Dinni et al., 2016 amongst others). However, the AMS investigation in volcanic rocks
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has been less frequently used than in plutonic rocks. Even though its potentialities are very
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high, given that the preferred orientation of minerals in such rocks is often very weak and can
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hardly be investigated by other conventional methods.
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The studied area is part of the Mount Bambouto, one of the major continental volcano
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of the Cameroon Volcanic Line (CVL) which is characterized by a mixture of volcanic
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products from different eruptive styles. Most of these volcanic products, along with their
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coexisting lava flows and pyroclasts have been widely characterized volcanologically and
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geochemically. However, only few works have been focused on the emplacement modes and
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flow direction. Such structural analyses using the AMS techniques have already been used
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only in the central part of the CVL, e.g. in the Dschang ignimbrites (Gountié Dedzo et al.,
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2011) and in the trachytic dome of the Foréké-Dschang escarpment (Bella Nke et al., 2014).
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The studied Zindeng phonolitic lava flow, about 300 m long and 100 m wide outcrop, was
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emplaced during late-Miocene Messinian age (6.61 ± 0.17 Ma, Nkouathio et al. 2008) on the
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mid-Miocene basalts (12.52 ± 0.29 Ma) that covers the Pan-African granitic basement.
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In this paper, we present field observations and structural measurements combined with
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AMS data to assess the internal fabric of the Zindeng phonolitic lava flow. The aim of the
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study is to model the emplacement and the magma flow regime and thus, to better constrain
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the volcanic history of the SW flank of the Mount Bambouto where the Zindeng phonolite
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was emplaced.
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2. Geological setting
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The CVL (Fig. 1) is a 1600 km long chain of volcanoes and anorogenic igneous
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complexes stretching from the Atlantic Ocean, across the Cameroon territory. This chain,
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Tertiary to recent in age (Fitton and Dunlop, 1985), extends from Pagalu Island in the Gulf of
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Guinea to Lake Chad, with a N30°E main orientation. The basement of the CVL in the
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continental sector is of Pan-African age and is made of gneisses and granites cut by numerous
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N70°E shear zones that divide the CVL into segments (Déruelle et al., 2007). More than 60
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anorogenic plutonic complexes and volcanoes were emplaced in the continental portion of the
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CVL. Plutonic rocks are mainly represented by granites and syenites associated with rocks of
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intermediate to basic compositions (Njonfang et al., 1992; Njonfang and Moreau, 2000;
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Njonfang et al., 2011). Volcanoes in the continental portion of the CVL (Figs. 1 and 2)
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comprise Mounts Etindé, Cameroon, Manengouba, Bambouto, Bamenda, Oku and volcanic
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plateaus (Bamiléké, Bamoun, Adamawa also known as Ngaoundéré, Kapsiki and Biu) having
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basalt, trachyte, phonolite, rhyolite and ignimbrites as main rock types (Marzoli et al., 1999,
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2000; Ngounouno et al., 2000; Nono et al., 2004; Rankenburg et al., 2005; Tamen et al., 2007;
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Kamgang et al., 2010, 2013; Gountié Dedzo et al., 2011; Tchuimegnie et al., 2015).
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The Mount Bambouto is a shield volcano (Nono et al., 2004; Nkouathio et al., 2008;
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Kagou Dongmo et al., 2010), lying between 10°00’–10°10’E and 5°35’–5°45’N, in the central
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part of the CVL (Fig. 1). It is elliptic in shape (45–50 × 20–25 km) and shows a large (13 × 8
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km) horseshoe-shaped well-preserved caldera at its summit. It displays various eruptive
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styles, ranging from calm effusive through extrusive (plugs and dykes), mildly explosive
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strombolian to highly vulcanian explosive that resulted in the ca. 10 km-wide caldera. The
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volcano displays an alternation of four eruptive styles: lava flows, domes, strombolian
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eruptions and ignimbrite-forming pyroclastic flows (Nono et al., 2004; Nkouathio et al., 2008
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and Kagou Dongmo et al., 2010). The nature and volumes of the volcanic rocks as well as
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their eruption time-scale are extensive. Volcanic rocks range from thick and widespread
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aphyric Ne-normative basanites coating the granito-gneissic basement, through alkaline
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basalts, scarce hawaiite, mugearite and phono-tephrite to felsic lavas (benmoreite, Q-trachyte
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and phonolite). Quantitatively, Q-trachytic lava flows of about 300 m thick are widespreaded
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(Marzoli et al., 1999), followed by aphyric mafic flows, rhyolitic ignimbrites, porphyritic
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mafic rocks and few trachytic and phonolitic plugs and dykes. According to Nkouathio et al.
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(2008), the large compositional range of Mount Bambouto lavas, together with their
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petrographic and geochemical characteristics (occurrence of xenocrysts, zoned crystals,
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coexistence of lavas of different chemical composition from basic to felsic) and
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morphological features (calderas, large lava flows, domes, necks) are in accordance with the
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concept of a large zoned magma chamber below the volcano.
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The reconstitution of the eruptive sequences reveals four phases: (i) the first phase that
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built up the initial basaltic shield volcano extended between ca. 21 and 18 Ma (N’ni, 2004);
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(ii) the second phase, between 16 and 11 Ma (Youmen et al., 2005), is marked by the collapse
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of the caldera subsequent to trachy-rhyolitic ignimbrites outpouring; (iii) the third phase, from
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9 to 4.5 Ma, which reconnects with basaltic effusive activity is coupled with post-caldera
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extrusions of trachytes and phonolites (Marzoli et al., 2000) and (iv) the last and shortest
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episode took place at ca. 0.5 Ma (Kagou Dongmo et al., 2010) at Totap and yielded the single
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known strombolian cone.
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Previous works carried out by Nono et al. (2004) and Kagou Dongmo et al. (2010) have
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studied the phonolitic lava flows associated with the third stage of the volcanic history of the
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Mount Bambouto (Kagou Dongmo et al., 2010). This stage is characterized by effusive
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activities that lead to a basanite-mugearite serie (15.1–4.5 Ma), associated with post-caldera
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extrusions of trachytes (15–8.8 Ma), and phonolites (amongst which, the studied Zindeng
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phonolitic flow) between 12.9 and 5.2 Ma.
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The Zindeng area is located at about 3 km from the Dschang city, between longitudes
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10°00’50’’ and 10°01’03’’ East and latitudes 5°25’41’’ and 5°26’06’’ North, covering a
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surface of 220000 m2. It lies on the low land of the southeastern slope of Mount Bambouto
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(Fig. 2). Here, the Pan-African basement consists mainly of (i) biotite megacrystic granite and
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(ii) banded amphibole gneisses and migmatic amphibole gneisses. This granitic magmatism,
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dated at 630 – 540 Ma (Tagné Kamga, 2003; Toteu et al., 2004; Kwékam et al., 2010, 2013),
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is a major feature of the central domain of the Pan-African fold belt in Cameroon.
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3. Petrology and structure of the Zindeng area
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The Zindeng area comprises banded amphibole gneisses, biotite megacrystic granite,
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basalt and porphyritic phonolite (Fig. 3). The porphyritic phonolite crops out as flagstones
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(Fig. 4a and b) and boulders on the flank of a granitic hill. The rock is greenish with
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microlithic porphyritic texture (Fig. 4c). It is made up of sanidine (Or15–55Ab35–85An0–6;
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Nkouathio et al., 2008), felsdpathoids, Na-clinopyroxene (En3–4Aeg33–35Fs62–65; Nkouathio et
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al., 2008), amphibole, plagioclase (An25–11Ab70–54Or22–19; Nkouathio et al., 2008), magnetite
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and other undetermined oxides.
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The basalt crops out also as flagstone and boulders. The rock is dark in color with
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microlithic porphyritic texture (up to 30% of phenocrysts). It is made up of large subhedral
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phenocrysts of olivine (Fo66-87), diopside (Wo45–48En43–52Fs8–17), augite (Wo38–44En43–53Fs10–23)
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and phenocrysts and microphenocrysts of plagioclase (An46–72) and oxides (Nkouathio et al.,
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2008).
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The biotite megacrystic granite crops out as flagstones in the valleys and as boulders on
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the flanks and the summits of hills. The rock is greyish or pinkish, fine- to medium-grained. It
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shows noticeable mineral preferred orientation and contains phenocrysts of perthitic alkali-
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feldspar. Other minerals are quartz, plagioclase and variable amount of hornblende and
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biotite. Magnetite, hematite, titanite, zircon, apatite and epidote are accessory minerals. The
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microstructure is heterogranular to granular. Locally, porphyroclasts of feldspar are moulded
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in a fine-grained matrix, sometimes with quartz ribbons, pointing to mylonitization.
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Banded amphibole gneisses are found as xenoliths in the phonolitic lava (Fig. 4d) and
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the biotite megacrystic granite. The rock is greyish, medium- to coarse-grained, with
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millimetric dark bands composed mainly of biotite and hornblende with few plagioclase and
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K-feldspar porphyroclasts. These dark bands alternate with light bands made up of quartz and
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porphyroclasts of K-feldspar. Accessory minerals are apatite, zircon, titanite and oxides.
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In the phonolite, the magmatic foliation plane is marked by a planar shape-preferred
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orientation of millimetric automorphic K-feldspar phenocrysts (Figs. 4b, c, f). Elongated
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gneissic xenoliths were transposed parallel to this foliation (Fig. 4d). Measurements made in
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the field and on thin sections cut from oriented samples indicate that the foliation strike varies
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from N19°E to N160°E with a mean value of N112°E (Fig. 5).
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4. Materials and methods
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In a low intensity magnetic field, as a first approximation, AMS is a second-rank tensor
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whose eigenvectors and eigenvalues, K1 ≥ K2 ≥ K3, define the principal axes of an ellipsoid
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(Jelinek, 1981; Tarling and Hrouda, 1993). The bulk magnetic susceptibility magnitude is
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given by Km = (K1 + K2 + K3)/3. The long axis of the AMS ellipsoid (K1), defines the
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magnetic lineation and the short axis (K3), is the pole of the magnetic foliation defined by the
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K1K2 plane.
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Oriented cylindrical cores, 22 mm high and 25.4 mm in diameter were taken at 29
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stations (Fig. 4), mostly in the phonolite (27 stations) and, to a lesser extent, in basalts (2
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stations). Two or three oriented cores were collected per station. Each core was cut into at
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least two cylinders, hence providing a minimum of four samples per station, with a total of
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127 samples analyzed in this study. AMS measurements were made at the LMTG
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(Laboratoire des Mécanismes et Transferts en Géologie, Toulouse, France), using a KLY-3
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Kappabridge (Agico, Czech Republic) working at a low alternating inductive field (4 x 10-4 T
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at 920 Hz) with a sensitivity of about 2 x 10-7 SI, allowing anisotropy discrimination below
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0.2% over a wide range of susceptibility.
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Hysteresis measurements were performed, up to 1 T, on two representative phonolite
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samples, on the Micromag VSM of the CEREGE (Centre de Recherche et d’Enseignement de
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Géosciences de l’Environnement, Aix-en-Provence, France), to further define the magnetic
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mineralogy.
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The AMS data are shown in Table 1, including the data for the two basalt stations that
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will not be discussed here. In this table, in addition to the orientation of K1, K2 and K3 axes,
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and to the values of the bulk susceptibility magnitude (Km), we found the total anisotropy
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percentage [P% = 100 x (P - 1) with P = K1/K3] and the shape parameter of Jelinek (1981) [Tj
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= 2ln (K2/K3)/(ln (K1/K3)) – 1].
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5. Bulk magnetic susceptibility
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Km in the Zindeng phonolite ranges from 196 µSI (station GZ24) to 15679 µSI (station
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GZ29; Table 1, Fig. 6). The two stations in the basalt (GZ28 and GZ29) have Km values of
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6971 µSI and 11039 µSI respectively. These values indicate, for the phonolitic rocks (Fig.
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6b), the coexistence of paramagnetic (Km ˂ 500 µSI; 7% of the stations) and ferromagnetic
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(sensu lato) behaviors (Km > 1000 µSI; 89% of the stations) with some intermediate cases
188
(500 µSI ˂ Km ˂ 1000 µSI; 4% of the stations) (Rochette, 1987; Bouchez, 2000). The
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paramagnetic behavior is carried by the iron-bearing silicates, namely Na-clinopyroxenes and
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amphiboles whereas the ferromagnetic behavior is due to the magnetite. Also, it is common to
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find at a given station, from one sample to another, para- or ferromagnetic behaviors. This
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indicates an uneven distribution of the magnetic minerals in the phonolite. It should be
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mentioned that the stations showing strictly paramagnetic and intermediate para-
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ferromagnetic behaviors are all located in the southeastern part of the lava flow (Fig. 6).
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6. Hysteresis data
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Magnetite, as identified through optical microscopy, is shown above to dominate the
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magnetic susceptibility in most samples. The two measured hysteresis loops should help to
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characterize the size, hence the crystal domain structure of this oxide.
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The shape of the GZ3A2 hysteresis loop is characteristic of pseudo-single domain
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magnetite, while the “S” shape of the GZ2C2 hysteresis loop is typical of multi-domain
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magnetite. Accordingly, the two studied samples plot respectively in the PSD and MD fields
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of a Day et al. (1977) diagram (Fig. 7c). These hysteresis data are consistent with the
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coexistence of fine and coarse-grained magnetite in the Zindeng phonolite.
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7. AMS data
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7.1. Anisotropy degree and shape of the magnetic fabric
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The degree of magnetic anisotropy (P%) of the Zindeng phonolitic lava flow varies
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from 0 (station GZ25) to 10% (station GZ13) (Table 1, Fig. 8) with a mean value of 4%. P%
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values ≥ 5% are mostly located on the northern border of the lava flow (Fig. 8a). It should be
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noted that P% vs. Km diagram (Fig. 8b) display a roughly positive linear correlation between
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P% and Km. This suggests that magnetic mineralogy exerts some influence on the
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eccentricity of the AMS ellipsoid.
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The shape parameter (Tj) in the Zindeng phonolitic lava flow varies from -0.2 to 0.9,
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and is positive for 96% of the stations (Table 1, Fig. 9a). Only one station (GZ18),
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representing 4% of the stations, shows a negative value of Tj. The AMS ellipsoids have thus
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dominantly an oblate shape, as illustrated by the Tj vs. P% diagram (Fig. 9b). This probably
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reflects a rock fabric which is also dominantly oblate (Hrouda, 1993; Borradaile and Henry,
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1997; Borradaile and Jackson, 2004)
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7.2. Directional data
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The magnetic foliation within the phonolitic lava is organized around four strike
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directions: one main direction, NW-SE (52% of the stations) and three subordinate directions,
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E-W (22% of the stations), N-S (15% of the stations) and NE-SW (11% of the stations)) (Fig.
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10). Based on this organization of the magnetic foliation, the phonolitic lava flow can be
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divided into two domains, referred to as Domain I and Domain II (Fig. 10). Domain I, situated
224
close to the granitic pluton, is characterized by a variable trend of the magnetic foliation,
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locally with strongly curved trajectories defining concentric patterns. Domain II, in the
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northwestern part of the lava flow, displays a dominant NW-SE trend with some oblique N-S
227
magnetic foliations. The lower hemisphere projection diagrams of the magnetic foliation
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show a best pole at 207°/68° for Domain I, 190°/51° for Domain II and 192°/56° for the
229
whole lava flow. Hence, in average, the foliation is WNW-ESE striking, i.e. roughly parallel
230
to the NW-SE elongation of the phonolite body, with a moderate dip to the NE
231
(N102°E/34°NNE).
232
It is important to note that the orientation of the magnetic foliation in the phonolitic lava
233
flow is concordant with the planar shape preferred orientation of feldspar phenocrysts that
234
defines the lava flow plane (Fig. 5). In particular, the mean strikes are sub-parallel (N112°E
235
vs. N102°E; Figs. 5 and 10). The magnetic fabric is thus flow related (Rochette et al., 1992).
236
The magnetic lineation (Fig. 11) is organized along N-S (15% of the stations), E-W
237
(29% of the stations), NE-SW (11% of the stations) and NW-SE (45% of the stations) strikes,
238
with plunges of variable values (2°–69°) and directions. Alike the magnetic foliation, the
239
magnetic lineation displays locally strongly curved trajectories close to the granitic pluton
240
(Domain I). In this domain, the lower hemisphere projection diagram indicates a best line at
241
95°/04°. In the northwestern part of the lava flow (Domain II), trajectories of the magnetic
242
lineation, as for the foliation, are oblique on each other, with a dominant NW-SE trend cut by
243
N-S and NE-SW lineations. The best line in Domain II is at 289°/08°. Therefore, the lineation
244
and the foliation trends display similar behaviors in the two domains. The best line for the two
245
domains is 277°/03°, hence, in average, the magnetic lineation is gently plunging to the W.
246
This mean magnetic lineation (N97°E/03°W) is parallel to the strike of the mean foliation
247
(N102°E/34°NNE)
248
8. Discussion
249
8.1. Construction of the Zindeng volcanic edifice
250
P% values in the Zindeng phonolitic lava flow are low (≤ 10%) to very low. According
251
to Henry (1988), P% parameter helps determining the emplacement mechanism of the rock
252
that can be syn-magmatic flow fabric (P% ˂ 20%) or post-magmatic linked to the deformation
253
(P% > 20%). The highest values of the Zindeng phonolitic lava are similar to those found in
254
the basaltic and rhyolitic flows or dykes by Tarling and Hrouda (1993) and Rochette et al.
255
(1999), but lower than those obtained by Morgan et al. (2008) in phonolites and trachytes
256
from the Mesa intrusion. This is consistent with a rather low strain induced by viscous
257
shearing within the flowing magma. The Tj parameter indicates that 96% of the stations have
258
a planar shape ellipsoid (Tj ≥ 0). According to Cañón-Tapia and Castro (2004) and Gountié
259
Dedzo et al. (2011), planar magnetic fabrics in volcanic products are typical of high viscosity
260
volcanic rocks that recorded a pronounced mineral fabric during crystallization. In short, both
261
the P% and Tj parameters has thus led to believe that the Zindeng phonolite is characterized
262
by weakly anisotropic and oblate magnetic fabrics typical of high viscosity volcanic rocks
263
that have recorded well defined mineral fabrics during crystallization.
264
The mean strike of the foliations (N112°E and N102°E, for the feldspars and magnetic
265
fabric, respectively) and the mean magnetic lineation (N97°E/03°E) make a small acute angle
266
with the NW-SE elongation of the phonolite body. This suggests a flowing direction for the
267
Zindeng phonolitic body close to this NW-SE direction.
268
8.2. Emplacement model
269
The AMS directional data (magnetic foliation and magnetic lineation) may be used to
270
reveal the building process of the Zindeng phonolitic lava flow. The magnetic lineation,
271
dominantly of low plunges (˂ 13°) either in Domain I or Domain II with an average E-W
272
direction, is consistent with magma being fed laterally from a source located at or close to, or
273
below one of the two lobes of the phonolitic lava flow. The arcuate, locally strongly curved
274
trajectories of the magnetic foliations and lineations in Domain I, together with the
275
dominantly moderate to steep dip of the magnetic foliations in this area suggest that Domain I
276
was emplaced as a dome above a feeding conduit at depth (Fig. 12). The magnetic fabrics of
277
Domain II are rather evocative of a flow domain sensus stricto characterized by more regular
278
foliation and lineation trajectories roughly parallel to the direction of the phonolite body
279
suggesting that the lava propagated towards the NW. Here, the magnetic foliations and
280
lineations have NW-SE, to locally E-W trends cut by N-S, to locally NE-SW trends (Figs. 10
281
and 11). The NW-SE to E-W trends may correspond to the direction of magma propagation,
282
while the N-S to NE-SW trends may be flow fronts. In short, the viscous phonolitic magma
283
rose from a feeding conduit located below the SE lobe of the Zindeng phonolite, close to the
284
granitic pluton, and propagated towards the NW as schematically illustrated in Fig. 12.
285
Conclusion
286
The Zindeng phonolite is a dominantly ferromagnetic lava flow characterized mostly by
287
weakly anisotropic and oblate magnetic fabrics typical of high viscosity volcanic rocks that
288
have recorded well defined mineral fabrics during crystallization. Feldspars and magnetic
289
foliations, as well as magnetic lineations strike close to the NW-SE direction of the phonolite
290
body pointing to (i) flow-related magnetic fabrics and (ii) a flow direction at low angle to the
291
phonolite elongation. The locally strongly curved trends of the foliation and lineation
292
trajectories, and the mostly moderate to steep foliation dips, close to the contact with the
293
granitic pluton in the southeastern part of the lava flow, points to emplacement of this domain
294
as a dome above a feeding conduit at depth. The northwestern part of the lava flow is
295
characterized by NW-SE to E-W foliation and lineation trajectories (flow trends) cut by N-S
296
to NE-SW trajectories (flow fronts). The phonolitic magma rose from depth below the SE
297
domain (dome) and propagated laterally towards the NW domain.
298
Acknowledgements
299
Gountié Dedzo is grateful to EGIDE (Centre français pour l'accueil et les échanges
300
internationaux), SCAC (Service de Coopération et d'Action Culturelle de la France au
301
Cameroun) and the French Government for funding its stay in UMR 5563-GET-OMP
302
(Toulouse – France). Field work was partially supported by the IRD-CORUS 2 project of M.
303
Jessell and J.L. Bouchez from LMTG (Toulouse). Warm thanks to P. Rochette for hyteresis
304
measurements and to R. Siqueira and F. de Parseval for technical assistance. Constructive
305
reviews by R. Ernst and one anonymous reviewer, as well as careful editing by R.B.
306
Mthanganyika Mapeo, are also greatly acknowledged.
307
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Table 1 Cordinates Station
Long (N)
(°)
Lat (E)
AMS parameters N
(°)
Km
P%
Mean eigen vectors
T
(µSI)
K1 Dec
Inc
K2
K3
Dec Inc
Dec Inc
Phonolite GZ01
10.0151
5.4321
4
6709
6 0.2
293
49
187
14
85
38
GZ02
10.0153
5.4321
5
11764
7 0.5
228
67
132
2
41
26
GZ03
10.0150
5.4319
6
7072
3 0.2
81
28
323
43
192
37
GZ04
10.0151
5.4321
3
12888
8 0.7
175
34
304
41
60
29
GZ05
10.0149
5.4322
5
4598
5 0.6
91
10
343
36
183
57
GZ06
10.0146
5.4324
2
6464
7 0.9
117
2
26
16
203
71
GZ07
10.0146
5.4322
6
14874
4 0.2
277
20
24
39
167
44
GZ08
10.0146
5.4320
2
7565
2 0.7
192
6
284
30
93
63
GZ09
10.0148
5.4321
5
10308
5 0.7
84
14
344
33
193
54
GZ10
10.0150
5.4318
5
3245
2 0.5
302
4
35
45
208
46
GZ11
10.0152
5.4316
6
4891
2 0.7
111
13
357
60
209
27
GZ12
10.0162
5.4313
4
11959
4 0.7
297
9
26
47
197
49
GZ13
10.0162
5.4313
5
14340
10 0.7
194
69
334
16
69
13
GZ14
10.0161
5.4313
5
9772
8 0.7
58
8
324
32
161
58
GZ15
10.0156
5.4316
5
15679
9 0.7
240
64
125
12
29
23
GZ16
10.0153
5.4317
4
1912
2 0.9
327
34
74
24
192
47
GZ17
10.0152
5.4318
5
2000
4
15
110
4
218
58
GZ18
10.0162
5.4310
4
6278
3 0.1 5 0.2
102
9
194
5
341
81
GZ19
10.0159
5.4310
3
12802
3 0.3
4
35
283
10
157
67
GZ20
10.0164
5.4309
6
13189
7 0.7
322
48
172
38
70
15
GZ21
10.0165
5.4321
4
11915
9 0.8
327
19
73
40
218
45
GZ22
10.0166
5.4304
4
663
3 0.0
41
25
159
46
292
33
GZ23
10.0164
5.4302
4
3883
3 0.4
357
44
208
64
234
3
GZ24
10.0161
5.4304
4
196
1 0.3
92
16
180
2
242
72
GZ25
10.0159
5.4306
5
266
0 0.1
327
16
217
36
82
58
GZ26
10.0160
5.4308
5
11530
5 0.5
79
21
341
18
216
61
GZ27
10.0162
5.4306
4
10681
5 0.8
73
8
342
19
191
70
GZ28
10.0135
5.4327
3
11110
4 0.6
224
10
314
5
71
77
GZ29
10.0136
5.4330
4
6971
2 0.0
222
4
314
19
192
36
Basalt
Table 2 Station GZ2C2 GZ3A2
m (g) 1.463 1.420
Km (Sample) Km (Station) (µSI) (µSI) 11747 7776
11671 7072
P% (Sample)
P% (Station) Khf (10-6 SI)
8 3
7 3
339 277
%para Ms 3 15
675 95
Mrs/Ms 0.043 0.058
Hcr/Hc 4.21154 3.67857
Hcr (mT) 21.9 20.6
Table 1. AMS data for the Zideng phonolitic lava flow and also the basalt flow. Km, mean magnetic susceptibility; P%, total anisotropy percentage; Tj, shape parameter of Jelinek (1981). K1, K2 and K3, long, intermediate and short axes of the AMS ellipsoid; Dec and Inc, declination and inclination of the axes in degrees.
Table 2. Rock magnetic parameters from hysteresis measurements of two ~1 cm3 samples. Khf, high field susceptibility; % para, percent of "paramagnetic" susceptibility; Ms, saturation magnetization; Mrs, remanent saturation magnetization; Hc, coercive field; Hcr, remanent coercive field.
Fig. 1.
Fig. 2
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Fig. 9.
Fig. 10.
Fig. 11.
Fig. 12.
Figure captions Fig. 1. Geological map of the CVL. (a) Location of Cameroon in Africa showing the different cratons; (b) Distribution of Cameroon Line volcanism (after Njonfang et al., 2011). Locations of seamounts are after Burke (2001). C.A.R = Central African Republic; SA = Study area
Fig. 2. Digital elevation model (DEM) of the central part of the Cameroon Volcanic Line (after Gountié Dedzo et al., 2011) showing the position of (i) Mounts Bambouto and Bamenda and the close surroundings and (ii) the Zindeng area. The different calderas are also indicated.
Fig. 3. (a and b) Field relationships between the Zindeng phonolitic lava flow, the basalt and the Dschang granite; (c) Geological map of the study area with location of the sampling stations.
Fig. 4. Field photographs and microphotographs (crossed polars). Field evidence of flow direction (a, b and d): (a) View of the phonolotic vent marked by rough vertical columnar jointing (Menouet river right bank). Note the vertical fractures within the rock; (b) Phonolitic flagstone outcrop; (c) Hand specimen of phonolite; (d) Gneiss xenoliths within the phonolite; Thin sections of porphyric phonolite (note the phenocrystals of sanidine within an aphyric matrix) (e) and aphyric basalt (f). LFD = trace of the planar shape-preferred orientation of millimetric automorphic K-feldspar phenocrysts; Sa = sanidine, Mgt = magnetite.
Fig. 5. Map showing the trace of the foliation defined by the K-feldspar phenocrysts, with lower hemisphere, equal-area projection diagrams.
Fig. 6. Magnetic susceptibility (Km). (a) Distribution map of the Km values and (b) susceptibility histograms.
Fig. 7. Hysteresis data. (a and b) Hysteresis loops for representative phonolite samples (GZ2C and GZ3A); (c) Mrs/Ms vs Hcr/Hc diagram of samples after Day et al. (1977) and Dunlop (1986). PSD = pseudo-single-domain; MD = multi-domain.
Fig. 8. Degree of magnetic anisotropy (P%). (a) Distribution map of the P% values and (b) P% vs Km diagram.
Fig. 9. Jelinek shape parameter (Tj). (a) Distribution map of the Tj values and (b) Tj vs P% diagram.
Fig. 10. Map of the magnetic foliations and equal-area projections of the foliation poles AMS parameter (lower hemisphere).
Fig. 11. Magnetic lineation. (a) Map and equal-area projections (lower hemisphere) and (b) plunge histogram.
Fig. 12. 3D emplacement model for the Zindeng phonolitic flow lava.
Highlights -
-
We investigated the fabric of the Zindeng phonolitic lava flow located on the southeastern flank of the Mount Bambouto through AMS measurements. The AMS scalar parameters (Km, P% and T) and directional data (magnetic foliation, normal to K3 and magnetic lineation K1) indicate a dominantly magnetite–dominated magnetic fabric, that is flow-related. A model of magma feeding from a source at depth followed by lateral flow is proposed for the Zindeng phonolite.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1464-343X(19)30383-8
DOI:
https://doi.org/10.1016/j.jafrearsci.2019.103728
Reference:
AES 103728
To appear in:
Journal of African Earth Sciences
Received Date: 11 April 2019 Revised Date:
29 November 2019
Accepted Date: 2 December 2019
Please cite this article as: Njanko, T., Gountié Dedzo, M., Tamen, J., Bella Nke, E.B., Kadji Kouémo, O.S., Fozing, E.M., Piankeu Doumsab, G., Tchakounté, J., Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon volcanic line: Constraints from the anisotropy of magnetic susceptibility (AMS), Journal of African Earth Sciences (2020), doi: https://doi.org/10.1016/ j.jafrearsci.2019.103728. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
1
Emplacement of the Zindeng phonolitic lava flow (West-Cameroon) in the Cameroon
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Volcanic Line: constraints from the anisotropy of magnetic susceptibility (AMS)
3
Njanko T. a,b*, Gountié Dedzo M. c, Tamen J. a, Bella Nke E.B. d, Kadji Kouémo O.S. a,
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Fozing E.M. a, Piankeu Doumsab G. a, Tchakounté J. e
5
a
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Dschang – Cameroon;
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b
Ministry of Scientific Research and Innovation, DPSP/CCAR, P.O. Box 1457, Yaoundé – Cameroon;
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c
Laboratory of Geology, High Teacher Training School of Maroua; P.O. Box 46, Maroua – Cameroon;
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d
Department of Earth Sciences, Faculty of Sciences, University of Maroua; P.O. Box 46, Maroua – Cameroon;
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e
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Cameroon.
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* Corresponding author. E-mail address: [email protected] (Njanko T.)
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Abstract
Laboratory of Environmental Geology, Department of Earth Sciences, University of Dschang, P.O. Box 67,
Department of Earth Sciences, Faculty of Sciences, University of Yaoundé 1, P.O. Box 812, Yaoundé -
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The Zindeng phonolitic flow forms a NW-SE elongated body located on the SE flank of
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Mount Bambouto (West-Cameroon), along the Cameroon Volcanic Line. It covers basalt
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emplaced on top of the Dschang biotite megacrystic granite. The magnitude of the magnetic
17
susceptibility (Km) in the phonolitic lava flow varies between 196 µSI and 15769 µSI,
18
indicating a dominantly ferromagnetic sensus lato behavior (Km > 1000 µSI; 89% of the
19
stations) due to the presence of pseudo-single to multi-domain magnetite grains as deduced
20
from hysteresis data. The low values of the degree of magnetic anisotropy, the dominantly
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oblate shape of the AMS ellipsoids and the concordance between lava plane and magnetic
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foliation point to flow-related magnetic fabrics. The pattern of the magnetic foliations and
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lineations indicate that the SE part of the phonolite was emplaced as a dome, whereas the NW
24
part corresponds to a flow domain sensus stricto. It is suggested that the viscous phonolitic
25
magma rose from a feeding conduit below the southeastern part of the lava flow and
26
propagated laterally towards the NW.
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Keywords: AMS; Phonolite; Lava flow; Zindeng; Mount Bambouto; Cameroon.
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1. Introduction
29
The emplacement of volcanic rocks and the development of the relevant magmatic
30
pathways may control the growth of the volcanoes along with magma composition and the
31
eruptive styles (Sparks, 2003). The understanding of magma flow in magmatic rocks is
32
imperative for our deep knowledge of crustal dilatation and volcanism (e.g. Hrouda et al.,
33
2002; Buck et al., 2006; Philpotts and Philpotts, 2007; Paquet et al., 2007). In rock samples,
34
the variation in magnetic susceptibility with direction is recognized as anisotropy of magnetic
35
susceptibility (AMS). AMS is one of the perfect methods for petrofabric analysis in igneous
36
rocks, even in visually isotropic rocks without well-developed field foliations and lineations
37
(e.g. Bouchez, 2000). It has been demonstrated that the magnetic fabric is representative of
38
the whole mineral fabric in plutonic rocks (Archanjo et al., 1995; Grégoire et al., 1998), as
39
well as in volcanic rocks, either of basaltic, trachytic or rhyolitic composition (Knight and
40
Walker, 1988; Gountié Dedzo et al., 2011; Bella Nke et al., 2014). In volcanic rocks, AMS
41
has been a very useful tool, used during the past three decades, to study the internal fabric and
42
commonly, to access the ill-defined fabric of pyroclastic flows, tabular bodies such as such as
43
sills, volcanic dykes or domes (Ernst and Baragar, 1992; Tarling and Hrouda, 1993; Raposo
44
and Ernesto, 1995; Callot and Guichet, 2003; Borradaile and Jackson, 2004; Silva et al., 2004;
45
Egydio-Silva et al., 2005; Gil-Imaz et al., 2006; Borradaile and Gauthier, 2006; Craddock et
46
al., 2008; Cañón-Tapia et al., 2009; Creixell et al., 2009; Bolle et al., 2010; Eriksson et al.,
47
2011; Yamato et al., 2012; Bella Nke et al., 2014; among others). It provides important
48
informations about their flow mechanism and can also help in locating feeder magma
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chambers (Ray et al., 2007; Silva et al., 2010; Gountié Dedzo et al., 2011; Bella Nke et al.,
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2014; Dinni et al., 2016 amongst others). However, the AMS investigation in volcanic rocks
51
has been less frequently used than in plutonic rocks. Even though its potentialities are very
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high, given that the preferred orientation of minerals in such rocks is often very weak and can
53
hardly be investigated by other conventional methods.
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The studied area is part of the Mount Bambouto, one of the major continental volcano
55
of the Cameroon Volcanic Line (CVL) which is characterized by a mixture of volcanic
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products from different eruptive styles. Most of these volcanic products, along with their
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coexisting lava flows and pyroclasts have been widely characterized volcanologically and
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geochemically. However, only few works have been focused on the emplacement modes and
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flow direction. Such structural analyses using the AMS techniques have already been used
60
only in the central part of the CVL, e.g. in the Dschang ignimbrites (Gountié Dedzo et al.,
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2011) and in the trachytic dome of the Foréké-Dschang escarpment (Bella Nke et al., 2014).
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The studied Zindeng phonolitic lava flow, about 300 m long and 100 m wide outcrop, was
63
emplaced during late-Miocene Messinian age (6.61 ± 0.17 Ma, Nkouathio et al. 2008) on the
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mid-Miocene basalts (12.52 ± 0.29 Ma) that covers the Pan-African granitic basement.
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In this paper, we present field observations and structural measurements combined with
66
AMS data to assess the internal fabric of the Zindeng phonolitic lava flow. The aim of the
67
study is to model the emplacement and the magma flow regime and thus, to better constrain
68
the volcanic history of the SW flank of the Mount Bambouto where the Zindeng phonolite
69
was emplaced.
70
2. Geological setting
71
The CVL (Fig. 1) is a 1600 km long chain of volcanoes and anorogenic igneous
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complexes stretching from the Atlantic Ocean, across the Cameroon territory. This chain,
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Tertiary to recent in age (Fitton and Dunlop, 1985), extends from Pagalu Island in the Gulf of
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Guinea to Lake Chad, with a N30°E main orientation. The basement of the CVL in the
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continental sector is of Pan-African age and is made of gneisses and granites cut by numerous
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N70°E shear zones that divide the CVL into segments (Déruelle et al., 2007). More than 60
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anorogenic plutonic complexes and volcanoes were emplaced in the continental portion of the
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CVL. Plutonic rocks are mainly represented by granites and syenites associated with rocks of
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intermediate to basic compositions (Njonfang et al., 1992; Njonfang and Moreau, 2000;
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Njonfang et al., 2011). Volcanoes in the continental portion of the CVL (Figs. 1 and 2)
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comprise Mounts Etindé, Cameroon, Manengouba, Bambouto, Bamenda, Oku and volcanic
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plateaus (Bamiléké, Bamoun, Adamawa also known as Ngaoundéré, Kapsiki and Biu) having
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basalt, trachyte, phonolite, rhyolite and ignimbrites as main rock types (Marzoli et al., 1999,
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2000; Ngounouno et al., 2000; Nono et al., 2004; Rankenburg et al., 2005; Tamen et al., 2007;
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Kamgang et al., 2010, 2013; Gountié Dedzo et al., 2011; Tchuimegnie et al., 2015).
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The Mount Bambouto is a shield volcano (Nono et al., 2004; Nkouathio et al., 2008;
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Kagou Dongmo et al., 2010), lying between 10°00’–10°10’E and 5°35’–5°45’N, in the central
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part of the CVL (Fig. 1). It is elliptic in shape (45–50 × 20–25 km) and shows a large (13 × 8
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km) horseshoe-shaped well-preserved caldera at its summit. It displays various eruptive
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styles, ranging from calm effusive through extrusive (plugs and dykes), mildly explosive
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strombolian to highly vulcanian explosive that resulted in the ca. 10 km-wide caldera. The
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volcano displays an alternation of four eruptive styles: lava flows, domes, strombolian
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eruptions and ignimbrite-forming pyroclastic flows (Nono et al., 2004; Nkouathio et al., 2008
94
and Kagou Dongmo et al., 2010). The nature and volumes of the volcanic rocks as well as
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their eruption time-scale are extensive. Volcanic rocks range from thick and widespread
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aphyric Ne-normative basanites coating the granito-gneissic basement, through alkaline
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basalts, scarce hawaiite, mugearite and phono-tephrite to felsic lavas (benmoreite, Q-trachyte
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and phonolite). Quantitatively, Q-trachytic lava flows of about 300 m thick are widespreaded
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(Marzoli et al., 1999), followed by aphyric mafic flows, rhyolitic ignimbrites, porphyritic
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mafic rocks and few trachytic and phonolitic plugs and dykes. According to Nkouathio et al.
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(2008), the large compositional range of Mount Bambouto lavas, together with their
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petrographic and geochemical characteristics (occurrence of xenocrysts, zoned crystals,
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coexistence of lavas of different chemical composition from basic to felsic) and
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morphological features (calderas, large lava flows, domes, necks) are in accordance with the
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concept of a large zoned magma chamber below the volcano.
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The reconstitution of the eruptive sequences reveals four phases: (i) the first phase that
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built up the initial basaltic shield volcano extended between ca. 21 and 18 Ma (N’ni, 2004);
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(ii) the second phase, between 16 and 11 Ma (Youmen et al., 2005), is marked by the collapse
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of the caldera subsequent to trachy-rhyolitic ignimbrites outpouring; (iii) the third phase, from
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9 to 4.5 Ma, which reconnects with basaltic effusive activity is coupled with post-caldera
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extrusions of trachytes and phonolites (Marzoli et al., 2000) and (iv) the last and shortest
112
episode took place at ca. 0.5 Ma (Kagou Dongmo et al., 2010) at Totap and yielded the single
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known strombolian cone.
114
Previous works carried out by Nono et al. (2004) and Kagou Dongmo et al. (2010) have
115
studied the phonolitic lava flows associated with the third stage of the volcanic history of the
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Mount Bambouto (Kagou Dongmo et al., 2010). This stage is characterized by effusive
117
activities that lead to a basanite-mugearite serie (15.1–4.5 Ma), associated with post-caldera
118
extrusions of trachytes (15–8.8 Ma), and phonolites (amongst which, the studied Zindeng
119
phonolitic flow) between 12.9 and 5.2 Ma.
120
The Zindeng area is located at about 3 km from the Dschang city, between longitudes
121
10°00’50’’ and 10°01’03’’ East and latitudes 5°25’41’’ and 5°26’06’’ North, covering a
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surface of 220000 m2. It lies on the low land of the southeastern slope of Mount Bambouto
123
(Fig. 2). Here, the Pan-African basement consists mainly of (i) biotite megacrystic granite and
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(ii) banded amphibole gneisses and migmatic amphibole gneisses. This granitic magmatism,
125
dated at 630 – 540 Ma (Tagné Kamga, 2003; Toteu et al., 2004; Kwékam et al., 2010, 2013),
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is a major feature of the central domain of the Pan-African fold belt in Cameroon.
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3. Petrology and structure of the Zindeng area
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The Zindeng area comprises banded amphibole gneisses, biotite megacrystic granite,
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basalt and porphyritic phonolite (Fig. 3). The porphyritic phonolite crops out as flagstones
130
(Fig. 4a and b) and boulders on the flank of a granitic hill. The rock is greenish with
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microlithic porphyritic texture (Fig. 4c). It is made up of sanidine (Or15–55Ab35–85An0–6;
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Nkouathio et al., 2008), felsdpathoids, Na-clinopyroxene (En3–4Aeg33–35Fs62–65; Nkouathio et
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al., 2008), amphibole, plagioclase (An25–11Ab70–54Or22–19; Nkouathio et al., 2008), magnetite
134
and other undetermined oxides.
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The basalt crops out also as flagstone and boulders. The rock is dark in color with
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microlithic porphyritic texture (up to 30% of phenocrysts). It is made up of large subhedral
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phenocrysts of olivine (Fo66-87), diopside (Wo45–48En43–52Fs8–17), augite (Wo38–44En43–53Fs10–23)
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and phenocrysts and microphenocrysts of plagioclase (An46–72) and oxides (Nkouathio et al.,
139
2008).
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The biotite megacrystic granite crops out as flagstones in the valleys and as boulders on
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the flanks and the summits of hills. The rock is greyish or pinkish, fine- to medium-grained. It
142
shows noticeable mineral preferred orientation and contains phenocrysts of perthitic alkali-
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feldspar. Other minerals are quartz, plagioclase and variable amount of hornblende and
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biotite. Magnetite, hematite, titanite, zircon, apatite and epidote are accessory minerals. The
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microstructure is heterogranular to granular. Locally, porphyroclasts of feldspar are moulded
146
in a fine-grained matrix, sometimes with quartz ribbons, pointing to mylonitization.
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Banded amphibole gneisses are found as xenoliths in the phonolitic lava (Fig. 4d) and
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the biotite megacrystic granite. The rock is greyish, medium- to coarse-grained, with
149
millimetric dark bands composed mainly of biotite and hornblende with few plagioclase and
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K-feldspar porphyroclasts. These dark bands alternate with light bands made up of quartz and
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porphyroclasts of K-feldspar. Accessory minerals are apatite, zircon, titanite and oxides.
152
In the phonolite, the magmatic foliation plane is marked by a planar shape-preferred
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orientation of millimetric automorphic K-feldspar phenocrysts (Figs. 4b, c, f). Elongated
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gneissic xenoliths were transposed parallel to this foliation (Fig. 4d). Measurements made in
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the field and on thin sections cut from oriented samples indicate that the foliation strike varies
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from N19°E to N160°E with a mean value of N112°E (Fig. 5).
157
4. Materials and methods
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In a low intensity magnetic field, as a first approximation, AMS is a second-rank tensor
159
whose eigenvectors and eigenvalues, K1 ≥ K2 ≥ K3, define the principal axes of an ellipsoid
160
(Jelinek, 1981; Tarling and Hrouda, 1993). The bulk magnetic susceptibility magnitude is
161
given by Km = (K1 + K2 + K3)/3. The long axis of the AMS ellipsoid (K1), defines the
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magnetic lineation and the short axis (K3), is the pole of the magnetic foliation defined by the
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K1K2 plane.
164
Oriented cylindrical cores, 22 mm high and 25.4 mm in diameter were taken at 29
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stations (Fig. 4), mostly in the phonolite (27 stations) and, to a lesser extent, in basalts (2
166
stations). Two or three oriented cores were collected per station. Each core was cut into at
167
least two cylinders, hence providing a minimum of four samples per station, with a total of
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127 samples analyzed in this study. AMS measurements were made at the LMTG
169
(Laboratoire des Mécanismes et Transferts en Géologie, Toulouse, France), using a KLY-3
170
Kappabridge (Agico, Czech Republic) working at a low alternating inductive field (4 x 10-4 T
171
at 920 Hz) with a sensitivity of about 2 x 10-7 SI, allowing anisotropy discrimination below
172
0.2% over a wide range of susceptibility.
173
Hysteresis measurements were performed, up to 1 T, on two representative phonolite
174
samples, on the Micromag VSM of the CEREGE (Centre de Recherche et d’Enseignement de
175
Géosciences de l’Environnement, Aix-en-Provence, France), to further define the magnetic
176
mineralogy.
177
The AMS data are shown in Table 1, including the data for the two basalt stations that
178
will not be discussed here. In this table, in addition to the orientation of K1, K2 and K3 axes,
179
and to the values of the bulk susceptibility magnitude (Km), we found the total anisotropy
180
percentage [P% = 100 x (P - 1) with P = K1/K3] and the shape parameter of Jelinek (1981) [Tj
181
= 2ln (K2/K3)/(ln (K1/K3)) – 1].
182
5. Bulk magnetic susceptibility
183
Km in the Zindeng phonolite ranges from 196 µSI (station GZ24) to 15679 µSI (station
184
GZ29; Table 1, Fig. 6). The two stations in the basalt (GZ28 and GZ29) have Km values of
185
6971 µSI and 11039 µSI respectively. These values indicate, for the phonolitic rocks (Fig.
186
6b), the coexistence of paramagnetic (Km ˂ 500 µSI; 7% of the stations) and ferromagnetic
187
(sensu lato) behaviors (Km > 1000 µSI; 89% of the stations) with some intermediate cases
188
(500 µSI ˂ Km ˂ 1000 µSI; 4% of the stations) (Rochette, 1987; Bouchez, 2000). The
189
paramagnetic behavior is carried by the iron-bearing silicates, namely Na-clinopyroxenes and
190
amphiboles whereas the ferromagnetic behavior is due to the magnetite. Also, it is common to
191
find at a given station, from one sample to another, para- or ferromagnetic behaviors. This
192
indicates an uneven distribution of the magnetic minerals in the phonolite. It should be
193
mentioned that the stations showing strictly paramagnetic and intermediate para-
194
ferromagnetic behaviors are all located in the southeastern part of the lava flow (Fig. 6).
195
6. Hysteresis data
196
Magnetite, as identified through optical microscopy, is shown above to dominate the
197
magnetic susceptibility in most samples. The two measured hysteresis loops should help to
198
characterize the size, hence the crystal domain structure of this oxide.
199
The shape of the GZ3A2 hysteresis loop is characteristic of pseudo-single domain
200
magnetite, while the “S” shape of the GZ2C2 hysteresis loop is typical of multi-domain
201
magnetite. Accordingly, the two studied samples plot respectively in the PSD and MD fields
202
of a Day et al. (1977) diagram (Fig. 7c). These hysteresis data are consistent with the
203
coexistence of fine and coarse-grained magnetite in the Zindeng phonolite.
204
7. AMS data
205
7.1. Anisotropy degree and shape of the magnetic fabric
206
The degree of magnetic anisotropy (P%) of the Zindeng phonolitic lava flow varies
207
from 0 (station GZ25) to 10% (station GZ13) (Table 1, Fig. 8) with a mean value of 4%. P%
208
values ≥ 5% are mostly located on the northern border of the lava flow (Fig. 8a). It should be
209
noted that P% vs. Km diagram (Fig. 8b) display a roughly positive linear correlation between
210
P% and Km. This suggests that magnetic mineralogy exerts some influence on the
211
eccentricity of the AMS ellipsoid.
212
The shape parameter (Tj) in the Zindeng phonolitic lava flow varies from -0.2 to 0.9,
213
and is positive for 96% of the stations (Table 1, Fig. 9a). Only one station (GZ18),
214
representing 4% of the stations, shows a negative value of Tj. The AMS ellipsoids have thus
215
dominantly an oblate shape, as illustrated by the Tj vs. P% diagram (Fig. 9b). This probably
216
reflects a rock fabric which is also dominantly oblate (Hrouda, 1993; Borradaile and Henry,
217
1997; Borradaile and Jackson, 2004)
218
7.2. Directional data
219
The magnetic foliation within the phonolitic lava is organized around four strike
220
directions: one main direction, NW-SE (52% of the stations) and three subordinate directions,
221
E-W (22% of the stations), N-S (15% of the stations) and NE-SW (11% of the stations)) (Fig.
222
10). Based on this organization of the magnetic foliation, the phonolitic lava flow can be
223
divided into two domains, referred to as Domain I and Domain II (Fig. 10). Domain I, situated
224
close to the granitic pluton, is characterized by a variable trend of the magnetic foliation,
225
locally with strongly curved trajectories defining concentric patterns. Domain II, in the
226
northwestern part of the lava flow, displays a dominant NW-SE trend with some oblique N-S
227
magnetic foliations. The lower hemisphere projection diagrams of the magnetic foliation
228
show a best pole at 207°/68° for Domain I, 190°/51° for Domain II and 192°/56° for the
229
whole lava flow. Hence, in average, the foliation is WNW-ESE striking, i.e. roughly parallel
230
to the NW-SE elongation of the phonolite body, with a moderate dip to the NE
231
(N102°E/34°NNE).
232
It is important to note that the orientation of the magnetic foliation in the phonolitic lava
233
flow is concordant with the planar shape preferred orientation of feldspar phenocrysts that
234
defines the lava flow plane (Fig. 5). In particular, the mean strikes are sub-parallel (N112°E
235
vs. N102°E; Figs. 5 and 10). The magnetic fabric is thus flow related (Rochette et al., 1992).
236
The magnetic lineation (Fig. 11) is organized along N-S (15% of the stations), E-W
237
(29% of the stations), NE-SW (11% of the stations) and NW-SE (45% of the stations) strikes,
238
with plunges of variable values (2°–69°) and directions. Alike the magnetic foliation, the
239
magnetic lineation displays locally strongly curved trajectories close to the granitic pluton
240
(Domain I). In this domain, the lower hemisphere projection diagram indicates a best line at
241
95°/04°. In the northwestern part of the lava flow (Domain II), trajectories of the magnetic
242
lineation, as for the foliation, are oblique on each other, with a dominant NW-SE trend cut by
243
N-S and NE-SW lineations. The best line in Domain II is at 289°/08°. Therefore, the lineation
244
and the foliation trends display similar behaviors in the two domains. The best line for the two
245
domains is 277°/03°, hence, in average, the magnetic lineation is gently plunging to the W.
246
This mean magnetic lineation (N97°E/03°W) is parallel to the strike of the mean foliation
247
(N102°E/34°NNE)
248
8. Discussion
249
8.1. Construction of the Zindeng volcanic edifice
250
P% values in the Zindeng phonolitic lava flow are low (≤ 10%) to very low. According
251
to Henry (1988), P% parameter helps determining the emplacement mechanism of the rock
252
that can be syn-magmatic flow fabric (P% ˂ 20%) or post-magmatic linked to the deformation
253
(P% > 20%). The highest values of the Zindeng phonolitic lava are similar to those found in
254
the basaltic and rhyolitic flows or dykes by Tarling and Hrouda (1993) and Rochette et al.
255
(1999), but lower than those obtained by Morgan et al. (2008) in phonolites and trachytes
256
from the Mesa intrusion. This is consistent with a rather low strain induced by viscous
257
shearing within the flowing magma. The Tj parameter indicates that 96% of the stations have
258
a planar shape ellipsoid (Tj ≥ 0). According to Cañón-Tapia and Castro (2004) and Gountié
259
Dedzo et al. (2011), planar magnetic fabrics in volcanic products are typical of high viscosity
260
volcanic rocks that recorded a pronounced mineral fabric during crystallization. In short, both
261
the P% and Tj parameters has thus led to believe that the Zindeng phonolite is characterized
262
by weakly anisotropic and oblate magnetic fabrics typical of high viscosity volcanic rocks
263
that have recorded well defined mineral fabrics during crystallization.
264
The mean strike of the foliations (N112°E and N102°E, for the feldspars and magnetic
265
fabric, respectively) and the mean magnetic lineation (N97°E/03°E) make a small acute angle
266
with the NW-SE elongation of the phonolite body. This suggests a flowing direction for the
267
Zindeng phonolitic body close to this NW-SE direction.
268
8.2. Emplacement model
269
The AMS directional data (magnetic foliation and magnetic lineation) may be used to
270
reveal the building process of the Zindeng phonolitic lava flow. The magnetic lineation,
271
dominantly of low plunges (˂ 13°) either in Domain I or Domain II with an average E-W
272
direction, is consistent with magma being fed laterally from a source located at or close to, or
273
below one of the two lobes of the phonolitic lava flow. The arcuate, locally strongly curved
274
trajectories of the magnetic foliations and lineations in Domain I, together with the
275
dominantly moderate to steep dip of the magnetic foliations in this area suggest that Domain I
276
was emplaced as a dome above a feeding conduit at depth (Fig. 12). The magnetic fabrics of
277
Domain II are rather evocative of a flow domain sensus stricto characterized by more regular
278
foliation and lineation trajectories roughly parallel to the direction of the phonolite body
279
suggesting that the lava propagated towards the NW. Here, the magnetic foliations and
280
lineations have NW-SE, to locally E-W trends cut by N-S, to locally NE-SW trends (Figs. 10
281
and 11). The NW-SE to E-W trends may correspond to the direction of magma propagation,
282
while the N-S to NE-SW trends may be flow fronts. In short, the viscous phonolitic magma
283
rose from a feeding conduit located below the SE lobe of the Zindeng phonolite, close to the
284
granitic pluton, and propagated towards the NW as schematically illustrated in Fig. 12.
285
Conclusion
286
The Zindeng phonolite is a dominantly ferromagnetic lava flow characterized mostly by
287
weakly anisotropic and oblate magnetic fabrics typical of high viscosity volcanic rocks that
288
have recorded well defined mineral fabrics during crystallization. Feldspars and magnetic
289
foliations, as well as magnetic lineations strike close to the NW-SE direction of the phonolite
290
body pointing to (i) flow-related magnetic fabrics and (ii) a flow direction at low angle to the
291
phonolite elongation. The locally strongly curved trends of the foliation and lineation
292
trajectories, and the mostly moderate to steep foliation dips, close to the contact with the
293
granitic pluton in the southeastern part of the lava flow, points to emplacement of this domain
294
as a dome above a feeding conduit at depth. The northwestern part of the lava flow is
295
characterized by NW-SE to E-W foliation and lineation trajectories (flow trends) cut by N-S
296
to NE-SW trajectories (flow fronts). The phonolitic magma rose from depth below the SE
297
domain (dome) and propagated laterally towards the NW domain.
298
Acknowledgements
299
Gountié Dedzo is grateful to EGIDE (Centre français pour l'accueil et les échanges
300
internationaux), SCAC (Service de Coopération et d'Action Culturelle de la France au
301
Cameroun) and the French Government for funding its stay in UMR 5563-GET-OMP
302
(Toulouse – France). Field work was partially supported by the IRD-CORUS 2 project of M.
303
Jessell and J.L. Bouchez from LMTG (Toulouse). Warm thanks to P. Rochette for hyteresis
304
measurements and to R. Siqueira and F. de Parseval for technical assistance. Constructive
305
reviews by R. Ernst and one anonymous reviewer, as well as careful editing by R.B.
306
Mthanganyika Mapeo, are also greatly acknowledged.
307
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Table 1 Cordinates Station
Long (N)
(°)
Lat (E)
AMS parameters N
(°)
Km
P%
Mean eigen vectors
T
(µSI)
K1 Dec
Inc
K2
K3
Dec Inc
Dec Inc
Phonolite GZ01
10.0151
5.4321
4
6709
6 0.2
293
49
187
14
85
38
GZ02
10.0153
5.4321
5
11764
7 0.5
228
67
132
2
41
26
GZ03
10.0150
5.4319
6
7072
3 0.2
81
28
323
43
192
37
GZ04
10.0151
5.4321
3
12888
8 0.7
175
34
304
41
60
29
GZ05
10.0149
5.4322
5
4598
5 0.6
91
10
343
36
183
57
GZ06
10.0146
5.4324
2
6464
7 0.9
117
2
26
16
203
71
GZ07
10.0146
5.4322
6
14874
4 0.2
277
20
24
39
167
44
GZ08
10.0146
5.4320
2
7565
2 0.7
192
6
284
30
93
63
GZ09
10.0148
5.4321
5
10308
5 0.7
84
14
344
33
193
54
GZ10
10.0150
5.4318
5
3245
2 0.5
302
4
35
45
208
46
GZ11
10.0152
5.4316
6
4891
2 0.7
111
13
357
60
209
27
GZ12
10.0162
5.4313
4
11959
4 0.7
297
9
26
47
197
49
GZ13
10.0162
5.4313
5
14340
10 0.7
194
69
334
16
69
13
GZ14
10.0161
5.4313
5
9772
8 0.7
58
8
324
32
161
58
GZ15
10.0156
5.4316
5
15679
9 0.7
240
64
125
12
29
23
GZ16
10.0153
5.4317
4
1912
2 0.9
327
34
74
24
192
47
GZ17
10.0152
5.4318
5
2000
4
15
110
4
218
58
GZ18
10.0162
5.4310
4
6278
3 0.1 5 0.2
102
9
194
5
341
81
GZ19
10.0159
5.4310
3
12802
3 0.3
4
35
283
10
157
67
GZ20
10.0164
5.4309
6
13189
7 0.7
322
48
172
38
70
15
GZ21
10.0165
5.4321
4
11915
9 0.8
327
19
73
40
218
45
GZ22
10.0166
5.4304
4
663
3 0.0
41
25
159
46
292
33
GZ23
10.0164
5.4302
4
3883
3 0.4
357
44
208
64
234
3
GZ24
10.0161
5.4304
4
196
1 0.3
92
16
180
2
242
72
GZ25
10.0159
5.4306
5
266
0 0.1
327
16
217
36
82
58
GZ26
10.0160
5.4308
5
11530
5 0.5
79
21
341
18
216
61
GZ27
10.0162
5.4306
4
10681
5 0.8
73
8
342
19
191
70
GZ28
10.0135
5.4327
3
11110
4 0.6
224
10
314
5
71
77
GZ29
10.0136
5.4330
4
6971
2 0.0
222
4
314
19
192
36
Basalt
Table 2 Station GZ2C2 GZ3A2
m (g) 1.463 1.420
Km (Sample) Km (Station) (µSI) (µSI) 11747 7776
11671 7072
P% (Sample)
P% (Station) Khf (10-6 SI)
8 3
7 3
339 277
%para Ms 3 15
675 95
Mrs/Ms 0.043 0.058
Hcr/Hc 4.21154 3.67857
Hcr (mT) 21.9 20.6
Table 1. AMS data for the Zideng phonolitic lava flow and also the basalt flow. Km, mean magnetic susceptibility; P%, total anisotropy percentage; Tj, shape parameter of Jelinek (1981). K1, K2 and K3, long, intermediate and short axes of the AMS ellipsoid; Dec and Inc, declination and inclination of the axes in degrees.
Table 2. Rock magnetic parameters from hysteresis measurements of two ~1 cm3 samples. Khf, high field susceptibility; % para, percent of "paramagnetic" susceptibility; Ms, saturation magnetization; Mrs, remanent saturation magnetization; Hc, coercive field; Hcr, remanent coercive field.
Fig. 1.
Fig. 2
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Fig. 9.
Fig. 10.
Fig. 11.
Fig. 12.
Figure captions Fig. 1. Geological map of the CVL. (a) Location of Cameroon in Africa showing the different cratons; (b) Distribution of Cameroon Line volcanism (after Njonfang et al., 2011). Locations of seamounts are after Burke (2001). C.A.R = Central African Republic; SA = Study area
Fig. 2. Digital elevation model (DEM) of the central part of the Cameroon Volcanic Line (after Gountié Dedzo et al., 2011) showing the position of (i) Mounts Bambouto and Bamenda and the close surroundings and (ii) the Zindeng area. The different calderas are also indicated.
Fig. 3. (a and b) Field relationships between the Zindeng phonolitic lava flow, the basalt and the Dschang granite; (c) Geological map of the study area with location of the sampling stations.
Fig. 4. Field photographs and microphotographs (crossed polars). Field evidence of flow direction (a, b and d): (a) View of the phonolotic vent marked by rough vertical columnar jointing (Menouet river right bank). Note the vertical fractures within the rock; (b) Phonolitic flagstone outcrop; (c) Hand specimen of phonolite; (d) Gneiss xenoliths within the phonolite; Thin sections of porphyric phonolite (note the phenocrystals of sanidine within an aphyric matrix) (e) and aphyric basalt (f). LFD = trace of the planar shape-preferred orientation of millimetric automorphic K-feldspar phenocrysts; Sa = sanidine, Mgt = magnetite.
Fig. 5. Map showing the trace of the foliation defined by the K-feldspar phenocrysts, with lower hemisphere, equal-area projection diagrams.
Fig. 6. Magnetic susceptibility (Km). (a) Distribution map of the Km values and (b) susceptibility histograms.
Fig. 7. Hysteresis data. (a and b) Hysteresis loops for representative phonolite samples (GZ2C and GZ3A); (c) Mrs/Ms vs Hcr/Hc diagram of samples after Day et al. (1977) and Dunlop (1986). PSD = pseudo-single-domain; MD = multi-domain.
Fig. 8. Degree of magnetic anisotropy (P%). (a) Distribution map of the P% values and (b) P% vs Km diagram.
Fig. 9. Jelinek shape parameter (Tj). (a) Distribution map of the Tj values and (b) Tj vs P% diagram.
Fig. 10. Map of the magnetic foliations and equal-area projections of the foliation poles AMS parameter (lower hemisphere).
Fig. 11. Magnetic lineation. (a) Map and equal-area projections (lower hemisphere) and (b) plunge histogram.
Fig. 12. 3D emplacement model for the Zindeng phonolitic flow lava.
Highlights -
-
We investigated the fabric of the Zindeng phonolitic lava flow located on the southeastern flank of the Mount Bambouto through AMS measurements. The AMS scalar parameters (Km, P% and T) and directional data (magnetic foliation, normal to K3 and magnetic lineation K1) indicate a dominantly magnetite–dominated magnetic fabric, that is flow-related. A model of magma feeding from a source at depth followed by lateral flow is proposed for the Zindeng phonolite.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: