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

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(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

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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

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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

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whole lava flow. Hence, in average, the foliation is WNW-ESE striking, i.e. roughly parallel

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to the NW-SE elongation of the phonolite body, with a moderate dip to the NE

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(N102°E/34°NNE).

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It is important to note that the orientation of the magnetic foliation in the phonolitic lava

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flow is concordant with the planar shape preferred orientation of feldspar phenocrysts that

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defines the lava flow plane (Fig. 5). In particular, the mean strikes are sub-parallel (N112°E

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vs. N102°E; Figs. 5 and 10). The magnetic fabric is thus flow related (Rochette et al., 1992).

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The magnetic lineation (Fig. 11) is organized along N-S (15% of the stations), E-W

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(29% of the stations), NE-SW (11% of the stations) and NW-SE (45% of the stations) strikes,

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with plunges of variable values (2°–69°) and directions. Alike the magnetic foliation, the

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magnetic lineation displays locally strongly curved trajectories close to the granitic pluton

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(Domain I). In this domain, the lower hemisphere projection diagram indicates a best line at

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95°/04°. In the northwestern part of the lava flow (Domain II), trajectories of the magnetic

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lineation, as for the foliation, are oblique on each other, with a dominant NW-SE trend cut by

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N-S and NE-SW lineations. The best line in Domain II is at 289°/08°. Therefore, the lineation

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and the foliation trends display similar behaviors in the two domains. The best line for the two

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domains is 277°/03°, hence, in average, the magnetic lineation is gently plunging to the W.

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This mean magnetic lineation (N97°E/03°W) is parallel to the strike of the mean foliation

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(N102°E/34°NNE)

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8. Discussion

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8.1. Construction of the Zindeng volcanic edifice

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P% values in the Zindeng phonolitic lava flow are low (≤ 10%) to very low. According

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to Henry (1988), P% parameter helps determining the emplacement mechanism of the rock

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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

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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

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from the Mesa intrusion. This is consistent with a rather low strain induced by viscous

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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

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The AMS directional data (magnetic foliation and magnetic lineation) may be used to

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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 -

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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: