First occurrence of pigeonite in the Cameroon Volcanic Line
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ORIGINAL PAPER
First occurrence of pigeonite in the Cameroon Volcanic Line Jacques-Marie Bardintzeff 1
&
Oumarou Faarouk Nkouandou 2 & Aminatou Fagny Mefire 2
Received: 18 November 2019 / Accepted: 16 May 2020 # Saudi Society for Geosciences 2020
Abstract Low-Ca clinopyroxene crystal of pigeonite composition has been discovered in the Tchabal Mbabo volcano massif of the Cameroon Volcanic Line (CVL), a long active N30° chain of volcanic centres and ring complexes in Central Africa. The Tchabal Mbabo is a huge Eocene-Oligocene volcano, at the junction with Adamawa Plateau (AP), made of lava flows and domes, which display a rather complete alkaline series from basanite and basalt to peralkaline trachyte and rhyolite. Pigeonite phase was found, as small crystals (30–50 μm) included in K-feldspar, which is in turn included in a biotite, in the Gouri peralkaline trachytic dome-flow. We describe in this paper for the first time the occurrence of modal pigeonite phase in the CVL. Occurrence of pigeonite phase in peralkaline trachyte does not evidence tholeiitic/transitional affinities but is considered resulting from silica saturation during differentiation of the alkaline series. Low (87Sr/86Sr)i (0.704277) and high εNdi (+ 1.25) of trachyte, close to OIB rather than FOZO reservoir, preclude any important crustal contamination. Keywords Cameroon . Tchabal Mbabo . Dome-flow . Peralkaline trachyte . Pigeonite . Silica saturation
Introduction Pigeonite is a Ca-poor monoclinic pyroxene, which contains between 5 and 20 wt% CaSiO3 (wollastonite (Wo)) (Deer et al. 1966; Morimoto et al. 1988). The occurrence of both modal and CIPW-normative clinopyroxene (pigeonite) and/or orthopyroxene (hypersthene) has been used as a petrogenetic marker for a long time (Kuno 1968; Irvine and Baragar 1971; Stout and Nicholls 1977; Le Maitre 2002). Pigeonite is described in tholeiitic, transitional, and calc-alkaline volcanic series (see Deer et al. 1963; Carmichael et al. 1974; Kuno 1969; Nakamura and Kushiro 1970a, b), but is absent or exceptional Responsible Editor: Domenico M. Doronzo * Jacques-Marie Bardintzeff [email protected] Oumarou Faarouk Nkouandou [email protected] Aminatou Fagny Mefire [email protected] 1
Sciences de la Terre, Volcanologie, Planétologie, UMR CNRS 8148 GEOPS, University of Paris-Saclay, Bat. 504, F-91405 Orsay, France
2
Faculty of Science, University of Ngaoundere, P. O. Box. 454, Ngaoundere, Cameroon
in alkaline series (e.g. in the French Massif Central, Maury and Brousse 1978). In addition, pigeonite occurs in some Lunar (Nagaoka et al. 2015) and Martian meteorites (shergottite, discussion in Christensen et al. 2004) and is suspected in the Martian crust (discussion in Poulet et al. 2009). Tchabal Mbabo massif is an Eocene-Oligocene voluminous volcano (Fagny et al. 2016a; Fagny 2017) in Adamawa Plateau (AP), part of the Cameroon Volcanic Line (CVL) in central Africa (Fig. 1). CVL is known worldwide as a long, still active, tectono-magmatic chain of volcanic centres and ring complexe
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