First occurrence of pigeonite in the Cameroon Volcanic Line

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 eNdi (+ 1.25) of trachyte, close to OIB rather than FOZO reservoir, preclude any important crustal contamination.