Volcanic history and magma systems of Lake Monoun Polygenetic Maar, Noun Plain, Western part of the Cameroon Volcanic Line: Constraints from stratigraphy, chronology and geochemistry

Abstract

Lake Monoun (LM) is a maar volcano located on the Cameroon Volcanic Line in West-Central Africa. In 1984, a limnic eruption occurred at LM, releasing large amounts of magmatic CO₂ gas that had accumulated in the lake. The CO₂ gas asphyxiated 37 people in the surrounding area. Despite this hazard, the volcanic history of LM maar remains unknown. This study presents the first comprehensive results of lithostratigraphy, physical volcanology, ¹⁴C dating and petrology of LM eruptive products. The lake has a WSW-ENE-aligned morphology with three main craters, the walls of which expose tephra and lava that can be divided into 5 stratigraphic units (A, B, C, D, and E from bottom to top). Unit A, a pyroclastic surge deposit in the NW, was produced by a phreatomagmatic eruption and deposited on a thick (ca.170 cm) paleosol developed on granitic basement. The paleosol yields a ¹⁴C age of ca 1.3 cal. ka. Units B (weathered scoria fall) and C (lava flow) in the NW part of the lake were respectively produced by strombolian and effusive eruptive activities. The SE side consists of unit D (scoria fall) a product of violent strombolian activity and unit E (voluminous surge) deposited by phreatomagmatic activity. The surge deposits of units A and E are rich in lithic fragments such as granite and lava, indicating subterranean excavation of country (accidental) and volcanic (accessory) rocks. A short time break represented by thin (ca. 15 cm, 0.1–0.3 cal. ka) paleosol can be recognized between units A and E in the distal facies. Based on the temporal and spatial distribution of these deposits, the eruption history of LM can be grouped into two stages. The first stage led to the development of the western and central craters and is recorded by units A to C in the NE section. The second stage proceeded after a short hiatus represented by the thin paleosol and a shift of the eruptive locus to the SE part of the lake. This gave rise to the largest eastern crater characterized by well-preserved units D and E. The LM case study reveals a potential time-space evolutionary pathway for crater formation, implying a polygenetic origin. Juvenile materials from the units are basanitic in composition with narrow chemical variation, suggesting a common magma system through the two stages. Silica content gradually increases from units A to E (SiO₂ = 43.4 to 45.8 wt%), suggesting that the magma was differentiated with time (from units A to E) and tapped from the whole part of the chamber at unit E. Accessory lava fragments in units A and E display three distinct geochemical trends, composed of alkali and subalkaline basalts, implying the presence of diverse magma systems around the area prior to the maar formation.