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The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources |
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| Authors: | MARZOLI A; PICCIRILLO E M; RENNE P R; BELLIENI G; IACUMIN M; NYOBE J B; TONGWA A T |
| Institution: | 1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITY OF TRIESTE, VIA E. WEISS 8, 34127 TRIESTE, ITALY 2BERKELEY GEOCHRONOLOGY CENTER, 2455 RIDGE ROAD, BERKELEY, CA 94709, USA 3DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF CALIFORNIA, BERKELEY, CA 94720, USA 4DIPARTIMENTO DI MINERALOGIA E PETROLOGIA, UNIVERSITY OF PADOVA, CORSO GARIBALDI 37, 35137 PADOVA, ITALY 5MINISTÈRE DE RECHERCHE SCIENTIFIQUE TECHNOLOGIQUE, B.P. 4110, YAOUNDÉ, CAMEROON 6LABORATORY FOR EARTHQUAKE CHEMISTRY, UNIVERSITY OF TOKYO, HONGO BUNKYO-KU, TOKYO 113, JAPAN |
| Abstract: | The volcanic activity of Mts Bambouto and Oku (Western Highlands)and of the Ngaoundere Plateau, in the continental sector ofthe Cameroon Volcanic Line, Equatorial West Africa, ranges inage from Oligocene to Recent. It is characterized by basanitic,alkali basaltic and transitional basaltic series. Mineral chemistry,major and trace element bulk-rock compositions, and geochemicalmodelling suggest that the magmatic series evolved mainly atlow pressure (2–4 kbar) through fractional crystallizationof clinopyroxene and olivine ± magnetite, at moderatelyhydrated (H2O = 0·5–1 wt %) and QFM (quartz–fayalite–magnetite)to QFM + 1 fO2 conditions. Basalts from Ngaoundere (Mioceneto Quaternary) and from the early activity (31–14 Ma)of the Western Highlands have incompatible trace element andSr–Nd isotopic compositions similar to those of oceanicCameroon Line basalts, pointing to a similar asthenosphericmantle source. By contrast, the late (15–4 Ma) WesternHighlands basanites and alkali basalts have anomalously highconcentrations of Sr, Ba and P, and low concentrations of Zr,which are exclusive features of continental Cameroon basalts.The genesis of these latter magmas is consistent with derivationfrom an incompatible element enriched, amphibole-bearing lithosphericmantle source. Western Highlands basalts show a continuous spectrumfrom high to low Sr–Ba–P compositions, and may resultfrom variable amounts of mixing between melts derived from ananhydrous lherzolite source (asthenospheric component) and meltsfrom an amphibole-bearing peridotite source (lithospheric HSrcomponent). New 40Ar/39Ar ages for Mts Oku and Bambouto basalts,combined with previous 40Ar/39Ar and K/Ar ages of basaltic andsilicic volcanics, and with volcanic stratigraphy, suggest aNE–SW younging of the peak magmatic activity in the WesternHighlands. This SW younging trend, extending from the Oligocenevolcanism in northern Cameroon (e.g. Mt Oku) to the still activeMt Cameroon, suggests that the African plate is moving abovea deep-seated mantle thermal anomaly. However, the age and locationof the Ngaoundere volcanism does not conform to the NE–SWyounging trend, implying that the continental sector of theCameroon Volcanic Line cannot be easily interpreted as the surfaceexpression of a single hotspot system. KEY WORDS: Cameroon Line basalts;40Ar/39Ar geochronology; lithospheric and asthenospheric mantle source; hotspot |
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