Tchabal Gangdaba massif in the Cameroon Volcanic Line: a bimodal association

Abstract

Tchabal Gangdaba (TG) volcanic massif, which is a part of the continental sector of the Cameroon Volcanic Line (CVL), is dated between 34.4 and 25.1 Ma. It displays mafic lavas (picrobasalt and basanite, 41–43 wt % SiO2) and felsic lavas (rhyolite, 68–73 wt % SiO2). The lack of intermediate rocks evidences a pronounced Daly gap between 43 and 68 wt % SiO2, which corresponds to an important time span of 3.4 Ma. It is interpreted as due to extensive fractional crystallization under peculiar thermodynamical conditions. Felsic lavas yield strong negative anomalies in Ba, Sr and Eu (0.1 < Eu/Eu* < 0.3) reflecting alkali feldspar and plagioclase fractionation and in Ti reflecting titanomagnetite and ilmenite fractionation. All TG rocks are enriched in LREE and mildly fractionated, which suggests enriched mantle sources. Sr initial isotopic ratios ranging from 0.7033 to 0.7059, eNd from +2.89 to +4.64 and Pb isotopic ratios of 18.988 < 206Pb/204Pb < 19.998, 15.592 < 207Pb/204Pb < 15.673 and 38.948 < 208Pb/204Pb < 39.648 are typical of an enriched mantle source close to HIMU and FOZO. A discrete Sr crustal leaching is evidenced.