Geochemistry of the Bayonplutonic Complex – Western Cameroon

Abstract

The BayonNeoproterozoic plutonic complex located in Western Cameroon intrudes gneisses of Paleo to Neo Proterozoic age. The complex is composed of gabbro, monzogabbro and monzonites frequently crosscut by trachytic and granitic veins. The primary mineral assemblages of the gabbro and monzogabbro is plagioclase (An₃₀ - An₆₉), clinopyroxene (En_{40 – 42} Fs _{12 – 18} Wo _{45 - 47}) hypersthene (En_{62 – 65} Fs_{34 – 37} Wo_{1- 4}) and orthoclase (Or₇₈-Or₉₁) whilebiotite, magnetite, Ilmenite and apatite constitute accessory minerals. Monzonite is formed of plagioclase (An22 to An₃₉), orthoclase (Or₈₀-Or₈₇),clinopyroxene (En_38-39 Fs_14-20 Wo_45-46), biotite and quartz. Amphiboles occur as secondary minerals. Ilmenite and apatite are accessory minerals in monzonite.The rocks are mafic to intermediate in composition (41 – 61 wt % SiO₂) and transalkaline with high-K and have shoshonitic features. Bayon plutonic rocks have high
abundance of Ba, Sr ,V and Zr but possesses low concentrations of Rb, Sc, Y and Th.Gabbroic rocks show moderately fractioned REE patterns (La_N/Lu_N= 14 – 27) with none or negligible Eu anomalies. The monzonite shows also moderate fractionatedpatterns (La_N/Lu_N= 20 – 27) with fairly positive Eu anomalies. All the studied rocks show flat HREE features. The primitive mantle- normalized element patterns are almost homogeneous with negative anomalie in Ta, Nb, Th, P and Ti. Sm/Nd-wr-Cpx-Pl ages of the complex are 580 ± 13 Ma; 553 ± 32 Ma for the monzogabbro and 547 ± 26 Ma for the monzonite. The Nd/Sr isotopic compositions show that the Bayon plutonic rocks were generated by partial melting of subcontinental lithospheric mantle. The depleted mantle Nd model age TD_M of 1.6 – 1.7 Ga indicates that the studied rocks originated by partial melting of Mesoproterozoic mantle.

KEYWORDS: West Cameroon, Panafrican, Pluton, Geochemistry, partial meting