FeO~*-Al_2O_3-TiO_2-Rich Rocks of the Tertiary Bana Igneous Complex, West Cameroon

摘要

FeO~*-Al_2O_3-TiO_2-rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plu-tono-volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron-titanium oxides, aluminosilicates and phosphates, and occur as layers 1-3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilrnenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe-Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al-rich fine-gained massive type. The first two types consist of dark globule or band enriched in Fe-Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilrnenite in the FeO~*-Al_2O_3-TiO_2-rich rocks suggest temperatures of crystallization in a range of 690-830 deg C at low pressures. The Bana FeO~*-Al_2O_3-TiO_2-rich rocks are characterized by low concentrations of SiO_2 (25-54.2 wt percent), Na_2O + K_2O (0-1 percent), CaO (0-2 percent) and MgO (0-0.5 percent), and high concentrations of FeO~* (total iron as FeO, 20-42 percent), Al_2O_3 (20-42 percent), TiO_2 (3-9.2 percent), and P_2O_5 (0.26-1.30 percent). TiO_2 is positively correlated with Al_2O_3 and inversely correlated with FeO~*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al_2O_3)-(CaO + Na_2O + K_2O)-(FeO~* + MnO + MgO) and (SiO_2)-(FeO~*)-(Al_2O_3), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high-temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO~*-Al_2O_3-TiO_2-rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas.