Petrology of ultramafic nodules from Sao Tome Island, Cameroon Volcanic Line (oceanic sector)

Ultramafic xenoliths were found in recent alkali basalts from Sao Tome Island. These include spinel peridotites (Iherzolites, harzburgites and dunites) and pyroxenites (orthopyroxenites and clinopyroxenites). Textures and mineral compositions indicate that pyroxenites originated from crystal/liquid separation processes operating on magmas similar to those giving rise to their present host rocks whereas spinet peridotite xenoliths had an accidental origin; Fo (>89) and Ni (>0.36 wt.%) contents in olivines, Mg# (91-95) of orthopyroxenes and low Ti in clinopyroxene (primary crystals: TiO2 < 0.06 wt.%) and in spinel (TiO2 < 0.1 wt.%) are within the range reported for abyssal peridotites, indicating Sao Tome spinet peridotites represent refractory residues of melting. Nevertheless, the lack of correlation between mineral chemistry and modal composition suggests that spinel peridotite xenoliths are not simple residues and were affected by infiltration of fluid/melts within the mantle. The wide temperature range obtained for spinel peridotites (700 to >1150 degreesC) is compatible with a long period of pre-entrainment cooling supporting Fitton's [Tectonophysics 94 (1983) 473] hypothesis that proposes oceanic lithosphere uprising in the Cameroon Volcanic Line prior to the initiation of the current thermal regime, related to Sao Tome magmatism. The association of upper mantle (peridotite) xenoliths with igneous cumulates (pyroxenites) suggests that the spinel peridotite suite originated in the uppermost mantle above the Sao Tome magma storage zone(s), probably in a region of high strain rate, near the boundary between the mantle and the overlying oceanic crust. (C) 2002 Elsevier Science Ltd. All rights reserved.