Low temperature grain-scale retrograde alteration of detrital minerals in Ajali formation from Benin Flank Of Anambra Basin, Nigeria

Abstract

Petrographical study of detrital and authigenic minerals from the Ajali Formation in the Benin flank of Anambra Basin was carried out with the aim of interpreting the textural characteristics and alteration patterns. Samples were collected from Ayowgiri sand quarry site and along Fugar-Agenebode road where good exposures of the Ajali Sandstone occurred. Heavy minerals were concentrated using bromoform according to standard procedures. The heavy mineral concentrates were mounted on glass slides with araldite glue. In addition, thin sections of sandstone were prepared. The samples were studied with the aid of polarizing microscopes. The textural features and relationships among the heavy minerals were used in the interpretations. The results indicate that the detrital heavy minerals (Fe-Ti oxides and garnet) have undergone a very low-temperatureretrograde changes rather than ordinary in-situ dissolution by weathering and burial diagenesis. The alteration of Fe-Ti oxides resulted into the formation of leucoxene and chlorite while that of garnet resulted only into the formation of chlorite. Detrital magnetite was partially dissolved with skeletal remains while the authigenic grains showed well preserved structures. The re-equilibration of these minerals and partial dissolution were probably due to rise in temperature and change in the chemistry of the formation water. Reducing and acidic conditions at elevated temperature probably favoured dissolution and alteration of the detrital Fe-oxides while the formation of authigenic hematite was aided by oxidizing condition. Temperature increase was probably induced by either underneath thin lithospheric plate or igneous activities at the center of the basin. The conversion of the detrital minerals to authigenic chlorite and partial replacement structures represent a typical case of retrograde reaction which could be described as evidence of a very low-temperature anchi-metamorphism at mineral grain level. Key words: Authigenic, chlorite, Fe-oxides, garnet, retrograde, anchi-metamorphism.