Trace element geochemistry and U–Pb ages of alluvial rutiles from a neoproterozoic coastal area, southwestern Cameroon: Implication for source rocks

Abstract

This study focuses on alluvial rutile from Nkolembonda in the southwestern Cameroon. The studied rutile grains were analyzed using mineral chemistry and geochronology. Rutile chemistry shows Cr, Nb and Zr concentrations varying from 418 to 582 ppm, from 201 to 4070 ppm and from 60 to 272 ppm, respectively. The source area discriminant diagram based on the Cr–Nb concentrations indicates that the majority of the investigated alluvial rutile is originated from metapelitic rocks, whereas few alluvial rutile grains are derived from metamafic rocks. The calculated rutile formation temperatures at P = 10 kbar (based on the Zr-in rutile geothermometry) vary from 528 to 635 °C, with an average temperature of 595 °C. Zr-in-rutile geothermometer yielded coincident temperatures for both metapelitic-and metamafic-derived rutiles. This demonstrates that the source rocks of the alluvial rutiles underwent similar metamorphic conditions and have similar metamorphic history.The alluvial rutile LA-ICP-MS U–Pb dating yielded ages in the time range of ca 534 to 760 Ma, consistent with the age of the Pan-African orogeny. Trace-element compositions, Zr-in-rutile geothermometry and rutile LA-ICP-MS U–Pb geochronology show that alluvial rutile grains were derived from Middle to Late Neoproterozoic rocks that underwent amphibolite-facies metamorphism. Amphibolite-facies metamorphic rocks in the Yaoundé Group and nepheline metasyenite outcropping in the southwestern Cameroon coastal zone seem to be the primary source rocks for the alluvial rutile in the study area.