Geochemistry of rutile from the Pan-African Yaoundé metamorphic group: Implications for provenance and conditions of formation

Abstract

Morphological and geochemical features of both in-rock and alluvial rutile within the Yaoundé Group of the Pan-African Central African Orogenic Belt (CAOB) were assessed, using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) in order to assess its provenance and condition of formation. Rutile in gneiss and micaschist is fine grained (<0.2 × 0.8 mm) and occurs as euhedral to subhedral crystals, either within the matrix or included in garnet, biotite, kyanite and plagioclase. The alluvial rutile is coarse-grained and ranges in size from several millimetres to 5 x 2 cm. The alluvial rutile shapes (angular to sub-blunt) indicate local sedimentation or more or less important hydromechanical transport. Alluvial rutile is Al, Fe and Sb-rich compared to in-rock rutile. The Cr and Nb abundances and the ≤0.02 232Th/238U ratios suggest a common origin (metapelitic and metamafic) for both alluvial and in-rock rutile. The Zr-in-rutile thermometer indicates that coarse-grained alluvial rutile formed under blueschist and greenschist facies temperature conditions (514–561 °C), while the in-rock rutile was formed under amphibolite to granulite facies temperature conditions (614–726 °C). Alluvial rutile temperatures are symptomatic of pegmatites and high-grade hydrothermal bodies formed during late-stage tectono-metamorphic episodes of the Pan-African Orogeny. The temperature evolution depicted from in-rock rutiles indicates a regional metamorphism dominated by retrogression from granulite to amphibolite facies from central to coastal Cameroon.