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

Morphological and geochemical features of both in-rock and alluvial ruffle within the Yaounde 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 x 0.8 mm) and occurs as euhedral to subhedral crystals, either within the matrix or included in garnet, biotite, kyanite and plagioclase. The alluvial ruffle is coarse-grained and ranges in size from several millimetres to 5 x 2 cm. The alluvial ruffle shapes (angular to sub-blunt) indicate local sedimentation or more or less important hydromechanical transport. Alluvial ruffle is Al, Fe and Sb-rich compared to in-rock ruffle. The Cr and Nb abundances and the <= 0.02 Th-232/U-238 ratios suggest a common origin (metapelitic and metamafic) for both alluvial and in-rock ruffle. The Zr-in-ruffle thermometer indicates that coarse-grained alluvial ruffle formed under blueschist and greenschist facies temperature conditions (514-561 degrees C), while the inrock ruffle was formed under amphibolite to granulite facies temperature conditions (614-726 degrees C). Alluvial ruffle temperatures are symptomatic of pegmatites and high-grade hydrothermal bodies formed during late-gage tectono-metamorphic episodes of the Pan-African Orogeny. The temperature evolution depicted from in-rock ruffles indicates a regional metamorphism dominated by retrogression from granulite to amphibolite facies from central to coastal Cameroon.