Garnet zoning in high pressure granulite-facies metapelites, Mozambique belt, SE-Kenya: constraints on the cooling history

摘要

Three metapelitic samples from the granulite facies Taita Hills, part of the Neoproterozoic Mozambique belt in SE-Kenya, contain nearly pure almandine-pyrope garnets. These garnets show a diffusional zoning of X_Fe = Fe/(Fe+Mg) at the rim over a distance of 200-500 µm if in contact with biotite. Garnet-biotite Fe-Mg exchange thermometry yields closure temperatures between 530–735°C. Diffusion zoning profiles in garnets are used to estimate cooling rates using a numerical model. For the calculations a metamorphic peak temperature and pressure of 820°C and 1.15 GPa are obtained from mafic granulites. Matching of numerically modelled and observed zoning profiles indicates cooling rates between 1–3°C/my. Comparison with cooling rates estimated with the analytical approach of Ehlers & Powell (1994) and with geochronologically derived cooling rates shows that the volumetric ratio of biotite to garnet was about 0.5 during closure. This is consistent with the volumetric ratio observed in thin section, but inconsistent with microprobe analyses that indicate that only biotite in the immediate vicinity of garnet equilibrated with garnet. Conversely, significant garnet zoning only occurs where in contact with biotite. We suggest that these inconsistencies can be explained with changes in the grain boundary processes during cooling: in the thermal evolution above the closure temperature around 735°C a fast grain boundary model applied so that all biotite in the thin section equilibrated with garnet. At lower temperatures local zoning developed, but did not influence the composition of the garnet grain centers. The change in grain boundary process from fast to slow diffusing grain boundaries may correlate with the solidus temperature of the rock.