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Modeling of retrograde diffusion zoning in garnet: evidence for slow cooling of granulites from the Highland Complex of Sri Lanka

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¶Diffusion modeling of zoning profiles in garnet rims from mafic granulites is used to estimate cooling rates in the Proterozoic basement of Sri Lanka, which represents a small, but important fragment of the Gondwana super-continent. Metamorphic peak temperatures and pressures, estimated with two-pyroxene thermometry and garnet–clinopyroxene–plagioclase–quartz (GADS) barometry, yield 875±20 °C and 9.0±0.1 kbar. These peak metamorphic conditions are slightly higher than results obtained by garnet-biotite Fe–Mg exchange thermometry of 820±20 °C. Reset flat zoning profiles were observed in most garnets. Only narrow garnet rims touching biotite exhibit retrograde zoning in terms of Fe and Mg exchange.

The garnet zoning observed requires a slow cooling history. Equilibrium was achieved along grain boundaries during or close to peak metamorphism. During subsequent cooling to lower temperatures, only local exchange between garnet and biotite occurred. A cooling rate of 1–5 °C/Ma is estimated. The estimated temperature-time history from garnet profiles is in good agreement with the cooling history inferred from mineral radiogenic ages in the literature.

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Received December 11, 2001; revised version accepted August 28, 2002

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Fernando, G., Hauzenberger, C., Baumgartner, L. et al. Modeling of retrograde diffusion zoning in garnet: evidence for slow cooling of granulites from the Highland Complex of Sri Lanka. Mineralogy and Petrology 78, 53–71 (2003). https://doi.org/10.1007/s00710-002-0224-1

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  • DOI: https://doi.org/10.1007/s00710-002-0224-1

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