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

摘要

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 deg 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 deg 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 deg 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.