Petrology and rare earth element geochemistry of clastic metasedimentary rocks from the Isua supracrustal belt, west Greenland.

Petrology and REE geochemistry of the clastic rocks from the approx 3800 m.y. belt provide constraints on the nature of early Archaean metamorphic regimes and on the sources of their sedimentary protolith. Garnet + staurolite + biotite and biotite + kyanite assemblages represent types common in younger metamorphic belts. Garnet-biotite geothermometry indicates T = 541 + or - 43oC for prograde metamorphism and T = 464 + or - 39o for retrograde metamorphism. Suggested metamorphic conditions of T approx 550oC and P approx 5 kbar imply burial to = or >15 km with metamorphic thermal gradients <40oC/km and argue against excessively steep early Archaean crustal thermal gradients. REE patterns for muscovite-biotite gneisses are strongly fractionated with variable Eu anomalies. Garnet-biotite schists have less fractionated light REE, and exhibit a slope reversal for the heavy REE. These represent a mixed felsic-mafic (-ultramafic?) protolith. Both sediment types could be the erosion products of a rapidly emergent volcanic structure shedding debris into a shallow basin.-W.S.W.