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

Abstract

An investigation of the petrology and rare-earth element (REE) geochemistry of clastic metasedimentary rocks from the ~ 3800 Ma Isua Supracrustal Belt has been carried out to provide constraints on the nature of early Archaean metamorphie regimes and on the sources of their sedimentary protolith. The assemblages garnet + staurolite + biotite and biotite + kyanite (both with qtz + muse + plag + Hm) characterize the Isua metasediments and represent types common in younger metamorphic belts. Secondary chlorite and sericite occur in most samples. Garnet-biotite geothermometry indicates T = 541 ± 43°C for prograde metamorphism and T = 464 ± 39°C for retrograde metamorphism. Suggested metamorphic conditions of T - 550°C and P - 5 Kb imply burial to at least 15 Km with metamorphic thermal gradients < 40°C/Km. These data argue against excessively steep early Archaean crustal thermal gradients. REE patterns for three museovite-biotite gneisses are strongly fractionated (CeN = 40-100; YbN = 2-8) with variable Eu-anomalies (Eu/Eu* = 0.48-0.95), not unlike patterns for Arehaean felsic voicanic rocks in other areas. Garnet-biotite sehists have less-fractionated light REE, and exhibit a slope reversal for the heavy REE (i.e., GdN< YbN. These most plausibly represent a mixed felsic-mafic (- ultramafic?) protolith. Both sediment types could be the erosion produets of a rapidly emergent voicanic structure shedding debris into a shallow basin.