VOLCANIC RESURFACING OF THE EARLY TERRESTRIAL CRUST: ZIRCON U-PB AND REE CONSTRAINTS FROM THE ISUA GREENSTONE BELT, SW GREENLAND

The oldest known bona fide succession of clastic metasediments occurs in the Isua Greenstone Belt, SW Greenland and consists of a variety of mica-schists and rare metaconglomerates. The metasediments are closely associated with a felsic metavolcanic lithology that has previously been dated to 3.71 Ga. Based on trace element geochemical data for >30 metasediments, we selected the six samples with highest Zr concentrations for zircon extraction. These samples all yielded very few or no zircon. Those extracted from mica-schists yielded ion probe U/Pb ages between 3.70 and 3.71 Ga. One metaconglomerate sample yielded one zircon of 3.74 Ga age.

The mica-schist hosted zircons have U/Pb ages, Th/U ratios, REE patterns and Eu anomalies indistinguishable from zircon in the associated 3.71 Ga felsic metavolcanic unit. Trace element modelling requires the bulk of material in the metasediments to be derived from variably weathered mafic lithologies, explaining the paucity of zircon in the mica-schists. The absence of older zircon in the mica-schists and the preponderance of mafic source material imply intense basaltic resurfacing of the early Earth. The implications of this process will be discussed.

Thermal considerations suggest that horizontal growth of Hadean crust by addition of mafic-ultramafic lavas must have triggered self-reorganisation of the protocrust by remelting. Reworking of Hadean crust may have been aided by burial of hydrated (weathered) metabasalt due to semi-continuous addition of new voluminous basalt outpourings. This could have led to Zr-saturated partial melts possibly with O-isotope compositions different from the mantle. The oldest zircons hosted in sediments would have been buried to substantial depth, from which it took billions of years for them to be exhumed and incorporated into much younger sediments.