THE ORIGIN OF METACARBONATE ROCKS IN THE ARCHEAN ISUA SUPRACRUSTAL BELT, WEST GREENLAND

Metacarbonate rocks, consisting of ankerite and dolomite together with combinations of amphibole, biotite, muscovite, chlorite, quartz, and clinopyroxene, are an important component of the roughly 3.8 Ga Isua s upracrustal belt. Along with layers of banded iron formation, felsic s chists, basic amphibolites, and variegated amphibolitic schists, they have previously been described as a single supracrustal suite intruded by ultramafic and Mg-rich basic sills. One of the formal stratigraphi c units recognized in this interpretation, the Calc-Silicate Formation , contains abundant metacarbonates and calc-silicates that were regard ed as the earliest known examples of metamorphosed calcareous chemical sediments. Field relations and geochemical models, however, suggest t hat the Isua metacarbonate units are metasomatic in origin and formed where fluids flowed across the contacts between ultramafic bodies and felsic or metabasaltic country rocks at deep crustal levels. Field evi dence in support of this includes the common occurrence of metacarbona tes at margins of ultramafic rocks, formation of metacarbonate assembl ages In veins near ultramafic contact zones, and the replacive nature of the contacts between metacarbonates and felsic or basaltic units. I n order to explore possible mechanisms for the origin of the metacarbo nates, metasomatic processes accompanying the advection of fluids thro ugh ultramafic rocks were simulated numerically using a model configur ation in which a column of dunite was sandwiched between layers of hos t rock. This allowed mineral zonation, bulk composition, and porosity changes to be studied at both upstream and downstream contacts. Fluids entering the ultramafic layer at the upstream contact react with oliv ine to form talc and magnesite +/- chlorite +/- phlogopite. The same f luids, modified by passage through the dunite, react with country rock at the downstream contact, forming assemblages analogous to those fou nd in the Calc-Silicate Formation. This process is due primarily to th e effect of reduced a(SiO2) in changing the saturation state of all ho st rock minerals of all at the point where fluids exit the ultramafic rock. Metasomatism of the Isua supracrustals probably took place under amphibolite facies conditions between 500 degrees and 600 degrees C i n the presence of water-rich fluids where time integrated fluid flux w as between 10(3) to 10(4) moles of water cm(-2). The recognition that the Gale-Silicate Formation is metasomatic in origin (and hence of no stratigraphic importance) considerably weakens previous ideas on the o rigin of the Isua belt as a well-defined stratigraphic package current ly exposed as a syncline. A corollary of this study is that metacarbon ate-ultramafic associations should be common In supracrustal rocks tha t originally contained peridotites that have reacted with fluids at de pth.