GEOLOGY OF 3830 MA FERRUGINOUS QUARTZ-PYROXENE (SUPRACRUSTAL) ROCKS FROM THE AKILIA ASSOCIATION, SOUTHERN WEST GREENLAND

Graphite with ¹³C/¹²C values akin to kinetic isotope fractionations via autotrophic metabolic pathways is present in Eoarchean volcanosedimentary rocks in the 3.81 Ga Isua Supracrustal Belt and the 3.83 Ga Akilia association (Aa) in West Greenland. Because the rocks have rarely been mapped in the appropriate detail (e.g. 1:100 scale) to interpret the geology, and the terrane experienced a protracted and complex history of multiple metamorphic events, acute disagreement abides their interpretation. It is significant that because in both geology and antiquity, rocks of this kind are not unique to one locale, any "special" explanation for one must apply to all. Similar enclaves are found scattered throughout southern West Greenland as well as in the 3.78 Ga Nuvvuagituq Supracrustal Belt in northern Québec, the Manfred Complex (Western Australia, 3.77 Ga), and Labrador (>3.7 Ga). The Akilia quartzites are coarse- to fine grained ferruginous Qtz + Cpx (Hd) ± Opx (Fs) ± Mag rocks with banding from the <mm to >1cm with Qtz-rich and Mag-rich layers, and variable Cpx, Opx and amphibole. Akilia sample G91-26 is Qtz + Cpx (hedenbergite) + Am (grunerite) ± Opx (ferrosilite) ± Mag ± Grt ± sulfide (Po and Ch) with minor apatite, graphite and (metamorphic) zircon. Hedenbergite (CaFe[Si₂O₆]) is a common Cpx phase in high-grade regionally metamorphosed iron-formations where it co-exists with orthoferrosilite. Multiple S- and Fe isotopes, trace elements, whole rock and mineral oxygen isotope data, and C isotopes on graphite (McKeegan et al., 2007; Papineau et al. 2009), annul the metasomatic/meta-igneous model mechanism of Fedo and Whitehouse (2002) and Lepland et al. (2005). Ages and metamorphic history are established by U-Pb zircon geochronology on primary (magmatic) zircons extracted from orthogneisses. Since the parent melts of such gneisses are highly undersaturated in Zr, zircon inheritance is very rare. Magmatic core zircon ages reflect emplacement ages of the gneisses. Low-strain domains preserve orthogneissic sheets dated from 3.77-3.83 Ga that structurally transect supracrustal lithologies. Geochronological, mineralogical and geochemical data as well as field relations of the quartz-pyroxene units are consistent with a marine sedimentary origin for rocks from the dawn of the geologic record.