The Akilia Controversy: field, structural and geochronological evidence questions interpretations of >3.8 Ga life in SW Greenland

Abstract

Abstract: Field relations, structure and geochronology demonstrate that controversial rocks on Akilia, SW Greenland, cannot host evidence of Earth's oldest life. Mafic–ultramafic gneiss that contains a layer of quartz–amphibole–pyroxene gneiss, a few metres thick, with purported biogenic graphite is not cross-cut by the protoliths of >3.82–3.65 Ga tonalitic gneiss. The contact between these gneisses is tectonic so their relative ages are unknown. Multiple episodes of intrusion and deformation in the tonalitic gneiss preceded the earliest deformation fabrics and structures seen in the mafic–ultramafic gneiss. Although previously interpreted as a volcano-sedimentary stratigraphy, the mafic–ultramafic gneiss displays no evidence of such an origin, and could be partly or wholly derived from igneous intrusions. Existing geochronology indicates that the mafic–ultramafic and qtz–am–px gneisses have ages <3.67 Ga. Metamorphic zircon in the qtz–am–px gneiss grew at c . 2.68 Ga during late Archaean high-grade events that complicate any protolith interpretation. U–Pb dating of apatite from a variety of rocks on Akilia shows that this mineral crystallized, or possibly recrystallized, at c . 1.75 Ga and it thus provides no indication of an early Archaean age for any associated graphite, regardless of whether or not the latter is biogenic. Supplementary material: Ion microprobe U–Pb analytical methods and isotope data tables are available at http://www.geolsoc.org.uk/SUP18333 .