ORIGIN OF >3.65 GA QUARTZ-AMPHIBOLE-PYROXENE ROCKS FROM AKILIA, SW GREENLAND: AN INTEGRATED FIELD AND MINERAL COMPOSITION INVESTIGATION

Some of Earth’s oldest (>3.7 Ga) known supracrustal rocks are exposed in SW Greenland, and termed the Akilia Association (AA). This assemblage is composed dominantly of mafic igneous rocks, with less common sedimentary units, including BIF. On the island of Akilia, AA rocks, including an ~5 m thick unit composed of banded quartz+amphibole+pyroxene (QAP), have had primary characteristics overprinted during multiple high-grade metamorphic events. Correct protolith identification is important because it has been interpreted as BIF and reported to contain isotopic evidence for a biosphere. Because the lithology cannot be dated directly, cross-cutting relationships have been used to establish an older relative age against the tonalitic Amîtsoq Gneiss (AG), which has been radiometrically dated at >3.65 Ga. In the field, the contact between the AG and AA is strongly foliated and lineated. Structural measurements, including orientation of foliations, mineral lineations, and fold axes in both units are identical in orientation, indicating the present relationship between the two units is tectonic; there is no field evidence for igneous intrusion of the AG protolith into AA. Furthermore, QAP is nowhere in contact with the AG, leaving its relative age and protolith uncertain. To investigate possible QAP protoliths, we analyzed its mafic mineral compositions, as well as those from the adjacent AA rocks. Outside QAP, mafic phases are dominated by enstatite, anthophyllite, and hornblende, and possess whole-rock trace-element signatures indicative of an igneous origin (i.e., Th/Sc and Cr/Th ratios and abundances of Cr, Y, TiO₂.,P₂O₅.). Within QAP, amphibole consists dominantly of actinolite, hornblende and cummingtonite, and pyroxene is dominantly diopside, hedenbergite and augite, which point to an original Mg-rich protolith. We argue that such amphibole and pyroxene compositions are likely to be derived from an original protolith consisting of ultramafic rocks with disseminated carbonate combined with a Fe-rich carbonate band brought to granulite facies metamorphism, rendering a BIF protolith unnecessary. Minor amounts of magnetite found in the QAP would have formed through the decarbonation of siderite. Similar lithologies are common in the lower grade Isua Greenstone Belt.