Geology, age and origin of supracrustal rocks at Akilia, West Greenland

Abstract

New mapping, geochronology, and geochemistry of the supracrustal enclave and associated orthogneisses at Akilia, West Greenland, support a ≥3825 ± 6 Ma age for the sequence and a chemical sedimentary origin for the controversial Fe-rich quartz-pyroxene gneisses. Lithologies of the enclave comprise laterally continuous, mappable units of mafic amphibolite, ultramafic rocks, and two Fe-rich quartz-pyroxene units. A minor anthophyllite-garnet rock with chemical characteristics suggesting a sedimentary protolith was identified. The earliest foliation parallels lithologic contacts. This foliation was isoclinally folded about a steep hinge surface, and then tightly refolded about a steep NS hinge plane. The latest folding produced limbs exhibiting contrasting strains. The full deformation history is shared by all lithologies of the enclave. Two previously unidentified orthogneiss sheets preserve deformed magmatic crosscutting relations with amphibolite and ultramafic units in the low-strain limb. The supracrustal anthophyllite-garnet rock experienced zircon growth at ∼3600 and ∼2700 Ma, in common with amphibolites. These ages are interpreted to reflect metamorphism at granulite and amphibolite facies, respectively, consistent with independent regional evidence. The crosscutting orthogneiss sheets preserve a complex history of zircon growth, but only earliest cores indicate Th/U exchange equilibrium with the bulk rock at magmatic conditions. Weighted-mean 207Pb/206Pb ages of cores of 3730 ± 7 Ma and 3825 ± 6 Ma date the crystallization ages of the sheets, indicating that the Akilia supracrustal rocks were deposited ≥3819 Ma. Quartz-pyroxene rocks on Akilia have been interpreted as metamorphosed chemical sediments that may preserve carbon-isotope evidence for life at the time of their formation. An alternative proposal is an origin by metasomatic alteration. Our results show that these rocks experienced a history of deformation and zircon growth identical to other lithologies of the supracrustal body. Metamorphic zircons give a minimum 207Pb/206Pb age of 3589 ± 13 Ma regardless of origin. The two models of formation of quartz-pyroxene rocks were evaluated using field, petrologic and geochemical tests. The distribution of the units, the absence of field or petrologic indicators of metasomatism, and geochemistry (oxygen and nontraditional stable isotopes, immobile elements and REE) together support a sedimentary, but not a metasomatic, origin. Because their protolith was sedimentary, the quartz-pyroxene rocks are a primary part of the supracrustal sequence and have the same minimum age. Despite strong deformation and polymetamorphism, the Akilia supracrustal enclave contains information about terrestrial surficial processes at ≥3819 Ma.