Geodynamic environment of the ca. 3800 Ma Outer Arc Group, Isua (Greenland)

Abstract

The arcuate, 35 km long Isua supracrustal belt (ISB, southern West Greenland) contains the world's largest remnants of Eoarchean volcanic and sedimentary sequences. The ISB is broadly divided into: (i) the northern Inner Arc Group of 3720 to 3690 Ma rocks, and (ii) the southern Outer Arc Group of ca. 3800 Ma rocks which is bounded on its northern side by the highly tectonized ca. 3750 Ma Dividing Sedimentary Unit. The boundary between the two groups is a mylonite formed between 3685 and 3660 Ma. Despite the generally high strain, amphibolite facies metamorphism and layer-parallel dislocations that can thin or altogether excise some units, domains of lower deformation comprising ≪1% (qualitative assessment) of the Outer Arc Group contain relict sedimentary and igneous structures. Combined with zircon U-Pb geochronology and whole rock geochemistry, this enables the Outer Arc Group lithological sequence and geodynamic setting to be reconstructed. The lower part of the Outer Arc Group is dominated by metabasaltic amphibolites of the Mafic Volcanic formation in which rarely-preserved pillow structures indicate both their predominantly subaqueous eruption and also their stratigraphic facing. They erupted >3800 Ma, because they were first intruded by subconcordant sheets of fine-grained hypabyssal tonalite dated at 3803±3 Ma (Crowley, 2003) and then by coarser-grained 3795 to 3791 Ma tonalite-granodiorite, which forms a large deformed pluton along the south side of the ISB. This formation is succeeded by the Sedimentary formation whose base consists of discontinuous rare, thin fuchsitic quartzites with 3890 to 3805 Ma detrital zircons. Overlying is a diverse package of dolostones, marls and siliceous rocks. Although they are extensively modified by metamorphism and metasomatism, producing widespread growth of talc or tremolite, relict graded sedimentary layering, chemical and isotopic signatures indicate originally sedimentary protoliths. Detrital zircons in these rocks range in age from ca. 3820 to 3805 Ma. This unit shows an upwards transition from ‘pure' chemical sedimentary rocks with distinct seawater-like trace element signatures into lithologies increasingly contaminated by felsic material that is locally preserved as graded layers, which are interpreted as an increasing volcanogenic input. Succeeding the sedimentary rocks is the Felsic Volcanic formation, an extensive unit of mostly schistose 3807 to 3802 Ma felsic potassic-altered rocks with carbonate-rich interludes and veins. Locally-preserved andesitic units with graded layering, massive vesicular lavas, polymict breccias, resorbed quartz phenocrysts and fiammé, attest to volcanic and volcano-sedimentary protoliths. Whole rock geochemistry and oxygen isotope analyses on these rocks and their zircons indicate predominantly felsic volcanic protoliths that experienced massive alteration in a surficial environment, probably following subaerial eruption. Massive volcanic rocks are commonest in the west of the ISB, suggesting this part of the belt was proximal to a volcanic center. Using these stratigraphic data, we conclude that the ISB volcanic and sedimentary rocks formed in a mobile geodynamic regime resembling plate tectonics, and not within a stagnant lid regime.