Reappraisal of purported ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) from detailed chemical and structural analysis

• Published in: Earth and Planetary Science Letters
• Main Authors: Zawaski, MJ, Kelly, NM, Orlandini, OF, Nichols, CIO, Allwood, AC, Mojzsis, SJ
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2020
• Subjects: Greenland
• Online Access: https://doi.org/10.1016/j.epsl.2020.116409; https://ora.ox.ac.uk/objects/uuid:9a487cdb-d7a0-4d18-9943-3781a5e44e60

The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. The structures in question are found within a suite of multiply deformed greenschist- to amphibolite-facies metamorphic rocks. To assess their premise as primary sedimentary features – as opposed to products of strain localization in layered, variably ductile rocks – we report new field mapping at the appropriate scale and resolution from the original discovery Sites reported by Nutman et al. (2016). Our new map was used to guide micro- and macro-structural investigations and comprehensive geochemical sampling. Here, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they are not deformed conical stromatolites. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron backscatter diffraction (EBSD) patterns collected on fresh surfaces cut perpendicular and parallel to the ridges attest to the lack of any residual sedimentary laminae (e.g., compositional layering) within these structures' cores. Internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceals granoblastic quartz ± dolomite cored boudins that sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. The morphology of boudins reflects viscosity contrasts of the different ductile layers during deformation. Therefore, these features are not of sedimentary origin. Furthermore, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results show that the Isua structures are the expected result of a tectonic fabric that preserves no ...