Abstract
The extension of subduction processes into the Eoarchaean era (4.0–3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics.
Highlights
The extension of subduction processes into the Eoarchaean era (4.0–3.6 Ga) is controversial
Rocks within the dunite bodies have chemical characteristics that distinguish them from other types of ultramafic rocks, including those formed as olivine cumulates, which are found within the ISB13
We study the microstructure and the lattice-preferred orientations (LPO) of olivine preserved in lenses of Isua Supracrustal Belt (ISB) mantle rocks using electron backscatter diffraction (EBSD)
Summary
The extension of subduction processes into the Eoarchaean era (4.0–3.6 Ga) is controversial. Lenses A and B preserve macroscopic high-temperature fabrics defined by the alignment of olivine grains (Figs 1c,d and 2a) Field relationships demonstrate these olivine fabrics predate the schistosity formed during juxtaposition of the dunites with adjacent crustal rocks and subsequent intra-crustal deformation under lower amphibolite facies conditions (r550 °C). The spatial association of lenses A and B with volcanic rocks with arc geochemical signatures, the tectonic contact between peridotite and adjacent rocks, their chemical compositions and the constraining U-Pb zircon geochronology has led to an interpretation that these lenses represent sub-arc mantle interleaved with supra-subduction ophiolitic material during the Eoarchaean era[6,13,15]
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