Geology

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Browsing Geology by Subject "Archean tectonics"

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    Emplacement history of mafic-ultramafic volcanic rocks in the northern Pontiac subprovince, Quebec, Canada
    (2020-05-15) Rehm, Adrian Gerhard Nutting
    Detailed geological mapping of volcanic, sedimentary and intrusive rocks in the northern Pontiac subprovince of the Superior Province has identified multiple volcanic-sedimentary contacts at the margins of a 30 km long volcanic package of high-Fe tholeiites and Munro- to Barbertontype komatiitic flows and sills. Breccias displaying blocky, globular and tendril-like juvenile clasts in a massive sedimentary matrix were observed along the contacts of mafic-ultramafic intrusions with their host sedimentary rocks. These textures are consistent with peperite, a rock formed by the mixing and mingling of magma with wet, unconsolidated sediments. Geochemical data for one of the peperites indicates that the igneous component has a similar trace element composition to spinifex-bearing komatiite flows and the sedimentary component has a similar composition to the Pontiac Group wackes. Additionally, conformable unstrained contacts were observed between effusive volcanic rocks and turbiditic wackes. These relationships indicate a previously unrecognized ca. 2682 Ma mafic and ultramafic volcanic event synchronous with the deposition of the Pontiac subprovince sedimentary rocks. This suggests that the mafic-ultramafic volcanic package is synsedimentary and autochthonous, which is not consistent with previous interpretations of these rocks as structural thrust panels. We propose that the mafic-ultramafic volcanic package was emplaced during a previously unrecognized plume-related extensional event that facilitated the upward flow of mantle-derived magmas along major, mantle-penetrating structures during Pontiac sedimentation. Shortly after, tectonic compression folded and imbricated the Pontiac rocks and accreted them to the southern margin of the Abitibi subprovince.