Origin of mafic and potassic to ultrapotassic rocks, Manicouagan area, central Grenville Province: geochemical and geochronological constraints on the evolution of the Mesoproterozoic SE Laurentian margin and the Grenvillian LHO

Maity, Barun (2019) Origin of mafic and potassic to ultrapotassic rocks, Manicouagan area, central Grenville Province: geochemical and geochronological constraints on the evolution of the Mesoproterozoic SE Laurentian margin and the Grenvillian LHO. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The southeastern margin of Laurentia preserves evidence for a long episode of juvenile crustal growth and recycling extending from the late Paleoproterozoic and throughout the Mesoproterozoic (~1.9–1.2 Ga). This was terminated by the onset of the Grenvillian continental collision (~1.1–0.98 Ga), forming a large, hot, long-duration orogen (LHO), comparable in scale to the modern day Himalayan-Tibet LHO. The Grenvillian LHO has been subdivided into the ca. 1090-1020 Ma Ottawan phase for which evidence is preserved in allochthonous rocks in the orogenic hinterland, and the ca. 1005-980 Ma Rigolet phase for which evidence is mainly preserved in the parautochthonous foreland (Rivers et al. 2012). Mesoproterozoic to early Neoproterozoic metamorphosed mafic and unmetamorphosed potassic to ultrapotassic rocks from the Canyon domain, part of the polycyclic allochthonous, medium-pressure (aMP) belt of the Grenville Orogen in the Manicouagan area, central Grenville Province, were investigated using integrated petrographic, geochronological, isotopic, and geochemical methods. The principal results of the thesis, presented as three Chapters (2 - 4), are summarized below. In Chapter 2, U-Pb TIMS geochronology of zircon, and whole-rock geochemical and Sm-Nd isotopic analyses of two granulite-facies mafic tholeiitic suites, provide constraints on: (i) the emplacement of crust-contaminated, depleted to enriched MORB-type, high Fe-Ti-P mafic sills at 1439⁺⁷⁶₋₆₈ Ma within a ca. 1.5 Ga supracrustal sequence that was under limited extension at that time, probably in a back-arc setting; and (ii) emplacement of a crust-contaminated, enriched MORB to arc-type mafic intrusive suite, previously dated at 1410 ± 16 Ma, in a transitional back-arc to arc setting. Integrated with published information, these results support the existing model of a long-lived continental margin arc on the southeastern margin of Laurentia, with intermittent back-arc opening and closure during Geons 15-14. In Chapter 3, U-Pb TIMS geochronology of zircon from two suites of granulite-facies mafic rocks has yielded ages of 1007.7 ± 2.0 Ma for the syn-metamorphic emplacement of dykes, and 997.0 ± 3.8 Ma for the emplacement of sills. Whole-rock geochemical and Sr-Nd isotopic compositions of the mafic rocks, coupled with petrogenetic modelling, indicate that they are tholeiitic intraplate basalts derived from ~4-10% decompression melting of upwelled asthenosphere, followed by ~4-20% contamination by crust or subcontinental lithospheric mantle (SCLM). Combined with published data, these results provide evidence for late-orogenic (post-Ottawan) lithospheric extension, which led to melting of Proterozoic SCLM, decompression melting of upwelling asthenosphere, and granulite-facies metamorphism in the crust. In Chapter 4, whole-rock geochemical and isotopic (Sr, Nd, Pb, and O) data for a suite of ca. 980 Ma, late- to post-tectonic, potassic to ultrapotassic dykes suggest two geochemically distinct groups, which formed by melting of spinel and garnet peridotite that were metasomatized by MARID-type vein assemblages containing amphibole, phlogopite, and accessory Fe-Ti-P phases. Their enriched EM I-type mantle sources were metasomatized by ancient subduction of carbonate, phosphate, and pelagic sediments. The late-tectonic group was derived from Proterozoic SCLM, whereas the post-tectonic group was derived from depleted late-Archean SCLM that had undergone early Paleoproterozoic metasomatism. Moreover, both the sources experienced an early Neoproterozoic metasomatism that is mainly attributed to late-Grenvillian continental subduction and asthenospheric upwelling. These data suggest that the terminal Rigolet phase convergence of the Grenvillian orogeny was intracontinental, characterized by foreland subduction, where progressively older components of SCLM underwent partial melting. Hence, with continued subduction, SCLM, initially of Proterozoic age and eventually of late-Archean age, became transported beneath the orogenic core, with the Archean Superior lithosphere being situated beneath the Grenvillian hinterland by ca. 980 Ma. The late-Grenvillian mafic rocks (Chapter 3) and potassic to ultrapotassic rocks (Chapter 4), together with published structural, geochronological and metamorphic data from the nearby Parautochthonous Belt, suggest that post-Ottawan lithospheric extension was followed by Rigolet intracontinental subduction beginning at ca. 1005 Ma, which in turn led to termination of the long-duration Grenvillian LHO by ca. 980 Ma.

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