Serpentinization and carbonatization of tectonic melanges in the Coy Pond Complex, Central Newfoundland

Timbal, Nicolai Adrian (1992) Serpentinization and carbonatization of tectonic melanges in the Coy Pond Complex, Central Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Mount Cormack Terrane, an elliptical tectonic window in the Dunnage Zone of central Newfoundland, is a structural dome comprising a central zone of variably metamorphosed metasediments surrounded by remnants of several ophiolitic complexes including the Coy Pond Complex, the subject of this study. The ophiolite complexes have been interpreted to represent remnants of the early Paleozoic Iapetus ocean floor, which was obducted onto the continental margin of Gondwana. -- The Coy Pond Complex, is an imbricated, but complete ophiolite which is oriented subvertically and dips and youngs outward from the tectonic window. -- Analyses of relict olivines, pyroxenes and spinels in the Coy Pond Complex indicate that it has an arc-related petrogenesis. -- The Coy Pond Complex is composed of two structural slices separated by two tectonic mélanges. From west to east the principal structural elements are: (i) basil structural slice composed of harzburgite and dunite; (ii) western tectonic mélange; (iii) central structural slice comprising pyroxenite, gabbro-diabase, pillow lava and sedimentary rocks; (iv) the eastern tectonic mélange. -- Locally the basal structural slice is missing and its place is taken by the mixed lithology slice, which is composed of exotic (i.e. non-ophiolitic) lithologies that were probably derived from the tectonically overlying and underlying units. -- The tectonic mélanges, are composed of ultramafic fragments up to 2 m in longest diameter enveloped in an ultramafic matrix. They display evidence of brittle (fragments) and ductile (matrix) deformation and are interpreted to have formed during fault movement and emplacement of the ophiolite. On the basis of the lithologies of the fragments, the tectonic mélanges are interpreted to have formed from the transition zone (dunite) and upper part of the mantle tectonite (harzburgite). -- Both fragments and matrix in the tectonic mélanges are extensively serpentinized and variably carbonatized. Petrographic evidence indicates that serpentinization initially involved hydration of the peridotite and formation of a pseudomorphic assemblage of lizardite ± brucite. Subsequently most of the original lizardite recrystallized to antigorite and to schistose serpentinite in the matrix. Veins of chrysotile are rare. -- Carbonatization resulted in replacement firstly of brucite by magnesite, and subsequently of the serpentine assemblages (lizardite and/or antigorite) by magnesite ± talc. Later, quartz-magnesite formed by replacement of talc or by breakdown of remaining lizardite. These assemblages indicate a progressive increase in XCO₂ in the fluid phase during formation of the metamorphic assemblages with temperatures <300ﾟC for quartz-magnesite and >300ﾟC for talc-magnesite assemblages. -- Carbon and oxygen isotopic compositions of magnesite coexisting with lizardite, antigorite, talc and quartz were measured from the eastern and western tectonic mélanges. δ¹³O values of magnesite vary only slightly, from 12.3 to 17.5, with higher values occurring in the quartz-magnesite assemblages. δ¹³O values show considerable variation in isotopic composition from -4.2 to -9.1 for magnesite associated with lizardite and talc, from -10.8 to -13.6 for magnesite associated with antigorite, and from -11.9 to -20.4 for magnesite associated with quartz (in the western tectonic mélange). These data are interpreted to indicate that the different magnesite assemblages formed from different carbon reservoirs, with magnesite associated with lizardite and talc having an oceanic and/or average crustal carbon isotopic signature, whereas magnesite associated with antigorite and quartz shows a progressively more organic (continental) carbon isotopic signature. -- Structural, metamorphic and isotopic information have been used to construct a tectonic model for the formation and evolution of the tectonic melanges, which involves several stages. -- (a) Early serpentinization of dunites during detachment of the ophiolitic nappe. -- (b) Initiation of the formation of the tectonic mélanges during imbrication? and obduction of the ophiolite nappe onto the continental margin, together with partial carbonatization. At this stage magnesite associated with lizardite and with talc was formed. -- (c) Emplacement of Coy Pond ophiolite on top of the continental margin, consisting of Spruce Brook Formation. -- (d) Extension of the continental margin induced heat transfer into the tectonically underlying sediments (Spruce Brook Formation) which were affected by a low-P metamorphism. -- (e) This induced isostatic instability in the lower crust, with mechanical and thermal doming of the Mount Cormack Terrane, followed by renewed imbrication and final emplacement of the Coy Pond Complex, by extensional faulting, into its present subvertical orientation. It is assumed that the formation of the tectonic mélanges was concluded during the extensional faulting, when magnesite associated with antigorite and with quartz assemblages was formed in equilibrium with fluids carrying carbon of organic origin.

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