The Cold Spring Melange and a possible model for Dunnage–Gander zone interaction in central Newfoundland

Abstract

Structural relationships at Cold Spring Pond and the recognition of ophiolitic melange bear on the important questions of timing and style of structural superpositioning of Dunnage Zone rocks above Gander Zone rocks in central Newfoundland. The latest models emphasize ductile shear boundaries and orogen-parallel movements. Previous models proposed west-to-east or head-on obduction of Dunnage ophiolitic rocks across the Gander Zone.At the Dunnage (Exploits Subzone) – Gander (Meelpaeg Subzone) boundary at Cold Spring Pond, discrete, outcrop-size ultramafic blocks and smaller quartzite blocks are randomly distributed, and they are surrounded by, or are embedded in, homogeneous black graphitic shale or phyllite. The ultramafic blocks are typical of nearby Early Ordovician Dunnage ophiolite suites, the quartzite blocks are typical of adjacent Early Ordovician or earlier Gander clastic rocks, and the matrix black shales are similar to those of Middle or Early Ordovician age that occur throughout central Newfoundland. This chaotic mixture of almost coeval lithologies at Cold Spring Pond is interpreted as an olistostromal melange; the Cold Spring Melange. It resembles melanges that are dated as Ordovician elsewhere in Newfoundland.The Cold Spring Melange is overprinted by the full range of structures and metamorphic effects evident in adjacent rocks of the Exploits (Dunnage) and Meelpaeg (Gander) subzones. These include the development of lineations, cleavages, schistosities, zones of ductile shearing, regional metamorphism, and contact metamorphism. The oldest of these effects are interpreted as Silurian, based on isotopic dating in southern Newfoundland.The formation of olistostromal, ophiolitic melange implies disruption of the oceanic tract (Exploits Subzone of the Dunnage Zone), and in the case of the Cold Spring example, juxtapositioning or transport of Exploits Subzone ophiolite suites against or across the supracrustal rocks of the Meelpaeg Subzone (Gander Zone). The age and provenance of Cold Spring components, lack of post-Ordovician components, overprinting structural relationships, and comparison with other Newfoundland melanges all support an Ordovician age of formation. Overprinting relationships indicate that major ductile shears at other Dunnage–Gander zone boundaries postdate initial Dunnage–Gander superpositioning.

Provenance of tectonic history of the Gander Zone in the Caledonian/Appalachian Orogen: Implications for the origin and assembly of Avalon

“…Significant is that this same Exploits cover also unconformably overlies the Early Ordovician ophiolitic rocks of the Exploits Subzone in Newfoundland (Kennedy and McGonigal, 1972;Colman-Sadd and Swinden, 1984) and probably also the Avalon Zone rocks of the Hermitage Flexure in southern Newfoundland (Chorlton, 1978;Dunning and O'Brien, 1989). Hence, the Middle Ordovician Exploits cover was interpreted as an overstep sequence (Williams and Piasecki, 1990; van Staal and Williams, 1991) deposited after early Arenig (Penobscot) obduction of the ophiolites on the Gander clastics, an interpretation confirmed through dating a stitching pluton (Partridgeberry Hills Granite: 474 +6/-3 Ma) by . These relationships support the premise that the Gander clastics form part of a Cambrian to Early Ordovician passive margin.…”

“…Just as were the Gander Zone rocks in the northern Appalachians, the quartz-rich clastic rocks along the Gondwanan margin of the Iapetus Ocean in the British Isles were involved in Early to Middle Ordovician deformation, including mélange formation (e.g., Max et al, 1990;Hughes et al, 1993), and are unconformably to disconformably overlain by Middle Ordovician volcanic and sedimentary rocks. The Arenig deformation in the eastern part of the northern Appalachians is temporally and spatially related to the southeastward Penobscot ophiolite obduction in Newfoundland and New Brunswick (Williams and Piasecki, 1990, van Staal, 1994. It is notable that ophiolitic, ultramafic-mafic slivers are tectonically juxtaposed against the Ribband and Duncannon groups in southeastern Ireland (Max et al, 1990) and possibly also preserved in the New Harbour Group in Anglesey (Gibbons and Ball, 1991;Church, 1992).…”

Paleogeography, accretionary history, and tectonic scenario: A working hypothesis for the Ordovician and Silurian evolution of the northern Appalachians

“…2) has received considerable attention, largely because of its excellent coastal exposure. A wealth of stratigraphic (e.g., Kay, 1967;Hibbard and Williams, 1979;Cocks, 1978,1981;Dean, 1978;Kean and others, 1981;Arnott and others, 1985;van der Pluijm and others, 1987), sedimentologie (e.g., Helwig and Sarpi, 1969;Home, 1970;Watson, 1981;Arnott, 1983;Pickering, 1987), geochemical (e.g., Strong, 1977;Strong and Dickson, 1978;Jacobi and Wasowski, 1985;Wasowski and Jacobi, 1985;Swinden and others, 1989;Fyffe and Swinden, 1991), and structural studies (e.g., Helwig, 1967Helwig, , 1970Home, 1969;Kennedy, 1975;Nelson, 1981;Karlstrom and others, 1982;van der Pluijm, 1986;Kusky and others, 1987;Blewett and Pickering, 1988;Elliott and others, 1989;Lafrance, 1989;Williams and Piasecki, 1990) have been carried out in this region. The results presented in this chapter are largely derived from this part of the northern Appalachians.…”

Time-Transgressive Salinic and Acadian Orogenesis, Magmatism and Old Red Sandstone Sedimentation in Newfoundland

“…This sequence disconformably overlies Cambrian-Tremadocian sandstone and shale of the Gander Group; the late Tremadocian hiatus marks obduction of the Penobscot ophiolites onto the Gander margin ( Fig. 2; Williams and Piasecki 1990;Colman-Sadd et al 1992;Zagorevski et al 2010). This part of the Exploits subzone mainly preserves a continuation of sedimentation on the Cambrian-Ordovician passive Gander margin following opening of the Tetagouche-Exploits backarc basin shortly after 475 Ma ( van Staal 1994 ).…”