Magmatic and tectonic evolution of the Cambrian-Ordovician Laurentian margin of Iapetus: Geochemical and isotopic constraints from the Notre Dame Subzone, Newfoundland

Abstract

Late Cambrian to Early Ordovician volcanic and hypabyssal rocks in the NotreDame subzone, Newfoundland Appalachians, are generally considered to have formed in a series of island arcs and back arc basins on the Laurentian side of Iapetus. Our compilation of new geochemical and Nd isotopic data provides a more detailed look at the nature of magmatism in these environments than has heretofore been possible. Nd isotopic data constrain the nature of the magma sources for the various volcanic episodes and, in particular, allow us to track the early Paleozoic interaction of mantle and continental lithosphere at the Laurentian margin. New and published geochronological data permit us to assign a time sequence to the various events that we recognize.Geochemical data show that volcanic rocks in the Notre Dame subzone can be divided into several groups, most of which have well-documented analogs in modern plate environments. In nonarc environments, tholeiitic rocks with characteristics of N-MORB (normal mid-ocean basalt ridge) and E-MORB (enriched-MORB) are present, as are tholeiitic and alkalic basalts with characteristics of oceanic island basalts. The geochemical and isotopic data indicate derivation from time-integrated depleted mantle sources. We interpret these to have formed in ensimatic back arc basins.Arc volcanic rocks include island arc tholeiites, transitional island arc tholeiites, calc-alkalic basalt and/or andesite, boninites, and felsic volcanics. Geochemical and Nd isotopic data indicate that the tholeiitic rocks were derived from time integrated LREE-(light rare earth element-) depleted mantle sources contaminated by subducted, hydrous oceanic lithospheric and continentally derived sediments. Calc-alkalic rocks and associated felsic volcanic rocks have Nd isotopic compositions that indicate the direct involvement of continental lithosphere in their petrogenesis.Integration of geochemical, geological, and geochronological evidence suggests the following sequence of events: (1

From Large Zones to Small Terranes to Detailed Reconstruction of an Early to Middle Ordovician Arc–Backarc System Preserved Along the Iapetus Suture Zone: A Legacy of Hank Williams

“…8A; Zagorevski et al 2006), detritus in associated sedimentary rocks (Fig. 8B), and low ε Nd values in both mafic (0.9) and felsic (-4.0 to -10.2) volcanic rocks (Table A3; Swinden et al 1997;Zagorevski et al 2006) indicate that the Floian arc was also built on a continental substrate. Although zircon inheritance data are limited, ca.…”

“…1; Dec et al 1997;Swinden et al 1997;O'Brien 2003). The Cottrell's Cove Group is divided into the volcanic-dominated Fortune Harbour Formation and the predominantly sedimentary Moore's Cove Formation (Dec et al 1997;O'Brien 2003).…”

The origin of mafic-ultramafic bodies within the northern Dashwoods Subzone, Newfoundland Appalachians

“…Both subzones are dominantly composed of the Late Cambrian to Lower Silurian Notre Dame Arc (Whalen et al 1997;van Staal et al 2003), paragneiss, arc-back arc complexes, and ophiolites (e.g., Swinden et al 1997;Lissenberg et al 2005 b). Post-tectonic late Lower to Upper Silurian magmatism is widespread, and is dominated by granite with subordinate mafi c rocks (Currie and van Berkel 1989;Whalen 1989;Dunning et al 1990;Whalen et al 1996Whalen et al , 2003.…”

“…1; Swinden et al 1997). The oldest generation of ophiolites, which are preserved in the Lushs Bight oceanic tract (508-501 Ma; Elliott et al 1991;Szybinski 1995;Kurth et al 1998), is comprised mostly of sheeted dykes and pillow basalts at Lushs Bight.…”

“…Crosscutting, continentally contaminated ca. 501-488 Ma dykes indicate that the tract was already emplaced onto Laurentian continental crust in the Late Cambrian-Tremadoc (Szybinski 1995;Swinden et al 1997). The ophiolitic Long Range Complex, which occurs near the southwest coast of Newfoundland (Fig.…”