Petrology and Isotopic Composition of a Grenvillian Basement Fragment in the Northern Appalachian Orogen: Blair River Inlier, Nova Scotia, Canada

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Mesoproterozoic metaplutonic rocks in northern Cape Breton Island, INTRODUCTION Nova Scotia, occur in a tectonic inlier within the Appalachian In eastern North America, Mesoproterozoic Laurentian orogen. Although they have been multiply metamorphosed and basement rocks and their Neoproterozoic to mid-Paleovariably deformed, the petrology and geochemistry of these rocks zoic volcanic and sedimentary cover preserve a record provide insight into the tectonomagmatic evolution of easternmost of the Proterozoic to mid-Paleozoic geologic history of Laurentia. Anorthosite, syenite, and granitoid plutons eastern Laurentia. The record includes Mesoproterozoic (1100–980 Ma) intruded the Sailor Brook and Polletts Cove River orogeny on the ancient North American craton, breakgneisses. New Nd isotopic data are presented from a biotite-rich up of a Neoproterozoic supercontinent, development of part of the Sailor Brook gneiss ( Ndi = −0·7), two anorthosite a passive continental margin, and mid-Paleozoic orogeny. samples ( Ndi = +2·1 and +2·8), and a charnockite unit Exposures of Mesoproterozoic Laurentian basement in ( Ndi = −0·4). New U–Pb zircon data from the anorthosite yield easternmost Canada and the USA occur in a disPb/Pb ages between 975 and 1095 Ma. Disrupted U–Pb continuous band of inliers in the Appalachian orogen systematics preclude a unique age interpretation, but the >1095 Ma from Newfoundland to Alabama (Fig. 1). They consist single-grain date is a minimum age for the anorthosite. Field relations, mostly of rocks formed or deformed during major and trace-element geochemistry, and isotopic characteristics >1190–980 Ma Grenvillian tectonic and magmatic indicate that the anorthosite and charnockite were probably coexisting events, and are distinct from Neoproterozoic and melts, but not differentiates of a single parent magma. The lithological Paleozoic units of accreted Appalachian terranes in that and chemical characteristics of the Lowland Brook Syenite are they include granulite-facies metamorphic rocks and, similar to those of a contemporaneous belt of ultrapotassic plutons in many cases, anorthosite–charnockite–mangerite suites in the southern Grenville Province, and both have the chemical (Bartholomew, 1984, and papers therein; Pettingill et al., characteristics of certain modern continental-margin arc magmas. 1984; Rankin et al., 1989, 1993; Owen & Erdmer, 1990; These data indicate that the Mesoproterozoic units in the Blair Hughes et al., 1997). River inlier were juvenile crustal additions to eastern Laurentia The sinuous trend of the band of basement inliers during Grenville-related orogenic events. broadly reflects promontories and re-entrants that formed