Brent V. Miller

The Blair River Complex of northern Cape Breton Island, Nova Scotia, includes four major Proterozoic units. The protolith of the tonalitic to dioritic Sailor Brook orthogneiss is no younger than 1217 Ma and metamorphic zircon associated with granulite-facies metamorphism of this unit crystallised at 1035 +12/-10 Ma Igneous zircon in the Lowland Brook Syenite crystallised at 1080 +5/-3 Ma, but this unit was not significantly affected by Proterozoic metamorphism and deformation. The Red River Anorthosite Suite is a Proterozoic massif-type anorthosite that contains a central core of massive anorthosite and grades outward, through leucogabbro and layered mafic rocks, to rare pyroxenite. High temperature, relatively low-pressure metamorphism of the suite occurred at 996 +6/-5 Ma, but its effects are rarely distinguishable due to a strong amphibolite-facies overprint and intense alteration. Charnockitic rocks are lithologically and chemically gradational with the layered unit, possibly due to contact metamorphism and metasomatism of the anorthosite suite during intrusion of the charnockite. The biotite rich, garnet bearing, granitoid Otter Brook gneiss yielded an igneous age of 978 +6/-5 Ma.

Silurian thermal activity in the Blair River Complex is recorded by magmatism associated with the undeformed Sammys Barren granite at 435 +7/-3 Ma and by metamorphic mineral ages. U-Pb analysis of metamorphic titanite from amphibolite-facies overprint assemblages in the Sailor Brook gneiss. Lowland Brook Syenite, Red River Anorthosite Suite, a gneissic anorthosite, and the Otter Brook gneiss. as well as igneous titanite from the Red River syenite and Fox Back Ridge diorite/granodiorite, all yield cooling ages of ca. 425 Ma Lower-temperature cooling ages are provided by hornblende from the Fox Back Ridge unit (417 ± 6 Ma), rutile from the Red River Anorthosite Suite (110 ± 2 Ma), muscovite from the Meat Cove marble (428 ± 7 Ma), and phlogopite from a calc-silicate lens in the Otter Brook gneiss (410 ± 6 Ma).

The Blair River Complex is interpreted to be a fragment of Grenvillian basement derived from a promontory on the proto-Atlantic continental margin of North America. Its tectonostratigraphic position at the western margin of a condensed section of the Appalachian orogen in Cape Breton Island is confirmed by geophysical data Similarities in rock types, ages, and isotopic characteristics to the Grenville Province support a cratonic Laurentian origin for the Blair River Complex and these features contrast with those of accreted terranes in the Appalachian orogen. The Blair River Complex preserves no indication of Taconian (Ordovician) events, and Acadian (Devonian) or Alleghanian (Carboniferous) events appear to be limited to high-level faulting associated with the amalgamation of the neighbouring Aspy terrane. Widespread Silurian metamorphism and localised magmatism are identical in age to similar events recognised elsewhere in the northern Appalachian orogen, and indicate involvement of the Blair River Complex in the Silurian culmination of Appalachian orogenesis.

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Supervisor: Rebecca A. Jamieson and Sandra Barr