Abstract

Sheeted dikes and brecciated dike rocks occur stratigraphically between underlying gabbros and overlying pillowed lavas of the Bay of Islands Complex at Blow-Me-Down Mountain and North Arm Mountain. The dikes trend northwesterly, dip steeply southwest or northeast and are approximately at right angles to the trend of the contacts between major rock units that comprise the complex. Similar dikes and dike breccias occur within other transported slices farther west (Little Port Complex) where they separate foliated gabbros, amphibolites, and sodic granites from adjoining relatively undeformed pillowed lavas.The dikes are metamorphosed but relict textures are everywhere preserved. Actinolite, epidote, chlorite, and zoisite are the commonest metamorphic minerals. Prehnite is abundant in some places and pumpellyite and other minerals of the zeolite facies assemblage are known locally in nearby volcanic rocks. The metamorphism within the dikes and overlying volcanic rocks can be related to depth of burial or geothermal gradient, and it is interpreted as an early hydration of the rocks that occurred in their place of formation at an Early Paleozoic spreading oceanic ridge.Brecciated dike rocks are much more common in places than sheeted dikes and cover an area of approximately 40 mi2 (64 km2) atop North Arm Mountain. The brecciation is chiefly localized in the dike horizon of the Bay of Islands Complex but it locally affects underlying gabbros and overlying volcanic rocks. In the Little Port Complex, the brecciation also affects foliated gabbros, amphibolites, and sodic granites. There is nothing to suggest that the brecciation is the result of brittle deformation. Rather the textures are comparable to those developed in igneous rocks through gas action or fluidization. The brecciation largely predates metamorphism in the dike rocks and it is therefore also interpreted as a feature related to Early Paleozoic plate accretion.The Bay of Islands Complex is a typical ophiolite suite. Recognition and definition of the Little Port Complex as a polygenetic assemblage distinct from the Bay of Islands Complex reconciles the contrasting structural styles between the two and removes a major obstacle to the interpretation of the Bay of Islands Complex as oceanic crust. The Twillingate Granite in Notre Dame Bay of northeast Newfoundland is thought to provide a direct analogy to deformed rocks of the Little Port Complex and all are interpreted as older deformed crustal remnants surrounded by ophiolitic rocks related to an Early Paleozoic ocean spreading episode.

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