The geology of the northern part of North Arm Massif, Bay of Islands Ophiolite Complex, Newfoundland: with application to upper oceanic crust lithology, structure, and genesis
Detailed mapping (1:15,800) of the northern half of North Arm Massif shows the area to be underlain with a complete, although thin, ophiolite assemblage consisting, from top to base, of basalts, sheeted diabase dikes, isotropic and layered gabbroic rocks, layered ultramafic rocks and harzburgite tec...
| Summary: | Detailed mapping (1:15,800) of the northern half of North Arm Massif shows the area to be underlain with a complete, although thin, ophiolite assemblage consisting, from top to base, of basalts, sheeted diabase dikes, isotropic and layered gabbroic rocks, layered ultramafic rocks and harzburgite tectonite. The assemblage represents a preserved piece of Cambrian oceanic crust analogous to present-day oceanic crust. Sheeted dikes and isotropic gabbros comprise the majority of the rocks exposed, with this sequence at present flat-lying to gently warped into open, upright but nonsystematic folds. Basalts occupy a shallow discontinuous N-S trending trough within the west-central part of the area. The remaining lithologies are exposed in lesser amounts along the western, eastern and northeastern edges of the surveyed area. Volcanics consist of pillowed lavas, massive flows and breccias plus intercalated red sediment, all crudely layered, with layer attitudes intersecting the lower horizon of the unit with large angles. Volcanics are fed by underlying diabase dikes, and grade into sheeted dikes by downward increase in numbers of dikes across a transition generally less than 50 m thick. The sheeted dikes unit consists entirely of multiple diabase dikes of 0.9 m mean thickness, subparallel throughout and trending NNW-SSE across the massif. The dike intrusion process includes a mode wherein several dikes each intrude the previous dikes, with the position of these very narrow, short term spreading axes episodically shifting within a wider but still narrow overall zone of intrusion and spreading. Dikes intersect the upper and lower contacts of the unit with mean angles of 60º and 75º respectively, and dip away from the spreading center, located to the west of the present position of the section on the basis of dikes attitudes and chill margin analysis. The sheeted dikes unit ranges between 400 and 600 m thick. An estimated 40% of the diabase shows extensive in situ fracturing in distinct zones subparallel to dikes trends. ... |
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