A microstructural investigation of the deformational conditions and kinematics of flow in peridotite tectonites from the North Arm Mountain massif, Bay of Islands ophiolite complex, Newfoundland

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1987

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Abstract

Microstructural and petrofabric investigations of cumulate and residual peridotite rocks from the North Arm Mt. massif, Bay of Islands ophiolite, Newfoundland suggest that the two units share a similar deformational history. Comparable stresses (350-425 bars from olivine neoblast and subgrain paleo- piezometry), strain rates (about 1 x 10 -14 sec), deformational temperatures (950[degrees]C+ from slip systems inferred through olivine petrofabric and kink band studies), mineral shape anisotropies, olivine petrofabrics and metamorphic textures (porphyroclastic-equigranular) are found in rocks from both sides of the cumulate-residual boundary. A low-temperature, high-strain, high-stress, high-strain rate deformation with NW (subduction direction) peridotite flow (from olivine Z-axis and spinel Iineation trends) characterizes the lowermost three km of North Arm massif rock. Stresses, strains and strain rates decline in magnitude up section while some flow directions above the three km level are found to trend NE (the sea floor spreading direction inferred from sheeted dike orientations). Basal North Arm massif rocks are considered to have undergone a subduction-related deformation that overprints original mantle upwelling- derived microstructures. Deformation is heterogenous in upper North Armmassif peridotites, with subduction-related strain partitioned into discrete shear zones.

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Keywords

Ophiolites--Newfoundland and Labrador, Geology--Newfoundland and Labrador, Peridotite

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