Abstract

Layering in appinites (hornblende-rich gabbros) of the ca. 610 Ma Greendale Complex, Antigonish Highlands, Nova Scotia, is defined by variations in texture and modal abundances of hornblede and plagioclase, and ranges in width from 5 to 50 cm. Regional studies indicate that the complex was probably emplaced during late Precambrian dextral shear on northeast-trending faults by brittle failure at the roof of the magma chameber. The origin and orientation of the layers may have been controlled by the regional tectonic setting. The layering is steep to vertically dipping and varies in strike from 080° to 180° (clockwise) with concentrations at 090° and 160°. The 090° layers are dilational and are thought to have developed perpendicular to north-south extension along the extensional plane of the instantaneous strain ellipsoid associated with progressive dextral shear. The 160° layers commonly display boudinage. They may have rotated clockwise from an original 090° attitude toward the plane of flattening (and hence into a stretching field) during progressive dextral shear. Late-stage cogenetic felsic veins fill conjugate shear fractures consistent with a dextral shear regime. This study emphasizes the potential importance of considering regional tectonic setting when analyzing the origin and orientation of igneous fabrics.

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