Internal structures in part of the South Mountain batholith, Nova Scotia, Canada

Abstract

The Devonian South Mountain batholith (SMB) in southern Nova Scotia provides an opportunity to examine the deformation produced in a magma during emplacement. The SMB was emplaced in three principal stages, from oldest to youngest, (1) granodiorite, (2) monzogranite, and (3) late granite porphyry, aplite, and pegmatite. This investigation focuses on the monzogranite that underlies the eastern end of the batholith, approximately 15 km south of the city of Halifax. The deformation consists of various types of mesoscopic and macroscopic flow folds, defined by mapping the preferred orientation of alkali feldspar megacrysts. The fold geometry is similar to that produced during melt-dominated (>30% liquid) flow in migmatites. The deformation did not affect the envelope of Cambrian-Ordovician metasedimentary rocks of the Meguma Group; hence, the stresses that caused the folding were internal. The response to the stresses was by ductile deformation (folding) and only locally by fracturing (schlieren, megacrystic dikes) during the late stages of consolidation. The magma behaved as a Bingham substance during emplacement, alternately plastic or brittle, depending on the stress.