Conjugate shear model for emplacement of midcrustal plutons during dextral transpression, southern Nova Scotia, Canada: tectonic and petrological consequences

Abstract

The South Mountain Batholith (SMB) is a syntectonic composite batholith emplaced in the upper crust within the Meguma terrane between 380 Ma and 370 Ma during the later stages of the Neoacadian orogeny. Coeval plutons in southern Nova Scotia are surrounded by mid-crustal (~4 kbar) andalusite-staurolite aureoles and are discordant to NE-trending, regional Neoacadian folds. Detailed field studies, combined with published results, indicate emplacement within a dextral transpressional regime during the transition from distributed (D1) to focused heterogeneous strain (D2), which provided vertical conduits that facilitated magma ascent. The Port Mouton Pluton (PMP) intruded along P-orientated crustal-scale fractures as a series of subvertical granitic sheets, which were progressively rotated and folded with ongoing dextral shearing. By contrast, the Barrington Passage pluton (BPP) intruded between crustal-scale, antithetic (sinistral) P-shear fractures and spread laterally between them as pulsed increments to form a layered, subhorizontal, sill-like complex resembling the laccolithic structure of the SMB. The SMB was emplaced below the Meguma Supergroup, with magma derived from underthrust Avalon terrane and Silurian Rockville Notch Group. The lack of mantle components in the SMB suggests transpressional orogenesis facilitated conductive crustal heating without significant mantle addition, consistent with low p-wave velocities of Meguma lower crust. 400-355 Ma zircons, recorded either as inherited grains in granites or in felsic granulite xenoliths, imply the Neoacadian thermal anomaly extended for 45 Ma, but magmatism represented only ~40% of that perturbation.