Early Carboniferous deformation and mineralization in the Cobequid shear zone, Nova Scotia: an 40Ar/39Ar geochronology study

Abstract

Latest Devonian to Early Carboniferous igneous rocks along the Cobequid shear zone of central Nova Scotia record a complex history of intrusion, volcanism, deformation, and hydrothermal alteration. Twenty new 40Ar/39Ar age determinations have been made on biotite and hornblende separates from rocks that constrain these events. The mafic plutons of the Cobequid shear zone are synchronous with the previously dated granitoid plutons (363–355 ± 4 Ma), with the exception of the younger Folly Lake gabbro (355–350 ± 4 Ma). High temperature shear deformation continued along the Cobequid shear zone during this entire period of pluton emplacement. Several samples from mylonitic and hydrothermally altered rocks with dates ca. 340 Ma indicate renewed movement along the Cobequid shear zone at this time, accompanied by minor magmatism. Younger ∼330–320 Ma ages reflect final movement along the Rockland Brook fault resulting in mid-Namurian uplift of the Cobequid Highlands. Following this regional Alleghenian event, there was no significant motion on the northern faults of the Cobequid shear zone and deformation moved southward to the Cobequid fault and its continuation in the Hollow and Chedabucto fault zones. Hydrothermal alteration, probably driven by a mid-crustal gabbro heat source, was widespread along the Cobequid and Rockland Brook faults in the early Carboniferous, resulting in albitization, potassic alteration, and iron mineralization.