Neoproterozoic plutonic rocks of the eastern Cobequid Highlands, N.S. (NTS 11E/10&11)

Abstract

Crystalline rocks east of Earltown in the Cobequid Highlands, Colchester County, Nova Scotia, have been mapped and representative samples studied for petrography, rock geochemistry and minerai chemistry. About 50 whole-rock analyses, 70 petrographic descriptions ofrocks, and 140 minerai analyses are presented. The rocks are plutonic or highly deformed plutonic rocks and almost ail petrographically and geochemically resemble Neoproterozoic plutonic rocks known elsewhere in the Cobequid Highlands, including gabbro, tonalite, granodiorite and granite. Minor Devonian-Carboniferous gabbro and intrusive rhyolite is found near the Cobequid Fault. Severa! types ofNeoproterozoic plutonic rocks are distinguished on the basis of petrography and geochemistry. These occurs in two groups, correlatable with the Bass River Block and the Jeff ers Block of the central and western Cobequid Highlands. The Mount Thom Complex comprises metasedimentary rocks of the Gambie Brook Formation intruded by late Proterozoic mafic plutonic rocks, and th us is not a basement to the A val on zone. lt has a similar origin to the Great Village River Gneiss. The Rockland Brook fault represents the Neoproterozoic suture zone of the Bass River and Jeff ers blocks. In the central Cobequid Highlands, this fault was reactivated in the late Paleozoic and was an important pathway for magma, but it was not reactivated in the eastem Cobequids Highlands, whether motion was taken up on the Cobequid and Millsville faults.