THE NEOACADIAN OROGENY IN THE MEGUMA TERRANE, NOVA SCOTIA, CANADA

The Meguma Terrane is exposed on land only in southern Nova Scotia, but drill hole and geophysical data show that it can be traced in the offshore from the southernmost Grand Banks to Cape Cod, Massachusetts. The terrane is characterized by mainly Cambrian turbiditic sandstone-shale of the Meguma Group, disconformably overlain by Silurian bimodal within-plate volcanic and siliciclastic rocks (White Rock Formation), and in turn by Early Devonian carbonate, siliciclastic, and volcanic rocks (Torbrook Formation). These younger rocks represent the onset of rifting of the Meguma terrane from Gondwana. Regional deformation is recorded by folding, cleavage development, and low-grade metamorphism at ca. 395 Ma, interpreted to be related to dextral oblique docking of Meguma with Avalonia along the Cobequid-Chedabucto Fault during the Neoacadian Orogeny. Deformation was followed by voluminous granitoid plutonism at ca. 380 Ma (South Mountain Batholith), ca. 376-372 Ma (‘peripheral' plutons), ca. 363 Ma (offshore Seal Island Pluton), and ca. 357 Ma (Wedgeport Pluton). Many of the ca. 376-372 peripheral plutons are associated with syntectonic high-temperature/low-pressure regional metamorphism. The chemical and isotopic similarities among the granitoid plutons suggest they are genetically linked, were derived from similar source areas, and experienced similar evolutionary processes. However, the large range in ages and some petrological differences indicate that they did not form from a single magma but represent similar processes continuing over a span of 25 Ma. The ca. 380 Ma and 376-372 Ma plutons and high-temperature/low-pressure metamorphism may have been related to delamination of a shallow-dipping Rheic Ocean lithosphere following accretion of Meguma to Avalonia. Rapid uplift and erosion in the Meguma terrane between 370 and 360 Ma resulted in deposition of the Upper Devonian to Lower Carboniferous Horton Group in the northern parts of the terrane. Uplift coincided with intrusion of the younger (ca. 363 and 357 Ma) plutons in southern Meguma terrane and reflects continued northwestward subduction of Rheic Ocean lithosphere under the Meguma terrane as Gondwana approached from the southeast. Widespread ca. 320 Ma deformation in southern Nova Scotia maybe related to the final juxtaposition of Gondwana.