New constraints on the timing and nature of metamorphism in the eastern Meguma terrane, Nova Scotia, Canada

Abstract

The Meguma terrane in Nova Scotia consists mainly of Cambrian to Ordovician metasedimentary rocks intruded by abundant peraluminous granite, granodiorite and minor diorite-tonalite-plutons between ca. 380 Ma and 360 Ma. The metasedimentary rocks were variably metamorphosed to greenschist- to amphibolite- facies during the early Devonian. In the eastern Meguma terrane, the regional M1 chlorite- and biotite- zone assemblages are overprinted by two types of high-temperature low-pressure M2 metamorphism. In contact aureoles around plutons the regional M1 assemblages are overprinted by cordierite- and andalusite-bearing M2 contact metamorphic assemblages, and away from plutons chlorite-bearing, (or biotite-bearing) regional M1 assemblages are overprinted by staurolite-, garnet-, and/or cordierite-bearing M2 regional assemblages that represent metamorphism driven by a regional elevated thermal gradient. The interpreted metamorphic conditions in the eastern Meguma terrane range from ca. 480 – 550 °C and pressures below 2.5 kbar up to 550 – 650 °C with pressures between 4 and 5 kbar. Based on a new garnet Lu-Hf age, regional metamorphism was ongoing at ca. 372 Ma, hence broadly coeval with pluton emplacement. Metamorphism and pluton emplacement took place in a largely transpressional tectonic setting during the collision of the Meguma terrane with Avalonia. After pluton emplacement, peak metamorphic assemblages were variably overprinted with retrograde chlorite and sericite M3 assemblages while deformation continued. Regional structures were transposed into a composite transposition foliation as a result of continued motion along the terrane-bounding Minas Fault Zone and associated ductile shear zones.