Abstract

Basement samples from offshore wells and geophysical data show that the Meguma terrane forms the basement of Mesozoic–Cenozoic sedimentary basins as far as 200 km offshore Nova Scotia. The data provide constraints on Avalon-Meguma terrane interaction during middle to late Paleozoic events in the Appalachian orogen. The basement consists of metasedimentary rocks intruded by late Paleozoic granitoid plutons, principally magnetite-bearing granodiorites, which contrast with more peraluminous ilmenite-bearing Devonian granodiorite and granite onshore. Offshore tonalites have a subduction-related trace element signature that is similar to the signature in minor Devonian gabbro and tonalite intrusions on land. Nd, Sr, and Pb isotopes suggest three sources for Meguma terrane plutons: (1) 2 Ga basement (possibly Saharan Shield) and sediments derived therefrom—this basement may be present beneath the southwestern Scotian Shelf; (2) Late Proterozoic underthrust Avalonian basement and basement autochthonous to the Meguma terrane; and (3) melts from mantle last remobilized in the Late Proterozoic Pan-African orogeny.

Seismic reflection profiles show Meguma terrane thrust northward over a wedge of Avalon basement extending 30 km south of the surface suture. There is no evidence that the Devonian tonalites are directly related to subduction. The lack of either ophiolites or Paleozoic pelagic sediment suggests that convergence of the Avalon and Meguma terranes was achieved principally by strike-slip motion resulting from low-angle continent-continent convergence, with only a minor thrust component. The widespread Devonian-Carboniferous igneous activity of the amalgamated Meguma and Avalon terranes was a consequence of regional extension, which resulted in decompression melting of upper mantle and production of mafic magmas variably mixed with crustal material.

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