CONSTRAINTS ON THE TECTONIC SETTING OF DEVONIAN MAGMATISM AND OROGENESIS

Acadian (Early Devonian) orogenesis is constrained by the tectonic processes during the Silurian. Accretion of Ganderia to Laurentia between 442 - 430 Ma implies that all intervening oceanic lithosphere had been subducted and that most of the northern Appalachians, with the exception of Avalonia and Meguma, by then formed part of composite Laurentia. Convergence between Avalonia and composite Laurentia (Ganderia) started at c. 442 Ma as a result of closure of the oceanic Acadian seaway. Evidence for this closure is preserved in the extensional Coastal arc and associated high-pressure fore-arc slivers formed at the leading edge of Ganderia. Arc magmatism lasted until c. 421 Ma when magmatism became collisional. The start of Acadian collision is constrained by inversion of the Mascarene and La Poile back-arc basins and inboard migration of the Coastal arc, both suggesting shallowing of the Acadian slab, probably due to subduction of the leading edge of Avalonia.

Between 416 - 400 Ma, the Acadian magmatic and deformation fronts progressively moved inboard at the latitude of Maine and New Brunswick, reflecting progressive retro-arc thickening, also evidenced by formation of late syn-Acadian felsic melts with both arc and non-arc characteristics. Inboard migration of arc magmatism may have been due to expulsion of the mantle wedge towards the thickening hinterland, but may also reflect melting of metasomatised mantle preserved in pockets above the dehydrating Avalonian flat slab. Non-arc magmatism probably formed by a combination of melting of the thickened lower crust induced by dehydration of the underlying flat slab, and in slab windows where the slab significantly changed dip and/or broke-off.

Meguma had started to accrete to composite Laurentia (Avalonia) by 400 Ma, by means of wedging. This led to local thickening of Meguma crust, which started to melt and produce c 380 to 360 Ma S-type granitoid rocks. This magmatism was synchronous with Middle to Late Devonian subduction under Meguma by the outboard Rheic Ocean. Areas of c. 373 Ma high temperature-low pressure regional metamorphism and related felsic and mafic magmatism in Meguma are related to parts of the subducting slab breaking or rolling-back, allowing hot asthenosphere to thin and melt Meguma crust. Subduction under Meguma continued well into the Carboniferous.