CONSTRAINTS ON THE TECTONIC SETTING OF DEVONIAN MAGMATISM AND OROGENESIS
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.