DOES PERI-GONDWANAN MEGUMIA EXTEND INTO SW ENGLAND?

Peri-Gondwanan Megumia, the type location of which occupies southern Nova Scotia, Canada, is the only major lithotectonic element of the northern Appalachian orogen that has no clear correlatives elsewhere in the Appalachians and lacks firm linkages to the Caledonide and Variscan orogens of western and southern Europe. This is in contrast with its immediate peri-Gondwanan neighbor, Avalonia, which shows similarities to portions of Carolinia in the southern Appalachians and has been traced from the Rhenohercynian Zone of southern Britain eastward around the Bohemian Massif to the South Carpathians and western Pontides. At issue is the tendency in Europe to assign all peri-Gondwanan terranes (like Megumia) lying north of the Rheic suture to Avalonia, characterized by relatively juvenile basement and detrital zircon ages that include Mesoproterozoic populations, and those to the south of the suture to Cadomia, characterized by a more evolved basement and detrital zircons with ages that match Paleoproterozoic and older sources in the West African craton.

The unexposed basements of Avalonia and Megumia are thought to be isotopically similar. But the sedimentary cover in Megumia contains scarce Mesoproterozoic zircons, such that felsic magmas produced by crustal melting in Megumia (e.g. the ~370 Ma South Mountain Batholith) are isotopically more juvenile (eNd = -5 to -1, T_DM = 1.3 Ga) than the rocks they intrude (eNd -12 to -7, T_DM = 1.7 Ga). This distinctive relationship is also recognized for the Sierra Norte Batholith (~330 Ma; eNd = +1 to -3, T_DM = 0.9–1.2 Ga) and its Late Devonian host rocks (eNd = –5 to –11) in southern Spain, where it has been used to suggest the possible presence of Megumia basement beneath the South Portuguese Zone. Available data for the Cornubian batholith of SW England (~270-290 Ma; eNd = -5 to -7, T_DM = 1.3–1.8 Ga) and the Devonian metasediments it intrudes (eNd = –8 to –11) suggests this may also be true of that part of the SW England (Rhenohercynian Zone) with which the South Portuguese Zone is traditionally correlated.