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The late Precambrian/early Cambrian Monian basement to the lower Paleozoic cover in Anglesey does not form one coherent stratigraphic unit but is divisible into at least three separate suspect terranes. Where they are exposed, the boundaries to these terranes range from ductile shear zones to brittle faults, all of which have an undetermined amount of displacement. Terrane 1 is represented by the Monian Supergroup: a thick sequence of marine sedimentary rocks with subordinate basic volcanic rocks, gabbros, and serpentinites, overlain by olistostromic mélange. Terrane 2 is identified as the Coedana Complex of central Anglesey: high-grade gneisses and the ca. 600-Ma Coedana Granite with an associated hornfels. Terrane 3 is represented by the southeast Anglesey schist belt and includes the ca. 550 to 560-Ma Anglesey blueschists. These suspect terranes are bounded to the southeast by the Menai Strait Fault System, to the southeast of which rocks of the Sarn Complex are placed within a separate terrane which may represent the northwest margin of the Avalon terrane in Britain. Terrane 1 and possibly terrane 2 are traceable into southeast Ireland (Cullenstown Formation and Rosslare Complex). Monian terranes are not known from elsewhere within the British Isles, although broadly coeval rocks occur in different terranes as the Avalonian/Cadomian rocks of southern Britain and the Channel Islands, and the Dalradian Supergroup of Scotland and northwest Ireland. No Monian rocks have yet been identified from the Tornquist line in the east, whereas several attempts at correlation have been made between Anglesey and the eastern seaboard of North America. Such attempts have failed to appreciate that there is not one “Mona Complex,” but a collage of disparate terranes, some of which are juxtaposed by shear zones comparable in scale to major crustal shears developed along transcurrent plate boundaries.

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