Tectonic emplacement of ophiolitic rocks in the Precambrian Mona Complex of Anglesey

Abstract

THE Mona Complex of Anglesey^1–5 is a thick succession (over 5,000 m) of metasedimentary and locally metavolcanic rocks (the ‘Bedded Succession’), gneisses of uncertain age and granite intrusions. The Monian Bedded Succession consists largely of flysch-type sediments deposited in a progressively shallowing sedimentary trough. The succession includes ferruginous chert and manganiferous shale, basaltic pillow lava and a serpentinite–gabbro suite of intrusions. The younger part of the succession has a mélange of regional extent, now interpreted as an olistostrome⁴. The sedimentary succession was complexly deformed and locally metamorphosed in high temperature/pressure (sillimanite–almandine facies) and low temperature/pressure (lawsonite–glaucophane facies) conditions, and intruded by granite during the latest Precambrian⁵. The structure and rock types present clearly indicate that the Mona Complex formed near to a late Precambrian destructive plate margin^6–9. In such a setting the association of deep ocean sediment, pillow basalt and a serpentinite–gabbro suite might represent progressively deeper oceanic crustal layers, and it has been proposed that these igneous rocks are fragments of the oceanic crust and upper mantle tectonically emplaced at the Monian destructive margin. However, recent detailed studies of the serpentinite–gabbro suite by Maltman^10,11 suggest that these rocks were emplaced magmatically rather than tectonically (for example, ref. 11). Here, I review the character of the Monian ophiolitic association and argue that the serpentinite–gabbro suite represents fragments of oceanic crust tectonically emplaced during oceanic subduction processes.