Dennis S. Wood, 1974. "Ophiolites, Melanges, Blueschists, and Ignimbrites: Early Caledonian Subduction in Wales?", Modern and Ancient Geosynclinal Sedimentation, R. H. Dott, Jr., Robert H. Shaver
Download citation file:
North Wales appears to be unique among pre-Mesozoic terranes in possessing a temporal and spatial association of all phenomena that are presently considered to be indicative of plate boundaries and zones of crustal subduction.
More than 30,000 feet of late Precambrian sedimentary and volcanic rocks belonging to the Monian System are preserved in Anglesey and Caernarvonshire. These rocks accumulated in a subsiding basin, which probably was one of several marking the onset of the Caledonian cycle. The lower part of this eugeosynclinal sequence consists of flysch sediments, whereas the upper part contains limestones, arenites, cherts, and basic pillowed lavas, and is characterized by the presence of melange over a region of several hundred square miles. The melange, which is the original example named by Edward Greenly in 1919, attains a thickness of at least 3,000 feet and is a deformed sedimentary slide rather than a tectonic breccia. The term melange should be descriptive only. The lower part of the Monian contains a suite of ultramafic and mafic intrusions, which are serpentinized and carbonated to varying degrees. Late Precambrian deformation was accompanied by metamorphism of variable intensity over short distances, so that unmetamorphosed sedimentary and volcanic rocks pass into sillimanite-bearing migmatites within a distance of a few hundred yards. Blueschist metamorphism proceeded in relation to major zones of contemporaneous shearing (slides) in eastern Anglesey where the Precambrian now passes beneath the lower Paleozoic rocks to the east. Between the upper Precambrian of Anglesey and the Lower Cambrian Series of the Welsh mainland is a thick sequence of ignim- brites, which pass conformably upwards into the Lower Cambrian. Both the Monian System and the earliest Cambrian (Arvonian) ignimbrites are intruded by potash granites, which, like the Monian metamorphic rocks, yield isotopic ages within the range of 580 to 610 my. These ages, for a region famed for its thick classical Cambrian sequence, demonstrate that metamorphism and intrusive activity occurred at a high crustal level, were probably shortlived, were virtually contemporaneous with ignimbrite activity, and were immediately followed by the development of the lower Paleozoic Welsh Basin to the southeast. The Arvonian ignimbrites are interpreted as recycled upper crustal material derived by melting from the Monian rocks during subduction near the edge of the incipient Welsh Basin. A continuation of the same process could well explain the great thickness of basic to acidic Ordovician volcanic rocks of the Snowdonia region of North Wales, which probably constituted a Middle Ordovician island arc system. It could also explain the lower Paleozoic gold and copper mineralization of North Wales. The Anglesey region was a moderately to strongly positive area throughout the lower Paleozoic, constituting an important cordilleran zone of internal sediment supply within the Caledonian mobile belt. Structures in the Monian basement controlled the development of large fault- bounded rotational blocks, which helped to localize the positions of several major unconformities.
The regional relationships invite analogy with the Mesozoic sequences of California. The Monian would be equivalent to the Franciscan Sequence, and the lower Paleozoic would be equivalent to the Great Valley Sequence. The greater time separation between oldest Monian and the youngest rocks of the Welsh Basin than between oldest Franciscan and youngest Great Valley rocks may be accounted for by invoking in North Wales a smaller amount of plate consumption and a slower rate of subduction.