The Monian ‘Penmynydd Zone of Metamorphism’ in Llŷn, North Wales

Abstract

In the Mona Complex of Llŷn, a narrow, steep belt of fine grained schists known as the Penmynydd Zone of Metamorphism lies between a deformed melange (Gwna Group) and a unit of gneisses and plutonic rocks (Sarn Complex). Rocks within and adjacent to this Zone are characterized by mylonitic textures. The highest grade Penmynydd rocks on Llŷn reached greenschist and, locally, glaucophanitic greenschist fades conditions. The schist belt is interpreted as a major shear zone formed as the Sarn Complex moved up against the lower grade Gwna Group. A previous interpretation of the schists (Shackleton 1956) as being the result of a prograde metamorphism from sub‐greenschist facies Gwna Group to anatectic granitic and gneissic rocks of the Sarn Complex is rejected.

Peri-Gondwanan terrane interactions recorded in the Cambrian–Ordovician detrital zircon geochronology of North Wales

“…Detrital zircons from this unit included a 522 ± 6 Ma grain (late Terreneuvian in the timescale of Peng et al, 2012) interpreted by Collins and Buchan (2004) as the maximum depositional age. The overlying New Harbour Group is characterized by pelitic rocks with subordinate serpentinite, gabbro, basalt, and chert (Gibbons, 1983). The youngest unit in the Monian Supergroup is the Gwna Group, a mélange that contains both deep-water and continental clasts including pillow lava, chert, sandstone, limestone, and granite (Gibbons, 1987).…”

Myths and recent progress regarding the Argille Scagliose, Northern Apennines, Italy

“…This is a north reoccurrence of Stainmann's famous 'trinity' that he first described in the Penninic Alps (Steinmann, 1907). More recent studies (Gibbons 1983;Thorpe 1978;Gibbons and Harris 1994), in fact, have interpreted the Gwna mélange as formed by processes directly associated with the subduction of an oceanic plate and arc beneath continental crust.…”

The Môn–Deemster–Ribblesdale fold–thrust belt, central UK: a concealed Variscan inversion belt located on weak Caledonian crust

“…One particularly distinctive tectonic unit in Anglesey, first recognised by Greenly (1919) a century ago, is the Penmynydd Zone (Gibbons, 1983). This is a megashear zone (Fig.…”

“…Its age of formation is not well constrained, but probably encompasses at least the first two events of the framework outlined above. The presence of protoliths with a wide range of densities, from felsic lavas and cogenetic granites, to blueschist and amphibolite (Gibbons, 1983), is likely to enhance the reflectivity of the zone. Another key element is the Carmel Head Thrust (Figs.…”