Groups and Networks

The Precambrian is sparsely distributed and generally occupies relatively small areas of southern Britain, however, these encompass an amazing diversity of rock from volcanic sequences, sedimentary strata ranging from deep-water to terrestrial environments, plutonic igneous rocks, and low- to high-pressure metamorphic rocks. Some of the sediments contain the enigmatic fossil impressions which are representative of the famous Ediacaran fauna. These rocks belong to the youngest part of the Proterozoic eon of the Precambrian, the ‘Neoproterozoic’. Precambrian rocks are dated by radiometric means to determine an absolute age using the decay schemes of the various radiogenic isotopes contained within the rocks.

The distribution of most of these rocks is in part determined by fault systems that have episodically uplifted the Precambrian from beneath coverings of younger (Phanerozoic) sedimentary strata. These fault systems have had a long and complex history that probably began in latest Neoproterozoic (about 600 Ma), and had repeated movements in response to compressive stresses during the Acadian and Variscan orogenies during the Palaeozoic, and extensional reactivations during the Mesozoic.

The Precambrian geology in England and Wales is sub-divided by the Menai Strait Fault system into two main terranes (Monian Composite Terrane and Avalonian Composite Terrane), with a number of sub-terranes within these. To the SW of this fault system the oldest igneous suites (the Stanner-Hanter, Malverns, Johnston complexes) are associated with an early phase ( c. 700– 640 Ma), of subduction-related calc-alkaline arc magmatism. The pre-existing continental crust is nowhere exposed, but isotopic dating evidence suggests that it may be as old as mid-Proterozoic (~1800-1100 Ma). Pervasive metamorphism, ductile deformation, uplift and erosion followed crystallization of these various plutonic suites at moderate crustal depths. This was followed after a considerable period of time by the ‘Main Magmatic Event’, during which several subduction-related volcanic-arc igenous suites (e.g. Padarn, Sarn, Charnian, Glinton) were erupted or emplaced between 620 and 585 Ma. Later rifting of some of these volcanic arc terranes, (e.g Wrekin Terrane), and is reflected in the late (570–560 Ma) eruption of transtensional and marginal basin volcanic suites (e.g. Uriconian, Coomb, Warren House). This was followed by voluminous sedimentation in rapidly subsiding transtensional pull-apart basins (Longmyndian, upper Arfon Groups) reflecting either the increasingly oblique nature of subduction convergence and/or the onset of terrane dispersal. Subsequent inversion of these basins was accompanied by deformation resulting in folding, mild metamorphism and weak cleavage formation. Hence, the amalgamation of the Avalonian Composite Terrane was a long-lived and progressive affair lasting some 200 Ma.

In Anglesey the similar age of the main calc-alkaline plutonic phase (represented by the Coedana Granite), to the Main Magmatic Event in the British Avalonian Composite Terranes is compatible with this overall history. The Athewy sub-terrane blueschists  and Monian Supergroup (and closely associated Gwna Melange and pillow basalts) are thought to be fragments of an accretionary prism caught between slivers of dismembered arc (e.g. Bwlch Gwyn Tuff on Anglesey and Padarn Tuff of the Arfon Group) and fore-arc (Central Anglesey Gneiss) crust, formed by SE-directed subduction.

These Precambrian events produced a heterogeneous amalgamation of disparate terranes at the margin of Gondwana by mid Cambrian times. In early Ordovician times part of this recently accreted crust was rifted away, producing a new terrane, known as ‘Avalonia’, of which the crust of southern Britain occupies the eastern part (extracts from Carney et al. 2000).

Further Information

Carney, J.N., Horak, J.M., Pharaoh, T.C., Gibbons, W., Wilson, D., Barclay, W.J., Bevins, R.E., Cope, J.C.W. & Ford, T.D., (2000). Precambrian Rocks of England and Wales , Geological Conservation Review Series, No. 20, Joint Nature Conservation Committee, Peterborough, 252p.