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Middle to Late Proterozoic Volcanic Suites of the North Atlantic Borderlands |
Deep Geology Research Group, British Geological Survey, Keyworth, Nottinghamshire, UK
Earth Sciences Department, Open University, Milton Keynes, Buckinghamshire, UK
Department of Geology and Physical Sciences, Oxford Polytechnic, Oxford, UK
The late Proterozoic basement of central England is largely concealed by a cover of Phanerozoic rocks. Proterozoic volcanic rocks are exposed as a result of localized uplift/folding in the eastern Midlands (the Charnian Supergroup) and adjacent to major crustal lineaments which include the Church Stretton and Pontesford-Linley Fault Zones (the Uriconian volcanic group) and the Malvern lineament (the Warren House volcanic group). The geochemistry of these volcanic associations has been examined through midocean ridge basalt (MORB)-normalized multi-element diagrams which emphasize the contrasted and variable relationships between large-ion lithophile (LIL) elements and high field strength (HFS) elements within/between these volcanic groups.
The Charnian Supergroup comprises over 2.5 km of bedded volcaniclastic and sedimentary rocks locally with interbedded lavas and high-level intrusions (termed ‘porphyroids’) of andesitic to rhyodacitic composition. The more mafic lavas are characterized by enrichment of the LIL elements K, Rb, Ba and Th, while HFS elements (Nb, Ta, Zr and Y) are depleted relative to MORB, and suggest that the Charnian lavas resulted from calcalkaline volcanism in an island arc. The Charnwood diorites, a minor intrusive suite emplaced into the bedded series, reflect increasing maturity of this same arc. The Uriconian volcanic group comprises a bimodal association of volcanic and pyroclastic rocks ranging from basalt to rhyolite in composition. They are enriched in HFS elements relative to MORB, which may indicate a within-plate component to the source, although their high LIL/HFS element ratios imply a subduction-related environment. The tectonic environment of eruption may have been a post-subduction, ensialic faulted basin between the Church Stretton and Pontesford-Linley Fault Zones. The Warren House group is in tectonic contact with the Malvernian (calcalkaline) plutonic complex and has LIL-element concentrations enriched in relation to MORB, but shows slight depletion of HFS elements similar to that exhibited by some marginal basin basalts and island arc tholeiites. These lavas may represent relics of marginal basin lithosphere trapped and uplifted in a Proterozoic suture between the Uriconian and Charnian terranes.
These new geochemical data suggest that the late Proterozoic basement of central England comprises a heterogeneous mosaic of geochemically-distinct volcanic terranes, formed in separate volcanic arc/marginal basin environments and juxtaposed by subsequent accretionary tectonism in the Avalonian/Cadomian Orogeny (cf. Thorpe et al. 1984).
