The timing of Ordovician magmatism in the English Lake District and Cross Fell inliers

Abstract

AbstractVolcanic and hypabyssal intrusive rocks of the Lower Palaeozoic English Lake District and Cross Fell inliers are elements of the Ordovician destructive plate margin system of microcontinental Avalonia. Two igneous sheets within the marine sedimentary Skiddaw Group of these inliers, previously described as lavas, are reinterpreted as sills. Sedimentary rocks enclosing these sills are of late Tremadoc-early Arenig (c. 493 Ma) and early Llanvirn (> 476 Ma) age, and breccias along the upper contacts of both were produced by steam explosivity and fluidization ahead of theadvancing tips of the intrusions. Previous interpretation of the breccias on the older sheet, as sediment deposited on the eroded top of a lava flow, implied an early Ordovician onset of arc magmatism. Such early magmatism would have been virtually coincident with the latest Tremadoc initiation of arc magmatism in Wales, but evidence for such a near synchronous response tothe putative onset of subduction is lacking. Respective onsets of magmatismwere probably separated by at least 17 m.y., and possibly by as much as 29 m.y. The apparent contemporaneity of mid and late Ordovician volcanic episodes in England and Wales, and similarities in extensional tectonic style, suggest that the two areas then were part of the same subduction system responding similarly to plate-scale magma-generating and tectonic processes. The early Ordovician situation is uncertain, but the absence of arc volcanic rocks of this age in the English Lake District suggests that this area and Wales are not tectonically juxtaposed elements of a former simple linear arc.