Palaeomagnetic data from the Borrowdale Volcanic Group: volcano-tectonics and Late Ordovician palaeolatitudes

Abstract

Nineteen palaeomagnetic sampling sites from the Borrowdale Volcanic Group between Borrowdale and Langdale (English Lake District) yield high unblocking temperature magnetization components which postdate the structural tilt, and therefore postdate the formation of the broad block-faulted syncline which characterizes this part of the central Lake District. The mean magnetization direction before tilt correction (Dec: 347.2°, Inc:–61.9°, α 95 : 6.9°) corresponds to a palaeolatitude of 43°S (+ 5°/–7°). The magnetization is associated with pervasive Late Ordovician hydrothermal alteration, and is certainly pre-Devonian based on comparison with available palaeomagnetic data from southern Britain. This implies that the block-faulted synclinal structure of the Borrowdale/Langdale region is pre-Acadian in age, consistent with recent interpretations of Late Ordovician volcano-tectonics based on mapping and volcanological studies.