The Great Whinscale Dacite — an enigmatic lava flow from the Borrowdale Volcanic Group, English Lake District

Abstract

SUMMARY The Great Whinscale Dacite (GWD) is a distinctive and important stratigraphic marker unit within the Ordovician Borrowdale Volcanic Group (BVG) of the English Lake District. Many features of the GWD are compatible with the unit being a lava — the origin proposed here — but with a low aspect ratio ( c .1: 100), a length of at least 13 km, and a minimum volume of 4 km 3 . The GWD is aphyric, remarkably homogeneous chemically, and is highly atypical of dacitic lavas in the BVG, which are distinctly porphyritic and of very restricted extent and volume. This paper presents field, petrographic, chemical and Rb-Sr isotopic age data from four transects through the GWD and one transect through a typical BVG dacite at Crowhow End for comparison. Sm-Nd isotope data are presented for selected samples. Although the overall chemistry of the GWD is very similar to other BVG dacites, the GWD has a distinctly more primitive Nd isotopic signature and is noticeably depleted in Ni and Cr. The aphyric nature of the GWD and its distinct chemical character may be attributed to an abnormally short crustal residence time for the GWD magma between source and eruption, and thus emplacement at a temperature above the liquidus. The implied short rise-time also resulted in a high effusion rate which controlled the unusual eruption geometry. Subsequently there were at least three distinct periods of isotopic and chemical disturbance.