The geochemistry and significance of sills within the Ordovician Borrowdale Volcanic Group around Black Combe, SW English Lake District

Abstract

The Borrowdale Volcanic Group (BVG) in the SW of the English Lake District (SWLD) consists predominantly of subaerial pyroclastic rocks that host syn-volcanic intrusions ranging in composition from basalt to dacite. The petrology and geochemistry of the sills verify a broad compatibility with the BVG in the central Lake District. The geochemistry indicates a marginal continental arc setting, a petrogenesis involving c. 15% partial melting of a lherzolite mantle source and subsequent fractionation of olivine, ortho-and clinopyroxene, plagioclase and Fe-Ti oxide. Three groups of basaltic sills are recognized in the SWLD, one of which bears a close resemblance, geochemically, to the lower BVG (Birker Fell Formation) basaltic lavas and appears to have been emplaced early in the SWLD succession. The other two groups, which may be cogenetic, appear to be somewhat geochemically distinct from the first, with trace element and rare-earth characteristics suggesting a source slightly more enriched in incompatible elements. It is suggested that the fault-dominated tectonic setting in the Millom Park area of the SWLD may account for the least-evolved magmas being allowed to migrate directly to a high structural level, accounting for a higher proportion of basic lithologies in the upper BVG succession of the SWLD than is found in other parts of the Lake District.