Baseline geochemistry of Devonian low-grade metasedimentary rocks in Cornwall: preliminary data and environmental significance

Abstract

Geochemical baseline data provide information on the state of the environment before modification by mining and other anthropogenic activities. A summary of the analyses of 41 elements on 100 samples from Devonian metasedimentary rocks away from areas of mineralization and granite intrusions in Cornwall are presented. The data show a remarkable consistency. Differences in major elements can be related largely to changes in mineral proportion expected from grain size variations within the sedimentary rocks. The trace element data indicate that similar types of sediment accumulated in the Gramscatho, Looe and Trevone basins and that subsequent metamorphism has not significantly re-distributed elements. Compared with an average UK Lower Palaeozoic shale and the North American Shale Composite, the Devonian metasedimentary rocks of Cornwall appear to be marginally depleted in many trace elements. Values for As, Ni and Cr frequently exceed published soil guideline values, especially in mudstones, which brings into question the issue of the identification of land, which is considered to be polluted.