Abstract
Synopsis The late Proterozoic Diabaig Formation consists of alluvial red breccias and sandstones, and lacustrine grey shales, which accumulated in the lower parts of palaeovalleys eroded in gneiss. The sediments are significantly depleted in Ca and Na with respect to the local gneisses which provided 80% of the sediment. Geochemical mass balance calculations, taken together with the mineralogy, show that these losses occurred mainly during breakdown of plagioclase and amphibole during sediment transport and reworking. Mobilisation of Ca and Na during diagenesis was quantitatively insignificant, despite clear evidence for albitization of detrital plagioclasc. There is no evidence of primary carbonates or evaporites in the shales. The Ca and Na removed from the sediments was lost to a hydraulically open, regionally extensive lake.
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