Metabentonite geochemistry: magmatic cycles and graptolite extinctions at Dob's Linn, southern Scotland

Abstract

The Moffat Shale Group is a condensed, variable and partly pelagic sequence of mudrocks of Llandeilo—Llandovery age. The sequence has a five-fold lithological subdivision based mainly on the occurrence of grey mudstones within a succession otherwise dominated by fully euxinic black graptolitic mudrocks. Associated with the black mudrocks, especially in the Llandovery, are metabentonite beds which achieve a climax, both in thickness and in number, within the top quarter of the mudrock sequence. A geochemical and mineralogical study has confirmed a volcanic origin for the metabentonites. Major element data highlight a carbonate-dominated environment above the gregarius—convolutus Zones boundary. Phosphorus levels reach a peak at the same boundary, as well as at the Caradoc—Ashgill boundary where phosphorite horizons are known from Wales and Norway. Immobile trace elements have highlighted regular changes in source magma composition. Prolonged periods of crystal fractionation in magmas of intermediate composition gave rise, on eruption, to large volumes of silicic ash which had a deleterious effect on graptolite species and led to local extinctions. Regular fluctuations in ash composition from silicic to intermediate are ascribed to alternating fractionation and magma mixing cycles.

Illitization and organic maturity in Silurian sediments from the Southern Uplands of Scotland

“…X-ray fluorescence analysis of bentonitic material from the Kirkcudbright area indicates a parent ash composition of a rhyodacite/dacite nature (Merriman & Roberts, 1990). Similar results have been obtained for the K-bentonites at Dob's Linn (Batchelor & Weir, 1988;Merriman & Roberts, 1990). It therefore seems unlikely that sufficient differences in initial smectite composition existed between the two regions to account for the observed differences in illitization.…”

The Lower Silurian Osmundsberg K-bentonite. Part II: mineralogy, geochemistry, chemostratigraphy and tectonomagmatic significance

“…For example, Huff, Morgan & Rundle (1997) report Llandovery through Ludlow K-bentonites in Great Britain typically contain R1 ordered I/S plus varying amounts of kaolinite, and Huff et al (1991) reported long-range or R3 ordered I/S in Llandovery K-bentonites from northern Ireland and the Southern Uplands of Scotland. Batchelor & Weir (1988) interpreted powder diffraction analysis of K-bentonite clays from the Southern Uplands as R0 I/S, but their XRD tracings clearly show R3 ordering. Silurian K-bentonites from Podolia, Ukraine, contain R0 ordered I/S in carbonate facies and R1-R3 ordered I/S in the shale facies (Huff, 1997).…”

Correlation of upper Llandovery–lower Wenlock bentonites in the När (Gotland, Sweden) and Ventspils (Latvia) drill cores: role of volcanic ash clouds and shelf sea currents in determining areal distribution of bentonite

“…Ray 2007;Ray et al 2011) and Scotland (e.g. Batchelor & Weir 1988;Batchelor 2009). In the East Baltic area ash beds from a total of 51 volcanic eruptions have been identified in the Telychian (Kiipli et al 2008b(Kiipli et al , 2008c(Kiipli et al , 2008d(Kiipli et al , 2010b and 55 in the Wenlock (Kiipli et al 2010a).…”