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Metabentonites in the Moffat Shale Group, Southern Uplands of Scotland: Geochemical evidence of ensialic marginal basin volcanism

Published online by Cambridge University Press:  01 May 2009

R. J. Merriman  and
B. Roberts
R. J. Merriman
Affiliation:
British Geological Survey, Keyworth, Nottingham, NG12 5GG, U.K.
B. Roberts
Affiliation:
Geology Department, Birkbeck College, 7–15 Gresse Street, London, W1P 1PA, U.K.
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Abstract

Metabentonites occur extensively in the Moffat Shale Group of the Southern Uplands of Scotland. At Dob's Linn 135 metabentonite beds, 1–50 cm thick, occur in Ashgill to Llandovery strata, representing an aggregate thickness of 6 m of compacted ash accumulated over approximately 25 Ma. The metabentonites are characterized by relatively high concentrations of trace elements, including Ba, Cs, Hf, Nb, Rb, Ta, Th, U, Y, Zr and REEs, which were inherited from evolved vitric ash. Immobile trace element data indicate that a spectrum of silicic ash compositions accumulated, ranging from subalkaline to mildly peralkaline. In the late Ordovician N. gracilis to G. persculptus biozones, subalkaline ash falls predominated, whereas peralkaline ash falls predominated in the Llandovery (Silurian) P. acuminatus to M. convolutes biozones, giving way to predominantly subalkaline ash falls during accumulation of the M. sedgwickii to R. maximus biozones. Changeovers in the dominant ash types are marked by increased proportions of ash. The magmas from which the ash types evolved were generated in an ensialic arc transitional to a back-arc setting, and involved attenuated sialic crust and mantle characterized by variable depletion in HFS elements. Lithological, petrological and REE characteristics suggest that the Moffat Shale Group is not exclusively pelagic in origin and probably accumulated in a back-arc basin bordering an ensialic arc terrane.

Type
Research Article
Information
Geological Magazine , Volume 127 , Issue 3 , May 1990 , pp. 259 - 271
Copyright
Copyright © Cambridge University Press 1990

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