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A cryptic Caledonian flysch terrane in Scotland

Department of Geology, University of Glasgow, Glasgow G12 8QQ, UK and Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, UK

A substantial part of the Lower Old Red Sandstone Crawton Group in the north-east Midland Valley of Scotland has a first cycle provenance from the south and east, i.e. from a source block within the Midland Valley. From the clast assemblages in proximal conglomerates, it is clear that this provenance comprised a flysch terrane cut by granodioritic and tonalitic plutons. Angular clasts of lithic arenite and metagreywacke derived from this cryptic flysch sequence show it to be distinct from a metasedimentary sequence which flanked the Lower Old Red Sandstone basin to the north and north-east. The basin must overlie, or lie adjacent to, a boundary between two quite different sedimentary assemblages. Petrographic examination and chemical analysis of the clasts show that the southerly cryptic succession includes an important mafic/ultramanc provenance, implying that ophiolitic source rocks may have originally contributed to it. Sr and Nd isotopic compositions for the clasts establish that the Midland Valley flysch is isotopically unrelated to either Southern Uplands shales and greywackes or Dalradian metasedimentary rocks. The clasts have lower Sr contents than either suite, and Sr isotopic compositions (⁸⁷Sr/⁸⁶Sr in the range 0.7099-0.7176 at 450 Ma) which are more radiogenic than the Southern Uplands, but less radiogenic than the Dalradian. Crustal residence ages (t_DM) have a narrow range of 1.76-1.94 Ga. The mean of 1.87 ± 0.06 Ga compares with a mean residence age of 1.49 ± 0.21 Ga for Southern Uplands greywackes and shales. Mixing calculations demonstrate that the incorporation of young ophiolite-derived basaltic detritus into the Caledonian sedimentary mass can explain properties of the Nd data previously attributed to a bimodal Proterozoic provenance. The distinct compositions of the Southern Uplands and Midland Valley flysch sediments suggest they originally tapped different parts of the Laurentian margin and have subsequently been brought together structurally as displaced fore-arc slivers. The diverse isotopic compositions of outboard flysch terranes has implications for the nature of the sedimentary component incorporated in inboard magma suites.

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