Compositional variation in Lower Old Red Sandstone detrital garnets from the Midland Valley of Scotland and the Anglo-Welsh Basin

Abstract

AbstractCompositional variation in Lower Old Red Sandstone (ORS) detrital garnets can be used to evaluate potential source areas and to trace the pattern of early Devonian sediment dispersal. Garnets in northwesterly derived fans along the northern flank of the Midland Valley appear to duplicate spatial variation in the compositions of garnets in the adjacent Dalradian block. Whilst metamorphic clasts ‘resembling’ Dalradian lithologies occur in these fanglomerates, the strike-extent of Dalradian-type crust (c. 1000 km) makes simply matching the clasts a rather imprecise way of constraining source–basin displacement. The garnet data imply that the Ordovician separation inferred for crustal blocks juxtaposed along the Highland Boundary was removed by early Devonian times. Major Silurian displacements are thus invoked along the Highland Boundary with amalgamation largely achieved prior to the ‘Acadian’ transcurrent deformation seen in the slate belts to the south. Vertical changes in the composition of garnets in the thick Lower ORS sequence in the northeast Midland Valley are slight and it is not known to what extent these reflect migration of a differentiated source block or the involvement of recycled additions which become increasingly important towards the base of the ORS succession. The axially dispersed sandstones which interdigitate with the northwesterly derived fans are dominated by spessartine-rich almandines which resemble garnets fed laterally to the basin in the northwest Midland Valley. Although the large scale inferred for the axial fluvial system suggests that its drainage basin extended outside Scotland, almandine-pyropes which might have come from exhumed eclogites in western Norway are absent. Such compositions are widespread in Mesozoic sandstones in the northern North Sea and it is evident that these sediments do not represent the final repository for the 'lost' sediment stripped from Caledonian metamorphic terranes in Scotland. Garnet populations from the Anglo-Welsh Basin do not resemble those in the coeval axial sandstones of the Midland Valley, suggesting that the two basins were not linked at this time. The scale of the Midland Valley axial sandbodies are inconsistent with internal drainage of central Scotland and a route to the early Devonian shoreline must be sought. It may be that 'Acadian' strike-slip motions have displaced the Scottish basins from the coastal alluvial plains they originally fed.