Abstract

Northerly-derived conglomerates dominate much of the thick Lower Old Red Sandstone (ORS) succession of the NE Midland Valley of Scotland. These contain a diverse suite of granitoid boulders, the provenance of which can be used to constrain the timing of the post-Caradoc displacements which juxtaposed the Highland Border Complex against the Grampian terrane.

Rb-Sr muscovite-whole rock ages for two-mica granite boulders from the Crawton Group are 473–457 Ma, confirming the persistence of an Ordovician two-mica granite source first recognized in the Dunnottar Group. The Rb-Sr muscovite ages are supported by a U-Pb age of 475 ± 5 Ma for monazite separated from one of the clasts. A Rb-Sr biotite age of 412 ± 4 Ma for a clast of porphyritic biotite granite and geochemical evidence linking this clast to many of the other biotite granite clasts, suggests that late Silurian granitoids and related hypabyssal intrusions have also made an important, hitherto unrecognized, contribution to the conglomerates. Mica-whole rock ages for accompanying psammitic clasts suggest the source included metamorphic rocks (of up to at least staurolite grade) whose uplift overlapped with the intrusion of crustally-derived melts.

The chemistry, chronology and Sr and Nd isotopic compositions of the various igneous clasts are shown to resemble closely the Ordovician through to Silurian magmatic history of a small area of the Grampian terrane in NE Scotland. This is taken as strong evidence for a pre-Lower ORS juxtaposition of the Highland Border Complex and the Grampian terrane, to within several tens of kilometres of their present position. Ashgill–Silurian terrane displacements are invoked to achieve this. Discrepancies in the proportions and types of clast occurring in the conglomerates are explained by: uplift and unroofing of the Grampian terrane prior to docking; the involvement of polycyclic sediment, perhaps as a local veneer of pre-existing sediment on the Grampian terrane; and the likelihood that the source involved parts of the local Grampian terrane which are no longer exposed.

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