TERRANE CHARACTERIZATION AND PROVENANCE ANALYSIS USING COMBINED U/PB ZIRCON AND AR/AR MICA: A CASE STUDY FROM RIVER BAR DEPOSITS IN THE OLD RED SANDSTONE, SCOTLAND

Detrital zircon geochronology is commonly used as a tool for characterizing terranes and drawing sedimentary and tectonic connections between basins and the terranes that provided the source for the sediments. Because zircon resists breakdown through multiple metamorphic and/or sedimentary cycles, many terranes in the Appalachian orogen have overlapping zircon age populations and confident identification of a particular source terrane on the basis of detrital zircon ages alone can be challenging. Ar/Ar detrital mica dating provides a useful additional tool for terrane characterization and provenance analysis due to the lower closure of the argon system in mica and the shorter lifetime of detrital mica in the sedimentary system. While the characteristic zircon population from a given terrane is dependent on the magmatic history and cratonic affinity, the characteristic mica population from an orogenic belt reflects only the most recent orogenic activity and is sensitive to variations in the timing of metamorphic activity between, and within, individual terranes. In the Appalachian-Caledonian orogen, in particular, these two geochronometers provide complementary information for provenance analysis since orogenesis at ca. 1.6 Ga and 1.0 Ga are recorded widely in zircons from many terranes, but the timing of latest orogenic activity as recorded by Ar/Ar mica ages varies around the Atlantic. Here we present preliminary results from a case study of the Old Red Sandstones (ORS) of Scotland, to test the hypothesis that major Devonian river systems entered Midland Valley, bringing sediment from distant, non-adjacent terranes such as the Caledonian (Scandian) highlands of Norway and Greenland. New detrital zircon and mica datasets from large river bar deposits in Midland Valley suggest that Lower ORS river deposits had a local source in the Grampian Highlands, but an Upper ORS river system was sourced in the Scandian Highlands.