GARNET PROVENANCE STUDIES, JUXTAPOSITION OF LAURENTIAN MARGINAL TERRANES AND TIMING OF THE GRAMPIAN OROGENY IN SCOTLAND

A study of the composition and zoning patterns of detrital garnets fro m Ordovician greywackes from the Northern Belt of the Southern Uplands terrane of Scotland reveals characteristics of the metamorphic source s very similar to the Dalradian Supergroup of the Scottish Grampian te rrane. The radiometric cooling and uplift ages of the Dalradian metamo rphic zones and the depositional ages of the Southern Uplands greywack es support the hypothesis for local provenance. Detrital metamorphic g arnet, identical to Dalradian garnet, has also been identified in the Upper Ordovican sandstones of the Highland Border Complex. These obser vations do not support proposals that the Grampian Midland Valley and Southern Uplands terranes were exotic to each other in the Late Ordovi cian time. These new results. together with a review of published age dates, clarify the Late Ordovician palaeogeography for this part of th e Laurentian margin. The distances between the Grampian, Midland Valle y and Southern Uplands terranes may have been similar to the present d ay. It is concluded that large rivers flowed out of the uplifting moun tainous Grampian terrane and across the Midland Valley into a Southern Uplands trench during the Late Ordovician time. The main orogeny (i.e . mountain building) in the Grampian terrane was therefore post-Cambri an, producing the first high mountains and resultant flysch in the Car adoc.