THE TECTONIC SIGNIFICANCE OF ORDOVICIAN BASIC IGNEOUS ROCKS IN THE SOUTHERN UPLANDS, SOUTHWEST SCOTLAND

In the northern part of the Southern Uplands, restricted volumes of basic igneous rocks occur at or near the base of the Ordovician sedimentary strata. These rocks have previously been interpreted as ocean-floor tholeiites representative of the subducted Iapetus oceanic plate, preserved as tectonic slivers in a fore-are accretionary prism. The alternative, back-are basin model proposed for the Southern Uplands on sedimentological evidence raises questions over the origin of these rocks. New geochemical data and previously published data clearly indicate that the volcanic material does not have a simple single source. The oldest (Arenig) volcanic rocks from the Moffat Shale Group associated with the Leadhills Fault include alkaline within-plate basalts and tholeiitic lavas which possibly display geochemical characteristics of midocean ridge basalts. In the northernmost occurrence, alkaline and tholeiitic basalts contained within the Caradoc Marchburn Formation are both of within-plate ocean island affinity. To the south, in the Gabsnout Bum area, the Moffat Shale Group contains lenticular bodies of dolerite and basalt which have characteristics of island-are to transitional basalts. This complex association of basaltic volcanic rocks is, at the present time, difficult to reconcile with either a simple fore-are or back-are setting for the Southern Uplands. However, the increasing are-related chemical influence on basic rock geochemistry towards the southeast may tentatively be used in support of a southern arc-terrane, and as a result, a back-are situation for the Southern Uplands basin. An alternative is that these volcanic rocks may represent the local basement to the basin and include remnants of an are precursor to the Southern Uplands basin.