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 ba sic igneous rocks occur at or near the base of the Ordovician sediment ary strata. These rocks have previously been interpreted as ocean-floo r tholeiites representative of the subducted Iapetus oceanic plate, pr eserved as tectonic slivers in a fore-are accretionary prism. The alte rnative, back-are basin model proposed for the Southern Uplands on sed imentologicaI evidence raises questions over the origin of these rocks . New geochemical data and previousIy published data clearly indicate that the volcanic material does not have a simple single source. The o ldest (Arenig) volcanic rocks from the Moffat Shale Group associated w ith the Leadhills Fault include alkaline within-plate basalts and thol eiitic lavas which possibly display geochemical characteristics of mid ocean ridge basalts. In the northernmost occurrence, alkaline and thol eiitic basalts contained within the Caradoc Marchburn Formation are bo th of within-plate ocean island affinity. To the south, in the Gabsnou t Bum area, the Moffat Shale Group contains lenticular bodies of doIer ite and basalt which have characteristics of island-are to transitiona l 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-terran e, and as a result, a back-are situation for the Southern Uplands basi n. 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.