The Bail Hill Volcanic Group: alkaline within-plate volcanism during Ordovician sedimentation in the Southern Uplands, Scotland

The Bail Hill Volcanic Group (Caradoc) represents the largest, single volca nic complex exposed within the Ordovician turbidite succession of the North ern Belt in the Southern Uplands of Scotland. The group comprises a heterog eneous sequence of submarine lavas, volcaniclastic and intrusive rocks (up to 2 km thick), and crops out in a small area (c. 4 km(2)) around Bail Hill , north of Sanquhar. The Cat Cleuch Formation (older) is dominated by a seq uence of autobrecciated basaltic lavas which contain large, zoned diopsidic clinopyroxene. The overlying Feat Rig Formation comprises a more mixed seq uence of plagioclase-amphibole-phyric lavas, volcaniclastic rocks and conte mporaneous volcaniclastic sedimentary rocks. The Cat Cleuch and lower part of the Feat Rig formations are cut by a vent breccia, the Bught Craig vent breccia, which formed part of the feeder to the upper part of the Feat Rig Formation. The Bail Hill volcanic rocks are alkaline in character, ranging from alkali basalt to trachyandesite in composition, possessing trace eleme nt characteristics and enrichment patterns typical of oceanic within-plate basalts. The Bail Hill Volcanic Group, although geochemically distinct, for ms part of a mixed assemblage of tholeiitic and alkaline within-plate lavas within the Southern Uplands which are of broadly similar age, some of whic h are intercalated within the greywacke sandstone sequence. This assemblage clearly indicates that a period of extension and within-plate volcanism oc curred during the early stages of the development of the Southern Uplands s edimentary basin.