LATE CALEDONIAN LAMPROPHYRE DYKE SWARMS OF SOUTH-EASTERN SCOTLAND

Calc-alkaline lamprophyre (minette) dykes in the eastern Southern Upla nds of Scotland form part of a swarm nearly parallel to the inferred I apetus Suture, stretching from the Ards Peninsula of Northern Ireland to St. Abbs Head in the east. The dykes are clustered close to several small granitoid bosses, but appear to be younger than the plutons and their associated porphyrite-porphyry dykes. Mica- (minette and kersan titic-minette) and hornblende-lamprophyres are present further west ne ar Hawick where no intermediate-acid plutons or dykes occur. The lampr ophyres have enrichments in LILE and LREE and relative depletions of H FS elements typical of subduction-related ultrapotassic magmas. These incompatible element enrichments are present in rocks with high Mg num ber and Ni and Cr contents, which combined with experimental constrain ts, their fine-grained nature and presence of chilled margins, imply a near-primary status for the least evolved varieties. High values of L REE, LILE, La/Nb, La/Yb, E Sr & low & Nd imply derivation from a previ ously metasomatised source. The minettes were probably derived from a source containing garnet and phlogopite, and the hornblende varieties from a shallower source in the stability field of amphibole. The minet tes of the eastern Southern Uplands have not provided a parental compo nent to the 410 Ma. granitoids which were derived from a more depleted source. The similarity of the lamprophyres to those in the Lake Distr ict south of the Iapetus Suture is taken to indicate underthrusting of Lake District lithosphere beneath the Southern Uplands. Emplacement o f lamprophyre dyke swarms is likely to be structurally controlled, and the presence of the main swarm in the Southern Uplands may indicate t he sub-surface trace of the Iapetus Suture.