CARBONATITE NEAR THE LOCH-BORRALAN INTRUSION, ASSYNT

A suite of intrusive carbonate rocks, some containing xenoliths of nep heline syenite and Durness Group dolomite, has been discovered near th e shore of Loch Urigill, Assynt, Scotland, c. 400 m outside the contac t of the strongly alkaline Loch Borralan intrusion. Four varieties occ ur: porphyritic white sovite, phlogopite sovite, sovite breccia and fo liated silicocarbonatite. Initially only the white sovite was found in situ, intrusive into Durness dolomites, but excavation has revealed a ll types, except the silicocarbonatite, in place. The rocks are largel y Sr-rich calcite, often enclosing phlogopite poikilitically, together with fluorapatite rosettes. Fluorite, titanite, pyrrhotite, pyrite, c halcopyrite and ilmenite also occur. Diopside, potassic richterite and chondrodite are found in the silicocarbonatite and in the vicinity of xenoliths of silicate-rock and dolomite in the other carbonatite type s. The carbonate rocks have the high Sr, Rb and Ba, rare earth element patterns, and carbon and oxygen isotopic ratios characteristic of car bonatites from a mantle source region and are chemically distinct from the Durness dolomites, even when the latter are sampled as xenoliths in nepheline syenite. Although the absence of Nb and the lack of signi ficant fenitization of accompanying silicate rocks are somewhat unchar acteristic, we conclude that the rocks at Loch Urigill are true carbon atites related to the Loch Borralan complex. The strongly alkaline Loc h Borralan intrusion and its associated carbonatite represent the most westerly expression of Caledonian magmatism in Britain, which support s the view that they represent the products of small-scale partial mel ting in old, cold lithospheric mantle on the flanks of the orogen.