Abstract

A suite of intrusive carbonate rocks, some containing xenoliths of nepheline syenite and Durness Group dolomite, has been discovered near the shore of Loch Urigill, Assynt, Scotland, c. 400m outside the contact of the strongly alkaline Loch Borralan intrusion. Four varieties occur: porphyritic white sövite, phlogopite sövite, sövite breccia and foliated silicocarbonatite. Initially only the white sövite was found in situ, intrusive into Durness dolomites, but excavation has revealed all types, except the silicocarbonatite, in place. The rocks are largely Sr-rich calcite, often enclosing phlogopite poikilitically, together with fluorapatite rosettes. Fluorite, titanite, pyrrhotite, pyrite, chalcopyrite 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 types. The carbonate rocks have the high Sr. Rb and Ba, rare earth element patterns, and carbon and oxygen isotopic ratios characteristic of carbonatites 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 significant fenitization of accompanying silicate rocks are somewhat uncharacteristic, we conclude that the rocks at Loch Urigill are true carbonatites related to the Loch Borralan complex. The strongly alkaline Loch Borralan intrusion and its associated carbonatite represent the most westerly expression of Caledonian magmatism in Britain, which supports the view that they represent the products of small-scale partial melting in old, cold lithospheric mantle on the flanks of the orogen

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