Synopsis

The Ben Vuirich intrusion is a small, elongate body of monzogranite that occurs in the Tummel Steep Belt, Perthshire. It was emplaced into Dalradian rocks (Appin Group) at 590 Ma, prior to the D1 phase of the Grampian Event (Caledonian Orogeny), and was strongly deformed during D2. Locally preserved cordierite- and andalusite-bearing hornfelses were altered to garnet ± kyanite-bearing assemblages during post-D2 regional metamorphism. A new study of the lower-grade hornfelses shows that the protolith was an undeformed, fine-grained, parallel-laminated sedimentary unit, confirming that the pluton is pre-orogenic with respect to the Grampian Event. Whole-rock and trace element analyses of 33 samples of the intrusion, together with rare earth elements, Rb–Sr and O-isotope data, show that it is an A2-group monzogranite. This finding supports the hypothesis that the granite, emplaced at a depth of 7–14 km, formed in the same extensional tectonic setting as the Tayvallich lavas at 600 Ma. Geochemical and isotope parameters point to a largely crustal source. The intrusion belongs to a swarm of rift-related, A-type granitoids that originally stretched from the Appalachians to Scotland, and includes foliated granitoids in the Moine. The granitoids formed in response to the early break-up of Rodinia, c. 50 Ma before the development of the Iapetus Ocean.

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