Fenites, breccia dykes, albitites, and carbonatitic veins near the Great Glen Fault, Inverness, Scotland

Abstract

In the Abriachan-Dochfour-Moniack, Rosemarkie and Foyers areas near Inverness, Moine metasediments and gneisses, amphibolites, Caledonian granitic rocks, and meta-limestones of uncertain age are veined or replaced by metasomatic assemblages of blue fibrous magnesio-riebeckite (crocidolite), green aegirine-rich pyroxene, albite, calcite, hematite, and anatase. The field characteristics, textures and parageneses of these assemblages duplicate those of classical fenites around igneous carbonatite complexes. In the Abriachan granite, the chemistry of metasomatism also recalls classical fenitization in correlating removal of SiO 2 , K 2 O and Rb with addition of Na 2 O, Fe, MgO, CaO, F, Zn, Zr, Ba, Ce and CO 2 , while sums of cations such as [Si + Na] and fSi + A1 + Fe + Mg + Ca] remain virtually constant. At Moniack and near Rosemarkie, the fenites border calcitic veins, whose high contents—especially of Sr, Nb, Ba, REE, and Mn—ally them with late-stage carbonatites rather than with limestones or hydrothermal carbonates. Associated dykes of ffuidized, often carbonate-rich breccias, and bodies of brick red, sometimes radioactive albitites, also have close analogues in carbonatite complexes. The fenites are distinguished by petrology, field relationships and age from other Scottish alkali metasomatized rocks. All these rocks are believed to have formed broadly coevally, during the emplacment of at least one carbonatite mass deep into the Great Glen Fault zone, most probably during early Devonian (c. 390 Ma) fault movements.