Sillimanite-potash feldspar assemblages in graphitic pelites, Strontian area, Scotland

Abstract

Graphitic pelites of the western Moinian were metamorphosed at the time of emplacement of the Strontian Granodiorite intrusion, at a late stage of the Caledonian Orogeny, producing a metamorphic zonation. The Sillimanite Zone (in which K feldspar does not occur with sillimanite) is succeeded by the Muscovite-Sillimanite-K feldspar Zone, Sillimanite-K feldspar Zone (without primary muscovite) and Cordierite-K feldspar Zone. Secondary muscovite from retrograde hydration of sillimanite+K feldspar is distinguished texturally from primary muscovite, but is compositionally similar. Primary porphyroblastic muscovite, inherited from the regional metamorphic textural evolution of the rocks, disappears abruptly at the “muscovite-out” isograd. Migmatites of earlier regional origin, with recrystallized textures, are distinguished from those associated with the late Caledonian metamorphism, which are confined to the Sillimanite-K feldspar and Cordierite-K feldspar Zones. Muscovite compositions are inferred to be very low in Fe3+. There are no marked changes in muscovite composition at the entry of sillimanite+K feldspar. Higher Na contents than in some other muscovites coexisting with sillimanite+K feldspar are interpreted in terms of relatively low P in the Strontian area. Andalusite is found at two localities. From cordierite-garnet-sillimanite-biotite-K feldspar-quartz assemblages, a P estimate of 4.1±0.4 kbar is obtained, with the aqueous fluid having \(x_{{\text{H}}_{\text{2}} {\text{O}}} \approx 0.5\), and the T at the cordierite-K feldspar isograd is estimated as 690° C. T at the muscovite-out isograd is inferred to the maximum for muscovite-quartz-sillimanite-K feldspar equilibrium with graphite at P≈4.1 kbar: T≈ 645° C, with \(x_{{\text{H}}_{\text{2}} {\text{O}}} \approx 0.84\). The well-defined lower boundary of the Muscovite-Sillimanite-K feldspar Zone is attributed to regionally rather homogeneous fluid composition at \(x_{{\text{H}}_{\text{2}} {\text{O}}} \approx 0.7\). The low P prevented melting in the Muscovite-Sillimanite-K feldspar Zone. Migmatites in the higher zones are attributed to partial melting, which accounts for the low \(x_{{\text{H}}_{\text{2}} {\text{O}}} \) necessary to produce the cordierite assemblages. Obscurity of these migmatites is attributed to deformation during and after the migmatization. A “breccia” structure in and near the Cordierite-K feldspar Zone, where pelites have flowed around disrupted pieces of psammite and amphibolite, can be explained by presence of grain-boundary melts in the pelites.