Petrogenesis of Dalradian albite porphyroblast schists

Abstract

A linear belt of schists within which abundant albite porphyroblasts are developed outcrops over approximately 2000 km 2 of the SW Scottish and NE Irish Dalradian. Other features which characterize these rocks are chloritization, the common occurrence of magnetite and development of quartz segregations. The albite porphyroblast schists occur in the biotite and garnet Barrovian metamorphic zones and occupy the crests of regional F3 antiforms. Textural, mineralogical and petrological data indicate that retrogression of original (Barrovian) metamorphic assemblages and concomitant hydrogen metasomatism occurred. As a consequence of these reactions pH gradients were induced between quartz-albite and mica-garnet-rich bands in the rocks which resulted in redistribution and recrystallization of albite as porphyroblasts. No input of sodium on a regional scale is necessary to explain porphyroblast growth. An influx of water was responsible for these processes. It is likely that the water was a product of Barrovian regional metamorphism and accumulated in the crests of F3 antiforms as a result of restricted fluid flow.