Crustal mass transfer and index mineral growth in Barrow's garnet zone, northeast Scotland

Abstract

The Barrovian metamorphic sequence of northeastern Scotland plays a critical role in our understanding of regional metamorphism at middle and lower crustal levels. The importance of regional temperature and pressure gradients in producing the sequence is well established, but the petrogenetic significance of mass transport of nonvolatile elements by metamorphic processes remains controversial. This paper focuses on the impact of fluid flow through fractures (now quartz veins) on the geochemical and petrologic evolution of metamorphosed clastic rocks in the northeastern part of Barrow9s garnet zone, Scotland. The veins constitute ∼13 vol % of the total rock mass and are generally surrounded by zones rich in plagioclase. A traverse along a typical metasedimentary bed cut by a vein reveals that, as one approaches the vein, (1) plagioclase porphyroblast size and An content increase, (2) the modal plagioclase/muscovite ratio increases, (3) bulk rock Na/K, Ca/K, Sr/Rb, (Na + Ca + K)/Al, Mg/Fe T , and (Mg + Fe T )/Al increase, and (4) bulk rock Si/Ti decreases. The plagioclase-rich zones are interpreted to be alteration selvages that developed as a result of fluid infiltration through fractures; the alteration reaction destroyed muscovite and produced plagioclase. Furthermore, in some rocks the assemblage garnet + biotite + chlorite was stable in the most intensely altered areas directly adjacent to vein margins, whereas chloritoid + biotite + chlorite was stable in less-altered and unaltered areas. Geochemical relations suggest that the fluid flow was in a direction of increasing temperature and that the vein quartz was mostly derived from local wall rocks. It appears that open-system transport of nonvolatile elements can exert important, previously unrecognized controls on rock chemistry and the growth of common regional metamorphic mineral assemblages.