Metamorphic reactions in the biotite zone, eastern Scotland: high thermal gradients, metasomatism and cleavage formation

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metas ediments from the biotite zone of eastern Scotland. Crystallization of musc ovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by si gnificant alkali metasomatism during active deformation and plays an import ant role in the formation of a pi-eminent spaced cleavage within the psammi tes. The Si content of most of these early-formed muscovites is partially b uffered by the quartz content of the rock. although close to the Highland B oundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjace nt to the fault are also indicated by a relatively high delta(18)O((SMOW)), signature in the rocks. The biotite-producing reaction in these greenschis t-facies rocks is linked to the later production of a celadonite-poor musco vite which formed as over growths around preexisting white micas. This reac tion is sensitive to the initial composition of muscovite and preferentiall y occurs in quartz-rich metasediments containing a celadonite-rich muscovit e. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent Co th e Highland Boundary Fault.