ADINOLES REVISITED - HYDROTHERMAL NA(CA)METASOMATISM OF PELITE SCREENS ADJACENT TO THOLEIITIC DYKES IN THE DUBLIN TERRANE, IRELAND

The albitisation of pelitic metasediments to form adinoles adjacent to minor mafic intrusions has in the past been attributed to Na-bearing metasomatizing fluids emanating from the intrusions themselves. The ch emistry and mineralogy of adinoles associated with dykes farming a hig h-intensity swarm in the Tallaght area, County Dublin, Ireland, confir ms a metasomatic origin for the adinoles described, with Na and to a l esser extent Ca introduced into the country rocks at the expense of K. We suggest, however, that the source of the metasomatizing fluids was external to the dykes - possibly involving a hydrothermal system driv en by an underlying magma reservoir that was parental to the dykes and also contributed volatiles to the hydrothermal system by degassing. H eat flow from the dykes was instrumental in promoting the reaction bet ween muscovite in the country rock pelites and the Na(Ca)-bearing flui ds to form albite, temperatures of similar to 350 degrees C in the cou ntry rocks within a metre of the contact being indicated. The mechanis m of adinolization proposed is shown to be compatible with recent expe rimental work on the hydrothermal alteration of greywackes and basalts and also with the likely temperature gradients adjacent to dykes.