CHEMICAL MODIFICATION OF ENCLAVE MAGMA BY POST-EMPLACEMENT CRYSTAL FRACTIONATION, DIFFUSION AND METASOMATISM

Hornblende-bearing microgranitoid enclaves from the Swifts Creek Pluton (SCP), SE-Australia display mineralogical and textural variations from their margins to their centers. Margins are fine grained and display quench textures and are enriched in amphibole, biotite and in some cases magnetite relative to their coarser grained centers. Enclaves of this type and their adjacent granitoid host rocks have been sectioned into 0.3 to 1 cm thick slabs in order to determine the chemical variations associated with these mineralogical changes. The fine grained margins are variably enriched in Ti, Al, Mg, Fe, Mg, K, Rb, Ba, Nb, Y, Sc, V, Ni and the REE up to a factor of three relative to the enclave centers. This enrichment is compensated by a depletion of Si and Zr. Elements like Ca, Na and Sr show less coherent variation from margin to center. Host rocks in immediate contact with these enclaves are depleted in Mg, Na, K, Rb and Ba relative to host rocks that are not exposed to enclaves. No one single process can account for all the variations. Instead, we propose that the chemical variations are related to a combination of in situ crystal fractionation of isolated magma globules, mass transfer by diffusion and metasomatic exchange. © 1990 Springer-Verlag.