Apatite and scapolite as petrogenetic indicators in granolites of Milford Sound,New Zealand

A network of 5 cm wide subplanar zones of garnet-granolite with accessory apatite as the dominant hydrous mineral, is overprinted on basic hornblende-granolites in an area of present and past tectonic uplift. Fracturing and the garnet forming reactions appear to be caused by destabilisation of hornblende, as a hydrous phase, through a critical drop in the PT ratio. Whereas the apatites from the hornblende-granolite have normal compositions, apatites from the garnet-granolite zones are among the most chlorine enriched hydroxyfluorapatites known. A later amphibolite facies event has depleted hydrous minerals of fluorine and chlorine, affecting hornblende more strongly than apatite. Based on literature data on F and Cl in coexisting minerals, original hornblende compositions are tentatively restored. It is then possible to derive all the chlorine of the garnet zone apatites from the original rock, with differential loss of H₂O and HF over chlorine during the dehydration reaction. Diffusion coefficients would have been larger for H₂O and HF than for the large chloride ion, and if the gas phase was in contact with even minor anatectic melts, activity gradients would also have been relatively small for chlorine. Low-Cl scapolite is present in quartz-free pegmatoid veins. Field evidence for a genetic tie between the garnet zones and these veins is inconclusive, but liberation of H₂O and HF under granolite facies conditions is likely to have caused limited fusion of the plagioclase. Since many garnet zones do not contain even small pegmatoid veins, melts related to their formation could have collected in veins only after initial diffusion or infiltration over a distance, and without leaving segregated mafic residues.