ANOMALOUSLY LOW-TEMPERATURE ORTHO-PYROXENE, SPINEL, AND SAPPHIRINE OCCURRENCES IN METASEDIMENTS FROM THE BAMBLE AMPHIBOLITE-TO-GRANULITE FACIES TRANSITION ZONE (SOUTH NORWAY) - POSSIBLE EVIDENCE FOR LOCALIZED ACTION OF SALINE FLUIDS

The question has been raised whether isolated granulite facies ''islan ds,'' which occur throughout the amphibolite facies of the regional am phibolite-to-granulite facies transition zone in the central part of t he Bamble sector, southern Norway are the result of prograde or retrog rade metamorphism. We have studied one of these islands, the sapphirin e granulites at Hauglandsvatn. Despite the preponderance of amphibolit e facies assemblages in the surrounding rocks and the small scale of d istribution of the granulite facies assemblages, the current assemblag e is considered to be prograde, and has developed at the expense of bi otite in the regional main foliation, leaving relict biotite separated from quartz by a continuous corona of orthopyroxene and K-feldspar. O ther high temperature, low a(H2O) reactions resulted in the formation of spinel +/- corundum + albite/oligoclase symplectites at the expense of biotite and sillimanite. The symplectites have partially been repl aced by symplectites of tiny blue sapphirine +/- albite/oligoclase. Qu artz contains fluid inclusions of highly saline brines. Relatively sma ll inclusions, usually without a salt cube being present at room tempe rature, have salinities of c. 25 wt % NaCl, but larger inclusions cont aining a salt cube with little fluid indicate that actual salinities m ay have been considerably higher. CO2-rich fluid inclusions are absent . As no feasible heat transport mechanism is available to cause an int ense, local increase in temperature, we suggest that breakdown of biot ite and development of granulite facies assemblages is due to the loca lized presence of highly concentrated brines at temperatures close to or lower than peak metamorphic temperatures. Recent experimental and t heoretical results demonstrate that salts will strongly reduce a(H2O) while maintaining alkali transport properties. The latter capacity of the fluids enabled K+ derived from biotite breakdown to be transferred to quartz-feldspar leucosomes, which, however, are non-anatectic. The occurrence lends support to recent petrogenetic grids that demonstrat e that sapphirine is not indicative of extremely high temperatures, ev en in quartz-rich rocks, and provides a possible illustration of the a ction of saline fluids in high-grade metasediments.