Anomalously Low Temperature Orthopyroxene, 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

Abstract

The question has been raised whether isolated granulite facies “islands,” which occur throughout the amphibolite facies of the regional amphibolite‐to‐granulite facies transition zone in the central part of the Bamble sector, southern Norway are the result of prograde or retrograde metamorphism. We have studied one of these islands, the sapphirine granulites at Hauglandsvatn. Despite the preponderance of amphibolite facies assemblages in the surrounding rocks and the small scale of distribution of the granulite facies assemblages, the current assemblage is considered to be prograde, and has developed at the expense of biotite in the regional main foliation, leaving relict biotite separated from quartz by a continuous corona of orthopyroxene and K‐feldspar. Other high temperature, low aH2O reactions resulted in the formation of spinel ± corundum + albite/oligoclase symplectites at the expense of biotite and sillimanite. The symplectites have partially been replaced by symplectites of tiny blue sapphirine ± albite/oligoclase. Quartz contains fluid inclusions of highly saline brines. Relatively small inclusions, usually without a salt cube being present at room temperature, have salinities of c. 25 wt% NaCl, but larger inclusions containing a salt cube with little fluid indicate that actual salinities may have been considerably higher. CO2‐rich fluid inclusions are absent. As no feasible heat transport mechanism is available to cause an intense, local increase in temperature, we suggest that breakdown of biotite and development of granulite facies assemblages is due to the localized presence of highly concentrated brines at temperatures close to or lower than peak metamorphic temperatures. Recent experimental and theoretical results demonstrate that salts will strongly reduce aH2O 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 demonstrate that sapphirine is not indicative of extremely high temperatures, even in quartz‐rich rocks, and provides a possible illustration of the action of saline fluids in high‐grade metasediments.