Thermal metamorphism of volcanic rocks on Barton Peninsula, King George Island, Antarctica

Abstract

Barton Peninsula is mainly composed of Tertiary metavolcanic rocks and granodiorite. The metavolcanic rocks underwent thermal metamorphism up to upper greenschist facies, producing calcic-amphibole (Amp), epidote, chlorite (Chl), calcite (Cc), plagioclase, prehnite and laumontite. Two metamorphic zones, Cc-Chl and Amp-Chl, are defined on the basis of mineral assemblages (+plagioclase, quartz, opaque minerals), characterized by epidote+chlorite±calcite and actinolite± hornblende+epidote+chlorite ±calcite, respectively. The distribution coefficient of Mg-Fe* (total Fe) between actinolite and chlorite, defined by KD=(Mg/Fe*)Act/(Mg/ Fe*)Chl, ranges from 0.56 to 1.03, suggesting low-pressure metamorphism. The coexistence of actinolite and oligoclase also supports low-pressure metamorphism. Chlorite geothermometry and T-XCO2 analysis suggest that metamorphic temperatures reached ca. 300°C at fluid pressures less than 1 kbar. Thermal metamorphism in Barton Peninsula probably was associated with magmatic-arc plutons emplaced during the Tertiary subduction of the southeast Pacific plate under Antarctic Peninsula.