Silicate and sulphide thermobarometry of low- to medium-grade metamorphic rocks from Holkham Bay, south-east Alaska

ABSTRACT The western metamorphic belt of the Coast Plutonic Complex, south-east Alaska and adjacent British Columbia, contains strongly deformed rocks and a prominent topographic low: the Coast Range megalineament. Near Holkham Bay, south-east Alaska, the lineament separates the western metamorphic belt into: a western low-grade (greenschist facies) terrane, and an eastern medium-grade (amphibolite facies) terrane. Sphalerite compositions of grains in direct contact with pyrite and pyrrhotite in chlorite-muscovite zone rocks in the low-grade terrane give pressures of about 8 kbar; compatible with pressures of 8-10 kbar at 500°C calculated from plagioclase-biotite-garnet-muscovite assemblages adjacent to the Windham Bay pluton about 15 km away. A pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in staurolite zone rocks east of the Coast Range megalineament. This is indistinguishable from pressures of 4.8 ± 1 kbar at 585°C and 5.1 ± 1 kbar at 680°C (plagioclase-garnet-aluminum silicate-quartz equilibria), and 4.1 ± 1 kbar at 585°C (plagioclase-biotite-garnet-muscovite equilibrium) determined for the medium-grade terrane. An identical pressure of 4.8 ± 0.7 kbar was calculated from sphalerite compositions in biotite zone rocks adjacent to the lineament; this is considerably higher than a pressure of 3.1 ± 1 kbar at 525°C obtained using plagioclase-biotite-garnet-muscovite geobarometry from shear zones within the lineament. The discrepancy may be explained by later equilibration of mineral phases within the shear zones. The geothermobarometry suggests relatively low temperatures and high pressures for the low-grade terrane (6-10 kbar), and intermediate temperatures and pressures for the medium-grade terrane to the east (4-6 kbar). Comparison of the barometers indicate that sphalerite can be used to estimate metamorphic pressures, similar to those estimated from silicate mineral chemistry when pyrrhotite-sphalerite-pyrite assemblages are used.