Low-grade, M (sub 1) metamorphism of the Douglas Island Volcanics, western metamorphic belt near Juneau, Alaska

The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Deformation, metamorphism, and plutonism range from about 120 to 50 Ma. Subgreenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska, record the earliest metamorphic event (M (sub 1) ). The protolith of the M (sub 1) , low-grade metamorphic mineral assemblages is mostly arc-affinity basaltic rocks of the Douglas Island Volcanics. The most common metamorphic mineral assemblages are chlorite-epidote-actinolite with or without pumpellyite and stilpnomelane. There is no systematic distribution of metamorphic mineral assemblages in the study area, and all assemblages are in the pumpellyite-actinolite facies near the transition to the lower greenschist facies. Different low variance assemblages can be attributed to minor differences in pressure (P), temperature (T), or X (sub CO (sub 2) ) . Mineral chemistry and phase equilibria suggest that thermal peak metamorphism of pumpellyite-bearing assemblages occurred at about 325 degrees C and 2 to 4.8 kbar. The geologic setting, the pumpellyite-actinolite to lower greenschist facies mineral assemblages, and the deduced P and T of peak metamorphism are all compatible with metamorphism of the Douglas Island Volcanics at a depth of 7 to 20 km. The low-grade rocks are contiguous with younger (M (sub 5) ), higher grade assemblages that define an inverted metamorphic gradient. The discontinuity in pressure indicated by the M (sub 1) mineral assemblages and M (sub 5) geobarometry (9-11 kbar) suggests juxtaposition of the two metamorphic sequences by vertical uplift along the Coast Range megalineament.