Low-grade, M1 metamorphism of the 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 as a result of tectonic overlap and/or compressional thickening of crustal rocks during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Sub-greenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska record the earliest metamorphic event (M1). These low-grade rocks are gradational with younger, higher-grade assemblages that define an inverted metamorphic gradient (metamorphic event M5). 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, temperature, or X[sub CO[sub 2]]. Mineral chemistry and phase equilibria suggest that thermal peak metamorphism of pumpellyite-bearing assemblages occurred at about 325 C and 2 to 4.5 kbar. The geologic setting, the pumpellyite-actinolite to lower greenschist facies mineral assemblages, and the deduced P and T of peak metamorphism are all more » compatible with burial metamorphism of the Douglas Island Volcanics at a depth of 15 to 20 km. Preservation of the low-grade metamorphic mineral assemblages during collisional crustal thickening and during establishment of the inverted metamorphic gradient is attributed to instantaneous thickening displacing the pre-thickening isotherms to substantially greater depths and rapid uplift after establishment of the inverted gradient. « less