Field relations and metamorphism of the Raspberry Schist, Kodiak Islands, Alaska

The glaucophane-bearing Raspberry Schist occurs as discontinuous, fault-bounded slivers along the northwest side of the Kodiak Islands in southern Alaska. The lithologies, predominantly metabasites and quartzites, experienced minor pre-metamorphic stratal disruption, extensive syntectonic deformation, and pervasive post-metamorphic disruption by faults. A major change in metamorphic assemblages and degree of recrystallization occurs across a fault zone, dividing the Raspberry Schist into units Js1 to the southeast and Js2 to the northwest. Js1 is characterized by relict igneous textures and phases and contains assemblages compatible with prehnite-pumpellyite, pumpellyite-actinolite, lawsonite-albite-chlorite, and lower greenschist transitional to blueschist facies. Rocks composing Js2 have been completely, dynamically recrystallized and contain assemblages described as transitional blueschist facies or high-temperature blueschist facies. Rb-Sr and K-Ar dates from the Raspberry Schist indicate that it was metamorphosed in Early Jurassic time. Lower Jurassic calc-alkaline mafic volcanics and associated plutons lie immediately northwest of the schist, juxtaposed with the schist by a wide brittle shear zone. Another calc-alkaline suite composed of Lower to Middle Jurassic plutonic and volcanic rocks lies 100 km to the northwest on the Alaska Peninsula. The metamorphism of the Raspberry Schist may be related to one of these calc-alkaline suites, but more evidence of the type and timing of fault movements in the region is needed to prove this conclusion.