Metamorphic facies map of southeastern Alaska – Distribution, facies, and ages of regionally metamorphosed rocks

Nearly all of the bedrock of southeastern Alaska has been metamorphosed to some degree. Much of it has been metamorphosed under mediumto high-grade conditions during episodes that were associated with widespread plutonism. The two oldest known metamorphic episodes in southeastern Alaska occurred during an early Paleozoic and a middle Paleozoic orogeny and affected probable arc-type volcanic, sedimentary, and plutonic rocks in the area of southern Prince of Wales Island. During Late Cambrian to Early Ordovician time, rocks were penetratively deformed, flattened, and recrystallized under greenschistto amphibolite-facies conditions. The subsequent metamorphic episode, which was Silurian to earliest Devonian in age, occurred under prehnite-pumpellyite-facies conditions and was not accompanied by penetrative deformation. The predominant period of metamorphism and plutonism occurred during the interval of Early Cretaceous to early Tertiary time. The oldest documented metamorphic episode during this interval took place in northern southeastern Alaska and apparently was associated with the intrusion of elongate bodies of highly foliated, 120to 110-Ma tonalite and diorite. Low-grade metamorphism of mid-Cretaceous age produced a weakly to moderately developed metamorphic fabric in rocks extending from Kupreanof Island in the north to the peninsula north and west of Revillagigedo Island (Cleveland Peninsula) and perhaps to Revillagigedo Island in the south. This episiode predated the intrusion of mafic-ultramafic bodies that have yielded K-Ar ages of 110-100 Ma. Low-grade metamorphism of melange and flysch north of Cross Sound and on Chichagof and Baranof Islands, southwest of the Peril Strait fault, also took place sometime during the Early Cretaceous to early Tertiary interval. Metamorphism of the melange occurred in a subduction environment and may have begun a s early a s latest Jurassic time. Metamorphism during the next episode or phase was associated with the intrusion of garnetand epidote-bearing plutons of early Late Cretaceous age (approximately 90 Ma). Experimental data on the composition of magmatic garnet and the pressure required to crystallize magmatic epidote have been used to infer a minimum 13-15 kb initial depth and a 6-10 kb final depth for crystallization of the magma body. Kyanite, indicative of intermediate-pressure conditions, occurs in extenManuscript approved for publication March 26, 1987. sively developed aureoles around some of the plutons within amphibolite-facies rocks in the central and southern part of the 90-Ma metamorphic belt. Other 90-Ma plutons that intrude lowgrade rocks have narrow low-pressure aureoles. Relict andalusite that has been replaced by kyanite occurs in aureoles within amphibolite-facies rocks near the northern end of the 90-Ma belt, near northernmost Wrangell Island. The early formation of andalusite, indicative of low-pressure conditions, is hard to reconcile with the high to intermediate pressures inferred for the associated garnetand epidote-bearing plutons. The final metamorphic episode or phase was mostly synkinematic with, but may have slightly preceded, the lastest Cretaceous and early Tertiary mesozonal intrusion of a 600-km long, northwestward-trending composite body herein referred to as the great tonalite sill. The northern two-thirds of the metamorphic belt produced in Alaska during this episode is composed of an intermediate-pressure (Barrovian) sequence in which metamorphic grade increases northeastward. Isograds marking the first appearance of biotite, garnet, staurolite, kyanite, and sillimanite are generally parallel to the sill and inverted. Kyanite has not been reported in the southern part of the belt, and metamorphic pressures may not have been as high as in the north. Intermediate and felsic epizonal plutons intruded the eastern part of the belt in Eocene time during the final phase of this metamorphic episode and produced low-pressure aureoles and zones of migmatite.