Pre-Cretaceous crystalline rocks of the western Chugach Mountains, Alaska: Nature of the basement of the Jurassic Peninsular terrane

Abstract

A narrow belt of Mesozoic and(or) Paleozoic crystalline rocks exposed immediately north of the Border Ranges fault of southern Alaska is the southern (trailing) edge of the Peninsular terrane, an exotic micro-continental block that collided with North America during the Cretaceous. The tectonic significance of this crystalline belt has been debated because previous K-Ar geochronology suggested that the belt contains both Early Jurassic high P/T metamorphic rocks—an apparent subduction complex—and Early Jurassic intermediate plutonic rocks—an apparent magmatic arc. Three major observations from detailed mapping in the western Chugach Mountains pertain directly to this apparent problem of mixed rocks association. First, despite major postmetamorphic faulting along the Border Ranges fault, all of the metamorphic rocks have been subjected to a regional, apparently low P/T metamorphism at the greenschist-amphibolite facies transition. The metamorphism is imprinted on rocks ranging from nearly undeformed ultramafic rocks to highly deformed amphibolites, yet regardless of structural histories of fault-bounded slices, the culmination of metamorphism appears to postdate deformation. Second, the metamorphic rocks are locally an argillite matrix melange in which main-phase structural fabrics appear to have formed at relatively low grade but are overprinted by growth of postkinematic, transitional greenschist-amphibolite facies minerals. Third, despite ambiguities in correlation across faults and equivocal absolute age relationships—presumably due to Cretaceous resetting of K-Ar ages—the transitional greenschist-amphibolite facies rocks are intruded by plutons that yield Early Jurassic K-Ar ages. Thus, the metamorphic protolith is presumably pre-Jurassic. In addition, the close association of low P/T metamorphic rocks with a large plutonic complex implies that the regional metamorphism is probably Early Jurassic in age and is associated with extensive plutonism. New age data from the western Chugach Mountains imply that a belt of Early Jurassic plutonism (and associated low P/T metamorphism) probably extends throughout the southern edge of the Peninsular terrane. The entire terrane probably represents a long-lived magmatic arc, because this Early Jurassic magmatic arc is separated from a parallel belt of Early to Middle Jurassic plutons by an Early Jurassic andesitic-volcanic pile. In the western Chugach Mountains, emplacement of alpine ultramafic bodies and development of chaotic rock bodies is premetamorphic, presumably pre-Jurassic in age. These observations of structural style, together with lithologic associations and regional occurrences of blueschist-facies rocks, suggest that a pre-Jurassic subduction complex may have formed parts of the basement for the Peninsular terrane arc. As a corollary of this conclusion, the blueschists of the southern Peninsular terrane are probably pre-Jurassic in age and represent scraps of the pre-Jurassic basement, not parts of an Early Jurassic high-pressure metamorphic belt.