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Roeske, S.M., 1988

Metamorphic and tectonic history of the Raspberry Schist, Kodiak Islands, Alaska

Bibliographic Reference

Roeske, S.M., 1988, Metamorphic and tectonic history of the Raspberry Schist, Kodiak Islands, Alaska: University of California, Santa Cruz, Ph.D. dissertation, 206 p., illust., maps.


The glaucophane-bearing Raspberry schist occurs as discontinuous, fault-bounded lenses in the Border Ranges fault system (BRFS), a major tectonic boundary in southern Alaska. The lithologies of the Raspberry schist, predominantly metabasites and quartzites, experienced minor pre-metamorphic stratal disruption, extensive syntectonic deformation, and pervasive post-metamorphic disruption by faults. Previous research and the results of this study support the interpretation that the Raspberry schist formed in a subduction complex. The purpose of this study was to determine the metamorphic age of the schist and the nature of its contact with a pluton to the northwest. The results clearly show that the current juxtaposition of the schist with the pluton is the result of post-Early Jurassic fault movements on the BRFS. New U-Pb and Rb-Sr isotopic dates from the subduction complex are 204 +/- 8 m.y. and 196 +/- 12 m.y. (2 sigma errors), respectively. The similarity of these dates to previously reported K-Ar dates support petrographic and petrologic observations that the subduction complex did not experience any metamorphism related to the intrusion of the now adjacent pluton and the oldest dates are interpreted as the approximate crystallization age of the schist. New U-Pb dates from the pluton are interpreted as a crystallization age of 215 - 220 m.y., significantly older than previously reported K-Ar dates of approximately 185 m.y. Previous studies have interpreted the pluton and the metavolcanics it intrudes as part of a Late Triassic to Early Jurassic primitive island arc complex. If the subduction complex is related to this coeval island arc, then the forearc and back part of the accretionary prism have been tectonically eroded, either by subduction erosion, strike-slip faulting during oblique convergence, or a combination of these processes. If, however, the subduction complex was juxtaposed with the island arc during strike-slip faulting that accompanied and post-dated the Cretaceous accretion of terranes of southern Alaska with North America, then the subduction complex could be related to an Early Jurassic island arc exposed on Vancouver island, where it is part of Wrangellia.

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