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Patrick, B.E., 1987

Petrological and structural studies in the Seward Peninsula blueschist terrane, Alaska

Bibliographic Reference

Patrick, B.E., 1987, Petrological and structural studies in the Seward Peninsula blueschist terrane, Alaska: University of Washington, Seattle, Ph.D. dissertation, 122 p.

Abstract

Blueschist and greenschist facies rocks of the central Seward Peninsula crop out over 8,000 km2. This protolith package is termed the Nome Group and maintains a coherent metamorphic stratigraphy wherever it is found on the Seward Peninsula. Several experimentally and empirically calibrated geothermobarometers have been utilized to derive a pressure-temperature (P-T) path for the metamorphism of the Nome Group. The results of this thermobarometry, coupled with previously obtained Rb-Sr phengite-whole rock isochrons yield a P-T-t path showing a steep burial segment followed by a prolonged period (55 Ma) of isothermal decompression. Peak pressures, obtained from the jadeite content of clinopyroxene in rare, isofacial eclogites, and from the celadonite content in phengite yield ~12 kbar. Temperatures at this pressure, gained from garnet-clinopyroxene and microcline-plagioclase pairs were 460 +/- 30 degrees C. Decompression temperatures were essentially the same, as evidenced by calcite-dolomite and garnet-biotite thermometry. Crystallization of blueschist facies minerals was synkinematic with development of a transposition foliation. This foliation parallels lithologic contacts and is axial planar to recumbent micro- to mesoscopic isoclinal folds. Fold hinges are subparallel to a well developed N-S stretching lineation. The deformation that produced these structures was noncoaxial, as indicated by microstructures and quartz c-axis fabrics. Kinematic indicators show unequivocal top-to-the-north shear sense, compatible with blueschist formation during mid-Jurassic collision between the downgoing northern Alaska continental margin and the overiding north-facing island arc (Yukon-Koyukuk). The Seward Peninsula blueschist terrane is structurally continuous with the medium-pressure, amphibolite-to-granulite facies metamorphic/plutonic complex of the Kigluaik Mountains. Structures that are synmetamorphic in the blueschists are premetamorphic in the higher-grade rocks (the Kigluaik Group). The coincidence in structures and protolith package link these seemingly contrasting baric types throughout the Mesozoic metamorphic cycle. The juxtaposition of baric types occurred via a regional contact metamorphism, whereby the structurally deepest portion of the protolith package underwent anatexis. The separation of these melts, and their subsequent emplacement and crystallization, gave rise to the substantial thermal overprinting observed in the Kigluaik Group.

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