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Lieberman, J.E., 1988

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Bibliographic Reference

Lieberman, J.E., 1988, Metamorphic and structural studies of the Kigluaik Mountains, western Alaska: University of Washington, Seattle, Ph.D. dissertation, 191 p., illust. (some color).

Abstract

On the Seward Peninsula, a close association of Jurassic high P/T Nome Group blueschists with underlying Cretaceous andalusite schists and granulite facies gneisses is exposed in the late structural arch of the central Kigluaik Mtns. This relationship exercises a unique constraint on the evolution of convergent orogens, by providing a structural link between rocks representing radically different metamorphic environments. A variety of metamorphic and structural approaches are combined in this dissertation to understand the quantitative conditions of metamorphism of each element in the association - P,T, and composition - and at the same time constrain those conditions to the encompassing time-framework required by the observed field relationships. Structural observations on a wide range of scales indicate that foliations and fold structures resulting from early high strain can be followed continuously downsection from glaucophane-bearing blueschists through a largely static thermal overprint to granulite facies (opx+cpx, opx+qtz) gneisses and synmetamorphic static/replacive pegmatites. Steeply dipping isograds suggest the form of a thermal dome approximately symmetric with the later structural arch. The static nature of high-grade metamorphism ties the highest grade gneisses to a maximum pressure of approximately 14-15 kb (by an observed 6-7 km section thickness) during Jurassic high P/T (11-13 kb) metamorphism, followed by concurrent uplift and heating to early Cretaceous peak conditions of 800-850 degrees C and 8-11 kb. This path may be recorded in patterns of Al-orthopyroxene content in garnet-spinel lherzolite. The lherzolite is a part of the high grade section, and probably formed in place in the crust in the most FeAl-rich parts of a spinel peridotite precursor. An uplift/heating path may also be recorded in zoned garnets whose cores formed near peak pressure conditions and whose zoning was preserved by low water activity. Peak staurolite (andalusite-sillimanite) temperatures of approximately 550 degrees C occurred approximately 6 km higher in the section and much later in the uplift than the granulite facies conditions, after the high-grade rocks had already cooled significantly. Late upward and outward spread of isotherms during uplift is probably related to the release of stored heat by cooling and crystallizing melts. Hydrous mineral equilibria record a modest water under-saturation under granulite facies conditions: XH2O ~ 0.3 in cordierite bearing gneiss, 0.5 in pelitic gneiss, and 0.75 in mafic gneiss: melt crystallization induced later sporadic infiltration of water-rich fluid.

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