Abstract

Metamorphic assemblages in pre-Tertiary rocks of northwestern Washington can be grouped into eight facies types which represent recrystallization under widely diverse P-T conditions. Consideration of these assemblages, together with their regional distribution and age, allows speculation concerning some aspects of the regional metamorphic and tectonic history: (1) high-pressure assemblages indicate that much of the metamorphosed rock has been affected by subduction; (2) two ages of subduction are represented, Carboniferous to Permian and Early Cretaceous; (3) during the Cretaceous event, rock masses of local to regional extent gained mutually distinctive metamorphic assemblages, indicating that the rock units acted as separate tectonic elements during the subduction process; (4) sea-floor metamorphism may be indicated by the occurrence of low-pressure assemblages in ophiolitic rocks; (5) in some rock units, low-pressure assemblages are overprinted by high-pressure minerals, suggesting a history of sea floor followed by subduction metamorphism; (6) some rock units contain only low-pressure assemblages and thus may have escaped subduction; (7) diverse origins of rock units and profound movement on unit-bounding faults are suggested by the disparity of ages and facies type.

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