Age of the Olympic Metamorphism, Washington: K-Ar Dating of Low-Grade Metamorphic Rocks

Abstract

The complexly deformed core rocks of the Olympic Mountains grade from mildly sheared and recrystallized graywacke and slate to semi-schist and mica phyllite. K-Ar ages of sand-sized and matrix-sized fractions from 14 samples (28 ages) of the graywacke-semischist sequence decrease with increasing rank of metamorphism. The ages plotted against the rank of metamorphism (as determined in thin section) define a curve which, in a rough way, flattens to an asymptote at approximately 29 m.y. The ages of 8 samples of the slate-phyllite sequence define the same asymptote, which is assumed to represent the age of regional metamorphism. Four samples from brecciated and quartz-veined mica phyllite indicate a local resetting of the clock by faulting and resulting recrystallization at about 17 m.y. Lines of equal age plotted from the average ages of samples from the graywacke-semischist sequence denote a crude metamorphic zonation which corresponds approximately to the observed metamorphic gradation. The ages of the metamorphic events are reasonable in light of known stratigraphic ages of the rocks and the regional tectonic setting, suggesting this method may be applied to other areas of low-grade rocks.