THE NATURE AND RELATIONSHIPS OF ULTRAMAFIC BODIES FROM MULTIPLE STRUCTURAL SETTINGS, NORTHERN NASON TERRANE, WASHINGTON

Ultramafic rock bodies ranging from small outcrop-scale bodies to ~2 km in length occur in multiple settings across the northern part of the Nason Terrane in the North Cascade Mountains of Washington State. Numerous bodies occur within the Wenatchee Ridge Orthogneiss (WRO) pluton; these tend to be roughly equidimensional, rarely if ever cut by the enveloping tonalite gneiss, range from dunite to harzburgite to serpentinite, and are commonly rimmed by blackwall formed during the Late Cretaceous metamorphism. A large body of peridotite occurs within the White River Shear Zone (WRSZ) cut by the Napeequa River valley (thus, the Napeequa Ultramafic Body, or NUB); this body consists of dunite and olivine-rich peridotite, containing minor pyroxene and Mg-amphibole in many locations, with very uncommon pyroxenite. Serpentinized portions of the NUB containing serp+carb+chl+talc+opq suggest an upper greenschist to low amphibolite facies overprint on higher T assemblages. Portions of the NUB are highly tectonized and ultramylonitic with olivine grain size of <5 mm, and flow stresses using olivine piezometry are at least 250 MPa.

Of special interest are two large bodies, one within the WRO above Nine Mile Creek (thus, the Nine Mile Creek ultramafic body, NMCUB), and another along the White River northeast of Lake Wenatchee (the Grave Ultramafic Body, or GUB). Both the NMCUB and GUB exceed 200 m in length and are characterized by ol+talc+amph, and in particular, by oblate patches of nearly pure chlorite. These rocks are also highly tectonized with extremely fine-grained olivine, but post-deformational, acicular, and undeformed cummingtonite and tremolite are common. In both bodies, a moderate to strong foliation is developed, and in the NMCUB, the foliation, with a shape fabric defined by the chlorite patches, locally shows increasing strain and curvature toward shear zones. Also in the NMCUB, tiny chromite grains define s-shaped trails indicate the chlorite patches may well be formed by the retrogression of garnet, i.e., these two bodies represent retrogressed garnet peridotites.

A structural model is proposed that may explain the proximity of these bodies, involving a deep-rooted WRSZ and synthetic zones, juxtaposition against the northern Nason Terrane, and engulfing or stoping by the WRO magmas.