GEOCHEMISTRY AND POSSIABLE TECTONIC ORIGIN OF MAFIC UNITS FROM THE MANASTASH INLIER, CENTRAL CASCADES, WASHINGTON

The Manastash inlier, Central Cascades, Washington, consists of a tectonic complex, Easton schist, amphibolite unit, Lookout Mountain fm and the ~ 157 Ma Quartz Mountain pluton. The tectonic complex and Easton schist are fault bound and occur along the northeastern portion of the inlier. The amphibolite unit maybe depositionally overlain by the semi-schist turbidites of the Lookout Mountain fm and the Quartz Mountain pluton intrudes these two units. The tectonic complex is undated, and consists of blocks of peridotite, sedimentary and meta-sedimentary rocks, greenstone, and amphibolite within a serpentinite mélange. Greenstone blocks occur as massive to well foliated blocks or preserve relic igneous textures such as well formed vesicular pillows. These pillows are associated with gray limestone and limestone breccia. The greenstones have a number of different magmatic affinities ranging from within-plate basalts (WPB), one enriched and normal mid-ocean ridge basalt (MORB), and calc-alkaline basalts (CAB). The pillow basalt blocks with limestone all have WPB magmatic affinities. These are the same magmatic affinities that are displayed by the mafic rocks of the Helena-Haystack mélange which has been correlated to this tectonic complex and occurs ~115 km to the northwest across the Straight Creek fault.

The amphibolite unit is undated, and consists of well foliated fine to coarse grained amphibolites and foliated tonalities. In places relic igneous features can be observed such as screens and chill margins. The amphibolites have island arc tholeiite and one has CAB magmatic affinities. The foliated tonalities have tholeiite and cal-alkaline magmatic affinities and have MORB-normalized patterns that appear to have fractionated from the same magmatic source as the amphibolites. The apparent depositional contact between the amphibolite unit and the Lookout Mountain formation, relic screens and chill margins, and chemistry suggest that the amphibolite unit represents a deformed ophiolite of ≥Mid-Jurassic age that may have originated in a supra-subduction zone setting. The screens, chill margins and foliated tonalite may represent the transitional level of an ophiolite between gabbro and diabase dikes where plagiogranites are commonly found.