GEOCHEMISTRY AND TECTONIC EVOLUTION OF THE POLYGENETIC INGALLS OPHIOLITE COMPLEX, CENTRAL CASCADES, WASHINGTON.

The Ingalls ophiolite complex, Washington Cascades, is dismembered and consists of ultramafic rocks that contain oceanic crust as meter- to kilometer-scale fault blocks in a serpentinite mélange. This mélange separates lherzolite in the north from harzburgite and dunite in the south and overprints mylonitic lherzolite. Mineral assemblages in the mylonitic lherzolite suggest T > 900° C and were interpreted to have formed in a fracture zone. Whole-rock serpentinite geochemistry and Cr-spinels in ultramafic rocks both plot in fields defined by modern abyssal and forearc peridotites. Crustal units in the large fault blocks are well preserved and divided into the: Iron Mt. unit; Esmeralda Peaks unit; and sedimentary rocks of the Peshastin Fm. The Early Jurassic Iron Mt. unit consists of pillow basalt and broken pillow breccia, with minor rhyolite, hyaloclastite, oolitic limestone and chert. Geochemical affinities are transitional between OIB and E-MORB. The Late Jurassic Esmeralda Peaks unit consists of pillows and massive flows, diabase with minor sheeted dikes, gabbro and rare tonalite and trondhjemite. Geochemical affinities of this unit are transitional between island arc tholeiite and N-MORB, and rare boninites exist.

We infer that the Iron Mt. unit formed as an off-axis seamount during the Early Jurassic. This seamount may have accreted onto an Early to Middle Jurassic forearc as the oceanic lithosphere it sat on was subducted. In this scenario, this forearc began rifting in the Late Jurassic, forming oceanic crust of the Esmeralda Peaks unit. This forearc ophiolite then transitioned into a back-arc basin that included a fracture zone. Based on age, lithologies, and magmatic affinities, we have correlated the Ingalls with the Josephine ophiolite and possibly its rifted basement in northern California and southern Oregon. The polygenetic Ingalls ophiolite was accreted onto the North American margin and translated to the north. It is not known when accretion occurred, however, the ophiolite was thrust over metamorphosed Late Jurassic and Early Cretaceous pelitic schists of the Cascade Crystalline Core in the mid- Cretaceous.