TECTONIC EVOLUTION OF THE POLYGENETIC INGALLS OPHIOLITE COMPLEX, CENTRAL CASCADES, WASHINGTON: A POSSIBLE RECORD OF JURASSIC FOREARC ACCRETION AND RIFTING?

The Ingalls ophiolite complex, Washington Cascades, is dismembered and consists of ultramafic rocks that contain oceanic crust as large-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. Crustal units in the large fault blocks are well preserved and divided into the: Iron Mt. unit; Esmeralda Peaks units; and sedimentary rocks of the Peshastin Fm. The Iron Mt. unit consists of pillow basalt and broken pillow breccia, with minor rhyolite, hyaloclastite, oolitic limestone and chert. A U-Pb zircon age from a rhyolite is ca. 192 Ma. Geochemical affinities of this unit are transitional between OIB and E-MORB. Sediments that overlie this unit are Early Jurassic (Pliensbachian). The Esmeralda Peaks unit consists of pillow and massive flows, diabase with minor sheeted dikes, gabbro and rare tonalite and trondhjemite. A U-Pb zircon age from a gabbro is ca. 161 Ma. Geochemical affinities of this unit are transitional between island arc tholeiite and N-MORB, and rare boninites exist. Late Jurassic (Oxfordian) sediments conformably overlie this unit and have geochemical affinities indicating a volcanic arc provenance.

It is interpreted that the Iron Mt. unit formed as an off axis seamount ca. 192 Ma. This seamount was then accreted onto an Early Jurassic forearc as the oceanic lithosphere it sat on was subducted. Early Jurassic sediments were then deposited on this accreted seamount. Rifting of this forearc began 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. Late Jurassic sedimentation probably initiated in the forearc and continued in the back-arc. Thus, the Iron Mt. unit and overlying Early Jurassic sediments formed the rifted basement for the Late Jurassic Esmeralda Peaks unit and overlying sediments. This polygenetic ophiolite was then accreted onto the North American margin and translated to the north. It was thrusted over the Cascade Crystalline Core in the Late Cretaceous.