The Ingalls ophiolite complex, central Cascades, Washington: Geochemistry, tectonic setting, and regional correlations

The polygenetic Ingalls ophiolite complex in the central Cascades, Washington, is one of several Middle to Late Jurassic ophiolites of the North American Cordillera. It consists primarily of mantle tectonites. High-temperature mylonitic peridotite, overprinted by serpentinite melange (Navaho Divide fault zone), separates harzburgite and dunite in the south from lherzolite in the north. Crustal units of the ophiolite occur as steeply dipping, kilometer-scale fault blocks within the Navaho Divide fault zone. These units are the Iron Mountain, Esmeralda Peaks, and Ingalls sedimentary rocks. Volcanic rocks of the Iron Mountain unit have transitional within-plate– enriched mid-ocean-ridge basalt affi nities, and a rhyolite yields a U-Pb zircon age of ca. 192 Ma. Minor sedimentary rocks include local oolitic limestones and cherts that contain Lower Jurassic (Pliensbachian) Radiolaria. This unit probably formed as a seamount within close proximity to a spreading ridge. The Esmeralda Peaks unit forms the crustal section of the ophiolite, and it consists of gabbro, diabase, basalt, lesser felsic volcanics, and minor sedimentary rocks. U-Pb zircon indicates that the age of this unit is ca. 161 Ma. The Esmeralda Peaks unit has transitional island-arc– mid-ocean ridge basalt and minor boninitic affi nities. A preferred interpretation for this unit is that it formed initially by forearc rifting that evolved into back-arc spreading, and it was subsequently deformed by a fracture zone. The Iron Mountain unit is the rifted basement of the Esmeralda Peaks unit, indicating that the Ingalls ophiolite complex is polygenetic. Ingalls sedimentary rocks consist primarily of argillite with minor graywacke, conglomerate, chert, and ophiolite-derived breccias and olistoliths. Radiolaria from chert give lower Oxfordian ages. Department of Marine and Ecological Sciences, Florida Gulf Coast University, 10501 FGCU Boulevard, South, Fort Myers, Florida 33965, USA; e-mail: jmacdona@fgcu.edu. 134 MacDonald et al. INTRODUCTION Seafl oor rifting and spreading within suprasubduction zones occurs in the western Pacifi c island arcs and in the East Scotia Sea in the southern Atlantic (e.g., Stern, 2002; Hawkins, 2003; Livermore, 2003; Martinez and Taylor, 2003). Seafl oor spreading within these suprasubduction zones can start either synchronously with arc formation (Scotia Sea; Larter et al., 2003) or by rifting of a previously active arc (Lau and Mariana Basins; Hawkins, 1995, 2003; Martinez and Taylor, 2003). Rifting within the forearc also occurs in the suprasubduction-zone environment (Bloomer, 1983; Bloomer and Hawkins, 1983; Hawkins and Melchior, 1985). The suprasubduction-zone setting is considered to be a likely place of origin for many, if not most, ophiolites (Dewey and Bird, 1971; Miyashiro, 1973; Pearce et al., 1984, Hawkins, 1995, 2003; Shervais , 2001; Pearce, 2003), including the belt of Middle to Late Jurassic ophiolites in the western United States (Fig. 1). These Cordilleran ophiolites have been proposed to represent oceanic crust formed by extension in a variety of suprasubduction-zone settings, including back-arc or forearc spreading and rifting of arc or forearc crust (Harper, 1984, 2003a; Harper and Wright, 1984; Wyld and Wright, 1988; Dilek, 1989a, 1989b; Saleeby, 1992; Stern and Bloomer, 1992; Miller et al., 1993; Harper et al., 1994, 2003; Dickinson et al., 1996; Metzger et al., 2002; Shervais et al., 2005a, 2005b). The origin for some of these ophiolites remains controversial (e.g., Coast Range ophiolite; Fig. 1) (Dickinson et al., 1996; Shervais, 2001; Shervais et al., 2004, 2005a; Hopson et al., this volume), while others are better constrained (e.g., Josephine ophio lite; Fig. 1) (Harper, 1984, 2003a, 2003b; Harper and Wright, 1984; Harper et al., 1994; Wyld and Wright, 1988). The Ingalls ophiolite complex, located within Washington State (Fig. 1), is a northern continuation of this Jurassic Cordilleran ophiolite belt. Understanding of the tectonic origin of the Ingalls ophiolite complex is critical in understanding the Jurassic tectonic evolution of the North American Cordillera. This ophio lite has had various tectonic settings proposed for its origin. Southwick (1974) suggested that the plutonic rocks formed in a marginal basin or open ocean, whereas the volcanic and sedimentary rocks originated in an island arc. Miller and colleagues The Ingalls ophiolite complex is similar in age and geochemistry to the Josephine ophiolite and its related rift-edge facies and to the Coast Range ophiolite of California and Oregon. The Ingalls and Josephine ophiolites are polygenetic, while the Coast Range ophiolite is not, and sedimentary rocks (Galice Formation) that sit on the Josephine and its rift-edge facies have the same Radiolaria fauna as Ingalls sedimentary rocks. Therefore, we correlate the Ingalls ophiolite complex with the Josephine ophiolite of the Klamath Mountains. Taking known Cretaceous and younger strike-slip faulting into account, this correlation implies that the Josephine ophiolite either continued northward ~440 km—thus increasing the known length of the Josephine basin—or that the Ingalls ophiolite was translated northward ~440 km along the continental margin.