Structure, Cr-chemistry, and age of the Border Ranges Ultramafic-Mafic Complex: A suprasubduction zone ophiolite complex

A distinctive yet enigmatic suite of fault-bounded ultramafic massifs occurs within accretionary complex mélange of the McHugh Complex on the Kenai Peninsula of southern Alaska. The largest and most significant of these include Red Mountain and the Halibut Cove Complex, consisting of dunite and pyroxenite with chromite seams and lesser quantities of garnet pyroxenite and gabbro. Several different hypotheses have been advanced to explain their origin. Burns (1985) correlated these fault-bounded ultramafic massifs with others known as the Border Ranges Ultramafic-Mafic Complex. Other parts of the Border Ranges Ultramafic-Mafic Complex are located several hundred kilometers away along the Border Ranges fault, marking the boundary between the Chugach terrane and the Wrangellian composite terrane in the northern and eastern Chugach Mountains. Burns (1985) suggested that this entire group of ultramafic bodies represents the deep roots of the Talkeetna arc developed on the southern margin of Wrangellia during Early Jurassic–Cretaceous subduction. In this model, bodies such as Red Mountain represent klippen thrust hundreds of kilometers southward over the McHugh Complex and now preserved as erosional remnants. Bradley and Kusky (1992) suggested alternatively that the Kenai ultramafic massifs may represent segments of a thick oceanic plate offscraped during subduction, and therefore might represent ophiolitic, oceanic plateau, or immature island arc crust as opposed to the roots of the mature Talkeetna arc. In this scenario, the Kenai ultramafic massifs would be correlative with the McHugh Complex, not the Talkeetna arc. A third hypothesis is that the Border Ranges Ultramafic-Mafic Complex may represent forearc or suprasubduction zone ophiolites formed seaward of the Talkeetna arc during early stages of its evolution and incorporated into the accretionary wedge during subsequent accretion tectonics. The implications of which of these models is correct are large because the Talkeetna arc section is the world's premiere example of a complete exposed arc sequence, including the volcanic carapace through deep crustal levels. Many models for the composition and evolution of the crust rely on the interpretation that this is a coherent and cogenetic section of arc crust.