HP/HT METAMORPHISM OF THE DEVIL'S GATE OPHIOLITE, SIERRA NEVADA, CALIFORNIA: WHERE IS THE UPPER PLATE?

A reconnaissance of the Devil’s Gate ophiolite (DGO) at the type section along Canyon Creek, North Yuba drainage, California shows that at least some of the rocks were metamorphosed at high pressure (P) and high temperature (T). These rocks are foliated epidote amphibolites, previously interpreted as meta-pillow basalts and meta-sheeted dikes even though no relict igneous textures exist. The meta-pillow interpretation is reinforced by the presence of intercalated meta-cherts. All metabasite mineral assemblages include: hbl + ep + ab +/- qtz. Some samples of meta-basalts and meta-dikes contain rutile, locally rimmed by titanite. Whereas others contain illiminite, locally rimmed by titanite. There is no systematic spatial distribution of the Ti phases and thus no metamorphic gradient from the meta-basalts into the meta-dikes. The presence of rutile in the meta-dikes and meta-basalts suggests that at least part of the Devil’s Gate ophiolite was subjected to high P metamorphism (>1.2GPa). Whether the irregular distribution of Ti-phases indicates significant vertical faulting and/or variations in bulk composition in the section is not clear. High-P, high T metamorphism of large (multi-km in lateral extent) ophiolite sheets is rare. Such sheets are commonly slightly metamorphosed and make up the upper plate of a paleo-subduction system; high-P metamorphic rocks are commonly found structurally beneath such sheets. The high P metamorphism along with the occurrence of metacherts requires burial rather than hydrothermal (spreading center) metamorphism and is most likely associated with subduction to attain sufficient pressure. This type of metamorphism of oceanic crust is commonly found in metamorphic soles structurally beneath ophiolites and is commonly thought to have formed at the inception of subduction beneath the ophiolite. The field relations and scale of the DGO differ from typical metamorphic soles, so a different tectonic mechanism is required to explain the metamorphism.