THE DEVIL'S GATE OPHIOLITE, NORTHERN SIERRA NEVADA, CALIFORNIA: NOT AN OPHIOLITE OR METAMORPHIC SOLE?

The Devil’s Gate Ophiolite (DGO), a 10 km by 15 km large, oblong mass of amphibolite, is associated with the Feather River ultramafic belt (FRB), a 150-km-long belt of ultramafic and mafic rocks in the northern Sierra Nevada of California. The DGO has been interpreted as comprising mainly sheeted dikes with some pillow basalts on the periphery of the exposures and small bodies of gabbro in the interior. Inspection of the outcrops, however, reveals ubiquitous siliceous metasediment layers (<1 m thick) throughout the interpreted dike and gabbro parts of the DGO. The metasediment layers suggest that the metabasites of the DGO are almost entirely sea floor basalts rather than being primarily intrusive rocks. Metamorphic grain sizes increase towards the interior of the body. No igneous minerals or textures are preserved. Rutile is present in most of the DGO except for the finest grained peripheral units. Al2O3 content of the calcic amphiboles range from 4-9.7 % in the peripheral units to 11.2-12.1% for those of intermediate grain size (formerly called sheeted dikes) to13.6-14.1. % in the coarsest grained rocks formerly called metagabbros. TiO2 content of calcic amphiboles ranges from 0.20 % in the finest grained rocks to 0.56% in the coarsest amphibolites. Plagioclase ranges from albite (An 2-4) in the finest grained peripheral metabasites to An 16-18 for medium-grained, and An 18-22 for the coarsest grained metabasites. Glaucophane (Al2O3 10.6-11.7%) locally rims calcic amphibole in the finest-grained metabasites. The three dimensional shape of the body appears bowl-like with the stucturally highest and highest grade rocks in the center and the structurally lowest and lowest grade rocks on the periphery. The mineral assemblages and compositions appear to reflect high-pressure (HP) metamorphism (1 GPa or more) at medium to high temperature (M/HT) (480-610°C) and an inverted pressure as well as inverted temperature gradient, apparently reflecting internal imbrication within the amphibolite body. Whereas the DGO does not resemble a true ophiolite, it also differs from a HP-HT metamorphic soles found beneath ophiolites because it is much thicker (>1km) than typical metamorphic soles (≤0.5 km). The origin of the DGO remains enigmatic, but the metamorphism suggests that is was partly subducted.