Spatial and temporal relationships between ophiolites and their metamorphic soles: A test of models of forearc ophiolite genesis

Abstract

Thin sheets of high-grade metamorphic rocks, called metamorphic soles, are found structurally beneath many ophiolites, apparently formed at the initiation of intraoceanic subduction, and may be useful in determining the tectonic setting of ophiolite genesis. Many ophiolites are interpreted to have formed in a supra-subduction zone setting largely on the basis of their petrology and geochemistry. Of these supra-subduction zone ophiolites, most have been interpreted to have formed in a forearc setting. Generation of ophiolites in a forearc setting indicates that such ophiolites must postdate the initiation of subduction beneath them and must therefore be younger than their metamorphic soles. However, the examples of pairs of ophiolites and their metamorphic soles reviewed herein show that the ophiolites are not younger than their soles. The spatial and temporal relationships reviewed herein may be incompatible with the generation of ophiolites in forearcs, although volumetrically subordinate igneous rocks that postdate the formation of the soles have been found in some of the examples. The apparent forearc setting of the ophiolites may be an artifact of their emplacement tectonics, not their igneous environment of formation. Although we know of no geochronologic confirmation of an older subduction zone structurally beneath purported forearc ophiolites, detailed geochronology has yet to be conducted on most ophiolite and metamorphic sole pairs. For the scenario in which an ophiolite is formed above one subduction zone but emplaced over a different one, either backarc formation or intra-arc formation of the ophiolite can be compatible with its regional geochronologic and structural relationships.