Evidence for subduction-related contamination of the mantle beneath the southern Chile Ridge: Implications for ambiguous ophiolite compositions

Abstract

The recent discovery of convergent-margin geochemical signatures along the southern Chile Ridge spreading center may have implications for the interpretation of ophiolite provenance. The location of the ridge near a ridge-trench-trench (RTT) triple junction suggests that the convergent-margin signatures may relate to processes associated with spreading-center subduction. The possibility that ophiolite obduction may also occur near RTT triple junctions where a spreading center is being subducted implies that mid-ocean ridge basalts (MORBs) with supra-subduction zone (SSZ) signatures may have been preferentially emplaced as ophiolite complexes in the past. There is evidence that this process may have occurred immediately south of the Chile triple junction in the region of the Taitao Peninsula. Controversy surrounding the provenance of some well-studied ophiolites (e.g., Bay of Islands, Semail massif) has recently tended to favor the view that these ophiolites formed above subduction zones, primarily on the basis of their trace element chemistry. The discovery of MORB with convergent-margin geochemical signatures near ophiolitic material with similar chemical systematics points to a modern example of a tectonic setting in which ocean crust with anomalous convergent-margin-like compositions is formed and may be emplaced as an ophiolite. As a result, our southern Chile Ridge findings suggest that the geochemical distinction between MORB and SSZ lavas may be less well defined than previously thought.