RECOGNITION CRITERIA OF OPHIOLITES FORMED DUE TO RIDGE COLLISION/SUBDUCTION

A mid-oceanic ridge system subducts underneath South American plate at latitude 46^oS off Chilean coast, forming a ridge-trench-trench type triple junction. At ~ 6 Ma, a short segment of the Chile ridge system subducted in south of the present triple junction. This ridge subduction event resulted in emplacement of a young ophiolite (5. 6 to 5. 2 Ma) and contemporaneous granite intrusions (5.7 to 4.9 Ma) and rapid crustal uplift (partly emerged after 4.9 Ma). This ophiolite, namely the Taitao ophiolite, provides criteria for the recognition of ridge collision/subduction-related ophiolites.

A ridge collision/subduction-related ophiolite has a short-life. In case of the Taitao ophiolite, two dating methods, U-Pb (date of mineral crystallization) and FT (cooling age at 200 ^oC) methods on zircons yielded the same age. High-temperature deformation structures were developed in plutonic section during the emplacement and superimposed on the structures related to the ridge processes. Such superimposed high-T deformations may result in complicated magnetic structures and demagnetization paths. A ridge collision/subduction-related ophiolite may exhibit a polarity of magmatism (change in age or geochemical signature toward one direction) reflecting obliquity between the subducting ridge and trench axis. In case of the Taitao ophiolite, centre of magmatism migrated from south to north with compositional changes from N-MORB to E-MORB. The width of the ophiolite is likely narrow, because deformations must be concentrated along hot, weak region near the ridge centre. The ridge collision/subduction related ophiolite may lack in the metamorphic sole.

A ridge subduction may accompany contemporaneous granite intrusions with various compositions. Our data indicates that the granitic melts started forming near the conjunction of the subducting ridge and transform fault. Generation of granitic melts continued as the spreading center of the same segment subducted, due to partial melting of the oceanic crust and subducted sediments. Eduction of fluids and melts from the subducting slab at shallow depth may result in the lack of the volcanic activities along the volcanic front, as exemplified as the volcanic gap in Chile.