ORIGIN AND EMPLACEMENT OF MAFIC–ULTRAMAFIC ROCK ASSEMBLAGES FROM DOWNGOING SLABS AT SUBDUCTION–ACCRETION COMPLEXES: PACIFIC RIM EXAMPLES AND ANCIENT ANALOGUES IN COLLISION ZONES

Phanerozoic accretionary and collisional orogenic belts include Penrose-type ophiolites derived from the upper plates of subduction zones (SSZ) as well as discrete bodies of mafic-ultramafic rock assemblages, derived from downgoing slabs and incorporated into accretionary prism complexes (APC). Unlike SSZ ophiolites, these assemblages may not have melt-residua petrogenetic kinship and are deformed, even metamorphosed at HP conditions. Recognition of their igneous origin and emplacement mechanisms into continental margins provide significant constraints for tectonic reconstructions of paleo–convergent margins. We review some salient examples of these assemblages in subduction-accretion complexes around the Pacific Rim and their ancient analogues in collision zones. Ridge-trench intersection (RTI) processes result in the emplacement of a young, buoyant MOR oceanic lithosphere into an active continental margin, as it decouples from the downgoing slab. Examples occur in Taitao (Chile) and Alaska. Ridge subduction may lead to asthenospheric window development beneath an upper plate and emplacement of gabbroic-doleritic dikes and lava flows into APCs. These igneous complexes in fossil APCs may be misinterpreted as ophiolites. Examples occur in Shikoku, Alaska, W Junggar and Central Tibet. Seamounts arriving at trenches may become decoupled from their oceanic subtratum, accreted beneath an APC, and transported to depths where they experience weak blueschist metamorphic overprint. Unroofing of accreted seamounts causes tectonic erosion, extensional collapse and slope failure, producing mass transport deposits and sedimentary mélanges overlying APCs. Examples include: Nankai Trough, Osa–Middle America Trench, Mount Diablo and Snow Mountain seamounts in the Franciscan APC, Ankara Mélange, YZSZ–Tibet, and W Junggar–CAOB.