Blueschists and eclogites: a possible plate tectonic mechanism for their emplacement from the upper mantle

Abstract

The most significant problem of blueschists and eclogite origin is explaining the mechanism by which they were transported from great depths to the surface. Three aspects of this problem are discussed: 1. (1) prograde and retrograde trajectories in blueschist- and eclogite-bearing belts and the preservation conditions for such minerals as aragonite, lawsonite and coesite; 2. (2) the mechanism by which low density crustal rocks (with coesite and diamond) descend to a depth of 100–150 km, and the high rate of return from these depths to the surface; 3. (3)the origin of a tectonic mixture of rocks from different depths in melange zones. Using estimations of P- T conditions and constructed P- T- t trajectories four types of blueschist belt are proposed. These include two of the Ernst types (the Alpine and Franciscan) and two additional types (the Dora-Maira and Sanbagawa types). Key examples of geologically and mineralogically well-studied complexes in the Soviet Union are discussed (the Kokchetav massif, Maksutov complex, Syumkeu belt, Borus belt). The various models for the origin of different types of high-pressure metamorphic belts are considered: 1. (1) slow uplift, or uplifting of the thickened crust after cessation of subduction; 2. (2) more rapid tectonic “pushing out”; 3. (3) the same as in case 2 but with an additional “champagne cork” effect; 4. (4) the most complicated process, with repeated deformation and metamorphism.