HIGH-PRESSURE/ULTRAHIGH-PRESSURE ROCKS IN THE CENTRAL ASIAN OROGENIC BELT: THEIR OCCURRENCE, AGE DISTRIBUTION, EXHUMATION AND EVOLUTION

The Central Asian Orogenic Belt (CAOB) contains about 50 occurrences of high-pressure (HP) blueschists and eclogites and at least three of ultrahigh-pressure (UHP) rocks. There are about 35 in the Early Paleozoic northwestern part of the orogen dominantly in southern Siberia including one UHP at Kokchetav, about 5 in the southern Late Paleozoic orogen with 4 in or near the terminal end-Paleozoic Tianshan-Solonker suture, and about 12 in the terminal Permian Uralian suture in the far west including one UHP at Maksyutov. The blueschists and eclogites typically occur in tectonic slices and lenses in subduction-accretion complexes in sutures in trench turbidites, HP paragneisses, augen orthogneisses, mafic dykes, mylonites, mélanges, ophiolites, MORB basalts, alkaline ocean island basalts, basic greenschists, garnet-bearing websterites and amphibolites, and in shelf marbles. UHP rocks in the Cambrian Kokchetav massif, Kazakhstan are unique, because they occur throughout a 150 km-long and 2 km-thick crustal slab of continental origin, in which coesites and diamonds occur as inclusions in almost all minerals in meta-pelites and psammites, marbles, amphibolites, orthogneisses, whiteschists, and garnet pyroxenites. Whereas sufficient structural work at Kokchetav has demonstrated that tectonic exhumation took place by wedge extrusion, unfortunately so little structural fieldwork has been undertaken in most other HP and UHP rocks that the tectonic mechanism by which they were exhumed is little known.