SIGNIFICANCE OF UHP MINERALOGY IN COLLISIONAL OROGENIC BELT; INSIGHT FROM THE KOKCHETAV MASSIF

Geology, petrology, geochronology and geochemistry of the diamond-bearing Kokchetav massif of northern Kazkhstan have been investigated by a Titech-USA-Kazakhstan Research Group since1998, after extensive studies by Russian scientists. We herewith summarize our findings to emphasize several new concepts of collision orogeny. (1) The Kokchetav massif consists of a series of subhorizonal nappes: a thin tectonic slice with P-T maximum occurs at structural intermediate. The UHP-HP unit is as thin as 1-2 km and bounded on the top and bottom by normal and reverse faults, respectively, and separated from the low-grade or low-P metamorphic rocks. (2) The underlying unit was thermally metamorphosed to form andalusite-sillimanite metamorphic rocks by the hot tectonic intrusion of UHP-HP rocks. (3) Metamorphic facies series ranges from the epidote-amphibolite, through quartz and zoisite eclogite to dry eclogite facies with a sharp kink point, indicating an anti-clockwise P-T path. (4) The P-T-time path calculated by inclusion mineralogy in garnet combined with zoned garnet yield the same P-T path with metamorphic facies series. (5) The extensive hydration at mid-crustal level obliterated the pre-existing UHP-HP mineralogy, except for mineral inclusions in garnet, zircon and omphacite. (6) Zoned zircons with UHP-HP minerals mantled by late-stage hydration stage with Barrovian minerals were dated as 537Ma and 507Ma; 30 m.y. long from 200 km depth to mid-crustal level, indicating slow tectonic exhumation. (7) Combining above constraints, a tectonic extrusion model is proposed and is quite different from that by England and Thompson (1984) for the Himalayan orogenic belt. These new constraints such as a thin tectonic slice, thermobaric structure, anti-clockwise facies series, P-T-time path, and slow exhumation of the Kokchetav massif may be common in most collisional orogenic belts. The continental subduction, the extrusion model and the Barrovian overprint and hydration can be modeled the processes of most collision orogeny.