Thermobaric Conditions for Exhumation of Ti-clinohumite Garnetites of the Kokchetav Subduction-Collision Zone (Northern Kazakhstan)

Abstract

Abstract —A new variety of Ti-clinohumite garnetites (‘garnet peridotites’) of the diamond-bearing Kumdy Kol terrane in the Kokchetav massif has been studied. An ultrahigh-pressure phase, Ti-chondrodite, orthopyroxene and contrast-zoned garnets of retrograde zoning have been found in these rocks for the first time. Ti-chondrodite and orthopyroxene were not described in these rocks earlier, and garnet composition was supposed to be homogeneous. Based on the character of chemical zoning of garnet and orthopyroxene, the peak and three retrograde parageneses for these minerals have been distinguished. Using the garnet–orthopyroxene geothermobarometry, for the Kumdy Kol terrane, based on the quantitative P–T estimates of Ti-clinohumite garnetites, a detailed retrograde P–T trend has been constructed for the first time; this trend includes the UHP peak stage (55–57 kbar, 1075–1120 °С), and three stages of retrograde metamorphism under UHP (32 kbar, 830 °С), HP (21 kbar, 750 °С), and MP (12 kbar, 740 °С) conditions. For the Kumdy Kol terrane this is the most detailed Р–Т trend indicating subisothermal decompression during crustal rock exhumation from the mantle depths (~170 km) to the lower-crust conditions (~36 km). The above-mentioned mineralogical findings allowed reconstructing all the described metamorphism stages, using the same rock and various generations of the same minerals (garnet and orthopyroxene) formed at different stages of tectono–metamorphic evolution of the Kumdy Kol terrane. Preservation of the relics for mineral zoning of the early metamorphic stages seemed to be due to high uplift velocities resulted in rapid decrease in P–T parameters and short duration of regressive metamorphism. The virtually complete identity of the obtained P–T trend for the Kumdy Kol terrane and the P–T trend for the Barchi terrane supports the idea of the common tectono–metamorphic evolution of the whole western (diamond-bearing) part of the Kokchetav massif.