Petrology of the diamond-grade eclogite in the Kokchetav Massif, northern Kazakhstan

The Kokchetav Massif of northern Kazakhstan is unique because of the abundant occurrence of microdiamond inclusions in garnet, zircon and clinopyroxene of metasediments. In order to determine precise pressure-temperature (P-T) conditions, we have systematically investigated mineral inclusions and the compositions of major silicates in Ti-clinohumite-garnet peridotite and diamond-grade eclogite from Kumdy-Kol. It was found that garnet peridotites from Kumdy-Kol contain assemblages of garnet, olivine, Ti-clinohumite and ilmenite. The garnet contains inclusions that are indicative of both ultrahigh pressure (UHP) and retrograde conditions. Inclusions of hydrous phases such as chlorite, amphibole and zoisite were formed at the post-UHP stage. The study also found that eclogite from Kumdy-Kol contains albite-augite symplectites after omphacitic pyroxene. The core of pyroxene (sodic augite) contains high K2O (up to 1wt%; average 0.24wt%). Phengite is included in the core. Applying the K2O-in-augite geobarometry, which is based on recent experiments, and the garnet-clinopyroxene (Grt-Cpx) geothermometer for peak metamorphism, the eclogites yield P-T estimates of > 6 GPa and > 1000 degrees C, and the diamond-grade eclogites yield lower temperature estimates at 900-1000 degrees C and 5 GPa.