Thermobaric structure of the Kokchetav ultrahigh-pressure-high-pressure massif deduced from a north-south transect in the Kulet and Saldat-Kol regions, northern Kazakhstan

To investigate the regional thermobaric structure of the diamondiferous Kok chetav ultrahigh-pressure and high-pressure (UHP-HP) massif and adjacent un its, eclogite and other metabasites in the Kulet and Saldat-Kol regions, no rthern Kazakhstan, were examined. The UHP-HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest l evel of the massif and consists of garnet-amphibolite and acidic gneiss wit h minor pelitic schist and orthogneiss. Unit II, which structurally overlie s Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kul et and Saldat-Kol eclogites are omphacite, sodic augite, garnet, quartz, ru tile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in ga rnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amp hibolite with local eclogite masses. The structurally highest unit, Unit IV , is composed of quartzitic schist with minor pelitic, calcareous, and basi c schist intercalations. Mineral assemblages and compositions, and occurren ces of polymorphs of SiO2 (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat-Kol regions indicate that the metamorphic gr ades correspond to epidote-amphibolite, through high-pressure amphibolite a nd quartz-eclogite, to coesite-eclogite facies conditions. Based on estimat ions by several geothermobarometers, eclogite from Unit II yielded the high est peak pressure and temperature conditions in the UHP-HP massif, with met amorphic pressure and temperature decreasing towards the upper and lower st ructural units. The observed thermobaric structure is subhorizontal. The UH P-HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low-pressure Daulet Suite to the south; boundaries are sub horizontal faults. There is a distinct pressure gap across these boundaries . These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP-HP unit during the exhumat ion process, and that all of the UHP-HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.