Geology of the Kokchetav UHP-HP metamorphic belt, Northern Kazakhstan

Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kaza khstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM roc ks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. P rimary UHPM minerals are poorly preserved due to partial obliteration by su bsequent Barrovian overprint during exhumation and later collision events i n Caledonian times. We report the results of detailed mapping of the Kokche tav Massif and use structural data to propose intrusion and exhumation mech anisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure-high-pressure (UHP-HP) units were form ed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The f irst order structure is sandwich-like; that is, the UHP-HP units are separa ted from underlying low-P metamorphic rocks of the Daulet Series and from f eebly metamorphosed to unmetamorphosed sedimentary strata on the top by sub horizontal faults. Kinematic indicators show top-to-the-south sense of shea r along the top, and top-to-the-north displacement along the bottom boundar ies. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumati on of the UHPM rocks.