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.