The diamond-bearing Kokchetav UHP massif in Northern Kazakhstan: exhumation structure

in the Kokchetav ultrahigh-pressure (UHP) massif in northern Kazakhstan, di amond-bearing UHP rocks occur exclusively in a western, rhomb-shaped domain , that differs from an eastern transpressional domain with coesite-bearing remnants indicating highest UHP conditions. Different mechanisms may have c ontributed to the early ascent of the UHP Kokchetav massif. The geometry an d structure of the diamond-bearing domain are interpreted as a sheath-like fold, coeval with early stage melting. In contrast, the coesite-bearing dom ain has a sheet-like geometry. At mid-crustal level this early difference i n the ascending UHP wedge is reflected in a western rhomb-horst and an east ern transpression structure, respectively. The latter extends to the east ( Borovoye) where the uppermost sequences of the UHP massif were defined by o thers. Sheath folding is postulated as a suitable mechanism contributing to the early buoyancy-driven ascent of the subducted rocks, and explains the selective spatial distribution of diamond-and coesite-bearing sequences pre served in the wedge.