Overview of the geology, petrology and tectonic framework of the high-pressure-ultrahigh-pressure metamorphic belt of the Kokchetav Massif, Kazakhstan

High- to ultrahigh-pressure metamorphic (HP-UHPM) rocks crop out over 150 k m along an east-west axis in the Kokchetav Massif of northern Kazakhstan. T hey are disposed within the Massif as a 2 km thick, subhorizontal pile of s heet-like nappes, predominantly composed of interlayered pelitic and psammi tic schists and gneisses, amphibolite and orthogneiss, with discontinuous b oudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyr oxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north-dipping, fault-bounded orogen-parallel units (I-IV, from base to top). Constituent metabasic rocks exhibit a systematic progres sion of metamorphic grades, from high-pressure amphibolite through quartz-e clogite and coesite-eclogite to diamond-eclogite facies. Coesite, diamond a nd other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered fr om the effects of fluids during retrogression. Inclusion distribution and c onventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780-1000 degrees C, P = 37-60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730-750 degrees C, P = 11-14 kbar; Unit IV: T = 530 degrees C, P = 7.5-9 kbar) and bottom (Unit I : T = 570-680 degrees C; P = 7-13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit oppo sing senses of shear at the bottom (top-to-the-north) and top (top-to-the-s outh) of the pile. The orogen-scale architecture of the massif is sandwich- like, with the HP-UHPM nappe group juxtaposed across large-scale subhorizon tal faults, against underlying low P-T metapelites (Daulet Suite) at the ba se, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V) . The available structural and petrologic data strongly suggest that the HP -UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjace nt lower-grade units at shallow crustal levels of around 10 km. The nappe p ile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the co re, and around 30-40 km at the margins. Consequently, wedge extrusion, perh aps triggered by slab-breakoff, is the most likely tectonic mechanism to ex hume the Kokchetav HP-UHPM rocks.