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 km along an east-west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet-like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. 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 progression of metamorphic grades, from high-pressure amphibolite through quartz-eclogite and coesite-eclogite to diamond-eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional 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 opposing senses of shear at the bottom (top-to-the-north) and top (top-to-the-south) of the pile. The orogen-scale architecture of the massif is sandwich-like, with the HP-UHPM nappe group juxtaposed across large-scale subhorizontal faults, against underlying low P-T metapelites (Daulet Suite) at the base, 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 adjacent lower-grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30-40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab-breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP-UHPM rocks.