GEOLOGY AND STRUCTURE OF DIAMOND-BEARING ROCKS OF THE KOKCHETAV MASSIF (KAZAKHSTAN)

Two crustal settings for microdiamond formation have been described from eclogite-bearing metamorphic areas: (1) the economic concentration of microdiamonds in metasedimentary gneisses and calc-silicate rocks (northern Kazakhstan); and (2) microdiamonds recently found in eclogite, garnet-pyroxenite and jadeitite from Dabie Shan Mountain, eastern China. The latter occurrence is interpreted to be the product of ultra-high-pressure metamorphism in a Mesozoic collision zone. There are, however, a number of discrepancies between the geological, structural and geochemical data for Kazakhstan microdiamond deposits and an interpretation in terms of a deep subduction zone model. The geodynamic setting of the Kokchetav massif can be defined as a continental rise prism environment related to a passive continental margin where rifting predominated during early Palaeozoic orogeny. The Kumdikol microdiamond province is closely associated with a tectonic melange zone involved in ductile to semi-ductile shearing abundant in graphite. Microdiamonds of the Kumdilkol area have a dual setting in the rocks. They appear to be included not only in refractory garnet and zircon but also in almost all rock-forming minerals as biotite, phlogopite, diopside, quartz and secondary sericite-chlorite and sericite-chlorite-calcite aggregates after garnet, pyroxene and plagioclase (?), and in spite of ''softness'' of the host phases microdiamonds are well preserved. On the other hand, the ore body extends along the shear zone and high concentrations of the microdiamonds within it are distributed without any lithological control along the local S-C surfaces of the main Kumdikol strike-slip shear zone. This duality of microdiamond settings in absence of practically all ultra-high-pressure minerals except diamond itself, weakens the interpretation of this occurrence in terms of very deep subduction and very fast uplift and exhumation during 15-10 Ma according to recent geochronological data. These microdiamonds could have been formed metastably under anisotropic stress conditions. In fact, the available data from the Kumdikol province provides certain support for either (metastable and stable) hypothesis.