12 - Diamond-bearing metamorphic rocks of the Kokchetav massif (Northern Kazakhstan)

Summary

Abstract

Primary mineral assemblages preserved mostly as inclusions in garnets and zircons have been found as a result of detailed investigations of diamondiferous metamorphic rocks of the Kokchetav massif, Northern Kazakhstan. Garnet and clinopyroxene are the principal minerals of these assemblages along with mica, kyanite, rutile, sphene, coesite and diamond. Estimation of the equilibration conditions using the mineral compositions shows that diamondieferous rocks were metamorphosed at temperatures not lower than 900°C. In establishing independent indicators of very high pressures (K in cpx, Al₂O₃ in sphene, diopside-magnesite assemblage, coesite inclusion in zircon, presence of diamond itself), all testify to the crystallization conditions of diamond in its stability field.

Introduction

The Kokchetav massif is the first locality where microdiamonds were found within metamorphic rocks of the Earth's crust (Sobolev and Shatsky, 1987, 1990). Information on alluvial diamonds in Northern Kazakhstan was known long ago (Esenov et al., 1968). Two hundred fifty microdiamond grains were found during the investigation of the titanium-zircon placer samples from the northern part of the Kokchetav massif. Kashkharov and Polkanov (1972) assumed the relative closeness of the native sources, based on the preservation of very delicate, fine-grained aggregates. Rozen et al. (1972) published the data on the diamond found in the weathered crust of eclogites at the Kumdy-Kol site. Later, eclogites were considered to be the native source of alluvial diamonds for this area (Rozen et al., 1979).