A comparison of the evolution of diamondiferous quartz-rich rocks from the Saxonian Erzgebirge and the Kokchetav Massif: are so-called diamondiferous gneisses magmatic rocks?

Abstract

The petrography and chemical composition of minerals of quartz-rich diamondiferous rocks from the Kokchetav Massif, especially the zonation of garnet, were studied and compared with diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge. Many compositional and textural features were found to be similar. For instance, microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks. On the basis of experimental data, a magmatic scenario was constructed to check if the quartz-rich diamondiferous rocks are of magmatic origin. By this, the P–T paths, derived here for the Kokchetav rocks, and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts. These melts originated by anatexis of deeply submerged metasediments (Erzgebirge: at T as high as 1200°C, Kokchetav Massif: at 50–100°C lower T) and ascended from at least 200 km depth. Relics of the pre-anatectic evolution are still present, for instance, as garnet cores. After ascent and emplacement of the magma in deep portions of thickened continental crust (Kokchetav Massif: 45–50 km close to 800°C, Erzgebirge: 55–60 km at 30–50°C lower T) considerable quantities of (white and/or dark) micas formed by peritectic reactions from melt. For instance, garnets could be resorbed at this stage and biotite grew instead. After the magmatic stage, retrogression took place much stronger in the Kokchetav Massif. This was accompanied by deformation transforming broadly the magmatic texture of quartz-rich diamondiferous rocks from the Kokchetav Massif to a gneissic texture.

Introduction

Microdiamonds in SiO₂-rich rocks, probably material from the continental crust, were unequivocally proven by ‘in situ’ studies from only two metamorphic complexes on Earth. The first detected location of this kind of rocks is in the Early Paleozoic Kokchetav Massif in northern Kazakhstan [1], [2]. Recently, a second location was recognized in the Late Paleozoic Saxonian Erzgebirge [3], [4] in the northwestern edge of the Bohemian Massif. A third occurrence of microdiamonds in felsic rocks, situated in the Norwegian Caledonides (island of Fjortoft), was reported some years ago [5]. However, so far, only some microdiamonds could be extracted from 50 kg Fjortoft gneiss. For the diamondiferous quartz-rich rocks from the Kokchetav Massif and the Saxonian Erzgebirge first peak pressure estimates of 4–4.5 GPa at temperatures close to 1000°C [2], [3], [6] were settled just above the graphite–diamond transition pressures. Later, because of observations such as SiO₂ exsolution and significant K contents in clinopyroxene, ultrahigh pressure (UHP) as high as 7 GPa was assumed for the diamondiferous rocks from the Kokchetav Massif [7]. The detection of TiO₂ with α-PbO₂ structure (UHP-TiO₂) [8] in the Erzgebirge even points to pressures in excess of 7 GPa according to the rutile–UHP-TiO₂ transition curve [9]. At such pressures, the temperatures of the Saxonian rocks with microdiamond might have been as high as 1200°C [10] because of the dT/dP slope (∼6°C/kbar) of the applied Ti-in-garnet thermometer [11]. At such high temperatures, melting of SiO₂-rich rocks in the absence of a hydrous fluid phase is not unlikely at UHP conditions. Experiments at pressures of 3 GPa and higher with pelitic, andesitic, and granitic compositions (dry with small amounts of H₂O added) gave silicate melt coexisting with phases such as garnet at temperatures above 1000°C [12], [13], [14]. A further hint of, at least, partially molten rocks at the above UHP conditions are inclusions in garnet from the Erzgebirge consisting of microdiamond, quartz, feldspars and micas. These inclusions represent COH-bearing silicate-rich (supercritical?) fluids (or melts depending on the pressure for the second critical end point in a relevant fluid–melt system) trapped during exhumation but still at UHP conditions [15].

In spite of the many studies on the UHP rocks from the Kokchetav Massif, for instance, by Ota et al. [16] and Theunissen et al. [17], a well-constrained prograde and retrograde path of the P–T evolution could not so far be derived. For the Saxonian occurrence, the information is better, at least, with regard to a P–T date for an early, pre-diamondiferous stage (1.8 GPa and 650°C, see [18]). In addition, we know that exhumation was accompanied by significant cooling [3]. Thus, the aim of this study is: (1) to improve the knowledge of the P–T evolution of UHP quartz-rich rocks from the Kokchetav Massif, (2) to compare these rocks with the diamondiferous Erzgebirge’s rocks, and (3) to answer the questions which degree of melting occurred and if it might have played a role in the exhumation of the diamondiferous rocks from both areas. The latter point also leads to a central problem, namely, are or were the so-called diamondiferous gneisses actually magmatic rocks?