Metamorphic evolution of diamond‐bearing and associated rocks from the Kokchetav Massif, northern Kazakhstan

Abstract

A B S TR A CT Representative diamond-bearing gneisses and dolomitic marble, eclogite and Ti-clinohumite-bearing garnet peridotite from Unit I at Kumdy Kol and whiteschist from Unit II at Kulet, eastern Kokchetav Massif, northern Kazakhstan, were studied. Diamond-bearing gneisses contain variable assemblages, including Grt+Bt+Qtz±Pl±Kfs±Zo±Chl±Tur±Cal and minor Ap, Rt and Zrn; abundant inclusions of diamond, graphite+chlorite (or calcite), phengite, clinopyroxene, K-feldspar, biotite, rutile, titanite, calcite and zircon occur in garnet. Diamond-bearing dolomitic marbles consist of Dol+Di±Grt+Phl; inclusions of diamond, dolomite±graphite, biotite, and clinopyroxene were identified in garnet. Whiteschists carry the assemblage Ky+Tlc+Grt+Rt; garnet shows compositional zoning, and contains abundant inclusions of talc, kyanite and rutile with minor phlogopite, chlorite, margarite and zoisite. Inclusions and zoning patterns of garnet delineate the prograde P-T path. Inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss. Other ultrahigh-pressure (UHP) indicators include Na-bearing garnet (up to 0.14 wt% Na 2 O) with omphacitic Cpx in eclogite, occurrence of high-K diopside (up to 1.56 wt% K 2 O) and phlogopite in diamond-bearing dolomitic marble, and Cr-bearing kyanite in whiteschist. These UHP rocks exhibit at least three stages of metamorphic recrystallization. The Fe-Mg partitioning between clinopyroxene and garnet yields a peak temperature of 800-1000°C at P>40 kbar for diamond-bearing rocks, and about 740-780°C at >28-35 kbar for eclogite, whiteschist and Ti-bearing garnet peridotite. The formation of symplectitic plagioclase+amphibole after clinopyroxene, and replacement of garnet by biotite, amphibole, or plagioclase mark retrograde amphibolite facies recrystallization at 650-680°C and pressure less than about 10 kbar. The exsolution of calcite from dolomite, and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c. 420°C and 2-3 kbar. A clockwise P-T path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to UHP subduction-zone metamorphism within the diamond stability field. Tectonic mixing may have occurred prior to UHP metamorphism at mantle depths. During subsequent exhumation and juxtaposition of many other tectonic units, intense deformation chaotically mixed and mylonitized these lithotectonic assemblages.

Titanium in phengite: a geobarometer for high temperature eclogites

“…Published analyses (Vavilov et al 1991;Shatsky et al. 1995;Zhang et al 1997;Hermann et al 2001;Zhang et al 2002a, b;Massonne 2003) show a large range of composition which arises from the changes of P-T conditions and the variability in protolith compositions. In a Ti versus Si diagram (Fig.…”

Intrusion of UHP metamorphic rocks into the upper crust of Kyrgyzian Tien-Shan: P-T path and metamorphic age of the Makbal Complex

“…The geological relationship between ultra -high pressure (UHP) pelitic schists and eclogites has been reported in many studies; however, there have been limited discussions on the ascending carrier (e.g., England and Holland, 1979;Platt and Wallis, 1991;Zhang et al, 1997;Kaneko et al, 2000). It seems that UHP metamorphic (UHPM) rocks rise rapidly from the upper mantle to the crust.…”

“…In contrast to the study of Hacker and Peacock (1995), three papers-by Gebauer et al (1997), Duchêne et al (1997), and Rubatto and Hermann (2001)-have reported that the exhumation rate of Dora -Maira is 20 -30 mm/year. According to Zhang et al (1997) and Hermann et al (2001), Kokchetav UHPM rocks rose at a rate of approximately 10 -30 mm/year. Do-bretsov and Shatsky (2004) proposed extremely high uplift velocities of 60 -200 mm/year for the Kokchetav UHPM rocks; however, the exhumation mechanism has not yet been understood completely.…”

OH defects in coesite and stishovite during ultrahigh-pressure metamorphism of continental crust

“…Therefore, the aim of the present work is to investigate the formation of OH defects in coesite and stishovite from a natural granitic material under experimental conditions that reflect a realistic geotherm of a subducting continental crust fragment. The experimental condition chosen here is based on natural examples such as the Dora Maira (Chopin 2003) and the Kokchetav (Zhang et al 1997) massifs. Finally, the results from the present study are compared to the defect inventory of quartz synthesized previously from the same starting material (Frigo et al 2016), to evaluate the potential of silica polymorphs as carrier for light elements (H, B, and Li) to the Earth's interior.…”