Thermobaric structure of the Kokchetav ultrahigh‐pressure–high‐pressure massif deduced from a north–south transect in the Kulet and Saldat–Kol regions, northern Kazakhstan

Abstract

To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh‐pressure and high‐pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO2 (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high‐pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low‐pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.

Is quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets

“…Diamond and coesite inclusions identified in the garnet porphyroblasts point to peak metamorphic conditions of P$4-6 GPa and T$950-1000 C (Sobolev & Shatsky, 1990;Hermann et al, 2001;Korsakov et al, 2007). Higher peak metamorphic conditions of P$6-8 GPa and T ¼ 1100 C have been reported by Ota et al (2000), Massonne (2003) and Dobrzhinetskaya et al (2006). Monocrystalline quartz inclusions occur in the core of garnet porphyroblasts similarly to the coesite-grade samples.…”

Aragonite-calcite-dolomite relationships in UHPM polycrystalline carbonate inclusions from the Kokchetav Massif, northern Kazakhstan

“…In the case of aragonite-tocalcite transformation the residual pressure would have been increased to 4.5 GPa (a ¼ 0.5). Taking into account the higher peak metamorphic conditions reported by Ota et al (2000) and Massonne (2003), the residual pressure would have been expected even higher (Fig. 6).…”

Structural location, composition, and geodynamic nature of diamond-bearing metamorphic rocks of the Kokchetav subduction–collision zone of the Central Asian Fold Belt (northern Kazakhstan)

“…The rocks of the Kulet and Sulu-Tuba terranes lack diamonds, though in places (southwest and south of Lake Kulet) coesite is found in them. In contrast to the Kumdy-Kol terrane, these ones lack metacarbonates but contain interbeds of talc-garnet, talc-phengite, and other highly magnesian rocks (Dobretsov et al, 1998;Ota et al, 2000;Shatsky et al, 1999;Theunissen et al, 2000).…”