Overview of the geology, petrology and tectonic framework of the high‐pressure–ultrahigh‐pressure metamorphic belt of the Kokchetav Massif, Kazakhstan

Abstract

High‐ to ultrahigh‐pressure metamorphic (HP–UHPM) rocks crop out over 150 km along an east–west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet‐like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north‐dipping, fault‐bounded orogen‐parallel units (I–IV, from base to top). Constituent metabasic rocks exhibit a systematic progression of metamorphic grades, from high‐pressure amphibolite through quartz–eclogite and coesite–eclogite to diamond–eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780–1000°C, P = 37–60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730–750°C, P = 11–14 kbar; Unit IV: T = 530°C, P = 7.5–9 kbar) and bottom (Unit I: T = 570–680°C; P = 7–13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit opposing senses of shear at the bottom (top‐to‐the‐north) and top (top‐to‐the‐south) of the pile. The orogen‐scale architecture of the massif is sandwich‐like, with the HP–UHPM nappe group juxtaposed across large‐scale subhorizontal faults, against underlying low P–T metapelites (Daulet Suite) at the base, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V). The available structural and petrologic data strongly suggest that the HP–UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjacent lower‐grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30–40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab‐breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP–UHPM rocks.

Geochemical data and zircon ages for rocks in a high-pressure belt of Chu-Yili Mountains, southern Kazakhstan: Implications for the earliest stages of accretion in Kazakhstan and the Tianshan

“…These were frequently assigned to the Archaean and Palaeoproterozoic, mainly on account of their high metamorphic grade and complex structure, but single zircon and Sm-Nd whole-rock dating demonstrated that many are as young as Palaeozoic (e.g., Shatsky et al, 1999;Salnikova et al, 2001;Kozakov et al, 2002;Hegner et al, 2010) and document high-grade events associated with terrane accretion and subduction. In this context HP-LT metamorphic belts, which commonly characterize ancient subduction zones and sutures (Coleman and Wang, 1995;Medaris, 1999), are of particular interest and provide important clues for reconstructions of the internal architecture and evolution of the CAOB accretionary collage (Tagiri et al, 1995;Gao et al, 1999;Kaneko et al, 2000;Maruyama and Parkinson, 2000;Gao and Klemd, 2003;Klemd et al, 2005;Dobretsov and Buslov, 2007;Zhang et al, 2007;Wang et al, 2010;Hegner et al, 2010). We present new data on the age, geochemistry, and origin of rocks from little known early Palaeozoic HP belt in the Anrakhai Mountains of southern Kazakhstan.…”

Boron cycling by subducted lithosphere; insights from diamondiferous tourmaline from the Kokchetav ultrahigh-pressure metamorphic belt

“…Recently, a K-rich tourmaline (K-tourmaline) with diamond inclusions was found from the Kokchetav UHPM belt, northern Kazakhstan (Shimizu and Ogasawara, 2005). The Early Cambrian Kokchetav UHPM belt, formed through collision of an island arc and a microcontinent (Zonenshain et al, 1990;Dobretsov et al, 1995), is composed of supracrustal rocks of pelitic-psammitic rocks and granitic rocks, with intercalations of amphibolite, eclogite, dolomitic marble and ultramafic rocks (Dobretsov et al, 1995;Kaneko et al, 2000;Maruyama and Parkinson, 2000). Metamorphic diamond and coesite are included in zircon, garnet, clinopyroxene, and kyanite in pelitic gneiss and dolomitic marble, mainly from the Kumdy-Kol region of the highest-grade UHPM unit (e.g., Sobolev and Shatsky, 1990;Sobolev et al, 1991Sobolev et al, , 1994Zhang et al, 1997;Katayama et al, 2000).…”

A synopsis of recent conceptual models on supercontinent tectonics in relation to mantle dynamics, life evolution and surface environment

“…9. Cartoon sketch illustrating the anatomy of a Pacific-type orogen (modified after Maruyama and Parkinson, 2000). The inset figure shows paired Pacific-type and collision-type orogens that develop on either side of the suture during continental amalgamation as proposed in this study.…”

“…center ( Fig. 9) (Maruyama, 1997;Maruyama and Parkinson, 2000;Santosh et al, 2009a,c;Sato et al, 2010). Such C-type and P-type paired orogens characterize the stratigraphy of double-sided subduction associated with continental amalgamation.…”