Preface to thematic issue: Petrotectonic characteristics of the Kokchetav Massif, northern Kazakhstan

Abstract

The Kokchetav Massif of northern Kazakhstan, compared to other recognized ultrahigh-pressure (UHP) terranes in the world, possesses three unique petrotectonic characteristics: (i) neoblastic microdiamonds, which are identified microscopically, are abundant as inclusions in garnet and zircon within biotite gneiss and pyroxene-bearing marble (e.g. Chopin & Sobolev 1995; Katayama et al. 2000 ); reported diamond occurrences in other UHP terranes, such as the Dabie Mountains and the Western Gneiss Region of Norway, are based on chemical dissolution of rocks and lack the unambiguous confirmation of microdiamond in situ; (ii) ultrahigh-pressure metamorphism of the Kokchetav Massif occurred at approximately 540–530 Ma, and probably reflects the apparent change in pressure/temperature conditions of the subduction-zone metamorphism at the Precambrian–Cambrian boundary ( Maruyama & Liou 1998); and (iii) some Kokchetav UHP rocks may have been recrystallized at the highest pressure conditions ever recorded for crustal rocks (P > 6 GPa), based on K-in-clinopyroxene geobarometry (e.g. Okamoto & Maruyama 1998; Okamoto et al. 2000 ). The Kokchetav Massif is a large, fault-bounded metamorphic complex of Proterozoic protolith age, surrounded by Caledonian rocks of the Ural–Mongolian fold belt. The Massif consists of amalgamated, differentially exhumed blocks situated between two major west-northwest-trending transcurrent shear zones. Several pre-Ordovician tectonic units (the Zerenda Series) of contrasting lithologies and metamorphic pressure–temperature (P–T) conditions occur, and have been interpreted as a tectonic megamelange (e.g. Dobretsov et al. 1995 , 1998; Shatsky et al. 1995 ). The highest-grade unit consists of crystalline schist, gneiss, eclogite, amphibolite, pyroxene granulite, quartzite, marble and associated amphibolite–granulite facies rocks. Diamond-bearing paraschist, paragneiss and marble, which carry biogenic carbon, clearly were laid down near the surface of the Earth ( Sobolev & Shatsky 1990; Chopin & Sobolev 1995). Diamond-bearing rocks crop out as thin, lenticular bands within felsic gneiss; studies prior to 1993 were mostly confined to the Kumdy–Kol diamond deposit area. Diamond-grade UHP rocks extend westward from Kumdy–Kol to Barchi–Kol ( Lavrova et al. 1996 ) and occur within an area no less than 80–100 km2 (for locations, refer to Fig. 1 of Kaneko et al. 2000 in this issue). Metamorphism of diamond-bearing paragneiss and schist and associated tectonic units took place in Early Cambrian time, as indicated by Sm–Nd and U–Pb ages between 530 and 540 Ma ( Claoue-Long et al. 1991 ). Adjacent units contain mineralogical evidence of slightly to markedly lower pressures and temperatures. Some represent a dismembered high pressure (HP)/UHP subduction collage, interpreted as a tectonic mélange composed of boudins of orthogneiss, eclogite and quartzite in a matrix of pelitic schist and paragneiss (e.g. Dobretsov et al. 1995 , 1998). The largest eclogite block is more than 1 km in length, forms the core of the Sulu–Tjube Mountain, and exhibits at least two stages of recrystallization (eclogite- and garnet amphibolite-facies). Some other units are coherent fault-bounded sheets consisting of recrystallized, gabbro–norite–diorite sills intruded into garnet + mica schist and kyanite/sillimanite-bearing aluminous gneisses. Pods of talc–kyanite–garnet whiteschist occur; their mineral parageneses have only been documented recently ( Zhang et al. 1997 ; Parkinson 2000). Other pre-Ordovician metamorphic series include slightly metamorphosed platform sedimentary strata—chiefly quartzite, feebly recrystallized basaltic rocks, and undifferentiated pre-Vendian ortho- and paragneisses. Russian and Kazakhstan geologists have extensively investigated the Kokchetav Massif over the past three decades. Previous studies have focused on the petrology and geochemistry of the diamond-bearing lithologies (e.g. Shatsky et al. 1995 ), the occurrences and chemical characteristics of UHP minerals including inclusions in zircon ( Sobolev & Shatsky 1990; Chopin & Sobolev 1995), and the geology and structure of the Massif (e.g. Dobrzhinetskaya et al. 1994 ; Dobretsov et al. 1995 , 1998). A subsequent US–Russian cooperative study in 1994–97 has resulted in the following important findings: (i) coesite pseudomorphs are present as inclusions in garnet of diamond-bearing gneiss, eclogite and calc–silicate rocks, hence providing unambiguous evidence for UHP metamorphism not only in diamond-grade terrane but also in other units extending further east to the Kulet area; (ii) clinopyroxenes from dolomitic marbles contain as much as 1.56 wt% K2O; (iii) mineral inclusions and textural relationships in whiteschists and eclogitic rocks establish both pro-grade and retrograde P–T paths of the UHP rocks, which is consistent with the later results by Parkinson (2000); and (iv) pressure–temperature estimates of whiteschists from other units indicate UHP conditions. These findings suggest that UHP conditions were not restricted to the diamond-grade unit in the Kumdy–Kol area (e.g. Zhang et al. 1997 ). In spite of intense mineralogic work conducted over the past three decades by Russian colleagues and by Kazakhstan geologists whose contributions are not yet published, the aereal dimensions and tectonic configuration of the Kokchetav Massif are poorly understood, reflecting limited surface exposures in the region. Thus, until 1998 (e.g. Dobretsov et al. 1998 ; Shatsky et al. 1998 ), these previous studies have documented neither the extent of the HP/UHP terrane nor the isotopic composition of the minerals and rocks. In order to better understand the geodynamics and geochemistry of the UHP Kokchetav Massif, a Japan–US–Kazakhstan cooperative project was established in 1997. The first step was to map the distribution and to determine structural relations of various tectonic units. This task was carried out by systematic field mapping by Japanese geologists during three field sessions (1997–99), led by Professor S. Maruyama (Tokyo Institute of Technology, Tokyo, Japan). The first workshop for the Task Group III-6, ‘Ultrahigh-Pressure Metamorphism and in Collision-type Orogenic Belts’ of the International Lithosphere Program (ILP), was held at Stanford University (CA, USA) in 1994, and the special issue of The Island Arc, ‘Ultrahigh-Pressure Metamorphism and Tectonics’, was published in December 1995. The second workshop was held in Beijing, during the International Geologic Congress 1996, and the second special UHP issue of The Island Arc contains many contributions from the Symposium 8-9, ‘Dynamic Metamorphic Rocks and High- and Ultrahigh-Pressure Metamorphism’, held in 1998. Field mapping of the Kokchetav Massif by the Japanese team has yielded many new geotectonic and petrochemical constraints, both in the field and laboratory. Preliminary data have been presented in two UHP workshops at: (i) Waseda University (Japan) in 1997; and (ii) Stanford University (CA, USA) in 1998, and in a special section of the American Geophysical Union (AGU) fall meeting in 1998. This special issue of The Island Arc focuses on petrology and geology of the Kokchetav Massif, Kazakhstan, and on the implications for collision-zone orogeny. It consists of 11 papers, resulting mainly from new geotectonic and petrochemical studies by the Japanese team. Previous contributions by Sobolev, Shatsky and other Russian scientists have provided a good basis for understanding the lithologic characteristics of the Massif. We hope that this special issue will enrich our understanding of subduction of supracrustal materials to depths of nearly 200 km. Kaneko et al. (2000) present the tectonic framework and a colored map of the regional geology of the Massif, and divide the HP/UHP Massif into four Units: I, II, III and IV, in ascending order of structural level. Unit I comprises the alternation of siliceous schist and amphibolite. Unit II is composed mainly of pelitic gneiss with abundant eclogite boudins and whiteschist in the coesite-bearing eclogite. Unit III consists of alternating orthogneiss and amphibolite, accompanied by large eclogite bodies. Unit IV comprises quartzite and siliceous schist. The HP/UHP Massif overlies the Daulet suite, a low-pressure metamorphic complex. Theunissen et al. (2000) followed the original idea of Dobretsov et al. (1995 , 1998) and considered that the Kokchetav Massif is composed of two megamelanges. The western domain includes the Kumdy–Kol megaunit, which is characterized by diamond-bearing metasediments, and the eastern domain comprises coesite-bearing metasediments and medium-P rocks. The two megamélanges are separated by a northeast-trending fault. All eclogites of the eastern domain are of the low-P type. Five sets of new 40Ar/39Ar ages are reported. Some are older than 560 Ma, due to excess argon, while others yield ages of the early exhumation stage at about 520 Ma, and shear development at 400 Ma. The early stage of formation of the Kokchetav megamélange was considered to be about 500 Ma. Structural features of the Kumdy–Kol unit in the western domain and the Kulet megaunit in the eastern domain were involved in early, rapid exhumation. Subsequent deformation reorganized the eastern domain. A uniform exhumation process for the whole HP/UHP Massif can not be established. In contrast, Japanese studies do not support the concept of mélange mixing for the Kokchetav Massif. For example, Yamamoto et al. (2000) describe subhorizontal structures and four distinguished stages of deformation in the central part of the Kokchetav Massif: (i) earliest penetrative, non-coaxial ductile deformation; (ii) upright folding with subhorizontal enveloping surfaces that refolded the earlier structures; the coesite- and diamond-bearing unit II has an opposite sense of shear direction to the overlying amphibolite unit, suggesting a north–westward extrusion of the UHP unit; (iii) large-scale bending around a subvertical axis; and (iv) local faulting. Similarly, Ishikawa et al. (2000) focus on the coherent structures of the lower part of the Kokchetav Massif. The HP/UHP unit is a thrust sheet emplaced onto the low-P Dulet Unit, with top-to-the-north movement. The Massif has nearly vertical schistosity, but the enveloping surface is subhorizontal, hence the lower boundary of the HP/UHP Massif is also subhorizontal. Ota et al. (2000) differentiate Kokchetav metabasites into epidote amphibolite, amphibolite, quartz eclogite, and coesite eclogite. The highest peak temperature occurs in diamond-grade eclogitic rocks of Unit II, and decreases toward structurally higher and lower units. A pressure gap of several tens of GPa exists between the UHP and its adjacent units. They used the wedge extrusion model of Maruyama et al. (1996) to explain the emplacement of these UHP/HP units, and suggested that the higher temperature (higher pressure) part of the HP/UHP unit extruded faster than the lower P–T slices of the same unit. Petrological data of eclogite and amphibolite from Units II and III of the Soldat–Kulet area are presented, and Al–Fe3+ partitioning between clinopyroxene and epidote is considered for temperature estimates. Masago (2000) describes parageneses of minerals of epidote amphibolite, amphibolite, quartz eclogite and coesite eclogite from the Barch–Kol area, that is, the western-most part of the Kokchetav Massif. In spite of its association with diamond-bearing garnet-bearing gneisses, eclogitic clinopyroxene is poor in Ca–Eskola and K–jadeite content. Chemical reactions related to transition of amphibolite to quartz eclogite are proposed and parageneses of minerals with bulk chemistry are shown in several tetrahedral diagrams. Okamoto et al. (2000) present petrogeneses of eclogites and garnet peridotites from the Kumdy–Kol area, where microdiamond occurs as inclusions in garnet and zircon, and clinopyroxene in metasediments and marbles. Most eclogites contain sodic augite with a maximum K2O content of 1.0 wt%, whereas peridotites are enriched in Ti–clinohumite. Estimated P–T conditions are > 6 GPa and > 1000°C for garnet peridotites, and 5 GPa and 900–1000°C for diamond-grade eclogites, which are consistent with previous estimates by Zhang et al. (1997) . Ogasawara et al. (2000) compares two types of marble from the Kumdy–Kol area. Dolomite-rich marble contains abundant microdiamonds in well-preserved garnet. The other carbonates lack diamond but contain aragonite, Ti–clinohumite and forsterite; garnet was nearly replaced by diopside + aragonite + spinel. Both types of carbonates probably formed at the same P–T conditions as the associated diamond-grade eclogite and garnet biotite gneisses. However, the difference in the fluid composition, specifically Xco2 was used to explain the differences in mineral assemblage for peak-stage metamorphism and in the preservation of garnet during retrograde recrystallization at exhumation. Katayama et al. (2000) systematically examine mineral species included in 6000 zircon separates from about 232 UHP/HP rocks of the Kokchetav Massif using micro-Raman spectroscopy, and textural relations using scanning electron microscopy (SEM) and cathode luminescence. More than 19 minerals representing various prograde stages were identified. Zircons are typically zoned, with probable detrital cores and prograde and peak metamorphic rim. Parageneses of included mineral species, such as albite–jadeite transition and diamond growth at the expense of graphite toward the outer zircon rims, constrain the prograde P–T path. De Corte et al. (2000) describe the nitrogen content, morphology and surface features of Kokchetav diamonds. They conclude that diamonds were formed and stayed at depth and 900–950°C for around 5 Ma, where fluid with H2O and CO2 had access to the growing diamond. The N content and C isotopic characteristics suggest the source of C for the diamonds is crustal in origin rather a mixture of crustal and mantle sources. Maruyama and Parkinson (2000) summarize the Japanese contribution to this issue and emphasize the subhorizontal structures of various HP/UHP units of the Kokchetav Massif. The P–T conditions of various units are summarized and plotted in various localities, and the pressure gap of the UHP unit at the contact point with the overlying and underlying lower pressure units amounts to 0.6 GPa or higher. The extrusion model was used to illustrate the exhumation of the HP/UHP units to crust levels, where HP/UHP rocks suffered amphibolite-facies re-equilibration. Ages of peak metamorphism occurred during mid-Cambrian time (530 Ma; zircon, Sensitive High mass-Resolution Ion Microphobe; 533 ± 30 Ma, garnet and clinopyroxene, Sm–Nd), and retrograde recrystallization took place at crustal depths at 515 Ma (muscovite, 40Ar/39Ar). Such rapid uplift was caused by slab break-off triggered by buoyancy of crustal materials enclosed in denser mantle materials at about 200 km depth. The uplift of the Kokchetav Massif has present-day analogs in Timor, where the Australian continent is colliding with the Banda Arc. The above-mentioned papers in this special issue of The Island Arc are examples of some of the exciting findings and research projects described by participants at the Waseda and Stanford workshops and the recent AGU meetings. It is our hope that The Island Arc will continue to publish a special issue on this increasingly recognized subject, which is essential to our understanding of continental subduction/collision, mantle dynamics and geochemical + fluid cycles. Our thanks and appreciation go to the many workshop participants, particularly to the authors of the papers in this special issue and to those who offered their time and expertise to review the manuscripts. The workshop held at Waseda in 1997 was supported by the Waseda University and the one at Stanford in 1998 was supported by the USA–Japan–Kazakhstan–Russian project of the National Science Foundation.