Home – Home – Jornals – Russian Geology and Geophysics 2006 number 4

The Kokchetav subduction-collision zone (KSCZ) hosting ultrahigh- and high-pressure (UHP-HP) rocks underwent the multistage Vendian-Early Ordovician geodynamic evolution. The subduction of the Paleoasian oceanic lithosphere bearing blocks of continental crust and the collision of the Kokchetav microcontinent with the Vendian-Cambrian island-arc system ultimately led to the formation and exhumation of UHP-HP rocks. In the Vendian-Early Cambrian the margin of the Kokchetav microcontinent deeply subsided into the subduction zone (150-200 km), which led to UHP-HP metamorphism (the maximum at about 535 Ma) and to partial melting of its rocks. In next stage (535-528 Ma), the generated acidic melts including blocks of UHP-HP rocks quickly, at a rate of 1 m/year, ascended to depths of 90 km for 1 Myr. During subsequent 5 Myr, the UHP-HP rocks ascending at a rate of 0.6-1 cm/year reached the base of the accretionary prism (depths of 60-30 km). Then, in the period from 528 to 500 Ma, the UHP-HP rocks ascended along the faulting structures of the lower crust as a result of jamming the subduction zone by the Kokchetav microcontinent. During the period from 500 to 480 Ma, the UHP-HP rocks became part of the upper crust. This process led to the KSCZ, which comprises terranes of the Vendian-Early Arenigian subduction zone occurring at different depths, separated by zones of garnet-mica and mica schists, blastomylonites and mylonites. In the same period there was a jump of subduction zone, which led to the formation of the Ordovician Stepnyak island arc. As a result of the Late Arenigian-Early Caradocian microcontinent-island arc collisions (480-460 Ma), the KSCZ overrided upon the fore-arc trough of the Stepnyak island arc to form a thick accretion-collision orogen, which having experienced anatectic melting was intruded by collisional granites of the Zerenda complex 460-440 Ma in age.

It has been shown by extensive mapping and sampling in the central Kokchetav subduction-collision zone that it is a terrane of a Vendian-Cambrian paleosubduction zone made up of tectonic sheets of Precambrian granite gneisses, Late Precambrian garnet-kyanite-sillimanite-biotite schists with boudins of garnet and garnet-plagioclase amphibolites, and Cambrian mylonitized granite gneisses with boudins of eclogites. The fault zones between tectonic sheets are formed by garnet-quartz-muscovite and quartz-muscovite schists. Along with the central Kokchetav belt, they are isoclinally overfolded southwestward. ⁴⁰Ar/³⁹Ar dating of muscovite from five mica-schist samples yielded close plateau and isochron ages of 480-485 Ma (Early Ordovician). The geological and geochronological data confirm the Early Ordovician formation of the nappe-imbricated structure of the Kokchetav subduction-collision

The siliceous rocks from structures of the junction zone of the Kokchetav massif and Stepnyak trough that occur in turbidites of the accretionary wedge, silica-volcanogenic sequence of the Stepnyak trough and syntectonic olistostrome were dated on the basis of findings of conodonts and radiolarians within the Middle-Upper Arenigian (conodont zones O. evae, B. navis-lower Par. originalis). This range of time is marked by a powerful tectonic rearrangement, involved with the rearrangement of the accretionary wedge and overriding of the Kokchetav massif upon the Stepnyak fore-arc trough.

Based on summarized and correlated structural, paleogeographical, and geochronological data, we propose new concepts of the intricate multistage Cambrian-Middle Ordovician evolution of an island-arc system in northern Kazakhstan. It is shown that in the Early Ordovician, the Selety island arc was rebuilt to give rise to the Stepnyak island arc. This rebuilding was intimately related to the collision of the Selety island arc with the Kokchetav microcontinent, which led to the formation of a nappe-thrust structure, exhumation of high- and ultrahigh-pressure rocks, and jump of subduction zone with the emergence of a new island arc.

The Shalkar ophiolite complex consists of a series of tectonic sheets, including lavas, a complex of parallel dikes, layered complex of gabbro and gabbro-pyroxenites intruded by dikes of plagiogranites. The tectonic sheets are subsided northward and are associated with the base of a large tectonic nappe made up of rocks of the Kokchetav microcontinent. The microcontinent and ophiolites are jointly thrust upon the Kokchetav megamelange complex with HP-UHP rocks. It is inferred from the chemical composition and structural position that the Shalkar ophiolites were formed under the conditions of suprasubduction zone of extension and belong to the setting of the initial stage of formation of the Stepnyak island arc. The upper age of the ophiolites is 485 ± 6 Ma as determined for zircons from plagiogranites using the U-Th-Pb SHRIMP method. The age of the Shalkar ophiolites is within 495-485 Ma. This interval is marked by a considerable rearrangement in the subduction zone, which can be due to a jump of subduction zone to the hinterland of the Kokchetav microcontinent and to the origin of a zone of extension within its limits with the formation of ophiolites.

Diamondiferous calc-silicate rocks occur at the Kumdy-Kol' deposit as partings and lenses among biotite gneisses and schists. The most frequent alternation of these rocks is observed in crosscut 45 of the adit driven during the exploration of this metamorphic diamond deposit. Our collection of samples taken from this crosscut permitted research into the geochemistry of rocks with contrasting contents of diamonds. Study was also given to carbonate rocks with distinct banding and garnet-pyroxene rocks with abnormally high contents of diamonds taken from the adit spoil heaps. These rocks were analyzed by ICP MS and ICP AES, which ensured a simultaneous determination of a wide spectrum of trace elements with low detection limits. The determined contents of incompatible elements evidence that the diamondiferous calc-silicate rocks are not a mixture of carbonates with clays only. It is not ruled out that carbonates are also mixed with an end-member compositionally different from protoliths of garnet-biotite gneisses and schists. It is shown that at different stages of metamorphic evolution, including exhumation, subducted continental crust might have been the source of high-density fluids or melts metasomatizing the upper mantle. At the same time, the interaction of these fluids with carbonate partings present among the subducted sediments might have hindered their far migration.

In the ultrahigh-pressure metamorphic (UHPM) Kokchetav massif (northern Kazakhstan), diamond-bearing clinozoisite gneisses of the western Barchi-Kol' area display three particular types of reaction textures involving melts: (1) kyanite + silicate-carbonaceous melt = clinozoisite-quartz symplectite + CO₂; (2) rutile + silicate-carbonaceous melt = titanite + CO₂; (3) silicate-carbonaceous melt = calcite + quartz + graphite. Recent experimental work provided constraining evidence on melt-carbonate interaction under UHP conditions and produced a clinozoisite-bearing rock at the interface between metacarbonates and metapelites. These results corroborate well our petrographic observations, illustrating silicate-carbonaceous melt formation under UHPM conditions. Since the amount of partial melts in pelitic material increases when carbonate is available, the carbonate-rich western part of the Kokchetav massif has endured a more extensive weakening. The melting under UHPM conditions induced rheological weakening and thus influenced the exhumation evolution of the UHPM Kokchetav massif.

In situ dating of zircon domains with a Sensitive High Resolution Ion Microprobe (SHRIMP) has been proved to be the best way to determine the age of peak metamorphic conditions in the diamondiferous, deeply subducted Kokchetav rocks. Three independent studies obtained within error-consistent results, providing evidence for peak metamorphic conditions at about 530 Ma. These studies demonstrated through mineral inclusions that the dated zircon crystals formed under peak metamorphic conditions, within the diamond stability field. Mineral inclusions indicate that zircon formed not only at the metamorphic peak, but also during the retrograde evolution under granulite- to amphibolite-facies conditions. The ages of the retrograde zircon domains are within error not distinguishable from the age of peak metamorphism. This provides evidence for very fast exhumation rates of minimum 1.8 cm/yr from diamond to granulite-facies metamorphism and high cooling rates > 40

Study of a series of samples of homogeneous and inhomogeneous (banded) calc-silicate rocks with contrasting diamond contents from the Kumdy-Kol' deposit of metamorphic diamonds (northern Kazakhstan) has shown an extremely wide diversity of compositions of pyrope-grossular garnets in association with dolomite, Mg-calcite, magnesite, diopside with up to 1.5 wt.% K₂O, forsterite, Ti-clinohumite, and phlogopite. Perovskite as an inclusion in forsterite has been discovered in these rocks for the first time. In some samples of complex composition interbeds with and without diamond alternate. Garnet forms a series of compositions with wide variations of Mg# [100 · Mg/(Mg + Fe)] from 70 to 95 and Ca# [100 · Ca/(Ca + Mg + Fe + Mn)] from 42 to 85, which has first been found for these rocks. Wide variations in garnet zoning have been revealed, with a decrease in Ca# towards the rims. Relict Mg-rich garnets which were firstly recognized under natural conditions persistently occur as light irregular spots, with uniquely low amounts of FeO, within 1-2 wt.% (Mg# is 92-95 and Ca# is 60-66). These relics make up 5 to 70 vol.% of the respective grain surfaces in thin section and are revealed only by cathodoluminescence (CL) approximately in only 10-20% of garnet grains for each sample. Thus, these unusual relics are typical of the diverse samples under study, and their composition is relatively homogeneous. The outer zones of most of these garnets completely correspond to typical well-studied compositions with Mg# of 70-80 and Ca# of 40-55. Supposedly, these relics are the earliest garnets formed under the conditions corresponding to the maximum of ultrahigh-pressure metamorphism. These garnets are often surrounded by reactionary rims irregularly developed around the grains made up of clinopyroxene and spinel symplectite. Occasionally, sapphirine and corundum are found in these rims. The symplectite paragenesis was formed as a result of retrograde metamorphism, in the early stages of exhumation, and is direct evidence of the granulite facies conditions, probably, at 1.8 GPa and about 900