Geology of the Kokchetav UHP-HP metamorphic belt, Northern Kazakhstan

Abstract Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure–high-pressure (UHP–HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich-like; that is, the UHP–HP units are separated from underlying low-P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top-to-the-south sense of shear along the top, and top-to-the-north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks.     

Subhorizontal boundary between ultrahigh-pressure and low-pressure metamorphic units in the Sulu–Tjube area of the Kokchetav Massif, Kazakhstan

Abstract The Kokchetav Massif of Kazakhstan includes high to ultrahigh-pressure (HP–UHP) metamorphic rocks (some of which were recrystallized at depths in excess of 150 km), juxtaposed against much lower pressure metamorphic components. We investigated the relationship between the HP–UHP metamorphic unit and the low pressure (LP) unit (Daulet Suite) in the Sulu–Tjube area, where the metamorphic rocks have previously been interpreted as constituting a megamelange with subvertical structural attitudes. Analyses of fold structures suggest that the HP–UHP metamorphic unit overlies the LP unit across a west-dipping subhorizontal boundary. In addition, kinematic indicators display top-to-the-north senses of shear along the tectonic contact between the two units, indicating that the HP–UHP unit has been extruded northward onto the LP unit. Following the juxtaposition of the two units, upright folds developed in both units, and these are associated with the previously reported steeply dipping metamorphic foliations. These data have important implications for the mode of exhumation of the UHP rocks from upper mantle to shallow crustal depths.     

Prograde pressure–temperature records from inclusions in zircons from ultrahigh-pressure–high-pressure rocks of the Kokchetav Massif, northern Kazakhstan

Abstract In the first extensive, systematic study of inclusions in zircons from ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic rocks of the Kokchetav Massif of Kazakhstan (separated from 232 rock samples from all representative lithologies and geographic regions), we identified graphite, quartz, garnet, phengite, phlogopite, rutile, albite, K-feldspar, amphibole, zoisite, kyanite, calcite, dolomite, apatite, monazite, omphacite and jadeite, as well as the diagnostic UHP metamorphic minerals (i.e. microdiamond and coesite) by laser Raman spectroscopy. In some instances, coesite + quartz and diamond + graphite occur together in a single rock sample, and inclusion aggregates also comprise polycrystalline diamond crystals overgrowing graphite. Secondary electron microscope and cathodoluminescence studies reveal that many zircons display distinct zonation textures, which comprise core and wide mantle, each with distinctive inclusion microassemblages. Pre-UHP metamorphic minerals such as graphite, quartz, phengite and apatite are common in the core, whereas diamond, coesite, garnet and jadeite occupy the mantle. The inclusions in core are irrelevant to the UHP metamorphism. The zircon core is of detrital or relatively low-grade metamorphic origin, whereas the mantle is of HP to UHP metamorphic origin. The zonal arrangement of inclusions and the presence of coesite and diamond without back-reaction imply that aqueous fluids were low to absent within the zircons during both prograde and retrograde metamorphism, and that the zircon preserves a prograde pressure–temperature record of the Kokchetav metamorphism which, elsewhere, has been more or less obliterated in the host rock.     

Metamorphic petrology of the Barchi–Kol metabasites, western Kokchetav ultrahigh-pressure–high-pressure massif, northern Kazakhstan

Abstract In the Barchi–Kol area, located at the westernmost part of the Kokchetav ultrahigh pressure (UHP) to high-pressure (HP) massif, northern Kazakhstan, metabasites from the epidote amphibolite (EA) facies to the coesite eclogite (CEC) facies are exposed. Based on the equilibrium mineral assemblages, the Barchi–Kol area is divided into four zones: A, B, C and D. Zone A is characterized by the assemblage: epidote + hornblende + plagioclase + quartz, with minor garnet. Zone B is characterized by the assemblage: garnet + hornblende + plagioclase + quartz + zoisite. Zone C is defined by the appearance of sodic–augite, with typical assemblage: garnet + sodic–augite + tschermakite–pargasite + quartz ± plagioclase ± epidote/clinozoisite. Zone D is characterized by the typical eclogite assemblage: garnet + omphacite + quartz + rutile, with minor phengite and zoisite. Inclusions of quartz pseudomorph after coesite were identified in several samples of zone D. Chemical compositions of rock-forming minerals of each zone were analyzed and reactions between each zone were estimated. Metamorphic P-T conditions of each zone were estimated using several geothermobarometers as 8.6 ± 0.5 kbar, 500 ± 30 °C for zone A; 11.7 ± 0.5 kbar, 700 ± 30 °C for zone B; 12–14 kbar, 700–815 °C for zone C; and 27–40 kbar, 700–825 °C for zone D.     

Diamond growth during ultrahigh-pressure metamorphism of the Kokchetav Massif, northern Kazakhstan

Abstract Characteristic features of in situ diamonds can be used to retrace diamond formation during ultrahigh pressure (UHP) metamorphism of the Kokchetav Massif, Kazakhstan. These features include the nitrogen aggregation state in diamond, dissolution features observed on diamond surfaces, and the carbon and nitrogen isotopic composition of the diamonds. The minerals in which the diamonds are included provide additional information and support the view that at least some of the Kokchetav microdiamonds are the products of prograde or peak UHP metamorphism. The coexistence of diamond and graphite is evaluated within this framework.     

Diamond-bearing and diamond-free metacarbonate rocks from Kumdy–Kol in the Kokchetav Massif, northern Kazakhstan

Abstract Dolomite marble from the Kumdy–Kol area of the Kokchetav Massif contains abundant microdiamond, mainly in garnet and a few in diopside. The mineral assemblage at peak metamorphic condition consists of dolomite + diopside + garnet (+ aragonite) ± diamond. Inclusions of very low MgCO₃ calcite and almost pure calcite occur in diopside and are interpreted as aragonite and/or aragonite + dolomite. Single-phase Mg–calcite in diopside with a very high MgCO₃ component (up to 21.7 mol%) was also found in diamond-free dolomitic marble, and is interpreted as a retrograde product from aragonite + dolomite to Mg–calcite. The dolomite stability constrains the maximum pressure (P) at < 7 GPa using previous experimental data, whereas the occurrence of diamond yields the minimum peak pressure–temperature (P–T) condition at 4.2 GPa and 980 °C at X co ₂ = 0.1. The highest MgCO₃ in Mg–calcite constrains the minimum P–T condition higher than 2.5 GPa and 800 °C for the exhumation stage. As these marbles were subjected to nearly identical P–T metamorphic conditions, the appearance of diamond in some carbonate rocks was explained by high X co ₂. A low X co ₂ condition refers to high oxidized conditions and diamond (and/or graphite) becomes unstable. Difference in X co ₂ for marble from the same area suggests local heterogeneity of fluid compositions during ultrahigh-pressure metamorphism.     

Geotectonic evolution of diamondiferous paragneisses, Kokchetav Complex, northern Kazakhstan: The geologic enigma of ultrahigh-pressure crustal rocks within a Paleozoic foldbelt

Abstract The Kokchetav Complex is a tectonic mega-melange consisting of seven pre-Ordovician units (units I-VII) of contrasting lithologies and P–T conditions of metamorphism, overlain and/or intruded by four post-recrystallization entities. Most of the constituent rock types display affinities with continental crust; paraschists and paragneisses, which carry biogenically produced carbon, clearly were laid down near the surface of the Earth. Microdiamond (and rare coesite) inclusions are contained in strong, refractory garnet, zircon, clinopyroxene, and kyanite, some of the constituent neoblastic phases of this metasedimentary unit. Systematic mineral parageneses and textural relationships support the hypothesis that the metamorphic assemblages represent a close approach to chemical equilibrium at the time of formation. Metamorphism of diamond-bearing paragneisses and schists transpired at 535 ± 5 Ma; physical conditions included minimum pressures of 40 kbar and temperatures exceeding 900 °C. Other associated units contain mineralogic evidence of somewhat lower to considerably lower pressures and temperatures: observed magnesite + diopside pairs, coesite, grossular-pyropic garnet, potassic clinopyroxene, Si-rich phengite, barroisite-crossite(?), aluminous titanite and/or Al-rutile, and the assemblage talc + kyanite + garnet all testify to relatively elevated pressures of formation. The metamorphosed lithotectonic units represent individual, discrete stages in what initially may have been a continuous P-T series, but intense post-metamorphic dislocation has resulted in the preservation of a chaotically mixed sequence rather than an unbroken gradation in preserved conditions of metamorphism. Only units I-III, and probably VIb may represent portions of a dismembered subduction zone lithologie assemblage. The uplift to mid-crustal levels and cooling of the mega-melange took place by about 515–517 Ma, at which time the complex was stabilized as a part of the Kazakhstan microcontinental collage. An hypothesized Late Vendian-Early Cambrian subduction of the Kazakhstan-North Tianshan(?) microcontinental salient to depths exceeding 125 km, followed by decoupling from the descending oceanic crust-capped lithospheric plate is held responsible for the ultrahigh-pressure metamorphism of the Kokchetav Complex. Inasmuch as vestiges of a calc-alkaline volcanic/plutonic arc of approximately Early Cambrian age are preserved as only scattered relics in the general region, the plate-tectonic setting may have involved an intra-oceanic, Marianas-type, incipient arc which was subsequently removed through transform faulting or erosion.     

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.     

Ubiquitous post-peak zircon in an eclogite from the Kumdy-Kol,Kokchetav UHP-HP Massif (Kazakhstan): Significance of exhumation-related zircon growth and modification in continental-subduction settings

U–Pb geochronological, trace-element and Lu–Hf isotopic studies have been made on zircons from ultrahigh-pressure (UHP) mafic eclogite from the Kumdy-Kol area, one of the diamond-facies domains of the Kokchetav Massif (northern Kazakhstan). The peak eclogitic assemblage equilibrated at > 900 °C, whereas the bulk sample composition displays light rare-earth element (LREE) and Th depletion evident of partial melting. Zircons from the eclogite are represented by exclusively newly formed metamorphic grains and have U–Pb age spread over 533–459 Ma, thus ranging from the time of peak subduction burial to that of the late post-orogenic collapse. The major zircon group with concordant age estimates have a concordia age of 508.1 ±4.4 Ma, which corresponds to exhumation of the eclogite-bearing UHP crustal slice to granulite- or amphibolite-facies depths. This may indicate potentially incoherent exhumation of different crustal blocks within a single Kumdy-Kol UHP domain. Model Hf isotopic characteristics of zircons (ε_Hf(t) +1.5 to +7.8, Neoproterozoic model Hf ages of 1.02–0.79 Ga) closely resemble the whole-rock values of the Kumdy-Kol eclogites and likely reflect in situ derivation of HFSE source for newly formed grains. The ages coupled with geochemical systematics of zircons confirm that predominantly late zircon growth occurred in Th–LREE-depleted eclogitic assemblage, that experienced incipient melting and monazite dissolution in melt at granulite-facies depths, followed by amphibolite-facies rehydration during late-stage exhumation-related retrogression.     

Petrology of the diamond-grade eclogite in the Kokchetav Massif, northern Kazakhstan

Abstract The Kokchetav Massif of northern Kazakhstan is unique because of the abundant occurrence of microdiamond inclusions in garnet, zircon and clinopyroxene of metasediments. In order to determine precise pressure–temperature (P–T) conditions, we have systematically investigated mineral inclusions and the compositions of major silicates in Ti–clinohumite–garnet peridotite and diamond-grade eclogite from Kumdy–Kol. It was found that garnet peridotites from Kumdy–Kol contain assemblages of garnet, olivine, Ti–clinohumite and ilmenite. The garnet contains inclusions that are indicative of both ultrahigh pressure (UHP) and retrograde conditions. Inclusions of hydrous phases such as chlorite, amphibole and zoisite were formed at the post-UHP stage. The study also found that eclogite from Kumdy–Kol contains albite–augite symplectites after omphacitic pyroxene. The core of pyroxene (sodic augite) contains high K₂O (up to 1wt%; average 0.24wt%). Phengite is included in the core. Applying the K₂O-in-augite geobarometry, which is based on recent experiments, and the garnet–clinopyroxene (Grt–Cpx) geothermometer for peak metamorphism, the eclogites yield P–T estimates of > 6 GPa and > 1000 °C, and the diamond-grade eclogites yield lower temperature estimates at 900–1000 °C and 5 GPa.     

Kinematic analysis of ultrahigh-pressure–high-pressure metamorphic rocks in the Chaglinka–Kulet area of the Kokchetav Massif, Kazakhstan

Abstract The central part of the Kokchetav Massif is exposed in the Chaglinka–Kulet area, northern Kazakhstan. The ultrahigh-pressure–high-pressure (UHP–HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I–IV) metamorphosed under different pressure–temperature (P–T) conditions. The coesite- and diamond-bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite-dominant Unit I on the bottom and the orthogneiss-dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP–HP metamorphic belt are affected by at least four deformational events post-dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non-coaxial ductile flow in a NW–SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top-to-the-northwest motions and those from Unit III provide top-to-the-southeast, south or south-west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear-indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large-scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike-slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north-westward extrusion of UHP Unit II.     

Mineral parageneses and metamorphic P-T paths of ultrahigh-pressure eclogites from Kyrghyzstan Tien-Shan

Abstract Eclogites occur in three districts of the northern and southern parts of Tien-Shan. Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P-T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite. Aktyuz and Makbal eclogites have not preserved the prograde path. An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600°C, P = 12 kbar) has recorded three stages of retrograde metamorphism. Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet-omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition. Garnet from a garnetchloritoid-talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low-temperature part of coesite eclogite fades metamorphism. Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample. The prograde path from stage I to stage III has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included. The peak metamorphism of stage III took place at about 660°C at 25 kbar. The stages IV and V are retrograde. UHP eclogite facies metamorphism took place twice in Kyrghyzstan. The Aktyuz and Atbashy eclogites gave Rb-Sr mineral-isochron ages of about 750 Ma and 270 Ma, respectively. The K-Ar age of paragonite from the Makbal eclogite is about 480 Ma.     

Tectonic evolution of the ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts from central China

Abstract In the Triassic suture between the Sino-Korean and Yangtze cratons, the Dabie metamorphic Complex in central China includes three tectonic units: the northern Dabie migmatitic terrane, the central ultrahigh-P coesite- and diamond-bearing eclogite belt, and the southern high-P blueschist-eclogite belt. This complex is bounded to the north by a north-dipping normal fault with a Paleozoic accretionary complex and to the south by a north-dipping reverse fault with Yangtze basement plus its foreland fold-and-thrust sequence. Great differences in metamorphic pressure suggests that these units reached different depths during metamorphism and their juxtaposition occurred by wedge extrusion of subducted old continental fragments. These units were subsequently subjected to (i) Barrovian type regional metamorphism and deformation at shallow depths; (ii) intrusion of Cretaceous granitic plutons; and (iii) doming and segmentation into several blocks by normal and strike-slip faults. A new speculative model of tectonic exhumation of UHP rocks is proposed.     

Metamorphism, partial preservation, and exhumation of ultrahigh-pressure belts

The Dabie-Sulu belt of east-central China, the Kokchetav Complex of northern Kazakhstan, the Maksyutov Complex of the South Urals, the Dora Maira Massif of the Western Alps, and the Western Gneiss Region of southwestern Norway lie astride intracontinental suture zones. All represent collisional mountain belts. Adjoining Eurasian regions exhibit little or no evidence of a coeval calc-alkaline arc. Each metamorphic complex contains mineralogic and textural relics of the presence or former existence of coesite ± diamond. Other ultrahigh-P, moderate-T metamorphic phases, including K-rich clinopyroxene, Mg-rich garnet, ellenbergerite, lawsonite, Al-rutile, glaucophane, high-Si phengite, and associations such as coesite + dolomite, magnesite + diopside, and talc + kyanite, diopside, jadeite, or phengite also testify to pressures approaching or exceeding 2.8 GPa. Each of the five well-studied Eurasian ultrahigh-pressure complexes consists chiefly of old, cool continental crust. Deep-seated recrystallization took place during the Phanerozoic. Subduction zones constitute the only known plate-tectonic environment where such high-P, low-T conditions exist. A model involving underflow of a salient of continental crust imbedded in oceanic crust-capped lithosphere explains the ultrahigh- pressure metamorphism. Partly exhumed ultrahigh-pressure terranes consist of relatively thin sheets 7 ± 5 km thick. During early stages of plate descent, hydration of relatively anhydrous units occurs, and volatiles are expelled from hydrous rocks. If present, aqueous fluids markedly catalyze reactions. Experimental studies on MORB bulk compositions demonstrate that, for common subduction-zone P–T trajectories, amphibole (the major hydrous phase in metabasaltic rocks) dehydrates at less than ~ 2.0 GPa; accordingly, mafic blueschists and amphibolites expel H₂O at great depth and, except for some coarse-grained, dry metagabbros, tend to recrystallize to eclogite. Serpentinized mantle beneath the oceanic crust devolatilizes at comparable pressures. In contrast, phengite and biotite remain stable to pressures exceeding 3.5 GPa in associated quartzofeldspathic rocks. So, under ultrahigh-pressure conditions, the micaceous lithologies that dominate the continental crust fail to evolve significant H₂O, and may transform incompletely to eclogitic assemblages. Although hydrous rocks expel volatiles during compaction and shallow burial, very deep underflow of partly hydrated oceanic crust + mantle generates most of the volatile flux along and above a subduction zone prior to continental collision. As large masses of sialic crust enter the convergent plate junction, fluid evolution at deep levels severely diminishes, and both convergence and dehydration terminate. After cessation of ultrahigh-pressure recrystallization, tectonic slices of sialic massifs return to shallow depths along the subduction channel, propelled by buoyancy; collisional sheets that retain ultrahigh-pressure effects lose heat efficiently across both upper (extensional, normal fault) and lower (subduction, reverse fault) tectonic contacts. These sheets ascend to midcrustal levels rapidly at average exhumation rates of 2–12 mm/year. Surviving ultrahigh-pressure relics occur as micro-inclusions encased in dense, strong, impermeable, unreactive mineralogic hosts, and are shielded during return towards conditions characteristic of midcrustal levels. Rehydration attending decompression is incomplete; its limited extent reflects the coarse grain size and relative impermeability of the rocks undergoing retrogression, as well as declining temperature and lack of aqueous fluids.     

Metamorphic zoning and thermal structure of the Dabie ultrahigh- pressure–high-pressure terrane, central China

The ultrahigh- and high-pressure (UHP–HP) metamorphic belt of the Dabie Mountains, central China, formed by the Triassic continental subduction and collision, is divided into four metamorphic zones; from south to north, the greenschist facies zone, epidote amphibolite to amphibolite facies zone, quartz eclogite zone, and coesite eclogite zone, based on metabasite mineral assemblages. Most of the coesite-bearing eclogites consist mainly of garnet and omphacite with homogeneous compositions and have partially undergone hydration reactions to form clinopyroxene + plagioclase + calcic amphibole symplectites during amphibolite facies overprinting. However, the least altered eclogites sometimes contain garnet and omphacite that preserve compositional zoning patterns which may have originated during their growth at peak temperature conditions of ∼ 750 °C, suggesting a short duration of UHP metamorphic conditions and/or consequent rapid cooling during exhumation. Systematic investigation on peak metamorphic temperatures of coesite eclogite have revealed that, contrary to the general trend of metamorphic grade in the southern Dabie unit, the coesite eclogite zone shows rather flat thermal structure (T = 600 ± 50 °C) with the highest temperature reaching up to 850 °C and no northward increase in metamorphic temperature, which is opposed to the previous interpretations. This feature, along with the preservation of compositional zonation, implies complicated differential movement of each eclogite mass during UHP metamorphism and the return from the deeper subduction zone at mantle depths to the surface.     

A comparison of the evolution of diamondiferous quartz-rich rocks from the Saxonian Erzgebirge and the Kokchetav Massif: are so-called diamondiferous gneisses magmatic rocks?

The petrography and chemical composition of minerals of quartz-rich diamondiferous rocks from the Kokchetav Massif, especially the zonation of garnet, were studied and compared with diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge. Many compositional and textural features were found to be similar. For instance, microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks. On the basis of experimental data, a magmatic scenario was constructed to check if the quartz-rich diamondiferous rocks are of magmatic origin. By this, the P-T paths, derived here for the Kokchetav rocks, and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts. These melts originated by anatexis of deeply submerged metasediments (Erzgebirge: at T as high as 1200°C, Kokchetav Massif: at 50-100°C lower T) and ascended from at least 200 km depth. Relics of the pre-anatectic evolution are still present, for instance, as garnet cores. After ascent and emplacement of the magma in deep portions of thickened continental crust (Kokchetav Massif: 45-50 km close to 800°C, Erzgebirge: 55-60 km at 30-50°C lower T) considerable quantities of (white and/or dark) micas formed by peritectic reactions from melt. For instance, garnets could be resorbed at this stage and biotite grew instead. After the magmatic stage, retrogression took place much stronger in the Kokchetav Massif. This was accompanied by deformation transforming broadly the magmatic texture of quartz-rich diamondiferous rocks from the Kokchetav Massif to a gneissic texture.     

Petrology and retrograde P-T path for eclogites of the Maksyutov Complex, Southern Ural Mountains, Russia

Abstract The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M_1-3) and two stages of deformation (S₁ and S₂) were observed in thin sections: M₁) garnet (Alm_55-60, Prp_22-28, Grs_16-20) + omphacite (Jd_46-56) + phengite (Si ≅ 3.5) + rutile; M₂) garnet + glaucophane ± lawsonite + white mica; and M₃) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.     

Prograde P–T path of kyanite eclogites from Junan in the Sulu ultrahigh-pressure province, eastern China

Kyanite-bearing ultrahigh-pressure (UHP) eclogites occur as blocks in orthogneisses at Yangzhuang, in the Junan area of the southwestern Sulu province, eastern China. Eclogites have variable bulk rock compositions, with Al₂O₃ = 16–27 wt%, FeO* + MgO = 6–22 wt% and CaO = 9–13 wt%. Major minerals are garnet, omphacite, phengitic white mica, zoisite, kyanite, rutile and an SiO₂ phase. Fe-rich staurolite (Mg ? Mg# = 0.24 ± 0.01) and paragonite–margarite aggregates are rarely included in the cores of prograde zoned garnet. Metamorphic conditions ranged from 520 to 650°C and <1.4 GPa at an early prograde stage, and mostly reached 660–830°C and 2.7–3.5 GPa at the peak UHP stage. The estimated dP/dT of the prograde P–T path is less than 0.25 GPa/100°C at earlier stages and increases to 0.7–1.4 GPa/100°C just before the UHP stage. The kink of the prograde P–T path closely resembles the steady-state P–T paths proposed, assuming a two-parameter brittle-plastic shear stress model. The estimated P–T path adequately explains the absence of prograde lawsonite and sodic amphibole and the common occurrence of coexisting zoisite, kyanite and sodic-calcic amphibole in the UHP eclogites throughout the Sulu province. Simple clockwise prograde P–T paths for Sulu UHP eclogites proposed in earlier studies should be carefully re-examined.