The Age and Composition of Alkaline Rocks of the Dugda Massif (Eastern Sayan): Patterns of the Manifestation of the Late Paleozoic Rare-Metal Magmatism in the Southwestern Fold Frame of the Siberian Platform

Doklady Earth Sciences - The age and geochemical characteristics of alkaline rocks of the Dugda massif (Eastern Tuva), attributed to the East Sayan Late Paleozoic rare-metal magmatic zone, have...     

Ancient mantle lithosphere beneath the Khanka massif in the Russian Far East: in situ Re–Os evidence

The Os‐isotope compositions of sulphides in mantle xenoliths hosted by Late Miocene alkali basalts from the Sviyaginsky volcano, Russian Far East, reveal the presence of Archaean–Proterozoic subcontinental lithospheric mantle beneath the Khanka massif. Their T_MA and T_RD model ages reveal similar peaks at 1.1 and 0.8 Ga suggesting later thermotectonic events in the subcontinental lithospheric mantle, whereas T_RD model ages range back to 2.8 ± 0.5 (2σ) Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those recorded in the crust of the Khanka massif. The sulphide Os‐isotope data show that the subcontinental lithospheric mantle beneath the Khanka massif had formed at least by the Mesoproterozoic, and was subsequently metasomatized by juvenile crustal‐growth events related to the evolution of the Altaids. The Khanka massif is further proposed to have tectonic affinity to the Siberia Craton and should originate from it accordingly.     

PGE distribution in sulfide ores from ultramafic massifs of the central East Sayan Mountains,Southern Siberia,Russia

Data on the composition of sulfide ores from ultramafic massifs in the central East Sayan Mountains and on the regularities of platinum group elements (PGE) in these ores are presented. It is found that the highest PGE contents are characteristic for net-textured and massive ores from the Zhelos massif: total PGE content there is up to 15 ppm, with Pd/Pt = 3–8, for Ni and Cu contents of 1.5–2.8 and 0.5–2.7 wt%, respectively. In the disseminated ores of the Zhelos massif, PGE contents vary from 1 to 7 ppm, at Ni and Cu contents varying in the ranges of 0.5–1.0 and 0.2–0.4 wt %, respectively. In the Tokty-Oi massif, disseminated ores are characterized by higher absolute PGE contents (1.6 to 3.3 ppm) at similar Ni content. PGE tenor of disseminated ores is higher compared to that of massive and net-textured ones. In the cross-sections of both massifs, net-textured and massive ores of an essentially pyrrhotine composition are found at the contact between ultramafic and host rocks. Total PGE in these ores is up to 12 ppm. The obtained data on sulfur isotopes indicate the common, well-homogenized sources, and close physical–chemical depositional conditions of all ore types.     

华北东部橄榄岩与岩石圈减薄中的地幔伸展和侵蚀置换作用

对比分析了华北不同时代捕虏体橄榄岩及其南部超高压地质体橄榄岩的矿物化学。具古老难熔岩石圈地幔特征的橄榄岩是古生代金伯利岩捕虏体和早中生代苏鲁变质带地质体的主要岩石类型。具这一性质的橄榄岩也构成了河南鹤壁上新世玄武岩捕虏体的主体部分,并可以在辽宁阜新晚中生代玄武岩中被发现。具饱满岩石圈地幔性质的橄榄岩则是阜新晚中生代火山岩、特别是郯庐断裂带(山旺)及其附近地区(栖霞)中新世玄武岩捕虏体的主要类型。从华北东部已有的捕虏体橄榄岩及地质体橄榄岩所表现出的新生饱满与古老难熔地幔的时、空分布特点,即有些地区捕虏体橄榄岩表现出不同性质地幔共存现象(如阜新、鹤壁)或橄榄石Mg#呈渐变关系看:克拉通岩石圈地幔因扬子板块俯冲所引起的早期(如早中生代)地幔伸展、和晚中生代—渐新世因太平洋俯冲所引起的热扰动的软流圈物质上涌对古老地幔产生强烈的侵蚀作用(引起岩石圈的巨大减薄);中新世以来的软流圈热沉降作用出现新生岩石圈地幔并表现为岩石圈的小幅增厚,从而实现地幔置换过程和华北东部岩石圈的整体减薄过程。岩石圈幔内薄弱带及岩石圈深断裂(如郯庐断裂带)起了软流圈物质侵蚀古老岩石圈地幔的通道作用并导引着深部物质运移和不规则减薄作用等。个别地区(如阜新)强烈的软流圈上涌于晚中生代就已经开始,显示地幔置换作用的强烈不均一性。     

The Talazhin plagiodunite–troctolite–anorthosite–gabbro massif (East Sayan): petrogeochemistry and ore potential

The petrology and ore potential of the Talazhin massif located in northwestern East Sayan are studied. The internal structure of the intrusion, the petrographic composition of its rocks, and their metallogenic, petrostructural, and petrogeochemical features are considered. The probable temperature and chemical composition of the parental magma of the pluton were computed using the KOMAGMAT-3.52 program on the modeling of equilibrium crystallization. The obtained data indicate that the Talazhin massif is a rhythmically layered plagiodunite–troctolite– anorthosite–gabbro intrusion formed from low-Ti high-alumina olivine–basalt melt. It is promising for Cu–Ni–PGE mineralization.     

Abundance and distribution of PGE and Au in the island-arc mantle: implications for sub-arc metasomatism

Ultramafic xenoliths from a veined mantle wedge beneath the Kamchatka arc have non-chondritic, fractionated chondrite-normalized platinum-group element (PGE) patterns. Depleted (e.g., low bulk-rock Al₂O₃ and CaO contents) mantle harzburgites show clear enrichment in the Pd group relative to the Ir group PGEs and, in most samples, Pt relative to Rh and Pd. These PGE signatures most likely reflect multi-stage melting which selectively concentrates Pt in Pt–Fe alloys while strongly depleting the sub-arc mantle wedge in incompatible elements. Elevated gold concentrations and enrichment of strongly incompatible enrichment (e.g., Ba and Th) in some harzburgites suggest a late-stage metasomatism by slab-derived, saline hydrous fluids. Positive Pt, Pd, and Au anomalies coupled with Ir depletions in heavily metasomatized pyroxenite xenoliths probably reflect the relative mobility of the Pd and Ir groups (especially Os) during sub-arc metasomatism which is consistent with Os systematics in arc mantle nodules. Positive correlations between Pt, Pd, and Au and various incompatible elements (Hf, U, Ta, and Sr) also suggest that both slab-derived hydrous fluids and siliceous melts were involved in the sub-arc mantle metasomatism beneath the Kamchatka arc.     

Age, composition of rocks, and geological setting of the Snezhnoe beryllium deposit: Substantiation of the Late Paleozoic East Sayan rare-metal zone, Russia

The Snezhnoe phenakite-beryl deposit is one of the highest-grade deposits in the Altai-Sayan beryllium province. This deposit is spatially associated with the alkali granite of the Ognit Complex and localized in the apical part of the granitic pluton. The trace element composition of granite, as well as of Be and Ta-Nb ores was studied. The Rb-Sr age of Be mineralization estimated at 305 Ma is consistent with the time of formation of numerous rare-metal alkali granitic plutons in the Eastern Sayan and the eastern Tuva. The region of these granitic plutons is outlined as the Late Paleozoic East Sayan rare-metal metallogenic zone specialized for Nb, Ta, Be, Li, Zr, Th, and REE mineralization. The East Sayan zone is localized in the marginal part of the Barguzin igneous province and is similar to the marginal zone of this province in composition of igneous associations and metallogenic specialization. The formation of the Barguzin igneous province and the East Sayan metallogenic zone is related to the evolution of the Late Carboniferous-Early Permian mantle plume.     

Redox state of deep off-craton lithospheric mantle: new data from garnet and spinel peridotites from Vitim, southern Siberia

Oxygen fugacity (fO₂) affects melting, metasomatism, speciation of C–O–H fluids and carbon-rich phases in the upper mantle. fO₂ of deep off-craton mantle is poorly known because garnet-peridotite xenoliths are rare in alkali basalts. We examine the redox and thermal state of the lithospheric mantle between the Siberian and North China cratons using new Fe³⁺/ΣFe ratios in garnet and spinel obtained by M?ssbauer spectroscopy, major element data and P–T estimates for 22 peridotite xenoliths as well as published data for 15 xenoliths from Vitim, Russia. Shallow spinel-facies mantle is more oxidized than deep garnet peridotites (average, ?0.1 vs. ?2.5 ΔlogfO₂(FMQ)). For intermediate garnet–spinel peridotites, fO₂ estimates from spinel-based oxybarometers are 1.5–3.2 ΔlogfO₂(FMQ) lower than those from garnet-based oxybarometers. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel–garnet reaction and concomitant changes in mineral chemistry do not keep up with P–T changes (e.g., lithospheric heating by recent volcanism) due to slow diffusion of trivalent cations and because gar-, gar-spl and spl-facies rocks may coexist on centimeter–meter scale. The spinel-based fO₂ estimates may not be correct while garnet-based fO₂ values provide conditions before the heating. The T (780–1,100?°C) and fO₂ ranges of the Vitim xenoliths overlap those of coarse garnet and spinel cratonic peridotites. However, because of a higher geothermal gradient, the deepest Vitim garnet peridotites are more reduced (by 0.5–2.0 ΔlogfO₂(FMQ)) than cratonic garnet peridotites at similar depths, and the “water maximum” conditions (>80?% H₂O) in the off-craton mantle exist in a more shallow and narrow depth range (60–85?km) than in cratonic roots (100–170?km). The base of the off-craton lithospheric mantle (≥90?km) at 2.5?GPa and 1,150?°C has fO₂ of ?3.0 ?logfO₂(FMQ), with dominant CH₄ and H₂O and minor H₂ in the fluid. Melting near the base of off-craton mantle lithosphere may be induced by increasing water share in migrating fluids due to oxidation of methane.     

The characterisation and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study

Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ¹³ C_PDB = ? 12.3 to ? ?3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO₂) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.     

Late Pleistocene glaciations in southern East Sayan and detection of MIS 2 terminal moraines based on beryllium (10Be) dating of glacier complexes

Analysis and summary of publications on southern East Sayan, eastern Tuva, and northern Mongolia have shown that the late Pleistocene glaciation covered a large area and had a complicated dynamics of glacier advance and retreat. Starting with MIS 5, the Todza Basin and, partly, the Oka Plateau, Azas Volcanic Plateau, Mondy Basin, and river valleys in southern East Sayan were periodically covered with ice. The thickness of ice in the eastern Todza Basin was 700 m, on the Azas Volcanic Plateau it reached 300–600 m, and in the valleys of southern East Sayan it is estimated as 700–800 m. The thickness of ice in the Mondy Basin was 300–350 m.Geological and geomorphological studies and isotope surface exposure dating (¹⁰Be method) of boulders from terminal moraine complexes have provided evidence for extensive MIS 2 glacier advance in the Mondy Basin and in the Sentsa, Jombolok, and Sailag river valleys (southern East Sayan). The average age of exposure for three groups of samples is 14, 16, and 22 ka.     

Nephrites of East Siberia: geochemical features and problems of genesis

The largest nephrite-bearing province of Russia is located on the southern folded periphery of the Siberian craton. Deposits of two formation types were established here: apoultrabasic (East Sayan and Dzhida areas, Parama massif) and apocarbonate (Vitim area). Nephrites compose schlieren and lenticular bodies usually in the zones of contact of serpentinous (lizardite–antogorite) dunite–harzburgite rocks and dolomitic marbles with aluminosilicate rocks of different compositions. Significant difference in composition has been established for nephrites of different formations. Apocarbonate nephrites are more magnesian and fluoric and less ferroan. The contents of Li, Be, Rb, and Cs in them are two orders of magnitude higher and the contents of Sc, Ti, and Mn are lower than those in apoultrabasic nephrites. The isotope data evidence that the fluid phase of apoultrabasic nephrites was released from serpentinites during metamorphism, whereas the fluid phase of apocarbonate nephrites is only meteoric water. Oxygen in minerals of metamorphosed carbonate rocks was borrowed from the matrix subjected to replacement, sometimes with the participation of formation waters.     

Non‐uniform splitting of a single mantle plume by double cratonic roots: Insight into the origin of the central and southern East African Rift System

Using numerical thermo‐mechanical experiments we analyse the role of an active mantle plume and pre‐existing lithospheric thickness differences in the structural development of the central and southern East African Rift system. The plume‐lithosphere interaction model setup captures the essential features of the studied area: two cratonic bodies embedded into surrounding lithosphere of normal thickness. The results of the numerical experiments suggest that localization of rift branches in the crust is mainly defined by the initial position of the mantle plume relative to the cratons. We demonstrate that development of the Eastern branch, the Western branch and the Malawi rift can be the result of non‐uniform splitting of the Kenyan plume, which has been rising underneath the southern part of the Tanzanian craton. Major features associated with Cenozoic rifting can thus be reproduced in a relatively simple model of the interaction between a single mantle plume and pre‐stressed continental lithosphere with double cratonic roots.     

Determination of Platinum‐Group Elements and Re‐Os Isotopes using ID‐ICP‐MS and N‐TIMS from a Single Digestion after Two‐Stage Column Separation

We report an improved procedure for the determination of the platinum‐group elements (PGE) and Re, and Os isotopes from a single sample aliquot by isotope dilution (ID) using inductively coupled plasma‐mass spectrometry (ICP‐MS) and negative thermal ionisation mass spectrometry (N‐TIMS), respectively. A two‐stage column method was used to purify PGE‐Re from their sample matrix and interfering elements (e.g., Mo, Zr and Hf) after Os had been separated by CCl₄ solvent extraction. The first column separation step used cation exchange resin (AG50W‐X8) to concentrate PGE‐Re and some potential interfering elements (e.g., Mo, Zr and Hf). In the second step, N‐benzoyl‐N‐phenylhydroxylamine (BPHA) extraction resin was used to separate PGE‐Re from the remaining interfering elements, which all remained strongly absorbed to the resin. The method was used to determine the PGE and rhenium, and Os isotope ratios in a range of geochemical reference materials (TDB‐1, WGB‐1, BHVO‐2 and UB‐N). The obtained results agree well with those previously published. This new method enables PGE‐Re abundances and Os isotopic ratios to be determined on the same sample digestion, and circumvents the problems created by sample heterogeneity when comparing PGE and Re‐Os isotope data.     

Opening and Tectonic Evolution of the Paleo-Asian Ocean

The Paleo-Asian ocean is defined by units located between the Russian (East European), Siberian, Tarim, and Sino-Korean (North China) continents. The study of the composition, age, and structural position of island-arc magmatic rocks, ophiolites, and high-pressure meta-morphic assemblages and their mutual correlations made it possible to identify similarities and differences in the evolution of the Paleo-Asian and Paleo-Pacific oceans. The initial stage of the evolution of the Paleo-Asian ocean defined its opening at 900 Ma, whereas opening of the Paleo-Pacific took place at 750 to 700 Ma. Closing of the Paleo-Asian ocean in the Carboniferous (NE branch) and the Permian corresponds to the main stage of reorganization and reopening of the Paleo-Pacific.

The maximal opening of the Paleo-Asian ocean occurred after or simultaneously with the first accretion-collision event at 600 to 700 Ma, resulting from the collision of microcontinents and the Siberian continent. Vendian-Early Cambrian boninite-bearing island-arc complexes occur as lavas, sheeted dikes, and sill-dikes associated with gabbro-pyroxenites and ultramafics. These complexes are widely distributed in the Gornyy Altay, East Sayan, and West Mongolian regions and can be considered fragments of a giant boninite-bearing belt.

In the late Early Cambrian, collision of seamounts with an island arc caused the squeezing of the subduction zone and return flows within the accretionary wedge. Serpentinite melange within fragments of ophiolites and high-pressure rocks are typical components of the Late Paleozoic accretionary wedges. Because of Middle Cambrian-Early Ordovician collisional events, two new oceans (Junggar-Irtysh-Kazakhstan and Uralian-South Tien Shan-South Mongolian) were formed. The junction of both oceans in East Mongolia opened to the Paleo-Pacific.     

The redox state of the continental lithospheric mantle of the Baikal-Mongolia region

The thermal and redox state of the upper mantle beneath the Baikal-Mongolia region was estimated on the basis of the investigation of the chemical composition (including iron oxidation state) of major minerals (olivine, orthopyroxene, clinopyroxene, and spinel) in spinel and garnet-spinel peridotite xenoliths from the Cenozoic alkali basalts of the volcanic fields of the Dariganga Plateau, Tariat Depression, and Vitim Plateau. At temperatures of 1030–1500°C and pressures of 29–47 kbar, the Δlog$ f_{O_2 } $ f_{O_2 } values relative to the FMQ buffer (calculated using the olivine-spinel oxygen barometer) range from −0.9 to −1.7 for the xenoliths of the Dariganga Plateau, from −0.9 to −1.8 for the Tariat Depression, and from −0.8 to −0.1 for the Vitim Plateau. The oxygen fugacity of peridotites from all of the areas is, in general, lower than that of the WM buffer. Oxygen fugacity is usually below the CCO and EMOD/G buffers in the peridotites of the Dariganga Plateau and the Tariat Depression and higher than these buffers in the peridotites of the Vitim Plateau. The T-PΔlog$ f_{O_2 } $ f_{O_2 } relationships in the xenoliths suggest the existence of spatial heterogeneity in the thermal and redox state of the upper mantle of the Baikal-Mongolia region. This heterogeneity is probably related to the influence of the plume that was responsible for the Late Mesozoic-Cenozoic intraplate magmatism of this region and reflects the different distance of the respective mantle domains from the plume head. The C-O-H fluids in equilibrium with the upper mantle peridotites are composed mainly of water and carbon dioxide. The mantle of the Dariganga Plateau and the Tariat Depression (Δlog$ f_{O_2 } $ f_{O_2 } < −0.9) is characterized by the dominance of H₂O, whereas CO₂-rich fluids are characteristic of the more oxidized mantle of the Vitim Plateau (Δlog$ f_{O_2 } $ f_{O_2 } is mostly higher than −0.8).     

Neoproterozoic to Early Ordovician Evolution of the Paleo-Asian Ocean: Implications to the Break-up of Rodinia

The paper reviews and integrates the recent geological and geochronological data, which allow us to recognize three stages of the evolution of the Paleo-Asian Ocean.The opening of the Paleo-Asian Ocean at 970-850 Ma is dated by the Nersin Complex in the Aldan shield, plagiogranites of the Sunuekit massif, enderbites of the Sludinsk Lake area, and passive margin sediments of the Patoma or Baikal series. The initial subduction (850-700 Ma) is marked by volcanic rocks, trondjemite and gabbro of the Sarkhoy island arc series. Collisions of microcontinents with Siberia at 660 to 620 Ma are evidenced by the exhumation of Muya eclogites (650 Ma), formation of migmatites and amphibolites of the Njurundukan belt (635 and 590 Ma), metamorphic units of the Near-Yenisei belt (640-600 Ma), and orogenic molasse (640-620 Ma). The Paleo-Asian Ocean maximally opened at 620-550 Ma, because at that time a long island arc composed of boninite volcanic rocks was formed. Primitive island arcs of that age have been reconstructed in Kazakhstan, Gorny Altai, West and East Sayan, and North Mongolia. HP and UHP rocks formed in two stages at 550-520 and 520-490 Ma. At 550-490 Ma oceanic islands and Gondwana-derived microcontinents (Kokchetav, Tuva-Mongolian, Central Mongolian and others) collided with the Cambrian-early Ordovician island arc of the Siberian continent. As a result, the island-arc system was extensively modified. Collision occurred twice at 550-520 and 520-490 Ma during which many HP and UHP rocks formed. At that time, the new oceans - the Junggar, Kazakhstan and Uralian - with an Ordovician island arc were formed.     

Ultra-depleted, shallow cratonic mantle beneath West Greenland: dunitic xenoliths from Ubekendt Ejland

Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al₂O₃ and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of ¹⁸⁷Os/¹⁸⁸Os values. The most unradiogenic ¹⁸⁷Os/¹⁸⁸Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean.     

Geodynamic Information in Peridotite Petrology

Systematic differences are observed in the petrology and majorelement geochemistry of natural peridotite samples from thesea floor near oceanic ridges and subduction zones, the mantlesection of ophiolites, massif peridotites, and xenoliths ofcratonic mantle in kimberlite. Some of these differences reflectvariable temperature and pressure conditions of melt extraction,and these have been calibrated by a parameterization of experimentaldata on fertile mantle peridotite. Abyssal peridotites are examplesof cold residues produced at oceanic ridges. High-MgO peridotitesfrom the Ronda massif are examples of hot residues producedin a plume. Most peridotites from subduction zones and ophiolitesare too enriched in SiO₂ and too depleted in Al₂O₃ to be residues,and were produced by melt–rock reaction of a precursorprotolith. Peridotite xenoliths from the Japan, Cascades andChile–Patagonian back-arcs are possible examples of arcprecursors, and they have the characteristics of hot residues.Opx-rich cratonic mantle is similar to subduction zone peridotites,but there are important differences in FeO_T. Opx-poor xenolithsof cratonic mantle were hot residues of primary magmas with16–20% MgO, and they may have formed in either ancientplumes or hot ridges. Cratonic mantle was not produced as aresidue of Archean komatiites. KEY WORDS: peridotite; residues; fractional melting; abyssal; cratonic mantle; subduction zone; ophiolite; potential temperature; plumes; hot ridges     

Main stages of tectonomagmatic activity of the Tuva–Mongolian microcontinent in the Precambrian: data on the U–Pb age of zircons

The U–Pb age of zircons from Ediacaran sandstones of the cover of the Tuva–Mongolian microcontinent and the rocks of its Early Precambrian basement (Gargan block) was analyzed by the LA–ICP–MS method. The major stages of tectonomagmatic activity of this block include the Neoarchean, Paleoproterozoic (no younger than 2 Ga), and Neoproterozoic. Comparison of the age of zircons from Ediacaran terrigenous rocks of the Tuva–Mongolian microcontinent and sandstones of the reference sections of the Ediacaran shelf of the Siberian platform undeniably indicates their independent accumulation.     

Upper mantle structure of the Saharan Metacraton

The ∼500,000 km² Saharan Metacraton in northern Africa (metacraton refers to a craton that has been mobilized during an orogenic event but that is still recognisable through its rheological, geochronological and isotopic characteristics) is an Archean–Paleoproterozoic cratonic lithosphere that has been destabilized during the Neoproterozoic. It extends from the Arabian–Nubian Shield in the east to the Trans-Saharan Belt in the west, and from the Oubanguides Orogenic Belt in the south to the Phanerozoic cover of North Africa. Here, we show that there are high S-wave velocity anomalies in the upper 100 km of the mantle beneath the metacraton typical of cratonic lithosphere, but that the S-wave velocity anomalies in the 175–250 km depth are much lower than those typical of other cratons. Cratons have possitive S-wave velocity anomalies throughout the uppermost 250 km reflecting the presence of well-developed cratonic root. The anomalous upper mantle structure of the Saharan Metacraton might be due to partial loss of its cratonic root. Possible causes of such modification include mantle delamination or convective removal of the cratonic root during the Neoproterozoic due to collision-related deformation. Partial loss of the cratonic root resulted in regional destabilization, most notably in the form of emplacement of high-K calc-alkaline granitoids. We hope that this work will stimulate future multi-national research to better understand this part of the African Precambrian. Specifically, we call for efforts to conduct systematic geochronological, geochemical, and isotopic sampling, deploy a reasonably-dense seismic broadband seismic network, and conduct systematic mantle xenoliths studies.