K. N. Egorov

Middle Paleozoic basic magmatism of the northwestern Vilyui Rift: Composition, sources, and geodynamics

Middle Paleozoic magmatism at the eastern Siberian platform was related to riftogenic processes, which were most clearly expressed in the Vilyui Rift and led to the formation of rift depressions filled with sedimentary-volcanogenic rocks and extended basaltic dike belts in rift shoulders. Two fields of diamondiferous kimberlites were found along with basaltic dikes in the Vilyui-Markha dike belt surrounding rift in the northwest. Active subalkali basaltic magmatism predated the emplacement of kimberlite bodies, which occasionally (Nyurba pipe) are cut by dikes of potassium alkali basalts. Based on geochemical and Sr-Nd isotopic characteristics, deep-seated sources were determined for the intrusive and volcanic basalts of the northwestern shoulder of the Vilyui rift. The REE distribution patterns of the studied rocks normalized to the primitive mantle are close to that of OIB, except for somewhat higher HREE. In the diagrams of indicator ratios of trace and rare-earth elements, the basalts are also plotted in the OIB field, being located between the end member of plume composition (FOZO) and enriched mantle sources. The rocks have positive εSr (+3.5 and +28.6) and εNd (+1.3 and +5.3). In a diagram εNd(T)-εSr(T), two fields with distinct content of radiogenic Sr are distinguished, which can be regarded as derived by mixing of the moderately depleted PREMA-type mantle and a source enriched in radiogenic Sr. Available isotope-geochemical data confirm that OIB type basalts of the region were generated by plume activity. The geodynamic setting of Middle Paleozoic magmatism and rifting in the eastern part of the Siberian platform is considered in light of plume-lithosphere interaction. The sequence of tectonomagmatic events during evolution of the Vilyui rift is consistent with the model of plume-lithosphere interaction or the model of active rifting.

Potassium basalts and picrobasalts from the devonian kimberlite fields of Western Yakutia, Russia, and their relations to kimberlite magmatism

Basic explosion pipes occur along with basic dikes, sills, and chonoliths within the Vilyui—Markha basic dike belt in the northwestern marginal part of the Vilyui Rift, characterized by widespread basaltic magmatism. The explosion pipes are of interest for exploration geology owing to their specific composition and tectonic setting, similar in many respects to the structural localization of kimberlite bodies in the sedimentary cover of the Siberian Platform. The basic explosion pipes from the Mirny district were referred to as tholeiitic and alkali-basaltic petrochemical rock series. Peculiar potassic and ultrapotassic rocks—potassium olivine basalts and picrobasalts—were identified in the alkali-basaltic series. These rocks were regarded as related to the deepest sources among basalts and were recommended for use as a prospecting guide for primary diamond sources. Our investigations allowed us to interpret the elevated K and Mg contents in basic fragments from some explosion pipes and associated intrusive bodies as a result of low-temperature metasomatic alteration. The explosion breccias and metasomatically altered basic rocks probably mark areas favorable for explosion activity and intrusion of both basic and kimberlitic rocks.

The northeastern boundary of the Siberian Craton and its formation peculiarities (derived from occurrences of early Cambrian and Devonian intraplate magmatism)

1252 The formation of the Siberian Craton ended in the Paleoproterozoic. Subsequently, it was repeatedly sub jected to destruction in response to interaction between the lithosphere and the mantle plumes. The most significant events of that time were the Neoprot erozoic and Middle Paleozoic rifting epochs, which determined the configuration of the Siberian Craton close to the present one. For example, the Neoprot erozoic rifting epoch is correlated with the breakup of Rodinia [1]. This epoch culminated with separation of the craton from the Laurentian part of Rodinia in the late Neoproterozoic [2] and formation of its southern (in modern coordinates) boundary. The Middle Pale ozoic rifting epoch resulted in the formation of the eastern boundary of the Siberian Craton [3]. The con tinental breakup was accompanied by the formation of a triple rift system, the best known branch of which is represented by the intracontinental Vilyui rift pinch ing out in the craton body [4]. Two other branches of this system determined the breakup boundary of the continent and separation of its eastern segments. These branches are represented by the Verkhoyansk and Sette Daban rifts, fragments of which are now observable in the Olenek (Kharaulakh segment) and South Verkhoyansk marginal zones of the Mesozoic Verkhoyansk fold–thrust belt (VFTB) [5].

Mineralogy and petrography of a kimberlite-picrite dike in the northwestern Urik-Iya Graben, the eastern Sayan region

Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism of Cr-diopside and high-Cr (up to 55.22 wt % Cr2O3) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite. The high content of Cr-spinel (45–55 wt % Cr2O3) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content (11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO2, 0.61 Cr2O3, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE, in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation of a common parental alkaline ultramafic magma.

Early Cambrian magmatism in the northeastern Siberian Craton (Olenek Uplift)

The Vendian–Lower Cambrian tectonomagmatic activation took place in the northeastern Siberian Craton, within the Olenek Uplift and in the Kharaulakh segment of the Verkhoyansk fold-and-thrust belt (the lower reaches of the Lena River). The Early Paleozoic volcanic activity in the Olenek Uplift is expressed in the form of basitic diatremes, small basaltic covers, and doleritic dikes and sills intruding and covering the Upper Vendian carbonate deposits. The material specificity of the Lower Cambrian basites and their mantle sources, jointly with the Vendian–Cambrian sedimentation history, gives reason to consider the Lower Cambrian riftogenesis and the associated magmatism as a consequence of the plume–lithosphere interaction in the northeastern Siberian Craton.

Rare-element composition of pyropes and lamproites and ancient dispersion haloes of the southwestern Siberian platform

The problem of heterogeneity of the mantle lithosphere of the southwestern portion of the Siberian Platform has been considered, and the diamond content in potential mother lodes within this area has been estimated based on original geochemical data on the rare-element composition of pyropes from diamondiferous lamproites of the Ingashin field within the Prisayan region and ancient dispersion haloes of minerals accompanying diamonds in the area between the Angara and Uda rivers. Pyropes from lamproites are characterized by low concentrations of Zr (0.18–9.05 ppm), Hf (0.03–0.37 ppm), and rare earth elements (Sm 0.04–0.49, Eu 0.02–0.16, and Dy 0.05–0.96 ppm). Pyropes from the Lower Carboniferous Baeron Formation within the Tangui-Chuksha area are significantly different from pyropes of the Ingashin lamproites in high contents of Zr (30.36–139.23 ppm) and Hf (0.4–2.22 ppm). These pyropes are characterized by elevated concentrations of rare earth elements (Sm 1.34–3.68, Eu 0.53–1.17, and Dy 1.0–2.05 ppm). The distribution patterns of rare incompatible elements in pyropes of the Lower Carboniferous Mura massif within the Mura area manifest even stronger differences with pyropes of the Ingashin lamproites and in many respects with pyropes from Lower Carboniferous sediments of the Baeron Formation within the Tangui-Chuksha area. The results obtained indicate that there is no large-scale regional spreading of pyropes from Mid-Riphean lamproite bodies in the course of washout of these bodies and that the mantle lithosphere in the southwestern portion of the Siberian Platform is laterally heterogeneous in mineralogical-geochemical terms. The chemical composition and the peculiar distribution pattern of rare elements in pyropes from lamproites of the Prisayan region indicate a depleted, primarily lherzolite composition of the upper mantle that was transformed through low-temperature potassium metasomatosis. In terms of the chemical and rare-element compositions, pyropes from Lower Carboniferous sediments of the Tangui-Chuksha and Mura areas belong to a wider range of mantle rocks: depleted peridotites, metasomatic peridotites under low (900–1000°C) and high (>1000°C) temperature conditions, and megacrysts. This suggests that the composition of the lithospheric mantle in this area of the southern portion of the Siberian Platform is characterized by a considerably differentiated stratification of mantle rocks, some of which were credibly formed in the diamond stability field.

The first find of impactites of a new astrobleme in the southwest of the Siberian Platform

Recently, finds of new impact structures or their manifestations on the Earth’s surface have gained principal importance. Firstly, many astroblemes are spatially connected with unique deposits of diamonds, gold, polymetals, and some other things [1–3 etc.]. Secondly, it is difficult to overestimate the influence of catastrophic explosions of space objects in the past on the formation of the modern topography of the Earth. In the Siberian Platform, reliable compositional features characteristic for impact structures are estab� lished only for the Popigai meteorite crater [4]. In the

Mineralogical-Geochemical Criteria of Diamond Potential of Kimberlites in the Yubileinaya Multiphase Pipe (Yakutia)

Pursuance and refinement of the available criteria for the assessment of diamond potential remain an urgent issue in diamond geology. The universality of mineralogical criteria, which determine the interrelation between the content of high-Cr low-Ca pyropes and high-Cr spinels (xenocrystals) with the diamond potential of kimberlitic breccia, has been confirmed for various diamondiferous provinces of the world [1‐3]. Other compositional criteria of the diamond potential of kimberlites are developed insufficiently as yet. Their application for assessment of the productivity of separate phases of kimberlites, pipes, or fields of kimberlite provinces has certain limitations [4‐6 and others]. Our paper shows for the first time that the complex of mineralogical and petrogeochemical criteria of diamond potential can be used for some kimberlite phases with the Yubileinaya Pipe as an example. The lithological composition of mineralogical‐petrographic varieties of kimberlites in autonomous emplacement phases of the Yubileinaya Pipe was investigated systematically during the mining of various levels of the deposit in the years 1986‐2006. In order to analyze the possible correlation between the productivity and compositional features of separate kimberlite varieties, we used the exploitation and geological exploration data on the diamond content (in conventional units) in kimberlite ores from different horizons of the pipe. Mapping of the southwestern and northeastern flanks of the Yubileinaya Pipe made it possible to outline three autonomous phases of kimberlite emplacement with different textural-structural, mineralogical, and petrogeochemical parameters and diamond potentials: coarse-porphyric micaceous kimberlite, kimberlitic breccia, and autolithic kimberlitic breccia. The central part of the pipe is characterized by a greater diversity of kimberlites. Based on the texturalstructural features and mineral compositions, they are divided into kimberlitic breccias, autolithic kimberlitic breccias, and kimberlites with various (schlieren‐ taxitic, aphyric, and sporadophyric) textures. This zone also includes some blocks of the porphyric kimberlite with the mineral composition similar to that of the coarse-porphyric kimberlite at flanks of the pipe. All these varieties are commercial kimberlite ores with a variable content of the useful component. The upper horizons of the central part are mainly composed of sheeted carbonate breccia with a small amount of the kimberlitic material. Their thickness varies from several meters at the margins of the pipe to 100‐130 m in the central part. The carbonate breccias grade downsection into kimberlite rocks of the “xenolithic belt” characterized by the presence of a great amount (up to 60‐70 vol %) of large xenoliths and “floating reefs” of sedimentary rocks. The large sedimentary xenoliths are cemented by the sporadophyric and schlieren‐taxitic kimberlites and the less common kimberlitic breccias and autolithic kimberlitic breccias. The xenolithic belt encloses the autolithic kimberlitic breccias that make up lower horizons of the pipe. The carbonate breccias (with sedimentary rock fragments up to 90%), sporadophyric and aphyric varieties, and some schlieren‐taxitic kimberlites are noncommercial ores.