Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone

Active deformation in the South Caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone. Rigid basement to the South Caspian Basin moves with a westward component relative both to stable Eurasia and Iran, and is beginning to subduct at its northern and western margins. This motion is oblique to the approximately north–south Arabia–Eurasia convergence, and causes oblique shortening to the south and northeast of the South Caspian Basin: thrusting in the Alborz and Kopet Dagh is accompanied by range-parallel strike–slip faults, which are respectively left- and right-lateral. There are also arcuate fold and thrust belts in the region, for two principal reasons. Firstly, weaker regions deform and wrap around the rigid block. This occurs at the curved transition zone between the Alborz and Talysh ranges, where thrust traces are concave towards the foreland. Secondly, a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence: west-to-east thrusts in the eastern Talysh represent underthrusting of the South Caspian basement, but pass via an arcuate fan of fold trains into SSW-directed thrusts in the eastern Greater Caucasus, which accommodates part of the Arabia–Eurasia convergence. Each part of the South Caspian region contains one or more detachment levels, which vary dependent on the pre-Pliocene geology. Buckle folds in the South Caspian Basin are detached from older rocks on thick mid-Tertiary mudrocks, whereas thrust sheets in the eastern Greater Caucasus detach on Mesozoic horizons. In the future, the South Caspian basement may be largely eliminated by subduction, leading to a situation similar to Archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust.     

Late Cretaceous-Eocene marginal seas in the Black Sea-Caspian region: Paleotectonic reconstructions

A series of seven reconstructions is presented to illustrate the evolution of marginal seas in the Black Sea-South Caspian segment of the margin of the Tethys Ocean from the Late Jurassic to the middle Eocene. After Middle Jurassic inversion and until the Aptian Age, no marginal (backarc) basins were formed in the region, while the Pontides-Rhodope margin developed in the passive regime. The retained relict of the Late Triassic-Early Jurassic backarc basin includes the southeastern part of the Greater Caucasus, the northern part of the South Caspian Basin, and the shallow-water Kopetdagh Basin. The basins of the southern slope of the Greater Caucasus, Balkans (Nish-Trojan Trough), and Dobrogea developed as flexural foredeeps in front of the Middle Jurassic fold systems. The next, Aptian-Turonian epoch of opening of marginal seas was related to the origination of subduction zones at the Pontides-Rhodope margin and to the incipient consumption of the Vardar Basin lithosphere with formation of the West Black Sea Basin and its western continuation in the Bulgarian Srednogorie. The backarc rifting in the Greater Caucasus resulted in transformation of the foredeep into the backarc basin. Two basins approximately 2000 km in total extent were separated by the bridge formed by the Shatsky and Andrusov rises. The last, late Paleocene-middle Eocene epoch of the formation of backarc basins was associated with the newly formed subduction zone south of the Menderes-Taurus Terrane that collided with the active margin in the early Paleocene. The Greater Caucasus Basin widened and deepened, while to its south the East Black Sea Basin, the grabens in the Kura Depression, and the Talysh Basin, all being separated by a chain of uplifts, opened. The Paleogene South Caspian Basin opened in the course of the southward motion of the Alborz volcanic arc at the late stage of closure of the Iranian inner seas.     

Sedimentary environments and geochemistry of Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus

Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus were examined in the most representative Chirkei section (Sulak River basin). Sharp lithogeochemical distinctions between them were revealed. The results of the study of nannoplankton demonstrated that the Eocene/Oligocene interface occurs slightly below the boundary between the Belaya Glina and Khadum formations. The studied section revealed a series of nannoplankton bioevents facilitating its stratigraphic subdivision. It has been established that organic matter (OM) in rocks of the Khadum Formation is characterized by a relatively high degree of maturity. Probably, the material of mainly marine genesis contains a terrigenous OM admixture. Positive oxygen isotope anomaly in the upper part of the Belaya Glina Formation reflects global climate changes (cooling) near the Eocene/Oligocene interface. Limitation of the anomaly by the upper boundary of the Belaya Glina Formation is likely related to changes in water salinity variations in the Early Oligocene basin and intense early diagenetic processes in rocks therein. Lithological, geochemical, and paleoecological data suggest that the Khadum paleobasin was depleted in oxygen. Such environment was unstable with periodic intensification or attenuation. Paleoecology in the Belaya Glina basin was typical of normally aerated basins.     

Organic matter in Oligocene Maikop Sequence of the North Caucasus

The results of the thorough study of organic matter (OM) in the Oligocene-Miocene Maikop sequence of the North Caucasus are considered. It is shown that its distribution within the Maikop sequence is characterized by irregular patterns. Despite that these rocks show the elevated TOC content, their hydrocarbon-generating potential is, as a rule, low. The low quality of OM is partly related to its intense anaerobic decomposition in anoxic environments with terrigenous sedimentation. The elevation of OM concentrations and hydrocarbon-generating potential could be connected with the reinforced phytoplankton influx into sediments. Sediments deposited in the second half of the Early Oligocene were synchronously enriched in OM. This enrichment chronologically corresponded to the restoration of connection with the World Ocean of the formerly partly isolated and freshened basin. Most probably, this event resulted from the sharp increase in biological productivity.     

http://www.sciencedirect.com/science/article/pii/S1674987114001078

The Caucasian-Arabian belt is part of the huge late Cenozoic Alpine-Himalayan orogenic belt formed by collision of continental plates.The belt consists of two domains:the Caucasian-Arabian Syntaxis(CAS)in the south and the EW-striking Greater Caucasus in the north.The CAS marks a zone of the indentation of the Arabian plate into the southern East European Craton.The Greater Caucasus Range is located in the south of the Eurasian plate;it was tectonically uplifted along the Main Caucasian Fault(MCF),which is,in turn,a part of a megafault extended over a great distance from the Kopetdag Mts.to the TornquistTeisseyre Trans-European Suture Zone.The Caucasus Mts.are bounded by the Black Sea from the west and by the Caspian Sea from the east.The SN-striking CAS is characterized by a large geophysical isostatic anomaly suggesting presence of mantle plume head.A 500 km long belt of late Cenozoic volcanism in the CAS extends from the eastern Anatolia to the Lesser and Greater Caucasus ranges.This belt hosts two different types of volcanic rocks:(1)plume-type intraplate basaltic plateaus and(2)suprasubductiontype calc-alkaline and shoshonite-latite volcanic rocks.As the CAS lacks signatures of subduction zones and is characterized by relatively shallow earthquakes(50e60 km),we suggest that the"suprasubduction-type"magmas were derived by interaction between mantle plume head and crustal material.Those hybrid melts were originated under conditions of collision-related deformation.During the late Cenozoic,the width of the CAS reduced to ca.400 km due to tectonic"diffluence"of crustal material provided by the continuing Arabia-Eurasia collision.     

The K–Ar and Rb–Sr Isotopic Systems in Rocks from the Jurassic Terrigenous Complex of the Greater Caucasus

The behavior of K–Ar and Rb–Sr isotopic systems in clayey rocks from the Jurassic terrigenouis complex of the Greater Caucasus sampled along the Terek River is discussed. It is shown that the rocks experienced intense postdiagenetic alterations accompanied by substantial changes in mineral composition and K–Ar and Rb–Sr isotopic systems. Lateral stress is the leading factor responsible for secondary mineral and geochemical transformations of rocks in cleavage zones. Rejuvenation of the radiological age of rocks, relative to their stratigraphic age, is 100 Ma and more. The age estimate of approximately 50 Ma obtained for samples from the southern limb of the anticlinorium reflects the Paleocene–Eocene phase of tectonic activity manifested in both the Caucasus and other areas of the Mediterranean foldbelt.     

Oligocene uplift of the Western Greater Caucasus: an effect of initial Arabia–Eurasia collision

The Greater Caucasus is Europe's largest mountain belt. Significant uncertainties remain over the evolution of the range, largely due to a lack of primary field data. This work demonstrates that depositional systems within the Oligocene–Early Miocene Maykop Series on either side of the Western Greater Caucasus (WGC) display a similar provenance and divergent palaeocurrents away from the range, constraining a minimum age for the subaerial uplift of the range as early Early Oligocene. An Eocene–Oligocene hiatus, basal Oligocene olistostromes and a marked increase in nannofossil reworking also point to initial deformation in the earliest Oligocene. The initial uplift of the WGC occurred during the final assembly of the Tethysides to its south. Uplift commenced after the Late Eocene final suturing of northern Neotethys and during the initial collision of Arabia with the southern accreted margin of Eurasia. This suggests that compressional deformation was rapidly transferred across the collision zone from the indenting Arabian plate to its northern margin.     

Geochemical constraints on the provenance of Oligocene–Miocene siliciclastic deposits (Zivah Formation) of NW Iran: implications for the tectonic evolution of the Caucasus

In this study, the whole-rock geochemistry of 35 Oligocene–Miocene sandstone and shale samples from the Zivah Formation, Moghan area (NW Iran) were collected and analyzed for evaluation of their provenance, tectonic setting and the intensity of paleo-weathering. Low to moderate values of the chemical index of alteration (mean CIA?=?53/68 for sandstones/shales) and relatively high values of index of compositional variability (mean ICV?=?1.23/1.08 for sandstones/shales) suggest weak chemical weathering and an immature source. These results support for the semi-arid and semi-humid paleoclimate conditions in the source area. The geochemistry results reveal that the sediments were deposited in a basin related to the island arc and active continental margin tectonic settings, probably indicating the time of initial collision between Arabia and Eurasia. The enrichment of Cr, Ni and V in the sandstone and shales are consistent with mafic input from the source area. However, La/Th vs. Hf and La/Sc vs. Co/Th plots reveal mixed source of felsic and intermediate volcanic rocks. The data indicate that the sediments most likely originated from a mixture of mafic, intermediate and felsic igneous source areas, possibly as the erosional products of localized topography of the Talysh and the Lesser Caucasus mountains (south to southwest), created by compression in the Moghan region during the syn-collisional development of the Caucasus.     

Geochronology of eruptions and parental magma sources of Elbrus volcano,the Greater Caucasus: K-Ar and Sr-Nd-Pb isotope data

Complex geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus volcano (Greater Caucasus) experienced long (approximately 200 ka) discrete evolution, with protracted periods of igneous quiescence (approximately 50 ka) between large-scale eruptions. The volcanic activity of Elbrus is subdivided into three phases: MiddleNeopleistocene (225–170 ka), Late Neopleistocene (110–70 ka), and Late Neopleistocene-Holocene (less than 35 ka). Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that hybrid parental magmas of the volcano were formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic upper crustal material of the Greater Caucasus. Mantle reservoir that participated in the genesis of Elbrus lavas as well as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle “Caucasus” source. Primary melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics: ⁸⁷Sr/⁸⁶Sr = 0.7041 ± 0.0001, ƒ_Nd = +4.1 ± 0.2, ¹⁴⁷Sm/¹⁴⁴Nd = 0.105–0.114, ²⁰⁶Pb/²⁰⁴Pb = 18.72, ²⁰⁷Pb/²⁰⁴Pb = 15.62, and ²⁰⁸Pb/²⁰⁴Pb = 38.78. The temporal evolution of isotope characteristics for lavas of Elbrus volcano is well described by a Sr-Nd mixing hyperbole between “Caucasus” source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus. It was shown that, with time, the proportions of mantle material in the parental magmas of Elbrus gently increased: from ∼60% at the Middle-Neopleistocene phase of activity to ∼80% at the Late Neopleistocene-Holocene phase, which indicates an increase of the activity of deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene-Holocene phase, increasing contribution of deep-seated mantle source in the genesis of Elbrus lavas with time as deduced from isotope-geochemical data, as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity were to continue.     

Paleodynamics of the southern margin of the East European craton

The tectonic and geodynamic evolution of the southern European part of Russia from the Donets Basin (Donbas) and the northern Caspian region in the north to the Mountainous Crimea and the Greater Caucasus in the south is considered. This territory embraces the southern margin of the East-European craton and the northern periphery of the Tethys Ocean, which originated in the Neoproterozoic, as well as its marginal seas till the formation of the modern Azov-Black Sea and Caspian basins.     

Stages of recent volcanism and problems of their correlation with landscape formation in the central Caucasus

The article presents a first comparison of the isotopic ages of Pliocene–Quaternary volcanic rocks of the Greater Caucasus with the time of creation of various forms of the modern relief. The latter are associated with lava flows and volcanic centers identified from the study of neotectonic movements, geomorphology, and glacial stages. It is demonstrated that the results of chronological subdivision of lava flows using geomorphological and neotectonic methods, in comparison with the isotopic data, generally agree with each other in this area and ensure more reliable dating of glaciation epochs in the Greater Caucasus. Despite the overall similarity of the data, some contradictions have been revealed and possible causes are considered.     

Organofacies and paleoenvironment of the Oligocene Maikop series of Angeharan (eastern Azerbaijan)

The Maikop Formation, deposited in eastern Azerbaijan during Oligocene and Early Miocene times, contains prolific source rocks with primarily Type II organic matter. Paleontological analyses of dinoflagellate cysts revealed a Lower to Upper Oligocene age for the investigated succession near Angeharan. A major contribution of aquatic organisms (diatoms, green algae, dinoflagellates, chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids and δ¹³C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulfur bacteria. Higher inputs of terrigenous organic matter occurred during deposition of the Upper Oligocene units of the Maikop Formation from Angeharan mountains. The terpenoid hydrocarbon composition argues for angiosperm dominated vegetation in the Shamakhy–Gobustan area.High primary bioproductivity resulted in a stratified water column and the accumulation of organic matter rich sediments in the Lower Oligocene units of the Maikop Formation. Organic carbon accumulation during this period occurred in a permanently (salinity-) stratified, mesohaline environment with free H₂S in the water column. This is indicated by low pristane/phytane ratios of all sediments (varying from 0.37–0.69), lower methylated-(trimethyltridecyl)chromans ratio in the lower units and their higher contents of aryl isoprenoids and highly branched isoprenoid thiophenes. Subsequently, the depositional environment changed to normal marine conditions with oxygen deficient bottom water. The retreat of the chemocline towards the sediment–water interface and enhanced oxic respiration of OM during deposition of the Upper Oligocene Maikop sediments is proposed.Parallel depth trends in δ¹³C of total OM, n-alkanes, isoprenoids and steranes argue for changes in the regional carbon cycle, associated with the changing environmental conditions. Increased remineralisation of OM in a more oxygenated water column is suggested to result in low TOC and hydrocarbon contents, as well as ¹⁵N enriched total nitrogen of the Upper Oligocene units.     

Evolution of an active continental margin as exemplified by the Alpine history of the Caucasus

A plate-tectonics model of the Alpine evolution of the Caucasus is suggested. According to the model, in the Jurassic-Neocomian the Caucasian territory comprised the shelf of the East European platform, the marginal sea of the Great Caucasus, the Pontian-Transcaucasian island arc, the Anatolian-Minor Caucasian oceanic basin (Tethys) and the Iranian-Turkish microcontinent. Along the northern margin of the oceanic basin a convergent plate juncture extended. Part of the Caucasus, situated north of this plate boundary, represented the West Pacific-type active margin of the East European platform. In the Middle Cretaceous the Iranian-Turkish microcontinent collided with the Pontian-Transcaucasian island arc and as a result the Transcaucasian-Minor Asian continental block originated. In the central part of the latter an extensive Paleogene andesitic belt formed, with the Black Sea-Adjara-Trialetian and Talysh-South Caspian basaltic rift troughs on its rear (northern) side (incipient Black Sea and South Caspian basins). Major plate boundary shifted south, into the Zagros-Taurus basin, though the Anatolian-Minor Caucasian suture zone remained mobile in the Upper Cretaceous and Paleogene. From the Oligocene, under conditions of ongoing convergence of the Eurasian and Afro-Arabian continental blocks, the present-day intracontinental mountainous foldbelt has developed.     

Organic geochemical evidences of early-diagenetic oxidation of the terrestrial organic matter during the Triassic arid and semi arid climatic conditions

The molecular character of organic matter (OM) present in Triassic clays of the Upper Silesia Basin and NW border of the Holy Cross Mountains was determined using GC–MS analysis. Oxidation processes were the major cause of the main changes of extractable OM molecular composition during sedimentation and early diagenesis of the Triassic clays. They resulted in a very significant decrease in the OM content of the clays and transformation of n-alkanes, triterpanes and steranes. Despite the changes in biomarker composition resulting from the disappearance of unsaturated and ββ hopanes and dominance of αβ and βα hopanes as well as preponderance of αββ over ααα steranes, the aromatic ketones were identified. Such PACs as benzophenone, fluorenone, cyclopenta(def)phenanthrenone, antracenone and benzanthrone were among the compounds identified in the red clays. These commonly originate during oxidation of sedimentary organic matter. Moreover, the aromatic fraction is characterized by the presence of phenyl derivatives (phenylnaphthalenes, terphenyls, phenyldibenzofurans and phenylphenanthrenes) that are also products of the abiotic oxidation of organic matter. Periods of terrestrial sedimentation have been interrupted by long phases of OM weathering and oxidation during arid conditions, and OM might only have survived without significant oxidation changes when the sedimentation was fast and/or long-lasting.     

http://www.sciencedirect.com/science/article/pii/S1674987113001382

The area from the Greater Caucasus to the southeast Turkey is characteri：;.ed and shaped by several major continental blocks. These are Scythian Platform, Pontian-Transcaucasu.,; Continent-Arc System （PTCAS）, the Anatolian-lranian and the Arabian Platforms. The aim of this paper is to define these continental blocks and describe and also compare their boundary relationships along the suture zones. The Scythian Platform displays the evidence of the Hercynian and Alpine orogens. This platform is separated from the PTCAS by the Greater Caucasus Suture Zone. The incipient collision began along this suture zone before middle-late Carboniferous whereas the final collision occurred before Oligocene. The PTCAS can be divided into four structural units： （1） the Georgian Block - northern part of the Pontian-Transcaucasian island-arc, （2） the southern and eastern Black Sea Coast-Adjara-Trialeti Unit, （3） the Artvin-Bolnisi Unit, comprising the northern part of the southern Transcaucasus, and （4） the Imbricated Bayburt-Garabagh Unit. The PTCAS could be separated from the Anatolian Iranian Platform by the North Anatolian-Lesser Caucasus Suture （NALCS） zone. The initial collision was developed in this suture zone during Senonian-early Eocene and final collision before middle Eocene or Oligocene-Miocene. The Anatolian-lranian Platform （AIP） is made up of the Tauride Platform and its metamorphic equivalents together with Iranian Platform. It could be separated from the Arabian Platform by the Southeastern Anatolian Suture （SEAS） zone. The collision ended before late Miocene along this suture zone. The southernmost continental block of the geotraverse is the Arabian Platform, which constitutes the northern part of the Arabian-African Plate. This platform includes a sequence from the Precambrian felsic volcanic and clastic rocks to the Campanian-early Maastrichtian fiyschoidal clastics. All the suture zones include MORB and SSZ-types ophiolites in different ages. However, the ages of the suture     

Estimation of the Deformation Value in Terrigenous Rocks by the Random Cross-Section Method (Belaya River,Northern Slope of the Greater Caucasus)

The deformation value in the Lower and Middle Jurassic terrigenous rocks from the valley of the Belaya River (the northern slope of the Greater Caucasus) is quantified by the morpholology of the cleavage zones. The numeral determination of the axis ratios of the deformation ellipses by the Fry method is performed as well. It is shown that for a statistical sample of the random cross-sections of the thin sections in the terrigenous rocks with cleavage structure the deformation value can be estimated correctly. The method of random cross-sections described in this work can be used for the quantitative estimation of deformation in nonoriented samples of terrigenous rocks.     

Two stages of explosive volcanism of the Elbrus area: Geochronology,petrochemical and isotopic-geochemical characteristics of volcanic rocks,and their role in the neogene-quaternary evolution of the Greater Caucasus

The chronology of evolution of the young explosive volcanism in the Elbrus area of the Greater Caucasus is revealed. The isotopic-geochronological data indicate that ignimbrites and associated volcanic rocks were formed during the Middle Pliocene (3.0–2.75 Ma) and Early Pleistocene (0.84–0.70 Ma) stages of magmatic activity of the Greater Caucasus. The presence of two groups of pyroclastic rocks significantly different in age and analysis of their location indicate two spatially combined volcanic centers different in age in this part of the Elbrus volcanic area: Pliocene Tyrnyauz center localized in the eastern and southern parts and Quaternary Elbrus volcanic center which is the only newest center of volcanic activity both in the Elbrus and in the entire neovolcanic area. The analysis of chronology of magmatic events and compositional peculiarities of the young igneous rocks of the Elbrus area for the period from 3 Ma to the Holocene shows that the caldera stage of the evolution of the Elbrus Volcano has not come yet and future catastrophic magmatism is highly possible.     

Geochemical Characteristics of Organic Matter from Maikop Rocks of Eastern Azerbaijan

Based on investigation of more than 170 samples taken from natural outcrops of the Maikop Formation (Oligocene–Lower Miocene) in eastern Azerbaijan, the genetic hydrocarbon potential and the organic matter (OM) maturity of these rocks were estimated. In the study region, sedimentary rocks of this formation were deposited under reductive or weakly oxidative conditions. Possessing a relatively high (1.9%, on the average) content of organic matter of a mixed (continental–marine) OM, these rocks are able to generate both liquid and gaseous hydrocarbons under favorable conditions. Contributions of both the continental and marine components to the total organic carbon (TOC) varied in time and space. The upper and lower subformations of the Maikop Formation differ in the qualitative and quantitative compositions of OM. Oligocene rocks have a relatively lower OM content and are characterized by better oil-generating properties, as compared to lower Miocene rocks.     

The age of young intrusions of Tsana Complex (Greater Caucasus) and isotope-geochemical evidence for their origin from hybrid magmas

This paper presents isotope-geochronological and petrological study of granitoids of the potentially ore-bearing (Au–As–Sb–Sn–Mo) Early Pliocene Tsana Complex, which are confined to the Main Caucasus fault zone (upthrow fault) in the central part of the Greater Caucasus Range. The Tsurungal and Karobi groups of magmatic bodies are distinguished based on spatial criterion. The Tsurungal group includes three small granite—granodiorite massifs (Tsurungal, Chorokhi, and Toteldash) and numerous acid and intermediate dikes in the upper reaches of the Tskhenistsqali River (Kvemo Svaneti, Georgia). The Karobi group comprises three subvolcanic rhyodacite bodies located in the upper reaches of the Chashuri River (Zemo Racha, Georgia) and numerous N–S-trending trachyandesite dikes near the axial zone of the Main Caucasus Range. The K-Ar and Rb-Sr isotope dating shows that the granitoid massifs and dike bodies of the Tsana Complex were formed in two different-age pulses of the Pliocene magmatism: phase I at 4.80 ± 0.15 and phase II at 4.15 ± 0.10 Ma. All hypabyssal rocks of the Karobi group, unlike those of the Tsurungal Group, were formed during the first pulse. Petrographic studies in combination with geochemical data indicate that most of the granitoids of the Tsana Complex are hybrid rocks (I-type post-collisional granites) and were derived through mixing of deep-seated mantle magmas with acid melts obtained by the upper crustal anatectic melting in the Main Caucasus fault zone. The granitoids of the Tsurungal Group define basic to acid evolution (diorite–granodiorite–granite–two-mica granite) possibly caused by both crystallization differentiation and increasing role of crustal contamination in the petrogenesis of the parental magmas of these rocks. This conclusion is also confirmed by the differences in the Sr isotope composition between granitoids of the early (⁸⁷Sr/⁸⁶Sr = 0.7053) and late (⁸⁷Sr/⁸⁶Sr = 0.7071) phases of the Tsana Complex. Main trends in spatiotemporal migration of magmatic activity in the central part of the Greater Caucasus in the Pliocene–Quaternary time were established using obtained and earlier published isotope-geochronological data.     

New Geochemical Data on the Organic Matter in Rocks of the Lower and Middle Cambrian Kuonamka Complex,the Lena–Amga Interfluve Area,Southeastern Siberian Platform

Saturated hydrocarbon biomarkers were studied in bitumens from organic matter (OM) in the Lower and Middle Cambrian Kuonamka Complex in the Lena–Amga interfluve of East Siberia. Their contents and distribution were analyzed. It was established that OM of siliceous and carbonate rocks from the lower part of the sequence differs from OM of overlying mainly mixed siliceous–carbonate rocks in terms of distribution of alkanes, steranes, tricyclanes, hopanes, and ratios of their homologs. It was concluded that the peculiarities of molecular composition of OM in the rocks are related to the biochemistry of microorganism communities, the remains of which were accumulated in sediments of Cambrian sea. It is possible that the microbiota changed its composition in response to a sharp change of sedimentation settings, which follows from biomarker proxies. It is suggested that sediments in the lower part of the sequence were formed under conditions of H₂S contamination. Catagenesis of OM and contribution of the Lower and Middle Cambrian potentially oil-generating rocks in naphthide generation on the northern slope of the Aldan anteclise are discussed.