Melange Zone Research Articles

Серпентиниты зоны меланжа на севере массива Рай-Из, вмещающие ювелирный гранат (андрадит-демантоид)

The chemical composition was studied; spectroscopic (FTIR, EPR) and X-ray diffraction analyses of serpentines from the host rocks of gem garnet andradite-demantoid occurrences of their melange zone in the northern part of the Ray-Iz massif were carried out. The quantitative species composition of the serpentinites was determined by the method of synchronous thermal analysis. We found that serpentines were represented by three varieties of lizardite-1T and chrysotile-2Mcl in sample PTM-1 and chrysotile-Orcl in sample PTM-2. The complex of conducted studies allowed specifying the mineral composition of the studied serpentinite samples and greenschist (chrysotile) facies of progressive contact metamorphism.

Gold mineralization in the diaphthorites of the Verkhne-Timptonskiy ore district (Aldan-Stanovoy shield)

In the Verkhne-Timptonskiy gold ore district, located where the Stanovoy and Aldan blocks intersect (Aldan-Stanovoy shield), substantial placer gold deposits have been found in the Gonam, Timpton, and Iengra rivers. The exploration of certain deposits in this area began at the end of the 19th century. Despite exploration efforts conducted in the second half of the 20th century, only minor gold ore findings were made, with uncertain links to greenstones and igneous formations from various periods. After conducting a thorough analysis of both existing and new data on the geology and metallogeny of the Verkhne-Timptonskiy gold ore district, we have concluded that gold occurrences in diaphthorites of the Kholodnikan belt display characteristic features of orogenic-type gold deposits. These features include disseminated pyrite and veinlet mineralization containing iron (Fe), lead (Pb), zinc (Zn), copper (Cu) sulfides, and sulfosalts. The near-ore alterations are represented by silicification, sericitization, carbonatization, and beresitization, characteristic of mesothermal fluid systems. Moreover, we identified common features of the geological structure and metallogeny of the Verkhne-Timptonskiy gold ore district and the Jiaodong province of the North China Craton. Thus, gold mineralization in the Verkhne-Timptonskiy gold ore district and the Jiaodong province is spatially associated with suture zones. In the Verkhne-Timptonskiy gold ore district, the Amga zone of tectonic melange in the Paleoproterozoic era was one of the regions where cratonization of blocks of the Earth’s crust occurred. Later, it became a zone of permeability and concentration of tectonothermal and metallogenic events of various ages. The results obtained will help assess the metallogenic potential of the Verkhne-Timptonskiy district and the Aldan-Stanovoy shield as a whole. They make it possible to increase the efficiency of search operations and focus on localizing research objects.

Anomalous position of Paleozoic faults of Western Donbas

Statement of the general problem. The oil and gas-bearing province of Eastern Ukraine is located in the Dnipro-Donetsk Basin (DDB). It covers the Paleozoic structural floor and is buried under younger sediments. The Paleozoic structural floor of the territory is represented as a long rift graben. The maximum thickness of the sedimentary cover in the graben is 15–17 km. According to geophysical data, the thickness of the earth's crust is 31–32 km under the axial part of the DDB and increases towards the sides. But according to the maximum thickness and facies characteristics of the Paleozoic deposits, the thickness of the Earth's crust below them could not exceed 20 km. The modern thickness is 1.5 times higher. Therefore, such a geological-geophysical model of the structure of the DDB is contradictory. Analysis of recent research and publications. The removal of the contradiction is possible due to the detection of a series of Hercynian thrusts with total amplitude of thrusts of the order of 50 km. Such thrusts began to be established on the northern and southern flanks of the DDB. Hercynian thrusts have a southwestern vergence. In the north, the Voronezh crystalline massif is pushed onto the DDB along the Noviy thrust with an amplitude of 9–10 km. In the south, the South Donbas melange zone is established, which pushes the folded Donbas onto the Ukrainian shield. On its continuation to the northwest is the southern side of the DDB - the so-called Southern monocline. The well-studied Pavlograd-Petropavlovsk coal mining district of Western Donbass with operating coal mines is located within its borders. An unsolved part of the overall problem. Numerous faults with relatively small amplitudes are established within the Paleozoic Southern Monocline. Together, they form a system of horsts and grabens with a north-western extension. But the bottom parts of the grabens are 1–3 km higher than the tops of the horsts. Such a geological situation is anomalous. This indicates the post-fault dislocation of the horst-graben system as a whole. The purpose of the article is identification of the causes of the anomalous structural position of the graben-horst system in the modern geological structure of the Southern Monocline and in the general model of the geological structure of Eastern Ukraine. Research materials and methods. Were involved more than 390 geological sections, constructed based on the data of more than 10,000 wells. Of them, 265 wells revealed the folds that are being analyzed. In addition, were used data of catagenetic transformations of the sedimentary section. Presentation of the main research material. The South Donbas mélange zone falls to the northeast and continues under the Southern Monocline. It is located in the upper block of this regional thrust structure and is in an allochthonous setting. According to geological data, the true amplitude of the thrust of Western Donbas on the Ukrainian shield is 15–20 km, and the vertical component of this amplitude exceeds 3 km. Such spatial parameters explain the abnormality of the structural position of the horsts and grabens of Western Donbass - their southwestern flanks were significantly raised relative to the northeastern flanks during thrusting. Scientific novelty. Therefore, in the modern geological structure, the Dnipro-Donetsk "basin" is not a rift, but a post-rift Hercynian folded region. It is formed by high-amplitude thrusts on the northern and southern flanks. The Hercynian fold region is buried under Mesozoic and Cenozoic sediments and is further complicated by Laramian and Attic fold-thrust dislocations of lesser intensity. Conclusions. The thrust model brings the geophysical parameters into the correspondence with the geological data and is confirmed by the actual material.

The Mal’dzhangarka Carbonatite Massif (Anabar Shield): The Age of Magmatism and Mineralization (U–Pb and Re–Os Isotope Systems)

Abstract—We present the first results of a comprehensive isotope-geochemical study of dolomitic carbonatites of the Mal’dzhangarka massif located in the southeast of the Billyakh melange zone (southeastern periphery of the Anabar Shield). Zircon grains separated from core samples from a depth of 6–30 m have a three-phase structure. All of them were trapped from the host metamorphic rocks and mark the age of the main stage of high-gradient metamorphism in the region, 2027 ± 9 Ma. Pyrochlore containing 38–705 ppm U and 5–21 ppm radiogenic Pb, with weak metamictization of the crystal lattice and an undisturbed U–Pb system, made it possible to estimate the concordant age of rare-metal mineralization in the massif, 167 ± 4 Ma, which is probably close to the crystallization age of the host carbonatites. The estimated age corresponds to one of the stages of kimberlite–carbonatite magmatism on the eastern slope of the Anabar dome. The Re–Os isotope system of pyrite from superposed late carbonate–sulfide veinlets in the carbonatites testifies to a close (within the error of determination) age, 179 ± 14 Ma, and a low initial Os isotope ratio, which indicates the contribution of mantle material to the formation of this isotope system.

Field and Petrographic Characteristics of Photang Thrust Sheet of Zanskar Tethys Himalaya, Ladakh, India

The Photang thrust sheet is a distinct complex of rocks that lies under the Spongtang ophiolite, showing almost complete ophiolite sequences. The pre-collision tectonic setting, emplacement age, mechanism of association of diverse litho-units within Spongtang klippe, and their regional correlation is still debatable and partially known. The controversy exists as to whether the ophiolite was obducted onto the northern passive margin of the Indian plate during the Eocene or the late Cretaceous. Moreover, the fundamental question emerges whether the Photang thrust sheet is a part of the independent Spong arc, has been tectonically derived from the Ophiolitic Melange Zone in the Dras island arc, or is a representative of the accreted seamounts of the Indus suture zone. This paper presents the application of fundamentals of field geology and petrography to understand the pre-collision tectonic setting of the Photang thrust sheet, Spongtang ophiolite, and associated rock sequences in the Photoksar region of the Zanskar Himalaya. Keywords: Photang thrust sheet, Spongtang ophiolite, Tectonic setting, Obduction, Ophiolitic Melange Zone, Indus suture zone, Himalaya

Geochemistry and tectonic setting of Middle Ordovician MORB-like basalts in the Kunlun Orogen: implications for a back-arc environment

The Central Kunlun ophiolitic melange zone, northern Tibetan Plateau, represents a remnant of the Early Paleozoic oceanic lithosphere. The Middle Ordovician Longshigeng basalts (LBs) are located in south of this melange zone and have important significance for reconstructing the opening and subduction processes of the Proto-Tethys Ocean. This study presents petrological, whole-rock geochemical, and zircon U-Pb data for these basalts. LA-ICP-MS U-Pb zircon data indicate that these basalts formed ca. 463 Ma. Geochemically, the basalts are characterized by low SiO2 (47.00–50.63 wt%) and K2O (0.22–1.05 wt%), intermediate Mg# (47–57) and high TiO2 (1.51–2.72 wt%), and are identified as tholeiitic basalts. Their chondrite-normalized REE patterns feature slight enrichments in LREEs (LREE/HREE = 2.11–3.08) with mild Eu anomalies (δEu = 0.89–1.06), and their patterns are similar to the reference line of enriched mid-ocean ridge basalt (EMORB). The primitive mantle-normalized trace element diagrams also show similarities with EMORB and are characterized by overall enrichments in large ion lithophile and high field strength elements, without notable Nb, Ta, and Ti depletions. Combining these data with the tectonic discrimination diagrams, we infer that these rocks formed in a back-arc environment in response to the northward subduction of the Proto-Tethys Ocean (South Kunlun Ocean) in the East Kunlun region, northern Tibetan Plateau.

ГЕОДИНАМІКА

Aim of the work is tectonophysical identify the totality of the deformation structures of the collisional evolutionary stage, which determine the tectonic style of the Transition Zone between Dnieper-Donets Basin and the Donbas Foldbelt. Methods. For the research, we used the author's technique for reconstructing the fields of tectonic deformations and tectonophysical analysis of geostructures. The analytical base of the research was made up of new materials of geological mapping of the territory of the transition zone between the Basin and the Foldbelt. Results. Inversion deformations of the Dnieper-Donets Paleorift were controlled by lattices of tectonites of regionally stable submeridional directions of movements. An analysis of structural patterns of tectonites indicates over the riftogenic faults of the basement in the sedimentary cover of the transition zone, echeloned stages of plumage are formed, composed of thrusts with a significant component of horizontal displacement. The tectonic style of the Transition Zone is determined by the pushing on the low dislocation autochthonous of the Basin of the repeatedly deformed, crumpled into the folds of sedimentary geomas from the Foldbelt. The allochthon structural and tectonic framework consists of thrusts, coulisse-jointed structural ensembles of thrusts, folded covers of transverse extrusion of geomas from axial to side zones, and folded covers of longitudinal thrust towards the depression. All together its form the Western Donets Cover-folded Region, the main structural element of which is the Segment of the Tectonic Wedging of geomass. The north-eastern flank of the Segment is formed by linear anticlinal zones - Torsky-Drobishivska, North-Donets, Matrossko-Toshkovska, south-western - Petrovsko-Novotroitska. The structural apex of the Segment is a tectonic junction at the ends of dynamically conjugated thrusts in the area of the joint of the salt-dome shafts of the axial part of the Basin. Scientific novelty. The tectonic inversion is responsible for the formation of three folded structural floors - the Herzinian, Laramian and Attic. According to the dynamically coupled lattice, a cover-folding system of tectonic thrusting was formed in them, which was first diagnosed as a Segment of Tectonic Wedging of geomas by the Donbas Foldbelt. On the basis of this, within the Transtition Zone, a Western-Donetsk cover-folded Region was separated, covering two tectonic areas in intensity and style of deformation of the sedimentary cover - Kalmius-Toretsky area of scaly covering in the southwestern part, which is limited to the South Donbass Melange Zone in the south, and the Lugansk-Kamyshuvakhsky area of the coulisse-jointed uplift-folding on the northeastern part, which from the north is limited by the low-folded Mesozoic-Cenozoic cover. They are separated by the Central Zone of Strike-slip control along the axial folded zone of large stage-jointed uplift-folds, which include Great-Kamyshuvakhska, Novotroitska, Druzhkovsko-Konstantinovska and Main anticlines. Practical significance. Based on the actual geo-mapping data, it is proved the riftogenic structure in the southeast of the Dnieper-Donets Paleorift is destroyed by folding at the stages of platform activation. Allocation of territory of the Western Donetsk Сover-folded Region allow to correct the scheme of tectonic zoning of the Dnieper-Donets Basin, which is the basis for modeling the geodynamics of the Transition Zone formation.

Accretionary complex: Geological records from oceanic subduction to continental deep subduction

Accretionary complex was usually formed by offscraping of the subducting crustal material over the trench and thus often referred to as subduction zone melange. The structure, composition and forming process of accretionary wedges can provide important insights into the evolution history of ocean basin, ocean-continent material cycle, continental accretion and thus contribute to understanding of the origin of plates and the growth of continents. Accretionary complex is characterized by a block-in-matrix structure associated with imbricate thrusts and isoclinal folds, diversified metamorphic types and intense water-rock interactions, which are distinct to the traditional stratigraphy. Since the proposal of the concept of accretionary wedge over a hundred years ago, great progress has been made in a variety of research focuses, such as the identification of the distribution of accretionary complexes, their compositions and formation mechanisms, the affinities of the matrix and igneous rocks, the recognition of the Ocean Plate Stratigraphy (OPS), the reconstruction of oceanic basin, the dynamic background of the tectonic evolution, the relationship between subduction zone and orogenic belt and, in particular, the accretionary complexes in continental subduction zones. These studies have significantly improved our understanding of the plate tectonic theory. Challenges remain in the identification of ancient accretionary complexes, the detailed analysis of accretionary complex zones, the accretion characteristics during continental collision, and the geochemical tracing of water-rock interaction during the accretion. China contains representative orogenic belts and accretionary complex zones in the world, and its geological records provide the best opportunity to make new breakthroughs in understanding of the plate tectonics.

Petrology and geochemistry of the late Mesozoic Dzheltula alkaline igneous complex, Aldan–Stanovoy Shield, Russia: constraints on derivation from the ancient enriched mantle source

Petrological, whole-rock major and trace element, and Sr–Nd–Pb isotopic data are reported for the late Mesozoic Dzheltula alkaline igneous complex in the Aldan–Stanovoy Shield, Russia. The alkaline rocks are emplaced into the Tyrkanda melange zone. The Dzheltula complex consists of monzonites, foid monzonites, and alkaline syenites; granite dykes intrude the complex. All alkaline rocks have high LILE contents (e.g., Ba and Sr), high light REE/HFSE ratios, strongly fractionated REE patterns, and typically lack Eu anomalies. Granite shows different major and trace-element characteristics in comparison to monzonites and syenites. The major and trace-element characteristics of the Dzheltula complex rocks and minerals are consistent with formation by combined assimilation and fractionation processes of an alkaline parental magma of lamproitic composition. The alkaline rocks have moderately radiogenic Sr (87Sr/86Sr(t) = 0.7057–0.7065) and unradiogenic Nd (ɛNd(t) = − 11.3 to − 15.2) and Pb (206Pb/204Pb = 17.17–17.26); granite has more enriched 87Sr/86Sr(t) value (0.707408) but similar ɛNd(t) = − 12.93 The trace element and Sr–Nd–Pb isotopic data for the Dzheltula complex indicate its mantle source experienced ancient metasomatic enrichment, probably associated with subduction.

Typomorphic Features of Placer Gold from the Billyakh Tectonic Melange Zone of the Anabar Shield and Its Potential Ore Sources (Northeastern Siberian Platform)

Precambrian shields and outcropped Precambrian rock complexes in the Arctic may serve as the most important sources of various types of mineral raw materials, including gold. The gold potential of the Anabar shield in the territory of Siberia has, thus far, been poorly studied. A number of primary and placer gold occurrences have been discovered there, but criteria for the prediction of and search for gold mineralization remain unclear. The main purpose of this paper was to study the typomorphic features of placer gold in the central part of the Billyakh tectonic mélange zone in the Anabar shield and to compare them to mineralization from primary sources. To achieve this, we utilized common methods for mineralogical, petrographic, and mineragraphic analyses. Additionally, geochemical data were used. As a result of this investigation, important prospecting guides were identified, and essential criteria for the prediction of and search for gold deposits were elucidated. The characteristics of the studied placer gold were specific for gold derived from a proximal provenance. These characteristics included the poor roundness of the native gold grains, a cloddy–angular and dendritic form, an uneven surface, and a high content of coarse-fraction native gold (0.5–2 mm), which was as high as 24% of the volume of analyzed native gold. In addition, we conducted a study on the mineralogical features of the gold-sulfide mineralization that was disseminated throughout a small exposure area of paleo-Proterozoic para- and orthogneisses in the Anabar shield basement. A comparison of mineral inclusions in the coarse-fraction native gold and mineral assemblages in the ore deposits showed that one of the possible primary sources for placer gold might be small bodies of metasomatically altered orthogneisses associated with large granitoid plutons.

Evaluation of hydrochemical data using multivariate statistical methods to elucidate heavy metal contamination in shallow aquifers of the Manipur valley in Indo-Myanmar Range

Descriptive statistics, factor analysis, correlation matrices, and cluster analysis are used to gain insights on hydrochemical processes and contamination in the shallow aquifers of Manipur valley. Groundwater has remained as a prime source of water supply for a population of nearly 3 million people living in this valley. Sixteen variables (pH, ORP, TDS, Ti, V, Cr, Cu, Ge, As, Rb, Sr, Nb, Mo, Hf, Ta, and W) are monitored from 28 shallow wells. Mean pH and TDS values (6.8 and 800 mg/l, respectively) suggest fresh quality water in terms of its acidity, alkalinity, or salinity. Oxidation reduction potential values (mean − 6.75 mV) indicate dissolution of metals in anoxic condition. The order of abundance of metals is Sr > As > Rb > Ti > Cu > V > Cr > Mo > Ge > W > Hf > Ta > Nb. Sr, As, Cr, Cu, and Mo are elevated than the WHO limit. Elevation of Sr is attributed to weathering of gypsum, evaporite, and rock salt which reflects in factor 5 of the factor analysis. Factors 2 represents Cr, As, and Mo elevations and signifies geogenic weathering from ultramafic rocks of Manipur Ophiolite Melange Zone. Factor 3 represents Rb, V, and Cu elevations owing to natural weathering of clay and Fe-oxyhydroxides along with dissociation of solid organic carbons. Factor 4 is related to a reduced environment under low pH condition. Factor 1 reflects dissolution of Nb, Hf, Ta, and Ti under anoxic environment as insoluble oxides. Analysis on Pearson correlation and hierarchical clustering strongly support observation made by factor analysis. Thus, the present study shows the accountability of multivariate statistical techniques in interpreting and delineating the sources of contaminations in shallow groundwater.

40Ar/39Ar ВОЗРАСТ ЩЕЛОЧНЫХ ПОРОД ВЕРХНЕАМГИНCКОГО МАССИВА (АЛДАНСКИЙ ЩИТ, ЮЖНАЯ ЯКУТИЯ)

Актуальность исследования обусловлена необходимостью расширения ресурсной базы полезных ископаемых, в том числе и золота. Крупные ареалы щелочного магматизма, такие как Алдано-Становой щит, представляют особый интерес, так как часто со щелочными породами ассоциируют золоторудные месторождения, в том числе крупные и гигантские. Верхнеамгинский массив расположен в Верхнеамгинском рудном районе Алдано-Станового щита, который, в свою очередь, входит в состав Чаро-Алданской металогенетической зоны, протянувшейся более чем на 700 км с запада на восток. Характерной особенностью Верхнеамгинкого рудного района является его приуроченность к Амгинской субмеридиональной структурной зоне тектонического меланжа, отделяющей Центрально-Алданский составной террейн с востока от расположенных к западу и югу от нее соответственно Западно-Алданского и Тындинского составных террейнов. Цели: определить основные типы магматических пород Верхнеамгинского массива, изучить особенности строения и состава и выяснить время внедрения различных фаз массива; сравнить полученные результаты с имеющимися геохронологическими данными по мезозойскому магматизму Алданского щита. Методы: петрографические исследования и 40Ar/39Ar датирование методом ступенчатого прогрева по монофракциям флогопитов. Результаты. В результате петрографических исследований показано, что основные фазы мезозойского щелочного магматизма в пределах Верхнеамгинского массива представлены лейкократовыми сиенитами и дайками мезократовых лампрофиров (минетт). Результаты 40Ar/39Ar датирования показали, что образование массива происходило в несколько этапов: (1) 129,1±2,5 млн лет внедрение сиенитов; (2) 117,7±3,4 млн лет внедрение даек лампрофиров. Мезозойский магматизм, проявленный в Верхнеамгинском районе показывает сходные возрастные рубежи с магматическими процессами, проявленными на Алданском щите в мезозойскую эпоху.

Characteristics of tectonic deformation of the melange zone in the Lachlan Orogen along eastern coast of Australia

ABSTRACT The Narooma-Batemans Bay (NBB) area along the southeast coast of Australia is a part of the eastern zone of the Early Paleozoic Lachlan Orogen. In the NBB, a set of rock association consisting of turbidites, siliceous rock, basic lava, and argillaceous melange zone is mainly developed. According to systematic field geological survey, the deformation of 3 stages (D1, D2, and D3) was identified in the NBB. At stage D1, with the original bedding S0 in a nearly east-west trending as the deformation plane, tight folds, isoclinal folds, and other structures formed in the NBB accompanied by structural transposition. As a result, crenulation cleavage developed along the axial plane of the folds and schistosity S1 formed. At stage D2, with north-south-trending schistosity S1 as the deformation plane, a large number of asymmetrical folds and rotated porphyroclasts formed owing to thrusting and shear. At stage D3, left-lateral strike-slip occurred along the main north-south-trending schistosity. Based on the analysis of the characteristics of tectonic deformation in the NBB and summary of previous research results, it is determined that the early-stage (D1) deformation is related to Ordovician Macquarie arc-continent collision and the deformation at stages D2 and D3 is the result of the westward subduction of Paleo-Pacific Plate. That is, it is not the continuous westward subduction of the Paleo-Pacific Plate that constitutes the evolution model of the NBB as previously considered.

Zun-Ospa Gold Deposit, Eastern Sayan: Geology, Ore Composition, and Genesis

The paper discusses the geology of Zun-Ospa gold deposit, which is situated near the Ospino ophiolitic nappe in the southeastern part of the Eastern Sayan, and the ore composition therein. The deposit is related to the tectonic melange zone and is characterized by distinct structural control. Three consecutive mineral assemblages formed within a temperature range of 380°–170°C: (i) native gold–quartz–pyrite, (ii) gold–quartz–polysulfide, and (iii) silver–sulfosalt. The ore was deposited from low-concentration (5.2–14.2 wt % NaCl equiv.) solutions without CO2, with the predominance of Mg and Fe chlorides and an admixture of Na and K chlorides. The major ore minerals are pyrite, chalcopyrite, galena, and sphalerite; identified subordinate minerals are pyrrhotite, pentlandite, heazlewoodite, fahlore (tennantite, freibergite), Ni and Ag sulfosalts (ullmannite, miargyrite, polybasite, stephanite), Ag sulfides (mckinstryite, argentite); Au minerals are represented by electrum, kuestelite, and native gold of medium to low fineness. The geological, mineralogical, geochemical, and isotopic characteristics of ore indicate a metamorphic–hydrothermal genesis of mineralization related to the formation of a melange zone in the duplex strike-slip structure. The sources of ore components are host rock complexes that have been subjected to tectonic deformations, among which rocks of an ophiolitic association predominate, along with fragments of initial hydrothermal–sedimentary ore, granitic, terrigenous, and carbonate rocks. The Late Paleozoic (352 Ma) age of mineralization corresponds to the stage of postcollision shear deformations within the entire Central Asian Foldbelt.

Oceanic and Continental Mantle Fragments in Ophiolites of the Northwestern Pacific Margins: Composition, Age, and Genesis of Sakhalin Peridotites

The paper presents the results from a study of original and published data on the chemical composition and age of mantle peridotites from Sakhalin Island ophiolites. The material and genetic proximity of peridotites from the Berezovsky and Shelting plutons, on the one hand, and melange zone serpentinites, on the other, have been established. In composition and Fe2O3 and MgO variations, Sakhalin peridotites differ radically from those of the Northeast Asia ophiolite complexes (Krasnogorsky Massif, Karaginsky Island Massif, etc.), which are fragments of Pacific Plate mantle. Conversely, Sakhalin peridotites have a subcontinental genesis and are compositionally close to xenoliths of lherzolites from Hankai Microcontinent mantle (southern Sikhote-Alin). The rythmics of alternation in compression and expansion at the margin of the Asian continent in the last 180 million years, caused by cyclical changes in the Pacific spreading rate, have been considered. According to data obtained by U-Pb dating of zircons, the formation of the Berezovsky Massif took place 169–154 Ma ago during Jurassic expansion of the continental margin. Matching age and composition data demonstrate that the Sakhalin ophiolites formed within the marginal sea basin during riftinduced destruction of the periphery of the Hankai Craton. The assumed tectonic setting was close to that reconstructed for the Jurassic Josephine ophiolites of the California margin of the North American continent. The continental genesis of the studied ophiolites agrees with the age and tectonic mode of ophiolite formation for Sakhalin Island.

Random forests algorithm in podiform chromite prospectivity mapping in Dolatabad area, SE Iran

The Dolatabad area located in SE Iran is a well-endowed terrain owning several chromite mineralized zones. These chromite ore bodies are all hosted in a colored melange complex zone comprising harzburgite, dunite, and pyroxenite. These deposits are irregular in shape, and are distributed as small lenses along colored melange zones. The area has a great potential for discovering further chromite resources. Therefore, the current work endeavors to delineate the favorable zones of podiform chromite mineralization to focus on the detailed exploration surveys. In order to achieve this goal, the machine learning random forests algorithm was adapted to integrate the footprints of mineralization in various exploration datasets. The genetic characteristics of podiform chromite deposits were used to define the exploration criteria. These defined criteria were then translated to a set of exploration evidence layers. The competent exploration evidence layers, i.e. those with remarkable positive spatial associations with mineralization, were then recognized using distance distribution analysis. Respecting the location of known chromite mineralizations and competent exploration evidence layers, a predictive random forests model was trained and then applied to predict the favorable zones of chromite prospectivity. The delineated targets were found to occupy 19% of the studied area, in which all the known chromite mineralizations were delimited. Consequently, it is worthy to follow up the detailed exploration surveys within the delineated zones.

Ankaramite: A New Type of High-Magnesium and High-Calcium Primitive Melt in the Magnitogorsk Island-Arc Zone (Southern Urals)

This work describes the geological position, mineral and chemical composition of high-Mg effusive ankaramites occurring as dykes and lava flows. They were found in the melange zone of the western margin of the Magnitogorsk island arc zone in the Southern Urals. Data on the liquidus association of phenocrysts and on the composition of the matrix of effusives are given. According to the data obtained, the conclusion was drawn that the ankaramites studied can be attributed to the primary island arc melts, which were not subject to essential differentiation. This type of effusives has not been distinguished previously among island arc volcanogenic formations of the Urals. It is shown that ankaramites can be considered to be primary melts parental for dunite–clinopyroxenites–gabbro complexes of Ural–Alaskan type. The occurrence of ankaramites in the Paleozoic island arc formations of the Urals indicates the wehrlite composition of the mantle as the reason for the extremely wide development of wehrlites and clinopyroxenites in different mafic–ultramafic complexes of the Urals.

On position of the Black Sea region in general scheme of Cenozoic tectonic movement in Eastern Europe and the Middle East

Purpose. To clarify a structural position of Cenozoic Black Sea basin in the overall scheme of geodynamics of Ukraine, Eastern Europe and the Middle East.Methodology. Within these regions structural relations between geological complexes (stratified and unstratified) and the analysis of change were identified. Modern geodynamic models and data of present-day seismology tectonic movements in the region were additionally used. The results are presented in the form of paleotectonic schemes.Findings. Cenozoic movement in Eastern Europe and the Middle East reflects the global tectonic processes. The closure of the paleo-ocean Tethys occurred in these regions by collision between the southern passive margin of the East European and microcontinents Tisia and Anatolia, approaching their northern active margins in a north-western direction. This is evidenced by the spatial distribution of paleoarc volcanics, melange zones and the degree of pre-Eocene sedimentary formation folding in Europe. As a result of collisional processes in the Paleocene there formed a vast area of Laramy folding in Europe. The northern border of this folding area on the territory of Ukraine and Poland has been established. Its shape shows the progress of fold deformations from south to north from the two microcontinents-indentors.Due to separation of Tisia from Anatolia a large marine gulf (future Carpathians) was formed between them on the southern edge of Eastern Europe, devoid of Laramy deformations and filled by Paleogene flysch from the north-east and south-west sides. In the Miocene general direction of tectonic movement in Europe has changed to the north-east, associated with opening of the Atlantic. It formed the attic orogenic structure of the Carpathians and complicated structure in other regions of Ukraine. At the same time closure of the Eastern Mediterranean basin continued with the subduction of the heavy oceanic plate under the Cretan-Hellenic continental edge. This led to Anatolia rifting from the southern margin of Eurasia in opposite southwest direction. As result, the Black Sea basin opened as a back-arc spreading sea between the oppositely moving plates.Implications. Good agreement between the structural and geological observations with seismic data, regional geological material and global models are shown.

Conditions of formation of gold-bearing magnetite-chlorite-carbonate rocks of the Karabash ultrabasic massif (South Urals)

Ilmenite, apatite, monazite, aeshynite-(Y), zircon, baddeleyite, thorianite, uraninite, and copper-containing native gold have been revealed in specific magnetite-chlorite-carbonate rocks and chloritolites in the Karabash ultrabasic massif in the South Urals. Dolomite from the magnetite-chlorite-carbonate rocks are characterized by a rather uniform isotope composition (513C = -0.9 to -1.9%c, 518O = 11.5-13.6%c, 87Sr/86Sr = 0.70422-0.70469) corresponding to a mixture of sources: marine limestones and mantle fluids. We determined the isotope compositions of antigorite from serpentinites (5D = -79.1 and -89.6%c, 518O = 7.4 and 7.6%c), of chlorite from chloritolites (5D = -57.8%c, 518O = 7.8%c), and of magnetite-chlorite-carbonate rocks (5D = -59.2 and -69.6%c, 518O = 6.4 and 5.9%c). The latter probably formed by the mechanism of filling of the free space at 480-280 °C, and chloritolites were developed after serpentinites. Oceanic serpentinites, gabbros, limestones, and mantle fluids can be considered the source of material during the formation of magnetite-chlorite-carbonate rocks. A comparative analysis of the latter and the massif rodingites (chlograpites) bearing copper-containing gold was carried out. The established common features of these types of rocks are the localization in zones of tectonic melange, the presence of chloritolite rims, geochemical specialization, thermal conditions of formation, and isotope parameters of minerals and fluids.

Petrogenesis of Late Cretaceous Volcanism in Kazhaba Area and its relationship with mantle plume activity of Reunion hotspot

Basaltic pillow lavas near the Kazhaba village in Balochistan are found in the tectonic slivers of the Bagh complex: the melange zone beneath the Muslim Bagh ophiolite complex. These vol-canics are mainly represented by alkali basalts. The petrography and chemistry suggest that these volcanic rocks belong to mildly to strongly alkaline, intra-plate volcanic rock series. Their low Mg# and low Cr, Ni and Co contents suggest that the parent magma of these volcanics was not directly derived from a partially melted mantle source, but resulted fractionation in an upper level magma chamber, en-route to eruption. Their LILE and HFSE, enriched primordial mantle-normalized patterns with marked positive Nb anomalies further confirm their within-plate geochemical signatures and are consistent with an enriched mantle source. Their highly enriched LREE patterns and high (La/Yb)N and (Ce/Yb)N ratios suggest a partially melted garnet-lherzolite parent magma source. The Zr versus Zr/Y studies suggest that these volcanics were derived from about 15% partially melted enriched mantle source. It is suggested that these Late Cretaceous intra-plate volcanics may represent the mantle plume activity of the Reunion hotspot, and were erupted during the passage of Ceno-Tethys Ocean floor prior to the passage of Indian Plate over it.