Dolerite Dikes Research Articles

Petrogenesis of mafic rocks from the Xigaze ophiolite, Southern Tibet: Insights into forearc extension induced by Neotethyan rollback

The Xigaze ophiolite in the central Yarlung Zangbo suture zone (southern Tibet) provides evidence of the tectonic processes that formed the Neotethyan Ocean between the Indian and Eurasian plates. In this study, we present zircon U Pb geochronological, mineralogical, and geochemical data for mafic rocks from the Xigaze ophiolite to determine their petrogenesis and tectonic environment. The crustal section of the Xigaze ophiolite includes mainly gabbro lenses, amphibolite dikes, dolerite sills or dikes, and basaltic lavas. The absence of a well-developed gabbroic crust and sheeted dike complexes in the ophiolite indicates slow to ultraslow-spreading conditions and a heterogeneous structure of the oceanic lithosphere, which is not atypical for layered ophiolites. Zircons from the amphibolite and dolerite dike reveal crystallization ages of 125.0 ± 1.8 Ma to 123.8 ± 1.3 Ma, respectively. Geochemically, all mafic rocks have mid-ocean ridge basalt (MORB)-like affinities with enrichment in large ion lithophile elements (LILEs, U) relative to high field strength elements (HFSEs, Nb, Ta and Ti) and light rare earth elements (LREEs). They also exhibit high εNd(t) values ranging from +8.4 to +8.8 at 206 Pb/ 204 Pb = 16.75–17.64. Moreover, chromite and clinopyroxene grains from the mafic rocks have a wide compositional spectrum and are mainly plotted in the supra-subduction zone (SSZ) fields. These features indicate that MORB-like magmatism was triggered by decompression melting of the upwelling asthenospheric mantle with minor mass transfer from the subducting slab. The Xigaze ophiolite lacks boninite, high‑magnesium andesites, island arc tholeiites, and calc-alkaline lavas, which indicates that it is incompatible with the complete Izu-Bonin-Mariana forearc sequence. Combined with previous studies, we speculate that the Xigaze ophiolite represents an ancient SSZ-type oceanic lithosphere formed in the Gangdese forearc during the Early Cretaceous, and the upper plate extension is linked to the southward retreat and rollback of the Neotethyan slab. Thus, the Xigaze and other Neotethyan SSZ ophiolites with no systematic change in composition with stratigraphic age provide new insights into the diversity of melt evolution in the forearc crust. • Amphibolite and dolerite dikes in the Xigaze ophiolite were formed during the Early Cretaceous. • The mafic rocks were derived from upwelling asthenosphere with minor slab input. • The Xigaze ophiolite originated from the Gangdese forearc setting. • The SIR model appears likely to be oversimplified for variable forearc ophiolites.

Geology, mineralogy, geochemistry and conditions of forming the Tukan gold-quartz deposit (Khudolaz trough, South Urals)

Research subject. Detailed mineralogical and geochemical studies of rocks of the Tukan gold-quartz deposit (quartz veins, dolerites, gabbro-diorites) were carried out. The deposit is one of numerous small gold-quartz occurrences in the Khudolaz trough of the West Magnitogorsk zone.Methods. The morphology and composition of native gold was studied using an Axios kop 40 A optical microscope and Tescan Vega 3 and JEOL-6390LV scanning electron microscopes. The chemical composition of rocks was determined by X-ray fluorescence (Carl Zeiss VRA-30) and atomic emission (Shimadzu ICPE-9000) analyzes. Fluid inclusions were analyzed using Linkam TMS-600 thermostage equipped with Link-System 32 DV-NC software and an Olympus BX51 optical microscope. The gas composition of fluid inclusions was examined by Raman spectroscopy (Horiba LabRam HR800 Evolution).Results. Gold-bearing quartz veins and dolerites intersect ing the gabbrodiorite intrusion of the Khudolaz complex are associated with submeridional strike-slip faults. Native gold has a various morphology and is confined to cracks in quartz veins and altered dolerites. The composition of gold is stable, with the average fineness of 871 ± 8.3‰. An analysis of fluid inclusions in a gold-bearing quartz vein showed that gold was formed at temperatures of no less than 186–230°С from a fluid with a salinity of 4–8 wt % NaCl-eq. The presence of gases СО2, N2 and CH4 was found in fluid inclusions. Dolerites and gabbrodiorites hosting gold-bearing veins contain sulfides (pyrite, pyrrhotite, chalcopyrite, pentlandite, violarite), sulfoarsenides (cobaltite and its analogs), as well as mine rals of Ag (hessite) and Pb (kuranakhite and phases similar in composition to minerals of the dugganite group and burckhardtite). Conclusions. The sustained composition of native gold and a narrow range of salinity and homogenization temperatures of fluid inclusions indicate the formation of gold during a single stage of mineralization. The main source of gold could have been the dolerite dikes of the Ulugurtau complex. The presence of CO2, N2 and CH4 gases in fluid inclusions from goldbearing quartz indicates that not only magmatic fluid, but also fluid from the host rocks took part in mineral formation. The low value of the ratio X(CO2)/X(CH4) ≤ 0.7 indicates a moderately reduced ore-forming fluid. Low Au concentrations (0.1– 0.4 ppm) in quartz veins, dolerites, and gabbro-diorites, including those containing visible gold, indicate a weak commercial potential of the deposit

Generation of Continental Alkalic Mafic Melts by Tholeiitic Melt–Mush Reactions: a New Perspective from Contrasting Mafic Cumulates and Dikes in Central Tibet

Abstract Alkalic basalts from continental regions have been widely used to fingerprint chemically and lithologically distinct components in the mantle. However, superimposed on mantle source effects are the effects of intra-crustal magma differentiation, which may be underestimated when exclusively considering fractional crystallization (FC) in melt-rich magma chambers. Here we present extensive mineralogical and geochemical data for mafic cumulates and dolerite dikes from central Tibet to demonstrate that tholeiitic magma differentiation occurred in crystal-rich mushes and melt-filled fractures. The mafic cumulates provide a snapshot of the percolation of water-poor (<0.9 wt% H2O), oxide-undersaturated, tholeiitic melts through slightly hotter mushes (50–60% crystals) with a framework of primitive olivine ± plagioclase crystals. The highly corroded olivines indicate that thermo-chemical disequilibrium between the later percolating melts and pre-existing crystal framework triggered melt–mush reactions in an open system. High-temperature (>1060°C) reactions were accompanied by a decreasing melt mass, causing the initial tholeiitic melts to evolve into alkalic mafic melts during slow cooling. This process involved the dissolution and reprecipitation of olivine, plagioclase, and clinopyroxene as well as the late crystallization of high-TiO2 (up to 3.4 wt%) clinopyroxene and kaersutite. In contrast, the same tholeiitic magmas (i.e. dolerite dikes) that intruded the cold country rocks along fractures produced silica-rich residual melts saturated in Fe–Ti oxides and sodic plagioclase by FC during rapid cooling. The residual melt was always in equilibrium with the instantaneous crystal during closed-system FC, so there was no reaction between the interstitial melt and adjacent crystal even after a high degree of crystallization. Alkalic mafic melts that can be formed by melt–mush reactions exhibit strong enrichment of incompatible element contents (e.g. Ti and alkali and rare earth elements) and ratios (e.g. Ce/Y), but little Si enrichment and Mg depletion, compared with the parental tholeiitic magma, which cannot be reproduced by simple FC. We argue that tholeiitic melt–mush reactions at mid- to lower-crustal pressures (≥0.5 GPa) are a new mechanism for the generation of interstitial alkalic mafic melts in continental mush reservoirs. Continental alkalic basalts could be produced by this mechanism if such interstitial melts were concentrated into melt lenses by compaction and then transported rapidly upward to the surface by focused flow.

Petrology and geochemistry of multiphase post-granitic dikes: A case study from the Gabal Serbal area, Southwestern Sinai, Egypt

Abstract Variable single and/or swarms of post-granitic dikes are widespread at Gabal Serbal, Southwestern Sinai, Egypt. The present article aims to identify and discriminate these multiphase dikes through detailed geological, petrographical, and geochemical examinations. These dikes are classified into two subphases: (1) acidic dikes (porphyritic dacite, microgranite, granophyre, and alkaline granophyre dikes); and (2) basic dikes (basalt and dolerite dikes). They range from vertical or steeply inclined bodies, 0.5–15 m wide, pink to black color, and NE–SW to N–S directions. Acidic dikes with different mineralogical constituents have medium to high k-characters, originating from calc-alkaline magma and extruded in a volcanic arc environment. In contrast, basic dikes have medium k-characters, originating from tholeiitic magma and developing within a plate environment. Basic dikes are enriched with opaque minerals, where the basaltic dike contains iron oxides (magnetite and hematite), such as apatite in addition to copper minerals. Dolerite dike comprises magnetite, titanomagnetite, and pyrite.

Basites of Vilyui-Markhinsky dike belt (Vilyui paleorift)
 and their relations with Nakyn field kimberlites

The Vilyui-Markhinsky dike belt (VMDB), which
 was formed as a result of Devonian rifting on the eastern
 margin of the Siberian Platform, is the marginal
 part (area of scattered rifting) of the Vilyui paleorift
 structure. The Nakyn field is located in the central part
 of the belt, but is controlled by an independent system of
 NNE-trending tectonic faults. The belt dyke intrudes the
 Nyurbinskaya kimberlite pipe. On their contact, specific
 breccias were formed resulting from the interaction between
 degassing products of basic magma with kimberlites.
 The typical zonality of the dyke endocontact indicates
 a later dyke introduction. Dolerite dikes thermally
 metamorphosed breccias in which high-temperature
 neoplasms of andradite, Al-lizardite, and clinochrysotile
 were generated. VMDB basites represent a single association,
 in which two series of rocks are distinguished:
 moderate-titanium (TiO2 ~ 2.5 wt. %) with normal alkalinity
 and low P2O5 content, and high-titanium (TiO2
 ~ 4.4 wt. %), occasionally with moderate alkalinity. The
 differences in the dike composition are insignificant and
 are the result of natural variations in the composition
 of individual bodies. 40Ar/39Ar dating of the VMDB basites,
 the method characterized by the best results convergence,
 shows that they formed in a narrow timeframe
 corresponding to the Upper Frasnian – Famenian stage
 of the Upper Devonian (368.5 to 376.3 Ma). The location
 of the Nakyn field basites and kimberlites is controlled
 by faults of various types, orientation and age. Kimberlites
 formed first, and VMDB intrusions followed.

U-Pb zircon age of a sheeted dike complex in ophiolites in the structure of the Revdinsky massif, Ural Рlatinum Belt

Research subject. A tectonic block of ophiolites, corresponding to the root zone of a sheeted dike complex, in the eastern part of the Revdinsky massif of the Platinum-bearing belt of the Urals (UPB).Materials and methods. Zircons for dating were collected in the first-generation dolerite dikes of the sheeted dike complex under study, which had been previously considered as Ordovician. U-Pb dating of zircons was performed by LA-ICP-MS on a NexION 300S quadrupole mass spectrometer with a laser ablation attachment NWR 213.Results. Zircons with an age of 425.6 ± 2.9 Ma are characterized by primary magmatic zoning and apparently correspond to the time of intrusion of the sheeted dike complex. In zircons with an age of 404.0 ± 2.9 Ma, inclusions of metamorphogenic minerals (amphibole, chamosite, quartz, clinozoisite) were discovered. This age reflects rock recrystallization or metamorphism, for example, during the intrusion of later vein rocks of the diorite-tonalite series or dolerite dikes of the second generation. Three points of determination showed a younger age of 362 ± 5.6 Ma, probably reflecting the time of metamorphism at the beginning of collisional processes. Conclusions. For the first time, the Silurian U-Pb age of zircons (LA-ICP-MS) was obtained for the ophiolite block (root zone of the sheeted dike complex) in the structure of the Revdinsky massif UPB. The obtained age of intrusion of the sheeted dike complex (425.6 ± 2.9 Ma) coincides with the age of zircons (428.5 ± 3.7 Ma) from gabbro screens in sheeted dikes of the East Ural zone determined earlier (Smirnov, Ivanov, 2010) and corresponds to the time of spreading over the subduction zone in the Middle Urals.

Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

Preliminary Petrological and Geochemical Studies of Dolerite Dikes at the Kara Astrobleme Central Uplift, Comparison with UHPHT Impact Melt Glasses (Pay-Khoy, Russia)

Recent find of the ultra-high pressure high-temperature (UHPHT) impact melt glasses among the impactites of the Kara astrobleme has a high interest in nicely preserved 70 Ma glass with potentially unusual structure and properties. By the moment, it is important to understand about the substance source for the UHPHT glasses. The Kara target is characterized with complicated rock material preferably presented with Paleozoic sedimentary units. At the same time, the target has in a sequence Devonian sills and dikes of gabbro-dolerites. The latter appear on the surface at the Kara dome being a material which probably have been affected by the most strong impact. Here we for the first time describe the results of preliminary analysis of petrological and geochemical features of the magmatic dikes of the central uplift with the aim to understand their probable genetic source for the UHPHT impact melt veins matter. The provided studies point to essential difference between the compared materials, that means the UHPHT impact melts do not correspond to the magmatic material of the Khengursky complex of gabbro-dolerites of the Pay-Khoy Ridge (Russia).

Petrophysical Property Modifications by Alteration in a Volcanic Sequence at IODP Site U1513, Naturaliste Plateau

AbstractPetrophysical properties of volcanic rocks are controlled by lithology and subsequent modification by alteration processes. Investigating the linkages, using a range of different techniques, are important to establish how petrophysical properties can inform about the alteration state of volcanic rocks. Here, we compile petrophysical data and correlate these with geochemical and mineralogical analyses acquired from a volcanic sequence on the Naturaliste Plateau, offshore southwest Australia (International Ocean Discovery Program Site U1513). The sequence consists of alternating basalt lava flows and volcaniclastic deposits, intruded by multiple dolerite dikes. Variable alteration intensities from fresh‐slight to strong are quantified using Chemical Index of Alteration. Intervals of slightly altered dikes exhibit low porosity and high values of bulk density, P‐wave velocity, and thermal conductivity. The increase of alteration intensity corresponds to decreases in bulk density to ∼2 g/cm3, P‐wave velocity to ∼2,000 m/s, thermal conductivity to ∼1.2 W/(m·K) and an increase in porosity up to 50%. Natural Gamma Ray and magnetic susceptibility vary downhole with rock composition and at lithologic boundaries. The distinct variations exhibit a good correlation with primary lithologic characteristics and secondary mineralogical and textural changes attributed to alteration processes. We provide synthesis models of petrophysical variation with alteration intensity. Although differences in primary lithology and alteration type introduce limitations and uncertainties, there is a reasonable applicability of our results to rapidly characterize the alteration intensity and volcanic stratigraphy in volcanic sequences and to calibrate wireline log‐based determinations. This will help others to develop strategies for exploration, drilling, and geophysical research of volcanic rocks.

The Influence of Trap Magmatism on the Geochemical Composition of Brines of Petroliferous Deposits in the Western Areas of the Kureika Syneclise (Siberian Platform)

Abstract —We present the results of study of the influence of trap magmatism on the geochemical composition of brines and on the geothermal regime of the Earth’s interior in the western areas of the Kureika syneclise. The Siberian trap province, which unites all cutting and layered tholeiite–basic magmatic intrusions and erupted basaltic lava, is the world’s largest Phanerozoic continental basalt province. Brines, hydrocarbon deposits, and organic matter of the sedimentary cover were subjected to a significant thermal impact as a result of the Permo-Triassic trap magmatism. During the trap intrusion, the maximum paleotemperatures in major Silurian (D’yavolskii), Ordovician (Baikit), and Cambrian (Deltula–Tanachi, Abakun, and Moktakon) productive horizons reached 650 °C. The Paleozoic and Proterozoic deposits of the study area contain brines with TDS = 50–470 g/dm3. By chemical composition, they are of Na, Na–Ca, Ca–Na, Ca–Mg, and Ca chloride types (according to the classification by S.A. Shchukarev), with mixed Ca–Na and Na–Ca chloride brines dominating. The studied brines can be divided into three groups according to the degree of metamorphism: low (S1), medium (S2), and high (S3). The first group includes mainly sodium chloride brines with TDS = 50–370 g/dm3 (rNa/rCl = 0.60–0.95; S ≤ 100). The second (dominating) group comprises Na–Ca, Ca–Na, Ca, and Ca–Mg chloride brines with TDS = 150–470 g/dm3 (rNa/rCl = 0.10–0.87; 100 ≤ S ≤ 300). The third group is Ca–Na and Ca chloride brines with TDS = 223–381 g/dm3 (rNa/rCl = 0.12–0.45; S ≥ 300). We have first established changes in the hydrogeochemical field (major- and trace-component and gas compositions) with distance from the contacts of intruded dolerite sills and dikes. Hydrocarbons (CH4, C2H6, C3H8, i-C4H10, n-C4H10, i-C5H12, n-C5H12, and C6H14) and water-soluble components I, B, and NH4 were most actively subjected to destruction. For example, at a distance of 100 m from the intrusion zone, the water-dissolved gases are dominated by CO2 (>90 vol.%), and CH4 amounts to 5 vol.%, whereas at a distance of 250 m, the concentration of CO2 decreases to 30 vol.%, and that of CH4 increases to 60–70 vol.%. In addition to the negative effect on the hydrocarbon preservation in the contact zone (≤400 m), the intrusive trap magmatism favored the formation of hydrocarbons in remote horizons. The reaction of intruding traps with brines of the sedimentary cover led to the saturation of the latter with iron, aluminum, and silica, which suggests extraction of metals in the form of salts from magmatic melts into an ore-bearing fluid.

Cretaceous to Miocene NW Pacific Plate Kinematic Constraints: Paleomagnetism and Ar–Ar Geochronology in the Mineoka Ophiolite Mélange (Japan)

AbstractThe Mineoka Ophiolite Mélange is located at the intersection of the Pacific, Philippine Sea, Eurasian, and North American plates. The Mineoka ophiolite origin is disputed, and it has been ascribed to a fully subducted plate or part of the Pacific and Philippine Sea plates. In this paper, we present a kinematic reconstruction of the Mineoka Ophiolite Mélange and its relation with the Pacific Plate, based on new paleomagnetic data and bulk‐rock 40Ar/39Ar ages of basaltic rocks. In addition to standard analyses for paleolatitudes, we performed a Net tectonic rotation analysis on sheeted dolerite dikes to infer the paleospreading direction that formed the ophiolite. The analysis shows that 85–80 Ma MOR pillow basalts erupted at a paleolatitude of N ∼16°, whereas ∼50 Ma basalts formed at N ∼34°. Net Tectonic Rotation analysis suggests that the spreading direction was NE 60°. Ar–Ar ages yielded 53–49 Ma for MORBs and 41–35 Ma for island‐arc basalts. The formation of this ophiolite occurred in the back‐arc spreading of the Nemuro–Olyutorsky arcs of the NW Pacific. It infers that the final consumption of Izanagi below Japan instigated a subduction jump and flipped its polarity. Subduction initiated parallel to the ridge, and a piece of the original back‐arc crust got trapped near the Japan trench during the northwards motion of the Philippine Sea Plate. The contrasting motion between the Pacific and the Philippine Sea plates generated a highly unstable setting followed by a subduction zone that left a small‐sized and short‐lived plate (“Mineoka”), surrounded by subduction zones.

Diachronous collision in the Seve Nappe Complex: Evidence from Lu–Hf geochronology of eclogites (Norrbotten, North Sweden)

AbstractThe collision of Baltica and Laurentia during the Caledonian Orogeny happened atc. 400–420 Ma. However, subduction and collision processes also took place before this main collisional phase and the tectonic history of these is still not fully resolved. The Seve Nappe Complex in Sweden has recorded these earlier phases. The Seve Nappe Complex in Norrbotten (North Swedish Caledonides) comprises four superimposed nappes emplaced by eastward thrusting (from base to top according to the conventional structural interpretation): Lower Seve Nappe, Vaimok, Sarek, and Tsäkkok Lenses. Eclogites occur in the Vaimok and Tsäkkok Lenses. The Vaimok Lens represents rocks of the Baltican continental margin intruded by Neoproterozoic dolerite dikes which were later eclogitized and boudinaged. By contrast, eclogites of the Tsäkkok Lens are former oceanic basalts associated with calcschists, possibly representing the ocean–continent transition between Baltica and Iapetus. Previous age determinations for eclogitization yielded various ages betweenc. 500 and 480 Ma, in contrast to younger (460–450 Ma) ages of ultra high‐Pmetamorphism in the Seve Nappe Complex further south in Jämtland. Eclogites from the Vaimok (one sample) and Tsäkkok (three samples) lenses were dated using Lu–Hf garnet geochronology. Garnet from all samples shows prograde zoning of major element and Lu contents and yielded well‐defined isochrons of the following ages: 480.4 ± 1.2 Ma (Vaimok); 487.7 ± 4.6 Ma, 486.2 ± 3.2, 484.6 ± 4.6 Ma (Tsäkkok). The ages from Tsäkkok are interpreted to date the burial of the Iapetus–Baltica ocean–continent transition in a west‐dipping subduction zone aroundc. 485 Ma, and the age from the structurally deeper Vaimok Nappe the following subduction of the continental margin. Previously reported ages of 500 Ma and older are not supported by this study. The age difference between eclogites in the Seve Nappe Complex in Jämtland (c. 460–450 Ma) and Norrbotten (c. 488–480 Ma) may reflect the collision of an island arc with an irregularly shaped passive continental margin of Baltica or alternatively the collision of a straight margin with a microcontinent (Sarek Lens) accreted to the upper plate.

Effects of Igneous Intrusion on Low-rank Coalbed Methane Reservoir Formation in Fuxin Basin, China

Researches of igneous intrusion in Fuxin basin on more complex formation mechanism of low-rank coalbed methane(CBM)reservoir are not only the focus and critical problem of basic geology, but also contribute to improvement of the exploration effectiveness of the same type of reservoirs, serving as one of rare achievement examples of low-rank CBM development. This paper presents a comprehensive research of the reservoir characteristics of the Wangying-Liujia CBM formation, such as the intense control action of the dolerite dike invasion so as to further confirm the effects of igneous intrusion on low rank CBM reservoir formation. The result shows that the dolerite dike invasion not only facilitates large hydrocarbon regeneration, but also obviously improves the Ro of the thick bituminite at the bottom of Fuxin formation. It also greatly improves the coal reservoir property, which helps the groundwater double replenishment along the coal seams outcrop and dolerite dike and the closed pressure water condition formation. Moreover, the research shows that there exists much free gas clearly for the closed fault-preventing gas diffusion. So the cokeite growing area is the most advantageous where the high production CBM wells will distribute in the girdle band along the dike. In summary, concerning low-rank CBM exploration, this paper proposes gas evaluation by using CBM and conventional means.

Early Cretaceous Dongbo ophiolite in the western part of the Yarlung Zangbo Suture Zone, Tibet: Constraints on multi-stages from ultra-slow spreading influenced by a mantle plume to intra-oceanic subduction

雅鲁藏布江缝合带西段出露东波和普兰等多个大型的超镁铁岩体，其成因还存在争议。为此我们近年来对东波蛇绿岩开展了地表填图并实施了一口1002m的科学钻探DSD-1。研究显示具有厚幔极薄壳特征的东波蛇绿岩记录了多期岩浆事件：（1）~137Ma的OIB型玄武岩和138~144Ma的OIB型辉绿岩和辉长岩，以及普遍出露熔融程度达30%以上的亏损型方辉橄榄岩，反映了早白垩世初期古洋盆受到地幔柱活动影响；（2）129Ma的薄壳均质辉长岩（I型）、128Ma的辉长岩脉（Ⅱ型）和科钻岩芯中厚18m辉绿岩（Ⅲ型）的地球化学成分均与西南印度洋洋脊玄武岩类似，它们是在慢速-超慢速扩张脊附近大洋核杂岩侵位过程中形成的；（3）地表辉绿岩脉（Ⅳ型）的SIMS和SHRIMP锆石U-Pb年龄为121~123Ma，全岩地球化学具有异常低的REE、HFSE含量及明显的Th、Nb、Zr、Hf负异常，类似于Albanide-Hellenide造山带蛇绿岩中的无壳源物质混染的中钛玄武岩，指示辉绿岩脉形成于洋内初始俯冲环境。总的来说，东波蛇绿岩记录了早白垩世地幔柱影响的超慢速扩张和初始洋内俯冲过程。;The genesis of several large ultramafic massifs outcropped in the western part of the Yarlung Zangbo Suture Zone, such as the Dongbo and Purang massifs, are still controversial. Geological mapping and an 1,002m-length scientific drilling (DSD-1) are carried out on the Dongbo ophiolite in recent years to solve this problem. Our study indicates that the Dongbo ophiolite is characterized by a thick mantle and very thin crust structure which recorded multi-stage magmatic events. The evolution of the major events of this massif can be diveided into three stages as follows: (1) The ~137Ma OIB-like basalt, 138~144Ma OIB-like dolerite and gabbro dikes, and their wall-rock depleted peridotite with a melting degree of more than 30% reflect the influence of mantle plume activity on the ancient ocean basin in the Early Cretaceous; (2) The 129Ma isotropic gabbro (type I), 128Ma gabbro dike (type II) and 18m thick dolerite (type Ⅲ) in the DSD-1drill core have similar geochemical compositions to the Southwest Indian ocean ridge basalts, which were produced during the emplacement of the ocean core complex near the slow-ultraslow spreading ridge; (3) The dolerite dikes (type Ⅳ) dated by SIMS and SHRIMP zircon U-Pb with ages of 121~123Ma have abnormally low contents of rare earth elements and high field-strength elements, and displayed obvious negative Th, Nb, Zr and Hf anomalies in their whole rock geochemical compositions. These features are similar to those of medium-titanium basalts without the addition of subduction components in the Albanide-Hellenide ophiolites, indicating that these dolerite dikes were formed in an initial intra-ocean subduction environment. In a word, the Dongbo ophiolite records an ultra-slow spreading influenced by a mantle plume and initial intra-ocean subduction process during the Early Cretaceous.

Styles of mesoscale brittle deformations associated with the Miocene folding of the Taishu Group, Tsushima, between the Japan Sea and East China Sea backarc basins

AbstractThe Taishu Group is a folded, Eocene–Lower Miocene, thick sedimentary package exposed widely on Tsushima Island between the Japan Sea and East China Sea. This location makes the strata important to understand tectonics and paleo‐environments in the Far East, but the timing of the folding is controversial. We studied the styles of brittle deformations of the strata. It was found that flextural‐slip folds were dominant. Mesoscale faults were classified into two groups: NE–SW trending reverse faults and NW–SE trending strike‐slip faults. Members of both the groups showed movements largely perpendicular to the fold axes. The latter group consisted of sinistral and dextral faults. Accordingly, we interpreted that they were transfer faults activated during the folding. Consequently, mesoscale faults and flexural‐slip faults evidence the map‐scale plane strain of the Taishu Group in the plane perpendicular to the NE‐trending fold axes. There were few transpressional deformations in the group. This is inconsistent with the transpression hypothesis for explaining the simultaneous folding and Japan Sea opening. Another hypothesis in which the folds in Tsushima are regarded as an onshore part of the Taiwan‐Shinji fold belt is inconsistent with the timing of folding suggested by mining geologists to be consistent with and contemporaneous with this deformation. On the other hand, we found that dolerite dikes and sills were involved in the folding. Therefore, we conclude that the folding began during the late Early Miocene time and climaxed during the ore mineralization at around 15 Ma. We suggest that the folding in Tsushima was the easternmost manifestation of the compressional regime around the Yellow Sea and East China Sea in the Early to early Middle Miocene, and that the compression was brought about by the arrival of the Philippine Sea plate to initiate buoyant subduction under Kyushu.

Behavior of TiO2 in Dolerits as a Possible Search Sign for Kimberlites

A generalization of studies of the chemical composition of dolerites of the Vilyui-Markha dike swarm (VMDS) in the Eastern part of the Siberian Platform is carried out. This belt stretches along the North-Western side of the Vilyui paleorift system (Eastern part of the Siberian platform) for almost 700 km. The formation of VMDS intrusives took place in the interval of 372 million years. The Devonian kimberlites, which formed three kimberlite fields: Malo-Botuobin, Nakyn, Suldyukar, are confined spatially to the VMDS. One is in the Southern part of the belt, and the last two are in its Central part. The formation of kimberlite pipes according to K-Ar, 40Ar / 39Ar, Rb-Sr dating is close to 362 Ma. When studying dolerites, it was found that near the kimberlites, the proportion of TiO2 increases in the first (almost 2 times - up to 4-5% versus 2.2-2.5%) and a number of heavy rare-earth elements, such as Th, Hf, Y, Nd. An increase in the content of these elements proceeds gradually from the edge of the kimberlite field to kimberlites and reaches a maximum of 7% (a single case) directly near kimberlites. The authors suggest that the latter is indirectly associated with kimberlites. The solubility of titanium in a basite melt sharply increases with increasing pressure. The kimberlite-forming system is long-lived, associated with diapir-like kimberlite protrusion, which subsequently produces kimberlite pipes. Long before the formation of kimberlites around the diapir, possibly located in the lower horizons of the earth’s crust, excessive stress pressures are created. A basite melt, penetrating through these abnormal (relative to pressure) regions, reacts with an increase in the content of titanium and HREE. Outside the kimberlite field contours, the content of the elements in question in dolerite dikes remains normal, i.e. 2-2.5%. Based on the above stated fact and the study of the geochemistry of dolerites of VMDS, the authors suggest the discovery of two more kimberlite fields within the swarm: Tenkelyakh, located 20-30 km North of the famous Nakyn field, and Orto-Kyulunke, whose location is possible in the North of the VMDS. The authors gave the names of the fields by the names of the rivers flowing through this territory.

Mineralogy and geochemistry of chloritholites from the Nepryakhino gold field, South Urals

The chlorite rocks (chloritolites) exposed in a bedrock of the Mokhovoe boloto (Moss swamp) gold placer (East Uralian Megazone, South Urals), which occurs on ultramafc rocks, are studied. The trace element composition of chloritolites is characterized by elevated contents of Mn, Ti, V (hundreds of ppm), Cu, zn, Ni, Co, Cr, zr, Li, Sc (tens of ppm), w, zr, Y and REE. Chloritolites contain up to 3 vol. % of disseminated magnetite, ilmenite and accessory minerals (rutile, xenotime, monazite, zircon, apatite, scheelite, U-bearing thorite) from a mineral assemblage, which cocrystallize with the main volume of chlorite. The mineralogical and geochemical features of the Mokhovoe boloto chloritolites and gold-bearing chloritolites of the Karabash massif in the Main Uralian Fault zone are slightly similar. The elevated Ti and P contents of the studied chloritolites, the level of REE contents corresponding to mafc rocks, and the lack of relict chromite indicate their possible metasomatic formation after dolerite dikes known within Chebarkul-Kazbai ultramafc complex. Figures 8. Tables 7. References 14.

Geochemistry of Paleozoic Dolerite Dikes in the Northeastern Kola Peninsula and Their Relations to Flood-Basalt and Alkaline Magmatism

The paper presents data on dolerite dikes in the Murmansk block in the northeastern Kola Peninsula. The dikes belong to a Middle Paleozoic flood-basalt association, which is a principal element of the large-scale plume–lithospheric process in the area of the Barents Sea. According to their geological, petrological, and geochemical features, the dolerite dikes are classified into two groups: older and younger, which pertain to the initial and final evolution of the large igneous province produced under the effect of the Barents Sea plume at 450–340 Ma. One of the results of the plume–lithosphere interaction was the origin of the Eastern Barents Basin, which is filled with large volumes of basalts, and this predetermined the further subsidence of the area in Meso-Cenozoic time and the accumulation of thick sedimentary sequences with significant hydrocarbon resources. The peripheries of the area with products of basaltic magmatism are marked by alkaline rock complexes. The largest of the latter are the intrusions of the Kola Province and kimberlites in Archangelsk oblast. The dolerites of the dikes abound in xenogenic grains of zircon, which were dated at 2.7, 1.8, 1.3, 0.7 Ga. These age values suggest that the parental melts may have been derived in the lower crust and that the magma-generating processes may have evolved in the root zone of an ancient rift system.

The first results of isotopic (U-Pb, ID-TIMS) dating of individual zircon grains from dolerite dikes of the Eastern zone of the Urals

Research subject. The article presents the results of dating two dolerite dikes differing in geochemical features from a section along the Iset river in the area of Smolinskoe settlement (the Eastern zone of the Middle Urals). Materials and methods. The dating was performed by an U-Pb ID-TIMS technique for single zircon grains using an artificial 205Pb/235U tracer in the laboratory of geochronology and isotope geochemistry of the Geological Institute of the Kola Science Centre of the Russian Academy of Sciences. The lead isotopic composition and uranium and lead concentrations were measured using a Finnigan-MAT (RPQ) seven-channel mass spectrometer in dynamic mode using a secondary electron multiplier and RPQ quadrupole in ion counting mode. Results. The dikes were dated 330 ± 3 Ma and 240 ± 2 Ma. Conclusions. The research results indicate different ages of dolerite dikes developed within the Eastern zone of the Middle Urals. The oldest of the two established age levels corresponds to the Early Carboniferous era. This fact, along with the proximity of the dolerites to the petrochemical features of the basaltoids of the Early Carboniferous Beklenischevsky volcanic complex, allows these bodies to be considered as hypabyssal comagmates of these volcanics. The youngest obtained age level – Triassic – indicates that the introduction of some dolerite dikes was associated with the final phases of the trapp formation developed rarely within the eastern outskirts of the Urals and widely further east in the foundation (pre-Jurassic basement) of the West-Siberian Plate.

Fragments of Paleoproterozoic Large Igneous Provinces in Northern Fennoscandia: Baddeleyite U–Pb Age Data for Mafic Dykes and Sills

New data on the age of dolerite dikes in the NE part of the Kola province of the Fennoscandinavian shield and the picrodolerite sills that cut the dikes are presented. The results of U–Pb ID-TIMS baddeleyite dating indicate that dolerites were formed between 2508 ± 6 and 2513 ± 16 Ma ago, simultaneously with the intrusions of the Monchegorsk group. A comparison of the composition of the dolerites studied with dykes of the same age found in other Archean cratons shows their significant similarity and suggests their formation at the same large magmatic province. The age of baddeleyite from the picrodolerites sills at 2403 ± 12 Ma ago indicates an event of basic magmatism that was not previously established in this part of the Fennoscandinavian shield. It is possible that, along with dolerite dykes with an age of 2405 Ma and komatiites of the Vetreny belt of the Karelian craton, sills of the Kola province are a component of a unified large magmatic event.