Dolerite Dikes Research Articles

Dike-induced aquifer models derived from high-resolution multi-spectral satellite imagery

The Main Karoo Basin in South Africa is a typical example of an expanding arid region dependent on groundwater resources. Dolerite dikes in the region, analogous to dolerite dikes worldwide, are known to influence subsurface groundwater flow and spatially relate to high-yielding boreholes. Here, the effect of dolerite dikes on groundwater flow is remotely assessed using the Modified Soil Adjusted Vegetation Index derived from high-resolution multi-spectral satellite imagery. From imagery collected during the wet and dry seasons of 2018 and 2021, two aquifer models relating to 505 dikes were identified; (1) barrier-controlled aquifers are induced by ~ 56% of dikes, (2) fractured aquifers are induced by ~ 35% of dikes. Surficial areas overlying aquifers are also shown to sustain vegetation growth through dry seasons. This research demonstrates the efficacy of vegetation indices to rapidly characterise dike-related aquifer models and their seasonal sustainability, critical for effective groundwater exploration and management.

The First Discovery of Archean Dolerite Dikes in the Western Part of the Aldan Shield

Dolerite dikes were studied in the western part of the Aldan terrane, in the middle reaches of the Tokko River. These dolerite dikes form a swarm of submeridional trend about 1 km wide. The dolerites of the thickest dike preserve their primary textural and structural features and mineral composition: plagioclase + pigeonite + augite + titanomagnetite. Dolerite in the chilled margins and central parts of the dike are homogeneous in composition, corresponds to low-Mg tholeiites, has low contents of Ti and other HFSE, with weak enrichment in light REE and small negative Nb anomalies. Sm–Nd isotope data on magmatic minerals of dolerite from the central part of the dike yield a good linear regression in an isochron diagram that gives to an age of 2510 ± 64 Ma, which probably corresponds to the crystallization age of the basalt. Metadolerites in a thin dike retain plagioclase porphyritic structures, but the pyroxenes are completely replaced by amphibole and chlorite. The metadolerites are contrastingly different in low contents of MgO, Cr, and Ni and in higher contents of TiO2, Fe2O3, P2O5, Nb, and all REE. The differences in the composition of the dikes may be explained by the longterm (about 65%) crystallization differentiation of the initial melt and the emplacement of the residual melt from a shallow intermediate magma chamber via opening cracks. Such conditions probably may have existed in tectonically stable intraplate settings. The age of the dolerites of the dike swarm is comparable to that of the anorogenic granites of the Nelyuki complex (~2.4–2.5 Ga), which are widespread in the western part of Aldan granulite–gneiss terrane. Our data bridge some gaps in characteristics of intraplate anorogenic magmatism that occurred in the western Aldan Shield in the Late Archean and marked the final consolidation of a large block of Archean crust in the Chara–Olekma granite–greenstone area.

The First Discovery of Archean Dolerite Dikes in the Western Part of the Aldan Shield

The First Discovery of Archean Dolerite Dikes in the Western Part of the Aldan Shield

Early Cretaceous Metasomatized Lithospheric Mantle beneath the Central Jiangnan Orogen in South China: Geochemical and Sr‐Nd Isotope Evidence from the Tuanshanbei Dolerite

AbstractIt is well established that Cretaceous magmatism in the South China Block (SCB) is related to the Paleo‐Pacific subduction. However, the starting time and the associated deep crust‐mantle processes are still debatable. Mafic dike swarms carry important information on the deep earth (including mantle) geodynamics and geochemical evolution. In the Jiangnan Orogen (South China), there is no information on whether the Mesozoic magmatic activities in this region are also directly related to the Pacific subduction or not. In this study, we present detailed zircon U‐Pb geochronological, whole‐rock element and Sr‐Nd isotope data for Early Cretaceous Tuanshanbei dolerite dikes, and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time. LA‐ICP‐MS zircon U‐Pb dating suggests that this dolerite erupted in the Early Cretaceous (∼145 Ma). All samples have alkaline geochemical affinities with K2O + Na2O = 3.11–4.04 wt%, K2O/Na2O = 0.50–0.72, and Mg# = 62.24–65.13. They are enriched in LILE but depleted in HFSE with higher initial 87Sr/86Sr ratio (0.706896–0.714743) and lower εNd(t) (–2.61 to –1.67). They have high Nb/U, Nb/La, La/Sm and Rb/Sr, and low La/Nb, La/Ta, Ce/Pb, Ba/Rb, Tb/Yb and Gd/Yb ratios. Such geochemical signatures suggest that the fractional crystallization is obvious but crustal contamination play a negligible role during magmatic evolution. Tuanshanbei dolerite were most likely derived from low‐degree (2%–5%) partial melting of a phlogopite‐bearing mantle material consisted of ∼85% spinel peridotite and ∼15% garnet peridotite previously metasomatized by asthenosphere‐derived fluids/melts with minor subduction‐derived fluids/melts. Slab‐rollback generally lead to the upwelling of the hot asthenosphere. The upwelling of asthenosphere consuming the lithospheric mantle by thermo‐mechanical‐chemical erosion. The lithospheric mantle may have partially melted due to the heating by the upwelling asthenosphere and lithospheric extension. It is inferred that the Tuanshanbei dolerite might be associated with the initial slab rollback and corresponding lithospheric extension occurred potentially at ca. 145 Ma.

Localization of High-Titanium Dolerites in Kimberlite Fields: Possible Causes and a New Criterion for Kimberlite Search

Abstract ––The generalized chemical composition data on the studied dolerites from the Vilyui–Markha dike swarm (Vilyui paleorift) allowed detecting geochemically anomalous areas among them. It has been found that percentage of TiO2 and a number of heavy rare-earth elements (Th, Ta, Hf, Y, Nd) in the dolerites of dikes located within the kimberlite fields show almost twofold increase compared to the normal content for these elements in the Vilyui–Markha dolerites. Similar behavior of these elements is also observed in the dolerite dikes near the Kuoika kimberlite field (the Molodo dike swarm, Olenek paleorift). It is inferred that there is a connection between the increase in abundances of titanium and rare-earth elements in the dolerites and that the latter occur within the lithosphere blocks hosting kimberlite fields. The areas of lithospheric mantle roots linked with the formation of proto-kimberlites may have had bearing on the composition of tholeiite melt both in the processes of its generation and during its ascent to the Earth’s surface, which led to its local enrichment in titanium and a number of other elements. While the processes of the alignment of the geochemical composition between the high-titanium basites and the rest of the melt beyond the contours of the lithospheric blocks with kimberlite fields were weakly pronounced, they were responsible for appearance of zones enriched in titanium. Kimberlite fields that formed before or after basite magmas intrusion overlap with placements of high-titanium dolerite dikes. Thus, high-titanium dolerites in dike swarms (belts) can be used as one of the criteria for kimberlite searches. In view of the above, two new potential zones (Tenkelyakh and Kyulenke areas) for kimberlite exploration have been identified within the Vilyui–Markha dike swarm.

Two-Stage Model of Devonian Basic Magmatism in the Vilyui Paleorift (Siberian Platform)

Abstract —We report study results of basic intrusive bodies in the middle Paleozoic Vilyui paleorift (eastern Siberian Platform). Geochemical data for basic sills penetrated by boreholes in the rift’s dike swarms are presented, as well as our data on the time of formation of sills and dikes. We also studied mineral-hosted melt inclusions from a dolerite dike of the Vilyui–Markha dike swarm on the northwestern flank of the Vilyui paleorift. Data on the compositions of homogenous glasses of mineral-hosted melt inclusions yielded the P–T parameters of mantle sources of basic melts responsible for the formation of the Vilyui paleorift gabbro-dolerites. Two depth levels of basic melt generation have been established: 95–65 km at 1480–1400 °C and 55–45 km at 1360–1320 °C. Crystallization of the melts occurred at a shallow depth of 12–4 km with a decrease in temperature from 1185 to 1125 ºС. This occurrence of basic melt sources at two different depths, as well as the heterogeneity of the chemical composition of melt inclusions and their host minerals explains the presence of two pulses of Devonian basic magmatism. Our new numerical thermomechanical model of magma rise during melting of the lithospheric mantle above a mantle plume supports the existence of two chambers at the spinel–garnet peridotite boundary and under the base of the crust, as well as the two-stage nature of Devonian magmatism.

ARKOSE SANDSTONES OF THE KEBEKTA GROUP OF THE UGUI GRABEN (ALDAN SHIELD): AGE, SOURCES, AND DEPOSITION SETTING

The paper presents the results of U-Pb (ID-TIMS) and U–Th–Pb (LA-ICP-MS) geochronological studies of baddeleyite from dolerite dikes of the Sulumat Complex and detrital zircon from red arkose sandstones of the Kebekta Group of the Ugui graben, the Aldan Shield. It has been established that the Kebekta Group have an age of 2.011.87 Ga. The sources of the Kebekta Group deposits were the Archean (2.92–2.52 Ga) igneous and metamorphic complexes of the Chara-Olekma geoblock and the zone of its junction with the West Aldan megablock of the Aldan Shield, as well as unidentified on the present erosion level the Paleoproterozoic (ca. 2.01 Ga) complexes. The obtained geochronological data do not contradict the correlation of terrigenous rocks of the Kebekta Group of the Ugui graben and the Kemen Group of the Kodaro-Udokan trough. The deposition of arkose sandstones of the Kebekta Group correlates in age with the glaciations in Australia and Sweden (ca. 1.9–1.8 Ga) associated with the formation of the Columbia/Nuna supercontinent. It also cannot be ruled out that the deposition of the rocks of the Ugui graben was related to the inflow of glacial material into the intracontinental extensional basin from the southern and/or western (in modern coordinates) framing of the Chara-Olekma geoblock at the stage of collapse of the Paleoproterozoic orogen.

An Assessment of the Impact of the Mining Industry on Soil and Plant Contamination by Potentially Toxic Elements in Boreal Forests

This study was conducted in the territory of the industrial site of the Udachny Mining and Processing Division (Yakutia, northeast Russia). The objects of study were permafrost soils and two species of shrubs (Betula middendorffii T. and Duschekia fruticose R.). Soil and plant samples were analyzed using atomic absorption spectrometry for the presence of PTEs (Pb, Ni, Mn, Cd, Co, Cr, Zn, Cu, and As). The bioaccumulation factor (BAF), frequency of occurrence (Hi), pollution index (PI), and pollution load index (PLI) were calculated. The PI and PLI are calculated for both soil and two plant species for the first time in this study. The results showed that the soils have a high Ni, Cr, Co, As, and Mn content. It has been established that high soil pollution naturally leads to an increase in the concentration of elements in the leaves of shrubs. The soils and vegetation are dominated by elements associated with trap magmatism—Cr, Co, Cu, and dolerite dikes—Mn and Zn. For Betula middendorffii, the PLI was classified as unpolluted to moderately polluted, and Duschekia fruticosa. was classified as unpolluted. The high level of contamination is typical for areas located near industrial sites, such as waste dumps, kimberlite pipes, tailings ponds, and roads. The BAF results confirmed that the leaves of Betula middendorffii are able to accumulate more PTEs and have the highest level of resistance to PTE contamination in mining environments. This analysis showed that the consistent application of the PI, PLI, and BAF indices is very efficient in the ecological and biogeochemical assessment of the situation in industrial development areas.

FORMATION CONDITIONS OF THE DEVONIAN BASITES OF THE VILYUI-MARKHA DIKE SWARM OF THE VILYUI PALEORIFFT (SIBERIAN PLATFORM)

Melt inclusions in the minerals of a sample from a dolerite dike of the Vilyui-Markha dike swarm, located on the northwestern shoulder of the Vilyui paleorift, have been studied. Data on the composition of homogeneous glasses of melt inclusions in minerals made it possible to establish the PT-parameters of mantle sources of initial deep melts responsible for the formation of gabbro-dolerites of the Vilyui paleorift. Two levels of generation of initial melts for dolerites have been established: in the intervals of 95–65 km at 1480–1400°C and 55–45 km at 1360–1320°C. The presence of two intermediate chambers in the crust, where melts crystallized, was identified at depths of about 12 and 9–4 km with a decrease in temperature to 1175–1125°С. The two-level arrangement of magma generation centers, as well as the heterogeneity of the chemical composition of the studied minerals, makes it possible to explain the presence of two pulses of Devonian basic magmatism.

Association of Fe–Ti oxides in subalkaline dolerites of the Rai-Iz massif (Polar Urals)

The relevance of the research. Subalkaline dolerites, Fe–Ti oxides of which were studied in this work, occur within the largest deposits of chromium ores of the Rai-Iz massif. The study of the material composition and age of these rocks was carried out by the authors for the first time. The purpose of the research is to reveal the patterns of changes in the chemical composition of the main accessory minerals of rocks – titanomagnetite and ilmenite, and on the basis of the obtained data to evaluate the conditions for the formation of subalkaline dolerites of the massif. Research methodology. The chemical compositions of minerals were determined using microprobe analysis on a CAMECA SX 100 and scanning electron microscopy on a Jeol JSM-6390LV setup with an INCA Energy 450 X-Max 80 energy dispersive attachment. Raman spectra were obtained on a LabRam HR 800 Evolution spectrometer equipped with an Olimpus BX-FM microscope. The studies were carried out at the “Geoanalyst” сenter for the collective use of scientific equipment of the IGG, UB RAS. Results. The typomorphism of the chemical composition of titanomagnetite and ilmenite has been established for petrographic varieties of dolerites allocated on the massif. The T–fO2 parameters of dolerite formation were estimated and compared with metaultramafites. It is shown that titanomagnetite in amphibole dolerites undergoes maghemitization. The highest temperatures (1076–1126 °C) are recorded in pyroxene dolerites. The oxygen fugacity in these rocks is FMQ +0.6–+1 units. Hyalodolerite from the chilled margin of the pyroxene dolerite dike gives a temperature of 811–818 °C and an oxygen fugacity of 1.8 units above FMQ. The compositions of the associated ilmenite and titanomagnetite from amphibole dolerites correspond to fO2 = –2–0 units relative to FMQ and temperature 580–720 °C. The trend of change in T–fO2 parameters shows that the injection of the mafic melt occurred under conditions of metamorphism of ore-bearing ultramafic rocks. Conclusions. The formation of dolerite veins occurred under conditions of ultramafic rock metamorphism. The increased fugacity of oxygen at a relatively low temperature led to the maghemitization of titanomagnetite, rather than to the redistribution of components in the magnetite-ilmenite paragenesis.

The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran

Abstract Several swarms of parallel E–W dikes cuts late Neoproterozoic- to early Paleozoic (540 Ma) basement in the Biarjmand area, NE Iran. The microgabbroic to doleritic dike are several hundred meters long and 0.5 to 3 meters wide. U–Pb dating of their zircon grains yields a crystallization age of 46.0 ± 7.1 Ma for the dike swarms. The dikes have SiO2 contents of 46.0–49.3 wt%, and Fe2O3 contents of 9.80–14.8 wt% with variable MgO (4.92–9.16 wt%), TiO2 (1.47–2.65 wt%), and K2O contents (0.33–1.13 wt%). The dikes have low contents of high field strength elements (HFSEs), and have some similarities to transitional mid-ocean ridge basalts (T-MORB) based on their chemical composition. Positive ${\varepsilon}_{Nd}(t)$ values of +5.2 to +6.0 for the dike samples imply that the basaltic magma was generated from depleted lithospheric mantle. After the closure of the Sabzevar ocean and obduction of the large mantle peridotite body over the continental crust in the late Cretaceous-Paleocene, the gravitational instability in the central Iran/Eurasia plate collision zone triggered exhumation of old basement coeval with injection of the mafic magma. During exhumation, mylonitic deformation with brittle-ductile structures affected the basement metagranite and the host rocks. The metagranite and dike swarms cut the Cretaceous Sabzevar ophiolite, and the disparting of ophiolite members probably occurred during basement exhumation in the Cenozoic. The similar ages of the mafic dike swarms and other Eocene magmatic rocks of the Urumiah (Urmia) Dokhtar magmatic arc (UDMA), western Iran, is consistent with a rapid extensional regime over the Sabzevar suture zone on an earlier collision zone in NE Iran. This process provided a suitable setting for the exhumation of the old basement, the emplacement of the parallel dike swarm, and the development of shallow basins in this area.

Geochemical Criteria for the Classification of Dolerite Dikes in the Central Part of the Baikal Basement Inlier of the Siberian Craton

The paper presents detailed geological, petrographic, geochemical, and isotope data on the dolerites from dykes of the Baikal dike subswarm in the central part of the Baikal basement inlier of the Siberian craton. The main geochemical and isotope criteria were identified for classifying the dolerites that are similar in geological–structural position and mineral composition into three geochemical groups. Provisional age estimates of the dolerites are presented. The dolerites of the first and, perhaps, second groups compose Neoproterozoic (715 Ma) dikes. The first group includes medium- to coarse-grained dolerites, which form thick (more than 5–10 m) dikes. The dolerites of this group are characterized by low concentrations of Th (0.6–2.1 ppm) and Nb (3.3–9.2 ppm) and by ɛNd(T) = ‒0.5 to ‒3.9. Considered together, the geochemical and isotope data show that these dolerites could be produced as a result of the melting of a mantle source produced by mixing mantle components close in composition to oceanic plateau basalts and subduction-related subcontinental lithospheric mantle. The second group includes fine-grained dolerites from thin (1–5 m) dikes, including dikes located in contact with the dikes of the first group. The dolerites of the second group are characterized by higher concentrations of Th (3.0–5.3 ppm) and Nb (9.8–21.1 ppm) and by ɛNd(T) = –5.3 to ‒6.0. Geochemical and isotopic data on the dolerites of the second group indicate that continental crustal material was added to the mantle source unified for the dolerites of first and second groups. The third group includes medium-grained dolerites, which make up individual dikes that are contrastingly different in geochemical and isotope characteristics from the Neoproterozoic dolerites of the first and second groups. Dolerites of this group show low concentrations of Th (0.6–1.6 ppm) and Nb (2.7–5.1 ppm) at low values of (Th/La)pm (0.29–0.71) and 143Nd/144Nd (0.511223‒0.511544), which indicates that these dolerites may have been generated as a result of the melting of the subcontinental lithospheric mantle enriched in subduction components. Dolerites of the third group exhibit geochemical characteristics close to those of Paleoproterozoic (1.84 Ga) dikes of the South Siberian post-collisional magmatic belt.

Geochemical Criteria for the Classification of Dolerite Dikes in the Central Part of the Baikal Basement Inlier of the Siberian Craton

Geochemical Criteria for the Classification of Dolerite Dikes in the Central Part of the Baikal Basement Inlier of the Siberian Craton

Organic geochemical and petrographic characteristics of the Cambrian-Ordovician organic-rich marine shales in Scandinavia

This work used organic geochemistry and organic petrology to study the depositional environment, organic matter characteristics, and thermal maturity of the Cambrian-Ordovician organic-rich marine shales in the Baltic Basin. The main macerals in Cambrian samples include alginite, bituminite and solid bitumen, while zooclastic macerals become the major proportion of organic matter in the Ordovician samples. As the maturity increase, solid bitumen becomes abundant and dispersed. Semifusinite-like maceral was observed only in Furongian of well DBH15/73, which probably indicates the local intrusion of Permo-Carboniferous dolerite dikes. The programmed pyrolysis results show that immature, early mature, and over-mature samples are developed. However, the data of high-uranium Furongian samples were greatly altered by igneous intrusives from local dikes. HIo calculation model is simulated based on pyrolysis data and fitted according to the least-square fitting method. The simulated fitting HIo: 400 mg HC/g TOC (369.5 mg HC/g TOC, 430.5 mg HC/g TOC as 95% confidence bounds) is within the worldwide marine shale HIo and indicates a marine anoxia and transgressive sea with shallow water column (organofacies B).

Geological processes defining the formation of plumasite-type corundum in the Paleoproterozoic Isertoq Terrane, South-East Greenland

Plumasite-type corundum occurrences in the Nattivit area in South-East Greenland offer a unique opportunity to study corundum formation in-situ where pegmatites intruded into metamorphosed lherzolite and dunite of the Archean-Paleoproterozoic continental crust. The Nattivit area, located in the Isertoq Terrane of North Atlantic Craton, forms part of the overriding plate during convergence of the Nagssugtoqidian orogen (1910-1840 Ma). New field observations and elemental and isotopic geochemical analysis provide further insights in the history of crustal convergence, its exhumation and how corundum was formed. The continental crust in the area consists of metamorphosed mafic to ultramafic rocks and tonalite-trondhjemite-granodiorite (TTG) gneisses, where the mafic rocks in the Isertoq Terrane yield a εNd TDM model age of 3000–2800 Ma. Dunite and lherzolite sills/dikes intruded the mafic rocks before the intrusion of the TTG sheets. The intrusion ages for the TTG obtained from zircon U-Pb geochronology are 2818 ± 8 Ma, 2760 ± 13 Ma to 2667 ± 7 Ma. U-Pb zircon data, zircon textures and Th/U ratios indicate metamorphism occurred at 2698 ± 7 Ma to 2629 ± 11 Ma, 2500–2400 Ma and 1900–1600 Ma. Whole rock geochemical data of mafic to ultramafic rocks show a continental arc affinity, with negative Ta, Nb and positive Pb anomalies. A metasomatic event at 2390 ± 70 Ma partly reset the isotopic signature in the mafic to ultramafic rocks. A marked absence of ages between 2350 and 2100 Ma in the TTG zircon age populations exists, indicating a period with minimal magmatic and/or metamorphic activity. The metamorphic mineral assemblages of the schist, amphibolite, ultramafic rocks and metasomatic reaction zones in ultramafic rocks indicate upper to medium–high amphibolite facies conditions. Kyanite in the metasomatic reaction zones in ultramafic rocks indicate the higher end of the temperature and pressure range above 4.2–10 kbar and 530–800 °C, similar to estimates from dolerite dikes in the Kitak area. The syn-tectonic pegmatites with an intrusion age of 1843 ± 4 Ma formed corundum. The new data indicate that the pegmatite melt/fluid and the geotectonic setting are defining factors for generating plumasite-type corundum.

MARBLE DIKES IN THE OLKHON COMPOSITE TERRANE (WEST BAIKAL AREA)

Linear or lens-like carbonate (marble) and carbonate-silicate bodies among gabbro and amphibolites within the Krestovsky subterrane of the Olkhon composite terrane (West Baikal Area) are identified as dikes. The dikes commonly dip almost vertically, range in thickness from 20 cm to a few meters, and are up to 100 m long. The Olkhon marble dikes quite often coexist with dolerite dikes and/or granite veins and show signatures of emplacement synchronously with the igneous bodies. The marble dikes differ from mantle carbonatites in mineralogy and chemistry and thus may be derived from sedimentary carbonate rocks molten during collisional events. The origin of the Olkhon carbonate and carbonate-silicate dikes may be explained with two possible geodynamic scenarios. They may be derived either from Neoproterozoic carbonate sediments upon the Early Precambrian basement of a cratonic block which was involved in collisional events, or from abundant carbonate sedimentary material in an island-arc terrane. Large-scale melting of silicate and carbonate rocks was maintained by heat released from mantle mafic magma intruding into the lower crust. The batches of both crustal (carbonate and granitic) and mantle (mafic) melts intruded late during the collision in a strike-slip tectonic setting.

Gold potential of the upper devonian basal conglomerates of the Kanin peninsula (Nenets autonomous district, Russia)

Native gold and garnets are described for the frst time in heavy concentrates from coarse-grained sediments of the Upper Devonian Tayaokuyakha Formation, which occurs on the eroded Upper Riphean schists with angular and azimuthal unconformity (middle reaches of the Murseyakha River, Kanin Peninsula, Nenets autonomous district, Russia). Particular attention was paid to indicators of primary source of native gold for the Upper Devonian conglomerates. The composition of weakly rounded almandine-spessartine garnets indicates their metamorphic origin and allows us to relate the primary source of native gold in conglomerates (D3tk) with underlying metamorphosed Riphean strata or intrusive rocks in them. Native gold is mainly characterized by a heterogeneous composition and contains Ag, Cu and Hg. The poorly rounded native gold grains, the absence of supergene rims and preservation of mineral inclusions in the margins indicate its proximal primary source. The presence of Cu suggests a link of native gold with mafc/ultramafc rocks: metamorphosed dikes of dolerites of the Nekhaiteyakha metagabbroid complex (m??RF3) or dolerite dikes of the Kanin-Timan complex (?D3kt), which intrude the Precambrian metamorphic schists. It is established that the Devonian conglomerates are intermediate reservoirs for the possible present-day placer formation.

Crust sequences of the Xigaze ophiolite in South Tibet: The water effect on crust accretion at a slow-spreading oceanic ridge and its implications

The tectonic setting of the Xigaze ophiolite of southern Tibet has been debated ever since it was first described 1981. It lies in the central part of the Yarlung Zangbo but differs from adjacent ophiolite by having a relatively thin crustal section, an oceanic core complex and complex geochemistry. It is overlain by radiolarian cherts suggesting and open ocean environment but also show geochemical evidence of suprasubduction zone magmatic activity. Our detailed study of this massif shows that the crustal sequence was formed by three stages of magma injection derived from a common parental source. Stage 1 is represented by heavily altered and deformed gabbros that form dikes and pods in the upper part of the peridotites, whereas stage 2 includes dolerite sills and extruded basalts, which form >80% of the crustal section. Finally, stage 3 is composed of dolerite dikes and gabbro intrusions, which crosscut rocks of stage 1 and 2 at high angles. The youngest rocks of stage 3 are plagioclase wehrlite intrusions, which are characterized by having anomalously high calcium plagioclase (An = 99–100) which was preceded by clinopyroxene. We suggest that the multiple magma injection were formed during tectonic extension and magma-fluid interaction at a slow-spreading rift in a distal fore-arc environment during the Early Cretaceous. Our study implies that caution must be exercised in applying subduction initiation models to ophiolite genesis, particularly when the geochemistry of the rocks contradicts the field observations and stratigraphic evidence. • Provides new information on the crustal structure of slow-spreading ridges. • Detachment faults served as fluid conduits changing the fractional crystallization. • Geochemical changes do not necessarily reflect the change of tectonic setting. • Both ridge spreading and subduction dynamics were active in a SSZ environment.

Episodic bimodal magmatism in Central Inner Mongolia, China: Insights from geochronological, geochemical, and Sr-Nd isotopic evidence

This study combines zircon U-Pb ages, whole-rock major and trace elements, and Sr-Nd isotope data to investigate the petrogenesis and tectonic implications of late Paleozoic magmatic rocks in central Inner Mongolia of the southeastern Central Asian Orogenic Belt. The results indicate the presence of two stages of bimodal magmatism from the late Carboniferous to early Permian. The late Carboniferous (ca. 323 Ma) bimodal associations consist mainly of gabbroic and granitic intrusive rocks, whereas the early Permian (ca. 273 Ma) bimodal suites are dominated by dolerite and granite porphyry dikes. The gabbroic intrusive rocks and dolerite dikes are calc-alkaline, slightly enriched in light rare earth elements and depleted in Nb, Ta, Ti, and have low initial 87 Sr/ 86 Sr ratios (0.70317–0.70510) and high ɛ Nd (t) values (+3.1 to +7.4). They are both interpreted to result from partial melting of a depleted spinel-lherzolite mantle source that was metasomatized by subduction-related components. The granitic intrusive rocks show I-type granite affinity and similar geochemical features to coeval gabbroic rocks, suggesting they were derived from a melt-induced anatexis of basaltic lower crust. The early Permian granite porphyry dikes show characteristics of LREE enrichment, negative Nb-Ta anomalies and positive Zr-Hf anomalies. They have higher La/Yb and Th/Zr ratios and ɛ Nd (t) values, and lower initial 87 Sr/ 86 Sr ratios than the coeval dolerite dikes. These features suggest that the early Permian felsic dikes were most likely generated by partial melting of a juvenile lower crust mixing with minor mantle-derived components. An integrated study with regional geological data suggests that the episodic bimodal suites in central Inner Mongolia formed in a post-collisional to within-plate rift setting after the closure of the Paleo-Asian Ocean. • Two stages of bimodal magmatism are identified in central Inner Mongolia. • The mafic rocks were derived from a depleted spinel-lherzolite mantle source. • The felsic rocks show a compositional variation from calc-alkaline to alkaline. • The bimodal suites formed in a post-collisional to within-plate rift setting.

Новые 40Ar/39Ar-данные долеритов Канино-Тиманской провинции внутриплитного магматизма (п-ов Канин)

The Kanin-Timan region is located in the north-east of the European part of Russia and represents the western edge of the Timan-Pechora Plate. In the Devonian time, the Kanin-Timan region experienced rifting processes caused by the influence of a mantle plume. As a result of these processes, the Kanin-Timan dolerite complex was formed, which includes dolerite dikes and basalt sheets. The rocks of the complex have been studied throughout the study region. In the Middle Timan, the basalt plagioclase of the Upper Vorykva cover was dated by the 40Ar/39Ar method, the dating result indicates an age of 389 ± 6 Ma. Dolerites of the Kanin-Timan complex were also dated by the K-Ar method in the last century. As a result of dating, conflicting data on the absolute age from 378 to 288 Ma were obtained. To clarify the age of dolerites, 40Ar/39Ar dating was carried out. According to the isotope age data, the dolerites of the Kanin Peninsula are 419 ± 8 Ma, the age of the basalts of the Upper Vorykva cover is 389 ± 6 Ma. Thus, the magmatic activity that led to the formation of the Kanin-Timan complex in the Kanin-Timan region began earlier on the Kanin Peninsula, and then on the Timan. According to the research results, we believe that the Kanin-Timan complex began its formation in the Early Devonian with the peak during the Middle and Late Devonian time.