Dyke Rocks Research Articles

Petrography and Rb-Sr mineral age of mafic dyke rock from Niban Iwa, Lützow-Holm Complex, East Antarctica

Petrography and Rb-Sr mineral age of mafic dyke rock from Niban Iwa, Lützow-Holm Complex, East Antarctica

LATE CENOZOIC MAGMATISM ON THE WILHELM ARCHIPELAGO, GRAHAM COAST OF THE ANTARCTIC PENINSULA

Manifestations of the recent magmatism were discovered in the Wilhelm Archipelago near the Ukrainian research station in the West Antarctica. There are small subvolcanic dykes that intrude Paleocene granitoids on the Barchans, Forge, Booth and Dannebrog Islands. The authors studied the occurrence, petrography and geochemistry of the dyke rocks in order to find out the peculiarities of their formation. The studied dykes are typical post-plutonic fissure intrusions injected in the host granitoids after their complete consolidation and cooling. Moreover, the host granitoids were not only cooled before the dyking but they were also exhumed at the beginning of the Neogene due to of the processes of tectonic uplift and erosion. Field observation and some other features point out to the Late Neogene or Quaternary age of the dykes. Their intrusion occurred at the shallow depths. During formation, at least part of the dykes was connected with the earth’s surface. So, they were conduits for fissured volcanic eruptions. The geochemical features of the most widespread basaltic dykes call into question their genetic connection with quantitatively subordinate dykes of andesitic and dacitic composition. They probably had different sources of magma generation that were related to different geotectonic processes. Andesitic and dacitic dykes may represent the final outburst of subduction-related calc-alkaline magmatism. On the other hand, the connection between subduction processes and Late Cenozoic basaltic dyking is not obvious and requires additional research.

Occurrences and chemical compositions of ultrapotassic mafic dyke rocks from Skallevikshalsen and Rundvågshetta, Lützow-Holm Complex, East Antarctica

Occurrences and chemical compositions of ultrapotassic mafic dyke rocks from Skallevikshalsen and Rundvågshetta, Lützow-Holm Complex, East Antarctica

The Kalba batholith dyke swarms (Eastern Kazakhstan): Mafic magmas effect on granite formation

The Kalba granitoid batholith formed in Early Permian is one of the largest in the western part of the Central Asian Orogenic Belt. Batholith granites are intruded by mafic and intemediate dykes forming several swarms. Previously it was believed that the dykes were Mesozoic and had no relationship with the formation of the batholith. Our geochronological studies allow to determine the age of the dykes at 279 Ma (also Early Permian). Dyke rocks are represented by dolerites, variolites, monzodiorites, and plagioclase porphyrites. The petrographic study of plagioclase porphyrites allow to find the sign of magma mixing. The another indicator of magma mixing are the contacts between mafic and felsic rocks in largest dykes of complex structure. A model for the interaction of mafic magmas with crustal substrates is proposed. Mafic magmas intruded from sublithospheric mantle, provoked a partial melting and then interacted with granitic melts at the basement level of granite batholith. Dyke swarms were formed after the cooling and consolidation of main volume of Kalba batholith by inrtusion of mafic and hybride magmas from the basement level of granite batholith. Chemical diversity of the mafic dykes was caused mainly by the differentiation of parental mafic magmas and also a lesser extent by their interaction with the granitoid magmas. Comparison with data on coeval magmatism in neighboring regions suggests that the Kalba batholith may be included in areal of Early Permian Tarim Large Igneous Province. • The dyke swarms cut the granites of the Kalba batholith in Eastern Kazakhstan. • The dykes and the Kalba batholith granites are Early Permian. • Dyke rocks composition was caused mainly by the differentiation of mafic magmas. • Parent mafic magmas formed in lithospheric mantle and provoked melting in crust. • Geodynamic setting was a post-orogenic extension with effect of Tarim mantle plume.

Secular evolution of the subcontinental lithospheric mantle beneath Indian cratons: Insights from geochemistry and geochronology of the Precambrian mafic dykes

The Indian Shield is comprised of the Dharwar, Bastar, Singhbhum, Bundelkhand, and Aravalli Cratons, intruded by distinct mafic dyke swarms of different generations (ca. 2.8–0.8 Ga). Most of these dykes are tholeiitic basalt to basaltic-andesite, including boninite. Some other subordinate dyke rocks are of komatiitic, picritic, and andesitic compositions. The vast areal extent of these dykes indicates that they are remnants of Precambrian Large Igneous Provinces (LIPs). The present study reviews the existing geochronological and geochemical data of various Precambrian mafic dyke swarms intruding in all Indian cratons, to track temporal changes in composition of the underlying subcontinental lithospheric mantle (SCLM). Most of these dykes have crust-like abundances of incompatible trace-element. Even the primitive dykes (Mg# = 82–64) exhibit crust-like incompatible element patterns. However, some also have depleted mantle-like abundances of incompatible elements. Most of the dykes also have high concentrations of compatible trace elements (e.g., Cr and Ni). The age-corrected radiogenic Nd isotope compositions (ɛ Nd(i) ) of these dykes vary between the upper continental crust and depleted mantle ɛ Nd growth arrays. The elemental composition and ɛ Nd(i) of these dykes suggest their derivation from a heterogeneous SCLM comprising enriched (metasomatized) and depleted mantle components. The negative as well as positive ɛ Nd(i) values exhibited by the ca. 2.8 Ga (oldest) dykes suggest the presence of enriched mantle materials distributed within a depleted-SCLM already by ca. 2.8 Ga. The enrichment must have occurred before ca. 2.8 Ga due to subduction-released fluids. Later, the SCLM beneath Indian cratons evolved with a composition consisting of an enriched/metasomatized mantle component and a depleted mantle component. • Geochemistry of 2.8–0.8 Ga mafic dyke swarms of Indian cratons have been reviewed. • They originated from heterogeneous SCLM containing enriched & depleted components. • Subduction-driven metasomatism of global SCLM occurred during the Archean.

Zircon SHRIMP U-Pb Dating and Significance From Weathered Residual Kaolin Deposits on the Northern Margin of the Qinzhou-Hangzhou Suture Zone, China

Weathered residual Kaolin deposits on the northern margin of the Qinzhou-Hangzhou Suture Zone are characterized by more feldspar-containing minerals and fewer dark minerals. These deposits have attracted increasing attention because of their high whiteness, low harmful impurity content, low depth of weathered mineralization, and ease for mining. The ore-forming rock masses mainly include granite (granite porphyry), quartz porphyry, eurite, haplite, and other acidic veins; the ore-bearing walls are mainly of Upper Proterozoic. Previous studies suggested that dyke rocks related to Kaolin deposits in this belt belong to the Mesozoic, but there is a lack of information regarding specific mineralization ages. In this study, two dyke wall-type weathered residual Kaolin deposits were selected from the northern margin of the Qinzhou-Hangzhou Suture Zone (Dongxicun and Yinbeidong, Jiangxi Province, China) for zircon SHRIMP dating. Two zircon U-Pb-based ages were obtained: 843 ± 10 Ma from Dongxicun granite veins that intrude into the Neoproterozoic Jiuling rock masses; 152.3 ± 1.8 Ma for Yinbeidong granite porphyry veins that intrude into the Neoproterozoic Anlelin Formation. We suggest that the dykes related to the Kaolin deposits were formed in the Neoproterozoic and late Jurassic. The Neoproterozoic dyke may be related to the convergence, formation, or pyrolysis of the Rodinia Supercontinent and the late Jurassic dyke may be related to the subduction of the Paleo-Pacific Plate. These results provide a chronological basis for the investigation and evaluation of weathered residual Kaolin deposits on the Qinzhou-Hangzhou Suture Zone.

Xigaze ophiolite (South Tibet) records complex melt-fluid-peridotite interaction in the crust-mantle transition zone beneath oceanic slow-ultraslow spreading centers

The formation and evolution of the crust-mantle transition zone (CMTZ) under oceanic slow-ultraslow spreading centers, compared with the well-documented examples that developed under fast spreading centers, remain largely unknown due to the lack of suitable targets. In this study, we systematically examine the CMTZ of the Xigaze ophiolite in the Yarlung Zangbo suture zone (south Tibet). This ophiolite represents a rare lithospheric fragment produced in oceanic slow-ultraslow spreading settings. We conduct detailed field mapping and petrological as well as geochemical studies on the CMTZ in the Dazhuka massif, the easternmost segment of the Xigaze ophiolite. Our aim is to characterize the lithological architecture and associated melt-fluid-peridotite interaction history of the CMTZ. The CMTZ (~1800 m thick) in the Dazhuka ophiolite consists of 4 subzones with complex lithological associations. They range successively from clinopyroxene-rich harzburgite with weak metasomatism and without amphibole in Zone 1 (the bottom) to clinopyroxene-poor harzburgite with strong melt impregnation and more amphibole in Zone 3 (the upper part, with Zone 2 transitional harzburgite in between), all cut by a wealth of dyke rocks (gabbro, dolerite and dunite). The Al2O3 contents decrease consistently in whole-rock (2.97–0.57 wt%), orthopyroxene (4.99–0.85 wt%) and clinopyroxene (5.25–1.22 wt%) for the harzburgites from Zones 1–3, contrary to the trends for the spinel Cr# (0.17–0.65) and clinopyroxene Li/Y, and bulk rare earth element (REE), Pb and Sr contents. The plagioclase-bearing peridotite (commonly enclosed in layered gabbro) in Zone 4 (the top) has remarkably elevated TiO2 contents (0.12–0.34 wt%) and Cr# values (0.45–0.53) in spinel, suggesting equilibration and strongest interaction with MORB-like melts among the CMTZ. The troctolite in Zone 4 has olivine Mg# (80.6–84.2) and NiO (0.37–0.45 wt%) and spinel Cr# (0.68–0.73). The fluid-mobile elements (U, Pb, Sr and Li) in both whole rock and clinopyroxene as well as amphibole abundances increase consistently from Zones 1–4, implying that the interaction of the mantle rocks with hydrothermal fluids became increasingly intensive from bottom to top of the CMTZ.These vertical variations collectively suggest the complex melt-fluid-peridotite interaction during the upward movement of deeply sourced melts and the downward penetration of seawater. We propose that such interactions are ultimately controlled by the relative slow spreading rates, where the less melt supply and very thin or even missing oceanic crust facilitate the downward seawater injection and enhance pervasive fluid metasomatism. This may represent a suitable explanation for the CMTZ with intriguing lithological and chemical heterogeneity under many other active or fossil oceanic slow-ultraslow spreading environments.

ІНТРУЗИВНО-МАГМАТИЧНІ УТВОРЕННЯ АРХІПЕЛАГУ ВІЛЬГЕЛЬМА ЗАХІДНОЇ АНТАРКТИКИ (ЧАСТИНА 2 – ГІПАБІСАЛЬНІ ТА СУБВУЛКАНІЧНІ ДАЙКОВІ ПОРОДИ)

The second part of the work "Intrusive-magmatic complexes of Wilhelm Archupelago, West Antarctica" is devoted to dykes. The authors studied dyke rocks in the Ukrainian Antarctic Station area in order to specify their petrographic diversity as well as to clarify the spatial distribution of the dykes, their geological position, age and geological relationships with other igneous formations in the region. It was found that hypabyssal and subvolcanic dykes of the Mesozoic-Cenozoic age are widespread on the Wilhelm Archipelago. The most common composition are mafic and intermediate rocks namely microgabbros, diabases, basalts and microdiorites. Felsic dykes are subordinate. Typically, studied rocks occur as small dykes that can concentrate in extended swarms. Sill-like intrusions are less common. The spatial distribution of dykes is controlled by the Lemaer – Penola fault zone of northeastern strike. Their greatest concentration was found on the Argentine Islands. The geological relationships of the dykes with isotopically dated granitoids as well as the sequence of intrusion of individual dyke bodies, which is established in the places of their distribution, allowed to distinguish, at least, two stages of dyke formation - namely Mesozoic and Cenozoic. The most ancient dyke formations of the Argentine Islands are hypabyssal dykes and sill-like intrusions of the Mesozoic age. They intruded after the accumulation of the Jurassic-Cretaceous volcanic strata of the Argentine Islands, but before the formation of the Paleogene granitoids on the Barchans and Forge Islands. The petrographic diversity of Mesozoic dyke rocks is represented by microdiorites and microgabbros, as well as their contact-metamorphosed equivalents. All hypabyssal and subvolcanic dykes intruding the Paleogene granitoids on Barchans and Forge islands are of Cenozoic age. Among them, the most common are diabase and basalt dykes. Microdiorites are of subordinate importance. The characteristic petrographic features of the Cenozoic dykes allow to identify their age analogs outside of the Barchans-Forge granitoid intrusion.

Deciphering syn- and post-emplacement processes in shallow mafic dykes using magnetic anisotropy

Dykes form key pathways for the transport and emplacement of magma within the crust. We have identified syn- and post-emplacement processes recorded across a ~2 m thick basaltic dyke on the Isle of Skye, Scotland. We measured the rock magnetic properties, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic magnetisation (AARM) across two dyke-thickness profiles spaced 13 m apart along the dyke strike (sites G5 and G6). At 20–25 cm intervals, our samples are very closely spaced compared to standard sampling protocols. Our results show that the dyke's magnetic fabrics originate from two mineral groups: titanomagnetite, which is abundant in the central dyke region at site G5, and iron sulphides (pyrite and pyrrhotite) that dominates the margin regions at both sites. The titanomagnetite occurs in unaltered dyke rock; its magnetic fabrics are primary, having formed during magma solidification, and record lateral magma flow. The pyrrhotite occurs in jointed and hydrothermally altered dyke rock; its petrological and magnetic fabrics are secondary, having originated from a sulphide-rich fluid which infiltrated cooling joints oriented perpendicular to the dyke margins and locally modified the primary magnetic fabrics. At site G6, pyrrhotite also occurs in the dyke centre, suggesting that locally the post-emplacement sulphide-rich fluid permeated into this region; this site is located close to a branch in the dyke and has increased joint frequency, which could explain the enhanced alteration. The presence of syn- (primary) and post-emplacement (secondary) fabrics was only identified due to our high frequency sampling regime and use of both AMS and AARM techniques. We highlight that future magnetic anisotropy studies of dykes may benefit from high sample frequency combined with sampling along-strike and across-thickness. Using both AMS and AARM techniques to detect more variations in magnetic fabrics can reveal more complete syn- and post-emplacement dyke histories.

Enigma of Dangoaposi and Jagannathpur Lavas of Singhbhum Craton, Eastern India: Possible Solution

Abstract In the Precambrian Singhbhum stratigraphy, some enigmatic aspects exist as regards to Dangoaposi and Jagannathpur lavas. The existing enigma embodies locational aspects of these lavas, structural characteristics, stratigraphic position, and geochemical affinities. Based on recent field studies by the authors and knowledge acquired from existing literature, a possible solution has been proposed in the present study, which may be followed by Precambrian geologists in order to avoid all existing confusion and dilemma. The present study clearly indicates that in Jagannathpur, there is the presence of dyke rocks only (and there is no lava) and as such stratigraphic position of Jagannathpur lava in Singhbhum Geology should be discontinued. Dangoaposi lava (described as Jagannathpur lava by some previous workers) explicitly represents a portion of western Iron Ore Group (IOG) lava and therefore, may be considered to represent a sub-domain within western IOG.

Phase petrographic, thermobarometric and petrochemical significance of Cretaceous mafic dykes along Nongchram Fault Zone of Swangkre–Rongmil area of Shillong plateau, NE India: Implications for genetic link to Kerguelen mantle plume

The present study elucidates the phase petrographic and petrochemical signatures of a group of Cretaceous mafic dykes emplaced in the Precambrian gneissic basement complex in the western part of the Shillong plateau, NE India, in order to trace their petrological, geotectonic, geothermobarometric and oxybarometric status. The whole rock geochemistry discriminates the dykes as basaltic andesites and basaltic trachyandesites genetically related to each other and derived from common parent magma. The enrichment in LREE relative to HREE and HFSE, systematic Nb anomalies, moderate MREE to HREE fractionation suggests variable depths of melting of slightly enriched mantle source in the garnet stability field. As per geochemical modelling, the studied dykes are derived by 3–5% non-modal batch melting of garnet peridotite source at melting depth of ~65–80 km. The clinopyroxene thermobarometry reveals a temperature span of 1250–800°C and < 2 kb pressure of crystallization for the dykes. The oxygen fugacity (16.82–18.25) indicates extremely reducing conditions at the time of cooling. The very good correlation of petrochemical and phase chemical data with Kerguelen plume derived Rajmahal Group II basalt, Sylhet volcanics and some ODP (ocean drilling project) sites from Kerguelen basalts implicate a genetic link of the studied dykes with Kerguelen mantle plume. Finally, the present study deciphers subalkaline nature of the studied dyke rocks that have been generated by tholeiitic magmas probably in an anorogenic extensional environment. But we need more geochemical data especially good isotope geochronologic data to get a clear picture of the studied dyke.

Petrographical and mineral chemistry characteristics of mafic dykes in Nhi Ha area, Ninh Thuan and its meaning

The NE-SW and NW-SE mafic dyke rocks in Nhi Ha area intruded through Cretaceous granitoid Dinh Quan or overlapped by Quaternary sediment materials. It has block type and blue grey in color in the field. Under microscope, based on minerals and textures, divided into 2 types of mafic dyke rock, such as diabase and porphyritic diabase. Minerals chemical analysis by EPMA method: plagioclase almost are labradorite (An 60.80%), oligoclase (An from 12.31 to 14.29%), some grain had albitation (An from 6.75 to 8.46%); pyroxene with augite (Wo41,80 En44,22 Fs13,98 to Wo43,40 En37,50 Fs19,10) and diopsite composition (Wo45,45 En42,48 Fs12,07 to Wo48,16 En40,02 Fs11,81); amphiboles in basalt are magnesio-hastingsite (Ca-amphibole group). Popular alteration minerals are chlorite, epidote and sericite. Chlorite is pycnochlorite type, ratio Fe/(Fe+Mg) &lt;0,6 show that it formed by hydrothermal alteration process with temperature from 150 to 3250C. Opaque minerals are pyrite, chalcopyrite, ilmenite, magnetite, titanomagnetite, titanite (sphene) and a little bit of rutile.&#x0D;

The “intraorogenic” Svecofennian Herräng mafic dyke swarm in east-central Sweden: age, geochemistry and tectonic significance

ABSTRACTThe Herräng mafic dykes form an E-W-trending dyke swarm within the Bergslagen lithotectonic unit of the Svecofennian orogen in east-central Sweden. They intrude the Svecofennian supracrustal rocks and early-orogenic granitoids, but are themselves cut by late Svecofennian pegmatites, and have undergone Svecofennian amphibolite-facies metamorphism, leading to their classification as “intraorogenic” Svecofennian dykes. They can be assigned an age between 1870 and 1850 Ma, with metamorphism of the dykes dated at 1848 ± 13 Ma by U-Pb in titanite. Their current mineralogy is dominated by metamorphic plagioclase and amphibole, with variable amounts of quartz and biotite, and minor to accessory titanite, apatite, epidote, pyrite, magnetite, ilmenite and zircon. Textures range from massive to strongly foliated. Twenty samples of dyke rocks from three subareas in the Roslagen region, including the Herräng type area, range in composition from basaltic to andesitic with 47 to 60 wt% SiO2, broadly similar to the Dannemora dykes and the Avesta-Östhammar gabbros and diorites. Initial 87Sr/86Sr ratios (at 1870 Ma) varies between 0.7026 and 0.7038, corresponding to initial εSr between +5 and +21, and initial εNd between −0.4 and +1.3, suggesting a slightly enriched to mildly depleted mantle source, similar to other Svecofennian mafic rocks. The dykes dominantly show a calc-alkaline volcanic arc signature related to subduction. They formed during an extensional episode, possibly related to incipient back-arc spreading or subduction roll-back following the main early-orogenic subduction-related Svecofennian magmatism, but penecontemporaneous with amphibolite-facies metamorphism in the area.

Kaersutite lamprophyres of the Pokrovo-Kyriivo massif (the Azov area, Ukraine): petrographic features, mineral composition, geochemistry

The paper represents results of the petrographical, geochemical and mineralogical studies of a dyke of porphyry-like mafic rock from the Pokrovo-Kyriivo massif of the Azov area, the Ukrainian Shield. This dyke of fine-grained rock has unique mineral composition as the main mineral in the groundmass is kaersutite. According to mineralogical and petrographic features, it can be identified as kaersutite lamprophyre. The Devonian Pokrovo-Kyriivo massif is located at a junction of the Azov Domain of the Ukrainian Shield and the folded structure of Donbas. This massif is composed of subalkaline (pyroxenite, gabbro) and alkaline (malignites, nepheline syenite) rocks which are cut by numerous dykes varying in composition from ultramafic to intermediate. The dyke of finegrained kaersutite lamprophyre was discovered in the bottom hole of the drillhole 36, where it is intruded into mediumgrained gabbroids and pyroxenites. The groundmass of the rock is composed of kaersutite, titanomagnetite, albite, chlorite, titanite, pyrite, chalcopyrite, apatite, monazite. The phenocrysts are represented by chlorite and talc pseudomorphoses after a diamond-shaped mineral with shortened obtuse angles. According to electron microprobe analysis, kaersutite has low TiO2 content and intermediate mg# values of 0.54—0.67. According to the chemical composition, studied kaersutites differ from the same mineral in the microgabro dyke in the Pokrovo-Kyriivo massif, and from kaersutites in camptonites of the Khlibodarivka open pit in the Azov area. At the same time, it is similar to kaersutites from camptonites of the Novoukrainka dyke field of the Inhul terrane. Kaersutite lamprophyre also differ from dyke rocks of the Pokrovo-Kyriivo massif by the absence of clinopyroxene, mica, ilmenite and low content of TiO2 in titanomagnetite, which does not exceed 6 %. This is due to a low TiO2 content in the rocks (up to 3.5 %), relative to other dyke gabbro of the area. According to mineralogical-petrographic and chemical characteristics, kaersutite lamprophyres are unique rocks that have no direct counterparts in the Azov area or on the Ukrainian Shield in the whole.

Petrochemistry and Sr, Nd, Pb Isotopic Characteristics of Basic Dykes of Mikir Hills, Assam

The basic dykes in Mikir Hills have gabbroic composition. The prominent trends of these dykes are E-W in Southern part of Mikir Hills and NE-SW trend in the Northern Part of Mikir Hills. The geochemical characterization of dyke rock shows that they are subalkaline to alkaline basalt in nature, medium to high potassic, high titanium, Fe enriched tholeiites. The evolved nature of magma is represented by low Mg# 39.34 to 49.76 and are characterized by relatively low abundances of MgO, Cr, Ni, Zr, Y and high Rb, Ba, Th, Nb, indicating the within plate basalt (WPB) and continental rift basalt (CRB) chemical affinity. This dyke activity may be related to the extensional tectonics related to Proterozoic rifting hence, showing characteristics of continental rift basalt (CRB) type. The nine samples of gabbro define a five-point Rb-Sr isochron age of 1200±67 Ma with 87Sr/86Sr ratio of 0.70477±0.00042 with MSWD of 0.85. The Sm-Nd model ages (TDM) varies from 1600 to 2018Ma (eNd +0.11 to −2.04) indicates that the protolith from which these gabbroid rocks are derived is mostly Palaeo- to Meso-Proterozoic in age. The isotopic evolution of initial Sr, Nd and Pb reveals crustal contamination.

Dyke magmatism in the evolution of the transform active continental margin of the Siberian Craton in the Ediacaran

An Ediacaran complex of dyke rocks has been identified for the first time in the Yenisei Ridge orogen. These igneous rocks are represented by basic, intermediate and acid variants and formed in the conditions of the transform active continental margin at the stages of its development before and during the interruption of subduction and slab breakoff: from dykes of the picrodolerite - ​dolerite-quartz diorite - leucogranite association with age in the range of 626-623 Ma (U-Th-U isotope data for zircons, SHRIMP-II method, this work) up to adakite - gabbro-anorthosite magmatism at the end of the Ediacaran.

Dike Magmatism in the Evolution of the Transform Active Continental Margin of the Siberian Craton in the Ediacaran

An Ediacaran complex of dyke rocks has been identified for the first time in the Yenisei Ridge orogen. These igneous rocks are represented by basic, intermediate and acid variants and formed in the conditions of the transform active continental margin at the stages of its development before and during the interruption of subduction and slab breakoff: from dykes of the picrodolerite - ​dolerite-quartz diorite - leucogranite association with age in the range of 626-623 Ma (U-Th-U isotope data for zircons, SHRIMP-II method, this work) up to adakite - gabbro-anorthosite magmatism at the end of the Ediacaran.

First data on U-Pb age of mafic dyke in the Mirny Station area (Pravdy Coast, East Antarctica)

We firstly report a SHRIMP zircon age on a post-metamorphic mafic dyke from the Mirny Station area in Pravdy Coast which is underlain by ca 1000 Ma metamorphic rocks that experienced thermal and magmatic activities at ca 500 Ma. A concordant age of 507 ± 6 Mа for zircon from post-metamorphic dolerite has been obtained. The analyzed grains display the REE and other trace element concentrations corresponding well to zircons from typical dolerites worldwide (e.g., low Hf, Y), which allows interpretation of the obtained date as dolerite dyke crystallization. Geochemical and Sr–Nd isotopic data on the dyke rocks show that these rocks have high-Ti, P concentrations, are enriched in other HFSE and also REE and have low εNd(500) of −13.2, and high 87Sr/86Sr(i) of 0.71206. These features are consistent with derivation from a strongly enriched subcontinental mantle source. Chemical and isotopic compositions of the dyke rocks correspond to those known from the continental flood basalt provinces, in particular to the Cambrian Antrim extensional Large Igneous Province in Australia. This magmatic event is thought to reflect an extension episode between East Antarctica, Australia, and Greater India.

Geochemistry and Fractionation of Zr and Hf in the Magmatic System of the Kugda Massif (Polar Siberia)

The patterns of the distribution and fractionation of strategic metals (Zr, Hf) in the Kugda intrusion (Polar Siberia) have been studied. The contents of these elements significantly exceed their concentrations in other rocks (Zr, 246 ppm; Hf, 7.4 ppm). A significant increase in Zr and Hf from early rocks (olivinite and melilite rocks) to later differentiation products, syenites with up to 570 ppm of Zr and 16 ppm of Hf, has been revealed. During the evolution of the Kuga magmatic system, notable fractionation of Zr and Hf occurred. The Zr/Hf ratios in the dyke rock, similar in composition to the primary Kugda Massif magma, and the early intrusions are fairly close to that of chondrite (Zr/Hf = 37 [1]), while in the latest phases this ratio increases by almost 5-fold.

Low-Titanium Lamproites of the Ryabinoviy Massif (Aldan Shield): Crystallization Conditions and Lithospheric Source

Obtained data shows that high-potassic dyke rocks of the Ryabinoviy massif (Central Aldan) belong to low-titanium lamproite series (Mediterranean type) and are distinct with “classic” high-titanium lamproites. Based on Al-in-olivine thermometer, temperature of olivine–chrome-spinel pair crystallization varies in range between 1100 and 1250°C. This suggests lithospheric mantle source for the parental melt and makes role of mantle plume insignificant. High-precision data on olivine composition and bulk rock traceelement composition imply mixed source for the parental melt, consisted of depleted peridotite and enriched domains, originated during ancient subduction.