Alkaline Magmatism Research Articles

Isotope-geochemical (Sm–Nd, Rb–Sr, REE, HFSE) composition of the University foidolite-gabbro pluton, Kuznetsk Alatau ridge, Siberia

The University foidolite-gabbro pluton is located among the Cambrian carbonate-volcanogenic deposits. Its composition is dominated by moderately-alkaline and alkaline mafic rocks, broken by dikes of ijolite-urtites, nepheline and alkaline syenites. The chemical composition of igneous rocks is characterized by low silica contents (SiO2 = 41–49 wt. %), wide variations of alkalinity (Na2O + K2O = 3–19 wt. %; Na2O / K2O = 1.2–7.2 wt. %), low titanium content (TiO2 = 0.07 1.59 wt. %) and high alumina content (Al2O3 = 15–28 wt. %), which corresponds to the K-Na derivatives of the basic alkaline formation. By the content of rare earth elements, alkaline rocks (104–246 ppm; La/Yb(n) = 5.79–12.73) are more differentiated derivatives than gabbro (94–111 ppm; La/Yb(n) = 6.87‑6.95). All varieties are characterized by low concentrations of most highly charged elements (Nb, Ta, Zr, Hf + Y), which in terms of accumulation are located between the basalts of oceanic islands and basalts of island arcs. The presence of negative Nb–Ta anomaly and the relative enrichment of Rb, Ba, Sr, and U indicate the probable interaction of plume material with previously formed accretionary complexes of subduction zones. The primary isotope 87Sr/86Sr ratio (~ 0.705 – 0.706) and a wide range of εNd (T) from +3.2 to +8.7 in the rocks also indicate a mantle-crustal nature and a complex geodynamic setting of the Paleozoic alkaline magmatism of the Kuznetsk Alatau. The obtained results of Sm‑Nd dating suggest the formation of moderately-alkaline gabbroids in the Early Paleozoic (494–491 ± 36 Ma), with the intrusion of dikes of alkaline rocks of the Middle Paleozoic age (394 ± 16 and 389 ± 37 Ma).

New Findings of Rare Minerals in Alkaline Rocks of Ukrainian Shield

Over recent years, new rare minerals have been discovered in the alkaline rocks of the Ukrainian Shield. Agpaitic varieties of alkaline magmatic and metasomatic rocks turned out to be especially abundant in rare minerals. Numerous findings are related to alkaline metasomatites which are considered to be fenites and apofenite albitites of the Dmytrivka quarry. It is well known primarily by the presence of various accessory (Nb, REE, and Zr) minerals, as well as silicate and oxide minerals that are rare for Ukraine. The most common albite microcline fenites of this quarry are characterized by rare-earth mineralization, whereas the concentration of REE decreases in apofenite albitites and Zr and Nb increase. New rare minerals were also found in the essentially albite rock with astrophyllite, alkaline pyroxene and amphibole of the Malatersa massif and agpaitic phonolites of the Oktyabrsky massif. In the rocks of the mentioned massifs and occurrences of alkaline rocks the most interesting are the findings of the perraultite — jinshajiangite series. They were found in three points of the Azov area and include 1) perraultite and jinshajiangite in the alkaline metasomatites of the Dmytrivka quarry; 2) only perraultite in agpaitic phonolites of the Oktyabrsky massif (Kam’yana gully); 3) jinshajiangite in a veined albite rock among the gabbro of the Malatersa massif. Baotite and minerals of the hejtmanite — bafertisite series were also found in the metasomatites of the Dmytrivka quarry. The latter belong to intermediate varieties in terms of MnO (10-17 wt.%) and FeO (10-17 wt.%) which distinguishes them from Fe-rich bafertisite from other regions. A silicate mineral with high content of Na, Zr, Mn and elevated Ti and Nb is rarely observed as small inclusions in the kupletskite grains from alkaline metasomatite of the Dmytrivka quarry. According to the chemical composition it was previously diagnosed as janhaugite. Tainiolite was found in some occurrences of alkaline metasomatites in the Azov region. In addition small aggregates of the REE-enriched epidote were found in fenites of the Kaplany village, which is probably the first finding in Ukraine. Two new Zr minerals have been found in the aegirine syenites of the Korsun-Novomyrhorod pluton: elpidite and mineral with a high content of Y2O3 (13-14 wt.%) (Y-hagatalite ?).

Relações de contato entre rochas alcalinas máficas e sieníticas na Praia do Jabaquara, setor norte da Ilha de São Sebastião, SP

Relações de contato entre as rochas sieníticas do stock de Serraria e as rochas máficas plutônicas e hipoabissais do setor norte da suíte alcalina da Ilha de São Sebastião (Ilhabela) são descritas pela primeira vez, permitindo a indicação de relações temporais entre diferentes pulsos magmáticos. Os afloramentos apresentam variedade sienítica hololeucocrática hospedando mega-, macro- e microxenólitos de diferentes rochas, formando agmatitos. Cinco unidades principais são descritas: álcali feldspato sienitos, melassienitos, diabásios, cumulatos máficos (melagabros/clinopiroxenitos) e gabros heterogêneos. Geoquimicamente,o magmatismo alcalino bimodal encontrado na ilha é bem representado pelas rochas dos afloramentos estudados. A sequência de eventos magmáticos consiste em: colocação e cristalização de pulsos de magmas básicos alcalinos, em ambiente de câmara magmática, gerando as variedades gabroicas cumuláticas, principalmente melagabros e clinopiroxenitos com olivina, e brechas de matriz gábrica com evidências de reequilíbrio textural por ação de temperatura; intrusão de novos pulsos de magma básico alcalino, representados pelos fragmentos de diabásios, que apresentam também, reequilíbrio textural (textura granoblástica predominante); intrusão de pulso sienítico na câmara máfica que fragmentou o gabro já parcial ou totalmente cristalizado, transportando ampla proporção de xenocristais e xenólitos, o que gerou rochas classificadas como melassienitos; nova intrusão de pulso sienítico, que fragmenta todo o sistema formando um agmatito com megaxenólitos e microxenólitos de gabro cumuláticos, diabásios, agmatitos de matriz gabroica (gabros heterogêneos) e melassienitos; veios e diques de pegmatito e aplito de sienitos com quartzo cortam todos os litotipos. Todos os tipos amostrados como xenólitos apresentam algum grau de recristalização e de reação química com o magma sienítico final que os hospeda.

Syenite from Ulleung Island: As a window for pre-eruptive shallow alkaline magma dynamics

Ulleung Island is an alkaline volcano that consists of Pleistocene basalts (Stage I), effusive trachytes (Stages II–III), and Holocene explosive phonolitic volcanics (Stage IV). Syenites are found as volcanic ejecta in Stage III and IV, hence, acting as a window for pre-eruptive shallow alkaline magma dynamics. We integrate geochemical data from Ulleung Island to track the magma batch interconnection underneath this island and discuss the petrological effect of magma mixing on the resultant eruption style through shallow preconditioning. The magma mixing characteristics of syenite with melanocratic enclaves are reported in this study from a textural and geochemical context. The textural assemblage from syenite is likely to indicate a couple of magmatic conditions including quenching of hotter magma, overgrowth of orthoclase-rich feldspar, and disequilibrium of kaersutite. The magma mixing conditions were traced through the predicting models from clinopyroxene, amphibole, biotite and alkali-feldspar chemistry, and constrained by ~690–740 °C at ~77–180 MPa. In addition, the Zr/Ce versus Zr/Y plot indicates that the syenites can be subdivided into the Group I as felsic cumulate from Stage IV, and the Group II as mixing product of Stage IV and II. Consequently, the magma mixing of the Stage IV and II produced the Stage III. The syenite ejecta from Ulleung Island indicate the petrological significance of the shallow alkaline magma dynamics: the explosive eruption of Stage IV is induced by the volatile (especially, magmatic water) transportation from Stage II and subsequent saturation in Stage IV. Furthermore, a discussion on the manifested eruption style through magma mixing from comparative perspective implies that the explosive eruption of a volcano requires the transportation and saturation of volatile components and a short stagnant period, avoiding stabilization of magma chamber prior to its eruption.

Age correlation of Large Igneous Provinces with Devonian biotic crises

Abstract Large Igneous Provinces (LIPs) are emerging as a significant driver of short duration climatic change including mass extinctions. Here we compare the record of LIPs against the timing of Devonian biotic crises that are well recorded in the numerous anoxia events throughout this period. The largest LIPs are two-pulse events at ca. 370 and ca. 360 Ma that are present in both Siberia (Yakutsk-Vilyui LIP) and Baltica (Kola-Dnieper LIP) and correlate with the Kellwasser anoxia events (late Frasnian) and Annulata or Dasberg and Hangenberg anoxia events in the late/latest Famennian. The regionally significant Altai Sayan LIP is only approximately dated at c.400 Ma and within uncertainties could be linked to either (or both?) the c. 405 Ma Chebbi or Atopus anoxia events near the basal Emsian (associated with graptolite extinction) or the c. 385–390 Ma group of anoxia events in the Eifelian or Givetian. Additional magmatic events that are less precisely dated are: 1) the c. 400–345 Ma Kedon magmatism of the Omolon craton, possibly linked with the Yakutsk-Vilyui event, 2) 380–330 Ma Maritimes (Magdalen) basin event of the Appalachian region of eastern Canada and adjacent US, 3) middle-late Devonian Selwyn basin alkaline magmatism of northwestern Canada, and 4) the Ordovician - late Silurian- possibly Devonian Soltan Maidan event Iran. However, more precise dating is required for each of these magmatic events before they can be properly compared with the Devonian biotic crisis record.

Petrographic, Geochemical and Isotopic (Sr–Nd–Pb–Os) Study of Plio-Quaternary Volcanics and the Tertiary Basement in the Jorullo-Tacámbaro Area, Michoacán-Guanajuato Volcanic Field, Mexico

AbstractThe origin of the large diversity of rock types erupted along the subduction-related Trans-Mexican Volcanic Belt (TMVB) remains highly debated. In particular, several hypotheses have been proposed to explain the contemporary eruption of calc-alkaline and alkaline magmas along the belt. The Michoacán-Guanajuato Volcanic Field (MGVF) is an atypical, vast region of monogenetic activity located in the western-central part of the TMVB. Here we present new petrographic, geochemical, and isotopic (Sr–Nd–Pb–Os) data on recent volcanics in the Jorullo-Tacámbaro area that is the closest to the oceanic trench. TMVB-related volcanics in this area are Plio-Quaternary (<5 Ma) and mainly form a calc-alkaline series from basalts to dacites, with rare (<5 vol. %) alkaline rocks that range from trachybasalts to trachydacites, and transitional samples. Crystal textures are consistent with rapid crystallization at shallow depth and processes of mixing of similar magma batches (magma recharge). All of the samples exhibit an arc-type trace element pattern. Alkaline and transitional magmas have higher Na2O and K2O, lower Al2O3, and higher concentrations in incompatible elements (e.g. Sr, K, Ba, Th, Ce, P) compared to calc-alkaline rocks. Calc-alkaline rocks are similar isotopically to transitional and alkaline samples, except for a few low 87Sr/86Sr samples. Sr, Nd and Pb isotopes do not correlate with MgO or 187Os/188Os, indicating that they were not significantly influenced by crustal contamination. Isotopic and trace-element systematics suggest that the Tacámbaro magmas are produced by melting of a mantle wedge fluxed by fluids derived from a mixture of subducted sediments and altered oceanic crust. Alkaline and transitional magmas can be derived from a lower degree of partial melting of a similar source to that of the calc-alkaline rocks, whereas the few low 87Sr/86Sr calc-alkaline rocks require a lower proportion of fluid derived from oceanic sediments and crust. Volcanism at the trenchward edge of the MGVF was thus driven purely by subduction during the last 5 Ma, hence discarding slab rollback in this sector of the TMVB.

Sapphirine-bearing pyroxenite xenoliths in Cenozoic alkali basalt from Jabel El Arab (Syria): Insights into the nature and composition of the lithosphere beneath the southern Syrian Rift, northern part of the Arabian Plate

Pyroxenites and composite xenoliths occur in the Cenozoic alkali basalts of Jabel El Arab (the southern extremity of the Syrian Rift). The anhydrous xenolith suite consists mainly of spinel garnet-sapphirine–bearing websterites, spinel-olivine–bearing websterite, as well as rare spinel plagioclase-garnet–bearing websterites and composite garnet-bearing websterite/anorthosite xenoliths. We report for the first time sapphirine coronas around spinel in spinel-garnet–bearing websterite xenoliths from two outcrops in this region. Their compositional and microstructural features are consistent with a reaction from spinel + orthopyroxene + anorthite to clinopyroxene + garnet + sapphirine. The full pyroxenite suite is divided into two groups based on mineralogical characteristics and geochemical composition: (A) tholeiitic to transitional mantle segregates having spinel ± garnet ± sapphirine characterized by a high Al2O3 and CaO content, low REE content, LREE depletion, and a highly variable Sr-Nd isotopic ratio; (B) alkaline segregates that include spinel-olivine–bearing websterite and composite xenoliths characterized by a high TiO2 and alkali content, relatively high REE, and low Ni and Cr. Slight LREE enrichments in clinopyroxenes from meta-igneous rocks of tholeiitic to alkaline affinity indicate that they were later metasomatized by small volumes of percolating silicate melts in the upper lithospheric mantle. The xenoliths have been re-equilibrated within the garnet-pyroxenite stability field. Combining xenolith-derived P-T estimates with mineral and chemical composition and seismic data, we show that the websterite xenoliths formed in the shallow lithospheric mantle near the Moho discontinuity at 28–44 km in depth. Nd-model ages and the geodynamic history of the Arabian Plate favor the scenario of tholeiitic-transitional and alkaline magmas being underplated below or intruded at the base of a thinned Syrian crust. This occurred around 200–100 Ma in response to rifting and magmatism beneath the Arabian Plate during the early assembly of the eastern Mediterranean region.

Geochemistry and mineral chemistry of gabbroic rocks from Horjand of Kerman province, Southeast of Iran: Implications for rifting along the northeastern margin of Gondwana

The Horjand area is part of the Kashmar-Kerman tectonic zone in the Central Iranian block, where the oldest continental crust of Iran can be found. Mafic igneous rocks associated with sedimentary units (gypsum, shale, limestone, dolomite and sandstone) belong to the magmatism of the Late Neoproterozoic–Early Cambrian Desu Series. Based on petrographic studies, the mafic igneous rocks comprise medium to coarse gabbro, micro-gabbro and hornblende-gabbro. The rocks mainly consist of plagioclase, olivine, clinopyroxene, biotite, amphibole and Fe-Ti oxides. The gabbroic rocks have a variable compositional range, with SiO2 (44.6–52.9 wt. %), MgO (2.94–13.83 %), CaO (5.56–15.23 wt. %) and TiO2 (1.19–2.68 wt. %) contents. Petrographic evidence and geochemical data suggest that fractional crystallization has played an important role in the evolution of the ascending magma of the gabbroic rocks. The chemical composition of the minerals is similar to those of gabbroic rocks that were formed in a non-orogenic environment. The crystallization of clinopyroxene of gabbros mainly occurred under temperatures ranging from 1045 to 1216 °C. The geochemical characteristics and mineral compositions of the gabbros show that the samples predominantly have alkaline to minor tholeiitic affinities. During magma ascent, crustal contamination was minor or absent. Rare earth element modeling demonstrates that the magma of the Horjand gabbros was produced from 10 to 15 percent partial melting of a spinel-garnet lherzolite mantle source. Chondrite-normalized rare earth element (REE) patterns are LREE-enriched with (La/Yb)N of 4.50-12.82. Both normalized REE and trace-element spidergrams are similar to patterns of HIMU-OIB (high mu (μ = U/Pb)-ocean island basalt) or intraplate alkaline magmatic rocks. The gabbros geochemically display an intra-plate tectonomagmatic setting without subduction geochemical characteristics. Based on the available data and mineral chemical composition the development of the Horjand gabbroic rocks could be related to an extensional phase, with upwelling melting of the asthenospheric mantle in the continental rift resulting in alkaline magmatism of Late Neoproterozoic–Early Cambrian age, in the Central Iranian micro-continent.

Comparing contiguous high- and low-elevation continental margins: New (U-Th)/He constraints from South Brazil and an integration of the thermochronological record of the southeastern passive margin of South America

The southeastern coast of South America is an example of the complexity of passive continental margins, as it displays both high- and low-elevation segments despite sharing a similar pre-rift geological history and structural configuration. As such, it is a prime candidate for investigating debated questions concerning the evolution of passive margins, such as the tectonic mechanisms driving uplift, their relationship to rifting and continental break-up, and why some margins are elevated when others are not. In this contribution, we present new (U-Th)/He data from a low-altitude portion of the South American passive margin in South Brazil, the Sul-rio-grandense Shield, and interpret it in the context of the regional thermochronological record. New results produce widespread apparent ages and reveal a complex exhumation history from the early Paleozoic onwards, including reheating during Paleo-Mesozoic sedimentation. For most of the study area, however, final exhumation was achieved at the latest during the rifting and early opening of the South Atlantic Ocean (135 Ma to 100 Ma). In spite of the presence of major Neoproterozoic shear zones, the inherited NE-SW structural framework seems not to have strongly influenced the thermochronological record. The new data were integrated into a large compilation of apatite fission track and (U-Th)/He results from southeast South America, in order to compare regional trends and investigate possible tectonic controls in the exhumation history. Low-elevation areas of the passive margin consistently record complex pre-rift cooling histories, while high-elevation areas experienced significant Upper Cretaceous/Paleogene uplift associated with the reactivation of Neoproterozoic shear zones. Because the inherited structural features of both segments are similar, plate dynamics alone cannot be responsible for the variating response. Hence, mantellic processes associated with post-rift alkaline magmatism may have affected the contrasting exhumation histories. This process was probably controlled by important South Atlantic fracture zones.

Deep structure of Parecis Basin, Brazil from 3D magnetotelluric imaging

The resistivity structure beneath the Parecis basin in central Brazil has been studied using broadband (0.001–5000 Hz) magnetotelluric (MT) data for 455 stations recorded along five lines in the Juruena sub-basin and over the Precambrian basement at the northern and southern basin borders. We found that the reconstructed resistivity-versus-depth profiles from our 3D MT inversion models satisfactorily match the average values of the resistivity well logs in the top 2200 m at two exploration well locations chosen for model validation. The low resistivity Parecis Formation at shallow depth can be traced across the region while an areally restricted conductive sequence at about 2000 m depth can be imaged in the central part of Juruena sub-basin and is interpreted as synrift deposits or remnants of a buried Neoproterozoic basin. Sedimentary formations deeper than 2200 m could be sensed but not individually resolved in our resistivity models. The deeper resistivity structure consists of a resistive upper crust, severely thinned over the Neoproterozoic fold belts bounding the basin to the north and south where it is underlain by conductive zones at mid-lower crustal depths. We interpret the deep crustal steep conductors as pre-existing shear-zones possibly associated with the Upper Cretaceous to Tertiary alkaline magmatism in the region.

Initial Pangean rifting north of the West African Craton: Insights from late Permian U-Pb and 40Ar/39Ar dating of alkaline magmatism from the Eastern Anti-Atlas (Morocco)

Numerous mafic dykes, sills and laccoliths crop out in the southern part of the Tafilalt basin (Eastern Anti-Atlas, Morocco). These rocks intrude the mildly folded Ordovician to Early Carboniferous formations, consisting mainly of lamprophyric dolerites and camptonites with minor gabbros and syenodiorites. Previous geochemical studies have shown that the Tafilalt magmatism of sodic-alkaline affinity has been produced by low degrees of partial melting from an enriched deep mantle source within the garnet stability field. However, the age and the geodynamic context of these rocks were presently unknown since no isotopic dating had so far been made of the Tafilalt dolerites. To resolve this issue, we present here the first 40Ar/39Ar biotite and U-Pb zircon dating from the Tafilalt alkaline magmatism. Three samples (biotite separates) yielded well-defined 40Ar/39Ar plateau ages of 264.2 ± 2.7 Ma, 259.0 ± 6.3 Ma and 262.6 ± 4.5 Ma whereas 206Pb/238U dating of zircon from one of these samples yielded an age of 255 ± 3 Ma. These ages coincide within the dating error, and indicate that this magmatism occurred in the late Permian. Considering geochronological and geochemical data, we propose that the Tafilalt magmatism reflects an early-rift magmatic activity that preceded the Triassic rifting heralded by the Central Atlantic Magmatic Province. This magmatic activity is recorded in both sides of the future Atlantic Ocean by small-volume alkaline magmatism that started in the late Permian and extends into the Triassic. The alkaline magmas are probably generated in response to an increase in the mantle potential temperature underneath the Pangea supercontinent.

Deep Carbon and the Life Cycle of Large Igneous Provinces

Carbon is central to the formation and environmental impact of large igneous provinces (LIPs). These vast magmatic events occur over geologically short timescales and include voluminous flood basalts, along with silicic and low-volume alkaline magmas. Surface outgassing of CO2 from flood basalts may average up to 3,000 Mt per year during LIP emplacement and is subsidized by fractionating magmas deep in the crust. The large quantities of carbon mobilized in LIPs may be sourced from the convecting mantle, lithospheric mantle and crust. The relative significance of each potential carbon source is poorly known and probably varies between LIPs. Because LIPs draw on mantle reservoirs typically untapped during plate boundary magmatism, they are integral to Earth's long-term carbon cycle.

Early mesozoic alkaline magmatism of the western framing of the Mongol-Okhotsk belt: age and structural position

A zonal igneous area appeared at the western end of the Mongol-Okhotsk belt in the early Mesozoic. Its central part forms a giant Khentey-Dauria batholith, which from the north and west is framed by rift-like structures with bimodal and alkaline granite magmatism. The geochronological studies of the peralkaline granites of the Choyr and Gal-Shara massifs were carried out. These massifs belong to the North Gobi (southern) border of the batholith. The age of the first was 215 million years, the age of the second was 213 million years. It is shown that associations involving alkaline igneous rocks are fairly well developed in the North Gobi zone. They are controlled by faults and grabens of the northeast strike. Thus, it was established that the formation of the zonal magmatic area was characterized by a specific mode. In its central part, which corresponds to the zone of the Mongol-Okhotsk suture, anatexis and batholite formation processes took place, and the periphery was involved in the processes of rifting. The data obtained indicate the closure of the western part of the Mongol-Okhotsk trough before the formation of the zonal magmatic area. The structure of the magmatic area was determined by the mechanisms of the plume-lithospheric interaction in the region characterized by a collision suture (anatectic melting zone) and its less tektonic tense framing (rifting zones).

Sulphur isotopes of alkaline magmas unlock long-term records of crustal recycling on Earth

Earth’s surface and mantle sulphur reservoirs are connected via subduction, crustal recycling and volcanism. Although oceanic hotspot lavas currently provide the best constraints on the deep sulphur cycle, their restricted age range (<200 Ma) means they cannot reveal temporal variations in crustal recycling over Earth history. Sulphur-rich alkaline magmas offer the solution because they are associated with recycled sources (i.e. metasomatized lithospheric mantle and plumes) and, crucially, are found throughout the geological record. Here, we present a detailed study of sulphur isotope fractionation in a Mesoproterozoic alkaline province in Greenland and demonstrate that an enriched subduction-influenced source (δ34S of +1 to +5‰) can be reconstructed. A global δ34S compilation reveals secular variation in alkaline magma sources which support changes in the composition of the lithospheric mantle and/or Ga timescales for deep crustal recycling. Thus, alkaline magmas represent a powerful yet underutilized repository for interrogating crustal recycling through geological time.

Об участии природных солей в щелочном магматизме. Статья 2. Эталонные объекты. Геологические аспекты модели

Research subject . The author suggests a new geologic and genetic model for alkaline magmatism. This model considers the saline complexes that are located along the paths of ascending deep magmas as additional sources of alkaline and volatile components. An analysis of the geological and genetic probability of the participation of salts in alkaline magmatism is based on the study of the space-time relationships of natural salts and alkaline magmatic complexes performed using global and regional data. This problem is investigated in a series of three articles. The first article was devoted to characterization of geological prerequisites and their tectonic types. This article sets out to describe the geological aspects of salt-magmatic interactions in greater detail. To this end, the standard alkaline-salt associations of three tectonic types - Italian (cover-fold type), Upper-Rhine (rift type) and North-West-African (passive-margin type) - were characterized. Methods. The most significant tectonic, lithological and petrological features of all the aforementioned types were described, along with the evolution of space-time relationships of alkaline magmatic complexes. The features confirming the participation of saline complexes in alkaline magmatism were generalized. Results. It is found that the presence of salt-bearing strata in deep zones of the Earth’s crust along the ascending routes of mantle magmas is a geological phenomenon. The places of their intersection are favourable for the injection of hot magmas and the formation of interstitial chambers, which form centres for interaction between these magmas and the components of saline (salt-carbonate) complexes. The assimilation of the alkaline and volatile components localized in these complexes can contribute to the formation of highly enriched (supersaturated) melt mixtures and a subsequent alkaline specialization of the magmas. Conclusion. On the basis of the data obtained, a geological model of salt-magmatic interactions has been developed. The author’s next article will discuss the genetic aspects of the proposed model along with a possible role of various alkaline and volatile halophilic components in the formation of alkaline magmas. In addition, a comparative analysis of various geological and genetic models of alkaline pet-rogenesis will be provided.

Clinopyroxene/Melt Trace Element Partitioning in Sodic Alkaline Magmas

Abstract Clinopyroxene is a key fractionating phase in alkaline magmatic systems, but its impact on metal enrichment processes, and the formation of REE + HFSE mineralisation in particular, is not well understood. To constrain the control of clinopyroxene on REE + HFSE behaviour in sodic (per)alkaline magmas, a series of internally heated pressure vessel experiments was performed to determine clinopyroxene–melt element partitioning systematics. Synthetic tephriphonolite to phonolite compositions were run H2O-saturated at 200 MPa, 650–825°C with oxygen fugacity buffered to log f O2 ≈ ΔFMQ + 1 or log f O2 ≈ ΔFMQ +5. Clinopyroxene–glass pairs from basanitic to phonolitic fall deposits from Tenerife, Canary Islands, were also measured to complement our experimentally-derived data set. The REE partition coefficients are 0·3–53, typically 2–6, with minima for high-aegirine clinopyroxene. Diopside-rich clinopyroxene (Aeg5–25) prefer the MREE and have high REE partition coefficients (DEu up to 53, DSm up to 47). As clinopyroxene becomes more Na- and less Ca-rich (Aeg25–50), REE incorporation becomes less favourable, and both the VIM1 and VIIIM2 sites expand (to 0·79 Å and 1·12 Å), increasing DLREE/DMREE. Above Aeg50 both M sites shrink slightly and HREE (VIri ≤ 0·9 Å ≈ Y) partition strongly onto the VIM1 site, consistent with a reduced charge penalty for REE3+ ↔ Fe3+ substitution. Our data, complemented with an extensive literature database, constrain an empirical model that predicts trace element partition coefficients between clinopyroxene and silicate melt using only mineral major element compositions, temperature and pressure as input. The model is calibrated for use over a wide compositional range and can be used to interrogate clinopyroxene from a variety of natural systems to determine the trace element concentrations in their source melts, or to forward model the trace element evolution of tholeiitic mafic to evolved peralkaline magmatic systems.

Early Mesozoic Alkaline Magmatism in the Western Framing of the Mongol–Okhotsk Belt: Age and Structural Position

A zonal igneous areal appeared at the western end of the Mongol–Okhotsk Belt in the Early Mesozoic. Its central part is comprised of the Khentei–Daurian giant batholith, which is framed on its north and west by riftlike structures with manifestations of bimodal and alkaline granite magmatism. Geochronological studies of the Choyr and Gal-Shara massifs of peralkaline granites, belonging to the North Gobi (southern) border of the batholith, are carried out. The ages of the former and latter are 215 and 213 Ma, respectively. It is shown that associations involving alkaline igneous rocks are fairly well developed in the North Gobi zone and controlled by NE-trending faults and grabens. Thus, it is established that the zonal magmatic area was formed under a specific regime. In its central part, which corresponds to the zone of the Mongol–Okhotsk suture, anatexis and batholith-formation processes took place, while the periphery was involved in rifting. The data obtained indicate the closure of the western part of the Mongol–Okhotsk trough before the formation of the zonal magmatic areal. The structure of the magmatic areal was determined by the mechanisms of the plume–lithosphere interaction between the collisional suture (anatectic melting zone) and its less tectonically strained framing (rifting zones).

New geochemical criterion of rare metal mineralization in the peralkaline magmas (Lovozero mineral deposit, Kola Peninsula)

Detailed studies have shown that changing the forms of eudialyte release (and the time of its crystallization) is a new geochemical criterion for the ore - bearing of alkaline magmas for rare metal (eudialyte ores). A new ore-bearing principle of alkaline magmas has been formulated: a prerequisite for the formation of an ore deposit is the early saturation of alkaline magmas with respect to the ore mineral. If the concentration of the ore component is significantly lower than the cotectic concentration (saturation), then the melt saturation and crystallization of the ore mineral will be carried out at the later stages of rock formation in a small volume of interstitial melt, when the phenomena of convective-gravity differentiation and segregation of mineral phases in the form of ore deposits are hampered. This leads to the dispersion of ore components in the form of xenomorphic forms of accessory minerals. Rocks of the differentiated complex (lower zone of the Lovozero deposit), and of the Khibiny massif, containing xenomorphic eudialyte, are not promising for eudialyte ores. Eudialyte deposits are associated with the upper zone of the Lovozero intrusion containing idiomorphic early eudialyte. The saturation of the initial magma in relation to eudialyte occurs after crystallization of about 80% of the intrusion. The proposed criterion is applicable to the largest alkaline massifs in the world. With the Ilimaussaksky massif (Greenland), in the rocks of which early, crystallized, idiomorphic eudialyte, there is a superlarge eudialyte ore deposit while in the Khibiny eudialyte ore is absent.

Sources of eocene magmatism of Western Kamchatka (according to geochemical and isotope Sr-Nd-Pb characteristics of basites)

The isotope-geochemical characteristics of the Eocene-Oligocene magmatic rocks of Western Kamchatka were studied. It is shown that the igneous rocks of the Eocene (45-53 Ma) Kinkil complex of Western Kamchatka are characterized by geochemical signs of super-subduction volcanism. Their isotopic composition of Sr, Nd and Pb, low concentrations of HFSE and HREE relative to the composition of MORB, suggest the formation of primary melts from depleted or poorly enriched in isotopic composition of the mantle wedge sources in different contaminated quartz-feldspath sialic sediments. From the end of the Middle Eocene on the territory of Western Kamchatka, K-Na alkaline-basalt magmatism (46-31 million years), whose geochemical characteristics are similar to E-MORB, as well as ultrapotassic alkaline-basalt magmatism, which continued to develop to the early Miocene (35-17 million years). The geodynamic nature of Late Paleogene alkaline magmatism involves the implementation of processes of diffuse rifting.

Early Devonian ultrapotassic magmatism in the North China Craton: geochemical and isotopic evidence for subcontinental lithospheric mantle metasomatism by subducted sediment-derived fluids

AbstractWe report new U–Pb zircon age data, zircon in situ oxygen isotope, mineral chemistry, whole-rock geochemistry and Sr–Nd isotopic compositions from the Early Devonian ultrapotassic Gucheng pluton in the North China Craton, and discuss its petrogenesis. The Gucheng pluton is exposed in the northern part of the North China Craton and forms a composite intrusion, consisting of K-feldspar-bearing clinopyroxenite, clinopyroxene-bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene (Wo43–48En19–35Fs18–38), sanidine (An0Ab3–11Or89–97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in situ oxygen isotopes (δ18O = 5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the North China Craton. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the North China Craton facing the Palaeo-Asian Ocean.