Rare Element Compositions Research Articles

The Underwater Esmeralda Volcano (Mariana Island Arch) and some Features of the Composition of its Composition Rocks

A generalization of the available original data and literature data on the geological and geophysical knowledge of the underwater volcano Esmeralda, located in the Mariana Island Arc, has been carried out. As a result of studying the rocks dredged during the 4th and 5th cruises of the R/V Vulkanolog at the present level, new data were obtained on the silicate and rare-element composition of the rock samples that make up this underwater volcano. It has been established that the studied volcanic edifice is composed of five types of rocks: basalts, basaltic andesites, dacites, gabbro, and basanites. For the first time, samples of dacite and basanite have been discovered, indicating that the petrochemical diversity of the underwater volcano Esmeralda is wider than previously thought. All dredged rocks are characterized by a slightly increased content of incoherent elements LILE and HFSE. The studies carried out made it possible to attribute the main part of the dredged rocks to the association of island-arc ferruginous tholeiites (IAB, IAT) and only the composition of a single sample of alkaline basalt (basanite) falls into the field of alkaline basalts of oceanic islands (OIB, OIA). The increased content of iron in plagioclase phenocrysts confirms that the rocks belong to the high-iron tholeiite association.

ПЕТРОЛОГО-ГЕОХИМИЧЕСКИЕ ОСОБЕННОСТИ БАЗАЛЬТОВ СТАНЦИЙ ДРАГИРОВАНИЯ 2776 И 2764 (ФИЛИППИНСКОЕ МОРЕ)

The article presents original data of the radioisotope age and composition of rare elements in basalts of dredging stations 2776 and 2764 located in the southern part of the Central Fault in the Philippine Sea. Researchs have shown that basalts of st. 2776 demonstrate the chemical features of tholeiites of middle-oceanic ridges and oceanic island arcs (N-MORB+OIAB). And basalts of st. 2764 combine the features of E-MORB+OIAB. A comparative analysis of the chemical composition of the volcanics of both stations and of the Kyushu-Palau Ridge shows that all rocks are united by the enrichment of melts with a «subductional» and withinplate (WPB) components. But WPB features in the rocks are mostly manifested at the level of the Primitive Mantle (PM), and these features have not reached to the level of alkaline basaltoids of the ocean islands (OIB).

Pliocene–Holocene Alkali-Basaltoid Volcanism of the Tsushima Basin of the Sea of Japan: New Geochemical and Geodynamic Data

Abstract—The paper contains original data on the rock-forming and rare element compositions in the Pliocene–Holocene alkaline basaltoids of the Tsushima Basin Sea of Japan, obtained by the “wet chemistry” method, atomic emission spectrometry, and inductively coupled plasma mass spectrometry. Based on interpretation of these data, the geochemical and geodynamic features of this stage of volcanism were established; the source of magmogenesis was revealed, represented by garnet peridotites. The withinplate plume–continental nature of the alkaline basaltoids was established, indicated by the high alkalinity and potassium content of rocks, Ta–Nb maximum, high normalized La/Yb (22–28) and La/Sm (3–6) values, as well as high Ti/V, Nb/Y, Zr/Y, Zr/Hf, Th/Yb, and Nb/Ta indices close to chondrite. The withinplate nature is also to some degree confirmed by the closeness of the chemical features of Tsushima basaltoids to rocks of Zinbochu and Udalyanche volcanoes of the Central Asian withinplate province (CAWP) located to west from the Sea of Japan, and it was formed in the Late Cenozoic under the influence of the North Asian superplume.

Geochemical evolution of the early Paleozoic collisional magmatism from autochthonous migmatites and granitoids to multiphase granite intrusions (Sharanur and Aya complexes, Baikal Region)

Overall petrologic and geochemical data indicate that the early Paleozoic magmatism in the Olkhon area of the Baikal Region exhibits diverse types of granitoids, whose time of formation is estimated at a narrow age interval of 500-465 Ma. This magmatism was responsible for the formation of both autochthonous gneiss-migmatite-granitoid suites (Sharanur complex) and multiphase intrusions (Aya complex) emplaced into the upper horizons of the continental crust. In major-element chemistry, K2O/Na2O values, and rare-element composition the migmatite-plagiogranites and calc-alkaline and subalkaline granitoids of the Sharanur complex are similar to the host gneisses and schists, as they were likely derived from melting of the ancient metamorphic substratum of the Olkhon series. In new isotope-geochemical characteristics (ICP MS method) the Sharanur granitoids are close to the first-phase biotite granites of the Aya massif, whose further geochemical evolution was governed mainly by intrachamber magmatic differentiation leading to the production of second-phase leucogranites enriched in HREE and HFSE (in particular, Ta and Nb) and depleted in Sr, Ba, Eu, Li, and LREE. The origin of the autochthonous and intrusive granitoids is related to early Paleozoic collision events within the Olkhon metamorphic terrane, while the formation of syncollisional granitoids is best explained by both melting of the crust protolith (Sharanur complex) and magmatic differentiation (multiphase Aya intrusion). All mineralogical and geochemical characteristics indicate that these granitoids are distinguished from rare-metal pegmatoid granites and Li-F and Rb-Be-Nb pegmatites, whose vein bodies crosscut the granitoids, and are regarded as middle Paleozoic rocks, which mark the transition to within-plate magmatism in the Baikal Region.

Cretaceous sedimentary–volcanogenic complexes of the Kamchatka Isthmus: Structure, composition and geodynamic formation conditions

New data on the chemical and rare-element composition and age of the rocks referred earlier to the Iruney suite of the Kamchatka Isthmus are received. In the recent structure these rocks compose the structural–strata complexes of the nappe-folded Lesnovsky Rise. Radiolarian analysis data substantiate that the deposits belonging to the Ening series and the middle and upper parts of the Iruney suite were formed in a single sedimentation basin in the Campanian time. The discovery of a new occurrence of Prunobrachidae representatives on the Kamchatka Peninsula allows us to draw wide interregional correlations and reconstruct the sedimentation conditions. The studied volcanites relate to different igneous series and were formed in geodynamic conditions of the marginal sea and the volcanic arc. The igneous rocks of the Ening stratum are similar to the N-MOR and OI basalts that were formed within the marginal sea (Iruney Marginal Sea) basin. The Upper Cretaceous formations of the eastern slope of the Sredinny Range were formed within the volcanic rise with the island-arc type of volcanism. The younger Eocene igneous rocks of the neo-autochthon (granites and granodiorites) and the volcanic rocks of the Kinkil suite mark a new orogenic stage of development of the Kamchatka margin.

Petrology of the tin-bearing granite-leucogranites of the Piaoak Massif, Northern Vietnam

The Piaoak tin-bearing granite-leucogranites located in the Caobang Province of Northern Vietnam compose a stock-like hypabyssal body. Host rocks are represented by Early Devonian carbonate sequences and Early Triassic “black” shales. The geochronological age of the Piaoak granite-leucogranites corresponds to the Late Cretaceous: T = 83.5 ± 6.2 Ma, 87Rb/86Sr method; T = 89.7 ± 1.0 Ma, 39Ar/40Ar method. The massif has a simple basic to acid order: two-mica and muscovite granite-leucogranite → raremetal aplites, pegmatites → tin-bearing greisens and hydrothermal veins. The petrographic and microstructural studies revealed a sharp change in crystallization conditions of the granite-leucogranite magma at the late magmatic stage and formation of muscovite via incongruent melting of protolithionite. The study of melt and coexisting fluid inclusions showed that solidus crystallization occurred under fluid-saturated conditions at 635–600°C. In composition, the granite-leucogranites of the Piaoak Massif correspond to the raremetal-plumasite geochemical type (according to L.V. Tauson), and reach Li-F facies in terms of their rare-element composition. The composition of aplites and pegmatites demonstrates that granite-leucogranite magma did not accumulate lithophile and volatile components in the residual melt during differentiation, but was initially enriched in rare-metals. It is most probable that the melt was generated from Proterozoic lithotectonic complexes and overlaying Lower Triassic “black” shales.

Rare-element composition of pyropes and lamproites and ancient dispersion haloes of the southwestern Siberian platform

The problem of heterogeneity of the mantle lithosphere of the southwestern portion of the Siberian Platform has been considered, and the diamond content in potential mother lodes within this area has been estimated based on original geochemical data on the rare-element composition of pyropes from diamondiferous lamproites of the Ingashin field within the Prisayan region and ancient dispersion haloes of minerals accompanying diamonds in the area between the Angara and Uda rivers. Pyropes from lamproites are characterized by low concentrations of Zr (0.18–9.05 ppm), Hf (0.03–0.37 ppm), and rare earth elements (Sm 0.04–0.49, Eu 0.02–0.16, and Dy 0.05–0.96 ppm). Pyropes from the Lower Carboniferous Baeron Formation within the Tangui-Chuksha area are significantly different from pyropes of the Ingashin lamproites in high contents of Zr (30.36–139.23 ppm) and Hf (0.4–2.22 ppm). These pyropes are characterized by elevated concentrations of rare earth elements (Sm 1.34–3.68, Eu 0.53–1.17, and Dy 1.0–2.05 ppm). The distribution patterns of rare incompatible elements in pyropes of the Lower Carboniferous Mura massif within the Mura area manifest even stronger differences with pyropes of the Ingashin lamproites and in many respects with pyropes from Lower Carboniferous sediments of the Baeron Formation within the Tangui-Chuksha area. The results obtained indicate that there is no large-scale regional spreading of pyropes from Mid-Riphean lamproite bodies in the course of washout of these bodies and that the mantle lithosphere in the southwestern portion of the Siberian Platform is laterally heterogeneous in mineralogical-geochemical terms. The chemical composition and the peculiar distribution pattern of rare elements in pyropes from lamproites of the Prisayan region indicate a depleted, primarily lherzolite composition of the upper mantle that was transformed through low-temperature potassium metasomatosis. In terms of the chemical and rare-element compositions, pyropes from Lower Carboniferous sediments of the Tangui-Chuksha and Mura areas belong to a wider range of mantle rocks: depleted peridotites, metasomatic peridotites under low (900–1000°C) and high (>1000°C) temperature conditions, and megacrysts. This suggests that the composition of the lithospheric mantle in this area of the southern portion of the Siberian Platform is characterized by a considerably differentiated stratification of mantle rocks, some of which were credibly formed in the diamond stability field.

A fragment of back-arc paleospreading in the Tunka terrane

High magnesium metavolcanites, which are found for the first time in the Tunka terrane (the Baikal-Khubsogul region), are described. By the content of MgO (12–16%), SiO2 (up to 52%), and alkali oxides, they are classified as metamorphosed picrites. There are two groups of them, divided by the TiO2 content and by the TiO2/Al2O3 ratio. When making a distinction between geochemical peculiarities of two groups, their rare-element composition is intermediate between picrites and boninites. The comparison between metavulcanites and their nonmetamorphosed analogues is made, some aspects of their genesis are considered, and the conclusion is made that these metavulcanites mark paleospreading of the back-arc basin.

Physico-chemical parameters of Neoproterozoic and Early Cambrian plume magmatism in the Paleo-Asian ocean ( data on melt inclusions)

The paper presents new data on physico-chemical parameters of the Neoproterozoic–Early Cambrian plume magmatism in the Paleo-Asian Ocean. The data on clinopyroxenes show the plume-related plateaubasalt magmatic systems of the Katun’ paleoseamounts, which interacted with mid-ocean ridge (MOR) magmas. The Kurai paleoseamount consists mainly of plateaubasalt systems, and the Agardag ophiolites represent products of OIB–type “hot-spot” within-plate magmatism. Our study of inclusions showed that the melts of the Katun’ and Kurai paleoseamounts crystallized at lower temperatures (1130–1190 °C) compared to the Agardag ophiolites (1210–1255 °C). The petrochemical analysis of the melt inclusions showed that the Katun’ and Kurai magmatic systems are different from the Mg- and Ti-richer melts of the Agardag ophiolites: the former are similar to the magmas of the Nauru Basin and Ontong Java Plateau (western Pacific), whereas the latter possess geochemical affinities to OIB-type magmatism. The rare-element compositions of the melts of the Katun’ and Kurai paleoseamounts correspond to those of the Ontong Java Plateau and Nauru Basin lavas. The numerically simulated parameters of the Katun’ paleoseamount primary magmas agree with the data on the magmatic systems of the Siberian Platform and Ontong Java Plateau. For the Kurai paleoseamount, the simulated results suggest interaction of deep-seated OIB-type magmatic systems with MOR ones. The Agardag ophiolites were formed in relation to mantle plume activity at the initial stages of paleo-oceanic complexes formation.

The ophiolites of the eastern slope of the South Urals

Silurian and Middle Devonian ophiolites of the South Urals are characterized. Data on the structure, composition, character of contact and age of the members of the ophiolitic section are listed. The similarity between the magmatic rocks of the South Urals and modern oceanic structures is shown on the basis of petrogenic data and composition of rare elements, including rare-earth elements.