Average compositions of magmas and mantle sources of mid-ocean ridges and intraplate oceanic and continental settings estimated from the data on melt inclusions and quenched glasses of basalts

Based on the generalization of the compositions of melt inclusions and quenched glasses from basaltic rocks, the average compositions of magmas were estimated for mid-ocean ridges (MOR), intraplate continental environments (CR), and ocean islands and plateaus (OI). These compositions were used to constrain the average contents of trace and volatile elements in mantle sources. A procedure was developed for the estimation of the average contents of incompatible elements, including volatiles (H₂O, Cl, F, and S), in the mantle. A comparison of the obtained average contents for the depleted mantle (DM) with the available published estimates showed that the contents of most incompatible trace elements (H₂O, Cl, F, Be, B, Rb, Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, Hf, Ta, Th, and U) can be reliably estimated from the ratio of K to the desired trace element in the MOR magmas and the average content of K in the DM. For Nb, Ti, P, S, Li, Y, and heavy REE, we used the ratios of their contents to an element with a similar degree of incompatibility in MOR magmas (U for Nb and Dy for the other elements). This approach was used to determine the average contents of incompatible elements in oceanic plume mantle (OPM) and the subcontinental mantle of intraplate settings or continental plume mantle (CPM). It was shown that the average composition of both suboceanic and subcontinental mantle plumes is moderately enriched compared with the DM in the most incompatible elements (K, U, Ba, and La) and volatile components (H₂O, Cl, and F). The extent of volatile component enrichment in the plume mantle (500–1500 ppm H₂O) is insufficient for a significant depression of the mantle solidus. Therefore, mantle plumes must be hotter than the ambient depleted mantle. The average contents of incompatible trace elements in the OPM are similar to those of the primitive mantle, which could be related either to the retention of primitive mantle material in the regions of plume generation or to DM fertilization at the expense of the deep mantle recycling of crustal materials. In the latter case, the negative anomaly of water in the trace-element distribution patterns of the OPM is explained by the participation of dehydrated crust in its formation. Variations in the compositions of magmas and their sources were considered for various geodynamic settings, and it was shown that the sources are heterogeneous with respect to trace and volatile components. The chemical heterogeneity of the magma sources and gradual transitions between them suggest that the mantle reservoirs interact with each other. Chemical variations in continental and oceanic plume magmas can be attributed to the existence of several interacting sources, including one depleted and at least two enriched reservoirs with different contents of volatiles. These variations are in agreement with the zoned structure of mantle plumes, which consist of a hot and relatively dry core, a colder outer shell with high contents of volatile components, and a zone of interaction between the plume and depleted mantle.     

Isotopic structure and evolution of the continental crust in the East Transbaikalian segment of the Central Asian Foldbelt

New data on the geology and tectonics of the main structural elements of the East Transbaikalian segment of the Central Asian Foldbelt are discussed. Correlation charts of the main stratified and igneous complexes are compiled. The rocks of the Baikal-Patom and Baikal-Muya belts, as well as the Barguzin-Vitim Superterrane, are characterized by new Nd isotopic data, which have allowed us to establish the sources of these rocks, to separate isotopic provinces, and to distinguish two stages of crust-forming processes: the Early Baikalian (1.0–0.8 Ga) and the Late Baikalian (0.70–0.62 Ga). The Early Baikalian crust was formed in relatively narrow and spatially isolated troughs of the Baikal-Muya Belt and probably in the Amalat Terrane, whereas the Late Baikalian continental crust was formed and reworked in the Karalon-Mamakan, Yana, and Katera-Uakit zones of the Baikal-Muya Belt. The Baikal-Patom Belt and most of the Anamakit-Muya Zone in the Baikal-Muya Belt are characterized by remobilization of the Early Precambrian continental crust and by a subordinate role of Late Riphean juvenile sources. Reworking of the mixed Late Riphean and Early Precambrian crustal sources is typical of the Barguzin-Vitim Superterrane. The origination and evolution of the continental crust in the studied region are considered in light of new data; alternative versions of paleogedynamic reconstructions are discussed.     

Late Cenozoic volcanic province in Central and East Asia

The paper presents materials on the inner structure of the Late Cenozoic within-plate volcanic province in Central and East Asia, in which two subprovinces are distinguished: Central Asian and Far Eastern, which comprise a number of autonomously evolving volcanic areas. Some of the volcanic areas are proved to have evolved for a long time, starting in the Late Mesozoic. In spite of differences in their age and structural setting, the volcanic areas evolved according to similar scenarios in the Late Cenozoic. Magmatism in the province was related to a mantle source of the within-plate type. The magmatic associations are dominated by mafic alkaline high-K rocks. The rocks are geochemically close to basalts of the OIB type, and their isotopic composition corresponds to a combination of mantle sources of the PREMA, EMI, and EMII types at the predominance of PREMA. Geological, geochemical, and isotopic lines of evidence suggest that magmatism in the province was related to mantle plumes. This is consistent with geophysical data, which testify that the volcanic areas are underlain by upwellings of the asthenospheric mantle or plumes. Seismic tomography data indicate that the “stems” of the plumes can be traced down to the upper and lower mantle. The province is thought to have been produced when the eastern margin of the Asian plate overlapped one of the branches of the Pacific superplume at approximately 160 Ma. This branch of the superplume is pronounced in the modern mantle structure as a cluster of mantle plumes that control (according to seismic tomography data) the interaction zone of the Pacific and Asian lithospheric plates.     

Ophiolites of Western Tyva as fragments of a late Vendian Island arc of the Paleoasian ocean

Doklady Earth Sciences -     

The Age and Composition of Alkaline Rocks of the Dugda Massif (Eastern Sayan): Patterns of the Manifestation of the Late Paleozoic Rare-Metal Magmatism in the Southwestern Fold Frame of the Siberian Platform

Doklady Earth Sciences - The age and geochemical characteristics of alkaline rocks of the Dugda massif (Eastern Tuva), attributed to the East Sayan Late Paleozoic rare-metal magmatic zone, have...     

Magmatism and formation conditions of the Urma helvite-bertrandite deposit,West Transbaikal berillium province

The relationships between mineralization and magmatism during the formation of the Early Mesozoic West Transbaikal beryllium province are exemplified in the Urma helvite-bertrandite deposit. The deposit is drawn toward granitoids of elevated alkalinity, which belong to the Tashir Complex. Mineralization is related to leucogranite and characterized by patched distribution controlled by localization of metasomatic alteration. The latter is identified owing to replacement of feldspar with microcline and albite followed by silicification related to fracture zones. Helvite and bertrandite are the major Be minerals at the deposit. The Be grade of the ore is nonuniform and varies from 740 to 25000 ppm. Zircon, malacon, monazite, allanite, bastnaesite, columbite, and xenotime occur in metasomatic rocks together with Be minerals. Geochemical characteristics of alkali granites and metasomatic rocks are similar in a wide range of incompatible elements. Both are characterized by lowered Ba, Sr, P, and Eu contents and enriched in Th, U, Pb, Zr, and Hf. The degree of enrichment is the highest in the ore. The Be content in the ore correlates with concentrations of a number of other rare metals typical of host granite, which form their own mineralization against the background of metasomatic alteration, including Zr and REE minerals. Similarity in geochemistry of granitic rocks and Be ore indicates that the Urma deposit was related to the evolution of magmatic melt. Regional correlation shows that the ore-magmatic system of the Urma deposit is close to that of the Orot deposit, one of the largest in the central segment of the West Transbaikal metallogenic province. Both deposits are characterized by a similar composition of granitoids and comparable localization of ore zones in the structure of plutons. This similarity supports the high ore resource potential of Early Mesozoic alkali granites in the western Transbaikal region. Taking into account that these granitoids are widespread in the West Transbaikal Rift Zone that controls the metallogenic province, one can expect the discovery of new deposits therein.     

Structure,age, and ore potential of the Burpala rare-metal alkaline massif,northern Baikal region

The Burpala alkaline massif is a unique geological object. More than 50 Zr, Nb, Ti, Th, Be, and REE minerals have been identified in rare-metal syenite of this massif. Their contents often reach tens of percent, and concentrations of rare elements in rocks are as high as 3.6% REE, 4% Zr, 0.5% Y, 0.5% Nb, 0.5% Th, and 0.1% U. Geological and geochemical data show that all rocks in the Burpala massif are derivatives of alkaline magma initially enriched in rare elements. These rocks vary in composition from shonkinite, melanocratic syenite, nepheline and alkali syenites to alaskite and alkali granite. The extreme products of magma fractionation are rare-metal pegmatites, apatite-fluorite rocks, and carbonatites. The primary melts were related to the enriched EM-2 mantle source. The U-Pb zircon ages of pulaskite (main intrusive phase) and rare-metal syenite (vein phase) are estimated at 294 ± 1 and 283 ± 8 Ma, respectively. The massif was formed as a result of impact of the mantle plume on the active continental margin of the Siberian paleocontinent.     

Crystallogenesis of zircon in alkaline granites and specifics of zircon U-Pb dating: A case study of the Khangai magmatic area

Data obtained on the Khangai zonal magmatic areole in the Central Asian Orogenic Belt indicate that accessory zircons in alkaline granitoids have crystallogenetic characteristics that generally reflect specifics of the evolution of the parental alkaline magmas. The late differentiation of these magmas was characterized by an increase in the concentrations of trace and rare-earth elements, water, and fluorine in the residual melts and aqueous salt-bearing fluids. Their action was associated with local transformations of the zircon, first and foremost, with local enrichment of zircon crystals in fluid inclusions and in crystalline inclusions of sulfides an other ore minerals. This disturbed the Pb isotopic composition of the zircon, for example, led to its enrichment in common (admixture) Pb and thus caused significant errors and uncertainties in the U-Pb zircon dates. Our recently obtained data indicate that one of the most efficient methods of preparing accessory zircons enriched in common Pb to their U-Pb dating is their preparatory treatment with acids (Makeev, 1981; Mattinson, 1994, 1997, 2005). The application of this technique makes it possible to rid the zircons of phases enriched in common Pb and usually to obtain reliable geochronologic data. The method of preparatory acid treatment of zircons is thus best suitable for geochronologic studies of granitoids of elevated alkalinity.