Magmatic sources and geodynamics of the early Mesozoic Northern Mongolia-Western Transbaikalia rift zone

The Northern Mongolia-Western Transbaikalia rift zone is the largest Mesozoic riftogenic structure in eastern Asia and extends for a distance of more than 1200 km. The zone consists of depressions and grabens, which were formed between 233 and 188 Ma and are filled with basaltic and basalt-comendite (bimodal) volcanic associations accompanied by numerous peralkaline granite massifs. Geochemical and isotope (Sr, Nd, and Pb) studies showed that mantle and crustal sources contributed to the formation of the magmatic rocks of the rift zone. The basalts were formed from incompatible element-enriched mantle sources. Geochemical and isotope-geochemical data suggest that the peralkaline salic rocks (comendites and peralkaline grantoids) and basalts are genetically related and were formed by the fractionation of a common parental magma. In addition, the magmatic associations contain peralkaline granites and comendites whose isotope signatures indicate their formation through the crustal contamination of derivatives of basaltic melts. The rift zone has arisen during the formation of the Mongolia-Transbaikalia zoned magmatic area in a complex geodynamic setting, combining collision in the Mongolia-Okhotsk suture with a mantle plume impact. The rift zone occupies the northern periphery of the area, being controlled by the Northern Mongolia-Transbaikalia fault system, which marks the boundaries (sutures) of large terranes in the lithosphere. Asthenospheric traps beneath suture boundaries served as pathways for the penetration of a mantle plume into the upper lithosphere, thus playing an important role in the localization of the riftogenic processes.     

High-pressure mafic granulites of the South Muya Block (Central Asian Orogenic Belt)

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.     

Isotope–Geochemical Evidence for the Nature of Protolite Eclogite of the Kokchetav Massif (Kazakhstan)

Doklady Earth Sciences - In the present paper, the results of our isotope–geochemical studies on eclogites of the ultrahighpressure metamorphic complex of the Kokchetav massif are reported....     

Space–time relationships between volcanic associations of different alkalinities: The Belogolovskii Massif,Sredinnyi Range,Kamchatka. Part II. Geochemistry of volcanic rocks and magma sources

Data are presented relating to volcanic series in the Belogolovskii Massif, Sredinnyi Range, Kamchatka. We discuss new geochronologic data, the distributions of rare elements and platinum elements in the rocks, and list the isotope characteristics of volcanic series with normal and moderate alkalinities. We show that the Late Pliocene to Early Pleistocene rocks that belong to the moderate alkaline series of the Belogolovskii volcanic massif are different from rocks in the normally alkaline series of the Late Miocene to Middle Pliocene volcanogenic basement in having higher concentrations of the HFSE and LILE components. We propose a model for the generation of moderate alkaline magmas involving a heterogeneous depleted and a heterogeneous enriched source of material. According to the isotope data, one of these sources may be the subducted oceanic lithosphere of the Pacific and the Commander-Islands type, while the other source was recycled material of the Indian MORB type.     

Isotopic signatures of strontium,neodymium, and lead in metamorphic rocks of the Khavyven highland in eastern Kamchatka

Metamorphic units of the Khavyven Highland that crop out in the northern portion of the Khavyven Uplift of the basement structures of the Central Kamchatka Trough are formed by rocks of the Khavyven Formation, which are metamorphosed in the green-schist facies. The formation comprises two strata: the lower part that consists of amphibole-micaceous ± garnet, epidote-micaceous ± garnet crystalline schists, and micaceous ± garnet quartzite schists has a total thickness of some 500 m, and the upper part, which is formed by epidote-amphibole and phengite-amphibole green schists and overlying epidote-amphibole-micaceous quartzites, with a visible thickness of some 750 m. The isotopic ratios of Sr, Nd, and Pb were determined in the examined rocks of the Khavyven Formation for the first time. The high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd ratios and the high K/La, Ba/Th, Th/Ta, and La/Nb ratios in combination with a deep Ta-Nb minimum indicate that the original volcanites of the crystalline schists of the lower rock mass had a subduction nature. The green schist of the upper rock mass, whose composition corresponds to that of spilitic basalts, have elevated ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, thus combining indications of depleted melts of the N-MORB and E-MORB types and those of subduction melts, which explains the deep Ta-Nb minimum and the low (La/Yb) _N ratios. The isotopic signatures of lead in rocks of the lower and the upper strata are similar. The composition points of the crystalline schists and the green schists are located near the trend of isotopic evolution of lead in the depleted mantle, which indicates that the rocks are closely related to this mantle source.     

Ore Potential of Granitic Rocks of the Gargan Block,East Sayan

Geology of Ore Deposits - An algorithm for identifying derivatives of potentially ore-bearing magmas among Precambrian rocks of tonalite–trondhjemite–granodiorite associations (TTGAs)...     

The new data on the origin of the Patom Crater (East Siberia)

It has been found that the origin of the Patom Crater is related to endogenous processes with the main role played by deep flow of fluid components, which determine formation of the ejecta cone at about 500 years ago or more. This is evidenced by the zonal structure of the crater and geochemical peculiarities of rocks, caused by the long formation time for particular zones. Sandstone and schist blocks that were included into eruptive breccia within the crater were affected by gaseous or fluid components and intensively carbonized. During carbonatization, these rocks within the crater were being enriched in Ca and Sr, but the shares of the ⁸⁷Sr and, consequently, ⁸⁷Sr/⁸⁶Sr ratio in them abruptly decrease. This is explained by the influence of deep fluids on terrigenous rocks, which were initially depleted in the radiogenic strontium isotope and might flow from a magmatic source with a low ⁸⁷Sr/⁸⁶Sr ratio. However, these fluids were enriched in CO2 and transported significant quantities of Sr, which led to enrichment of all terrigenous rocks in the crater in this element. The discovery of individual sandstone blocks with high concentrations of summarized rare earth elements (up to 557 g/t) and higher Sr and Ba contents among the fragments of host stratum within the Patom Crater allows us to suppose that there is a magmatic source enriched in fluid components at depths. The effect of the active fluid phase with low strontium isotopic ratios on rocks during the Patom Crater formation might lead to an abrupt decrease in values of the initial ⁸⁷Sr/⁸⁶Sr ratio in carbonized sandstones and schists.