Trondhjemite-granodiorite intrusive magmatism of the Losevo lithotectonic zone of the Voronezh crystalline massif

The mineralogical-petrographic, petrochemical, and geochemical characteristics of the trondhjemite-granodiorite intrusions widespread among the volcanoterrigenous rocks of the eastern part of the Voronezh crystalline massif are studied. The rocks are characterized by cotectic quartz-plagioclase differentiation and differentiated (La/Yb = 31) REE pattern and are enriched in LILE with a decrease in the K/Rb and Zr/Hf ratios and an increase in the Th/U ratio by 1.5–2 times relative to subduction trondhjemites and their volcanic analogs from the modern and young island arcs. These data allow us to conclude that generation of the trondhjemite magma is a result of melting of the rocks of the continental crust during collision. The concordant (SHRIMP) age of trondhjemites is 2047 ± 11 Ma, and the age of zircon cores probably entrapped from the host rocks by the trondhjemitic melt is 2172 ± 17 Ma.     

Phase equilibria of bastnaesite,allanite, and monazite: Bastnaesite-out isograde in metapelites of the Vorontsovskaya group,Voronezh crystalline massif

Paleoproterozoic metapelites of the Vorontsovskaya structure contain accessory REE phosphates (monazite, xenotime, and REE-apatite), fluorine-carbonates (bastnaesite and synchysite), and silicate (allanite). Analysis of phase equilibria involving REE-bearing minerals indicates that bastnaesite is stable only in the greenschist facies and decomposes with the synthesis of monazite at temperatures below the staurolite isograde (490–500°C) at a pressure of 3 kbar. Monazite first appears in the greenschist facies, and its stability expands with increasing temperature, including the granulite facies. A diversity of reaction textures suggests that the mineral is formed in the garnet zone by a reaction of bastnaesite with apatite and by the partial decomposition of REE-bearing chlorite. Monazite is produced in the garnet and staurolite zones by a reaction of allanite with apatite and by a decomposition reaction of REE-bearing apatite.     

Paleoproterozoic sequences and magmatic complexes of the Losevo suture zone of the Voronezh crystalline massif: Geological position,material composition,geochemistry, and paleogeodynamics

In order to resolve the contradictions associated with uncertainty in the identification of the material composition, subdivision, and conditions of formation of the Paleoproterozoic intrusive, metavolcanogenic, and metasedimentary sequences of the Losevo suture zone of the Voronezh crystalline massif, this work presents geological, petrographic, petrochemical, and geochemical features of these sequences. The stratigraphic and magmatic scheme of the central part of the Losevo suture zone is clarified. In particular, the Paleoproterozoic Losevo Series is divided into two sequences: Strelitsa (marginal sea) and Podgornoe (island arc). A new hypabyssal Novo-Voronezh metagabbro-diabase complex, comagmatic to metatholeiites of the Podgornoe sequence, is distinguished. The isotope age of the Strelitsa sequence is assumed to be 2172 ± 17 Ma on the basis of the results of age dating of zircon cores from the Usman plagiogranites, intruding this sequence. The upper age boundary of the Strelitsa sequence corresponds to the age of premetamorphic gabbro of the Rozhdestvenskoe complex, comagmatic to metavolcanites (2120 ± 11–2158 ± 43 Ma). The age of the Usman plagiogranite complex is clarified. On the basis of geological-structural and petrographic-mineralogical analyses of metavolcanogenic rocks, lithological analysis of metasedimentary formations, and new geochemical data obtained from metavolcanites and metamorphosed deposits, the pattern of paleogeodynamic evolution of the Losevo suture zone in the first half of the Paleoproterozoic is proposed. The next stages are distinguished: (1) intrusion of tholeiites of transition T-MORB type in spreading zones and deposition of terrigenous strata in the marginal sea basins; (2) intrusion of Nb-depleted tholeiites and plagiorhyolites, the geochemical characteristics indicating their formation in the subduction setting; (3) intrusion of gabbroids of the Rozhdestvenskoe complex; (4) formation of an island arc synchronously with stage 2, tholeiitic and calc-alkaline (Podgornoe sequence) volcanism; (5) intrusions of gabbro-diabases, subsynchronous to volcanism, of the Novovoronezh complex and diorite-granitoides, crystallization of granitoides of the Usman complex; (6) a break in sedimentation and formation of molasses of the Voronezh (Somovo) Formation.     

Grothite from garnet-micaceous crystalline schist of the Shikhtinskaya suite in the Middle Kamchatka massif

Doklady Earth Sciences -     

长城堡岩体西段放射性水文地质特征及其找矿方向

文章论述了长城堡岩体西段放射性水文地质特征、水化学异常特征和铀矿化特征，探讨了三者之问的相关关系；指出该区铀矿化受水化学异常区和水文地球化学垂直分带的控制，铀主要富集在氧化-还原过渡带；根据该区铀易浸出的特点，认为适合于用溶浸和地浸法开采。     

山西地块的深部构造和金伯利建造:兼论金刚石矿与稳定地块的深部过程

稳定地块的深部,软流圈上拱,岩石圈破裂,产生三联结构超壳深断裂系,地幔热流使上地幔岩局部熔融,形成碱性-偏碱性系列岩浆,沿深断裂系侵位过程中演化为金伯利岩建造.金刚石聚集于该建造的终端-(角砾状)碱性超基性岩类,爆发式侵入上地壳,迅速冷凝,金刚石得以保存而成为有用资源.我国的稳定地块面积颇广,显生宙经历多次深部过程,已查明多处金刚石矿藏,仍有巨大找矿潜力.     

The geology and petrology of the metavolcanic rocks of the Kvakhona Formation of the Sredinnyi Range crystalline massif in Kamchatka

The metavolcanic rocks of the Kvakhona Formation exposed on the western slopes of the Sredinnyi Range metamorphic massif are represented by two sequences. The lower sequence occupying the most part of the exposed formation is dominated by porphyric and aphyric clinopyroxene-plagioclase metabasalts and their tuffs with subordinate metapicritic basalts, metaandesites and their tuffs, and metadacites. The latter form isolated bodies in the northern part of the Kvakhona exposures. The upper sequence is composed of metaandesites, metabasalts, and their tuffs intercalated with terrigenous rocks (siltstones, sandstones, and carbonate graywacke) and metadacite bodies. The rocks were subjected to intense metamorphism under the greenschist facies conditions (t = 250–420°C, P _s around 1 kbar) with the replacement of clinopyroxene phenocrysts (or their clasts in the tuff varieties) by actinolite, chlorite, and epidote, while plagioclase phenocrysts are replaced by albite, muscovite, chlorite, and epidote. In the metabasalts enriched in ore minerals, clinopyroxene is replaced by very thin veinlets and the finest grains (about 20–30 μm) of Ca-Na and Na amphiboles (winchite, ferrowinchite, glaucophane, and ferroglaucophane). The groundmass of the rocks (or tuff cement) consists of variable combinations of titanite, magnetite, chlorite, epidote, silpnomelane, and albite. The metabasalts of the formation belong to the high-Fe and often high-Ti rocks, which makes them similar to the tholeiitic basalts of mid-ocean ridges or rifting zones. The metaandesites and metadacites also preserved an elevated Fe content and belong to calc-alkaline series rocks typical of island arcs and active continental margins. It is suggested that the primary rocks of the Kvakhona Formation were formed within volcanic centers on the floor of a vast Cretaceous epicontinental marginal basin, which accumulated thick sequences of terrigenous rocks. The detrital material for these rocks was supplied from the northeastern Asian continent. The geological and geochemical data testify to the similarity of the Kvakhona metavolcanic rocks and the greenstone altered volcanic rocks of the Pensantain Range of Western Kamchatka, which are dated by the U-Pb SHRIMP method at 90–100 Ma. The protolith of the metavolcanic rocks of the Kvakhona Formation was presumably formed within the same age interval.     

Trace elements in various genetic types of zircon from syenite of the Sakharjok massif,Kola Peninsula

Zircon is an accessory mineral in alkali and nepheline syenites of the Neoarchean Sakharjok intrusion. Zircon in association with britholite and pyrochlore forms orebodies in nepheline syenite of this massif. Zircon crystals reveal an inhomogeneous zonal, occasionally mosaic structure comprising fragments and zones related to magmatic, hydrothermal, and metamorphic stages of mineral formation. Magmatic zircon differs by a high REE concentration (1769 ppm, on average), distinct Ce maximum (Ce/Ce* = 105, on average), and Eu minimum (Eu/Eu* = 0.19) as compared with other genetic types. No correlation between these parameters has been established. Hydrothermal zircon is characterized by a low Ce/Ce* ratio (0.7–3.9 and 2.0, on average), elevated LREE contents, and lowered ratios of MREE and HREE to La. Metamorphic zircon differs from magmatic by a sharply lower REE concentration (385 ppm, on average), lowered Th/U (0.32) and Ce/Ce* (31.9, on average) ratios. In the Ce/Ce* versus MREE/La plot, the lowest values of these ratios are typical of hydrothermal zircon, while the intermediate and maximum values are inherent to metamorphic and magmatic zircons, respectively. These variations make it possible to delineate reliable fields of their compositions. The distribution of data points in the above-mentioned plots shows that REE chemical activity depends on the redox conditions of zircon crystallization.     

Carbonatites of the Dara-i-Pioz alkaline massif,central Tajikistan,and their compositional features

Carbonatites found in the Dara-i-Pioz alkaline massif, Tajikistan, form linearly-elongated, steeply-dipping and, sometimes, rounded in plan pipe-like bodies of mainly calcitic composition. As compared to reference carbonatites, they have lower contents of Fe, Mg, Ti, Sr, Ba, Mn, P, REE, Zr, Nb, Ta, and other elements but elevated contents of Rb, Cs, Al, Si, Na, K, and others. However, the Dara-i-Pioz carbonatites are characterized by such typomorphic features of carbonatites as the sharp predominance of LREE over HREE and high ΣCe: ΣY, La: Yb, and Sr: Ba ratios.     

Behavior of iron-group elements,oxybarometry, and genesis of unique chromite deposits in the Kempirsai massif

Ultramafic rocks and high-Cr chromite ore from the Almaz-Zhemchuzhina deposit, the largest in the Main ore field of the Kempirsai massif, have been studied. The detailed mineralogical and geochemical examination of deep structure test and exploratory boreholes allowed us to establish the rough stratification of ultramafic rocks and to demonstrate the position of unique chromite deposits in the generalized vertical section of the southeastern Kempirsai massif. From top to bottom, a barren harzburgite-lherzolite series gives way to an ore-bearing dunite-harzburgite complex with the largest chromite deposits, including the unique Almaz-Zhemchuzhina deposit, in its upper portion and then to pyroxene-free dunite densely impregnated with chromite in the upper part and containing sparsely disseminated chromite at its base. The lower unit is composed of a barren lherzolite-harzburgite series transformed into blastomylonites near the contact with dunite, suggesting a tectonically doubled section in the southeastern part of the massif. The synore asymmetric geochemical zoning developed in the course of formation of chromite deposits as a result of removal of oreforming iron-group elements from the underlying and wall ultramafic rocks into the overlying rocks. Host rocks with disturbed initial proportions of Cr, Fe, Ni, and Mn, together with orebodies, made up ore-bearing zones no less than 1 km in thickness and subdivided into supra-, inter-, and subore subzones. The subore and wall rocks are characterized by partial loss (wt %) of Cr₂O₃(0.1), NiO (0.04), FeO_tot(0.5), and MnO (0.02) and their removal into the interore and supraore (0.03 NiO) subzones. Thus, the subore ultramafic rocks served as a source of ore-forming components, while the interore zone with orebodies occurring therein served as a zone of discharge of these components. Using Mössbauer spectroscopy, the crystal chemistry of iron ions was studied in a representative selection of Cr-spinel samples from rocks and ores of the southeastern and western blocks (the Almaz-Zhemchuzhina and Geophysical XII deposits). The degree of iron oxidation in the samples varies from 8 to 33%. In most cases, a difference in degree of iron oxidation is established in stoichiometric approximation and from Mössbauer data. In other words, the integral stoichiometry of ferrous and ferric ions is disturbed. Such a disturbance may be related not only to partial inversion of the Cr-spinel structure but also to local heterogeneity of the mineral at the micro- and nanolevels with clustering of cations and formation of their associates. An empirical correction of the olivine-Cr-spinel geothermometer and oxybarometer has been performed. The inverse correlation between oxygen fugacity and degree of depletion of ultramafic rocks indicates that these rocks were formed in a closed system with participation of a water-methane fluid. Along with stratification of ultramafics, this correlation testifies to a powerful asthenospheric source of reduced fluids. The retention of low oxygen fugacity in central portions of orebodies does not rule out that after a break this source participated in the formation of unique chromite deposits in the Kempirsai massif.