Sr-Isotopic Characteristic of the Typical Carbonate Neoproterozoic Siberia Sections

First Sr isotopic data for the type of Neoproterozoic carbonate sediments of the South Siberia are reported. Upper Riphean complex is represented by terrigenous -carbonate rocks of Tungysic Formation,Yenisey Mountain Ridge.Isotopic age of this Formation shows a wide range,from 924±40 Ma(K-Ar method)to 753±6 Ma(U-Pb method).Sediments of Zabit Formation,Bokson Series(East Sayan)and

Tourmalinization at the Darasun goldfield,Eastern Transbaikalia:Compositional,fluid inclusion and isotopic constraints

Zoned tourmaline （schorl-dravite） in the matrix of hydrothermal explosive breccia and ore veins in gold deposits, Chita region, Eastern Transbaikalia, Russia, are associated with Na- and K-rich porphyry-type subvolcanic intrusives. 61So values of tourmaline from three gold deposits （Darasun, Talatui, Teremkinskoye） are ＋8.3%, ＋7.6%, and --6.0% and calculated 6180 values of fluids respon- sible for the tourmalinization are ＋7.3%, ＋7.7%, and ＋4.2%, respectively. These data imply an igneous fluid source, except at the Teremkin deposit where mixing with meteoric water is indicated. Wide ranges of Fe3＋/Fetot and the presence of vacancies characterize the Darasun deposit tourmaline indicating wide ranges off（O2） and pH of mineralizing fluids. Initial stage tourmalines from the gold deposits of the Darasun ore district are dravite or high mg schorl. Second stage tourmaline is characterized by oscillatory zoning but with Fe generally increasing towards crystal rims indicating decreasing temperature. Third stage tourmaline formed unzoned crystals with XMg （mole fraction of Mg） close to that of the first stage tourmaline, due to a close association with pyrite and arsenopyrite. From Fe^3＋/Fetot values, chemical composition and crystallization temperatures, logf（O2） of mineralizing fluids ranged from ca. -25 to 20, much higher than for the gold-bearing beresite-listvenite association, indicating that tourmalinization was not related to gold mineralization.

Tectonometamorphic Cycles in Different Geodynamic Conditions

Formation and attenuation of crust thermal anomalies, accompanied by development of geological structures, are governed by special laws, together with metamorphism and other endogenic process, form tectonometamorphic cycle (TMC). Because of rock rheology under metamorphic conditions, the structures, concurring with metamorphism, reflect geodynamic conditions much more perfect than those of the structures of nonmetamorphosed rocks. For this reason, structural investigations open wide, sometimes unique, possibilities for geodynamic research into regional metamorphosed terrains. The TMC features under different geodynamic conditions are shown with examples of California (subduction), Himalaya and Junggar Alatau (collision), and Northwest environs of the White Sea (polycyclic development). The analysis of these units serves as basis of some general conclusions. TMC, the steady pattern of crust thermal anomalies manifestation, does not practically depend on peculiarities of metamorphism. At lower and middle crust levels, occur the high-temperature complete cycles that include following two stages. The first is represented by nappes and paragenes of parallel bedding flow, the second, by linear folds, domes and faults. At top levels of fold complexes display the low-temperature reduced cycles, expressed only by structures of the second stage. There are gradual transitions between the complete and reduced cycles. Thermal anomalies in the first stage of complete cycles devolop against the background of large horizontal displacements at contacts between interacting plates or are genetically connected with their gently sloping fault planes. Of all structural elements of the cycles, in compressional environments (nappes, linear folds, domes etc.), only the structures of the first stage of complete cycles can be considered as indications of plate tectonics. Presence of such structures at the most ancient supracrustal complexes point to existence of plate tectonics already at the early stages of geological development of the earth.

Zircon Xenocrysts from the Shaka Ridge Record Ancient Continental Crust:New U-Pb Geochronological and Oxygen Isotopic Data

Over the past two decades, a significant number of discoveries of ancient zircon xenocrysts in ocean-floor magmatic rocks have been reported. These findings provide compelling evidence for the presence of ancient continental crust within young oceanic lithosphere. Almost all finds of ancient zircon xenocrysts within oceanic crust are from the Mid-Atlantic Ridge. For other localities, however, similar data are very limited. This report presents the first age determinations(U-Pb, SHRIMPII) and isotope-geochemical data(oxygen, trace and rare earth elements) for zircon xenocrysts from gabbro-diorites of the Shaka Ridge, in the vicinity of the Shaka fracture zone, near the western end of the Southwest Indian Ridge. The work is based on a study of bottom rock material dredged during expeditionary research on the R/V"Akademik Fedorov"(Russia) in 2016. The U-Pb isotope system of the zircon xenocrysts recorded a crystallization age of ~2.8 Ga at an upper discordia intercept and an age of ~600 Ma interpreted as the timing of a superimposed thermal event at a lower discordia intercept.The zircon xenocrysts show geochemical signatures of magmatic origin, i.e., fractionated REE distribution spectra with an increase in chondrite-normalized values from light to heavy REE, positive Ce anomalies and negative Eu anomalies, and high Th/U ratios(0.59-7.77). In discrimination diagrams based on a series of inter-element relationships, zircon compositions fall into the fields of zircons from rocks of continental crust, mostly granitoids. The Li content of the zircons is high(1.8 ppm-50 ppm),adding further evidence to their derivation from rocks of continental crust. During their residence within young oceanic crust, the zircon xenocrysts experienced alterations under the influence of submarine high-temperature hydrothermal fluids, which selectively affected the distribution of trace elements in the zircons and reduced the δ;O values to 1.75‰-3.15‰. The presence of obviously older zircons in Shaka gabbro-diorites clearly demonstrates the presence of ancient continental fragments and their recycling into the mantle at the western end of the Southwest Indian Ridge.

Zircon U-Pb Geochronology Recorded Late Cretaceous Fluid Activation in the Central Aldan Gold Ore District, Aldan Shield, Russia: First Data

The gold mineralization in the Central Aldan ore district is genetically related to potassic calc-alkaline and alkaline magmatism dated at 115–150 Ma.The objective of this study is to establish the age of hydrothermal processes that accompanied the formation of Au-Te mineralization at the Samolazovsky Deposit.Based on the isotope-geochemical study of zircons from quartz-feldspar metasomatic rocks of the deposit,the granitoids and charnokites of the Nimnyr Complex(1900–1960 Ma)at the contact with the Yukhta monzonite-syenite massif(~127 Ma)were studied.Zircon U-Pb dating was performed on a SHRIMP-II ion microprobe,and rare-earth and trace elements analyses of zircon were carried out in the same craters by secondary-ion mass spectrometry on a Cameca IMS-4f ion microprobe.It is revealed that the studied zircons have heterogeneous structures:dark core and lighter rim,which differ greatly in isotope-geochemical parameters.Zircon rims are cut by a network of fractures,extending into the central part of zircon grains.The rims yield a subconcordant age of 1937±24 Ma,with an average total REE content of 550 ppm,which corresponds to the formation age of the Nimnyr Complex.All zircon cores yield a discordant age of 83±11 Ma and are characterized by a higher total REE content(~1812 ppm),as well as higher contents of U and non-formula elements(Ca,Sr,and Y)with respect to rims,due to the effect of fluid on zircons.Despite the limited number of zircon grains,the additional geochronological study of zircons from syenites of the ore-bearing Ryabinovy Massif has revealed the presence of two distinct age clusters:~125–138 and 76–83Ма.The older ages of zircons from syenites are typical for the Central Aldan ore district.Until now,there is still no explanation for an age range(76–83 Ma)of single zircon grains from ore-bearing syenites of the Ryabinovy Massif.The obtained data suggest that the processes of activation(the effect of fluid)within the Central Aldan ore district continued until Late Mesozoic.With regards to the equivocal geotectonic position of the Mesozoic potassic magmatism in the study area and its high metallogenic potential,there exists an absolute necessity to determine the geochronological age of the rock formations.Therefore this study presents the Late Cretaceous geochronological data for the first time which can constrain the time-frame for the formation of gold-bearing magmatic and metasomatic rocks of the Aldan ore district.