A thermal event in the Dolpo region (Nepal): a consequence of the shifting from orogen perpendicular to orogen parallel extension in central Himalaya?

In the Lower Dolpo Region (central Himalaya), structurally above the South Tibetan Detachment System (STDS), blastesis of static micas have been recognized. Nevertheless, until now, very little work has been done to constrain the tectonic meaning and the timing of this static mica growth. In this work we investigate samples from the STDS hanging wall, characterized by three populations of micas, defining (i) S1 and (ii) S2 foliations, and (iii) M3 static mineral growth cutting both foliations. New geochronological 40Ar/39Ar analyses on the microtexturally-different micas, complemented by microstructural and compositional data, allow to place temporal constraints on the static (re)crystallization at the STDS hanging wall. Results point out homogeneous chemical compositions and ages of micas within the investigated samples, irrespective of the structural positions. Phlogopite and muscovite on S1 and S2, and post-kinematic biotite yielded 40Ar/39Ar ages within 14-11 Ma with decreasing ages upward. We suggest that mica (re)crystallized under static conditions during a late thermal event at low structural levels (c. 15-18 km), after cessation of the ductile activity of the shear zone. We hypothesize that this later thermal event is kinematically linked to the switch from orogen perpendicular to orogen parallel extension in central Himalaya.Supplementary material: [Electron microprobe analyses of biotite and white mica] is available at https://doi.org/10.6084/m9.figshare.c.5509998Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Geochronology of Himalayan shear zones: unravelling the timing of thrusting from structurally complex fault rocks

Dating structurally complex fault rocks often results in internally inconsistent ages, as several mineral generations are intergrown at scales << 10 µm and almost always altered to various degrees. Firstly, electron probe microanalysis is necessary to assess both inventory and spatial distribution of minerals and their retrogression/alteration phases. We then used 40Ar/39Ar stepheating combining two independent indicators that allow the discrimination of coexisting mica generations from each other: (i) mica stoichiometry, which is proxied by 39Ar concentration in combination with 37Ar/39Ar and 38Ar/39Ar (Ca/K and Cl/K) ratios; (ii) furnace temperature, at which the degassing peak accompanying dehydration and structural collapse is observed. As dehydration rates depend on average bond strength in the crystal structure, it is predicted and observed that the temperature of the differential Ar release peak is variable among different minerals. We observe that the Ca/Cl/K signatures of pure micas coincide with the Ar release peak. The Main Central Thrust zone in the Garhwal Himalaya records a protracted history. Foliation of Vaikrita Thrust formed at 15-8 Ma, followed by static decompression at 7 Ma; foliation of structurally lower Munsiari Thrust formed around 5 Ma. Our elaborate and time-consuming petrochronological procedure should become routine whenever analysing polydeformed metamorphic rocks.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5357212Thematic collection: This article is part of the Isotopic Dating collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Geological features and the first isotopic data of volcanic rocks in the Iset river basin, East-Urals megazone

Relevance of the work. The Iset river basin contains the most extensive outcrops of volcanogenic formations of the Beklenishchevsky complex of the East Ural megazone, the age of which is determined as Early Carboniferous by the ratio of volcanic rocks with faunistically characterized sedimentary deposits. Volcanics here compose flows of andesite-basaltic and andesitic lavas and lava breccias. There are no geochronological dates specifying the age of the rocks, which makes it difficult to assess their role in the formation of the megazone. Therefore, isotopic dating of these formations is very important. Methods. The U – Pb age and data on the geochemistry of zircons were obtained by laser ablation (LA – ICP – MS). Purpose of the research is to study the features of the geological structure, the material composition of volcanic rocks in the Iset river basin, the geochemistry of zircons from andesites and the determination of their isotopic age. Results of the work and the scope of their application. Lava flows of andesites and basaltic andesites with minor amounts of basalts and dacites have tectonic contact with sedimentary rocks of the Early Carboniferous age. The distribution of rare elements in volcanics is typical of supra-subduction formations. Zircons in andesites are represented by prismatic and isometric crystals. Prismatic differences in the nature of the distribution of REE and the content of Li, Ti, Sr, Th, U refer to zircons of magmatic genesis, isometric – to “hydrothermal”. According to the U / Yb – Y ratios, the former correspond to the zircons of the ocean floor, while the latter are related to the continental ones. Isotopic dating of zircons from andesites was carried out for the first time. Their age was 311 million years. The data can be used in geological mapping, as well as in the compilation of large-scale geodynamic maps and diagrams. Conclusions. Volcanic rocks in the Iset river basin were formed in supra-subduction continental-marginal geodynamic conditions that took place in the Urals in the Carboniferous. The obtained value of the age of zircons from andesites, possibly, fixes the stage of their transformation. Keywords: East-Ural megazone, volcanic rocks, zircon, isotopic age.

Uranium-Lead Geochronology on Monacite of Granites of the Lypnyazka Massif and its Framing (Ingul Megablock of the Ukrainian Shield)

The Lipnyazka massif is located in the Dobrovelychkivsky district of the Kirovohrad region, v. Lipnyazka and further south. Structurally, it is located within the Bratskyy Synclinorium of the Ingul megablock and coincides with the Mikhailovsky anticline. The main petrotype of the massif rocks are porphyry-like granites, often with a gneiss-like texture, aplito-pegmatoid, pegmatoid granites and pegmatites. The latter most often form secant vein bodies. Uranium-lead isotopic dating of granitoids of the Lipnyazka granite massif has been performed, with which a number of deposits and ore occurrences of rare elements, primarily lithium, are spatially and probably genetically connected. The age of porphyry-like granites (2032 ± 6 million years), which is the main petrotype of rocks distributed in the area of the village of Limestone and pegmatoid granites (2027 ± 1 million years), which cut porphyry-like in the form of vein bodies. Aplithoid framed granites (2046 ± 8 million years old), common in the area of the mouth of the Sukhyi Tashlyk River (Dobryanka village), are somewhat older. Based on the results of determining the isotopic composition of strontium in the accessory apatite of granites, a conclusion was made about the upper crust source of granites of the Lipnyazka massif (87Sr/86Sr – 0.730-0.785).

AGE AND PETROGENESIS OF ACID VOLCANITES OF THE ALGANSKY MOUNTAINS OF THE KORYAK HIGHLANDS (NORTHEASTERN OF RUSSIA)

The isotopic dating of U-Pb by the zircon method of volcanic acid zircons of the Konachan complex (Algansky mountains, northwestern part of the Koryak highlands) confirmed their Late Eocene age (34.6 ± 0.5-38.8 ± 0.4 Ma, Priabon). It has been shown that silicic rocks are represented by moderately potassium, peraluminous (ASI = 1.23–1.30), moderately and high magnesian differences, often with a high content of Cr and Ni. The contents of Sr, Y and Yb are adakite. High ratios of 143Nd/144Nd and low 87Sr/86Sr in the rocks of the complex, the distribution pattern of rare and rare-earth elements suggest that the source of salite melt, to some extent contaminated by sedimentary material, was metabasites. The formation of the Konachan complex is caused by magmatism during accretion of terranes of the Olyutor-East Kamchatka island-arc system [27]; compression processes led to the blocking of the upwelling front of the continental asthenosphere, initiating intense crustal anatexis and silicate magmatism.

Uranium-Lead by Monacite Geochronology of Granites Distributed in the Middle Current of ihe River Ros (Ros-Tikich Megablock Ukrainian Shield)

Granitoids play a leading role in the geological structure of the Ros-Tikich megablock. The rocks of the Ros-Tikich series, which form the supercrystalline base here, have survived among the granitoids only in the form of some remnants of different sizes. In the coastal outcrops of the middle course of the river Ros (east of Bila Tserkva) and in the quarries located near the river, the most common are two-feldspar granitoids, isolated as part of two complexes – Uman and Stavyshche. Plagiogranitoids and supercrustal rocks have survived among them in the form of separate fragments. Of the accessory uranium-containing minerals, zircon and titanite are present in two-field spar granites, and monazite was found in single samples of granites in which titanite was absent. The age of obscure porphyry-like granite discovered by the Ostrivsk quarry – 2063.5 ± 1.2 million years and uniformgrained granite, distributed in the quarry of the village of Bovkun – 2042.5 ± 8.6 million years. The obtained values of the isotopic age of these granites are in good agreement with the results of uranium-lead isotopic dating of zircons from twofeldspar granites of the Ros-Tikich megablock (1990-2080 million years). This allows us to propose to combine the granites of the Stavyshche and Uman complexes into one Paleoproterozoic complex.

Overview in situ U - Pb isotopic dating method on cassiterite. Application to determine for Sn - W mineralization age of the Lung Muoi deposit in the Pia Oac region, Cao Bang province

In recent years, the U - Pb isotopic dating method for cassiterite minerals has been used by many scientists around the world in the field of mining and mineral research. This paper presents an overview of the history, development and results achieved using this dating method in the field of mining and mineral research in the world. The LA - ICP - MS U - Pb isotopic dating method for cassiterite minerals was used at the Lung Muoi Sn - W deposit in Pia Oac region, Cao Bang province. The result of LA - ICP - MS U - Pb cassiterite isotopic dating shows the Sn - W mineralization in the Lung Muoi deposit crystallized at 88 Ma. The new age results in this paper are very consistent with the previously published results of the Pia Oac granite.