Geochemical, Isotopic and Petrological Constraints on the Origin and Evolution of the Recent Silicic Magmatism of the Greater Caucasus

Significant volumes of rhyolites and granites of the Pliocene-Pleistocene age are exposed in the collision zone of the Greater Caucasus, Russia. The volcanic history of the region includes ignimbrites and lavas associated with the Chegem caldera (2.9 Ma) and Elbrus volcano (1.98 and 0.7 Ma) and rhyolitic necks and granites in Tyrnyauz (1.98 Ma). They are characterized by a similar bulk and mineral composition and close ratios of incompatible elements, which indicates their related origin. The 1.98 Ma Elbrus ignimbrites, compared to the 2.9 Ma Chegem ignimbrites, have elevated concentrations of both compatible (Cr, Sr, Ca, Ni) and incompatible elements (Cs, Rb, U). We argue that the Elbrus ignimbrites were produced from magma geochemically similar to Chegem rhyolites through fractionation crystallization coupled with the assimilation of crustal material. The 1.98 Ma Eldjuta granites of Tyrnyauz and early ignimbrites of the Elbrus region (1.98 Ma) are temporally coeval, similar mineralogically, and have comparable major and trace element composition, which indicates that the Elbrus ignimbrites probably erupted from the area of modern Tyrnyauz; the Eldjurta granite could represent a plutonic reservoir that fed this eruption. Late ignimbrites of Elbrus (0.7 Ma) and subsequent lavas demonstrate progressively more mafic mineral assemblage and bulk rock composition in comparison with rhyolites. This indicates their origin in response to the mixing of rhyolites with magmas of a more basic composition at the late stage of magma system development. The composition of these basic magmas may be close to the basaltic trachyandesite, the flows exposed along the periphery of the Elbrus volcano. All studied young volcanic rocks of the Greater Caucasus are characterized by depletion in HSFE and enrichment in LILE, Li, and Pb, which emphasizes the close relationship of young silicic magmatism with magmas of suprasubduction geochemical affinity. An important geochemical feature is the enrichment of U up to 8 ppm and Th up to 35 ppm. The trace element composition of the rocks indicates that the original rhyolitic magma of Chegem ignimbrites caldera was formed at >80%–90% fractionation of calc-alkaline arc basalts with increased alkalinity. This observation, in addition to published data for isotopic composition (O-Hf-Sr) of the same units, shows that the crustal isotopic signatures of silicic volcanics may arise due to the subduction-induced fertilization of peridotites producing parental basaltic magmas before a delamination episode reactivated the melting of the former mantle and the lower crust.

Insights into Magma Storage Beneath a Frequently Erupting Arc Volcano (Villarrica, Chile) from Unsupervised Machine Learning Analysis of Mineral Compositions

A key method to investigate magma dynamics is the analysis of the crystal cargoes carried by erupted magmas. These cargoes may comprise crystals that crystallize in different parts of the magmatic system (throughout the crust) and/or different times. While an individual eruption likely provides a partial view of the sub-volcanic plumbing system, compiling data from multiple eruptions builds a picture of the whole magmatic system. In this study we use machine learning techniques to analyze a large (>2000) compilation of mineral compositions from a highly active arc volcano: Villarrica, Chile. Villarrica's post-glacial eruptive activity (14 ka–present) displays large variation in eruptive style (mafic ignimbrites to Hawaiian effusive eruptions) yet its eruptive products have a near constant basalt-basaltic andesite bulk-rock composition. What, therefore, is driving explosive eruptions at Villarrica and can differences in storage dynamics be related to eruptive style? We used hierarchical cluster analysis to detect previously undetected structure in olivine, plagioclase and clinopyroxene compositions, revealing the presence of compositionally distinct clusters. Using rhyolite-MELTS thermodynamic modeling we related these clusters to intensive magmatic variables: temperature, pressure, water content and oxygen fugacity. Our results provide evidence for the existence of multiple discrete (spatial and temporal) magma reservoirs beneath Villarrica where melts differentiate and mix with incoming more primitive magma. The compositional diversity of an erupted crystal cargo strongly correlates with eruptive intensity, and we postulate that mixing between primitive and differentiated magma drives explosive activity at Villarrica.

Alpine subduction zone metamorphism in the Paleozoic successions of the Monti Romani (Northern Apennines, Italy)

The hinterland of the Cenozoic Northern Apennines fold-and-thrust belt exposes the metamorphic roots of the chain, vestiges of the subduction-related tectono-metamorphic evolution that led to the buildup of the Alpine orogeny in the Mediterranean region. Like in other peri-Mediterranean belts, the tectono-metamorphic evolution of the Paleozoic continental basement in the Apennines is still poorly constrained, hampering the full understanding of their Alpine orogenic evolution. We report the first comprehensive tectono-metamorphic study of the low-grade metasedimentary (metapsammite/metapelite) succession of the Monti Romani Complex (MRC) that formed after Paleozoic protoliths and constitutes the southernmost exposure of the metamorphic domain of the Northern Apennines. By integrating fieldwork with microstructural studies, Raman spectroscopy on carbonaceous material and thermodynamic modelling, we show that the MRC preserves a D1/M1 Alpine tectono-metamorphic evolution developed under HP-LT conditions (~ 1.0-1.1 GPa at T ~400 °C) during a non-coaxial, top-to-the-NE, crustal shortening regime. Evidence for HP-LT metamorphism is generally cryptic within the MRC, dominated by graphite-bearing assemblages with the infrequent blastesis of muscovite ± chlorite ± chloritoid ± paragonite parageneses, equilibrated under cold paleo-geothermal conditions (~ 10 °C/km). Results of this study allow extending to the MRC the signature of subduction zone metamorphism already documented in the hinterland of the Apennine orogen, providing further evidence of the syn-orogenic ductile exhumation of the HP units in the Apennine belt. Finally, we discuss the possible role of fluid-mediated changes in the reactive bulk rock composition on mineral blastesis during progress of regional deformation and metamorphism at low-grade conditions.

Quartz-in-garnet barometry constraints on formation pressures of eclogites from the Franciscan Complex, California

Determining pressure and temperature variations between high-pressure/low-temperature (HP–LT) eclogite blocks is crucial for constraining end-member exhumation models; however, it has historically been challenging to constrain eclogite pressures due to the high variance associated with this bulk-rock composition. In this work, we utilize quartz-in-garnet elastic barometry to constrain formation pressures of eclogites from the northern (Junction School, Ring Mountain, Jenner Beach) and southern Franciscan Complex (Santa Catalina Island). Multiple eclogite blocks from Jenner Beach are analyzed, and a single eclogite from the other localities. By comparing garnet growth conditions from within a single outcrop and between distinct outcrops, we evaluate the local and regional spatial distribution of P conditions recorded by eclogites. We compare the mean, median, and max pressures between different garnet zones and eclogites. Pressures sometimes exhibit systematic changes across garnet zones; however, some eclogites exhibit no systematic pressure variations across garnet zones. Pressures from northern Franciscan eclogites range from $$\sim $$ ∼ 1.4–1.8 GPa, at an estimated temperature of 500 $$^{\circ }$$ ∘ C; pressures from the Catalina eclogite range from $$\sim $$ ∼ 1.2–1.5 GPa, at an estimated temperature of 650 $$^{\circ }$$ ∘ C. Mean and maximum pressures of different eclogites from the northern Franciscan exhibit negligible differences (< 0.1 GPa). The results are inconsistent with models that propose exhumation of metamorphic blocks from different structural levels, and suggest that now exposed HP–LT eclogites from the northern Franciscan Complex may represent rocks that were coherently underplated, and exhumed from similar structural levels.

Theoretical and experimental analysis on environmental pollution with exhaust gases at the mechanical dislocation of rocks, located in a protected nature environment

The paper approaches the theoretical and experimental analysis of the level of exhaust gas pollution in a protected nature environmental, located in the mountain area, for excavation a trench. Due to the legislative provisions prohibiting the use of explosives in the nature reservation or other protected nature environmental areas, the mechanized excavation solution remains the only viable option. Excavation the trench through the mechanized solution must be carried out, taking into account the maximum level of pollution with exhaust gases provided in the legislative norms. According to the geotechnical data, the rock composition to be excavated on the route will be of different hardness, which involves impactors of various powers to displace the rock, on the one hand, and on the other, different excavators and bulldozers for the handling the material resulted from excavation. The theoretical research required to solve the problem has several stages, of which we mention: choosing the correct mechanization solution for displacing the hard and very hard rock; determination of fuel consumption, and energy consumption, respectively, transformation of the energy consumed in the liquid fuel necessary for the tool driving engines, and which generate exhaust gases.

Physical and Mechanical Behaviors of Red Sandstones and Marbles after High-Temperature Treatment

Deep geothermal energy is of great strategic importance for the development of the energy industry. In the process of geothermal energy extraction, temperature changes will significantly affect the physical and mechanical properties of the rock mass. To investigate the influence of temperature on the physical and mechanical properties of red sandstones and marbles, the uniaxial compression test, variable-angle shear test, mercury intrusion porosimetry (MIP) test, and SEM test were conducted on the red sandstone and marble specimens treated by 9 temperature levels (from 25°C to 800°C). The results show that the porosity is positively correlated with the temperature regardless of rock types. The peak strength of red sandstones during uniaxial compression increases first when temperature increases from 25°C to 400°C and then decreases when temperature increases from 400°C to 800°C, whereas the peak strength of marbles exhibits a first decreasing (from 25°C to 300°C), then increasing (from 300°C to 600°C) and finally decreasing (from 600°C to 800°C) trend. Similarly, the shear strength and cohesion of red sandstones increase first and then decrease as temperature rises from 25°C to 800°C, despite of the predesigned shearing angle, which is opposite to the variation in frictional angle. The variations in physical and mechanical behavior are closely related to the expansion of the constituent grains or groundmass which make up the rock composition and closure of pores. Additionally, the temperature in the range from 400°C to 600°C plays an important role to evaluate the variations in the physical and mechanical characteristics of red sandstones and marbles after high-temperature exposure, because of the stress, strain, and porosity change dramatically.

Experimental Study on the Anisotropy of Layered Rock Mass under Triaxial Conditions

Weak and hard inhomogeneous rock formations are typically encountered during tunnel excavations. The physical and mechanical properties and geological conditions of these rock formations vary significantly; thus, it is crucial to investigate the mechanical characteristics of deep bedded composite rock formations. Three-dimensional (3D) scanning and 3D printing were used to prepare composite rock specimens to simulate natural rock laminae. Triaxial compression tests were conducted to determine the influence of the bedding angle, rock composition, and confining pressure on the mechanical properties of the composite rock specimens. The anisotropic strength characteristics and the damage patterns of the composite rock specimens were analyzed under different confining pressures, and the failure mechanism during triaxial loading was revealed. The results show that the damage of the composite rock specimens with a bedding structure depends on the bedding dip angle and the rock formation. The stress-strain curves and peak strengths of the composite rock specimens have anisotropic characteristics corresponding to their failure modes. As the bedding dip angle increases, the peak strength of the three groups of specimens first decreases and then increases under different confining pressure levels. The compressive strength has a nonlinear relationship with the confining pressure, and the difference between the compressive strengths of specimens with different inclination angles decreases as the confining pressure increases. The Hoek–Brown strength criterion is a good predictor of the nonlinear increase in peak strength of the composite rock specimens under different confining pressures. The specimen with a β = 60°dip angle shows the most significant increase in the strength difference with increasing confining pressure. The results can be used as a reference for testing and analyzing the anisotropic mechanical properties of bedded rock masses.

Outcrop Analogues to Fractured Reservoirs, UAE

Fractures were not the focus of reservoir studies in Abu Dhabi for the last decades, although its importance in enhancing production, as the general understanding considering fractures are not contributing to production. The fractured carbonate outcrops provide useful analogue observations, data and concepts to support subsurface hydrocarbon reservoir characterization from well and seismic data. The fracture orientation, size, porosity, length, spacing, crosscutting relationships, fracture density versus lithology and bed thickness and connectivity are difficult to measure directly from subsurface well and core data. The understanding of fracture formation and distribution and their effects on fluid flow has been greatly improved by the use of outcrop analogue data through the current work. This paper address the fracture geometry, kinematics and mechanical properties based on outcrops matching Abu Dhabi subsurface reservoir analogues. Integrating outcrop data with fracture orientation and fracture density from core and borehole image data, and seismic capturing fractures characteristics. The outcrop analogues constrain the uncertainty and developing new concepts in characterizing the interplay of rock matrix and fracture networks relevant to fluid flow and hydrocarbon recovery. Analysing the fractures with fracture lengths, aperture, spacing per each interval and relate them to the tectonic event are extracted strictly in the reservoir section. The results showing developed highly dipping shear fractures with short length, small spacing and bimodal aperture distribution that related to fracture orientation. Fracture porosity is dependent on size and controlled by lithology, bed thickness, paleostress and rock composition. Understanding fractures and their behaviour will optimize production greatly and they create exploration targets in otherwise tight reservoir zones, including under-explored sections.

Organic geochemical and petrographical characteristics of the major lower cretaceous petroleum source rock (Makhul Formation) in Kuwait - Arabian Gulf

The Lower Cretaceous Makhul Formation is one of the major petroleum source rocks in Kuwait. This study aims to evaluate the Makhul source rock for its organic matter richness and its relation to the rock composition and depositional environment. A total of 117 core samples were collected from five wells in Raudhatain, Ritqa, Mutriba, Burgan, and Minagish oil fields north and south Kuwait. The rock petrographical studies were carried out using a transmitted and polarized microscope, as well as SEM and XRD analyses on selected samples. Total organic matter TOC and elemental analyses were done for kerogen type optically. The GC and GC-MS were done as well as the carbon isotope ratio. The results of this study show that at its earliest time the Makhul Formation was deposited in an anoxic shallow marine shelf environment. During deposition of the middle part, the water oxicity level was fluctuating from oxic to anoxic condition due to changes in sea level. At the end of Makhul and the start of the upper Minagish Formation, the sea level raised forming an oxic open marine ramp depositional condition. Organic geochemical results show that the average TOC of the Makhul Formation is 2.39% wt. High TOC values of 6.7% wt. were usually associated with the laminated mudstone intervals of the formation. The kerogen is of type II and is dominated by marine amorphous sapropelic organic matter with a mixture of zoo- and phytoplankton and rare terrestrial particles. Solvent extract results indicate non-waxy oils of Mesozoic origin that are associated with marine carbonate rocks. The formation is mature and at its peak oil generation in its deepest part in north Kuwait.