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The deformation and metamorphic history of the Precambrian high-grade rocks in the Key Afer area, southwestern Ethiopia within the Mozambique belt is described. It comprises poly-deformed, metamorphosed, and migmatized rocks with intrusion of granitoids and overlain by Quaternary sediments. A combination of field litho-structural mapping, metamorphic mineral assemblages, and microstructural analysis there are three metamorphic events and four phases of ductile deformation, and one Cenozoic brittle fracture (D5) are recognized. The development of the relatively steep NNW-SSE trending S1 relict gneissic banding and the rise of pyroxene and anhydrous minerals indicate that the peak metamorphism (M1) is synchronous with D1. Subsequently, the hydration of M1 assemblages leads to the formation of amphibolite facies (M2). This is followed by the development of amphibolite facies (M2) caused by the hydration of M1 assemblages synchronous with the D2 deformation. It is defined by the major regional fabric (S2) of the area trending NW-SE, tight to isoclinal upright F2 folds, and local L2 lineation. These D2 upright folds are orthogonally superimposed by another upright F3 folds during D3 resulting in a type-I fold interference pattern. The replacement and breakdown of hornblende to epidote, biotite to chlorite, and plagioclase to sericite give a retrogressive event to greenschist facies (M3) syn-D4. It gave rise to NNE-SSW-oriented S4 mylonitic foliations associated with F4 drag folds. Both sinistral and dextral shear sense is recorded but dextral shear sense appears dominant. The fifth phase of deformation (D5) is characterized by brittle fracture and joint structures of the area with varying orientations. The three metamorphic events with deformational episodes of the study show a clockwise P-T path loop from burial to uplift similar to the collision-parallel shearing orogenic setting. [ABSTRACT FROM AUTHOR]

Melt migration through Earth's crust drives well‐documented melt–rock reactions, locally changing rock assemblage and geochemistry. However, melt–zircon interaction remains understudied. We report on three zircon‐melt interaction events from the Pembroke Granulite, New Zealand. Primary zircon from gabbroic gneiss which was subject to minor post‐emplacement melt migration and primary zircon from younger dykes exhibit straightforward microstructures, microchemistry, and age data. In contrast, zircon from melt‐mediated reaction halos adjacent to the dykes and from melt‐fluxed high‐strain zones display dissolution modification of grains, micro‐porosity and blurred or truncated internal zoning typical of replacement by coupled dissolution‐precipitation. Replaced zircon domains show changed rare earth element patterns and redistributed or lost radiogenic Pb that generates ambiguous apparent spot date arrays, smeared over tens of millions of years. We conclude that the metamorphism and three melt–rock interaction events were brief, and the arrays misrepresent the true age and duration of the metamorphism. Pb‐loss persisted beyond the metamorphism, with porosity and inclusions formed during coupled dissolution‐precipitation making replaced zircon domains more susceptible to subsequent Pb‐loss compared to the structurally intact, primary magmatic zircon in the host gabbroic gneiss or dykes. We recommend conducting high‐resolution microstructural investigations upon recognition of spot date arrays observed in single samples to rule out the possibility of spurious arrays resulting from coupled dissolution‐precipitation. [ABSTRACT FROM AUTHOR]

The Mingshan reservoir of the Dabie Orogen has a number of Neoproterozoic bimodal intrusive rocks. We focused on the zircon U-Pb chronology, Hf isotopes, and bulk-rock geochemistry of these rocks. The results showed the following: (1) The bimodal intrusive rocks mainly consist of monzogranitic gneiss and plagioamphibolite, with zircon U-Pb ages of 785.0 ± 7.1 Ma and 787.3 ± 6.1 Ma, respectively. These ages indicate that they were formed in the late Qingbaikou epoch of the Neoproterozoic era. (2) The monzogranitic gneiss was dominated by peraluminous features and displayed a strong right deviation of REE (rare-earth element) patterns and a negative δEu anomaly. It is enriched in the LILEs (large-ion lithophile elements) Rb, Ba, and K, but slightly depleted in Nb, Sr, P, and Ti, with low 10,000* Ga/Al values, indicating that it is similar to Al-type granite. The plagioamphibolite belongs to the metaluminous, peraluminous series. It has a Mg# (molar ratio of Mg to Mg + Fe) of 36.1~55.9 and is enriched in the LILEs Rb, Ba, and K, with a slight positive anomaly of Ba, and is depleted in Nb and Sr. (3) The monzogranite shows negative zircon εHf(t) values ranging from −13.4 to −7.2 and a Paleoproterozoic TDM2(Hf) (two-stage depleted mantle model age) of 1969–2298 Ma. The zircon εHf(t) values and TDM2(Hf) of the plagioamphibolite were concentrated around 2.9–5.7 and 1257–1410 Ma, respectively. The geochemistry and Hf isotopes show that the monzogranitic gneiss and plagioamphibolite have distinct magmatic sources. The plagioamphibolite formed from mantle and partial continental crustal materials. The monzogranitic gneiss, on the other hand, was formed as a result of the partial melting of the shallow ancient felsic crust caused by mafic rock heating or upwelling. Taking into account regional correlation, the middle Neoproterozoic bimodal intrusive rocks originated in the structural framework of an extensional setting. [ABSTRACT FROM AUTHOR]

The anisotropic characteristics of rock have important value for construction safety and evaluation of rock properties. The purpose of this article is to study the variation law of drilling parameters in different rocks and their anisotropy characteristics. This article investigates drilling characteristics by conducting digital drilling experiments on six different types of rocks in three different directions. The variation law of drilling parameters (drilling speed, torque, rotational speed, thrust force) and the special energy with drilling depth are analyzed as well as the anisotropy variation of six types of rocks. The influence of anisotropy on the specific energy of six types of rocks is investigated. A special energy index f is proposed to characterize the anisotropy for evaluating different rocks. The test results show that the anisotropy of granite, slate, gneiss, red sandstone, argillaceous sandstone, and sandstone is weakened accordingly by using the special energy index f. The specific energy index after data normalization can better judge the anisotropy difference for different rocks, such as granite, slate, gneiss, and red sandstone. [ABSTRACT FROM AUTHOR]

Multi-component crustal sources are universally acknowledged as the overriding factor in causing geochemical heterogeneities of the Cenozoic Himalayan leucogranites. In previous studies, metasedimentary rocks from the Greater Himalayan Crystalline Complex were always underlined to be the dominant origin for leucogranites after the Eocene and Oligocene transition (ca. <36 Ma). However, given that the petrological diversity of the Greater Himalayas, especially the widespread and high-grade metamorphosed granitic gneisses; this traditional standpoint has been increasingly questioned nowadays. To further demonstrate the role of granitic gneiss in leucogranite generation, a rounded compilation of geochronological and geochemical data for leucogranites, granitic gneisses, and other related rocks from the specified N – S striking Yardoi – Cuonadong – Tsona transect has been conducted. After making comprehensive comparison and discussion between leucogranites and granitic gneisses, we argue that Cenozoic Himalayan leucogranites may not be pure metasediments derived S-type granites as orthogneisses could be another important endmember for the provenance of them based on the following evidence: (1) Abundant relict zircons within the Himalayan leucogranites display two evident U – Pb age clusters at ca. 850–800 Ma and ca. 520–470 Ma, which are contemporaneous with the Neoproterozoic and early Paleozoic granitic magmatism, respectively. (2) Zircon Hf isotopes of Himalayan-aged rims (−11.21 to −4.82) could be perfectly constrained by two evolution lines derived from Neoproterozoic (−6.40 to 0.16) and early Paleozoic (−2.37 to 6.15) zircon groups. (3) In terms of whole-rock Sr – Nd isotopes (all corrected to 20 Ma), there is a notable overlap between leucogranites (0.7142 to 0.8429 for Sr; and −17.34 to −9.86 for Nd) and granitic gneisses (0.7703 to 0.8716 for Sr, and −16.27 to −9.80 for Nd). (4) Although the fertility of granitic gneisses should be poorest in the absence of a separate aqueous phase; however, the evolutionary P – T – XH2O conditions triggered by compressional thrust activity of the Main Central Thrust and arc-parallel extension have remarkably modified original source structures (infiltration by LHS-derived fluids) and melting behaviours (more recognized fluid-present partial melting cases). Consequently, the role the granitic gneisses would be strengthened because of the greatly improved fertility via fluid-present melting; and the Sr – Nd isotopic signatures of Himalayan leucogranites would be spatially-temporally evolved. [ABSTRACT FROM AUTHOR]

The present study provides multidisciplinary research on the alteration of petrographic, mineralogical, geochemical characteristics, and physical properties of Himachal gneiss due to weathering. Three different weathering grades fresh, slightly, and moderately weathered gneiss are collected from Kullu, Himachal Pradesh, India. The petrographic and mineralogical analyses are done by thin section and XRD analyses. The microstructural and geochemical studies of gneiss samples were performed using scanning electron microscopy (SEM) and X-ray fluorescence (XRF) respectively. The petrographic images revealed a large amount of alteration in the plagioclase due to weathering. The pervasive sericitization of plagioclase is observed in moderately weathered gneiss samples. The quartz grains are intact and only minor fracturing in some quartz grains is observed in moderately weathered gneiss samples. The SEM images indicate that the inter-granular and intra-granular micro-cracks increase with the weathering grades. The XRF results are used to obtain the different chemical weathering indices and their suitability for Himachal gneiss. Among twenty-five chemical weathering indices, the CIA, CIW, alkaline ratio, PIA, LOI, and WI show a suitable trend and can be used for assessing the weathering of Himachal gneiss. In addition, alterations in the physical properties of gneiss are observed due to weathering. The bulk, dry, saturated densities and slake durability index decrease while the porosity increases with the progressive weathering of gneiss. The progressive weathering significantly reduces the gneiss's uniaxial compressive strength (UCS). For all three weathering grades of gneiss, the maximum and minimum values of UCS were observed at 90˚ and 30˚ foliations, respectively. [ABSTRACT FROM AUTHOR]

This study aims to evaluate the Whole-Body Vibration (WBV) exposure of rock breaker operators in surface mining, focusing on limestone, banded hematite quartzite, and granite gneiss rocks. Using a seat pad tri-axial accelerometer, operators were monitored across varying impact powers, Rock Mass Cuttability Index (RMCI), and net breaking rate (NBR). Results indicate that operators exceeded the Exposure Action Value (EAV) of 0.5 m/s² and the Vibration Dose Value (VDV) of 9.1 m/s1.75 while operating in limestone and granite gneiss rocks. Moreover, hydraulic rock breaker operators surpassed the Exposure Limit Value (ELV) of 1.15 m/s² and the VDV of 21 m/s1.75 for the granite gneiss rocks, based on A(8) and VDV(8) derived limits. Notably, operators breaking banded hematite quartzite rocks experienced lower vibration amplification. RMCI and impact power emerged as critical factors in WBV exposure, underscoring the need for optimal settings to mitigate excessive vibrations. These findings underscore the importance of managing WBV for operator health and safety in surface mining operations. [ABSTRACT FROM AUTHOR]

With the continuous development of underground engineering construction in China, it is particularly important to study the identification of structural plane characteristics of rock masses. In this study, three types of pseudo-rock specimens with structural plane interlayers were fabricated to analyze the patterns of drilling parameter changes in rock bodies with structural planes during the drilling process and to explore the characterization and identification methods of rock body structural planes. Gneiss, granite, and sandstone were used as rock materials, with gypsum mortar as the interlayer material for the structural planes in these three types of specimens. The indoor digital drilling equipment was utilized for conducting indoor digital drilling experiments. The variation patterns of drilling parameters in rock bodies with structural surfaces under different interlayer inclinations and thicknesses were analyzed. The relationship between the ratio of the change in rotational speed and drilling speed during the stable phase of the upper rock mass and the peak torque and peak drilling pressure has been established. The relationship between the structural plane inclination angle and the ratio of change in rotational speed and drilling speed has been determined. By utilizing the variation in these ratios, the impacts of the structural plane inclination angle and the thickness of the structural plane on the peak torque and peak drilling pressure have been elucidated. The research results provide a theoretical basis for the stability evaluation of rock masses with structural planes under drilling action. [ABSTRACT FROM AUTHOR]

This study presents an updated interpretation of geological data collected between 1984 and 2022. The area under consideration holds significant regional importance as it is located between the Internal Nappe Zone (INZ) and the Regional Mylonitic Complex (RMC). Re-evaluation of the geological data has highlighted a more intricate structural framework than what is currently documented in the existing literature. This paper aims to illustrate, through structural analysis, that the Posada Valley Shear Zone (PVSZ) does not serve as the transitional boundary between the Inner Nappe Zone and the Regional Mylonitic Complex or High-Grade Metamorphic Complex (HGMC) as traditionally thought. Instead, the authors' findings indicate that the transition boundary is confined to a shear band with a variable thickness ranging from 10 to 70 m at its widest points. The development of the Posada Valley Shear Zone is characterized by a series of transitions from mylonite I S-C to mylonite II S-C, extending over approximately 5 km. The formation of the Posada Valley Shear Zone is chronologically confined between the development of the East Variscan Shear Zone (EVSZ) and the emplacement of the Late Variscan granites. The differing orientations of Sm and S3 observed in the mylonitic events of the Posada Valley Shear Zone and the Regional Mylonitic Complex, respectively, are likely attributable to an anticlockwise rotation of the shortening directions during the upper Carboniferous period. Furthermore, this study proposes that the Condensed Isogrades Zone (CIZ), despite its unclear formation mechanism, should be recognized as the true transition zone between the Inner Nappe Zone and the Regional Mylonitic Complex or High-Grade Metamorphic Complex. This new interpretation challenges the previously accepted notion of increasing Variscan metamorphic zonation toward the northeast. This conclusion is supported by the identification of the same NE–SW orientation of the D2 tectonic event in both the Old Gneiss Complex (OGC in the Regional Mylonitic Complex) and the lithologies of the Inner Nappe Zone and the Condensed Isogrades Zone. The comprehensive analysis and new insights provided in this paper contribute to a refined understanding of the geological relationships and processes within this region, offering significant implications for future geological studies and interpretations. [ABSTRACT FROM AUTHOR]

The Yangtze Craton is one of the crucial cratons constitued the eastern Asia continent. It is characterized by Archaean to Paleoproterozoic basement rocks, making it an excellent area to study the Precambrian geologic and tectonic regime. Recently, abundant Archaean rocks have been reported from the Dabie Orogen, which providing important information to understanding the early evolution of the Northern Yangtze Craton. In this contribution, we present petrological, whole-rock geochemical, and zircon U – Pb – Hf isotopic analyses on newly discovered Neoarchean granite in the Dabie Orogen. We identified 2744 ± 10 Ma and 2715 ± 11 Ma (~2.74 Ga) Na-rich granitic gneisses, 2691 ± 6 Ma and 2696 ± 8 Ma (~2.69 Ga) A2-type granitic gneisses, and 2664 ± 8 Ma, 2664 ± 6 Ma, 2672 ± 7 Ma, 2642 ± 10 Ma (~2.66 Ga) A1-type granitic/syenite gneisses from the Daqishan area of the Dabie Orogen. Geochemical and zircon Hf isotope data indicate that the ~ 2.74 Ga Na-rich granitic gneisses were partial melting products of thickened mafic crust with limited depleted mantle input, while the 2.69 Ga and 2.66 Ga A-type granitic gneisses represent felsic magmatism in a post-collisional to an extensional tectonic setting. Based on our research, we categorized the Northern Yangtze Craton into the Kongling domain and Dabie domain by comparing with the distribution of the zircon U-Pb magmatic ages and metamorphic events. We suggest that the initial unified basement of the northern proto-Yangtze Craton formed during the Nuna supercontinent cycle at ~ 2.0 Ga. [ABSTRACT FROM AUTHOR]

Identifying deformational mechanisms and associated structures at various scales, ranging from regional‐scale structures to microscopic fabric, is crucial for the assessment of tectonic development. Thirty‐three samples were taken from the Qazzaz metamorphic core complex to estimate the finite strain for felsic and mafic minerals. These samples included gneisses rocks, monzogranite, and metavolcano–sedimentary rocks for both the Thalbah and Bayda groups. Using the Rf/j and Fry methods, the axial ratios (XZ) range about 2.20 to 7.10 and 1.90 to 9.10, respectively. For various rock units, the strain measurements show moderate to highly deformation. Most of the observed samples show shallow WNW dipping along a N to WNW trend of finite strain (X). The short axes (Z) based to be subvertical foliation related with a subhorizontal foliation. The results demonstrate that contacts generated at semi‐brittle to ductile deformation and that the strain of magnitude has the same value for different lithologic units. It concluded that nappe generation in orogens results from pure shear deformation. [ABSTRACT FROM AUTHOR]

We present significant findings of the Kausani Granite Gneiss within the Inner Lesser Himalayan Sedimentary Zone (iLHSZ) just north of the North Almora Thrust (NAT). The Kausani Granite Gneiss body lies within the quartzite of the Someshwar Formation and has a tectonized contact and a discordant relationship on the north side. Detailed seismic profiling across the body also confirms a similar result. Cathodoluminescence images of zircon from the Kausani body show no inheritance of older cores. The U–Pb ages from the zircon populations separated from the Kausani body give a crystallization age of 1866 ± 3 Ma. Along with the Upalda granite gneiss and Toneta granite gneiss near the Alakhnanda Thrust in the Garhwal Himalaya and the Dungeshwari granite gneiss near the Dailekh Thrust in Nepal, the Kausani Granite Gneiss north of NAT Kumaun Himalaya forms a major terrain boundary. These gneissic bodies mark the southernmost extent of felsic magmatism at NAT, rather than the Main Central Thrust. An about 1.8 billion years‐old magmatic event in the LHSZ suggests that it is a currently active continental margin inside the 'Greater India' region, now situated in the Himalayan domain. However, the Pb‐loss modelling of the U–Pb zircon data reveals thermal events during the Himalayan Orogeny (~45 Ma). [ABSTRACT FROM AUTHOR]

In the rugged terrain of the highway project Struma, Lot 3.2, east of the Kresna gorge, a variety of risky geological processes have been recognized. They include strong deep weathering of granites and gneisses, tectonic rework of rocks, and unfavorable orientation of joints and small faults. This features facilitate kinematic instability of rock blocks and rock wedges. These processes act together, threatening the design and construction of the highway. In this work, brief description of the risky processes is presented in order to emphasize the importance of careful predesign geological work for finding an optimal engineering solution. It also serves to emphasize that the geological environment is not permissible for cheaper solutions that have been applied in other highway projects in Bulgaria. [ABSTRACT FROM AUTHOR]

A cyclic loading may be responsible for reversible and irreversible movements of rock blocks along pre-existing fractures, serving as a preparatory factor for rockfall with cumulative effects over time. The mechanism associated with this hypothesis posits that thermally induced deformations may be sufficient to generate irreversible movements through the contraction and expansion of rock blocks. This study aims to investigate the mechanical and hydraulic behavior of discontinuities in gneissic rock under prepeak cyclic shear loading to identify irreversible displacements for the study of rock block falls. Cyclic shear tests were conducted on fractured rock samples, varying the values of the Joint Roughness Coefficient (JRC). The applied normal stress ranged from 100 to 1,000 kPa, while the adopted frequencies were 0.1 and 0.01 Hz. This approach ensured an almost static character in the tests, reproducing the conditions of block displacement on a rock slope. The overall analysis of the results indicated that pre-peak shear stress, associated with low normal stress (100 kPa) and low frequencies (0.1 and 0.01 Hz), was sufficient to generate irreversible accumulated displacements over cycles. Additionally, it was observed that samples with higher JRC values exhibited larger hydraulic openings. [ABSTRACT FROM AUTHOR]

This paper reports the metamorphic texture of cordierite megacrysts and the metamorphic P–T path of a newly exposed section of gneiss in East Antarctica. We used mineral textures and pseudosection modeling to reconstruct the metamorphic P–T path of cordierite- and spinel–garnet-bearing gneisses from Botnnuten, an isolated nunatak located ~ 60 km from the southern edge of Lützow-Holm Bay in East Antarctica. The gneisses underwent low-P granulite-facies metamorphism at 5.0–6.1 kbar and 850 ± 20 °C followed by isobaric cooling. The isobaric cooling path implies long residence in the middle to shallow crustal level without rapid exhumation. This contrasts with the widely recognized clockwise P–T path of basement rocks of the Lützow-Holm Complex. The rocks at Botnnuten have long been considered part of the Lützow-Holm Complex based on their petrographical features and geothermobarometric data. However, the present results, combined with a reevaluation of available data, indicate the metamorphic history of the Botnnuten gneisses is more comparable to that of the Yamato Mountains, located southwest of the study area. [ABSTRACT FROM AUTHOR]

Using new and published data, we synthesize the tectonic evolution of the Ottawa River Gneiss Complex (ORGC), the metamorphic core and detachment zone of a large mid- to late-Ottawan metamorphic core complex in the western Grenville Province. Field and petrologic data indicative of retrogression and exhumation, combined with maps and schematic crustal-scale sections, are used to document spatial and temporal relationships of multi-scale structures developed during its formation, of which the largest, termed mega-cross-folds and megaboudins, occur within and define the detachment zone. Mega-cross-folds, orogen-normal structures up to 70 km in length with coaxial constrictional fabrics in their hinge-lines, formed in a single phase of deformation during retrogression and exhumation. A cluster of asymmetric megaboudins, individually from 10–50 km long with granulite-facies cores and high-strain amphibolite-facies rims, similarly formed during syntectonic retrogression and exhumation of granulite-facies precursors. We argue the mega-cross-folds developed in a regime of regional transtension, whereas the megaboudin cluster formed by extensional inversion of an anastomosing early-Ottawan thrust system, with the strain patterns of both suggesting the detachment zone was the site of intense ductile flow between the stronger metamorphic core and cover. Comparison of these results with generic numerical models of extensional collapse of overthickened continental crust suggests the first-order tectonometamorphic features of the ORGC developed during necking of the upper crust and associated large-scale extensional flow of the mid and lower crust into the domiform necked region during collapse of the early-Ottawan thrust stack. [ABSTRACT FROM AUTHOR]

Fractures in a rock mass have an important influence on the mechanical response and hydraulic properties. Investigating and modeling fractures is crucial, especially with regard to the geological disposal of high-level radioactive waste, where long term stability is necessary to avoid a direct threat to society. This study sought to classify joints and faults in the Hongcheon gneiss, South Korea, based on their geometric characteristics. A generic classification model for joints and faults is proposed, dividing them into four types based on propagation pattern and development of fault core and fault effect zone. More than 4,000 joints and 34 faults were analyzed using samples of deep drillcore to investigate the relationships between the properties of these structures and depth or lithology. To validate the model, it was applied to other area where different lithologies are found. A damage index is utillized to visually represent the impact of a fault on the quality of rock mass. The joint patterns can be correlated with lithology, as the mineralogical characteristics and internal structures influence the patterns. The fault zone patterns show a relationship with depth, and the damage index provides a reliable indication of the fault impact. The validation of the fault model will be conducted in subsequent studies. With estimation of the hydromechanical properties of each joint and fault pattern, a practical approach would be provided to characterize fractures from drillcore data and enhance the accuracy of fluid flow and stability models for any given rock mass. [ABSTRACT FROM AUTHOR]

Fractures in a rock mass have an important influence on the mechanical response and hydraulic properties. Investigating and modeling fractures is crucial, especially with regard to the geological disposal of high-level radioactive waste, where long term stability is necessary to avoid a direct threat to society. This study sought to classify joints and faults in the Hongcheon gneiss, South Korea, based on their geometric characteristics. A generic classification model for joints and faults is proposed, dividing them into four types based on propagation pattern and development of fault core and fault effect zone. More than 4,000 joints and 34 faults were analyzed using samples of deep drillcore to investigate the relationships between the properties of these structures and depth or lithology. To validate the model, it was applied to other area where different lithologies are found. A damage index is utillized to visually represent the impact of a fault on the quality of rock mass. The joint patterns can be correlated with lithology, as the mineralogical characteristics and internal structures influence the patterns. The fault zone patterns show a relationship with depth, and the damage index provides a reliable indication of the fault impact. The validation of the fault model will be conducted in subsequent studies. With estimation of the hydromechanical properties of each joint and fault pattern, a practical approach would be provided to characterize fractures from drillcore data and enhance the accuracy of fluid flow and stability models for any given rock mass. [ABSTRACT FROM AUTHOR]

The reconstruction of the Early Paleozoic collision metamorphism of the Khan-Khukhei Block (Northern Mongolia) based on 3D modeling of thermal state of the crust and isotope dating is presented. The age of garnet–biotite schist from a metamorphic complex of the Khan-Khukhei Block of 517.4 ± 7.4 Ma is determined for the first time. 3D modeling was conducted to compare this age of metamorphism with the available age determinations of (post-) collision granite formation in the range of 513‒505 Ma. The model considers radiogenic heating at an elevated heat production of rocks of the thickened crust of the Khan-Khukhei block as the cause of the migmatization and granitoid melts formation. 3D modeling was conducted for the first time for thermal-dome type of metamorphism. The results provide a realistic conception of magma generation in typical collision settings in the absence of mantle magmatic heat sources. The time interval between metamorphism and the stage of anatectic magma formation is 5–12 Ma. [ABSTRACT FROM AUTHOR]

The paper presents geochemical and geochronological data on gneisses and granitoids from three deep boreholes (Yalykskaya-4, Danilovskaya-532, Srednenepskaya-1) in the basement of the southwestern part of the Nepa-Botuoba anteclise. Based on U-Pb zircon dating, three stages of granitoid magmatism were identified: ∼2.8, 2.0 and 1.87 Ga. At ca. 2.8 Ga magmatic TTG protoliths of biotite–amphibole gneisses (Yalykskaya-4 borehole) were formed, these rocks represent the Mesoarchean crust and experienced thermal effects typical of the Tungus superterrane of the Siberian craton at the terminal Neoarchean (∼2.53 Ga). Biotite gneissic granites (∼2.0 Ga) (Danilovskaya-532 borehole), which correlate in age with the granitoids of the basement of the Magan terrane and the Akitkan orogenic belt, were derived from a metasedimentary source formed by the erosion of predominantly Paleoproterozoic juvenile crust rocks. The 1.88 Ga A-type granite (Srednenepskaya-1 borehole) corresponds to the main stage of post-collision granite magmatism within the South Siberian magmatic belt. The ca. 2.8 Ga biotite–amphibole gneisses mark the eastern boundary of the Archean crust with Paleoproterozoic juvenile crust in the south of the Tungus superterrane, which are separated by a transitional zone intruded by granites having intermediate isotopic characteristics. The isotopic composition of Paleoproterozoic gneisses and granitoids indicates that marginal southern Magan terrane in contact with the Tungus superterrane includes blocks of both Archean and Paleoproterozoic crust, thus showing similarity with the Akitkan orogenic belt and accretionary orogens. The final amalgamation of the Tungus superterrane with blocks of the eastern part of the Siberian platform basement corresponds to a milestone of 1.88 Ga. [ABSTRACT FROM AUTHOR]

Flexural toppling occurs when a series of layered rock masses bend towards their free face. It is important to evaluate the maximum bending degree and the requirement of supports of flexural toppling rock mass to prevent rock mass cracking and even failure leading to a landslide. Based on the rock tensile strain-softening model, this study proposes a method for calculating the maximum curvature (Cppmax) of flexural toppling rock masses. By applying this method to calculate Cppmax of 9 types of rock masses with different hardness and rock layer thickness, some conclusions are drawn: (1) the internal key factors affecting Cppmax are E⋆ (E⋆= Ess/E0, where E0 and Ess are the mean deformation moduli of the rock before and after reaching its peak tensile strength, respectively), the strain εt corresponding to the tensile strength of rock, and the thickness (h) of rock layers; (2) hard rock layers are more likely to develop into block toppling than soft rock layers; and (3) thin rock layers are more likely to remain in flexural toppling state than thick rock layers. In addition, it is found that Cppmax for flexural toppling rock masses composed of bedded rocks such as gneiss is related to the tensile direction. [ABSTRACT FROM AUTHOR]

Fluid infiltration into Proterozoic and Early Palaeozoic dry, orthopyroxene-bearing granitoids and gneisses in Dronning Maud Land, Antarctica, has caused changes to rock appearance, mineralogy, and rock chemistry. The main mineralogical changes are the replacement of orthopyroxene by hornblende and biotite, ilmenite by titanite, and various changes in feldspar structure and composition. Geochemically, these processes resulted in general gains of Si, mostly of Al, and marginally of K and Na but losses of Fe, Mg, Ti, Ca, and P. The isotopic oxygen composition (δ18OSMOW = 6.0‰–9.9‰) is in accordance with that of the magmatic precursor, both for the host rock and infiltrating fluid. U-Pb isotopes in zircon of the altered and unaltered syenite to quartz-monzonite indicate a primary crystallization age of 520.2 ± 1.0 Ma, while titanite defines alteration at 485.5 ± 1.4 Ma. Two sets of gneiss samples yield a Rb-Sr age of 517 ± 6 Ma and a Sm-Nd age of 536 ± 23 Ma. The initial Sr and Nd isotopic ratios suggest derivation of the gneisses from a relatively juvenile source but with a very strong metasomatic effect that introduced radiogenic Sr into the system. The granitoid data indicate instead a derivation from Mid-Proterozoic crust, probably with additions of mantle components. [ABSTRACT FROM AUTHOR]

The Tectono-Magmatic Lull (TML) refers to the period between 2.37 and 2.24 Ga when magmatic activity declined globally, the cause of which remains debated. The North China Craton (NCC), which was formed by assembling several micro-blocks in Neoarchean to Paleoproterozoic, preserves the most records of magmatism across the TML. In this contribution, we report petrological, geochemical, and zircon U-Pb-Hf isotopic data of Paleoproterozoic basement rocks from the Xuhuai (XH) micro-block in the southeastern part of the NCC, and use them to constrain the geodynamic regime at the onset of the TML. Our data reveal that the central part of the XH micro-block is mainly composed of biotite granodioritic gneiss and plagioclase amphibolite, which is divided into a ca. 2510 Ma suite of low- and medium-pressure TTGs and a ca. 2430 Ma bimodal igneous suite including high-pressure TTG and plagioclase amphibolite. Variable magmatic zircon εHf(t) values (1.9 to 7.2), Nb, Ta and Ti anomalies for the 2510 Ma TTGs indicate that the parental magma, which was involved with mantle materials, was generated in an active subduction environment in this pre-TML era. The felsic component of bimodal igneous rocks with high Na/Ta ratios (27 to 30) and stable Mg# (41 to 44) suggests their magma source region was as deep as the rutile stability field, and the delamination-derived asthenosphere mantle ascending. The influx of hot mantle induces thermal weakening and mechanical stretching of the overlying crust, leading to back-arc extension. The results of this study support the hypotheses that the NCC formed from micro-block amalgamation and the trigger of the TML could be the large-scale delamination of the lower crust into the mantle. [ABSTRACT FROM AUTHOR]

This study employs a combination of texture and composition of allanite to investigate the chemical evolution of rare earth elements (REEs) in the hydrothermal veins within the Chhotanagpur Granite Gneiss Complex around Kalapathar area, Purulia district, West Bengal, India. Allanite occurs as pockets and lumps in the hydrothermal veins. Locally, the allanite pockets/lumps have high concentrations of REEs (∑REEs ~9.8 wt.%), Th (up to 1427.2 ppm) and U (up to 642.1 ppm). Three hydrothermal fluids and associated REE mobilization have been distinguished and characterized. The hydrothermal fluid1 causes metasomatic addition of REEs that leads to the formation of early REE-enriched allanite. Hydrothermal fluid2 imposed changes in the texture and composition of the allanite where the early allanite was replaced by relatively REE-depleted allanite following the substitution scheme REE+3 + (Fe+2, Mg+2) → Ca+2 + (Al, Fe+3). The hydrothermal fluid3 is marked by the alteration of allanites to the bastnaesite and monazite. REEs have been transported mainly in aqueous chloro-complexes with high to moderate salinity and low homogenization temperature (257°–361°C). The timing of the hydrothermal event is established through U–Th–total Pb dating of monazite, yielding at around 875 ± 43 Ma. Research highlights: The allanite clots and lumps in hydrothermal veins within CGGC around Kalapathar, Purulia District, West Bengal display high ∑REEs up to 9.8 wt.% along with Th and U up to 1427.22 and 642.14 ppm, respectively. The hydrothermal event has been characterized linked with two distinct varieties of allanite growth. The transportation of REEs occurs in aqueous chloro-complexes during the hydrothermal event. The timing of hydrothermal events and growth of the allanites has been determined. REE-enriched magmatic intrusion. [ABSTRACT FROM AUTHOR]

Bhilangana granitoid is a batholithic size body exposed in Bhilangana Valley and later thrusted over rocks of Lesser Himalaya. It comprises mylonitized porphyritic granite gneiss, psammitic gneiss, and its variants, containing primary biotite and muscovite, and S-type in nature. Bhilangana granitoid is intruded by basic intrusive. The variation in chemical signatures and associated petrogenetic issues of Bhilangana granitoid are dealt with detailed petrographic, geochemical and mineral chemical study. In the vicinity of basic intrusive, the porphyritic granite gneiss shows enrichment of MgO, whereas its evolved part, psammitic gneiss shows more boron enrichment with the presence of tourmaline nodules surrounded by quartzo-feldspathic rim. The major mineral assemblage in Bhilangana granitoid is quartz–K-feldspar–plagioclase–biotite–muscovite with tourmaline, epidote, zircon, ilmenite, apatite and monazite. The granite is peraluminous with ASI values always >1. Biotite shows variation from Fe-rich siderophyllitic to Mg-rich eastonitic composition, indicating involvement of both Al-rich crustal and Mg-rich mafic sources, inferring source heterogeneity or mixing of felsic-mafic magma. Granitoid melt was buffered below QFM to NNO and partly above HM, indicating a strong reducing to the oxidizing condition of melt. The melt was saturated with the presence of 3.3–6.8 wt.% H2O content. The crystallization temperature of Bhilangana granitoid (TZr) ranges between 697° and 807°C with a solidification pressure of about 2.92–4.31 kbar, implying crystallization at a depth of 8.76–13 km. The presence of greisens assemblage of quartz–mica–tourmaline–epidote, along with the presence of fluorite, is a representative greisens assemblage for Sn–W mineralization. Thus, the Bhilangana granitoid represents the fertile nature and potentiality of Sn–W minerals. Research highlights: Bhilangana granitoid is S-type, peraluminous granitoid, formed by anatexis of crustal material The biotite chemistry reveals the variation in the redox condition and modification in the oxidation state, preserved in the different variant of granite representing the magma mixing or source heterogeneity in protolith. The Bhilangana granitoid has attended the crystallization temperature (TZr) of 697–807 °C under pressure of about 2.92–4.31 kb implying the depth of crystallization of 8.76–13 km. The presence of greisenization favours the potential of Sn–W minerals. The geochemical results shows abnormal enrichment of W and Sn in the Bhilangana granitoid. The Bhilangana granitoid appears to have fertile nature for Sn–W mineral potential. [ABSTRACT FROM AUTHOR]

In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (>50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems. [ABSTRACT FROM AUTHOR]

The Middle Benue Trough (MBT) in Northcentral Nigeria is a geologically significant area with vast mineral resource potential. Employing airborne magnetic and radiometric data, this study utilized the Centre for Exploration Targeting on enhanced total magnetic intensity data to reveal geologic structures, lithological units and mineralization zones. Lineaments predominantly trended in NE-SW direction, with noteworthy orientations in NNE-SSW and E-W. Radiometric anomalies correlated with distinct lithological units, pinpointing granitic gneiss, alluvium, shale, siltstone and sandstone. A magnetically concentrated and potassium-rich area indicated potential polymetallic-magnetic mineralization. The 2D model illustrated igneous intrusions influencing prevalent geologic structures, such as sediment baking and doming. Thorough analysis, including source parameter imaging, standard Euler deconvolution and 2D forward modelling, revealed sediment thicknesses below 1500 m. This research enhances understanding of the MBT's geological features, offering valuable insights for mineral exploration and resource assessment in the region. [ABSTRACT FROM AUTHOR]

Canoe Reach is a region of high geothermal potential on a segment of the Southern Rocky Mountain Trench fault (SRMTF) with highly metamorphosed and structurally complex wall rocks, near Valemount, British Columbia. This study contains analyses of magnetotelluric data collected at Canoe Reach accounting for electrical anisotropy, which is not often considered during geothermal exploration. Isotropic and anisotropic 3D inversions are used due to signs of electrical anisotropy in the Canoe Reach magnetotelluric data and the presence of visibly anisotropic geological structure. At Canoe Reach North, the anisotropic model is preferred for its simpler structure and consistency with the mapped geology. An anisotropic feature in the footwall of the steeply southwest-dipping SRMTF has a low resistivity in the fault-perpendicular direction and a high resistivity in the vertical direction, which is more easily explained by conductive minerals than by fluids in the highly metamorphosed gneiss. An exploration well in the SRMTF footwall encountered two graphite seams with thicknesses ≥1 m, supporting the interpretation of anisotropic resistivity due to conductive minerals. A strong resistivity contrast across the SRMTF suggests juxtaposition of different lithologies, challenging existing interpretations of SRMTF displacement at Canoe Reach. At Canoe Reach South, anisotropic features near the Canoe River thermal spring with a high resistivity in the fault-perpendicular direction and low resistivity in the vertical direction are consistent with fault core and damage zone models. Magnetotelluric data may be sensitive to permeability anisotropy of fault zones, and the use of electrically anisotropic inversions should be considered for these settings. [ABSTRACT FROM AUTHOR]

This study focusses on the scarce resources of high-quality silicated rock aggregates in Central Chad, a wide area almost devoid of hard rock outcrops. We provide an original study of the petrographic and mechanical characteristics of Alfallé massif together with a review of quarries in activity or potentially exploitable. The resistance to degradation of the prepared aggregates were evaluated using the standard test of Los Angeles and micro-Deval. The Alfallé massif is made up of three lithologies: alkali feldspar coarse granite, biotite-rich fine-grained quartz monzonite and biotite-rich fine-grained gneiss. The mechanical test results obtained indicate that the materials studied have excellent to satisfactory properties, and that the Alfallé massif provides the best characteristics among six different quarrying areas in Central Chad. Moreover, the three quarries that were once isolated islands in the mega-Chad paleolake yield on average higher resistance. We thus propose that these rare residual reliefs within the mega-Chad paleolake area reached optimal mechanical properties through a natural selection process, specific of the paleolake erosional conditions. [ABSTRACT FROM AUTHOR]

The study presents measurements of 222Rn exhalation rates, 226Ra contents and radiological exposure doses of granite, gneiss, shale, sandstone and soil samples across the seismically active Kopili Fault Zone, India using LR-115 (II) track detectors. The granites were found to have higher 222Rn exhalation rates than the other types of rocks. The study shows higher 222Rn exhalation rates and radiological exposure doses of soil samples collected near the fault than those collected away from the fault. However, the 226Ra contents in every geological sample were within the global average value of 40 Bq kg−1 for materials used in common buildings. [ABSTRACT FROM AUTHOR]

New U-Pb zircon ages are preliminarily reported here from various high-grade units within the overall low-grade Balkan fold-and-thrust belt of northern Bulgaria. The Stakevtsi gneisses yielded Cadomian ages interpreted as magma crystallization ages ranging from 651±8 Ma to 601±3 Ma, some of the samples exhibited a significant proportion of Mesoproterozoic ages with peaks at 1.6 and 1.5 Ga. Paragneisses in the Barzyia Massif exhibit complex U-Pb zircon age patterns, with age peaks typically at 580 Ma and 540 Ma, but also Paleo- and Mesoproterozoic and younger at 430 Ma to 310 Ma. The Divchovoto gneisses yielded age spectra with a minor contribution of Archean, Paleo- and Mesoproterozoic grains and a dominant population of Cadomian ages at 610 Ma. In summary, most of the samples point to the north African provenance, however, some of the Stakevtsi gneisses indicate rather a Baltican provenance. This suggests that the pre-Variscan tectonics and juxtaposition of different terranes was complex in this segment the former Cadomian orogenic belt, but also the possible proximity of Baltica to Gondwana at the end of Neoproterozoic. [ABSTRACT FROM AUTHOR]

Sardinia (Italy) represents one of the most comprehensive cross-sections of the Variscan orogen. The metamorphic and structural complexity characterizing its axial zone still presents many unresolved issues in the current state of knowledge. The data presented from the structural study of the entire axial zone of this area have allowed the authors to propose a subdivision into two new structural complexes. In particular, a younger complex is identified as the New Gneiss Complex, containing remnants of an older and higher-grade metamorphic complex defined as the Old Gneiss Complex. The structural and geometric relationships between the two complexes suggest the redefinition of the axial zone of Sardinia as part of the intracontinental East Variscan Shear Zone/medium-temperature Regional Mylonitic Complex. Comparable relationships are also highlighted in many other areas of the Variscan chain (e.g., Morocco, Corsica, Maures Massif, and Argentera Massif). Extending this new structural interpretation to other axial zones of the South European Variscan orogen could provide new hints for reconstructing the collision boundaries between Gondwana and Laurussia in the late Carboniferous to the early Permian periods. [ABSTRACT FROM AUTHOR]

The retreat of two roadside cut slopes in weathered gneiss in the Middle Himalaya of east Nepal has been assessed by engineering geological mapping, sequential photography and slope profiling over a 40 year period, from 1979 to 2018, allowing approximate degradation rates to be calculated. The rate of removal of completely weathered gneiss and residual soil was about twice that of highly weathered material, primarily reflecting increased micro-cracking and breakdown of feldspars and micas in the weathered fabric. Surface water erosion by rilling and gullying in the summer monsoon wet season was the dominant process affecting the slopes, with landsliding (mainly by soil falls and debris slides) playing a secondary role. Drainage and bio-engineering protection measures applied to the slopes within the first 3–12 years after excavation were effective in reducing the degradation rate by between one and two orders of magnitude once the slopes had developed a moderate vegetation cover. The total volume of material removed from the slopes over four decades is equivalent to average rates of ground lowering in the range of 150–320 and 2–4 mm a−1 before and after protection works, respectively. The latter range is similar to long-term denudation rates measured in drainage catchments in the region with similar settings. Supplementary material: One figure in three parts showing slope retreat over time for a complete set of 28 measured cross-sections and two tables giving details of calculated cross-sectional area losses and approximate intersectional volumetric losses of slope material are available at https://doi.org/10.6084/m9.figshare.c.7005885 [ABSTRACT FROM AUTHOR]

The timing of partial melting in high-grade metamorphic rocks is critical for constraining tectonic histories and processes. However, uncertainties exist about the behavior of monazite and zircon during partial melting, especially about the timing of crystallization with respect to melting reactions. This study is focused on a single sample (16TG143) of finely layered, migmatitic gneiss from the Adirondack Highlands, New York, interpreted to have undergone extensive biotite dehydration melting (i.e., Bt + Pl + Als + Qz = Grt + Kfs + melt). The rock contains one distinct leucosome layer. The non-leucosome (gray gneiss) portion of the migmatite has millimeter-scale sublayers with distinct differences in modes and mineralogy. The layers are interpreted to reflect the differential preservation of reactants and products formed during the forward and reverse progress of the melting reaction. Monazite and zircon modes, and to some degree, texture, composition, and geochronology all vary from layer to layer. Both minerals have up to three domains: ca. 1150 Ma anhedral cores, ca. 1050 Ma monazite mantles/fir tree textured zircon, and ca. 1030 Ma rims. The heterogeneous layered gray gneiss provides robust constraints on the timing of melting (ca. 1050 Ottawan orogenesis), melt crystallization, and post-melting retrogression, in addition to information about earlier metamorphic events. Early-formed monazite and zircon grains were largely dissolved during progressive melting, except where preserved as relicts or inclusions. Monazite mantles and fir tree zircon grains precipitated upon cooling during progressive melt crystallization between temperatures of 800 and 750 °C. Rims are interpreted to have precipitated during subsolidus, solid-state retrogression after ca. 1050 Ma. Correlations between the gneissic layering, melting reactions, and the character of geochronometers emphasize the importance of characterizing the layer-forming and chronometer petrogenesis processes as a critical part of deconvoluting the history of migmatitic gneisses. [ABSTRACT FROM AUTHOR]

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Mabel Creek Ridge, in the northern Gawler Craton, is a granulite‐facies domain recording early Mesoproterozoic metamorphism, flanked by less metamorphosed rocks and dissected by crustal‐scale divergent structures. The nature of early Mesoproterozoic events is poorly understood due to the lack of basement outcrop. Calculated metamorphic phase diagrams and geochronology are used to decipher the tectonic regime of a potential gneiss dome. Pressure–temperature (P–T) modelling of metapelites from five drill holes across Mabel Creek Ridge suggests it has experienced conditions of ~6.4–7.4 kbar and 800–850°C and the growth of suprasolidus cordierite after garnet indicates subsequent decompression. In situ U–Pb monazite and Lu–Hf garnet geochronology constrains the granulite‐facies metamorphism of Mabel Creek Ridge to ca. 1600–1560 Ma. In contrast, drill hole GOMA DH4 located to the north of Mabel Creek Ridge records conditions of 2.2–5.4 kbar and 710–740°C at ca. 1520 Ma, with no evidence for 1600–1560 Ma metamorphism. Our new P–T pseudosection results and geochronology data from Mabel Creek Ridge and adjacent crust, coupled with the regional seismic and airborne magnetic data, reveal that Mabel Creek Ridge represents a record of early Mesoproterozoic extension in the Gawler Craton, during which thermally perturbed lower crustal rocks were exhumed within a gneiss dome. Early Mesoproterozoic extension took place within a complex geodynamic regime resulting from the interplay between final Nuna convergence along the margin of northeast Australia at ca. 1600 Ma and subduction to the southwest at ca. 1630–1610 Ma. [ABSTRACT FROM AUTHOR]

There are 25 Li-rich geothermal fields (>15 mg/L) are displayed throughout nine geothermal zones in Tibet. The Kawu geothermal system is one of the representative high-enthalpy and Li-rich geothermal systems and is located in the Skaya dome, the Himalayas. Their boiling and hot and cold waters are Cl-HCO3-Na and HCO3-Ca types, respectively. The parent geothermal fluids are characterized by high Na, Cl, B, Si, As, Li, Rb+, Cs+ and F, and the Cl− concentration and enthalpy of the parent geothermal liquid are 515 mg/L and 1,485 J/g, respectively (corresponding to a temperature of 323.5°C). The stable oxygen and hydrogen isotopic compositions of geothermal fluids range from −19.7 to −23.4‰ and −155 to −181.2‰, indicating that the ratio of magmatic water mixed into geothermal fluid is between 11.7% and 19.5%. The lithium content in the geothermal system is high (13.04 to 20.8 ppm), but the δ7Li is relatively low (1.06 to 1.72‰). The high Cl, Li and B contents, constant Cl/Li and Cl/B ratios and low δ7Li indicate that they mainly originate from magmatic fluid, rather than from water rock reaction with the leucogranite and gneiss. While ascending from the reservoirs to the surface, the parent geothermal fluids gradually mixed with the meteoric waters infiltrated via brittle tensional small-scale faults, in turn forming four cooling processes upwelling patterns, i.e. conductive cooling, adiabatic cooling, mixing with cold water, and the combination of these processes. [ABSTRACT FROM AUTHOR]

Magnetic susceptibility is widely applied in geology, but its use in the study of metamorphic rocks has been limited due to the complex nature of metamorphism. In this study, analyzing a partially retrogressed mafic enclave within Archean TTG gneiss from the Jiaobei Terrane, North China craton, we incorporate magnetic susceptibility with conventional metamorphic (petrographic and major element) proxies in order to investigate profiles of metamorphic retrogression. The magnetic susceptibility results show more complete retrogression in the direction parallel to the tectonic fabric and partial retrogression perpendicular to it. This geophysical observation, which is broadly consistent with the petrological observations and P--T estimates, suggests that the availability of fluids and perhaps even non-lithostatic pressure play roles in the preservation of such differential retrogression. This study thus introduces magnetic susceptibility as a novel proxy in this context, revealing its utility in rapidly and quantitatively identifying variable retrogression gradients consistent with, but with more precision than, measured metamorphic pressures. [ABSTRACT FROM AUTHOR]

This study was carried out in the weathering profile on gneiss in the Mandoga area (south Cameroon plateau), on the petrological characterization of the weathered phases. Gneiss is made up of quartz, feldspars, biotite, muscovite, garnet, kyanite, and zircon, with negative Eu anomaly (Eu/Eu* = 0.63). The profile shows three groups of phases: coarse saprolite at the bottom of the weathering horizon, the phases of the fine saprolite, and those of the uppermost part of the weathering profile. The mineralogical composition differs from the coarse saprolite to the top soil. All phases comprised kaolinite, quartz, and goethite. They characterize a pedoclimatic environment-rich in water, favorable to the stability of hydrated minerals. The negative Eu anomaly in all phases reflects the lithodependance of horizons at the expense of the gneiss, the parent rock. The Ce anomalies are linked to the pedodynamic conditions. In the same way, the behavior of REE in the different phases confirms the variation of the geochemical processes of the two pedodynamic domains. The phases from the fine saprolite are enriched in LREE than in HREE and match an overlapping of geochemical pedodynamic processes. Thus, the phases are marked by the geochemical signature along the weathering profile. [ABSTRACT FROM AUTHOR]

In this paper, we applied two kinds of knowledge-driven methods, one using the fuzzy logic and another using geometric average to evaluate 3D mineral prospectivity for Sansong district, Yongyu apatite deposit, in the Phyongnam Basin, DPR Korea. Based on the ore geology studies of apatite deposits and the available spatial datasets in the study area, we used four independent evidential maps for 3D apatite deposit prospectivity modeling. They include (1) carbonatite, (2) biotite gneiss, (3) granitic gneiss, and (4) P2O5 values from the borehole data. The evidential factors were modeled into 3D space, and 3D P2O5 values from the borehole data were transformed into continuous values of the [0, 1] range using logistic sigmoid. In 3D MPM just as 2D MPM, it is very economic and efficient to simultaneously apply the fuzzy logic and geometric average methods for mineral prospectivity modeling of the study area because two predictive models can use the same fuzzification methodology based on fuzzy membership function. Our strategy is to fuzzify the evidential maps before applying the geometric average as well as in the fuzzy logic. The results for the two predictive models were validated by the prediction efficiency method. The results demonstrated that most of the validation data were distributed in voxels with high prospectivity values. Although the validation results were slightly worse than those in 2D MPM case study, our case studies suggested that both predictive models and their modeling results are useful for evaluating 3D prospectivity of apatite deposits in Sansong district, Yongyu apatite deposit, DPR Korea. [ABSTRACT FROM AUTHOR]

This article focuses on the issues related to the primary nature and metamorphism of gneisses from the Larba block of the Dzhugdzhur–Stanovoy fold area, which are based on the study of the mineralogical composition of supracrustal rocks. The PT-parameters for the highest temperature and pressure metamorphic transformations of rocks of the Larba block are estimated as P = 6.5–8.5 kbar and T > 800°C. Relatively low-temperature garnet–biotite and garnet–biotite–chlorite mineral parageneses were formed at the regressive stage of metamorphism at ~4–6 kbar and temperatures up to 500–600°C. Wide variations in paragneiss compositions indicate that the studied gneisses were derived from protoliths of different composition. Overall, Larba metasedimentary rocks had weakly and moderately weathered source areas. Most of the compositions of paragneisses are located in the field of chlorite–montmorillonite, montmorillonite–kaolinite–hydromica, and chlorite clays formed from the mafic protolith. The low alkalinity in combination with a high MgO content indicates the presence of rock erosion products with a significant admixture of basic volcanic material in the detrital material. Migmatization of the rocks is the most significant factor that distorts the reconstruction of the original rock composition. [ABSTRACT FROM AUTHOR]

The oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (>77.5°C/kbar). [ABSTRACT FROM AUTHOR]

The Western Gneiss Region (WGR) is a Precambrian basement domain in the Scandinavian Caledonides and one of the world's largest high‐ and ultrahigh‐pressure terranes. The south–central WGR underwent regional eclogite facies metamorphism 415–400 Ma ago when Baltica subducted beneath Laurentia, during the Scandian orogeny. Eclogites in the WGR group into two traditional types: (1) Precambrian mafic intrusions metamorphosed in situ during Scandian continental subduction and (2) eclogites, garnet peridotites and garnet pyroxenites within ultramafic complexes derived from the subcontinental mantle beneath Laurentia. We document, using field relations, petrography, whole‐rock geochemistry and secondary ion mass spectrometry (SIMS) zircon geochronology, a hitherto unrecognized third type of eclogite in the WGR that places new constraints on its tectonic architecture: an eclogitized fragment of oceanic crust from the Iapetus Ocean. The Kråkfjord eclogite complex is a km2‐sized body with an interior consisting of kyanite eclogite (meta‐troctolite) and subordinate layers and lenses of garnet peridotite, garnet websterite and kyanite–garnet leucotonalite. This interior is capped by Fe–Ti‐rich eclogite, which locally contains subordinate pockets of migmatitic aluminous gneiss. The elemental abundances and isotopic compositions of the Fe–Ti‐rich eclogites resemble those of mid‐ocean ridge basalt (MORB). In contrast, the interior kyanite eclogites, peridotites and pyroxenites have compositions similar to the gabbroic cumulates in the lower oceanic crust of slow‐spreading ridges. U–Pb SIMS dating of igneous zircon cores from a leucotonalite pod in the interior of the Kråkfjord complex yields Cambro‐Ordovician igneous ages of 500–440 Ma, with the ~500 Ma age interpreted as the isotopically undisturbed age. This age matches those of Iapetan oceanic rocks exposed elsewhere in the mountain belt. Metamorphic zircon from an Fe–Ti‐rich eclogite in the carapace of the Kråkfjord complex dates the eclogite facies metamorphism at 421.9 ± 2.2 Ma, synchronous with the continental collision. Zircon from a leucosome in Fe–Ti‐rich retro‐eclogite indicates an age of 408.5 ± 2 Ma for the crystallization of partial melt following the decompression. Detrital zircon core ages from a pocket of aluminous migmatitic gneiss in the carapace indicate derivation of sediment from the Baltic crust. Collectively, the data show that the eclogite complex (1) originated at an Iapetus spreading centre near the continent Baltica, (2) subducted to eclogite conditions during Scandian continental collision and (3) was tectonically intercalated with the Precambrian Baltica basement of the WGR. [ABSTRACT FROM AUTHOR]

Slave Province in Canada is an Archean granite–supracrustal terrane at the northwestern corner of the Canadian Shield. It is bordered by the Thelon–Taltson orogen (2.0 to 1.9 Ga) to the southeast and the Wopmay orogen (1.9 to 1.8 Ga) to the west. Acasta gneisses, exposed in the westernmost Slave Province, and the Wopmay rocks, located close to the gneisses, were systematically collected for K–Ar and laser step-heating 40Ar/39Ar single-crystal analyses of the biotite and amphibole. The K–Ar biotite ages of the four Wopmay samples range from 1816 ± 18 Ma to 1854 ± 26 Ma. The 40Ar/39Ar biotite analyses of the three Wopmay samples yield plateau ages of 1826 ± 21 Ma, 1886 ± 13 Ma, and 1870 ± 18 Ma. These ages fall within the reported U–Pb zircon age range of the Wopmay orogen. The K–Ar biotite ages of the fifteen Acasta gneisses range from 1779 ± 25 Ma to 1877 ± 26 Ma, except for one younger sample (1711 ± 25 Ma). The 40Ar/39Ar analyses of the biotite crystals from three samples give the plateau ages of 1877 ± 8 Ma, 1935 ± 14 Ma, and 1951 ± 11 Ma. The K–Ar amphibole ages from twelve samples range from 1949 ± 19 Ma to 1685 ± 25 Ma. Two samples of them give ages older than the zircon U-Pb age of Hepburn plutons. The 40Ar/39Ar analyses of the amphibole crystals show varied age relations. The two samples give plateau ages of 1814 ± 22 Ma and 1964 ± 12 Ma. Some samples exhibit apparent old ages of ~2000 Ma in the middle temperature fractions. These old fractions result from the amphibole crystals, originally formed in the Archean, being affected by the thermal events during the Wopmay orogeny but not fully resetting. These observations suggest that the K–Ar system ages of the biotite and amphibole in the Archean Acasta gneiss were rejuvenated during the Paleoproterozoic ages. The Discussion explores the possibility that the heat source rejuvenating the K–Ar system ages may have arisen due to asthenospheric extrusion into the wedge mantle, a process likely triggered by subduction rollback. [ABSTRACT FROM AUTHOR]

In-situ dating of the zircons, titanites, and apatites in the rock fragments of approximately 1 cm2 in size from the Acasta Gneiss Complex was performed using secondary ion mass spectrometry or laser ablation inductively coupled plasma mass spectrometry to provide constraints on the thermal history of the Acasta Gneiss Complex. The zircons in these rock fragments typically exhibit multiple age populations, reflecting the presence of inherited zircons or the post-crystallization process of zircon overgrowth. Combined with previous studies, our zircon dating results reveal multiple magmatic events that occurred in the Acasta Gneiss Complex, specifically at >3.96, ∼3.72, and ∼3.57 Ga, respectively. A titanite Pb-Pb isochron age of 2911±22 Ma (95% confidence level, MSWD=1.5) for sample AY199 is identified, consistent with the timing of the latest Archean magmatism in the Acasta Gneiss Complex. The titanite U-Pb ages for samples AC478 and AY066 are 1932±270 Ma (95% confidence level, MSWD=2.3) and 1813±45 Ma (95% confidence level, MSWD=2.3), respectively. The apatites in sample P090803-C exhibit a Pb-Pb isochron age of 1833±26 Ma (95% confidence level, MSWD=1.4). The apatite U-Pb ages for samples AC478, AY199, and AY066 are 1850±20 Ma (95% confidence level, MSWD=1.3), 1827±100 Ma (95% confidence level, MSWD=6.3), and 1807±58 Ma (95% confidence level, MSWD=3.9), respectively. Titanites in samples AC478 and AY066, as well as the apatites in all four investigated samples, show a uniform age (Pb-Pb or U-Pb age) of ∼1.9–1.8 Ga, indicating U-Pb system reset in these minerals due to the Wopmay orogeny and documenting that the peak temperature condition associated with the Wopmay orogeny exceeded the apatite U-Pb closure temperature and approached or exceeded that of titanite. [ABSTRACT FROM AUTHOR]

Sillimanite-bearing gneisses in the Romsdal region of the Western Gneiss Region (south Norway) have been investigated to document the presence, formation, composition and petrological evolution of the sillimanite-bearing assemblages. Sillimanite is found in augen gneiss, as nodular gneiss, and in well-foliated sillimanite–mica gneiss. Lenses and layers of eclogite occur within the gneiss units. The sillimanite-bearing gneisses are heterogranular and dominated by quartz, plagioclase (An 29–41), K-feldspar and biotite (Mg# = 0.48–0.58; Ti = 0.16–0.36 a.p.f.u.), with variable amounts of white mica (Si = 6.1–6.3). K-feldspar occurs as porphyroclasts in augen gneiss, and garnet constitutes resorbed porphyroblasts. Garnet (Alm 46–56 Sps 24–36 Prp 10-20 Grs 4–6 ; Mg# = 0.22–0.29) shows rimward-decreasing Mg#, together with a smaller grossular decrease and a marked spessartine increase up to Sps 36. The foliation is defined by crystal-preferred-orientation micas, elongation of shape-preferred-orientation coarse K-feldspar phenocrysts and a modal banding of phases. Sillimanite occurs as coarse orientation-parallel matrix porphyroblasts, as finer grains and as fibrolitic aggregates. Quartz constitutes coarser elongated grains and monomineralic rods. Pseudosection modelling suggests that the peak-metamorphic mineral assemblage of garnet–sillimanite–feldspar–biotite–quartz–ilmenite–liquid equilibrated at temperatures up to 750 °C and pressures of 0.6 GPa. Subsequent retrogression consumed garnet. Mineral replacement and melt crystallization involved sillimanite, white mica, K-feldspar and quartz. The results document a metamorphic retrogression of the sillimanite gneisses in accordance with the presence of remnants of eclogites and high-pressure granulites in this northwestern part of the Western Gneiss Region. [ABSTRACT FROM AUTHOR]

Geological mapping of over 50 km2 area has been carried out in and around Obulapuram area, SE of Sandur Schist Belt. The Sandur Schist Belt (SSB), a part of Eastern Dharwar Craton (EDC) is a NW–SE trending lens-shaped late Archaean volcano-sedimentary belt. The litho-types exposed include peninsular gneiss of PGC, fuchsite quartzite, meta-basalt with intercalation of amphibolite, phyllite, metagabbro and quartzite, banded magnetite quartzite (BMQ), banded hematite quartzite (BHQ) belonging to the Donimalai Formation of Sandur Group. Granites, dolerite dykes, gabbro dykes and quartz veins are younger intrusives. Basement Peninsular Gneiss and intrusive granite are exposed towards the southern and eastern side of the study area. The area has undergone two phases of deformation, of which the earlier D1 deformation is quite pronounced producing the NW–SE trending S0∥S1 regional structures of the area. D1 deformation is responsible for the development of tight isoclinal and macroscopic (F1) folds. D2 deformation resulted in the development of open to close mesoscopic (F2) folds and broad warps with axial traces along ENE-WSW direction and crenulation cleavage (S2). The area is predominantly characterised by regional green schist facies of metamorphism, which increases to amphibolite facies near the contact of the schist belt with gneisses and intrusive granites. [ABSTRACT FROM AUTHOR]

This report unveils the outcomes of measurements of 222Rn exhalation rates, effective 226Ra content, and the annual effective dose for rocks and soil collected in the vicinity and far from the seismically active Kopili Fault Zone, India. LR-115 (II) nuclear track detector has been used to investigate radiation exposure in the seismically active fault zone and gneissic complex of the Shillong Plateau, which have significant geological characteristics and frequently produce earthquakes. In this study, 222Rn mass exhalation rates for granite, gneiss, and soil samples collected near and far from the fault were calculated to be in the ranges of 73.2–83.0, 11.0–14.8, 21.1–70.7 and 34.3–65.5, 6.4–9.9, 19.1–28.9 mBq kg−1 h−1, respectively. Surface exhalation rates of 222Rn for granite, gneiss, and soil samples collected near and far from the fault varied between 2428.8–2751.7, 365.6–491.9, 698.3–2345.9, and 1138.6–2172.7, 211.9–328.2, 634.9–960.0 mBq m−2 h−1, respectively. The 226Ra contents of granite, gneiss, and soil collected in similar distributions were found to vary from 9.76–11.06, 1.47–1.98, 2.8–9.4, and 4.58–8.74, 0.85–1.32, 2.6–3.6 Bq kg−1, respectively. The annual effective doses attributed to 222Rn inhalation from exposure to the samples were calculated to be in the ranges of 176.3–199.8, 26.5–35.7, 50.7–170.3 and 82.7–157.7, 15.4–23.8, 46.1–69.7 μSv y−1, respectively, for granite, gneiss, and soil. The lifetime fatality-risks have also been analysed in this study. The samples that were collected in close proximity to the fault showed higher values than those that were collected farther away. However, the results reveal that the measured values are within the globally recognised permissible range. [ABSTRACT FROM AUTHOR]