Gneissic Rocks Research Articles

Effect of land\xa0use, season, and soil depth on soil microbial biomass carbon of Eastern Himalayas

BackgroundSoil microbial biomass, an important nutrient pool for ecosystem nutrient cycling is affected by several factors including climate, edaphic, and land-use change. Himalayan soils are young and unstable and prone to erosion and degradation due to its topography, bioclimatic conditions and anthropogenic activities such as frequent land-use change. Through this study, we tried to assess how soil parameters and microbial biomass carbon (MBC) of Eastern Himalayan soils originated from gneissic rock change with land-use type, soil depth and season. Chloroform fumigation extraction method was employed to determine MBC from different land-use types.ResultsSoil physical and chemical properties varied significantly with season, land-use and soil depth (p < 0.001). The maximum values of soil properties were observed in the rainy season followed by summer and winter season in all the study sites. Annual mean microbial biomass carbon was highest in the forest (455.03 μg g− 1) followed by cardamom agroforestry (392.86 μg g− 1) and paddy cropland (317.47 μg g− 1). Microbial biomass carbon exhibited strong significant seasonal difference (p < 0.001) in all the land-use types with a peak value in the rainy season (forest-592.78 μg g− 1; agroforestry- 499.84 μg g− 1 and cropland- 365.21 μg g− 1) and lowest in the winter season (forest − 338.46 μg g− 1; agroforestry – 320.28 μg g− 1 and cropland − 265.70 μg g− 1). The value of microbial biomass carbon decreased significantly with soil depth (p < 0.001) but showed an insignificant increase in the second year which corresponds to a change in rainfall pattern. Besides, land-use type, season and soil depth, soil properties also strongly influenced microbial biomass carbon (p < 0.001). Microbial quotient was highest in the agroforestry system (2.16%) and least in the subtropical forest (1.91%).ConclusionsOur results indicate that land-use, soil depth and season significantly influenced soil properties and microbial biomass carbon. The physical and chemical properties of soil and MBC exhibit strong seasonality while the type of land-use influenced the microbial activity and biomass of different soil layers in the study sites. Higher soil organic carbon content in cardamom agroforestry and forest in the present study indicates that restoration of the litter layer through retrogressive land-use change accelerates microbial C immobilization which further helps in the maintenance of soil fertility and soil organic carbon sequestration.

A multi-criteria decision analysis for groundwater potential evaluation in parts of Ibadan, southwestern Nigeria

Multi-criteria decision analysis based on Saaty’s analytical hierarchy processing technique has been used to establish groundwater potential distribution pattern across some highly populated parts of Ibadan metropolis in southwestern Nigeria. The technique weighted and ranked seven sets of thematic hydrological parameters derived from Landsat 8 OLI satellite imagery, 143 vertical electrical sounding (VES) geophysical data, and geological and topographical data. Filtered and enhanced Landsat 8 OLI satellite imagery, quality-checked and inverted VES data, and categorized geological and other ancillary data were analyzed and used to generate lineaments, subsurface geoelectric parameters and other terrain information employed to extract thematic hydrogeological parameters used to characterize the subsurface in terms of groundwater potential. Weighted, normalized and ranked derived thematic hydrogeological parameters (lineament density, drainage density, coefficient of anisotropy, aquifer thickness, overburden thickness, aquifer resistivity and lithology) were employed to generate groundwater resource potential map. The map delineates the study area into very low (6.5%), low (41.0%), medium (38.1%), high and very high (14.4%) groundwater resource potential zones. Regions underlain by quartzite/quartz schist rocks present medium-to-high groundwater resource potential, while regions underlain by migmatite and granite gneiss rocks mostly have very low–low groundwater resource potential. This study indicates that variation in groundwater resource potential across Ibadan situated within the basement complex terrain is mostly influenced by the heterogeneity of subsurface geology which varies rapidly in terms of rock distribution and associated hydrogeological indices.

Statistical variability of radiation exposures from Precambrian basement rocks, NW Nigeria: Implication on radiogenic heat production

The heat flux system within the Precambrian basement complex in northwestern Nigeria has been poorly studied and little is known about the potential of the constituent petrologic units as regards geothermal resources. The earth crust gets heated up by a combination of processes, prominent among them is radioactive decay of 40K, 232Th and 238U radionuclides. Radioelement concentrations were measured within the rock units that dominate the basement complex namely quaternary sediments, carbonates, metasediments, younger granites, metavolcanics, older granites, migmatite and gneissic rock. Statistical parameters were estimated to deduce the radioactive absorbed dose of the various rock units. Quaternary sediments recorded the lowest mean dose while gneissic rock recorded the highest mean dose given as 112.09 nGy.h−1 and 264.54 nGy.h−1 respectively. One-way analysis of variance performed in this study showed distinct statistical significant variations of the radioelement concentrations in the dose assessment of the rock units. The contributing percentages of the radioelements for all the studied rock units were in the order 40K > 232Th > 238U. Two major classes of rock unit namely clastic rocks and plutonic rocks were assessed for their heat production capacity. The clastic rocks comprising quaternary sediments and carbonates presented mean heat production units given as 0.528 µWm−3 and 0.784 µWm−3 respectively while the plutonic rocks comprising granite and gneissic rock presented mean heat production units given as 1.404 µWm−3 and 1.982 µWm−3 respectively. Subsequently, heat generation unit estimations were extrapolated from the heat production values with gneissic rock showing the highest value given as 5.5660 HGU. These results which may assume theoretical pilot status towards geothermal resource investigation in northwestern Nigeria are in tandem with other similar studies performed around the globe.

Evaluation of springs and waterfalls as Geomorphosites and proposition of strategies to develop Geotourism at Ajodhya hill, Puruliya district, Eastern India

Ajodhya hill is an extended part of Chota Nagpur Granite Gneissic Complex located in Puruliya district, western part of West Bengal, Eastern India. This uplifted plateau is primarily composed of Archean granite and gneissic rock with the sharp break at the plateau fringe. Ground water comes out as spring through the fractures of the granitic rock base situated beneath the permeable red soil. Waterfalls are prominent along the steep slope of the plateau fringe. Thirteen springs and four waterfalls have been identified as important Geomorphosites to analyse their Geotourism potential of Ajodhya hill in response to its geologic and geomorphic background. SWOT analysis has been used for estimating the Geotourism potentialities of Ajodhya hill. To analyse the potentiality, weight and score has been given under each factors and weighted score has been calculated. Few strategies have been suggested for enriching the Geotourism potential of the region with the help of TOWS matrix. Prioritization of the strategies has been ascertained by QSPM (quantitative strategies planning matrix) method through which the Ranking of Total Attractiveness Score (RTAS) has been estimated.

Petrogenesis and tectonic setting of the Paleoproterozoic Kelle Bidjoka iron formations, Nyong group greenstone belts, southwestern Cameroon. Constraints from petrology, geochemistry, and LA-ICP-MS zircon U-Pb geochronology

ABSTRACT The Kelle Bidjoka Iron Formations (IFs) occur at the northmost margin of the Neoarchean to Paleoproterozoic Nyong group greenstone belts in SW Cameroon. This contribution presents for the first time the petrographical, geochemical, and geochronological data of the Kelle Bidjoka IFs and associated metaigneous (metatonalite) and metasedimentary (pyroxene gneiss) rocks. The studied IFs form a thin unit of 3.21 m thick and comprise magnetite pyroxene gneiss, garnet-bearing magnetite pyroxene gneiss, pyroxene BIFs, and grunerite BIFs. Petrographic studies have revealed mineral assemblages indicating retrograde amphibolite facies metamorphism. Magnetite pyroxene gneiss and pyroxene BIFs show signatures of both seawater and hydrothermal fluids, with magnetite pyroxene gneiss deposited closer to the vent source. In contrast, garnet-bearing magnetite pyroxene gneiss and grunerite BIFs are devoid of hydrothermal characteristics and show significant crustal contamination. The lack of Ce anomaly in most samples indicates anoxic to suboxic conditions during their deposition. The geochemical characteristics of the associated metaigneous and metasedimentary rocks indicate that Kelle Bidjoka IFs were deposited in a submarine volcanic arc environment, similar to the Algoma-type BIFs. Detrital zircon LA-ICP-MS U-Pb dating suggests that the Kelle Bidjoka BIFs were deposited during early Paleoproterozoic (ca. 2423 Ma) and were metamorphosed up to granulite facies during the Eburnean/Transamozian (ca. 2050 Ma) orogeny.

The Velocities of Modern Horizontal Movements of Earth Crust in the South Sector of Yenisei Ridge According to GNSS Observations

The paper presents the results of GNSS observations, provided by the authors in 2010-2019 years, in the zone of contact of tectonic structures of the Siberian platform, West-Siberian plate and West-Sayan orogenic area. We presented the first time assessment of the velocities of modern horizontal movements and the structural-kinematic model of block movements in a southern sector of the Yenisei Ridge. This model enables to assess the geodynamic safety of disposal of high-level radioactive waste in granite gneisses rocks of the Nizhne-Kansk massif.

Sugarloaf Land in south-eastern Brazil: a tropical hotspot of lowland inselberg plant diversity

BackgroundIsolated monoliths of granitic and/or gneissic rock rising abruptly from the surrounding landscape are known as inselbergs. Dome-shaped inselbergs are common throughout the Atlantic Forest in south-eastern Brazil, a region known as Sugarloaf Land (SLL). This study aimed to create the first checklist of vascular plant species occurring on lowland inselbergs in SLL, with a focus on vegetation islands. We used information from online databases, our own field sampling and data from previously-published studies. We found 548 vascular plant species (505 angiosperms; 43 ferns and lycophytes) belonging to 69 families and 212 genera. Of all identified species, 536 are native and 12 are naturalised.New informationWe updated the information currently available in Flora do Brasil 2020, as 59% of the angiosperms and 63% of the ferns and lycophytes on our checklist were not previously characterised as occurring on rock outcrops. As a first step towards generating a Virtual Herbarium of lowland inselberg vascular plants, we added barcode vouchers with images available online for 75% of the total number of vascular species. In the official lists of endangered species, 115 angiosperms and five ferns and lycophytes are mentioned. However, the conservation status of many species have not yet been evaluated (77% angiosperms; 88% ferns and lycophytes), thus this list is an important step towards their conservation. The information provided herein is essential for management programmes related to rock outcrops in Brazil as they are facing serious threats to conservation.

Correcting indirect strain measurements in laboratory uniaxial compressive testing at various scales

Various devices can directly or indirectly measure strain and record the deformational response of rock materials in uniaxial compressive strength tests. Strain gauges glued to specimens directly measure electrical resistance, while displacement sensors, typically linear variable differential transformers (LVDTs), indirectly measure linear displacements. However, no guidance is available regarding how to position the LVDTs to accurately measure axial or radial strain. A common practice is to measure axial strain with displacement sensors on the basis of the reduction in span between steel platens in relation to specimen height. If the deformational response for axial strain measured using strain gauges and displacement sensors is compared, however, it may be that neither maximum axial strain nor elastic constants are the same. This paper describes an approach to correcting platen-to-platen displacement measurements based on energy calculations at different scales in Olkiluoto gneissic rock specimens, with control specimens tested by combining strain gauge and displacement sensor measurements. It is suggested that the approach may be adaptable to other rocks.

Significance of viscous folding in the migmatites of Chotanagpur Granite Gneiss Complex, eastern India

ABSTRACTTo understand the physico-chemical processes associated with migmatisation is an interesting petrological problem. New developments in microfluidics and chaotic mixing experiments have helped us to better perceive these processes from the migmatic rocks of the Proterozoic Chotanagpur Granite Gneiss Complex (CGGC), eastern India. The migmatic rocks of CGGC have preserved folded leucocratic veins in amphibolites representing viscous folding. The viscous folding phenomenon occurred due to the interaction between leucosome and melanosome. Based on textural features and mineral chemical data interpretations, we infer that when granitic and pegmatitic magmas intruded the gneissic rocks and amphibolites of our study area, diffusion of heat and volatiles from the hotter felsic magmas to the colder country rocks initiated partial melting in the amphibolites, forming melanosomes. After their formation, the highly viscous felsic magmas veined into the melanosomes, by progressively melting them and then interacting, leading to chaotic mixing dynamics. The development of chaotic mixing allowed the leucosome to venture into the melanosome as veins by stretching and folding dynamics. As the leucocratic veins or leucosome traversed through the partially molten rock or melanosome due to advection, the veins underwent viscous folding owing to the exertion of compressional stress brought about by the viscosity difference between the two mediums. The occurrence of viscous folding exponentially increased the contact area between the leucosome and the melanosome, eventually leading to enhanced diffusion and augmented mixing between the two mediums. Evidence of mixing through elemental diffusion is well documented by the compositions of amphibole and biotite occurring in the leucosome and melanosome. These minerals show substitution of magnesium and ferrous ion that show linear variation between the endmember compositions.

Monsoon rainfall and contrasting source rocks influenced sediment composition of peninsular basins along the east coast of India (western Bay of Bengal)

The peninsular India is characterized by remarkable variation in lithological composition as well as contrasting rainfall distribution patterns. The erosional and weathering history of these source rocks along with monsoonal changes are archived in sediments deposited in the basins along the east of coast of India. Here, we present a 60 ky record of Sr-Nd isotope ratios coupled with major elemental compositions of a sediment core off Krishna-Godavari (K-G) basin, western Bay of Bengal. The marked temporal variation in 87Sr/86Sr, εNd, and Mg/Al ratios are attributed to the changes in relative contribution of sediments from the Deccan Basalts (DcB) and Archean-Proterozoic gneissic complexes (APGC). The enhanced contribution of APGC associated with an increase in monsoon precipitation may be attributed to heightened erosion of the granitic gneissic rocks. On the other hand, a close correlation of 87Sr/86Sr ratios with Rb/Sr ratio, δ18OG-ruber, and CIA profiles indicate the possible influence of weathering intensity on the sediment composition.

Archean basement components and metamorphic overprints of the Rangnim Massif in the northern part of the Korean Peninsula and tectonic implications for the Sino-Korean Craton

The geology of the Korean Peninsula’s northern part is little known to the international community. Even though this area is a critical link between various eastern Asia terranes, its tectonic architecture and geochronological framework are still gaps in knowledge. This study reports integrated field, geochemical and geochronological characteristics of the Precambrian gneissic rocks from this region, focusing on the Rangnim Massif. The studied samples are divisible into three groups according to their field occurrence as granitic gneiss, fine-grained biotite gneiss and leuco-veins. The essential mineralogy of all these rocks is biotite (Bt), muscovite (Mu), plagioclase (Pl), K-feldspar (Kf) and quartz. The analyzed samples show high SiO2 (65.92–77.63 wt%) and Al2O3 (10.23 to 17.56 wt%) contents, low MgO and CaO. Their K2O contents is mostly higher than Na2O. SIMS (Secondary Ion Mass Spectrometry) zircon U-Pb dating of the eleven samples yields magmatic protolith ages of 2454–2567 Ma and multiple metamorphic overprintings during the earliest Paleoproterozoic (~2.45 Ga), Paleoproterozoic (1.87–1.93 Ga) and Jurassic (160 Ma). The presence of these potassic granitoids in different locations indicate the high extent maturation of Neoarchean continental crust in the Rangnim Massif. Zircon Lu-Hf isotopic results of these samples show large variations, mostly plotting between the Depleted mantle linear model line and Chondritic Uniform Reservoir (CHUR) line, but some have negative εHf(t) values. Considering that their protoliths are all crust melting products, it can be concluded that the Neoarchean magmatism incorporated both juvenile crustal components as well as a reworking of older crust. This is supported by the detection of rare early Archean xenocrysts. Based on Juvenile crust having εHf values of ~0 before 3500–3600 Ma, it is concluded that the reworked components were Eoarchean in age, not Hadean. Magmatic as well as Precambrian metamorphic pulses of the Rangnim Massif show broad similarities with the eastern North China Craton, Gyeonggi and Yeongnam Massifs and southwest Japan, which seem to suggest that they might share a coherent Precambrian basement and evolutionary history, at least before the end of late Paleoproterozoic.

Origin and stability of uranium accumulation-layers in an Alpine histosol

Uranium (U) accumulation in organic soils is a common phenomenon that can lead to high U concentration in montane wetlands. The stability of the immobilized U in natural wetlands following redox fluctuations and re-oxidation events, however, is not currently known. In this study, we investigated a saturated histosol that had accumulated up to 6000 ppm of U at 30 cm below ground level (bgl). Uranium in the waters feeding the wetland originates from the weathering of surrounding gneiss rocks, a process releasing trace amounts (<3 ppb) of soluble U into nearby streams. Redox oscillations in the first 20 cm bgl led to the accumulation of U, Ca, S in low permeability layers at 30 and 45 cm bgl. XRF measurements along the core showed that U strongly correlates with sulfur (S) and calcium (Ca), but not iron (Fe). We tested the stability of uranium in the histosol over a nine-month laboratory amendment of a large core of the histosol (∅ 30 cm; length 55 cm) with up to 500 ppm nitrate. Nitrate addition was followed by complete nitrate reduction and re-generation of oxidizing Eh conditions in the top 25 cm of the soil without U release to the soil pore waters above background levels (1–2 ppb). Our results demonstrate that, fast reduction of nitrate, sulfate, and Fe(III) occur in the soil without U release. The remarkable stability of sorbed U in the histosol may result from buffering by sulfide and Sn° and/or strong U(IV)-OM or U(VI)-OM enhanced by organic S moieties or bridging complexation by Ca. That U in the soil was immobile under nitrate addition for up to 9 months can inform remediation strategies based on the use of artificial wetlands to limit U mobility in contaminated sites.

Mineralogy and geochemistry of the Morro dos Seis Lagos siderite carbonatite, Amazonas, Brazil

The Morro dos Seis Lagos niobium rare earth element, Ti-bearing lateritic deposit (Amazonas, Brazil) is derived from a primary siderite carbonatite. The complex is the only example of a Nb deposit in which Nb-rich rutile is the main Nb ore mineral. Apart from the laterites, at the current level of exposure the complex consists only of siderite carbonatite; silicate rocks are absent. Three types of siderite carbonatite are recognized: (1) a brecciated and oxidized core siderite carbonatite consisting of up to 95 vol% siderite together with: hematite; pyrochlore; Nb-brookite; Ti-maghemite; and thorobastnäsite; (2) a REE- and P-rich variety of the core siderite carbonatite consisting of siderite (up to 95 vol%), hematite, minor pyrochlore, monazite and bastnäsite; (3) a border hydrothermal siderite carbonatite with ~70 vol% siderite, barite (~15 vol%), gorceixite (~7 vol%) and minor rhabdophane and pyrochlore. The country rock gneiss in which the carbonatite was emplaced was affected by potassic fenitization, with the formation of phlogopite and orthoclase together with monazite, fluorapatite and bastnäsite. The siderite carbonatites exhibit a wide variation of δ13C (−5.39‰ to −1.40‰), accompanied by a significant variation in δ18O (17.13‰ to 31.33‰), especially in the REE-rich core siderite carbonatite, and are explained as due to the presence of both H2O and CO2 in the magma. The core siderite carbonatite is the richest in Fe (48.64–70.85 wt% Fe2O3) and the poorest in Ca (up 0.82 wt% CaO) example of a siderite carbonatite yet recognized The ferrocarbonatite has significant contents of Mn, Ba, Th, Pb and LREE, and a very high Nb (up to 7667 ppm) content due to the presence of Nb-brookite. The substitution 3Ti4+ = Fe2+ + 2Nb5+ recognized in Nb-rich brookite explains enrichment of Nb in the core siderite carbonatite and indicates formation in a reducing environment. The high Nb/Ta ratio (1408–11,459) of the carbonatite is compatible with residual liquids derived by fractional crystallization. The 87Sr/86Sr (0.70411–0.70573) and 144Nd/143Nd (0.512663–0.512715) isotopic data suggest the carbonatite is mantle-derived with essentially no crustal contamination and is younger than the maximum age of 1328 ± 58 Ma (UPb in zircon). We suggest that the Morro dos Seis Lagos carbonatite complex represents the upper-most parts of a differentiated carbonatite magmatic system, and that the siderite carbonatite is related to late-magmatic-to-carbo-hydrothermal processes.

The Theodul Glacier Unit, a slab of pre-Alpine rocks in the Alpine meta-ophiolite of Zermatt-Saas, Western Alps

The Theodul-Glacier-Unit (TGU) is a 100 m thick and 2 km long slab of pre-Alpine schist, gneiss and mafic rocks tectonically emplaced in the eclogite-facies Zermatt-Saas meta-ophiolite nappe (ZSU). The meta-sedimentary rocks occur mostly as garnet-phengite schists with locally cm-sized garnet porphyroblasts. The metavolcanic basic rocks are present as variably retrogressed eclogites showing a continental basalt signature and contain abundant zircon, which is unusual for basalts. The zircons dated with the U–Pb system yield an upper intercept age of 295 ± 16 Ma and a lower intercept age of 145 ± 34 Ma. The early Permian age is interpreted to represent the age of high-grade granulite facies metamorphism, evidence of which is also preserved in the cores of garnet porphyroblasts of the Grt-Ph schists. The lower intercept age corresponds to the time of continental breakup and the initiation of the Tethys in the Mid-Jurassic; these events may have created the TGU as an extensional allochton. Eclogite facies metamorphism recorded by the TGU rocks occurred during Alpine subduction at 57 Ma, the Lu–Hf age of TGU eclogite garnets. The TGU reached a depth of about 53 km at P–T conditions of 1.7 GPa and 520 °C derived from both, eclogite and Grt-Ph schist. This is in contrast to the ZSU surrounding the TGU with a reported subduction depth of more than 80 km at 43 Ma. It is proposed here that TGU and ZSU were subducted separately out of sequence. After juxtaposition of the two units during late Alpine thrusting and folding forming the present day geometry of nappes in the Zermatt-Saas region both units were progressively metamorphosed to about 650 MPa and 470 °C. This late prograde metamorphism at 34 Ma produced oligoclase + magnesio-hornblende in the matrix of Grt-Ph schists and eclogites. The derived TGU data document a complete Wilson Cycle.

Ediacaran-Cambrian far-field hydrothermal event in the southeast of the São Francisco Craton, Brazil: Evidence from zircon U-Pb SHRIMP, trace elements, Lu-Hf and oxygen isotopes

Iron formation-bearing sequences from the Guanhães Tectonic Block and Southern Espinhaço Belt of the Southeastern São Francisco Craton, were affected by the Ediacaran-Cambrian Brasilian Orogeny, when the Guanhães Block was upthrusted onto Statherian metasedimentary units of the Lower Espinhaço Supergroup. The iron formation contain a number of hydrothermal high-grade iron ore bodies (e.g. Horto-Baratinha, Cuité, Morro Escuro and Morro do Pilar deposits). Three different types of zircons were studied in order to characterize the temporal and geochemical evolution of the late-orogenic, hydrothermal event: detrital zircons (Lower Espinhaço Supergroup – Orosirian-Statherian maximum depositional age), zircons from the gneissic basement (Archean Guanhães Complex), and zircons from metagranitoid (Statherian Borrachudos Suite). All populations include zircons with <5 μm to 50 μm wide areas (mainly rims) that are brighter in BSE images. These zones are much younger than the rest of the grains (U-Pb SHRIMP ages from 492 Ma to 554 Ma) and display higher U contents and lower Th/U ratios. Zircons from pegmatites that crosscut the Paleoproterozoic sequence show similar ages, as well as high U contents and low Th/U ratios. Some of the pegmatite zircons contain inherited cores from the Borrachudos Suite. A general ZrO2, SiO2 and Th depletion and U, Pb, Hf, Mg, Ca, Fe, Yb and P enrichment was observed in the zircon rims in comparison with their cores. Lower εHf(t) values and higher 176Hf/177Hf(t) ratios were obtained in the zircon rims relative to the unaltered cores. Higher δ18O values (above 7‰) were found in the rims of the detrital and pegmatite zircons, being approximately 1‰ higher than the cores, indicating low-temperature water-rock interactions. Uplift and juxtaposition of Neoarchean gneissic rocks of the eastern São Francisco Craton, Statherian granites and metamorphic supracrustal units occurred during the late to mainly post-collisional stages of the Brasilian Orogeny (560–490 Ma). This uplift and decompression promoted partial melting of the Borrachudos Suite, generating pegmatite intrusions and a magmatic-hydrothermal fluid that affected the entire sequence. Zircons from metasedimentary rocks, granite and gneiss underwent recrystallization generally at sub-solidus conditions during interaction with the moderate- to low-temperature hydrothermal fluid. A concentric porous fabric is present in some grains and registers the fluid pathways during crystal alteration and recrystallization. This recrystallization process had a large impact on the chemical and isotopic compositions of the zircons. The pegmatite zircons represent new anatectic, zircon growth or anatectic overgrowth on older cores in the presence of fluid-saturated melts.

Importance of Tourmaline Gneiss and Vein near Main Central Thrust in Sikkim Darjeeling Himalaya

Lingtse gneiss (LGn) and Higher Himalayan crystallines (HHC) comprise parts of main central thrust (MCT) in the Darjeeling Sikkim Himalaya. Tourmaline bearing gneiss and quartz tourmaline veins are reported in immediate contact with the LGn and some lesser Himalayan rocks in this study. Their importance is inferred via their comparative occurrence, micro-texture and chemistry. Flow of ductile crust was proposed to expose deep crustal rocks in the Himalayas in form of these gneissic rocks. Generation of paragneissic HHC from the protolith like the lesser Himalayan rocks like biotite-muscovite schist was proposed and documented in previous studies. The main central thrust where the principal motion is reported to date at circa 20 Ma appears in the contact regions of the HHC and lesser Himalayan rocks. Whether the tourmaline bearing gneiss or veins is a product during the episode of generation of the Higher Himalayan crystallines, which is taken as a component of the higher Himalayan crystallines episode remains a question as both concordant and discordant tourmaline bearing gneiss and/or quartzo-feldspahic veins appear respectively. The mm-cm scale tourmaline in the occasionally discordant quartz tourmaline veins shows strong zonation and less effects of shearing. Those are strongly zoned indicating magmatic hydrothermal character. The matrix tourmaline shows a separate composition. However, evidences of a less prominent shearing in them might signify rejuvenation. Lower temperature activity and fluid movement in this thrust zone are signified from the microstructure signifying that the high grade main central thrust was probably rejuvenated during or after the veining.

Experimental Study and Numerical Simulation Using Extended Finite Element Method (XFEM) Combined with Cohesive Zone Model (CZM), of Crack Growth Induced by Non-Explosive Expansive Material on Two Neighboring Circular Holes of A Gneiss Rock

The Non-explosive expansion material (NEEM) is a method more environmentally friendly than the harmful conventional rock fracturing techniques. However, it is slower and very costly. Thus, any means of economizing their use is very desirable. This paper investigates the crack growth between two neighboring holes of a gneiss rock internally pressurized by NEEM mixed with water with the aim to evaluate the influence of holes spacing (center-to-center distance), on the initiation and growth of cracks. Field experimental results reveal that crack starts earlier and grows faster with increasing ambient temperature. But when the ambient temperature is above 28°C, the NEEM is “blown out” of the holes. At these ambient temperatures, the surrounding rocks are hot and cannot dissipate efficiently the heat generated by the hydration reaction. The best filling time was found to be in the evening when the daily hot temperature has drooped. The time to first crack increases as hole diameter decreases. The 3D numerical modeling and simulation of crack growth between two neighboring holes internally pressurized by NEEM using ABAQUS (XFEM/CZM) software shows a good agreement with the theoretical and experimental results.

Petrography, microthermometry, and isotopy of the gold veins from Vetas, Santander (Colombia)

The gold mineralization located in Vetas, Santander, consists of auriferous quartz veins hosted in Bucaramanga gneiss rocks, intrusive Jurassic rocks, and intrusive to porphyritic Miocene rocks. This study identified four mineralizing events: (1). Sericite, carbonate (ankerite and calcite?), massive and microcrystalline quartz, sphalerite, adularia, albite, galena, thin pyrite, pyrrhotite, chalcopyrite. The age for this stage is 10.78 ±0.23Ma (Ar/Ar on sericite). (2). Molybdenite, magnetite with exsolution of ilmenite, As-pyrite, sphalerite, fine-grained pyrite and little chalcopyrite quartz with huge, feathery, fine mosaic, flamboyant and microcrystalline textures and, tourmaline and sericite. (3). Gold and tennantite associated with sphalerite, fine- and coarse-grained pyrite, As-pyrite, chalcopyrite like inclusions, and quartz with flamboyant, mosaic, massive and “comb” textures, and tourmaline. Stage 2 and 3 happened from 7.58 ±0.15 Ma to 6,89±0,41Ma (Ar/Ar on sericite). (4). Thick, thin, and pyrite with arsenic, hematite and microcrystalline quartz (forming breccia texture), and sericite. The age for this stage is 5.24 ±0.10 (Ar/Ar on sericite). Post-mineral: quartz comb, alunite, halloysite, kaolinite, and ferrum hydroxides. The stable isotopes, ∂18O, ∂D, and ∂34S and fluid inclusions analysis infer that fluids were producing a mixture of meteoric and magmatic fluids with low salinity and minimum trapping temperatures between 200°C to 390°C. The mineralogy association, and fluid inclusions, in the first event show characteristic of low sulfidation epithermal. The second stage was hottest and with more magmatic signature over printed an intermediate sulfidation system; show a little more salinity on the fluids and more mineralogical diversity, the third and four events, could show an evolution of this fluid, where it was cooling and impoverishing on metals. Two initials stages are contemporaneous with two magmatic Miocene pulses on the area: the first one of granodiorite composition 10, 9± 0.2 Ma (U/Pb zircon), and the other one rhyodacite with 8.4 ±0.2 y 9.0 ± 0.2 Ma.

A multidisciplinary approach to investigate weathering processes affecting gneissic rocks (Calabria, southern Italy)

The paper focuses on the weathering processes affecting gneissic rocks of the western Sila Grande Massif (Calabria, Italy) through the development of an interdisciplinary research based on field studies and investigations, minero-petrographical analyses and geochemical modeling. Both physical and chemical weathering affect gneissic rocks of the study area, where the combination of tectonic and past climatic conditions played an important key role in the development of complex and deep weathering profiles. Field surveys and cut slope analyses highlight articulated and complex geometric relationships between various classes of weathering (i.e., out-of-sequence weathering horizons giving rise to a partial, or even complete inversion of the “normal” weathering profile). The weathering profile has turned out particularly intense, as classes IV, V and VI are widespread. Among the class VI, the colluvial soils are prominent in comparison whit the residual soils. Borehole logs, that confirm the intensity and complexity of weathering profiles in deep, allow to estimate the presence of weathered rocks to be at least 70 m in thicknesses.The main mineralogical modifications linked to weathering processes concern the partial transformation of biotite and the partial destruction of feldspars (mainly plagioclases), associated with the neoformation of secondary mineralogical phases (clay minerals and Fe-oxides). Neoformed clay minerals (such as sericite, illite and mixed layer phases) and ferruginous products replaced feldspars and biotite during the most advanced weathering stage. Results of XRD analyses and geochemical modelling provide a good indication on the secondary mineral assemblage due to the increase of the weathering intensity (form class III to class VI) that reflects the different contributions of chemical elements provided by dissolution of silicate minerals into the surrounding groundwater system. The chemical composition of the studied rock samples and the oxides variations suggest a removal of some alkali (Na and K) and alkaline earth (Ca and Mg) into solution as a consequence of weathering reactions. The chemical analysis and the weathering indices (CIA, PIA, CIW and CIW’) show a marked alteration process, with ferromagnesian minerals and feldspars probably dissolved and leached into the surrounding groundwater system. The CO2-controlled dissolution of plagioclase appears to be the most important reaction during chemical weathering. The progressive dissolution results then dominated by biotite, followed by a minor amounts K-feldspar, chlorite and garnet; whereas the sillimanite shows a neglectable amounts. The secondary solid phases observed during the geochemical modeling (illite, followed by vermiculite, ferrihydrite and saponite) are similar to those found in this natural system. The proposed approach could be used to characterize weathered crystalline rocks and related weathering profiles in similar geological setting, and the obtained results represent a key point for the evaluation of the control exerted by weathering on landscape evolution under current environmental settings in terms of sediment generation, soil erosion rates, and mass movements, and for the mechanical characterization of weathering profiles for engineering geological purposes.

Mineralogy and micromorphology of soil from gneissic rock in East Luwu, South Sulawesi

Different type of rock, climate, and topography will produce specific mineralogical and micromorphological characteristic in soils. There were still a few research related to the influenced of several types of rocks to soil mineralogy and micromorphology, especially from metamorphic rocks. The objective of this research was to examine the characteristics of mineralogical and micromorphological of soil formed from gneissic rock in East Luwu, South Sulawesi. The observation of soil profiles were carried out in four profiles. The mineralogical analysis was carried out by X-ray diffractometer, and micromorphologogical analysis was carried out by using the thin section. Gneissic rock consisted of quartz-gneiss and muscovite-quartz gneiss. Minerals in the soil had mesomorphous alteration stage with class of alteration of 2nd and 3rd that had been altering in 25-75%. The minerals were quartz, goethite, muscovite, gibbsite, hematite, boehmite, and ilmenite. Alteration mineral formed sesquioxide horizon in soil. The high intensity of weathering of gneissic rock was influenced by the high intensity of rainfall, reached 3389mm/year. Nodules as the weathering product dominated in typic soil type. The c/f related distribution was enaulic, related distribution was porphyric and b-fabrics were granostriated and stippled speckeld. High content of oxide mineral with stippled speckeld b-fabric showed the advanced stage of pedogenesis process from the gneissic rock.