Phyllite Samples Research Articles

Study on improvement of impermeability of strongly weathered phyllite by microorganisms

The technology of strengthening weakness rock based on microbial mineralization has been widely concerned by researchers and engineers in recent years. The uniaxial compressive strength, permeability and microscopic physical properties of strongly weathered phyllite treated by microorganisms were investigated in this study. The microstructure study were conducted using nuclear magnetic resonance (NMR), scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that effective cementation is generated in weathered phyllite due to the microbial induced carbonate precipitation in the process of microbial modified weathering of phyllite, which can increase the strength up to 39 % of treated phyllite. The NMR results proved that the stone content can significantly affects the pore distribution of improved phyllite samples. The porosity is significantly reduced after adding microorganisms for improvement, the permeability changes from 0.019 to 0.022 md with different treatments in this study. A high stone content can promote the adhesion of microorganisms, which is conducive to the formation of calcium carbonate. Both excessively high or low levels of calcium chloride and urea can significantly affect on microbial and enzyme activities. The XRD results confirm the calcite which generated by the microbe-induced carbonate precipitation. Although the calcite content is low but with a wide distribution range, which can effectively decrease the porosity and permeability. That is to say, the wide distribution of calcite can improve the strength of weathered phyllite.

Investigating the Creep Damage and Permeability Evolution Mechanism of Phyllite Considering Non-Darcy’s Flow

This study investigates the intricate interplay of hydraulic coupling, creep damage, and permeability evolution mechanisms in phyllite, focusing on their relevance to tunnel engineering design and long-term stability in soft rock formations. To achieve this, conventional triaxial tests were conducted on saturated phyllite specimens under creep-seepage acoustic emission conditions. The results were systematically analyzed to unveil the inherent characteristics of creep deformation, seepage rate evolution, and damage progression in phyllite when subjected to stress–seepage coupling. Furthermore, a permeability model for representative volume element (RVE) was developed based on meso-mechanics principles, considering the distinctive attributes of low-permeability non-Darcy’s flow in rock. Consequently, a novel relationship between effective damage and permeability was established. We determined that seepage pressure induces three key effects on the creep damage mechanism of phyllite samples: (1) It adds to the existing stress through the superposition of osmotic pressure and axial load, (2) it induces tensile expansion stress because of pore water pressure, and (3) it softens the fracture surfaces to some extent. More importantly, this study validates the relationship between effective damage and permeability through the fitting of creep and damage parameters, which were obtained from the test results, and it reveals a square relationship between subsequent damage and permeability under stress conditions. The findings of this study provide a robust theoretical foundation for comprehending the evolution of damage and permeability characteristics during the creep process of rock under seepage conditions. We obtained essential insights and quantitative analyses for comprehending the damage mechanisms in rock creep under hydraulic coupling, which has significant implications for tunnel engineering and long-term rock stability assessments in soft rock environments.

Influence of lithology and bedding orientation on failure behavior of “D” shaped tunnel

To explore the influence of lithology on the failure behavior of layered tunnel, true triaxial compression experiments were undertaken on cubical phyllite and yellow sandstone samples containing a “D” shaped hole. The failure progress of the hole sidewalls was monitored and captured using a miniature camera. The results reveal that the initial vertical failure stress of the phyllite samples presents a “U” shaped change as the bedding angle increases. At the bedding angle of 45°, compared with the initial vertical failure stress of the yellow sandstone tunnel, that of the phyllite tunnel is lower, resulting in larger rock fragments and deeper V-shaped grooves. The failure pattern of the phyllite tunnel is primarily manifested as extensive shear sliding failure, and the failure is more severe. The failure of the yellow sandstone tunnel is primarily characterized by the sequential laminar fracturing along the maximum principal stress direction, predominantly manifesting as tensile failure. The primary factors influencing the failure of two types of layered rocks are the significant variations in the clay mineral content within the rocks. For the phyllite, it contains nearly one-third of montmorillonite (a clay mineral). This results in the formation of weak bedding planes within surrounding rocks, which induces shear slip failures along these bedding planes. In contrast, the yellow sandstone has a lower clay mineral content, leading to the absence of distinct weak bedding planes within the surrounding rock. In this case, bedding planes present ignorable effect on the surrounding rock.

Triaxial compression experiment and damage constitutive model of microbially modified strongly weathered phyllite

Microbial improvement of strongly weathered phyllite roadbed can effectively solve the problems of insufficient filling and waste soil in roadbed construction while protecting the environment and considering economic benefits. This paper uses microbially induced carbonate precipitation (MICP) technology to improve the strongly weathered phyllite in the Nanchang area and conducts the porosity and triaxial compression tests on the samples before and after the improvement. According to the strain strength theory of rock and the Druker-Prager yield criterion, the damage constitutive model of the improved sample is established considering the effects of the initial porosity and cementation degradation coefficient on the sample’s mechanical properties. The results show that the strengths of the samples improved by MICP technology are increased by more than 35%, and the porosities are reduced by 15%, which meets the requirements of roadbed filling. The stress–strain curves of the modified samples under triaxial compression can be divided into three stages: elastic stage, elastic–plastic stage and ductile flow stage. The triaxial compression test results of the improved samples show that the damage constitutive model established can describe the stress–strain relationship of the samples well, which verifies the rationality of the model. This model provides a theoretical basis for studying damage softening and mechanical properties of microbially modified strongly weathered phyllite samples.

Dynamic Mechanics and Energy Dissipation of Saturated Layered Phyllite

This paper is a study of the dynamic mechanics and energy dissipation of saturated layered phyllite. Using the Split-Hopkinson pressure bar system, the mechanical properties and energy dissipation law of the sample during dynamic loading in the test were analyzed. The results show that the weakening effect of water on the phyllite rock body will have a great impact on its mechanical properties, strain rate sensitivity damage mode, and fracture energy dissipation. The values of mechanical parameters such as the modulus of elasticity and compressive strength of the specimens in the test varied with the dip angle of the layer with 0° > 90° > 30° > 60°, and the mechanical parameters of the specimens in the saturated state were smaller than those of the dry specimens. The damage of the sample is mainly in the form of crushing damage at 0° dip angle, shear damage along the laminar surface at 30° and 60°, and destabilization damage of the compression bar at 90° dip angle, with higher fragmentation of the sample in the saturated state. The energy dissipation densities of different inclination samples in the saturated state of schist are greater than those in the dry state, with the highest energy dissipation density in the 0° inclination sample and the lowest in the 90° inclination sample. Both the mean strain rate and compressive strength of the samples showed a multiplicative power relationship with the crushing energy dissipation density of the samples, showing a strong strain rate correlation. These results indicate that the use of rock crushing energy dissipation density can better reflect the strength characteristics of phyllite samples under dynamic loading.

Application of wavelet transform in evaluating the Kaiser effect of rocks in acoustic emission test

The Kaiser Effect is amongst the phenomena that can be detected through the so-called acoustic emission method. This effect can be used to evaluate in-situ stress of the rock. Identification of the point at which the Kaiser Effect is applied is problematic in particular cases where the acoustic parameters increase gradually, and this is where signal analysis methods come into play. In this research, Brazilian test was performed on Phyllite samples and acoustic data was recorded simultaneously. Following an energy-based and count-based parametric approach, the occurrence time and the Kaiser Effect stress were obtained for the test specimens. Then, wavelet transform method was used to estimate the occurrence time of the Kaiser Effect. Continuous wavelet transform analysis was undertaken to analyze the signals. The signal analysis was performed by considering the peak frequency as desired parameter and db5 wavelet as the mother wavelet. In continuous wavelet analysis method, the results were classified into five cluster using the K-means clustering algorithm. Considering the mechanism of the Kaiser Effect, the fifth category was considered for further investigations. The results showed that the Phyllite specimens exhibited good capabilities for recovering the stress memory and evaluating the Kaiser Effect-driven stress, the results further showed a good agreement between the occurrence times obtained from the parametric method and those resulted from continuous wavelet transform technique, so that the corresponding differences in the Kaiser Effect-driven stress are negligible. The level of Kaiser Effect-driven stress obtained from the proposed method were acceptably in agreement with the preloading stress levels.

THE MECHANISM OF THE KAISER EFFECT IN PHYLLITE UNDER INDIRECT TENSILE LOADING

Determination of in-situ stress serves as an important step in the design and construction of civil and mining projects, among others. Conventional methods of the in-situ stress measurement are time- and cost-intensive. Therefore, the application of low-cost yet rapid methodologies for in-situ stress evaluation has been increasingly regarded by researchers. The Kaiser effect-based acoustic emission method is one of such novel approaches to the in-situ stress evaluation. Not only the point at which the Kaiser effect occurs, but also the mechanism of the Kaiser effect is of paramount importance. In this research, acoustic emission tests were conducted on phyllite rock samples under Brazilian tensile loading to collect a variety of acoustic data, including the amplitude, rise time, count, duration, and energy. Then, the Kaiser effect point was determined using the collected data on acoustic parameters, with its occurrence mechanism investigated. In addition, mathematical transformations were adopted to transform the acoustic signal from the time domain to the frequency domain, where the peak frequency was analyzed. The results of the RA/AF ratio analysis showed that the acoustic emission was sourced from tensile micro-cracks. Moreover, the high level of energy indicated a high intensity of crack formation at the Kaiser effect point. The large number of received hits showed that the count of generated cracks increases abruptly within the range of the Kaiser effect. In addition, the obtained high value of the peak frequency implied that the crack growth rate is high at the Kaiser effect point.

Progressive ocean oxygenation at ~2.2 Ga inferred from geochemistry and molybdenum isotopes of the Nsuta Mn deposit, Ghana

Recent geochemical data suggest the occurrence of an O2 overshoot during the mid-Paleoproterozoic (~2.3–2.0 Ga). This O2 overshoot appears to be consistent with carbon isotope records that suggest high burial rates of organic carbon during that period, the so-called Lomagundi Event. However, little is known about the changes in the ocean redox conditions associated with the O2 overshoot. To better understand the mid-Paleoproterozoic ocean chemistry, we investigated the microstructures, major and trace element concentrations, Re-Os and Mo (δ98/95Mo) isotopes, and total organic carbon contents of Mn-ore and phyllite samples from the Nsuta Mn deposit in the Birimian Supergroup of Ghana which were deposited during the O2 overshoot (at ~2.2 Ga). The Mn-ore samples contain early diagenetic rhodochrosite (Mn carbonate). The trace element compositions and Re-Os isotopes of the Mn-ore samples suggest that the rhodochrosite originated from primary manganese oxides (MnO2) deposited at ~2.2-Ga. The δ98/95Mo values of the least-altered Mn-ore samples range between −1.10‰ and −0.55‰ (relative to NIST3134), suggesting seawater δ98/95Mo values of 1.85 ± 0.18‰ (1SD) during the O2 overshoot. Such high seawater δ98/95Mo values can be best explained by enhanced removal of isotopically light Mo through adsorption onto Mn oxides. To form extensive Mn-oxide deposits, bottom seawater with O2 concentrations of > 10 μM would have expanded at ~2.2 Ga. The oxidizing conditions might have supported the emergence of stem group eukaryotes during the mid-Paleoproterozoic.

Quantitative Study on Empirical Strength Parameters of Extremely Fractured Phyllite Based on Fractal Theory

Extremely broken phyllite is widely distributed and is easily seen in engineering construction. Aiming at the problem that the empirical strength parameters of extremely broken surrounding rock have intense subjectivity and significant difference, based on the characteristics of the existence of particle group state in the highly broken surrounding rock, the fractal theory and large‐scale direct shear test are used, the fractal description of polar broken phyllite samples with different pile numbers in fault fracture zone of Qinyu Tunnel is carried out, and the fractal dimensions and empirical strength parameters of each sample are determined. Based on a fractal description and large shear test, the functional relationship between fractal dimension and empirical strength parameter of extremely broken phyllite in fault fracture zone is established, and the quantitative value method of empirical strength parameter determined by fractal dimension of extremely broken phyllite is given, which provides ideas for the quantitative value of empirical strength parameter of the similar extremely broken surrounding rock.

Influence of stress amplitude on the dynamic characteristics of phyllite samples under triaxial multi-stage cyclic loading

The generation of a rockslide in an earthquake event is strongly dependent on the dynamic characteristics of the rock materials constituting the rock slope. In this study, phyllite samples were extracted from the Donghekou landslide area of Qingchuan County. The landslide occurred during the 2008 Wenchuan earthquake and the samples were made to undergo multi-stage cyclic loading and unloading tests under triaxial compression conditions. The confining pressure during the tests was 10 MPa, the loading frequency was 2 Hz, the loading waveform was sine wave, and 60 loading cycles were carried out at each level of stress. From the dynamic axial stress–strain curves, the effect of stress amplitude on the hysteresis loop area, the dynamic elastic modulus, the dynamic damping ratio, and the damping coefficient of the phyllite samples was analyzed. The results are as follows. (1) The strain–stress hysteresis loop area experienced a rapid increase as the stress amplitude exceeded a certain value, indicating that the rock samples have entered an accelerating crack propagation stage under high levels of cyclic loading. (2) As the stress amplitude increases, the dynamic elastic modulus, the dynamic damping ratio, and the damping coefficient of the rock sample followed a similar two-phase evolution law. There was a rapid reduction in the three parameters relative to the initial value in Phase I. Moreover, there was a minor change in Phase II. (3) The effect of the number of loading cycles number on dynamic elastic modulus and damping parameters was much more obvious under the lower levels of cyclic loading than under the higher levels.

Study on anisotropic characteristics of dynamic deformation parameters of phyllite

The acoustic wave test was carried out by selecting the joints of the joint surface and the horizontal plane at 0°, 30°, 45°, 60°, 75°, and 90° phyllite samples. According to the corresponding waveform diagram, the starting point is selected, and the dynamic deformation parameters of thousands of rock samples under various angles are calculated. The results show that: The anisotropy characteristics of the dynamic deformation parameters are obvious, and the influence of the variation of the joint surface inclination on the dynamic deformation parameters is achieved by affecting the wave impedance. P-wave impedance and S-wave impedance have different effects on rock dynamic deformation parameters. Generally, P-wave and S-wave impedance should be considered comprehensively for the dynamic deformation parameters of rock samples. Acoustic waves are in the elastic deformation law of the rock sample caused by the propagation in the rock sample is similar to the elastic deformation law caused by the external load acting on the rock sample.

Mineralogy and organic geochemistry of phyllite from the Dewon–Pokrzywna deposit, the Opava Mountains (SW Poland)

Phyllites from the Dewon–Pokrzywna deposit in the Opava Mts., SW Poland, were investigated by XRD (Rietveld method), XRF, EPMA, SEM, and ATR-FTIR from the perspective of their potential usage as a buffer and/or backfill material in a geological repository of radioactive waste. Organic matter dispersed in the phyllite matrix was analysed by GC-MS. Fine-grained Mg-Fe-muscovite (13 to 29 wt.%), Fe-ripidolite (10 to 25 wt.%), detrital quartz (20 to 46 wt.%), and albite (7 to 28 wt.%) ± microcline, illite or illite/smectite, and kaolinite are major minerals in phyllite samples. The chlorite/muscovite ratio ranges from 0.65 to 1.1. Mg-annite inherited from the precursor rock is a minor constituent. Detrital ilmenite is a dominant accessory mineral. Ancylite-(Ce) occurs in quartz-calcite-ripidolite veins. Two types of phyllite have been distinguished based on the proportion of phyllosilicates to silt fraction: argillaceous (47 to 55 wt.% phyllosilicates) and silt-rich (28 wt.% phyllosilicates). Argillaceous phyllite shows elevated content of alumina and moderate concentration of silica. It is highly enriched in Fe compared to phyllites from other localities worldwide. The BET specific surface area of argillaceous phyllite ranges from 1.73 to 3.64 m 2 /g. Whole-rock chemical composition, mineral assemblages, chlorite geothermometry, and the occurrence of aliphatic hydrocarbons suggest that argillaceous phyllite originated from a pelagic pelite protolith under low-temperature (260–370°C) greenschist to subgreenschist facies conditions. Persistent biomarkers are indicative of bacterial degradation of planktonic organic matter suspended in a high water column. Enrichment in Fe-rich chlorite and Mg,Fe-muscovite, low volume of interconnected pores with dominant mesopores suggest that argillaceous phyllite from the Dewon–Pokrzywna deposit is a potential candidate for a buffer and/or backfill material.

Petrographic Characteristics of Manganese Bearing Rocks of Banswara Manganese Ores Belt, District Banswara, Rajasthan (India)

In this paper, we described the petrographical characters of the Mn-bearing rock types outcropping in the Kalakhuta, Ghatia, Gararia, Itala, Wagaicha, Tambesara and Talwara villages of Banswara district. The Mn associated rocks in the study area are phyllite, schist, quartzite and limestone. The mineral assemblage in phyllite formed under greenschist facies metamorphism. Petrographic studies of various phyllite samples also reveal that porphyroblasts of biotite sat in the fine-grained micaceous matrix (Biotite, muscovite, chlorite) along with quartz, opaque and ore minerals. Petrographic studies of schist reveal its formation under prograde metamorphism. Mineral assemblages observed in schist are typical of greenschist to lower amphibolite facies. The various schist samples at places show clusters made up of perfect rhombshaped garnets which suggest the increase in the grade of metamorphism. The quartzites are disposed conformably with the phyllites, and are at times brecciated and re-cemented by secondary chert and manganese. These Mn-bearing quartzites are with sutured/serrated and straight grain boundaries. The quartzite samples are fully composed of quartz showing undulose extinction with subordinate feldspar, micas and carbonatic materials. The petrographic characteristics explain that mostly limestones are the dolomitic composition but some ferruginous varieties are also noticed at places. The purer variety of limestone consists of calcite, dolomite and some magnetite but tremolite, quartz and some minute flakes of biotite also occur in impure variety.

Multivariate Statistical Analysis of Phyllite Samples Based on Chemical (XRF) and Mineralogical Data by XRD

It is presented the results obtained of a multivariate statistical analysis concerning the chemical and phase composition, as a characterization purpose, carried out with 52 rock phyllite samples selected from the provinces of Almeria and Granada (SE Spain). Chemical analysis was performed by X-ray fluorescence (XRF). Crystalline phase analysis was performed by X-ray powder diffraction (XRD) and the mineralogical composition was then deduced. Quantification of weight loss (100? and 1000?C) was carried out by thermal analysis. The aims of this investigation were to analyze and compare the chemical and mineralogical composition of all these samples and to find similarities and differences between them to allow a classification. Several correlations between results of the characterization techniques have been also investigated. All the data have been processed using the multivariate statistical analysis method. The XRF macro-elements (10) and microelements (39) data generate one macrogroup with two new subgroups (1 and 2), and an isolated sample. In subgroup 1 of macroelements, a positive correlation was found between XRF results and geographic location characterized by lower MgO content, which is associated to its geological origins. When multivariate statistical analysis is applied to results obtained by XRD, two groups appear: the first one with a sample with zero percentage of iron oxide and the second one with the rest of the samples, which is classified in two groups. A correlation is observed between the alkaline content (XRF) and illite (XRD), CaO and MgO with dolomite and indirectly between the weight loss after heating at 1000?C and the contents of phase minerals that lose structural water (illite + chlorite) or carbon dioxide (dolomite). The present investigation has interest and implications for geochemistry and analytical chemistry concerning earth rocks and silicate raw materials.

The tectonic significance of K/Ar illite fine-fraction ages from the San Luis Formation (Eastern Sierras Pampeanas, Argentina)

The Sierra de San Luis forms the southern tip of the Eastern Sierras Pampeanas in central Argentina. Two narrow belts of low-grade phyllites and quartz arenites, i.e. the San Luis Formation, have accommodated part of the strain-related differential exhumation of the medium- to high-grade metamorphic domains that constitute to the basement complex of the sierra. Eleven phyllite samples were subjected to the K/Ar fine-fraction dating technique. Results are interpreted in relation to the Kubler index of the illites, which indicate epimetamorphic conditions for the majority of the samples. Obtained ages between 330 and 290 Ma cover a period of compressional tectonics in the late Mississippian (Visean/Serpukhovian boundary) followed by the subsidence during the formation of the Paganzo Basin in the provinces of La Rioja and San Luis. These tectonic movements are coincident with the Toco orogeny in northern Chile and southern Bolivia. This suggests that the older K/Ar ages document the compressional stage and that younger ages record the cooling of the basement during the subsequent extensional uplift of the basement.

Single crystal U–Pb zircon age and Sr–Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites

The 1.07Myr old Bosumtwi impact structure (Ghana), excavated in 2.1–2.2Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U–Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145±82Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb–Sr and Sm–Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb–Sr and Sm–Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n=29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846±160Ma, with an initial Sr isotope ratio of 0.703±0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible “metamorphic age” of ∼1.8–1.9Ga tentatively derived from our U–Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb–Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event.

Phyllites used as waterproofing layer materials for greenhouses crops in Spain: multivariate statistical analysis applied to their classification based on X‐ray fluorescence analysis

AbstractThe results are presented for the chemical characterization carried out with 53 raw material samples with phyllosilicates from outcrops located between the provinces of Almería and Granada (SE Spain) in order to use them as waterproof layers under the floors of greenhouses. For this purpose, the samples have been studied by these techniques: X‐Ray Diffraction (XRD), X‐Ray Fluorescence (WDXRF), Scanning Electron Microscopy (SEM), chemical analysis by energy‐dispersive X‐Ray detection (EDX), and thermogravimetry. To isolate groups of phyllite samples with similar chemical profiles and correlations between the samples, the WDXRF data (11 chemical elements) have been processed using the methodology of exploratory multivariate statistical analyses: cluster analysis, main‐component analysis, and discriminating canon analysis. This study was performed as a screening test and as a means of finding similarities and correlations among all 53 phyllite samples, allowing the isolation of groups of phyllite samples with similar chemical profiles. The results indicate that the 53 phyllite samples can be divided into two main groups. The first group is subdivided into two subgroups (1 and 2), one of which includes most of the samples. The latter is further classified into three blocks with the same chemical composition. This allows to search for the raw material with potentially the best waterproof characteristics within the five groups. The classification is of validity as a screening test for subsequent experimental determinations concerning the physical properties of these samples. Copyright © 2009 John Wiley & Sons, Ltd.

Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south‐west Japan: evidence from 40Ar/39Ar whole‐rock phyllite ages

Abstract Five whole‐rock 40Ar/39Ar plateau ages from low‐grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole‐rock phyllite samples from the Mikabu greenstone belt record well‐defined 40Ar/39Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high‐pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub‐belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north (c. 110 Ma) to south (c. 215 Ma). The northern Chichibu sub‐belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole‐rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.Accretion of Sambagawa‐Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma‐Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa‐Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.

Soil contamination due to open cast mining in Goa: A statistical approach

Abstract Soil samples were collected during April 1990 (pre‐monsoon) from the mining and nonmining areas of Goa region and were analysed for chemical data viz. major and trace element. A set of iron ore and phyllite samples were also used for comparison with the soil chemical data. Discriminant analysis on normalised major and trace element data of soil, ore and phyllite samples suggested a clear discrimination between chemical data of soil, ore and phyllite samples. From the R‐mode factor analysis four common factors were retained from the chemical data of soil. First and second rotated factors were explained as natural processes such as weathering, secondary alteration of clay minerals and lateritisation process. The third and fourth rotated factor were interpreted as anthropogenic sources such as fertilizers, atmospheric fallout and vehicular exhausts.

Tectonic implications of 40Ar/ 39Ar ages from a pre-Mesozoic metamorphic basement penetrated on Leg 77 of the deep sea drilling project in the southern Gulf of Mexico

Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the south-eastern Gulf of Mexico. At Site 538A located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite records 40Ar/ 39Ar plateau dates of c. 500 Ma, indicating early Paleozoic metamorphism. Biotite from the gneiss yields a plateau age of c. 350 Ma which likely dates a superposed late Paleozoic thermal event. A whole-rock diabase sample records a crystallization date of 190.4 ± 3.6 Ma. A phyllitic metasedimentary sequence was penetrated at Site 537 drilled 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display variably discordant 240Ar/ 39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at c. 500 Ma. The age and lithologic character of the basement terrane penetrated at Sites 537 and 538A suggest that the drilled fault block are underlain by attenuated fragments of continental crust of “Pan-African” affinity. This support pre-Mesozoic tectonic reconstructions which locate Yucatan in the present Gulf recess during amalgamation of Pangea.