Schist Research Articles

Growth zoning of garnet porphyroblasts: Grain boundary and microtopographic controls

AbstractChemical zoning in the outer few 10s of microns of garnet porphyroblasts has been investigated to assess the scale of chemical equilibrium with matrix minerals in a pelitic schist. Garnet porphyroblasts from the Late Proterozoic amphibolite facies regional metamorphic mica schists from Glen Roy in the Scottish Highlands contain typical prograde growth zoning patterns. Edge compositions have been measured via a combination of analysis of traverses across the planar edges of porphyroblast surfaces coupled to X‐ray mapping of small areas within polished thin sections at the immediate edge of the porphyroblasts. These approaches reveal local variation in garnet composition, especially of grossular (Ca) and almandine (Fe) components, with a range at the edge from <7 mol.% grs to >16 mol.% grs, across distances of less than 50 µm. This small‐scale patchy compositional zoning is as much variation as the core–rim compositional zoning across the whole of a 3 mm porphyroblast. Ca and Fe heterogeneity occurs on a scale suggesting a combination of inefficient diffusive exchange across grain boundaries during prograde growth and the evolving microtopography of the porphyroblast surface control garnet composition. The latter creates haloes of compositional zoning adjacent to some inclusions, which typically extend from the inclusion towards the porphyroblast edge during further growth. The lack of a consistent equilibrium composition at the garnet edge is also apparent in the internal zoning of the porphyroblast and so processes occurring during entrapment of some mineral inclusions have a profound influence on the overall chemical zoning. Garnet compositions and associated zoning patterns are widely used by petrologists to reconstruct P–T–t paths for crustal rocks. The evidence of extremely localized (10–50 µm scale) equilibrium during growth further undermines these approaches.

Filtering of hydraulically significant lineaments from lineament map of precambrian metamorphic terrain in NE India using set theory

Identification of hydraulically potential lineaments directly from lineament map required for studies of groundwater resource development in crystalline metamorphosed rock terrain, which is considered as a poor groundwater province area, is a big challenge. Lineament study from satellite image gives all available lineaments present in the location including the hydraulically potential lineaments. But there are no existing methods present at our disposal which can detect only the hydraulically potential lineaments. This study attempts to delineate the hydraulically significant lineaments those are actively promoting groundwater flow in Purulia district, West Bengal, NE India, where the exposed rock-types are porphyritic granite gneisses, quartzite, mica schist, garnetiferous-sillimanite-biotite schist and phyllite, using Set theory. For the study, standard FCC image setup has been used to view the satellite imagery on computer screen within Erdas Imagine software and lineaments are examined visually on-screen, without taking any print out of the map. The azimuth directions of the lineaments are diagrammed using GEOrient software. Azimuth rose diagrams of the trend of foliation planes and fracture planes, obtained from direct measurement at rock exposures, are compared with the lineament trends to yield hydraulically significant lineaments, concordant to the trends of hydraulically significant fractures. Hydraulically significant lineaments are filtered from the total lineament map by using the laws of set theory, like universal set, sub-set, union, intersection, subtraction and equality. Validation of the obtained result with ground truth reveals that intense deformation of brittle rocks, that allow fluid movement, have a significant role in the development of hydraulically significant lineaments. This study successfully distinguishes the hydraulically significant lineaments, exhibiting a directional concurrence to hydraulically significant fractures, implying that set theory can be successfully used to extract hydraulically significant lineaments directly from lineament map.

A Modified Engineering Classification Method for Schistose Surrounding Rocks of Tunnel

For schistose surrounding rocks of tunnel, engineering classification of rock masses by the widely used BQ method in China are often inconsistent with the reality. In this study, microtests and uniaxial compression test were performed on three types of quartz mica schist. Microtests reveal that the schist is characterized by natural orientation and interbedded distribution of minerals and microfissures are prone to be distributed directionally along the smooth edge of flaky minerals. Mechanical test shows that the compressive strength of schist samples has U-shaped change with the varied loading direction. Moreover, the strength anisotropy degree of the schist is positively correlated with the development of schistosity planes, and the strength anisotropy increases significantly after saturation. When the direction of uniaxial load applied to the samples changes gradually, the splitting-tensile, shear-slip and shear failure occur respectively, indicating that the dominant factors affecting the strength of schist specimens are not uniform. In particular, the strength of specimens with transverse schistosity planes is less affected by the weak planes. Based on the experimental study, a new model considering the schistosity development degree was constructed, which is served as the evaluation of the basic quality of schistose rock mass. By regression analysis on 168 sets of practical data, an optimized BQ equation was further developed. Then a correction coefficient table was put forward based on numerical simulation, which is helpful to correctly determine the final engineering grade of surrounding rock. Case study shows that the improved BQ method is more reliable for classifying engineering grade of schistose surrounding rock.

SYN-EXHUMATION COUPLING OF OCEANIC AND CONTINENTAL UNITS ALONG THE WESTERN EDGE OF THE ALPINE CORSICA: A REVIEW

The Alpine Corsica represents a segment of the Alpine collisional belt. In its western edge, it is characterized by the close association of continental unitsdeformed under high-pressure metamorphic conditions (Lower Units) and oceanic units showing a metamorphism ranging from high-pressure (Schistes LustresComplex) to very low-grade conditions (Upper Units). This paper provides a complete review of the relationships between the continental and oceanicunits in selected five areas where the stratigraphic features, deformation history, metamorphic P-T path and tectonic setting are available for each unit. Thecollected data indicate that the oceanic units occur not only at the top of the continental ones, as generally proposed in the literature, but also intercalated withinthem. Such relationships were achieved at shallow structural level during the late stage of exhumation, when the continental units were tectonically coupledwith the oceanic units which were dragged as slices from the orogenic wedge. The coupling probably occurred immediately before the transition from syn- topost-orogenic geodynamic regime that affected the whole Alpine-Apennine collisional system in the early Oligocene. After the coupling, the stack of oceanicand continental units experienced a further exhumation-related deformation before their final exposure at the surface.

The Facies and Metamorphism Types Determination of Metamorphic Rock in The Part of Mekongga Complex

Lambolemo Village is geomorphologically dominated by hilly slopes morphology. Lithology in the form of metamorphic rocks are mica schists and chlorite schists. The present research aimed to determine the facies and metamorphism types of metamorphic rocks in Lambolemo Village. The research methods engaged were field observation and laboratory observation utilizing petrographic analysis. Observations stage generated texture data and set of mineral both macroscopically and microscopically. The minerals were discovered within metamorphic rock of the research area, namely: muscovite, quartz, biotite, and chlorite. These findings then classified into two zones, there were chlorite zone and biotite zone. These zones were embodied into greenschist facies with chlorite mineral index. The greenschist facies was characterized with chlorite, albit, epidote, and actinolite minerals appearance. The metamorphic rock had a foliation structure. The structure was commonly generated by a regional metamorphic process, which was particularly a feature of the clear alignment, translucent minerals from flat minerals, such as : mica, chlorite and minerals that obtained fibers or scents.
 Keywords: Determination; Facies; Metamorfism Type
 
 Copyright (c) 2020 Geosfera Indonesia Journal and Department of Geography Education, University of Jember
 This work is licensed under a Creative Commons Attribution-Share A like 4.0 International License

A Spatial Autologistic Model to Predict the Presence of Arsenic in Private Wells Across Gaston County, North Carolina Using Geology, Well Depth, and pH

Chronic exposure to arsenic-contaminated drinking water is detrimental to human health. We develop an autologistic regression model to evaluate if the geology, pH, and well depth can improve our ability to predict the presence of arsenic at and above detectable levels (≥ 5 µg/L) found in private wells. We use arsenic samples measured in private well water across Gaston County, North Carolina, from 2011 to 2017. We use kriging to map the probability of arsenic at detectable levels across Gaston County. Arsenic at detectable levels was reported at 78 private wells. The median pH for samples containing detectable levels of arsenic was 7.3 and for samples with arsenic < 5 µg/L was 7.1. Our spatial autologistic model suggests that arsenic at detectable levels is positively associated with pH. In addition, private wells set in Mica schist (ЄZms) were associated with arsenic, suggesting a local-scale geologic source influence of arsenic in the county. Our kriging map shows that the northwestern section of the county has more than a 50% probability to have arsenic at detectable levels. In conclusion, based on our results, we recommend increased testing for wells in the Mica schist area. The map of probability of arsenic at and above detectable levels can be used to implement cost-effective targeted interventions.

Two episodes of REEs mineralization at the Sin Quyen IOCG deposit, NW Vietnam

The Sin Quyen iron oxide-copper–gold (IOCG) deposit in northwestern Vietnam contains a proven reserve of 52.8 Mt ores at 14% Fe, 0.91% Cu, 0.7% REEs (rare earth elements), and 0.44 g/t Au. REEs mineralization is hosted within Ca-silicate alteration zone and associated with Fe-Cu ore bodies in the gneiss, mica schist, and marble of the Upper Sin Quyen Formation. Two mineralization episodes are identified based on the field and petrographic relations. The first episode includes sodic alteration (stage I-1), calcic alteration and associated REE mineralization (stage I-2), and localized K-Fe alteration (stage I-3). The second episode is main stage mineralization and contains four main stages of alteration: pre-ore Na alteration (stage II-1), Ca-Fe alteration and the associated Fe-REE mineralization (stage II-2), K-Fe alteration and associated Cu-Au mineralization (stage II-3), and the final carbonate stage of quartz-carbonate veins (stage II-4) crosscutting the above-mentioned alteration phases and ores.Primary allanite and type 1 titanite (Tnt-1) from episode 1 yield weighted mean 206Pb/238U ages of 882 ± 3 Ma and 881 ± 8 Ma (2σ), respectively, which are interpreted as the timing of REE mineralization in episode 1. Secondary allanite and type 2 titanite (Tnt-2) associated with iron-oxide and Cu-Au ores, however, have U-Pb ages of 840 ± 7 Ma and 838 ± 5 Ma (2σ), respectively. These ages are hence considered to represent the timing of polymetallic IOCG mineralization in episode 2. ~880 Ma titanite grains have εNd(t) values ranging from −7.00 to −5.43, which are intermediate between the Neoproterozoic felsic intrusions and ore-hosting meta-sedimentary rocks in the region and likely reflect interaction between magmatic fluids and ore-hosting rocks. In contrast, ~840 Ma titanite have εNd(t) values ranging from −4.31 to −1.97, similar to those of the Neoproterozoic felsic intrusions, indicating a possible magmatic fluid source for the episode II IOCG mineralization. Multiple episodes of mineralization at Sin Quyen were likely formed during the prolonged tectonothermal events related to backarc extension during the Neoproterozoic subduction along the western margin of the Yangtze Block.

Multiphase and multicomponent simulation of acid mine drainage in unsaturated mine waste: Modeling approach, benchmarks and application examples

Acid mine drainage (AMD), caused by weathering of sulfide-rich mine wastes, is arguably the most critical environmental issue in the mining sector. Despite the fact that individual mechanisms responsible for the generation/propagation of AMD are fairly well-known, the prediction of overall dynamics in a mine waste system is challenging due to the nonlinearity and multilevel coupling associated with the key physico-chemical processes. This work presents a new three-phase multicomponent reactive transport model for AMD simulations, taking into account variably saturated water flow, multicomponent advective-dispersive transport both in aqueous and gaseous phase, and kinetic and equilibrium chemical reactions. The model is capable to capture the species-species coupling due to solute/surface charge or gas phase pressure by solving Nernst-Planck equation for the aqueous transport, and Maxwell-Stefan equation for the gaseous transport. Furthermore, the unsaturated flow and transport code is coupled with PHREEQC, utilizing the PhreeqcRM module, to simulate a wide range of geochemical reactions. The capabilities of the code are benchmarked against other software, previously published laboratory data, and pilot-scale drainage experiments in mica schist containing waste rocks performed in this study. The results are analyzed in terms of spatial and temporal profiles. The simulations reveal that the proposed model, called AMD-PHREEQC, is capable of accurately reproducing the experimental results, and thus it can be effectively used in environmental risk assessment, long-term drainage quality predictions, and data interpretations in mine waste.

Berit Dağları’nda (Kahramanmaraş) Litolojik ve Jeomorfolojik Faktörlerin Bitki Örtüsünün Dağılışına Etkisi

Bu çalışmanın amacı, Berit Dağları’nda litolojik ve jeomorfolojik faktörlerin bitki örtüsünün dağılışına etkisini açıklamaktır. Bu amaçla litolojik ve jeomorfolojik faktörlere ait alt kriterler ile bitki örtüsü katman haritaları oluşturulmuştur. Oluşturulan katman haritaları, Coğrafi Bilgi Sistemleri (CBS) teknikleriyle, ArcGIS 10.7 yazılımı kullanılarak çakıştırılmıştır. Çakıştırma işlemi sonunda her bir jeomorfolojik faktörün alt kriterinde bulunan bitki türü ve alanı tespit edilmiştir. Buna göre, Berit Dağları’nda 1600,01-1800 m yükseltileri arasındaki yüksek plato ve yamaçlarda şist anakayası üzerinde 45,01º> eğimli alanlarda kuzeydoğu, doğu ve güneydoğu yönlerde bitki çeşitliliği ve yoğunluğu fazladır. Buna karşılık 2400 m’den yüksek alanlardaki zirve düzlükleri üzerinde, ofiyolitik melanj ve 0-15ºeğimli güney ve güneybatı yönlü alanlarda bitki tür çeşitliliği ve yoğunluğu azdır. Çalışma sonucunda, Berit Dağları’nda litoloji, yerşekilleri, yükselti, eğim ve bakı gibi jeomorfolojik faktörlerin bitki örtüsü çeşitliliğini ve dağılışını etkileyen önemli unsurlar olduğu anlaşılmıştır.

Brief communication: The influence of mica-rich rocks on the shear strength of ice-filled discontinuities

Abstract. A temperature- and stress-dependent failure criterion for ice-filled rock (limestone) joints was proposed in 2018 as an essential tool to assess and model the stability of degrading permafrost rock slopes. To test the applicability to other rock types, we conducted laboratory tests with mica schist and gneiss, which provide the maximum expected deviation of lithological effects on the shear strength due to strong negative surface charges affecting the rock–ice interface. Retesting 120 samples at temperatures from −10 to −0.5 ∘C and normal stress of 100 to 400 kPa, we show that even for controversial rocks the failure criterion stays unaltered, suggesting that the failure criterion is transferable to mostly all rock types.

Mesoarchean accretionary mélange and tectonic erosion in the Archean Dharwar Craton, southern India: Plate tectonics in the early Earth

Accretionary orogens are hallmarks of subduction tectonics along convergent plate margins. Here we report a sequence of low-grade metasediments carrying exhumed blocks of ultramafic, mafic and felsic rocks from Sargur in the Western Dharwar Craton in India. These rocks occur along the southern domain of the Chitradurga Suture Zone, which marks the boundary between the Western and Central Dharwar Cratons and thus provide a window to explore Archean convergent margin processes. We present zircon U-Pb and Lu-Hf data from Sargur metasediments including quartz mica schist, fine-grained quartzite, and pelitic schist, as well as from blocks/layers of trondhjemite, garnet amphibolite, and chromite-bearing serpentinite occurring within the metasedimentary accretionary belt. The detrital zircon grains from the metasediments show multiple age groups, with the oldest age as 3482 Ma and an age peak at 2862 Ma. Magmatic zircons in trondhjemite show 207Pb/206Pb weighted mean age of ca. 2972 Ma, whereas those in the chromite-bearing serpentinite display multiple age populations of ca. 2896, 2750, 2648, 2566 and 2463 Ma, tracing zircon crystallization in an evolving mantle wedge adjacent to a subducting oceanic plate. Metamorphism is dated as ca. 2444 Ma from zircon grains in the garnet amphibolite. Zircon εHf(t) in the mafic-ultramafic rocks and trondhjemite are mostly positive, suggesting a juvenile (depleted mantle) source. The detrital zircon Lu-Hf data suggest that the sediment source involved Paleoarchean juvenile and reworked components. Based on our findings, we propose that the Sargur sequence represents an accretionary mélange which forms part of a major Mesoarchean accretionary orogen that witnessed multiple stages of tectonic erosion at least during three periods at ca. 3200–3000 Ma, 3000–2800 Ma and 2800–2500 Ma removing a large part of the accretionary prism along the convergent margin. We correlate the processes with prolonged subduction-accretion cycle culminating in the final collision between the Western and Central Dharwar cratonic blocks.

Paired metamorphism in the Neoarchean: A record of accretionary-to-collisional orogenesis in the North China Craton

Paired metamorphism is the hallmark of asymmetric subduction at Phanerozoic convergent plate margins, yet spatially and temporally linked paired metamorphic belts have rarely been documented in the Archean rock record. Here, we investigate the Neoarchean Dengfeng Complex, a typical granite–greenstone belt in the southern part of the North China Craton (NCC). Using petrography, geochronology, and phase equilibrium constraints on rocks from different lithostructural units, we suggest the ca. 2.54–2.50-Ga Dengfeng Complex represents a remnant of a spatially and temporally linked Neoarchean paired metamorphic belt. In the western part of the complex, a garnet amphibolite from a dominantly metabasic unit (MBU) records near peak P–T conditions of 6.3–10 kbar and ∼675–750°C, corresponding to a high thermobaric ratio (T/P) of ∼720–1200°C/GPa. Tonalite–trondhjemite–granodiorite (TTG) gneisses preserve evidence for partial melting, also consistent with relatively high T/P (∼875–1400°C/GPa). In the eastern part of the Dengfeng Complex, two garnet amphibolites and three garnet quartz–mica schists from a dominantly metasedimentary unit (MSU) record peak P–T conditions of >9.8 kbar and ∼525–655°C, corresponding to intermediate thermobaric ratios of ∼425–600°C/GPa. Zircon and titanite U–Pb dating, coupled with existing ages of post-kinematic intrusions, constrain the metamorphic age to ca. 2.52–2.50 Ga. The metamorphic P–T data indicate that the MSU was buried to >30 km then exhumed to the near-surface by the early Paleoproterozoic. Our metamorphic results, when combined with recent structural and geochemical data, suggest the Dengfeng Complex records Neoarchean paired metamorphism, in which the higher thermal gradients reflect the arc–forearc region and the lower thermal gradients correspond to the accretionary complex. Our data indicate that the southern NCC experienced accretionary-to-collisional orogenesis with intra-oceanic subduction, forearc accretion (ca. 2.54–2.50 Ga), and subsequent arc–continent collision (ca. 2.50–2.47 Ga). The recognition of paired metamorphism in the NCC is consistent with widespread subduction and the operation of a plate tectonic regime in the late Archean.

Fluid Inclusion Characteristics of Tungsten Mineralization in the Agargaon Area of Sakoli Fold Belt, Central India

The Lower to Middle Proterozoic Sakoli fold belt in Central India forms a triangular belt with significant mineralization of strategic minerals. The Sakoli fold belt comprises metasediments, felsic and mafic volcanics with metabasalts bounded by the gneissic-migmatitic terrain. The last pulses of granitic activity in the form of quartz lenses intrude the metasediments and are associated with tungsten mineralization. The metasediments are intruded by the quartz veins and tourmaline breccias trending 60°N to 65°E and 60°S to 65°W and are parallel to the regional structural foliations. The tungsten mineralization in this area is restricted to tourmaline-quartz mica greisens and quartz veins. The NE-SW trending foliated contact zones of chlorite mica schist and porphyritic granite/gneisses have served as easy channels for the mineralizing vapours and solutions to percolate, which formed ore bearing greisens and quartz veins. This mineralization is erratic and manifested by sparse and sporadic disseminations of wolframite and scheelite associated with minor amount of molybdenite and chalcopyrite. The fluid inclusion microthermometry on mineralized quartz veins and quartz-tourmaline veins reveals the existence of a metamorphogenic aqueous- gaseous (H2O-CO2+NaCl) fluid that underwent phase separation and gave rise to gaseous (CO2) inclusion. The salinity of tungsten mineralizations varies from low to high (1.32 wt.% to 40.44 wt.% NaCl eq.). The estimated P-T range of tungsten mineralization varies from 1.2 to 2.2 kbar at 280 to 390 °C. Raman spectroscopy reveals that the fluid inclusions mainly contain H2O and CO2 with rarely H2S and CH4. Stable isotopic data reveal that the sulfur isotope fractions from the deposits δ34S ranging from +3.1‰ to +3.35‰, suggesting the deep crustal source for the sulfur, which can be further interpreted as a single (magmatic) supply of sulfur during magmatic-hydrothermal mineralization. The studies reveal the presence of chlorides such as FeCl2/MgCl2 and CaCl2, indicating the involvement of chloride complexes in transportation of tungsten to the fluid system and the evolution of the ore-forming fluids by mixing or immiscibility of high-temperature, high-salinity magmatic fluids and low-temperature, low-salinity fluids in hydrothermal system, and also representing magmatic-hydrothermal interactions contributed wolframite and scheelite with minor amount of molybdenite and chalcopyrite.

Re-Mapping the Neoproterozoic Basement Rocks of Um Gheig area, Central Eastern Desert, Egypt based on Remote Sensing, Petrography and Field Investigation

Landsat 8 (OLI) ETM+ satellite imagery data are evaluated forimproving and re-mapping the geologic map of Um Gheig area, CentralEastern Desert of Egypt. Different remote sensing techniques includingFCC, band ratio, PCA, supervised classification and spectralcharacteristic analysis were applied on satellite imagery to facilitate thediscrimination of the widely exposed Neoproterozoic basement rocks inthe study area. Best color composite 146 FCC, 762 FCC, band ratio 6/2,4/3, 7/3 and 7/6, 6/5, 4/2 in RGB, as well as PCA 213 and 314 in RGBwere selected and used for detailed mapping of the different lithologicalunits. Spectral characteristic analysis was used to discriminate the rockforming minerals through the different rock units in Umm Gheig area(e.g. amphibole b7/b5, serpentine b5/b7, Quartz b7/b6, muscovite b6/b4and PCA4). The rock units of Um Gheig area are divided four mainlithological units according to field and petrographical studies; ophioliticassociations, foliated metavolcanics, post-granitoids and hammamatsediments, as well as small parts of unmapple metavolcanics andmetgabbro. Ophiolitic associations are subdivided serpentinites,metagabbro, volcanoclastic metasediments and hormblende schist.Granitoids are subdivided El-Deilihimmi granite and Abu Shaddadgranite. Petrographically, serpentinites prevailing by antiguriteserpentinites, metagabrro includes quartz and hornblende gabbro, VCMcomprised different varieties range from biotite schist, biotitehornblende schist, garnet biotite hornblende schist, intercalations ofactionolite schist and highly mylonitic schist, foliated metavolcanicsrepresented by schistose metatuffs and mylonitic schist, metavolcanicsrepresented by meta-andesite and fine metatuff, El-Deilihimmi graniteincludes granodiorite and monzogranite while Abu shaddad granitecomprised alkali feldspar granite and syenogranite, hammamatsediments represented by pebbles of dacite. A new modified geologicalmap is produced for Um Gheig area based on the remote sensing data,field investigation and petrographical description.

Nepheline syenite intrusions from the Rengali Province, eastern India: Integrating geological setting, microstructures, and geochronological observations on their syntectonic emplacement

Ten nepheline syenite intrusions have been concordantly emplaced within or close to a polydeformed, regionally metamorphosed sedimentary sequence in the Rengali Province, eastern India during the Mesoproterozoic. The intrusions and the country rocks share a common deformation history and record three phases of folding. The nepheline syenites preserve deformation structures and textures indicative of continuous deformation that started in the presence of a melt and ended under sub-solidus conditions. This deformation in nepheline syenites is divided on the basis of mechanical behavior into three stages: an early one with the development of varied types of magmatic layering and flattened magmatic enclaves, a middle one with the development of asymmetrical magmatic folds and intramagmatic shear zones, and a late stage with melt-enhanced embrittlement. Igneous zircons from two of the intrusions, namely Chhatabar and Gunjighara are dated at 1356 ± 5 Ma and 1358 ± 4 Ma (U–Pb concordant age, LA–ICP–MS), respectively. Electron microprobe U–Th–total Pb geochronology of monazites from two samples of sillimanite grade mica schists in the host metasedimentary sequence of the Chhatabar nepheline syenite yields metamorphic ages of 1362 ± 12 Ma and 1376 ± 23 Ma, which indicate broad synchronism of regional metamorphism of the country rocks and the nepheline syenite magmatism. Contrary to the common proposition that nepheline syenite intrusions in eastern India and in some other regions of Precambrian crust were emplaced anorogenically in intracontinental rifts and subsequently deformed by continental collision, we develop a structural interpretation in which the nepheline syenites were emplaced syntectonically in the sense that magma transport and cooling occurred during regional metamorphism and deformation. The study shows that an integration of structural, textural and geochronological data from both nepheline syenite and country rock is crucial for an understanding of the spatial and temporal relationships between contractional deformation and alkaline magmatism. Our results imply that deformed nepheline syenites may not always be useful in constraining the initiation of a Wilson Cycle heralded by intracontinental rifting.

Growth mechanism of garnet megaporphyroblasts of the Passos Nappe, Southern Brasília Orogen, Brazil

The Passos Nappe is an important tectonic unit of the Southern Brasília Orogen with inverted metamorphism, grading from greenschist facies at the base to granulite facies in the upper part. In the central part, of amphibolite facies, mica schist layers contain garnet megaporphyroblasts (3–9 cm diameter) that contrast with the common-size garnet (0.2–1.5 cm diameter) present in other mica schist layers above and below. This paper intends to describe this textural feature in detail and to discuss its genesis. The megaporphyroblasts are ellipsoidal, with major axis parallel to the mineral and stretching lineation, associated with the main foliation (S2). Inclusion trails analysis in forty-six megaporphyroblasts, demonstrate straight internal foliation (Si = S1) in the core, progressively crenulated towards the rim by D2. Si is discontinuous with the external foliation (Se = S2), which wraps around the megaporphyroblasts, indicating early syn-D2 growth. The angle between the axial planes of the included crenulations and S2 in the matrix indicates that the D2 deformation continued after porphyroblast growth. Asymmetric deformation shadows reveal non-coaxial D2 deformation, associated with the emplacement of the nappe (top-to-the SE). The megaporphyroblasts are almandine-rich (73–82%) and spessartine-poor (0.1–2.5%) exhibiting prograde growth zonation. The common-size garnet porphyroblasts of the adjacent micaschists show similar inclusion patterns and chemical zonation indicating a similar tectonometamorphic evolution. A possible origin of the megaporphyroblasts by introduction of fluids along shear zones is discarded because the megaporphyroblasts are distributed along S0//S1 planes that make an angle with S2. Since the porphyroblasts grew early syn-D2, these shear zones should be parallel to S2 which does not match the observations. The chemical data indicate that megaporphyroblast formation is related to a high-Al protolith composition, associated with scarce nucleation sites.

Alanya Birliği Metamorfitlerinin Stratigrafik ve Petrografik Karekteristikleri ve Sugözü Karışığında (Orta Nap) Porfiroblast - Foliasyon İlişkileri, Anamur-Mersin-Türkiye

The study area is located in the southern part of the Central Taurus in Anamur (Mersin) vicinity. The Alanya Unit consisting of completely metamorphites consists of threeimbricate metamorphic nappes. Lower-Middle Cambrian aged the Mahmutlar group (lower nappe) is composed of widespread mica schists containing crystallized limestone, cherty dolomite, quartzite interlevels and greenschist lenses. Precambrian aged Sugözü complex (middle nappe) contains kyanite-sillimanite-staurolite mica schists and widespread garnet-mica schists, which contain greenschist-amphibolite-amphibol schist bands-lenses and metagabro-metadiabase dykes-interlevels. In the middle nappe, Cambrian and Permian blocks belonging to Alanya Unit are observed. The Yumrudağ group (upper nappe) begins with Lower-Middle Cambrian aged mica schists containing cherty dolomite, crystallized limestone, quartzite interlevels and greenschist lenses. Higher up with an unconformity pass into the Upper Permian aged alternation of quartz schist - calc schist - crystallized limestone - phyllite and thick bituminous crystallized limestone containing greenschist lenses. Barite, copper and galenite formations take place in upper nappe lithologies (Ağzıkara formation). Upper Paleocene - Middle Eocene aged Kötekler formation unconformably overlies the Alanya Unit. Alanya Unit has been subjected to multi-stage deformations in connection with intense tectonic movements. Especially, biotite (brown, green) + muscovite + garnet (prop-almandine-grossular) + quartz ± chlorite (ripidolite-picnochlorite, pennin-clinochlore) ± kyanite ± staurolite ± sillimanite + plagioclase (albite-oligoclase-andesine) ± epidote ± zoisite/clinozoisite ± orthoclase + tourmaline (green, brown) ± graphite ± sphene ± rutile ± apatite mineral paragenesis are observed in metapelitic and metasemipelitic rocks belonging to Sugözü complex (middle nappe). Amphibolites and amphibole schists of this complex contain hornblende (tchermacite, ferro-tchermacite, magnesio-hornblende and edenite) + garnet + plagioclase (albite-oligoclase-andezine) ± relic clinopyroxene (augite-pigeonite) + chlorite (ripidolite, clinochlore) ± orthoclase ± epidote ± zoisite/clinozoisite ± biotite (green, brown) ± actinolite ± muscovite ± quartz ± calcite ± sphene ± apatite ± rutile mineral assemblage. Hornblende (tchermacite, ferro-tchermacite) + plagioclase (albite-oligoclase-andesine) + relic clinopyroxene (augite) ± garnet + quartz + chlorite (ripidolite, pennin-clinochlore) ± actinolite ± zoisite/clinozoisite ± epidote ± calcite ± sphene ± apatite is observed in metagabbros and metadiabases. The greenschists are characterized by tremolite + actinolite + chlorite (ripidolite-picnochlorite) + plagioclase (albite) ± garnet + zoisite / clinozoisite + epidote ± quartz ± microcline ± relic clinopyroxene (augite) ± calcite + sphene ± apatite mineral assemblage. Especially when the index minerals, different mineral paragenesis and deformation-metamorphism relations within the Sugözü complex take into account, it has been seen that Alanya unity underwent three phases of metamorphism which follow one another.

Determination of Residential Soil Gas Radon Risk Indices Over the Lithological Units of a Southwestern Nigeria University

Radiation dose from natural sources is mainly from exposure to radon in the environment. Radon has its origin from uranium-bearing bedrocks and overburden. In the present study, assessment of the level of radon over the three lithological units upon which the residential areas of ObafemiAwolowo University Campus, Ile-Ife (OAU) was situated was carried out. Soil gas radon concentration measurement was carried out at a constant depth of 0.80 m across the three lithologies (granite gneiss, grey gneiss and mica schist) using a RAD7 electronic radon detector. A total of 138 in-situ soil gas radon measurements were carried out. Obtained experimental data were analysed and summarised using descriptive and inferential statistics with statistical significance set at p < 0.05. A radon potential map was also developed using existing permeability data of the soils in the area. Soil radon concentration varied across the different lithologies ranging from 0.04 kBq/m3 – 190 kBq/m3 with a mean value of 14 kBq/m3. The mean value of Rn-222 concentration obtained in the three lithologies are 3.5 ± 5.9, 11.5 ± 25.8 and 28.4 ± 37.4 kBq/m3 for granite gneiss, grey gneiss and mica schist respectively. There is a statistically significant difference (p < 0.001) in the mean concentration of radon-222 measured on the three lithologies. The granite gneiss and grey gneiss lithologies have been designated into low radon index, while mica schist lithology has been designated as medium radon index. 34% of the sampled areas exhibit high radon risk based on Swedish risk criteria, thereby warranting protective actions.

Rapid fluid infiltration and permeability enhancement during middle–lower crustal fracturing: Evidence from amphibolite–granulite-facies fluid–rock reaction zones, Sør Rondane Mountains, East Antarctica

Permeability is a key control on fluid infiltration in the crust. However, quantitative geological constraints on crustal permeability are limited, particularly with regards to its temporal evolution. Here we constrain the permeability evolution in the middle–lower crust, based on metamorphic processes associated with fluid infiltration and crustal fracturing. We investigated mafic granulite and orthopyroxene–hornblende schist (opx–hbl schist) samples from Mefjell, Sør Rondane Mountains, East Antarctica. Millimetre-scale amphibolite-facies reaction zones occur along fractures in these rocks. In the mafic granulite, two zones were identified: (i) reaction zones (1–2 mm thick); and (ii) mafic granulite host rock. The opx–hbl schist sample can be divided into the following three zones: (i) actinolite–cummingtonite zones (1.4 mm thick); (ii) actinolite–orthopyroxene zones (1.6 mm thick); and (iii) host rock. These zones are evident from the modal mineralogy, reaction textures, and trace element profiles. The P–T conditions of fluid infiltration are estimated to be 0.55 GPa and 620 °C for the mafic granulite, and 0.3 GPa and 450 °C for the opx–hbl schist, respectively. Chlorine concentrations in apatite grains show a gradual decrease from the fractures towards the host rocks. Chlorine concentration profiles suggest that the dominant processes of chlorine transport were advection with minor diffusion for both samples. Based on these results, the timescales of fluid infiltration are constrained to be ~8 h for the mafic granulite and ~10 h for the opx–hbl schist. The pressure gradient across the reaction zones was estimated from the H2O activity in the reaction zones to be 2–15 MPa/mm. The permeability of the host rock and fractures were estimated to be 10−21–10−23 and 10−8–10−9 m2, respectively. Our results show that rapid infiltration of Cl-bearing fluids (~10 h) occurred due to a limited fluid flux from low-permeability (10−21–10−23 m2) host rocks. The low-permeability media led to fluid accumulation and further fracturing. The spatio-averaged permeability then increased by more than several orders of magnitude (10−10–10−16 m2) and the fluid pressure decreased. The contrasting permeability between the host rocks and fractures reveals permeability enhancements associated with crustal fracturing on timescales comparable to geophysical observations. Compared with average long-term (Myr) permeability estimates from previous studies (e.g., 10−18 m2), the permeability obtained from the fracture–reaction zone systems shows large fluctuations in the middle–lower crust (10−21–10−23 to 10−10–10−16 m2).

Rare and new compounds in the Ni-Cu-Sb-As system: first occurrence in the Gomati ophiolite, Greece

The Gomati ophiolite (Northern Greece) is located in the Serbo-Macedonian Massif, in the Hellenides orogenic belt. It consists of altered peridotites hosting scattered chromitite bodies. The ultramafics are enclosed in Silurian gneisses and schists, and are partially in contact with late Cenozoic granites. The present work focuses on accessory minerals in the Ni-Cu-Sb-As system, found in a chloritized clinopyroxenite in contact with chromitite. The electron microprobe analyses revealed the presence of known minerals such as orcelite (Ni 5 As 2 ) and breithauptite (NiSb) and other new phases that cluster around the following stoichiometries: (Ni,Cu) 7 (Sb,As) 3 , (Ni,Cu) 2 (Sb,As), (Ni,Cu) 11 (Sb,As) 8 , Ni 3 As, Ni 5 (As,Sb) 2 and Ni 7 (As,Sb) 3 . As orcelite is a non-stoichiometric mineral, (Ni,Cu) 7 (Sb,As) 3 , Ni 5 (As,Sb) 2 and Ni 7 (As,Sb) 3 may correspond to Cu and/or Sb-rich terms of this phase. A mineral phase corresponding to (Ni,Cu) 2 (Sb,As) was first described in the Tulameen complex of Canada. A phase with stoichiometry Ni 3 As was found in the Bon Accord oxide body of South Africa. The (Ni,Cu) 11 (Sb,As) 8 probably represents a Cu-rich Sb analogue of the mineral maucherite (Ni 11 As 8 ). Such mineral assemblage in the Gomati ophiolite is puzzling. While ultramafic rocks contain Ni and As of magmatic origin, the presence of Sb, Ag, Au and Cu minerals could be indicative of a metasomatic enrichment, probably linked to the presence of fluids emanating from the granite body in contact with the Gomati ophiolite.