Dolerite Dykes Research Articles

Engineering geological and geotechnical evaluation of Sathya Sai Prasanthi Nilayam railway tunnel, Andhra Pradesh, India

Detailed engineering geological investigations were carried out for a railway tunnel which was constructed more than two decades ago. 3D engineering geological mapping was carried out using Brunton Compass and Total Station Surveying instruments in 1:100 scale. Coarse-grained pink and grey granite, hornblende-biotite gneiss and dolerite dyke of Archaean age and Lower Proterozoic age were mapped. Rock mass was intersected by sub-horizontal, inclined, and vertical joint sets, which were continuous and persistent, smooth, and planar with thick filling of decomposed and crushed sheared material or with thin coating of clay material. Based on the Q-system, rock mass was classified into different classes. On the basis of large-scale engineering geological mapping and Norwegian Method of Tunnelling, a support system was recommended which includes rock bolt, fibre reinforced shotcrete, grouting and reinforced ribs of sprayed concrete and the same is implemented by the agency. As per the best knowledge of the authors, reinforced ribs of sprayed concrete are first time used for transportation tunnels in India and it will be more effective if it will be compared with ISMB or Lattice Girder.

Petrographic and geochemical study of Precambrian and Paleozoic rocks in the western Anti-Atlas belt, Morocco: Facies, weathering, provenance and economic exploitation

Detrital and volcanic-detrital rocks from the Ifni Buttonhole and Lakhssas Plateau were analyzed to determine their provenance, compositional maturity, and alteration source. Geochemically, the sediments were classified as arkoses, lithic arenites, grauwackes, sandstones, lithic arenites, and Fe-rich sands, indicating low compositional and mineralogical maturity. A high average SiO2 concentration and low Al2O3 were consistent with a low abundance of shale and clay components. The geochemical signatures of the detrital and volcano-detrital (RDVD) rocks indicate that they have undergone a moderate to low degree of chemical alteration. The CIA study also suggests that the granitic, granodioritic rocks represent the source provenance which, during weathering and transport, supplied the detritus to the supra-crustal units. The major trace element data suggest that these rocks are largely derived from felsic igneous rocks, namely granitoids, with a minor contribution from intermediate sources. The carbonate rocks do not represent a wide variety of facies: dolomitic limestone, calcareous limestone, and calcaro-dolomitic chert. Calcitic and dolomitic samples show a linear increase in SiO2, regardless of their CaO/LOI ratio values, which remain relatively constant. The highest SiO2 contents are observed in the calc-dolomitic chert. Geochemical analysis of RDVD from the Ifni buttonhole determined their origin, maturity, and alteration. Major oxides decreased with higher silica content, indicating quartz control. Samples, formed under semi-arid conditions, show maturity under stable deposition. They suggest a felsic, recycled source, with moderate alteration and zircon enrichment during recycling. In the study area, limestones and dolomites serve as materials applicable in the building sector, suitable for all types of concrete. The Taliwine Formation harbors Lower Cambrian dolomites and limestones, ideal for mosaic aggregates. Described as variable in color, compact, homogeneous, very hard, and resistant to alteration, the plutonic rocks form prominent peaks. They exhibit both subalkaline characteristics in granitoids and an alkaline trend in dolerite dykes. Most samples display minimal alteration, indicating the reliability of their major element compositions for geochemical analyses. These granitoids constitute valuable deposits for ornamental and building rock purposes.

Geophysical signatures on gold and base metal mineralisation in the Ibaga area, northeast of Iramba, central Tanzania

This study provides an in-depth geophysical interpretation of the gold and base metal mineralisation of the Ibaga area related to hydrothermal alteration using high-resolution aero-geophysical and induced polarisation ground survey datasets. Despite its potential for gold and base metal and the available artisanal mines, the Ibaga area has remained poorly understood on geophysical signatures of ore deposits and other subsurface geological structures controlling the mineralisation. The magnetic results on the major structures show high magnetic signatures, supported by gravity, radiometric signatures and petrophysical analysis. The potential mineralised structures are interpreted as dyke for gold, and contacts for copper and the depth to the basement varies from 0.113 to 1.556 km. The gravity anomaly shows high values on the gold mineralisation similar to high-density values of dolerite dykes and gabbro and low to medium radiometric concentrations associated with mineralised zones. The magnetic signatures on the copper deposits coincide with the relative positive gravity anomaly of high-density intrusive bodies and relatively moderate conductive bodies with elevated potassium (K) alteration. The copper deposit also has intermediate K and low to intermediate K/Th and K/U ratios caused by K alteration within the geological structures. The good spatial correlation of the hydrothermally altered zones, igneous intrusions and geological structures implies that the geological features associated with gold and copper mineralisation lie within the regions of high-density intrusive bodies and geological structures (faults, fractures and dykes). These features are also associated with relatively moderate conductive bodies, low to intermediate radiometric concentrations and low to moderate magnetic signatures. Thus, the study reveals that the delineated geophysical signatures on gold and base metal mineralisation are associated with quartz–sericite–muscovite ± chlorite hydrothermally altered areas, deformed structures, and intrusive bodies, which act as pathways for the migration and accumulation of hydrothermally altered fluids.

ASTER data processing and fusion for alteration minerals and silicification detection: Implications for cupriferous mineralization exploration in the western Anti-Atlas, Morocco

Alteration minerals and silicification are typically associated with a variety of ore mineralizations and could be detected using multispectral remote sensing sensors as indicators for mineral exploration. In this investigation, the Visible Near-Infra-Red (VNIR), Short-Wave Infra-Red (SWIR), and Thermal Infra-Red (TIR) bands of the ASTER satellite sensor derived layers were fused to detect alteration minerals and silicification in east the Kerdous inlier for cupriferous mineralization exploration. Several image processing techniques were executed in the present investigation, namely, Band Ratio (BR), Selective Principal Component Analysis (SPCA) and Constrained Energy Minimization (CEM) techniques. Initially, the BR and SPCA processing results revealed several alteration zones, including argillic, phyllic, dolomitization and silicification as well as iron oxides and hydroxides. Then, these zones were mapped at sub-pixel level using the CEM technique. Pyrophyllite, kaolinite, dolomite, illite, muscovite, montmorillonite, topaz and hematite were revealed displaying a significant distribution in relation with the eastern Amlen region lithological units and previously detected mineral potential zones using HyMap imaging spectroscopy. Mainly, a close spatial association between iron oxides and hydroxide minerals, argillic, and phyllic alteration was detected, as well as a strong silicification was detected around doleritic dykes unit in Jbel Lkest area. A weighted overlay approach was used in the integration of hydrothermal alteration minerals and silicification, which allowed the elaboration of a new mineral alteration map of study area with five alteration intensities. ASTER and the various employed processing techniques allowed a practical and cost effective mapping of alteration features, which corroborates well with field survey and X-ray diffraction analysis. Therefore, ASTER data and the employed processing techniques offers a practical approach for mineral prospection in comparable settings.

Petrology and Geochemistry of the Ultramafic Rocks of Yerrabali Schist Belt, Eastern Dharwar Craton, Telangana, India

Abstract The Yerrabali schist Belt (YSB) is a NW–SE trending 10 km long and 4 km wide belt located at the north eastern corner of Eastern Dharwar Craton (EDC) consisting of Banded Magnetite Quartzite (BMQ), ultramafic rocks, quartz–cordierite– gedrite ± garnet bearing schist, sillimanite–garnet–cordierite schist, metadolomite, amphibolite, ferruginous quartzite, tremolite schist and dolerite dyke. The ultramafic rocks consist of olivine, orthopyroxene, clinopyroxene and spinel. The Fe content of olivine varies from 88-91 and consists of high amount of NiO (0.3-0.511%). Orthopyroxone are enstatitic and clinopyroxene are Mg-augite in nature. The spinel grains are rich in Al2O3 (48.32-59.09%) and show very low Cr# values (0.08-0.29). Thermo-barometric analyses show equilibrium condition of 700°-1100°C temperature and 19-21 kb pressure. Oxygen fugacity of the ultramafic rocks range from -0.035 to -1.356 log units below FMQ buffer. Low Cr# values of spinel along with low oxygen fugacity suggest their formation by low degrees of partial melting from a less refractory Mid Oceanic Ridge (MOR) type mantle.

Subsurface linear structures and potential zones of mineralisation of the Iramba-Sekenke greenstone belt, central Tanzania with implications for the structural-controlled mineralisation

This study aims to delineate concealed subsurface linear structures and their relationship with mineralisation in the Iramba-Sekenke greenstone belt. Results reveal three different types of linear structures and their concealed continuations, which were previously not identified. These structures include dykes trending in NE-SW and NS; shear zones with NW-SE trending and faults that trend in NE-SW, NW-SE and N–S. The faults and shear zones, which were previously shown to terminate at the northwestern end of the Iramba Plateau due to sediment cover, have been revealed to continue into Wembere depression. Moreover, the aeromagnetic and gravity results show that some faults are intruded by dykes in the same trends, which were not previously identified. The geological relationship between these linear structures shows that the youngest structures are the western dolerite dyke trending N–S and the oldest is the NE-SW Wembere-Iramba Plateau fault. The delineation of geological structures from magnetic and gravity data using different analysis techniques reveals that the mining pits are located close to geological structures trending in NW-SE, NE-SW and N–S directions. The integration of the findings from CET grid analysis techniques, together with magnetic maps and geological information reveals a good correlation between known occurrences of minerals and promising exploration target areas. The good correlation between the artisanal mining pits with the location of the potential exploration targets is consistent with the location of lineaments and provides future promising prospects for mineral exploration. The correlation suggests that mineralisation and ore deposits in the study area are respectively structurally controlled and hosted in deformed rocks. This study is potentially useful as a guide for the delineation of concealed subsurface linear structures and integration of structural geometry and geophysical methods for exploration of hidden mineral deposits in the Iramba-Sekenke greenstone belt.

Geochemistry, Petrology and Tectonic Settings of Dolerite Dykes of Ranipet District, Northern part of Southern Granulite Terrain, Tamil Nadu, India


 
 
 The dolerite dykes and their associated rock are collected from the Ranipet district which is located in the northern part of the Southern Granulite Terrain and South of the Dharwar craton. The major dyke in the Ranipet district shows WSW-ENE trending dykes intruded the hornblende-biotite gneiss with gabbroic diorite and granite gneiss. The dolerite in this region shows typical dolerite composition with high alteration like chloritization and sericitization. Mineralogically, the rock shows laths of plagioclase, anhedral to euhedral clinopyroxene with basal cleavage, and quartz minerals. The opaque minerals ilmenite and magnetite crystals interstice the major minerals and it has altered boundaries. Texturally, the rock shows subophitic and intermediate gabbroic texture (granular texture). TAS diagram shows that the dykes plot in the basalt, basalt andesitic, and rhyolitic field (highly sericitized dolerite). The host and other associated rocks plot within the sub-alkaline field. The bivariate plot for the studied dolerite shows a positive correlation with SiO2, CaO, Na2O, K2O, and P2O5 and a negative correlation with TiO2, Fe2O, and Al2O3. The AFM and Jensen plots show that the dolerites samples plot in the tholeiitic field with Fe enrichment and a High Fe tholeiitic character respectively. The primitive mantle diagram shows the depletion of Nb, P, and Ti showing possible interaction of crust during the emplacement. The Chondrite normalized plot for the dolerite shows enrichment of LREE and Slight depletion of HREEs indicates OIB character. Tectonically, the dolerites formed emplaced in the continental and alkaline arc OIB settings with E-MORB affinity.
 
 

Multi‐geophysical methods for characterizing fractures in an open pit mine, western Bushveld Complex, South Africa

AbstractIn the Bushveld Complex, South Africa, open pit mines are faced with a challenge of rock slope stability due to geological structures (fractures, faults and dykes) that compartmentalize the rock mass. Geophysical surveys (seismics, magnetics and electrical methods) were conducted in a 0.2 km2 area at Tharisa mine, with the goal to delineate fractures that may be potential conduits for water migration into the pit. Special processing techniques were applied to the dataset to obtain good quality seismic, magnetic and resistivity models. The P‐wave velocity models show distinct low velocities in the centre of the seismic profile, indicating the presence of weak zones associated with faulting or fracturing. Seismic reflection method was used to image the deeper discontinuities and mineralization contacts. Near surface reflections are observed throughout the profiles and are correlated with the contact between the chromitite and host rock. Ground magnetic surveys were conducted to delineate dykes and fractures. De‐trending and de‐culturing techniques were applied on the magnetic data for correcting regional and temporal variations. The low magnetic regions indicate the presence of fracture systems in the subsurface, whereas the high magnetic region is correlated with the dolerite dyke that crosscuts the pit. The electrical resistivity tomography exhibits linear low resistivity contrast zones that differentiate between the fractured and undisturbed hard rock at an estimated depth of 4–10 m. Resistivity shows discontinuities that suggests the presence of fracturing and dyke‐host rock contacts. Correlation among magnetics, P‐wave velocity models, resistivity section and seismic data is evident when overlaying the different datasets, implying that the low magnetic regions are highly weathered and prone to fracturing. The integration of geophysical data is encouraging, because it was able to image the depth to the bedrock, fractures within the host rock and dyke in a complex mining environment.

The Ophiolite-Hosted Cu-Zn VMS Deposits of Tuscany (Italy)

Several Jurassic, ophiolite-hosted Cu-Zn VMS deposits occur in Tuscany. They are hosted by tectonic units of oceanic affinity (Ligurian Units), such as the well-known deposits of nearby Liguria. Industrial production was small and definitively ceased in the 1960s. Locally, massive ore (chalcopyrite-bornite-chalcocite) with an exceptionally high grade was found. The Montecatini Val di Cecina mine exploited the largest “bonanza” and, for few decades in the 19th century, became one of the most profitable copper mines in Europe. This study provides an updated review of these deposits. Tuscan Cu-Zn VMSs mostly occur in proximity of the contact between the serpentinite-gabbro basement and the overlying basalts. Chalcopyrite-pyrite stockworks occur in serpentinite-gabbro cut by dolerite dykes, while the largest massive sulphide bodies are hosted by polymictic-monomictic breccias at the base of pillow basalts. Early chalcopyrite ores were mechanically–chemically reworked and upgraded to bornite-rich nodular ore embedded in a chlorite, calcic amphibole, Fe-rich serpentine, quartz, andradite, ilvaite, and xonotlite assemblage. This bornite-rich ore contains substantial amount of sphalerite and pyrite and ubiquitous grains of clausthalite, hessite, tellurium, and gold. They represent a prime example of the sub-seafloor portion of a hybrid mafic-ultramafic oceanic hydrothermal system formed in an OCC along the slow spreading ridge of the Jurassic Piedmont-Ligurian Ocean. The peculiar mineralogical–textural character of the bornite-rich ore was driven by an interface coupled dissolution–precipitation process mediated by fluids.

Amphibole Group Minerals in the Ozren Massif Ophiolites of Bosnia and Herzegovina as Petrogenetic Indicators

The Ozren ophiolite complex (OOC) of the Dinaridic Ophiolite Belt is one of the six ophiolite complexes in Bosnia and Herzegovina. This paper deals with the mineral chemistry of amphiboles determined by electron probe micro-analysis and micro-Raman spectroscopy. The detected amphibole generations and types in mafic, ultramafic, and metamorphic rocks suggest a polystage evolution and are therefore useful petrogenetic indicators of the investigated OOC. Most gabbroic rocks and dolerites contain primary magmatic amphibole1 (magnesio-hornblende to pargasite, occasionally hastingsite) and prismatic to needle-like aggregates of late magmatic amphibole2 (magnesio-hornblende), while plagiogranite contains ferri-winchite and ferro-ferri-winchite as primary magmatic amphibole. Post-magmatic amphiboles were detected in dolerites, troctolites, and lesser in peridotites. The Na-(Ti)-rich amphibole3 (ferri-winchite and ferro-ferri-winchite to katophorite and ferri-katophorite) with amphibole4 (grunerite) rim formed along the grain boundaries of clinopyroxene, amphibole1, and plagioclase in dolerites. A part of these amphiboles grows into amphibole1, 2. Kaersutite to ferri-kaersutite, associated with phlogopite, occur in troctolites and dunites, while Mhbl was detected in harzburgite. The ultramafic rocks (lherzolites, harzburgites, and dunites) and the gabbroic layer are crosscut by clinopyroxene–plagioclase gabbroic and clinopyroxene–plagioclase–amphibole gabbro–dolerite dykes, suggesting ‘dry’ and ‘hydrated’ percolating melts generated in inferred subridge and supra-subduction settings, respectively. The amphibole3 and 4 in gabbros and dolerites and similar amphibole types in ultramafic rocks could be related to inferred arc-type basaltic and plagiogranitic percolating melts and fluids. Low-Al amphibole5 (tremolite and actinolite) and associated chlorite, albite, and clinozoisite represent the ocean-floor alterations in mafic rocks. Amphibole6 (magnesio-hornblende to pargasite) was identified in metamorphic sole amphibolites. Micro-Raman spectroscopy provided typical Raman spectra for the studied amphiboles, highlighting distinct features such as bands related to CMg content, CFe3+ presence, TO4 ring-breathing mode, TiO6 stretching mode, presence > 0.3 apfu of CTi, and TO4 stretching indicating CFe2+ in the structure. Applied amphibole geothermobarometry revealed the formation P–T conditions of amphibole (Amp)1 (avg. 863 °C at 0.23 GPa), Amp2 (avg. 747 °C at 0.17 GPa), Amp in the mantle rocks (avg. 853 °C at 0.64 GPa), Amp5 (avg. 349 °C at 0.03 GPa), and Amp6 (avg. 694 °C at 0.46 GPa).

Zircon and apatite U-Pb geochronology of the Paleoproterozoic (Eburnean) basement and late Neoproterozoic (Pan-African) metamorphism and magmatism from Port-Béni, Armorican Massif (France)

By re-examining the historical outcrops of Port-Béni located in the Trégor unit of the North Armorican Cadomian belt, the present work delivers four new ages that provide additional constraints on the Proterozoic history of northern Brittany. It is established that granitic, porphyritic rocks crystallized at the end of the Rhyacian (Paleoproterozoic), 2038 ± 12 Ma ago, before being transformed into orthogneisses at a late Neoproterozoic (Ediacaran) age of 621 ± 2 Ma, which is a minimum age, given the retrograde alteration these rocks underwent. The age of ca. 1.8 Ga previously proposed for the protolith of the Port-Béni orthogneiss should be discarded, and these two new ages are consistent with most of those yielded so far by the other Icartian (i.e., Eburnean) basement relics from the Armorican Massif. The gneissic basement was then intruded and disrupted into xenoliths by a granodioritic magma that crystallized 604.5 ± 2.0 Ma ago. This age, slightly younger than previously thought, corresponds to the emplacement age of one of the main units of the North Trégor batholith − the Pleubian-Talbert unit −, part of the Trégor volcano-plutonic complex, which may have built up over a longer period than that indicated by the uncertainty associated with this age. Caution should be exercised in extrapolating this age to that of the whole complex. Finally, doleritic dykes, possibly resulting in two swarms previously thought to be Paleozoic in age, have crosscut this complex. One of the latest yielded an age of 597 ± 15 Ma, indicating that the Trégor doleritic dyking episodes also occurred during the late Neoproterozoic, in between ca. 605 Ma and ca. 580 Ma. As the doleritic dykes are of tholeiitic composition, which distinguishes them from the earlier calc-alkaline magmas, they suggest that the intra-arc extension, documented in the southern, adjacent Saint-Brieuc unit of the belt, also affected the Trégor unit. They may likely have fed northern equivalents of the lava flows from the Paimpol Formation (exposed in between the Saint-Brieuc and the Trégor units), when magma production became moderately influenced by the Cadomian (i.e., Pan-African) subduction and mostly dominated by extension, possibly as a result of a steepening of a north-dipping subduction slab. Indeed, a re-examination of the available geochemical and geochronological data in the light of our new results documents that arc-magma production moved progressively from north (Trégor unit) to south (Saint-Brieuc unit) over time, in the interval 605-580 Ma.

Geochemical characteristic of rocks and isotope dating of zircon from the andesite dyke of the rudyanskaya sequence (D2 rd) of the Middle Urals Eastern zone

Relevance. In the basin of the r. Pishma subvolcanic formations, localized in the fields of spreading of the Middle Devonian Rudyanskaya sequence, are presented by different on composition small bodies, sills and dykes, forming with volcanogenic rocks of the Rudyanskaya sequence a basalt-andesite-rhyolite complex, the formation of which is associated with volcanic activity, that took place in Middle Devonian in the environments close to the modern island arcs. Their isotope dating and geochemical characteristic are rather relevant for specification of the composition, volume and paleogeodynamic environment of the vulcanite formation. In the paper new data are presented on geochemistry of andesites of the dyke from the Diviy Kamen rock, considered as part of the Rudyanskaya sequence, as well as the age of zircon from them. Methods. Chemical analyses of vulcanites have been performed in the Centre of collective usage “Geoanalyst” of the Institute of Geology and Geochemistry of the UB of RAS (Ekaterinburg) by X-ray fluorescent method and ICP–MS method. Zircons were studied with scanning electron microscope JSM-6390L of the Jeol firm and with electrone-probe microanalyser Cameca SX100. The data on the U and Pb isotopes, as well as on elements-impurities in zircon grains were obtained by mass-spectrometry method with inductively coupled plasma with laser ablation (LA–ICP–MS) at the quadrupole ISP–MS NexION 300S with the attachment for laser ablation LA NWR 213. Purpose of researches. Getting new data on geochemical composition and age of the andesites of dyke in the section along the r. Pishma. Results. The andesite dyke intersects medium clastic unstratified tuffs and dipply occurring fragment of the lava flow of porphyry dacite. Andesites present themselves porphyry rocks with 20–30% of plagioclase phenocrysts. At the normalized many-component diagram the Ta–Nb minimum is displayed. For zircon grains are characteristic differentiated spectra, demonstrating the growth of compositions from light to heavy REE, positive Ce and negative Eu anomalies, typical of the zircon of magmatic genesis. For the first time geochronological study of zircons from andesites has been maded by method of LA–ICP–MS. 4 age groups of zircons, Ma: 390–387, 369–362, 337–345, 327–329 were revealed. Conclusions. The data obtained testify that andesites were formed under suprasubduction environments, and magmatic stage of rock formation corresponds to the upper Devonian varieties of zircons. Early Carboniferous datings can be associated with the thermal impact caused by the intrusion of a later dolerite dyke.

Enrichment of rare earth elements in epigenetic dolomite occurring in contact metamorphosed Witbank coals (South Africa)

The emplacement of igneous intrusions into coal alters both the organic and inorganic components, and may result in the addition of specific components, including rare earth elements (REEs), into the country rock. Previous studies on the intruded coals of the Main Karoo Basin (South Africa) have mainly focused on understanding changes to their organic fraction despite the likelihood that the inorganic composition is also affected, as demonstrated by others studying intruded coals elsewhere in the world. This study reports on the effects of a 0.4 m thick dolerite dyke on the mineralogy and geochemistry of Witbank coals, South Africa. Coal samples were collected at 0.25 m intervals on both sides of the dyke (west and east); in addition to the intruded coals, an unaltered coal from the same seam and a sample of the dolerite were included. The samples were assessed using petrography, X-Ray Diffraction, X-Ray Fluorescence, Scanning Electron Microscopy, and Inductively Coupled Plasma Mass Spectrometry following sequential leaching, focusing on REEs for the latter analysis. Inertinite is the main maceral group in the coal samples; as a result, the impact of the intrusion on the organic fraction is compared using mean random total reflectance (%RoTmr). Total reflectance increases from 1.65 %RoTmr (for the unaltered coal) to 4.17 %RoTmr closer to the dyke on the western side. In contrast, the eastern side reports lower values (max. 1.94 %RoTmr). Similar to reflectance, dolomite is higher in samples from the western side of the dyke. Dolomite is also found in the unaltered coal (4 wt%) and the dolerite sample (15 wt%). Syngenetic dolomite and a weak positive Gd anomaly for the unaltered coal (ΣREE = 83.7 μg/g) suggest the presence of acidic groundwaters during peatification. Subsequently, the coals were also affected by hydrothermal activity, related to the intrusion of the dolerite dyke. For the intruded coals, dolomite (fracture-infilling) and ΣREEs are higher on the western side, at a maximum of 24 wt% and 153.0 μg/g, respectively. Carbonate associated REEs are higher for the intruded coals than the total for the dolerite (ΣREE = 23.3 μg/g). This reflects the incompatibility of REEs with early-formed minerals in the dolerite, concentrating them in the residual melt. Increased reflectance and dolomite (and associated REEs) in coals from the western side suggest pronounced hydrothermal activity, related to the availability of accommodation space (i.e., fractures).

Subduction of solidifying melt lens beneath ultraslow-spreading ridges revealed by (Grt-)amphibolites and dolerites in the Xiugugabu ophiolitic mélange, southwestern Tibet

Ridge subduction is an inevitable process during the evolution history of a vanished ocean. Due to its special characteristics, such as buoyant density, high heat fluxes and active magmatism, subduction of a ridge segment would leave specific fingerprints of high geothermal gradients and unique metamorphism. Therefore, investigations on ridge subduction could provide invaluable details of the evolution of oceanic lithosphere, and also help to understand the emplacement and preservation mechanism of worldwide ophiolites. This study mainly focuses on the mafic blocks of garnet-(Grt-)free and Grt-bearing amphibolites, foliated dolerites and dolerite dykes intruding into other blocks outcropped in the Xiugugabu ophiolitic mélanges of the west segment of the Yarlung-Tsangpo Suture Zone (YTSZ). On the basis of the detailed field, petrographical and geochemical investigations, combined with previous research on the other YTSZ ophiolites, this study highlights that the (grt-)amphibolites in the ophiolitic mélange are metamorphosed products under conditions of high geothermal gradients with protoliths to be subducted cumulate layered gabbros, which were likely to be newly crystallized in solidifying melt lenses beneath ridges. Hence, the generation of the XGGB ophiolite was contemporary with Neo-Tethyan ridge subduction. Intruding dolerite dykes show identical UPb ages to their intruded metamorphic soles, indicating that these amphibolites had gone through extremely rapid exhumation after peak conditions. Finally, scenarios of ridge destruction and subduction were proposed to complete the evolution history of the Neo-Tethyan Ocean and to provide an explanation for ophiolite emplacement and extremely rapid exhumation of metamorphic soles.

Petrogenesis and geochemistry of WNW-ESE to NW-SE trending doleritic dykes of the Paleoproterozoic Liptako basement (West African Craton, West Niger)

Doleritic dykes with regional orientations WNW-ESE to NW-SE, cut the Paleoproterozoic Birimian formations of the Liptako basement (Ouest-Niger). Mineralogical compositions studied from these doleritic dykes consist of augite (Wo21-40-En24-45-Fs13-54), pigeonite (Wo8-12-En45-59-Fs31-41), plagioclase, alkali feldspars, titano-magnetite, ilmenite, edenite, apatite, biotite and chlorite. The geochemical affinity of the doleritic dykes in the two trends is similar to oceanic plateau basalts (OPB), with high iron content relative to Mg. Negative Nb, Ti and Ta anomalies and the Y/Nb ratio (1.59–3.15) indicate a continental tholeiitic character for while light rare earth element enrichment, low to moderate Ti contents and the Th/Yb (0.28–0.59) and Nb/Yb (3.25–7.46) ratios suggest sources similar to E-MORB. Their Th/Nb (0.06–0.10), Th/Yb (0.28–0.59), TiO2/Yb (0.54–0.82) ratios suggest that the magma was generated by partial melting (5–17%) of a residual spinel-garnet lherzolite, that could have been part of a mantle plume. Relatively low Mg# (36–51) reflect some degrees of fractional crystallization, with some of this differentiation recorded between the margins and interiors of dykes. This differentiation was accompanied by unusually low degrees of crustal contamination, as indicated by relatively low Th/Ta ratios (1.18–2.03). This petrological and geochemical information provides new constraints on the origin of these WNW to NE trending mafic dykes of the West African Craton.

Hydrological Evaluation of the Groundwater Potential in the Fractured Karoo Aquifer Using Magnetic and Electrical Resistivity Methods: Case Study of the Balfour Formation, Alice, South Africa

The study is aimed at evaluating the groundwater accumulations present in Alice using magnetic and electrical resistivity measurements to examine the trends of structural elements and characterize the groundwater resource for borehole drilling. The magnetic maps show a low magnetic linear structure moving northwest to southeast direction, which may be caused by fractures. The linear high intensities were probably caused by dolerite dykes, while dolerite sills caused broader high-intensity areas. The depth slices show that the near-surface magnetic structures are visible to a depth of about 19 m, and the deep-seated structures are found at a depth of about 31 m, possibly deeper. Twenty-five vertical electrical soundings (VES) of the Schlumberger array were measured with AB/2 varying between 1.5 m and 250 m across the study area. The VES interpretation showed four geoelectric layers composed of HK and HA curve types. The geoelectric layer’s thicknesses are (1) topsoil from 0.4 to 1.8 m, (2) weathered layer from 0.8 to 17.5 m, and (3) weathered/fractured layer from 9.9 to 143.9 m; the third layer could be the productive water-bearing zones, and (4) bedrock layer has an infinite thickness. The layers have resistivity values of 20-5752 Ωm, 3-51 Ωm, 136-352 Ωm, and 44-60428 Ωm, respectively. A correlation of the VES with the borehole log indicated a well-matched result. The magnetic and electrical resistivity surveys provided a detailed subsurface structure and helped identify possible fractures that could act as a passage for groundwater.

Paleogeography of India at ∼1.8 Ga: New constraints from baddeleyite geochronology and paleomagnetism of mafic dykes from the Dharwar craton

The paleogeographic position of Dharwar craton/India within Paleoproterozoic supercontinent configurations has been well demonstrated using highly reliable key paleopoles from ∼2.37 Ga to ∼1.86 Ga mafic events in recent studies. Here, we report a paleomagnetic pole at ∼1.8 Ga along with a precise Pb-Pb baddeleyite age. The emplacement age of an NW-SE trending dolerite dyke yields a weighted mean age of 1799 ± 0.8 Ma (MSWD = 0.7), which coincides with earlier reported U-Pb baddeleyite age (1794 ± 7 Ma) and confirms the existence of significant emplacement of Pebbair dyke swarm at ∼1.8 Ga. Paleomagnetic results from eleven sites shows mean D = 306°, I = 31°, α95 = 10° (k = 23), and corresponding paleopole is calculated at 38°N, 357°E (dp = 6.1° and dm = 11°). These remanent directions are carried by the pseudo-single domain (PSD) magnetite. This pole passes the paleosecular variation test of the geomagnetic field and fulfills the reliability criteria (R = 5) of Meert et al. (2020), so, suggested to be a key pole for Dharwar craton/ India (∼1.8 Ga). The paleopositon of Dharwar craton is occupied at shallow paleolatitude and drifted towards the north with a minimum velocity (∼5.7 cm/yr) from ∼1.86 Ga to ∼1.80 Ga. The paleogeographic reconstruction model at ∼1.8 Ga has been attempted with available coeval paleopoles from other global cratons. The Dharwar craton (South India block), the Amazonia craton, and the Baltica craton have assembled at ∼1.86 Ga, and North Australia, Sao Francisco, Rio de la Plata cratons came in close to the South India block, Amazonia, and Baltica assembly at ∼1.80 Ga.

Mapping subsurface geological structures in the Birimian Supergroup, Ghana using airborne magnetic and radiometric data: Implications for gold exploration

Rarity of outcrops is a typical feature of considerable parts of the Birimian Supergroup in Ghana as a result of deep weathering which leads to the formation of thick surficial cover and laterization. This hampers the precise mapping of lithological boundaries and mafic dykes. These boundaries are tectonic contacts that may have the potential to host orogenic gold deposits. This study aims to map these subsurface geological features using regional airborne geophysical data. Derivatives of magnetic data such as analytic signal, vertical derivative, horizontal gradient and the ternary map of the radiometric data were employed to interpret subsurface geology. The general correlation of the magnetic and radiometric maps with the existing geological map confirms their suitability as geological mapping tools. Tectonostratigraphic contacts of several geological domains within the Birimian Supergroup were delineated. These zones host world-class gold deposits and are potential corridors to focus exploration activities to ensure efficient fund utilisation and minimisation of exploration risks. Additionally, SSE-NNW, WSW-ENE and WNW-ESE trending dolerite dyke swarms were outlined.

Combined bulk-rock Lu-Hf and Sm-Nd isotopic study of Archean granitoids and mafic rocks from Sangmelima terranes (Ntem Complex, south Cameroon): Geodynamic implications

Hafnium and neodymium isotopes provide robust tool tracers for tracking the mechanisms of crustal generation and differentiation, from mantle extraction to the late geological processes undergone by the studied rocks. Though these two isotope systems commonly behave similarly and define a positive correlation between 143Nd/144Nd and 176Hf/177Hf referred to as the “terrestrial array”, they may differ in terrains having a complex geological history, such as Archean cratons. This study presents the first combined whole-rock Hf and Nd isotopic analyses of Archean mafic and felsic igneous rocks from the Sangmelima terranes in the Ntem Complex (located in the Northwest of Congo Craton (NW CC) in southern Cameroon). It mainly consists of Mesoarchean charnockites and tonalite-trondhjemite-granodiorite (TTG) suites, greenstone belts, mafic enclaves or dykes, and potassic granitoids. Mesoarchean charnockite and TTG suites have subchondritic initial 176Hf/177Hf and 143Nd/144Nd ratios with a mean initial (suffix (i)) ε176Hf(i) and ɛ143Nd(i) of –3.30 ± 1.40 and –3.85 ± 0.65 (2SD) respectively, implying the involvement of Eo- to Paleoarchean components in their petrogenesis. The bulk rock initial Hf and Nd isotope compositions of the gabbro and gabbronorites are suprachondritic (ε176Hf(i) = +3.4 to + 8.2; ε143Nd(i) = +0.9 to + 1.6), indicate that they were derived from a depleted mantle source. Coupled with pre-existing data from the literature, these new Hf-Nd isotopic data indicate that two episodes of mantle-derived mafic magmatism in the Ntem Complex occurred during the Mesoarchean period (i.e., at ∼ 3.1 and 2.86 billion years (Ga)). Neoarchean dolerite dykes show broadly chondritic to supra-chondritic ε176Hf(i) and ε143Nd(i) values of –0.28 to + 4.53 and TCHUR ages of 2.74 – 2.76 Ga, suggesting the derivation of doleritic magma from a depleted mantle source with either none or limited crustal residence time. Considering other Archean domains of the NW CC, there were multi-stage magmatism events and crustal growth events occurred at ∼3.75 – 3.31 Ga, ∼3.26 – 3.0 Ga, ∼2.92 – 2.85 Ga, and ∼2.75 – 2.72 Ga from Eoarchean to Neoarchean in the NW Congo Craton.

Petrographic, mineralogical and microstructural studies in the mafic dyke occurred in the area of Mettur, Salem District, Tamil Nadu, India

A mafic dyke is a mass of rock generated in either sedimentary or igneous terrain and intrudes in pre-existing cracks. It was common to see many enormous dyke swarms in the southern granulitic landscape made up of archean rocks. Dykes tend to be younger than the surrounding rock body. Generally, mafic dolerite dyke has an ophitic texture and is medium to fine grained. In the study area country rock and dolerite dyke rocks petrographical microscope, XRD, and FE-SEM techniques were analysed. Plagioclase feldspar, augite, biotite, hornblende, and quartz minerals presented in the study area rocks. Alternating wedges, augite/labradorite grain boundaries are shown in the microstructure as a white rectangle. Two augite wedges were observed with rounded ends. The mineral percentage of quartz is rich in dolerite rock because of its intergrowth texture and low cooling temperature and pressure. So, the magma crystallized rapidly on the surface thus, the crystallizing temperature of the quartz is low. Thus, the rock mineral shows a high percentage of quartz. The rock is quartz-dolerite.