Granodioritic Rocks Research Articles

Toeslope ferricretes in the tropical rainforest of southern Cameroon: Internal properties, mechanisms and conditions of genesis

The formation of toeslope ferricretes under humid tropical rainforest is common. However, no specific work has been devoted to the toeslope ferricretes to determine the mechanism of formation and the exact time. The study pointed out the internal properties (petrology, mineralogy and geochemistry) of southern Cameroon toeslope ferricretes, their spatial distribution in relation to the landscape morphology, and the mechanisms and conditions of their genesis. Fieldwork included the study of the soil properties and patterns, as well as of their geometric relationships. Laboratory analysis included X-ray powder diffraction (XRD), scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), X-ray fluorescence spectrometry (XRF) and inductively coupled plasma (ICP-AES). The main results revealed that, at the bottom of the studied toeslope soil profiles, iron oxides (Al-goethite and hematite) accumulate in situ, starting from domains in which the parental granodiorite rock structure is still preserved. This relative accumulation is relayed towards the top of the profiles by an absolute accumulation of Fe from the upslope ferricrete dismantling. New ferruginous accumulation patterns occur in varying degrees of rubification and show various degree of hardness from soft soil patterns to carapace, as precursor of two types of toeslope ferricrete genesis: spotted massive carapace (with a continuous framework) on the moderate slope and variegated vesicular carapace (ferruginous network with vacuoles) on the gentle slope. Ferricretes on the moderate slope are thick, with limited lateral extension and light induration, while those on gentle slope are thinner, but more hardened and more extended on the slope. Such processes are clearly linked to the present-day humid climate with flattening of landscapes and rising of groundwater table.

Semiautomated Statistical Discontinuity Analyses from Scanline Data of Fractured Rock Masses

Quantitative statistical discontinuity analysis of fractured rock masses is commonly applied in the fields of engineering geology, rock mechanics, slope stability, and hydrogeology. This study presents a workflow for the semiautomatic determination of basic discontinuity parameters, such as spacing, frequency, trace length, and termination, from scanline surveys written for the open-source software Octave. The aim is to provide theoretical background information and scripts for a quick introduction to all interested parties from academia and consulting, in order to promote the use of widely known and accepted statistical evaluation methods. Data from a study site in granodioritic rock are analyzed in the context of the proposed workflow. These test data and all scripts (m-files) used in the study are provided in order to minimize initial training time. The scripts provided herein are kept short and simple, but can be used as a basis for advanced automation of the workflow and presentation of the results.

Processing of micro-CT images of granodiorite rock samples using convolutional neural networks (CNN), Part I: Super-resolution enhancement using a 3D CNN

X-ray micro-computed tomography (micro-CT) is widely used for three-dimensional analysis of many rock types. However, the practical implementation of this method for micro-porous samples requires a compromise between the resolution of the images and the obtainable field of view (FOV). Generally, resolution enhancement results in a reduction of the FOV. The generation of high-quality micro-CT images is an expensive and time consuming task due to the competing requirements of a large FOV and fine resolution. To alleviate this, super-resolution processing, based on deep learning, is proposed to improve the quality of low-resolution images that can obtain a large FOV. In this research, a super-resolution technique employing the three-dimensional U-Net convolutional neural network (CNN) architecture was applied to enhance the resolution of granodiorite rock sample images. This was undertaken using two sets of micro-CT image triplexes, where the first triplex contained 3-, 6-, and 12-micron resolution sets, and the second triplex contained 1-, 2-, and 4-micron resolution sets. For each triplex, 80% of the images were used for training the neural network with the remaining 20% used for validation. Further validation was performed by comparing the processed results to images obtained from scanning electron microscopy (SEM). It was observed that super-resolution processing can significantly improve the low-resolution micro-CT image quality without physically reducing the sample size typically required for high-resolution scanning. It is expected that this technique could assist practitioners reveal features absent in small samples (e.g. large fractures and or rock textures). Furthermore, images restored through super-resolution processing maintain the FOV of the lower resolution scan, a task that would be comparatively expensive and time consuming to acquire in a high-resolution scan. The workflow proposed in this study has a significant impact on a range of fields including the numerical prediction of rock permeability, and segmentation for advanced mineral analysis.

Finite element simulation of permeable fault influence on a medium deep borehole thermal energy storage system

Solutions for seasonal energy storage systems are essential for the reliable use of fluctuating renewable energy sources. As part of the research project SKEWS, a medium deep borehole thermal energy storage system with a depth of 750 m is under construction at Campus Lichtwiese in Darmstadt, Germany, to demonstrate this innovative technology. Prior to the design of SKEWS, the geological context in the surroundings of the project location was investigated using archive drilling data and groundwater measurements. The geologic survey suggests the assumption that the uppermost part of the intended storage domain is crosscut by a normal fault, which displaces the Permian rocks east of Darmstadt against granodioritic rocks of the Odenwald crystalline complex. A 3D finite-element numerical model was implemented to estimate the effect of the potentially higher hydraulic conductivity of the fault zone on the planned storage system. For this purpose, a storage operation over a time span of 30 years was simulated for different parametrizations of the fault zone. The simulations reveal a limited but visible heat removal from the storage region with increasing groundwater flow in the fault zone. However, the section of the borehole thermal energy storage system affected by the fault is minor compared to the total depth of the system. This only constitutes a minor impairment of the storage efficiency of approximately 3%. In total, the amount of heat extracted varies between 320.2 GWh and 326.2 GWh for the different models. These findings can be helpful for the planning and assessment of future medium deep borehole thermal energy storage systems in fractured and faulted crystalline settings by providing data about the potential impact of faults or large fractures crosscutting the storage system.

Late cretaceous magmatism in the Northern Lhasa subterrane, central Tibet: geodynamic implications

ABSTRACT Compositional changes of successively emplaced magmatites can be used to infer changes in their petrogenetic evolution and geodynamic setting. The central-northern Lhasa terrane hosts extensive Late Cretaceous magmatism, however, the mechanism that triggered the magmatism and its relationship to the directly preceding Late Cretaceous tectonic transition in the central Tibetan Plateau is still unresolved. Therefore, we conducted a detailed petrological, geochronological, geochemical, and isotopic study on recently identified Late Cretaceous doleritic and granodioritic rocks in the Xiongmei area, northern Lhasa subterrane. The potassic (K2O/Na2O ratios >1) dolerites (ca. 80 Ma) display a post-collisional basaltic shoshonitic signature with uniform moderately enriched whole-rock Sr–Nd and zircon–Hf isotopic compositions ((87Sr/86Sr)t = 0.7070 to 0.7095, whole-rock εNd(t) = – 2.4 to – 1.3, and zircon εHf(t) = – 5.9 to +1.9), suggesting that they evolved from a similar parental magma and were likely derived by partial melting of an enriched lithospheric mantle metasomatized by hydrous fluids. The granodiorites are geochemically divided into medium-K and high-K calc-alkaline rocks. The medium-K samples (ca. 80 Ma) are generally characterized by low K2O/Na2O ratios <1, whereas the high-K samples (78–76 Ma) display high K2O/Na2O ratios >1. The medium-K samples have positive zircon εHf(t) values from +4.8 to +6.5 and uniform whole-rock ɛNd(t) values from – 2.4 to – 1.3, which significantly differ from those of the high-K samples that have negative to positive zircon εHf(t) values from – 1.9 to +2.8 and negative whole-rock ɛNd(t) values from – 8.4 to – 5.1. The medium-K samples are characterized by high Sr contents and Sr/Y ratios, coupled with low Y and HREE contents, similar to adakitic compositions, and were interpreted to have been derived from deep partial melting of the foundering juvenile garnet-bearing mafic lower crust at relatively low temperatures. Compared to the medium-K samples, the high-K samples have lower Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and distinct negative Eu, Sr and Ti anomalies and are most likely derived from shallower partial melting of the K-rich juvenile lower crust at relatively high temperatures. Considering the occurrence of variable magmatism in the central-northern Lhasa subterrane during the Late Cretaceous, the change in magma composition and lower crustal melting conditions (cold-deep vs. hot-shallow) suggest that the lithospheric mantle keel was partially detached by small-scale mantle convection in response to the low-angle Neo-Tethys-associated subduction between 85 and 70 Ma.

Pore system evolution in arenaceous regoliths - Case study from the Sila Massif (southern Italy)

Porous and permeable crystalline rocks have the potential of being hydrocarbon reservoirs. In many cases, reservoir properties are created by rock-altering physico-chemical processes occurring in the near-surface environment. Hydrocarbon reservoirs in altered granitoids may comprise both arenaceous (clay-poor) and argillaceous (clay-rich) palaeoregoliths, and hence outcrop analogues from various environments are required. In the Sila Massif, southern Italy, the arenaceous weathering of medium to coarse grained granodioritic rocks has been studied. The pore system, dominated by microfractures, was described and quantified at each stage in the weathering process (altered coherent rock facies → saprock facies→ lower and upper saprolite facies → soil) in relation to changes in chemical and physical properties. The transformation of rock to saprolite was close to an isochemical process, as demonstrated by the low variations in the bulk mass transfer coefficient (τbulk). At the same time, trans-, intra- and intermineral microfractures increased in frequency and aperture upwards through the regolith profile. It was the main reason for the reduction of bulk densities (ρb) from 2.66 g/cm3 in the altered coherent rock to 1.88 g/cm3 in the saprolite, corresponding to the formation of up to 30% porosity (φc). The results suggest that a volume increase of >50%, represented by the volumetric strain factor (ε), was the main cause for the transformation of solid crystalline rock to porous saprolite. Anisovolumetric arenaceous regoliths with minimal mass loss, are the result of one of the major weathering processes, notwithstanding that they are much less understood than isovolumetric, anisochemical, argillaceous regoliths. Arenaceous saprolites potentially can have excellent porosity and permeability properties, with open and well-connected microfractures. Reservoir quality of arenaceous saprolites is in stark contrast to the poor permeability observed in some argillaceous saprolites where porosity is dominated by isolated dissolution voids within a clay-rich matrix.

Fe isotopic evidence that “high pressure” TTGs formed at low pressure

Tonalite–trondhjemite–granodiorite (TTG) rocks consitute the dominant felsic lithology of the Archean crust in most cratons, but their petrogenesis and geodynamic setting are still matters of great debate. Here, we report new Fe isotopic analyses for ca. 2.5 Ga diorites and TTGs with compositions typically interpreted to reflect a high-pressure affinity in eastern Hebei Province of the North China craton. Whole-rock Fe3+/∑Fe and δ56Fe of the trondhjemites show strong correlations with MgO and Fe2O3T, both indices of igneous differentiation, supporting the radical reinterpretation that the apparent “high pressure” signatures in the trondhjemites instead formed by amphibole-dominated fractionation at low pressure. Likewise, for the tonalites, their light Fe isotopes make them unlikely to be the melting products of enriched mafic rocks at high pressure within the garnet stability range. Thus, Fe isotopes support that “high pressure” TTGs in verity formed at lower pressure with little involvement of residual garnets. Our results thus challenge the evidence once purported to infer Archean subduction.

Mineralogy of a rare Ediacaran epidote‐bearing trondhjemitic complex from the Nubian Shield: Insights into P‐T conditions of magma emplacement

Igneous epidote occurring in tonalite–trondhjemite–granodiorite (TTG) rocks provides valuable insight into the physiochemical parameters of their magmatic system. Epidote has been identified within a newly discovered, rare, Ediacaran, high‐Al TTG‐suite at the Fannani Igneous Complex (FIC), Nubian Shield. The objective is to assess P–T conditions of magma emplacement using petrographic‐, mineral chemical data, and thermobarometry methods. The TTG‐suite is made up of ubiquitous oligoclase, variable contents of quartz, K‐feldspar, amphibole, biotite, epidote, accessory titanite, zircon, apatite and magnetite. Feldspars vary from An14 to An26 in plagioclase, with K‐feldspar having near‐end‐member compositions (Or91–Or95). Amphiboles are Al‐rich (AlT = 1.65–2.04 atoms per formula unit [apfu]), calcic (Ca = 1.83–1.93, apfu), with average Mg/(Fe + Mg) of 0.44, and are ferroedenite–edenite–ferropargasite. Biotites are moderately Fe‐rich (Fe/(Fe + Mg) = 0.50–0.58) characteristic of type ‘C’–biotite of calc–alkaline orogenic rocks. Epidote exhibits pistacite components [Ps = Fe3+/(Fe3++Al)] of Ps24–Ps33, typical of magmatic epidote. The average crystallization pressure of 5.41 kbar (calculated using the Al‐in‐Hb barometer) is consistent with the presence of magmatic epidote and reflects the mesozonal crustal levels of emplacement. The Hb–Plag–geothermometer produced an average crystallization temperature of 721°C. These values fall within experimentally‐determined P–T ranges of stability of magmatic epidote with fO2 buffered from nickel‐nickel‐oxide (NNO) to hematite (HM). The FIC rocks were developed from a wet, subsolvus, orogenic, Al‐rich magmatic system. Findings reported here could have implications for conditions of magma emplacement and mineralogical characterization of TTGs occurring within various continental masses.

On the Physical and Mechanical Responses of Egyptian Granodiorite after High-Temperature Treatments

In the design and stability of thermal engineering applications, a thorough understanding of the evolution of damage in the rock following high-temperature treatments is crucial. Hence, this study investigates the influence of high temperatures on Egyptian granodiorite rock properties, given its widespread use as ornamental stones and aggregate material for roadways. Temperature effects up to 800 °C on its physical and mechanical responses were examined in conjunction with microstructure alterations. The results show that the density of granodiorite decreases after heat exposure due to a gain in volume and a loss in mass, with volume expansion being the most important component. In addition, the uniaxial compressive strength increases up to 400 °C before reducing linearly as the temperature increases, while the elastic modulus and P-wave velocity show a reducing trend with the temperature. This study suggests that granodiorite has a thermal damage threshold of 400 °C, beyond which its microstructure and physical and mechanical characteristics deteriorate, and granodiorite becomes less brittle and more ductile. Hence, at the mutation range (between 400 and 600 °C), the physical and mechanical responses shift from a stable to an unstable state. As a result, the microstructure of the granodiorite samples was destroyed at 800 °C, resulting in a significant drop in compressive strength and dilemmas in measuring the P-wave and elastic modulus. Accordingly, the findings of this study can be used to aid in the safe handling of this rock in high-temperature conditions.

Comparative analysis of exploration potential within the Urumieh Dokhtar Magmatic Arc, Iran, with a detailed example from mineral chemistry of the Marshenan intrusion

The Early Miocene Marshenan intrusion, in the central part of the Urumieh-Dokhtar magmatic arc (UDMA), central Iran, includes granodiorite-granite and diorite; it is a classic example of a CuAu barren intrusive system. Zircon U-Pb dating indicates that the diorite display an age of 20.32 ± 0.36 Ma, coeval with granodiorite rocks (20.5 ± 0.8 Ma). These rocks are composed of feldspar, quartz, amphibole, biotite, titanite, and magnetite. In the granodiorites, the plagioclase composition ranges from oligoclase to bytownite, the amphiboles are magnesio-hornblende and biotites is Mg rich, related to calc-alkaline orogenic suites in the region. Plagioclase phenocrysts exhibits oscillatory zoning and marked changes in the abundance of elements, such as Ba, Sr, and Fe, suggesting magma mixing/mingling may have had a role in generating these parental magmas. The average calculated P–T conditions of the granodiorite and diorite rocks are about 730 °C and 2.1 kbar and 733 °C and 1.7 kbar, respectively, corresponding to near solidus conditions equal to emplacement depths of ~7 to 8 km. Magmatic H2O contents and ƒO2 calculated from crystallized amphiboles indicate that the Marshenan granodiorite had initial magmatic H2O contents ~5 wt% and relatively high ƒO2 (ΔNNO; ave. 1.3) and the diorite had initial magmatic H2O contents ~4.7 wt% and relatively high ƒO2 (ΔNNO; ave. 1.5), both consistent with the presence of phases, such as hornblende, biotite, magnetite, and titanite. In comparison with other intrusions in the UDMA, the Marshenan intrusion formed via the same physico-chemical mechanisms like other barren intrusions, whereas the fertile intrusions exhibit slightly higher temperatures, pressures, ƒO2, and H2O values than the barren intrusions. These compiled data suggest that, in spite of the high magmatic H2O and ƒO2 contents, the Marshenan and other barren intrusions in the UDMA will not produce porphyry Cu mineralization, unlike the giant Kerman deposit, probably due to magma source, magmatic evolution processes, timing of volatile exsolution, pre-existing crustal-scale fractures, coeval volcanism, and extended duration of volatile saturated crystallization to subsolidus conditions.

Diffusion and Sorption Studies of Cs, Sr and Co in Intact Crystalline Rock

Three cationic tracers, Sr2+, Co2+ and Cs+ were tested with a modified electromigration device by applying 2V, 3V and 4V voltage gradients over an intact Grimsel granodiorite rock sample. An ideal plug-flow model and an advection-dispersion model were applied to analyze the breakthrough curves. Matrix characterization by C-14-PMMA autoradiography and scanning electron microscopy showed that in the centimeter scale of Grimsel granodiorite rock, the interconnected matrix porosity forms a well-connected network for diffusion. Micrometer-scale fissures are transecting biotite and chlorite minerals, indicating sorption of the studied cations. The ideal plug-flow model indicated that the effective diffusion coefficients (De values) for Sr2+, Co2+ and Cs+ tracer ions within the Grimsel granodiorite rock were 3.20 × 10−13 m2/s, 1.23 × 10−13 m2/s and 2.25 × 10−12 m2/s, respectively. De values were also derived from the advection-dispersion model, from which 2.86 × 10−13 m2/s, 1.35 × 10−13 m2/s and 2.26 × 10−12 m2/s were calculated. The diffusion speed for the tracers was in the sequence of Cs+ &gt; Sr2+ &gt; Co2+ that is in the same sequence as their diffusion in diluted water. The distribution coefficients (Kd values) calculated from the models covered the range of two magnitudes (from 10−7 m3/kg to 10−5 m3/kg). The result indicated that the sorption process of the studied elements did not reach equilibrium during the electromigration process, mainly due to the too much acceleration of the migration speed by the voltage gradients applied over the rock sample.

Late Indosinian granitoids in the Zhacanggou, Guide Basin, NW China: Whole‐rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes, and implications for magma mixing

The Guide Basin, located in the West Qinling orogen of central China, is a typical geothermal field. However, the granodiorites occurring in the Zhacanggou area have been poorly restricted. Herein, we report new isotopic compositions of whole‐rock major‐trace elements, zircon U–Pb, and Lu–Hf, on the host granodiorites and their microgranular enclaves (MEs) in the Zhacanggou area. Zircon U–Pb dating for the host rocks (porphyritic granodiorite and granodiorite) revealed ages of 230.5 ± 1.4 Ma and 229.0 ± 1.6 Ma, similar to those (230.5 ± 1.5 Ma and 231.4 ± 1.4 Ma) of the MEs. Zircon Hf isotope analyses for the granodiorite and MEs yielded εHf(t) values from −11.0 to −1.5 and from −7.8 to −0.7, with corresponding TDM2 ages of 1,355–1,959 and 1,304–1,755, respectively. The MEs possess plagioclase phenocrysts, acicular apatites, and chilled borders. They were characterized by relatively low SiO2 (58.39–65.24 wt%) and high Cr, Ni, and MgO contents, leading to relatively high Mg# values (44–59) and low Sr/Y (14.23–19.84) ratios. The host granodioritic rocks show similar adakitic geochemical characteristics with SiO2 = 64.23–68.57 wt%, MgO = 1.29–2.22 wt%, Mg# = 48–53, Sr = 340–596 ppm, Y = 11.3–13.7 ppm, Sr/Y = 28.39–47.89, and no obvious Eu anomalies. These geochemical and isotopic data, combined with the mineralogical features, indicate that the granodiorites in the Zhacanggou area were produced by the mixing of high‐Mg# melts originating from an enriched lithospheric mantle source, and felsic magmas deriving by the partial melting of the normal lower continental crust with the tectonic regime transition from a syn‐collision to a post‐collision setting.

Geochemistry of aeolian sand in the Taklamakan Desert and Horqin Sandy Land, northern China: Implications for weathering, recycling, and provenance

Geochemical studies examining surface aeolian sediments are significant for understanding Earth’s surface processes and determining sediment provenance in arid and semi-arid regions. However, there is limited research on the geochemical comparison of aeolian sediments formed under distinctive tectonic and climatic settings. Surficial sand samples from the Taklamakan Desert and Horqin Sandy Land in northern China were collected, and the major, trace, and rare earth elements (REEs) in the bulk samples, fine fractions (<75 µm) and coarse fractions (>75 µm) of sand were analyzed. This study aimed to explore the spatial variation in the geochemical composition of aeolian sand and its drivers. The results show that differences between the two regions are primarily caused by the intensity of chemical weathering, sedimentary recycling, and provenance. The mean chemical proxy of alteration (72.3 vs. 77.8) and the Parker's weathering values (37.8 vs. 55.8) both suggest the bulk samples from the Horqin Sandy Land experienced more intense chemical weathering, whereas physical weathering is a dominant process in the Taklamakan Desert. These differences may be attributed to the different climatic conditions in the two regions and the exposure time to weathering. A combination of the index of compositional variability and immobile trace element ratios shows that aeolian sand in the Horqin Sandy Land underwent more intense sedimentary recycling than sediments in the Taklamakan Desert. REEs ratios (such as Cr/V and Y/Ni) and the Al2O3-CaO*+Na2O-K2O (A-CN-K) diagram indicate aeolian sand in the Taklamakan Desert is probably derived from mixed source rocks of tonalite, granite, and granodiorite. Eu anomaly vs. (La/Yb)N diagrams suggest that the primary source-rocks contributing to the coarse and fine fractions in the Horqin Sandy Land are different but mainly derived from the Great Hinggan Mountains. Differences in the provenance of the fine- and coarse-grained fractions are primarily controlled by interactions between wind and fluvial systems. This study confirms that surface processes and sediment provenance in drylands of northern china have distinct spatial differences, implying that geochemical signatures in aeolian sand need to be interpreted in the specific climatic and tectonic contexts.

Quantifying the Porosity of Crystalline Rocks by In Situ and Laboratory Injection Methods

The porosity and pore geometry of rock samples from a coherent granodioritic rock body at the Grimsel Test Site in Switzerland was characterised by different methods using injection techniques. Results from in situ and laboratory techniques are compared by applying innovative in situ resin impregnation techniques as well as rock impregnation and mercury injection under laboratory conditions. In situ resin impregnation of the rock matrix shows an interconnected pore network throughout the rock body, consisting mainly of grain-boundary pores and solution pores in magmatic feldspar, providing an important reservoir for pore water and solutes, accessible by diffusion. Porosity and pore connectivity do not vary as a function of distance to brittle shear zones. In situ porosity was found to be about 0.3 vol.%, which is about half the porosity value that was determined based on rock samples in the laboratory. Samples that were dried and impregnated in the laboratory were affected by artefacts created since core recovery, and thus showed higher porosity values than samples impregnated under in situ conditions. The extrapolation of laboratory measurements to in situ conditions requires great care and may not be feasible in all cases.

Relation between intrusive and deformational processes in oblique subductive margins. The case of the zoned Flamenco pluton in northern Chile

Oblique convergent margins, like the subduction of Phoenix beneath the South American plate during Jurassic and Early Cretaceous times, are characterized by strain partitioning and a positive feedback loop between strike-slip deformation and magma ascent along the magmatic arc.Located in the Coastal batholith of northern Chile, the Flamenco pluton is one of the youngest Andean intrusives emplaced in the western active margin of South America. Besides an older SW domain of granodioritic rocks (c.a. 213 Ma), the NW and E domains of the Flamenco pluton were emplaced between 194 and 186 Ma. They present a normally zoned structure constituted by external gabbroic to Qtz-dioritic magmatic facies and tonalites and granodiorites located in inner areas of the intrusive body. These domains are separated by a central strip of stretched coarse-grained Crd-schists that presents ductile asymmetrical folding and S–C structures that point to the NW-directed displacement of the E domain of the pluton. This syn-emplacement shear zone shows kinematic compatibility and continuity to the north and south with folded and mylonitic metasediments out of the contact aureole of the pluton. Together, these segments constitute a large, steeply dipping sigmoidal structure of average N–S direction; the called here Chañaral transcurrent shear zone.Contemporary to the emplacement of the Flamenco pluton, slight variations in the trend of the crustal-scale structure generated strike-slip and transpressive sectors along the Chañaral shear zone, which favored the access of intruding magmas to the final emplacement level. As a paradigmatic example, the curviplanar Flamenco shear zone, an internal, magmatic branch of the main structure that traverses the E domain of the pluton, is defined by the sinistral and reverse shearing under magmatic conditions of the previously mingled mafic and felsic batches. Consequently, the transpressive Flamenco shear zone is interpreted as an ascent conduit where gabbroic and granodioritic liquids interacted during the building of the intrusive body. In addition, these sheared rocks were affected by late textural coarsening processes that evidence the slow and cyclical cooling of the growing magma reservoir. In contrast with the steeply dipping contacts and structures found to the east, the NW domain of the pluton shows sharp and gently dipping contacts between almost horizontal magmatic layers. We suggest that the western block of the Chañaral shear zone was a relatively passive footwall dominated by horizontal flow trajectories and lower replenishment rates according to the inverse emplacement sequence, i.e., late external mafic batches intruded along the margins of the felsic core. The variable structural arrangement of the crustal rocks that hosted the Flamenco pluton was the result of the complex interaction between far-field and local, magmatic forces during the emplacement process, besides the interference with pre-Andean structures.The presence of the Chañaral shear zone favoring the emplacement of the Flamenco pluton demonstrates that the Late Jurassic to latest Early Cretaceous Atacama Fault System had earlier precursors and both the magmatic arc axis and the transcurrent shear zones migrated landward during Jurassic times.

Field Occurrence, Petrography and Structural Characteristics of the Basement Rocks in the Northern Part of Kushaka and Birnin Gwari Schist Belts, Northwestern Nigeria

Field, studies and geological mapping on a scale of 1:50000 were carried out to determine the lithologic framework and structural features of the Basement Complex rocks in northern parts of the Kushaka and Birnin Gwari schist belts (Kushaka Sheet 122). The area is underlain predominantly by five main rock types mainly (i) Migmatite-Gneiss-Quartzite suite comprising dioritic, granodioritic and granitic gneisses with fissile and ferruginous quartzites and banded iron formations (BIF); (ii) Kushaka graphite and sulphur bearing biotite and muscovite quartz schist inter-banded in places with iron formations; (iii) Birnin Gwari biotite-staurolite quartz schist; iv) the Kushaka Gneiss Complex composed of basalts (which is being reported for the first time), staurolite and muscovite gneisses and banded iron formations (BIF), and (v) syn-tectonic and late-orogenic biotite-hornblende syenite (BHS) and biotite-hornblende granite (BHG) in the Kushaka schist belt and biotite muscovite granite (BMG) in the Birnin Gwari schist belt area. Petrographic studies have revealed that essential minerals are quartz, K-feldspars (orthoclase, microcline), plagioclase, pyroxene, epidote, hornblende, biotite and muscovite while the accessory minerals are titanite, zircon, apatite, iron oxide (magnetite and hematite). With pyroxenes occurring in the dioritic and granodioritic rocks, metamorphism may have locally reached grannulite facies. Imprints of Pan-African thermo-tectonic events have shown observable migmatization as the first thermo-tectonic event resulting in plastic deformation D 1 and regional S 1 foliation, demostrated by presence of tight isoclinals fold, compositional banding and N – S preferred orientation of mafic minerals. The D 2 deformation is co-axial with D 1 and resulted in the formation of decimeter sized F 2 isoclinal folds, B 2 boudins and eye ball structures that are parallel to S 1 plane schistocity. Strike-slip faults with dextral sense of movements were mapped in a number of places. D 3 deformation is concentrated in the Kushaka Gneiss Complex with near circular deep fractures south of the Kalangai fault. Here granitization and fragmentation of proto-mylonitic staurolite resulted in brittle deformation and F 3 open fold that refolded or transposed the earlier tight isoclinal F 2 folds. The D 4 deformation resulted in N-S and NW-SE quartz veins and pegmatite dykes which serve as channels for epigenetic gold-sulphide and rare metal bearing ore fluids. Keywords: Basement Complex, Pan-African, metamorphism, deformation, Kushaka, Birnin Gwari, Nigeria DOI: 10.7176/JNSR/12-12-02 Publication date: June 30 th 2021

Fluid source, element mobility and physicochemical conditions of porphyry-style hydrothermal alteration-mineralization at Mirkhani, Southern Chitral, Pakistan

The Cretaceous calc-alkaline granodioritic rocks in north-western Kohistan island arc host porphyry-type mineralization at Mirkhani in the Khyber Pakhtunkhwa province of Pakistan. In this paper, we present whole-rock geochemistry, mass-change calculations, and type and degree of element mobility regarding the hydrothermal mineralization-alteration processes. There are two distinct types of pyrite mineralization: (1) euhedral to subhedral pyrite (Py1) and cataclastic pyrite (Py2). To comprehend the source of hydrothermal fluids and their physicochemical conditions, trace element and S (sulfur) isotope compositions of the two types of pyrite were determined. Besides, the pyrite and silicate minerals were investigated through electron probe microanalysis (EPMA).The average low Ni (46), Zn (5), Cd (below detection level), and Bi (1) contents (in ppm), high Co/Ni ratio (>10) of both Py1 and Py2, and the range of sulfur isotope values for Py1 (−0.84‰ to 1.49‰) and Py2 (−0.93‰ to 1.84‰) indicate affiliation of the mineralization with felsic magma-derived hydrothermal fluids. The average low and high contents (ppm) of Se (0.7), As (2.6) and Zn (2), and Co (1089), respectively, in Py1 than Py2 (Se = 1.3, As = 4, Zn = 6 and Co = 747) indicates that former pyrite grains crystallized at a higher temperature than the latter, however, both types of pyrites were formed in medium to high-temperature conditions.The observed strong mobility of REE (rare earth elements) and HFSE (high field strength elements), high La/Yb ratios (avg. 23), and low (<1) Nb/La and Th/La ratios in the altered rocks reflect mobilization by slightly acidic solutions dominated by chloride and fluoride-rich complexes (±SO4 and OH). The gain-loss calculations reveal that SiO2, CaO, and TiO2 were leached out, whereas Al2O3, Fe2O3, MgO, P2O5, K2O, Cu, As, Mo, Ag, Au, Bi, and REE, HFSE, and LILE (large-ion lithophile elements) were variably added to the rocks during the hydrothermal process.Temperature calculation for chlorite formation and the type of gangue and ore mineral assemblage (epidote + sericite + albite + pyrite) implies a mineralization temperature range between ~ 215 °C and ~ 322 °C. The porphyry style of hydrothermal alteration-mineralization, trace element and S isotope composition of pyrite, and elemental gain and loss indicates that the mineralization at Mirkhani could be a part of a major porphyry Cu-Au system.

U - Pb geochronology and composition of zircon mineral from granodiorite in the G18 Gold mine, Quang Nam and its significance in regional tectonics

The Tam Ky - Phuoc Son suture zone (TPSZ) is located in central Indochina and is referred to as a amalgamation site between the Truong Son and Kon Tum terranes during the early Paleozoic. The amphibolite and ultramafic rocks within the region were considered as a part of the Tam Ky - Phuoc Son ophiolitic complex. In this study, the authors present results of the U - Pb dating and trace element composition of the zircon grains derived from a granodioritic sample collected in the G18 gold mine in Quang Nam province in order to clarify the timing of magma emplacement and tectonic setting. The U - Pb dating data indicates that the granodioritic rocks formed at 447,4±2,9 Ma while the U/Yb ratio is ̴1 (average: 1.32) and the Sc/Yb ratio is ̴ 1.04, high Hf content (Average Hf: 10937 ppm) and low Yb content (Average Yb: 308 ppm). These geochemical values are comparable with the zircon, which is formed in the continental magmatic arc. Combination with the pre - existing data allowed us to confirm the existence of two contrast magma members in the North Kontum massif: The Middle Cambrian island arc complex and the Middle Ordovician continental complex.

Geochemical, geochronological, and isotopic constraints for the Archean metamorphic rocks of the westernmost part of the Carajás Mineral Province, Amazonian Craton, Brazil

The Archean nucleus of the Amazonian Craton, represented by the Carajás Mineral Province (northern Brazil), crops out as a broad exposition of igneous, sedimentary, and metamorphic rocks showing a complex polyphase geologic history. A succession of orthogneisses (metagranitoids), amphibolites, and metavolcano-sedimentary rocks that show greenstone belt characteristics occur in Tancredo Neves Village district, northeast of São Félix do Xingu city (state of Pará), is the focus of this study. Detailed field, petrographic, and stratigraphic data reveal that these metamorphic rocks underwent two different tectono-metamorphic events: (i) the regional metamorphism (>2.8 Ga), which occurred under greenschist-to mid-upper amphibolite-facies conditions; and (ii) the dynamic metamorphism (>2.6 Ga) which reached amphibolite-facies conditions and overprinted previous metamorphic textures. Associated regional strike-slip NW–SE to EW-trending ductile shear zones generated mylonitic, ultra-mylonitic, and migmatitic zones. Whole-rock geochemistry discriminates four groups of metagranitoid protoliths. G1 group is composed of peraluminous trondhjemitic rocks samples, calc-alkaline, weakly fractionated, and enriched in HREE. G2 group comprises metaluminous and alkaline monzogranite samples with high Zr, Y, Nd, HREE, and intermediate fractionation. U–Pb (LA-MC-ICP-MS) dating of G2 zircons yields a crystallization age of 2745.6 ± 5.0 Ma. Groups G3 and G4 encompass peraluminous, calc-alkaline to alkali-calcic, anorogenic, fractionated monzogranitic and granodioritic rocks. G3 and G4 groups are further distinguished by high and low La/Yb(N) ratios, respectively. The results obtained for Tancredo Neves Village district very much resemble the Canaã dos Carajás (Carajás Mineral Province) area characteristics, comprising intense magmatism (2.7–3.0 Ga) and reworking related to at least two distinct metamorphic events in the Archean. As Canaã dos Carajás, Tancredo Neves Village can be a promising area for mining exploration.

Geochemistry and Petrogenesis of the Banded and Granitic Gneisses in Parts of Kushaka Schist Belt, Northwestern Nigeria

The Kushaka schist belt in the Kwona Mutua and Kushaka areas is one of the 12 well recognized N-S trending belts composed of igneous, metamorphic and metasedimentary rocks in varying proportion. The major rocks of the belt comprise the Migmatite-Gneiss-Quartzite Complex, schists and Pan-African granitoids. Banded and granite gneisses constitute the bulk of the Migmatite-Gneiss-Quartzite Complex. Banded gneiss consists of paleosome and leucosome of dioritic, tonalitic, granodioritic and granitic composition while granite gneiss is composed of biotite, staurolite–biotite and staurolite-muscovite gneiss. Major mineral constituents of these rocks are quartz, orthoclase, plagioclase, pyroxene, biotite, muscovite, orthoclase, microcline and staurolite while the accessory minerals are titanite, apatite and iron oxides. Metamorphism may have reached grannulite facies locally with pyroxenes crystallizing in the dioritic and granodioritic rock. Analysis of geochemical data reveals marked variation in the abundance of SiO 2 (60.77-77.53 wt %), Al 2 O 3 (12.9-15.99 wt %), Fe 2 O 3 (0.78-7.04 wt%), Na 2 O (1.67-5.15 wt%) and K 2 O (1.64-6.13 wt %), typical of rocks of hybrid sedimentary–igneous protoliths. Igneous protolith reveals decreasing P 2 O 5 content with increasing silica content, low K 2 O/ Na 2 O ratio (0.24 – 0.87), low K 2 O values (&lt; 2.5 wt %) displaying mixed tholeiitic and calc-alkaline, metaluminous and peraluminous, ferroan and magnesian character, and high-K calc-alkaline to shoshonitic affinities. By contrast, the rocks of sedimentary protolith shows high K 2 O/ Na 2 O ratio (1.19 – 2.5), high Na 2 O values (&gt; 3 wt %) and staurolite mineral. Their pelitic and mafic attributes were derived essentially from a quartz-diorite, granodiorite and granite-quartz monzonite source. Fractional crystallization and partial melting of older dioritic-granodioritic-tonalitic source rock derived from upper mantle materials contaminated by continental crust played important roles during their genesis. They are enriched in Large Ion Lithophile Elements (LILE) but depleted in Nb, P and Ti typical of volcanic arc and syn-collisional settings. Keywords : Banded gneiss, Pan-African, sedimentary, igneous, granodiorite, protolith, Kushaka, Nigeria DOI: 10.7176/JEES/11-4-05 Publication date: April 30 th 2021