Gneiss Complex Research Articles

Controls on Groundwater Fluoride Contamination in Eastern Parts of India: Insights from Unsaturated Zone Fluoride Profiles and AI-Based Modeling

Groundwater fluoride (F) occurrence and mobilization are controlled by geotectonic, climate, and anthropogenic activities, such as land use and pumping. This study delineates the occurrence and mobilization of F in groundwater in a semi-arid environment using groundwater, and an artificial intelligence model. The model predicts climate, soil type, and geotectonic as major predictors of F occurrence. We also present unsaturated zone F inventory, elemental compositions, and mineralogy from 25 boreholes in agricultural, forest, and grasslands from three different land use terrains in the study area to establish linkages with the occurrence of groundwater F. Normalized unsaturated zone F inventory was the highest in the area underlain by the granitic–gneissic complex (261 kg/ha/m), followed by residual soils (216 kg/ha/m), and Pleistocene alluvial deposits (78 kg/ha/m). The results indicate that the unsaturated zone mineralogy has greater control over F mobilization into the groundwater than unsaturated zone F inventory and land-use patterns. The presence of clay minerals, calcite, and Fe, Al hydroxides beneath the residual soils strongly retain unsaturated zone F compared with the subsurface beneath Pleistocene alluvial deposits, where the absence of these minerals results in enhanced leaching of unsaturated zone F.

Occurrences of Rare Earth Element (REE) Bearing Minerals in Migmatitic Gneiss and Granitoids of Chhotanagpur Granite Gneissic Complex, Bihar, India

Abstract This paper presents the geochemical characteristics, petrogenetic processes and source characteristics of Rare Earth Elements (REE) bearing migmatitic gneiss and granitoids near Tangeshwar village, Banka district, Bihar, in order to demarcate the potential zone of REE mineralization. These migmatitic gneiss and granitoids belongs to the Chhotanagpur granite gneissic complex (CGGC). The study of migmatitic gneiss and granitoids of CGGC suggest that they have high content of SiO2, low contents of Fe2O3 and MgO and SiO2 vs. Na2O and K2O shows positive correlation which suggest, they are dominated by sodic and alkali rich minerals and were derived from a felsic (granitic) source. Also LREE-LILE enrichment, HFSE depletion and calc-alkaline nature indicates that the magmatism is related to the subduction zone. Petrographic and EPMA studies of these migmatitic gneiss and granitoids confirms the presence of REE bearing mineral phases such as allanite, xenotime, monazite, and basnasite. They are generally present at the rim of biotite, as an inclusion in biotite and also along the grain boundaries and within the cracks of major silicate minerals. This suggest that the REE formation takes place at the deuteric stage of magma crystallisation followed by the formation of granite. Chemical analysis of bed rock samples for rare earth elements using ICPMS shows that the highest concentration is in migmatitic gneiss and granitoids, which varies from 47.85- 925.67 ppm and 47.85- 626.48 ppm respectively.

Geochemistry and Sr-Nd isotopic studies of Precambrian gneisses from central Aravalli Craton, NW India: Implications for crustal evolution and reworking

The Aravalli Craton of the Indian shield constitutes heterogeneous basement lithologies (Banded Gneissic Complex; BGC), and among them, the granitoids are the most voluminous lithology. The BGC comprises two lithotectonic units, viz., BGC-I and BGC-II. The BGC-II has been further classified as amphibolite facies Mangalwar and granulite facies Sandmata Complexes. In the present study, the gneisses of the Mangalwar Complex are geochemically categorized into (i) low-and high-pressure sodic gneisses and (ii) potassic gneisses. The sodic gneisses are metaluminous and characterized by high Sr/Y and LaN/YbN ratios; and exhibit subduction-related negative anomalies of Nb and Ti. The εNd (t = 2992 Ma) ranges from +2.3 to +3.1, with an average Nd-depleted mantle model age (TDM) of 3.06 Ga. The whole-rock Sm-Nd isochron age is ∼3.0 Ga (2992 ± 340 Ma). Genetically, the sodic gneisses originated from the melting of an enriched precursor (oceanic plateau) in an arc environment. These gneisses show strong correlations with the gneisses from BGC-I depicting similar geochemical signatures. In contrast, the potassic gneisses are characterized by slightly higher SiO2 along with high K2O and high large-ion lithophile elements and negative Eu anomalies along with negative εNd (t = 1.7 Ga) (−13.2 to −3.9), higher initial 87Sr/86Sr isotopic ratios and average TDM = 2.87 Ga. These geochemical features of the potassic gneisses indicate that they were derived from the reworking of the pre-existing TTG-like (sodic gneisses) crust during the Paleoproterozoic Era.

Hydration, melt production and rheological weakening within an intracontinental gneiss dome

The Entia Gneiss Complex (EGC) in central Australia represents a deeply exhumed Paleoproterozoic basement terrane that underwent fluid-catalysed transformation culminating in the formation of a migmatitic gneiss dome during the 450–300 Ma intracontinental Alice Springs Orogeny (ASO). Foremost amongst the changes the EGC experienced during this event was the regional-scale conversion of nominally anhydrous granulite and igneous crust to hydrous amphibolite facies migmatite, together with the emplacement of locally voluminous mica-bearing pegmatite swarms. LA–ICP–MS U–Pb dating of zircon, monazite and xenotime demonstrates that pegmatite emplacement dominantly occurred late in the Alice Springs Orogeny (c. 360–300 Ma). The oldest generation of pegmatites are transposed into the regional Palaeozoic gneissic fabric and formed at c. 1770 Ma, indicating they belong to protolith rocks of the EGC. Three subsequent generations of pegmatite have ages that range between 360 and 300 Ma. The oldest of these is parallel to the migmatitic gneissic fabric and dated at c. 360 Ma, whereas the younger two generations are largely undeformed, discordant and intruded throughout the period c. 350–300 Ma. The capability for a comparatively dry intracratonic orogenic core to repeatedly produce melt over an interval of c. 60 Myr requires explanation. We suggest crustal-scale duplex development during the latter stages of the ASO resulted in the juxtaposition of dry and mechanically strong Paleoproterozoic basement over deeply buried Neoproterozoic–Cambrian sedimentary sequences. Fluid derived from the metamorphic dehydration of these sequences led to hydration, partial melting and rheological weakening of the basement slab. Prior to its hydration at c. 360 Ma, the EGC was metamorphically unresponsive, accounting for its comparatively late timing of deformation and partial melting compared to the overall history of the Alice Springs Orogeny. The development of the crustal-scale duplex system, combined with hydrous melting of the low-density basement slab beneath high-density supracrustal rocks, resulted in a thermomechanically unstable architecture that facilitated the formation of an orogen-scale gneiss dome during the waning stages of intracontinental orogenesis.

Nature and origin of anorthosite enclaves within Proterozoic granite of Chotanagpur Granite Gneiss Complex of Eastern India

The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated, forming the Greater Indian Landmass. The Chotanagpur Granite Gneiss Complex (CGGC) represents the eastern extension of the CITZ and exposes several granite plutons. Enclaves of diverse origin are present as minor constituents within these granitic bodies. This study reports new major and trace element data for anorthosite enclaves hosted within granites in the Pattharkatti and Rajgir area from the northern margin of CGGC to get modern insights into the petrogenesis of anorthosites. Anorthosite enclaves show sharp contact with the host granite. They contain cumulus plagioclase (An87–94), intercumulus amphibole (magnesiohornblende and ferrotschermakite), and biotite (Mg-biotite and phlogopite) along with minor iron oxides. Amphibole crystallization pressure and temperature are constrained between 0.5 and 6.4 kbar and 653–780°C for the anorthosites. The studied anorthosites display a very gradual and steady increase in whole-rock rare earth element (REE) contents from Lu to La [(La/Yb)N = 1.22–13.08]. They also show a sharp decline in Fe2O3(t) and MgO, whereas Al2O3 increases with increasing silica contents from 45.69 to 51.16 wt%. In the chondrite normalized REE diagram, plagioclase exhibits LREE enriched patterns with strong positive Eu anomaly. The composition of parental liquid for anorthosite from the study area was estimated by adopting the equilibrium distribution method. Parental melt curves from Sm to La are near parallel and constrained broadly between trapped melt fractions (TMF) = ∼5%–15%. Anorthosites of the study area may have formed from the plagioclase-saturated basaltic melt.

Genesis of the gneissic core complexes in the Arabian-Nubian Shield and its tectonic implications: A regional overview

Metamorphic core complexes (MCCs) are a domed structures cored by high grade gneiss overlain by low grade supracrustal rocks. They are characterized by some common features such as extensional fabrics, ultra high-grade metamorphic facies, low angle normal fault (detachment fault), strike-slip shear zones surrounded the core complexes and ductile mylontitic shear zone (thrust zone) that separate the overlying low-grade rocks from the lower high-grade rocks. There is a debate about the presence or absence of metamorphic core complexes in the Arabian-Nubian Shield (ANS) especially in its northern part. The gneissic complexes in the ANS are considered as strike-slip core complexes like the Qazaz and Krish Domes whereas, those in the Egyptian Nubian Shield (ENS) are interpreted as antiformal stacks (e.g. Meatiq and Hafafit) formed during thrusting, or core complexes formed during orogen-parallel crustal extension. Some metamorphic core complexes are domes or contain gneiss domes within them, but not all gneiss domes possess the essential elements of a true metamorphic core complex. The most important points that negate the existence of MCCs in the ANS are absence of ultra-high grade metamorphic facies, absence of real low-angle normal faults, not all the gneisses have a domal structures, adjacent syn-extensional basins have fill that is older than the gneissic complexes, and models of ANS core complex exhumation include strike-slip faults with slip senses recently found to be inconsistent with the models. The gneissic complexes in the ANS differ from the Cordilleran-type or Aegean-type metamorphic core complexes. The origin of gneiss domes in the ANS is controversial, and many of them are presumably produced by mechanisms other than horizontal extension. During the oblique convergence of East and West Gondwana, the gneissic complexes in the ENS evolved from pure shear to simple shear-dominated transpression due to oblique convergence between East and West Gondwana along the Mozambique belt.

Production Potential Appraisal of Soils of for Part of Palamaner Division in Chittoor District, Andhra Pradesh, India

The soil profile samples that are present in upper slopes of the Palamaner agricultural division are evaluated to know the production potential of the study area. The soils have moderate slope and are affected by erosion. Besides, soils have excessive drainage limitation. Geologically, soils are developed on the quartz-migmatite gneiss complex. All the soils are studied for morphological, physico-chemical and chemical properties. The results shown that, soils are slightly acidic to neutral in soil reaction (pH), non saline (EC), moderately deep to deep (90-135 cm) in depth. The texture of soils varied from moderately well drained with no erosion to severe erosion. The soils are placed under IVes land capability class. The actual productivity and the potential productivity are calculated; the maximum potential productivity was 81.23 and the crop improvement factor was 1.59.

A New Method for Relating Zircon Crystallisation to Petrogenetic Events

Abstract The trace element contents of zircon can provide unique insights into tectonothermal events, however, interpreting these data and identifying correlations with specific magmatic/metamorphic events can be challenging. This limits our ability to construct temporally constrained petrogenetic histories of complex metamorphic terranes. Unlocking the information that the rare earth element (REE) patterns of zircon contain is difficult because of the need to quantify differences. We have parametrised the shape of zircon REE patterns in terms of three independent parameters: average abundance, slope, and curvature. Quantifying REE patterns using independent shape parameters is similar to the use of REE ratios but is an improvement as (1) it uses information from all 14 REE rather than just two; (2) the use of two independent parameters (e.g. slope and curvature) is a more robust discriminant than the use of a single ratio; and (3) subtle variations in shape are easily distinguished enabling trends in the REE patterns of large datasets to be identified. Quantitative models were constructed showing how the shapes of the REE patterns of zircon change due to the co-crystallisation of other metamorphic minerals (monazite, apatite, and garnet). Diagnostic changes in shape enable the REE contents of zircon crystals or crystal zones to be accurately related to the growth of specific minerals and hence metamorphic events. The results were used to interpret the REE patterns of zircons from high-grade metamorphic terranes, which have experienced multiple deformation events (Val Malenco, Italy; Betic Cordillera, Spain; Seram, Indonesia; Lewisian Gneiss Complex, Scotland; Napier Complex, East Antarctica) and clearly identified zircon that crystallised in the presence of garnet. Quantitative comparison enabled zircon that crystallised prior to, synchronously with, or after garnet to be identified. Similar models can be used to interpret the REE patterns of monazite. This allows the relative timing of the growth of these minerals to be accurately constrained, which given the importance of zircon for geochronology and garnet for geobarometry has the potential to provide insights into the evolution of a metamorphic event.

Archean crustal evolution of the Saglek-Hebron Complex, Northern Labrador, revealed from coupled 147−146Sm-143−142Nd systematics

The Saglek-Hebron Complex in Northern Labrador, Canada, hosts some of the oldest rocks on Earth. Recording a protracted crustal history over 1 billion years, the complex includes multiple generations of felsic rocks formed through episodic magmatic activity spanning the whole Archean Eon, between ∼3900 Ma and ∼2700 Ma. Coupled 147−146Sm-143−142Nd systematics from these granitoids show a complex crustal history, including melting of older mafic crustal precursor sources and reworking of Eoarchean felsic crust. Eoarchean granitoids are characterized by high initial ε143Nd up to +4 and μ142Nd up to +15. Combined long- and short-lived Sm-Nd isotope systematics suggest the crustal precursor source of the oldest Saglek-Hebron granitoids was derived from an early incompatible element depleted reservoir formed between 4200 Ma and 4300 Ma, evolving with a 147Sm/144Nd ratio of ∼0.265. The source involved in the formation of the Saglek-Hebron Eoarchean felsic crust appears to be younger and more depleted compared to that of the Saglek-Hebron mafic-ultramafic rocks, but similar to the source of the Eoarchean granitoids from the Itsaq gneiss complex in SW Greenland. This may suggest the formation of a Hadean highly depleted mantle reservoir at the scale of the North Atlantic Craton, perhaps resulting from the extraction of Hadean mafic crust. Paleoarchean ∼3300 Ma granitoids from the Illuilik magmatic episode exhibit initial ε143Nd between -1.6 and -1.3, which could be consistent with the reworking of the Saglek-Hebron Eoarchean felsic crust. However, the whole-rock geochemical composition of some Iluilik samples rather suggests derivation from a mafic crustal source, and thus their initial ε143Nd values imply derivation from an Hadean mafic precursor. This scenario would also account for the lower μ142Nd of ∼+6 found for the Iluilik granodiorite, compared to the older TTG, and consistent with derivation from a distinct crustal source. Most granitic rocks exhibit initial ε143Nd and μ142Nd consistent with reworking of the oldest Saglek-Hebron TTG, throughout the Archean, between ∼3700 Ma and ∼2700 Ma. Finally, coupled 147−146Sm-143−142Nd systematics suggest a shift in precursor sources at the end of the Paleoarchean, with the ∼3200 Ma Lister gneiss exhibiting more juvenile initial ε143Nd and no deviation of μ142Nd from the terrestrial standard, as opposed to most other Saglek-Hebron granitoids. This shift to more juvenile precursor material in the Paleoarchean has also been observed in other Archean cratons, suggesting a planetary-scale transition for crustal formation processes.

Testing the importance of sagduction: insights from the Lewisian Gneiss Complex of northwest Scotland

Archean cratons often contain rock units that are interpreted as having been juxtaposed from different structural levels. A range of uniformitarian and non-uniformitarian processes have been invoked to explain these occurrences, prominent amongst which is the density-driven ‘sagduction’ of high-density upper-crustal lithologies into the underlying dominantly felsic mid-crust. In this paper we test the geological importance of sagduction, using a combination of petrology, phase equilibria modelling, and mechanical modelling. We use the Lewisian Gneiss Complex of northwest Scotland as a test case, analysing the range of observed subordinate felsic–ultramafic bodies within the dominantly felsic crust, but our approach is applicable to Archean terranes globally owing to their analogous lithological ranges. We find that for our thermodynamically-estimated densities of the lithologies present in the Lewisian Gneiss Complex, unrealistically hot temperatures are required for sagduction to be important, given the observed body sizes and available constraints on event durations, geotherms, crustal rheology and emplacement depths. These results suggest sagduction is not responsible for emplacement of the observed subordinate lithologies within the Lewisian felsic mid-crust, and instead support uniformitarian tectonic interpretations. Additionally, our results cast doubt on the importance of sagduction in the structural evolution of granite-greenstone belts. Overall, our study indicates that sagduction was not an important Archean tectonic process.

On the origin of shear-band network patterns in ductile shear zones

Ductile yielding of rocks and similar solids localize shear zones, which often show complex internal structures due to the networking of their secondary shear bands. Combining observations from naturally deformed rocks and numerical modelling, this study addresses the following crucial question: What dictates the internal shear bands to network during the evolution of an initially homogeneous ductile shear zone? Natural shear zones, observed in the Chotonagpur Granite Gneiss Complex of the Precambrian craton of Eastern India, show characteristic patterns of their internal shear band structures, classified broadly into three categories: type I (network of antithetic low-angle Riedel (R) and synthetic P-bands), type II (network of shear-parallel C and P/R bands) and type III (distributed shear domains containing isolated undeformed masses). Considering strain-softening rheology, our two-dimensional viscoplastic models reproduce these three types, allowing us to predict the condition of shear band growth with a specific network pattern as a function of the following parameters: normalized shear zone thickness, bulk shear rate and bulk viscosity. This study suggests that complex anastomosing shear-band structures can evolve under simple shear kinematics in the absence of any pure shear component.

Reappraisal of 75 Years of Exploration for Atomic Minerals in India and the Way Forward

Abstract In India, Atomic Minerals Directorate for Exploration and Research (AMD), the oldest unit under the Department of Atomic Energy (DAE) is mandated with the exploration and augmentation of atomic mineral(s) resources of the country to support the Nuclear Power Programme (NPP) of India. AMD, till date have established four (04) generations of uranium metallogeny in India, distributed in 46 deposits and several other occurrences, major share of which are contributed by the globally unique, stratabound carbonate hosted, syn-diagenetic Tummalapalle Group of deposit (~59%), metamorphite type deposits along the Singhbhum Shear Zone (~21%), unconformity related deposits in northern part of Cuddapah Basin (~5%) and sandstone type deposits in Cretaceous Mahadek Basin (~6%). Besides these, metasomatite type deposits along the North Delhi Fold Belt (4%) and granite related-structurally controlled, high grade deposits in Bhima Basin (2%) along with some small hydrothermal/metamorphite/migmatite type deposits in Aravalli, Kotri-Dongargarh, Higher Himalaya belts and Chhotanagpur Granite Gneiss Complex contribute to the national inventory which stands at 3,76,000 tonne uranium oxide. AMD have stockpiled substantial quantities of Nb-Ta, Be and Li minerals needed for the NPP of India from prospective pegmatite belts in the country. AMD have established immense potential for REE mineralization in carbonatite complexes of Ambadungar in Gujarat and per-alkaline granite-rhyolite of Siwana Ring Complex, Barmer district, Rajasthan. Exploration along the coastal and riverine placers of the country has established 130 heavy mineral placer deposits with substantial thorium and REE resources. The progressive advancements by AMD through adaptation of integrated multidisciplinary exploration techniques coupled with major thrust in airborne/ground geophysics and expansion of state-of-the-art analytical facilities have facilitated systematic planning for future augmentation of atomic mineral(s) resources. Exploration inputs are envisaged to be intensified in the brownfield target areas for resource augmentation while R&D and phase-wise exploration inputs will be focused for developing the identified greenfield areas following a planned roadmap to ensure selfsufficiency in atomic mineral resources for sustained growth of the NPP to reach the target of net zero carbon emissions by 2070.

Tectono‐stratigraphic evolution of Shillong Plateau, North East India through the Permian‐Eocene window

The Shillong Plateau in the north‐eastern Indian Plate signifies a cut‐off patch of the Indian Peninsular Shield, bordered by fault and thrust sheets, with thick alluvium cover along its periphery. The basement rock in the plateau accomplished about 2 km relief difference with the adjoining Sylhet Trough of Bangladesh in the south. The plateau witnessed Permo‐Carboniferous Gondwana sedimentation at its western margin, with Early Cretaceous basaltic traps and continuous fluvio‐marine sedimentation along its southern edge since the late Cretaceous. Although it is largely believed that strike‐slip movement along the Dauki Fault shifted the plateau about 250 km eastward, however, the driving mechanism for Shillong Plateau detachment and conversion of rift‐intercratonic basin to platformal configuration has drawn more attraction. This article focuses on the tectono‐stratigraphic evolution of Shillong Plateau and its adjoining regions on the basis of previous works, to try to understand the distinct tectonic and environmental factors. Early Permian continental rifting to the south‐west of the plateau and south‐east and south‐west of the Malda High presumably separated the Shillong Plateau from Chotanagpur Granite Gneiss Complex (CGGC). Extrusion of Rajmahal Traps west of the Malda High and Bengal‐Sylhet‐Mikir volcanism along an Early Cretaceous geofracture probably accelerated the east‐northeast migration of Shillong Plateau with respect to the CGGC. Since the Late Cretaceous, sedimentation at the southern fringe of Shillong Plateau commenced with an intracratonic basin which gradually transformed into a passive margin configuration during the Late Palaeocene‐Eocene Epoch during growth of the Indian Ocean. Differential subsidence in southern Shillong Plateau caused clastic sedimentation without considerable transportation. At the beginning, the Late Cretaceous intracratonic basin had received volcanoclastic, fluvio‐lacustrine sediments in a series of graben/half‐grabens associated with extensional faulting and volcanism, as represented by the Lower Mahadek and Jadukata formations in the plateau. The upper Mahadek and later Palaeogene sequences along the southern margin of the plateau record marine sedimentation, initiated with an estuarine‐marshy environment during the Late Cretaceous to Palaeocene, followed by a platformal marine succession since the Mid‐Eocene. The Calcutta‐Mymensingh Eocene Hinge Zone that separates a stable shelf from the deep basinal part might be traced through the south‐eastern fringe of Shillong Plateau and presumably continues further north‐east between the Naga and Disang thrusts in the eastern part of Dhansiri Valley.

Titaniferous-Vanadiferous, Magnetite-Ilmenite Mineralization in a Mafic Suite within the Chhotanagpur Gneissic Complex, Bihar, India

Titanium or vanadium metals or their alloys are important industrial metals/alloys. Because these resources are in short supply, the investigation of potential titaniferous-vanadiferous deposits needs special attention to bridge the supply-demand gap. The study integrates geological, geochemical, remote sensing, and geophysical data for assessing the potentiality of titaniferous-vanadiferous, magnetite-ilmenite mineralization in and around the Sudamakund and Paharpur areas, Gaya and Jehanabad districts, Bihar, India, and delineation of specific targets for detailed exploration. Field visits for large scale mapping on (1:12,500 scale) were used to conduct a reconnaissance survey for magnetite-ilmenite mineralization in parts of toposheet number 72G/04 in the Gaya and Jehanabad districts of Bihar, as well as the collection of bedrock samples (BRS), pit/trench samples (PTS), petrographic samples (PS), and petrochemical samples (PCS), followed by petrographic and ore microscopic study, and interpretation of chemical results. Signatures of oxidized iron-bearing sulphides (iron-oxides ratio) and other ferrous-iron-bearing minerals surrounded by altered rocks (clay bearing minerals) are visible in remote sensing images. The geological work was followed by ground geophysical gravity and magnetic surveys in selected blocks by the Geophysics Division, eastern region (ER) on a 1:12,500 scale. The magnetite ore is hard, compact, crystalline, and at some places, granular in nature. The analytical value of these magnetite ore bodies indicates average Fe content at 49.53% (range 25.85–60.78%), with a considerable amount of TiO2 (average 15.85%, range 1.47–26.77%), and V (average 144.79 ppm, range 30.00–256.00 ppm, from PTS). The trends of these magnetite ore deposits correspond to the major lineaments (NE-SW and NW-SE). The superimposition of gravity and magnetic contour maps with the geological map (1:12,500 scale) helps explain the observed geophysical anomalies, and the possible subsurface (horizontal and vertical) expansion of magnetite ore deposits in alluvium cover regions warrants further investigation.

Crustal growth/reworking and stabilization of the western Superior Province: Insights from a Neoarchean gneiss complex of the Winnipeg River terrane

AbstractLong-term stability of the continental lithosphere is attained through a cumulative increase in net buoyancy and rigidity due to progressive compositional differentiation (i.e., cratonization). As stable cratons provided the nucleus for the subsequent accretionary growth and tectono-magmatic reworking that produced modern continental crust, the geodynamic processes that facilitated the stabilization of cratons are critical for understanding the evolution of Earth’s lithosphere. This study uses a portion of the Winnipeg River terrane, one of the oldest terranes of the western Superior Province, as a natural laboratory to investigate Archean crustal growth (partial melting of mantle) and reworking (partial melting of crust) and provides insights into the geodynamic processes driving mantle depletion and crustal remelting. Zircon U-Pb data obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) from an extensive Winnipeg River terrane gneiss complex reveal six major magmatic events at ca. 3060 Ma, 2930–2920 Ma, ca. 2910 Ma, 2830–2800 Ma, 2735–2730 Ma, and ca. 2700 Ma and regional metamorphism at ca. 2900 Ma. Whole-rock geochemistry and zircon Lu-Hf and trace element data indicate that (1) the magmatism at ca. 3060 Ma and ca. 2930–2920 Ma represents reworking of the isotopically evolved components of the incipient Winnipeg River terrane at shallow depths, (2) the ca. 2910 Ma magmatism features a step-change of Hf isotopic compositions from subchondritic to suprachondritic and records the formation of new juvenile magmas and the first reworking of existing juvenile crust, and (3) the magmatism after ca. 2830 Ma largely reflects reworking of the juvenile components of the incipient Winnipeg River terrane at medium to shallow depths prior to the ca. 2700 Ma trans-crustal magmatism associated with the convergence of the Winnipeg River terrane and western Wabigoon terrane. Juvenile magmatism and crustal growth in the Winnipeg River terrane at ca. 2910 Ma are inferred to correspond with significant mantle depletion below the Winnipeg River terrane, which led to a more stable lithospheric mantle in this part of the western Superior Province. Zircon trace element data support a mantle upwelling model rather than lithosphere recycling models for the depletion of mantle at ca. 2910 Ma. This study suggests that crustal growth and mantle depletion bracketed by prolonged, episodic crustal reworking may be a fundamental characteristic of the cratonization process.

Persistent mildly supra-chondritic initial Hf in the Lewisian Complex, NW Scotland: Implications for Neoarchean crust-mantle differentiation

The composition of the mantle source from which continental crust is ultimately derived informs both the timing and rate of continental growth. This composition is, however, poorly constrained during the first billion or so years of Earth's evolution. Typically in zircon Hf isotope studies, a linearly evolving depleted mantle back-calculated from modern MORB-source mantle to chondrite at 4.57 Ga has been used to calculate model ‘crust formation’ ages, which are themselves used in crustal growth models. Yet, an increasing number of studies of Eo- to Paleoarchean continental crust suggest crust extraction from a relatively undifferentiated mantle. Zircon Hf isotope studies of ca. 2.9 Ga gneisses from the Neoarchean Lewisian Gneiss Complex of Scotland have previously revealed initial Hf isotope compositions that suggest derivation from such an undepleted mantle source. Here, we present combined zircon U-Pb and Lu-Hf isotope data from seventeen representative grey gneiss samples from across the Lewisian of the Scottish mainland and the Outer Hebridean archipelago, ranging in age from 3.1 to 2.7 Ga. Regardless of location and exact age, initial Hf isotope signatures are predominantly near- to slightly supra-chondritic. Several models are investigated to explain this observation, our favoured one involving episodic extraction of the grey gneiss precursors from a mildly depleted mantle that began to diverge from chondritic composition at ca. 3.5 Ga. Samples from the southern Outer Hebrides record slightly more radiogenic initial Hf signatures, consistent with a possible terrane boundary along the Paleoproterozoic South Harris shear zone. This study provides further confirmation that domains of Hadean to Paleoarchean, and possibly even locally Mesoarchean, mantle remained chondritic with respect to Hf isotopes. Initiation of a depleted mantle source at this later time and its limited divergence from chondritic mantle, even into the Neoarchean as suggested here, has implications for crustal growth models based on detrital zircon Hf model ages. On a regional basis, the signatures recorded in our data are consistent with contemporaneous gneisses across Fennoscandia and East Greenland and the mildly depleted mantle source of continental crust furthermore persists into the Mesoproterozoic.

The geochemistry and isotopic compositions of the Nakdong River, Korea: weathering and anthropogenic effects.

The Nakdong River is the longest river in South Korea, and flows through various geological terrains with different land use characteristics; therefore, the geochemistry of its water is expected to be influenced by many factors. In this work, the geochemical characteristics of the Nakdong River were examined, and its chemical compositions, δD, δ18O, and δ13CDIC values, and 87Sr/86Sr ratios were determined to investigate the geological and anthropogenic effects on the geochemistry of the Nakdong River water. The obtained concentrations of major ions were strongly affected by both the anthropogenic activity and weathering of the rocks. With increasing the flow distance, the ion concentrations slightly increased; and after the inflow of the Kumho River, which was the largest tributary running through Daegu (the fourth largest city in South Korea), the concentrations of Na and SO4 ions abruptly increased and decreased again, suggesting the existence of strong anthropogenic effects caused by sewage treatment plants and dyeing industrial complex. Other activities such as agricultural ones also increased the NO3 concentration. In July, the high precipitation level from tropical cyclones and downpours decreased the ion concentrations as well as the δD and δ18O values. The δ13CDIC magnitudes showed that the dissolved inorganic carbon mainly originated from mineral weathering upstream, while the oxidation of soil organic materials influenced by agricultural activity became more important downstream. The 87Sr/86Sr ratios revealed that in the upstream regions, the weathering of granite and gneiss complex was dominant, while in the downstream regions, the weathering of sedimentary rocks became more important. The weathering and anthropogenic effects on the river water chemistry were also demonstrated using statistical analysis, which revealed that the water geochemistry was mostly influenced by the anthropogenic sources, including industrial complex, represented by Na, Cl, and SO4. The obtained results show that, as compared to the geochemistry of the Han River (which is also a major river in Korea), the geochemistry of the Nakdong River is more influenced by anthropogenic activities (including agriculture and the industrial complex) due to the different land use.

Comparative Evaluation of Weathering Indices of Rock–Soil Sequences in parts of Peninsular Gneissic Complex, Western Dharwar Craton, Karnataka, India

Comparative Evaluation of Weathering Indices of Rock–Soil Sequences in parts of Peninsular Gneissic Complex, Western Dharwar Craton, Karnataka, India

Petrogenesis and geochemistry of fayalite and fluorite-bearing granite from the Assam Meghalaya Gneissic Complex, West Khasi Hills, Meghalaya, India: their implication towards Rodinia Supercontinent amalgamation

Petrogenesis and geochemistry of fayalite and fluorite-bearing granite from the Assam Meghalaya Gneissic Complex, West Khasi Hills, Meghalaya, India: their implication towards Rodinia Supercontinent amalgamation

Rift‐related multistage evolution of the Mangalwar Complex, Aravalli Craton (NW India): Evidence from elemental and Sr–Nd isotopic features of Proterozoic amphibolites

The Banded Gneissic Complex (BGC) of the Aravalli Craton (India) comprises Archean BGC‐I (3.3–2.5 Ga) and Proterozoic BGC‐II. The BGC‐II is a mosaic of amphibolite facies namely, (a) Mangalwar Gneissic Complex (MGC), (b) Mangalwar Metasedimentary Complex (MMC), and (c) granulite‐facies Sandmata Metamorphic Complex. Here we present field, petrography and geochemical study of the Proterozoic amphibolites from the MGC and MMC. Based on field and geochemical data, the amphibolites have been characterized into three types related to rift settings (G1, G2 and G3). The G1 type occurs as dykes in the MGC and bears ocean island basalt‐type rare earth element (REE) patterns along with negative Nb and Ti anomalies, negative to positive values of εNd(t) (−0.02 to +3.96) and slightly variable initial 87Sr/86Sr (ISr) ratios. They are derived from deep mantle sources and correspond to the pre‐rift magmatic phase. The G2 type occurs as isolated patches associated with chert and is characterized by light REE (LREE) depleted and almost flat heavy REE (HREE) patterns suggesting that they were emplaced in an oceanic setting and were derived from a shallower mantle bearing positive εNd(t) (+2.87 to +6.27) and ISr = 0.7002–0.7083. This phase corresponds to the opening of the Mangalwar sedimentary basin (MMC). The G3 type occurs intercalated with metasedimentary rocks of the MMC and marked by LREE‐enriched and HREE‐depleted to flat patterns that resemble Upper Continental Crust signature, their εNd(t) mostly negative values and variable ISr also corroborate this explanation. They are believed to be derived from heterogeneous sources and represent syn‐sedimentary volcanic phases. All these signatures indicate that the amphibolites distinctly represent three phases of magmatism that occurred during pre‐rift (1.72 Ga), opening of basin (1.62 Ga) and syn‐sedimentary volcanism (1.6–1.3 Ga) in the rift‐basin and they were formed during the Proterozoic. These rifting events might have been connected with the fragmentation of Columbia.