Charnockitic Rocks Research Articles

The role of Ediacaran synkinematic anatectic rocks and the late-orogenic charnockitic rocks in the development of the hot Araçuaí belt

The orogenic evolution of the hot Araçuaí belt is characterized by continuous magmatic activity, with magmas of different compositions recording a long-lived (∼630 to ∼530 Ma) tectono-thermal evolution in response to convergence between the Congo and São Francisco continents during the West Gondwana amalgamation. Geochemical data from the Carlos Chagas domain (CCD) and the Nova Venécia Complex (NVC), in the hinterland of this belt, show that these rocks contain high amounts of heat producing elements – HPEs – (Th, U, and K) and have a dominant peraluminous signature, suggesting that they result from partial melting of continental crust. The CCD is intruded by post-collisional charnockites that have a dominant shoshonitic signature, interpreted as representing magmas from an enriched mantle reservoir with a variable amount of crustal contamination. Detailed morphological investigations at the macro- and micro-scales show that the CCD contains remnants of residuum material from metamorphic reactions associated with textures that attest to melt crystallization. This reinforces the interpretation that the CCD is locally derived, i.e., it represents an in-source subhorizontal rheologically weak layer of migmatites and parauthoctonous granites that formed during the orogenic thickening. The CCD likely triggered the formation of an orogenic plateau and a geological setting in which high temperature conditions could be sustained for tens of millions of years. In such a setting, continuous heat supply from radiogenic decay might have been the main heat source for the compositionally diverse magmatism during most of the orogenic evolution of the Araçuaí belt.

High pressure granulite facies metamorphism at the interface of the Archean Bastar craton and the Proterozoic Eastern Ghats Belt, India

Nd model ages show that the suture between the Archean Bastar craton and Proterozoic Eastern Ghats Province (a part of the Eastern Ghats Belt), India is marked by a broad shear zone that contains a mixture of rocks from both the blocks. In this suture zone, amphibolite facies rocks of the Bastar craton were subjected to high pressure granulite facies metamorphism possibly at ~500 Ma (published isotopic data). New petrological, geothermobarometric and P-T pseudosection studies show that dehydration-melting of amphibole (in mafic protolith) and of biotite (in quartzofeldspathic protolith) along a clockwise P-T path led to peak metamorphic conditions of 9.75 ± 0.5 kbar; 875 ± 30 °C. Mafic granulites from the Eastern Ghats Province had an earlier metamorphic history of isobaric cooling from unknown peak metamorphic conditions, but currently record P-T maximum values of 9.25 ± 0.25 kbar; 825 ± 25 °C. Followed by cooling through ~100 °C from the peak metamorphic conditions, mafic granulites from the Bastar craton were exhumed to mid-crustal levels by decompression-cooling, probably as a tectonic wedge. This decompression-cooling is also shared by charnockitic rocks of the Bastar craton and mafic granulites of the Eastern Ghats Province. We interpret further burial of the Bastar craton rocks as a result of underthrusting beneath the Eastern Ghats Province, consistent with available seismic data. Total absence of any evidence of ocean opening or closure between the Bastar craton and the Eastern Ghats Province at around 500 Ma, coupled with interpretation of deduced P-T paths, suggest the shear zone resulted from far-field stress related to the Kuunga orogeny in an intraplate setting. This would support a model of Tonian age amalgamation of the Bastar craton and the Eastern Ghats Province as a part of the Greater Indian Landmass.

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yielded εHf(t) values from −1.17 to 0.46 with TDM and TDMC and age peaks at 2392 Ma and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.

Imaging lateritic bauxite bearing zones in Ekiti, Southwestern Nigeria, using magnetic and electrical resistivity tomography techniques

This research uses geophysical techniques such as magnetic and electrical resistivity tomography to image the lateritic bauxite bearing zones in Orin Ekiti, Southwestern Nigeria. The high-resolution aeromagnetic data were utilized for generating the total magnetic intensity (TMI) map. Different filtering techniques such as Euler deconvolution and total horizontal derivative were also carried out to enhance better understanding of bauxite deposit, with the generation of different maps therein. Low magnetic intensities were observed over charnockitic rock which is a host material for bauxite occurrence. Magnetic low trending NE–SW was observed in the study area. The (EUD and THDR) maps were then superimposed on the geologic map to geo-reference areas that have dominant charnockite. The depth extent and structural signature interpreted from magnetic interpretations show that significant deposit in the study area is at shallow depth which tallies with the geology of bauxite as being a supergene enrichment (surficial deposit). 2D ERT results obtained from five traverses with inter-traverse separation of 45 m and traverse length of 100 m each, established in the NW–SE direction to image the lateritic bauxite deposit based on the prospect shown from the aeromagnetic analysis. The images display the constant resistivity regimes beneath the traverses with different colour bands. The 2D ERT profiles and 3D inversion results show bauxitic zones to be of high resistivity which ranges from 154 to 3814 Ω m, the migmatite gneiss and granitic rocks (unaltered parent rocks) are of considerably lower resistivity than the weathered charnockitic rock and by extension bauxitic zones. These different techniques used in this study have however shown that the area of interest for bauxitic deposit in Orin Ekiti trends in the NE–SW direction with a dominant concentration in the north-eastern (NE) part. By the same token, this research has been able to substantiate the fact that bauxite deposits occur as supergene enrichment.

Physicochemical characteristics and spatial distribution pattern of the Yercaud Lake surface sediments, South India

Freshwater lakes and sediments need to be monitored continuously for environmental assessment and geochemical explorations. Lake sediments are major sinks for organic matter and for all geochemical elements. In the present study, total organic matter (TOC), nitrogen (TN), and geochemical analysis of major oxides was carried out on Yercaud Lake surface sediments, located at a higher elevation in South India to infer sediment distribution pattern and source for organic matter. Particle size distribution of lake surface sediments suggests that the sediments are predominantly silty sand and sandy silt, poorly to very poorly sorted and symmetrically skewed. TOC/TN ratio (6.54–12.65) reveal that the organic matter (OM) is derived from the in situ lake sources particularly from the vascular plants and lake biota accumulation. Weathering indices that fall from [chemical index of alteration (CIA)] 88.59–93.08 values and A‐CN‐K plot reflects intense chemical weathering of the bedrock exposures in the source area, which was accelerated under warmer and humid climate conditions in the sub‐tropical regions. Al2O3/TiO2 ratio (6.94–26.62) indicates weathering particularly of the exposed charnockite rocks around the lake basin.

Geochemical processes influencing stream water chemistry: a case study of Ala River, Akure, Southwestern Nigeria

Ala River is the main river that drains Akure metropolis in southwestern Nigeria. The river takes its sources from deep-seated fractures in granite and charnockite rock bodies at the North and North-western regions of Akure town, flowing through quartzite and granite gneiss exposures. This study was carried out to assess the geochemical processes influencing the quality of Ala river water. The dendritic river system, covering an area of about 427 km2, was studied and water was sampled at thirty-seven (37) points for in-situ determination of some physicochemical parameters and dissolved concentrations of the major ions. pH values obtained varied between 7.16 and 7.75, electrical conductivity ranged from 123 to 520 µs/cm, total dissolved solids varied between 62 and 260 mg/l-CaCO3 while average temperature value was 26.65 °C. Relative concentration of cations and anions were in the order of Ca2+ > Mg2+ > Na+ > K+ and HCO3− > SO42− > Cl− > NO3−. Mean values of dissolved Na+, K+, Ca2+, Mg2+, Fe2+, Cl−, HCO3−, SO42−, NO3−, and PO42− were 34.028, 8.872, 34.175, 18.289, 0.428, 16.543, 56.41, 12.503, 0.495 and 2.029 mg/l, respectively. Two hydochemical facies namely Calcium-Bicarbonate facies and mixed calcium–magnesium–chloride facies dominated the chemistry of the water. CaHCO3 facies revealed the contribution of geogenic processes while the mixed Ca(Mg)-Cl type reflected domestically induced anthropogenic activities influencing chemistry of the water. Gibbs diagrams identified rock–water interaction as an important geochemical process in the study area. Silicate weathering of rocks and carbonate weathering were identified as the important geochemical processes which influence the water chemistry of the study area.

Comparison of PIXE Technique with Chemical Analysis and NAA -Geo chemistry of High Grade Metamorphic Charnockite Rocks

Comparison of PIXE Technique with Chemical Analysis and NAA -Geo chemistry of High Grade Metamorphic Charnockite Rocks

Geophysical and Petrographical Study of Apatapiti Charnockitic Rock, Akure, Southwestern Nigeria

Geophysical and petrographic evaluation of the earth’s subsurface is of vital concern, especially prior to geotechnical and hydrogeology works. Vertical Electrical Sounding (VES) of Ire-Akari Estate of Apatapiti Community in Akure South was carried out to assess the effects of the petrographic properties of charnockitic rocks on the geotechnical and hydrogeologic systems. The three curve types identified within the study area are AA, KH and HA, with a predominance of KH curve type, which suggests that the predominant geologic sequence comprising the topsoil has a clayey content alternating with laterite. This is underlain by weathered layer and fresh bedrock. Petrographic analyses revealed the presence of plagioclase (Albite-Anorthite), hypersthene, biotite, quartz, hornblende, microcline, pyroxene and dark-coloured minerals, with an average modal composition of 31%, 15%, 11%, 18%, 13%, 1%, 5% and 6%, respectively. A correlation of both results showed that the topsoil is mostly of about 0.4 m to 2 m thick layer of clay, indicative of weathering of feldspars. This is underlain by 0.8 m to 7 m thick layer of lateritic clay which responds as a low resistive layer. However, the mineral composition at location 1 is characterized by rocks that are more resistant to weathering due to the presence of low plagioclase in the rock sample compared to samples from other locations with a higher percentage of plagioclase and hypersthene minerals. This implies that weathering condition at all locations except location 1 could be favorable to hydrogeology if there are interconnected fractures in the parent rock (charnockite). However, since the area is predominantly covered by clayey materials which are established as poor foundational materials, appropriate ground improvement techniques and in-depth geotechnical analyses should be performed to forestall hazards associated with them.

Petrological and Geochronological Constraints on the Evolution of Charnockitic Rocks in the Massifs of Cauvery Shear Zone, Southern Granulite Terrain, India

Abstract Cauvery shear zone of the southern granulite terrane, India, is sandwiched between the amphibolite to granulite transition zone in the north and the Neoproterozoic granulite belt in the south. Two massifs of this shear zone consist of charnockitic rocks of tonalitic to granodioritic composition and include hornblende-mafic granulite enclaves. Petrology and geochemistry clearly indicate dehydration partial melting of mafic rocks as the mode of origin of the charnockitic rocks. However, high MgO, Ni and Cr contents and negative Nb anomalies suggest some interaction with the mantle in a subduction zone environment. U-Pb and Lu-Hf isotopic data of zircons in mafic granulite enclave and charnockite-enderbite host rocks indicate ca. 2.5 Ga granulite-cum-anatectic event. Juvenile mafic magmatism at ca. 2.7 Ga could represent the precursor of the mafic granulites. With the chemical and isotopic evidence of subduction zone environment, the Cauvery shear zone could be appropriately described as a suture along which Archaean-Paleoproterozoic granulite terrain and Neoproterozoic granulite terrain had collided.

Health risks associated with fluoride intake from rural drinking water supply and inverse mass balance modeling to decipher hydrogeochemical processes in Vattamalaikarai River basin, South India

Preliminary investigation reveals that fluorosis is reported due to the continuous intake of fluoride-rich groundwater in Vattamalikarai River basin, Tamil Nadu, India. A detailed study was attempted for evaluating the health risks associated with the intake of fluoride-rich groundwater supplied to the rural community. Groundwater samples were collected from 60 and 58 dug and tube wells during winter and southwest (SW) monsoon seasonsrespectively. The samples were analyzed for the determination of fluoride and other chemical parameters to examine the fitness for drinking water. Spatio-temporal variation maps reveal that fluoride concentration is high during SW monsoon season when compared with the winter season in this region. The fluoride bearing minerals present in hornblende-biotite gneiss and charnockite rock formations leached into the groundwater during rock-water interaction. To understand the subsurface hydrogeochemical reactions, inverse mass balance model was developed using NETPATH code. The model output indicates that calcite dilution, silicate (hornblende and biotite) weathering, ion exchange (Ca/Na and Mg/Na) and illite precipitation are the dominant processes controlling the groundwater chemistry along the flow paths. Non-carcinogenic risks to children and adults (women and men) were evaluated by working out intake exposure of groundwater. Hazard quotient (HQ) based on fluoride intake was calculated for children and adults. It varied from 0.08 to 2.21 with an average of 1.07 for adults. For children, it varied from 0.01 to 2.99 with the mean of 1.44. About 78%, 69% and 61% of the samples fall under the risk category for children, women and men during winter season. However, more number of samples possessed health risks (83% of samples for children, 73% of samples for women and 64% of samples for men) during SW monsoon season.

Assessment of Rock Aggregate for Construction Using Comminution Theory

In this research, grinding energy of selected rock samples collected from South West Nigeria was determined using comminution theory in order to evaluate suitability of rock for aggregate production. Bond Work Index (BWi), Aggregate Impact Value (AIV) and Aggregate Crushing Value (ACV) of samples was characterized and correlated. The work index of the charnockitic rock, granite gneiss, porphyritic granite –labelled PG1 and porphyritic granite –labelled PG2 of samples was found to be 17.12 kWh/t, 13.72 kWh/t, 13.64 kWh/t and 12.76 kWh/t respectively. The ACV of the charnockitic rock, granite gneiss, porphyritic granite (PG1) and porphyritic granite (PG2) was determined to be 26.2 %, 27.3 %, 27.6 % and 27.8 % respectively; while the AIV of the samples, in same order, was 11.2 %, 13.2 %, 19.1 % and 18.4 % respectively. Following high correlation coefficient of 0.98% between BWi and ACV, hardness of rock materials are classified as ‘very difficult’, ‘difficult’, ‘medium’, ‘easy’ and ‘very easy’ for grinding energies in the range of >18 kWh/t, 14-18 kWh/t, 10-14 kWh/t, 7-10 kWh/t and 0-7 kWh/t respectively. Based on the classification, rock materials with grinding energy >10 kWh/t could be suitable for aggregate production and usable for civil construction purposes.Keywords- Aggregate, Aggregate impact value, Aggregate crushing value, Comminution, Grinding energy.

SHRIMP U-Pb Geochronology of the Socorro Batholith and implications for the Neoproterozoic evolution in SE Brazil

The Socorro Batholith is one of the most expressive granite manifestations associated with the Neoproterozoic evolution in SE Brazil, occupying large areas (~1,200 km 2 ) in the southern portion of the Socorro-Guaxupe Nappe. A U-Pb zircon SHRIMP dating program was developed to determine the ages of the main components of this batholith, identified in previous detailed mapping projects. High-K calc-alkaline (HKCA) porphyritic biotite-hornblende granites with relatively low (60–67 wt%) SiO 2 are the most voluminous component of this and other large “syn-tectonic” batholiths in the SGN (Agua Limpa and Pinhal-Ipuiuna) and neighboring domains located south of it in the Apiai and Sao Roque Domains of the Ribeira Fold Belt. Two samples collected in widely separated localities at the northern and southern part of the Socorro batholith yield similar ages of magmatic crystallization, respectively 610.1 ± 7.0 and 608.3 ± 6.6 Ma. A more fractioned (> 72 wt% SiO 2 ) granite reported in the literature as related to a younger event (“Socorro II magmatism”, as opposed to the previous “Socorro I” HKCA granites) yield a precise age that is clearly older (624.4 ± 3.6 Ma), and contemporary to anatectic granites and migmatites that were produced during a prolonged period of high-grade metamorphism (635–605 Ma) that affected the SGN. Our data thus indicates that at least part of the HKCA magmatism that constitutes the Socorro batholith post-dates the high-P metamorphism associated to continental collision, and may have been a source of heat and volatiles to the high-T metamorphism responsible for partial melting of the upper portions of the crustal section represented by the SGN. Two charnockitic rocks that show transitional contacts with granites of the Socorro batholith were also dated. The Socorro Charnockite is aged 641.6 ± 4.1 Ma, which overlaps those of regional orthogneisses (in part also of charnockitic character) considered as associated with a pre-collisional tectonics (subduction-related?). However, it is reported to transition to granites that are very similar to the HKCA granites of the Socorro batholith, which are yet undated. The Atibaia Charnockite has distinct geochemical affinity (lower mg# and Sr content; higher Zr), a younger age (633.3 ± 6.2 Ma), and may signal a different tectonic setting at the end of the period of plate consumption as yet poorly characterized.

U-Pb zircon and U-Th-total Pb monazite ages from the Phulbani domain of the Eastern Ghats Belt, India: Time constraints on high-grade metamorphism and magmatism in the lower crust

We present zircon U-Pb (SHRIMP) and monazite U-Th-total Pb (EPMA) ages from a suite of high-grade rocks of the Phulbani domain of the Eastern Ghats Belt, India. These ages are integrated with the metamorphic and magmatic histories of the Phulbani domain to put precise time constraints on the tectonothermal evolution of the northern part of the Eastern Ghats Belt. Zircon from the coarse-grained charnockite rock shows crystallization age of ca. 970 Ma. Aluminous granulite contains spinel and quartz-bearing mineral assemblage indicating ultrahigh temperature metamorphism (>900 °C at 8–10 kbar) which possibly occurred during ca. 987 Ma as revealed from monazite included in porphyroblastic garnet. Monazite in the aluminous granulite and the migmatitic felsic gneiss grew dominantly at 966 ± 4 Ma and 968 ± 4 Ma ages respectively, which are interpreted as the cooling ages subsequent to peak metamorphism. Oscillatory-zoned zircon grains of the felsic augen gneiss yield ca. 1173 Ma age which is interpreted as the crystallization age of the granitic protolith. This ca. 1173 Ma age granite possibly composed a part of the Proterozoic basement of the Eastern Ghats Province. A monazite age of 781 ± 15 Ma from the aluminous granulite may indicate the timing of a localized shear-induced thermal process in the Phulbani domain. The presently studied rock suite thus recorded three distinct events (ca. 1173 Ma, ca. 1000–900 Ma and ca. 781 Ma) of magmatism, metamorphism and deformation of Eastern Ghats Belt. Metamorphic and magmatic histories of the Phulbani domain are similar to those of the adjacent Visakhapatnam domain and the Rayner Province of East Antarctica during the time frame ca. 1000–900 Ma.

Geophysical lineaments of Western Ghats and adjoining coastal areas of central Kerala, southern India and their temporal development

A family of geophysical lineaments have been identified in ∼15,000 km2 in central Kerala between 9°30′N to 10°45′N and 76°00′E to 77°30′E, integrating geophysical data with geological and geomorphological features. The characteristics of these lineaments in the magnetic and gravity fields and their derivatives have been analysed. The evolution of the lineaments has been traced to the temporal phases of global evolution of the region. A group of these faults have formed by reactivation of the deep-seated distensional fractures associated with and accompanying dyke emplacements during the episodic breakup of Gondwana at ∼90 and 65 Ma under distinctive mantle thermal regimes. It is possible that reactivation of these distensional faults may have started during the cooling interval of time between the two distensions in the 90 and 65 Ma and post 65 Ma periods and later in the Cenozoic, when the lineaments were enlarged to their present dimension, possibly under the influence of forces that led to the uplift of the western Ghats. These may extend down to the crust-mantle interface. A cluster of younger geophysical lineaments has been generated by reactivation along the weak planes of transformation of the charnockitic rocks of the Precambrian. They seem to have a strike-slip character. They are devoid of any dyke association and were formed on a cold crust. They may be confined to the upper-middle crust. They were generated in the high intensity intra-plate palaeo-stress fields of the triple forces arising from (1) the back-thrust from the Himalayan Collision; (2) the impact of epeirogenic forces and related isostatic uplift of the Western Ghats and (3) the flexural isostatic uplifts due to surface loads of late Mesozoic basaltic lavas and Cenozoic sedimentation in the coastal rifted basins in late Cenozoic, probably in the time span of 20 Ma to the present, when the palaeostress fields were most intense.

Mineralogy, geochemistry, and petrology of Neoarchean ferroan to magnesian granites of Carajás Province, Amazonian Craton: The origin of hydrated granites associated with charnockites

2.75 to 2.73Ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the Carajás Province. The Vila Jussara Suite was recognized recently and its geologic, mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the Estrela Complex, Serra do Rabo and Igarapé Gelado granites and Planalto Suite. Monzogranites and syenogranites are dominant in most units but in the Igarapé Gelado Granite and Vila Jussara Suite granodiorites and tonalites are also relatively abundant. The main mafic phases in all these granitoids are amphibole [potassian hastingsite with subordinate magnesiohastingsite; 0.95≥Fe/(Fe+Mg)≥0.47] and biotite [0.88≥Fe/(Fe+Mg)≥0.52] with rare occurrence of clinopyroxene relict crystals. Magmatic epidote (% of ‘pistacite’ component from 26 to 31) occurs only in the Vila Jussara Suite. The rocks of Estrela, Serra do Rabo, and Planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced-ferroan granites [whole rock FeOt/(FeOt+MgO)≥0.89] that evolved at low fO2 (FMQ±0.5). The Igarapé Gelado Granite needs additional studies but it is formed apparently by reduced-ferroan and oxidized-ferroan granites. The Vila Jussara Suite is also composed of reduced-ferroan granites that are similar to those of the other granite units. However, it has additionally oxidized-ferroan and magnesian granitoids which contain magnetite (±ilmenite) as the main Fe-Ti oxide phase and evolved, respectively, at moderate fO2 (NNO±0.5) and at comparatively higher fO2 (NNO to NNO+1). The magmas of these granites were formed at temperatures≥900°C and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc-alkaline in composition. Plagioclase, orthopyroxene, clinopyroxene and Fe-Ti oxides were the main phases in the melt residues and the liquids were enriched in Ba, Zr, Y, and REE (except for Eu), but not in Sr compared to their sources. The magmas should have relatively high water content (>4wt.%). The magmatic evolution was strongly influenced by partial melting processes. The different varieties of the Vila Jussara Suite are not linked by fractional crystallization and derived from distinct magmas. The Neoarchean granites of the Carajás Province were emplaced in a collisional tectonic setting. They are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling. The magmas were generated in the crust at pressures of ~800 to 1000MPa and emplaced at pressures of 200 to 500MPa. The studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites. The magnesian variety of the Vila Jussara Suite is similar to the pyroxene-free granitoids of the Neoarchean Matok Pluton of the collisional Limpopo Belt. The Neoarchean granites of Carajás are hydrated granites emplaced at comparatively higher crustal levels when compared with the Matok pluton.

Charnockite Bedrock, Chennai Coast, Tamil Nadu: Micromorphology and Geochemical Signatures in Stages of the Weathering Processes

Abstract: Mineral weathering of bedrock in the context of landscape evolution provides useful insights in understanding the physico-chemical and climate processes. In this paper, we report mineral and petrographical features supported by geochemical results on the weathering of charnockite rocks exposed along the east coast, Chennai. The exposed rocks consist of medium to fine grained charnockite and are intensely weathered, fractured with weathering rind. Micromorphology of the unweathered charnockite core rock, weathering rind and soil show that the weathering processes are characterised by progressive physical and chemical attack on the labile minerals with the formation of neoformed clay minerals substituting the original rock fabric. The alteration of minerals has produced Fe oxides and clay minerals such as halloysite and kaolinite. Ferruginous oxides etch and impregnate into the plagioclase and othropyroxene microcracks and fractures during the advanced weathering stage. Microfabric alterations in microfractures and its variations occur in the weathering rind thereby, affect the chanrnockite weathering landscape. Geochemical data and chemical weathering indices (CIA, CIW, PIA and CWI) indicate insitu weathering due to stronger physical weathering compared to chemical alteration. UCC and chondrite normalized trace and REE data of the core rock, weathering and soil show similar spider spectra again pointing to insitu weathering with minor recycling. Negative Eu anomaly is due to the alteration of the alkali feldspars. Weathering of the core rock to soil grades towards the increase of quartz grains and Fe oxides in the weathering rind and soils. Weathering rind exhibits formation of clays such as kaolinite, halloysite and gibbsite formed under the humid, alternate dry and wet conditions.

An application of PIXE technique to Proto Crustal Rocks: Geo chemical evaluation of Granulitic Charnockites of Eastern Ghats, Andhrapradesh, India

In this work, PIXE has been used for a variety of Precambrian proto crustal rocks in the form of granulitic charnockites,Eastern Ghats AP,India.In previous study of exterior part of the charnockite hill elemental analysis using electron microprobe analysis, the elements in host charnockites no Na, traces of Mn,Ca and high Ti,Cl,F (Biotite), no Mn, low Na,and high K,Cl (Hastingsite), and Cl, Fe (Apatite) were only detected, but by using PIXE technique in addition to the above twenty two trace elements are identified. PIXE is highly sensitive and non-destructive method for multi elemental analysis in a variety of Precambrian charnockite rocks down to levels of a few parts per million. The samples chosen for analysis from the central portion of a charnockite hill near Visakhapatnam airport. A big reticular mass of relict litho logical body which is compositionally and physically different from host chatnockite was observed in the central portion of the hill. These experiments are carried out using a 3MV pelletron accelerator facility at the Institute of physics, Bhubaneswar. A collimated proton beam of 2mm diameter is made to fall on to the sample, and the beam current is kept at 20na. A high resolution Si(Li) detector(160ev FWHM at 5.9kev energy) is employed in the present experiments and the Guelph PIXE (GUPIX) software package is used to analyze the spectra. The elements identified in this Precambrian charnockite rock are Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ru, Ag, Pb..From this study these rocks are early crust(proto crust) rocks. These samples are to belong to a very important geological phase and further work on petrography and REE (Rare Earth Elements in Geology)of the rock is indeed to firmly establish its exact parentage.

Mineral chemistry and magnetic petrology of the Archean Planalto Suite, Carajás Province – Amazonian Craton: Implications for the evolution of ferroan Archean granites

The Planalto Suite is located in the Canaã dos Carajás subdomain of the Carajás Province in the southeastern part of the Amazonian Craton. The suite is of Neoarchean age (∼2.73 Ga), ferroan character, and A-type affinity. Magnetic petrology studies allowed for the distinction of two groups: (1) ilmenite granites showing low magnetic susceptibility (MS) values between 0.6247×10−3 and 0.0102 × 10−3 SI and (2) magnetite-ilmenite-bearing granites with comparatively higher but still moderate MS values between 15.700×10−3 and 0.8036 × 10−3 SI. Textural evidence indicates that amphibole, ilmenite, titanite, and, in the rocks of Group 2, magnetite also formed during magmatic crystallization. However, compositional zoning suggests that titanite was partially re-equilibrated by subsolidus processes. The amphibole varies from potassian-hastingsite to chloro-potassian-hastingsite and shows Fe/(Fe + Mg) > 0.8. Biotite also shows high Fe/(Fe + Mg) ratios and is classified as annite. Plagioclase porphyroclasts are oligoclase (An25-10), and the grains of the recrystallized matrix show a similar composition or are albitic (An9-2). The dominant Group 1 granites of the Planalto Suite were formed under reduced conditions below the FMQ buffer. The Group 2 granites crystallized under more oxidizing conditions on or slightly above the FMQ buffer. Pressures of 900–700 MPa for the origin and of 500–300 MPa for the emplacement were estimated for the Planalto magmas. Geothermometers suggest initial crystallization temperatures between 900 °C and 830 °C, and the water content in the magma is estimated to be higher than 4 wt%. The Neoarchean Planalto Suite and the Estrela Granite of the Carajás Province reveal strong mineralogical analogies, and their amphibole and biotite compositions have high total Al contents. The latter characteristic is also observed in the same minerals of the Neoarchean Matok Pluton of the Limpopo Belt but not in those of the Proterozoic rapakivi A-type granites. On the other hand, in terms of the degree of magma oxidation, the Planalto and Estrela granites approach the reduced Mesoproterozoic rapakivi granites and the reduced to moderately oxidized Paleoproterozoic granites of the Velho Guilherme and Serra dos Carajás Suites, respectively, and differ from the oxidized granites (Jamon Suite) of the Carajás Province as well as those of Matok pluton. The high total Al content of amphibole and mica could be caused by crystallization at high pressures that, in turn, can be a reflex of the association of the studied granites and Matok with charnockitic rocks.

Assessment of fluoride contaminations in groundwater of hard rock aquifers in Madurai district, Tamil Nadu (India)

The fluoride contamination in drinking water is already gone to the alarming level and it needs the immediate involvement and attention of all people to solve this problem. Fluoride problem is higher in hard rock terrains in worldwide and Madurai is such type of hard rock region. Totally 54 samples were collected from the Madurai district of Tamilnadu with respect to lithology. The samples collected were analysed for major cations and anions using standard procedures. The higher concentration of fluoride is noted in the Charnockite rock types of northern part of the study area. 20 % of samples are below 0.5 ppm and 6 % of samples are above 1.5 ppm exceeding the permissible limit. The affinity between the pH and fluoride ions in groundwater suggests that dissolution of fluoride bearing minerals in groundwater. The higher concentration of fluoride ions are observed in the lower EC concentration. The isotopic study suggests that fluoride is geogenic in nature. In factor scores, fluoride is noted in association with pH which indicates the dissolution process.

Zircon U–Pb–Hf isotopes and geochemistry of two contrasting Neoarchean charnockitic rock series in Eastern Hebei, North China Craton: Implications for petrogenesis and tectonic setting

The Zunhua–Qinglong microblock in Eastern Hebei, North China Craton, exposes two contrasting charnockitic rock series in the Yuhuzhai–Taipingzhai and Cuizhangzi areas: charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks, and charnockite series showing tonalitic and granodioritic components. Petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks. Medium-fine grained anhedral orthopyroxenes are preserved in both series, and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series. In situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene, whereas the medium-fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements (REE) and no obvious Eu anomalies, resulting in left-inclined REE patterns. Combined with petrographic features, these new data indicate that the medium-fine grained anhedral orthopyroxenes were formed by granulite–facies metamorphism caused by dehydration reactions, whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530±17 and 2523±12Ma, and those of the charnockite series were emplaced between 2527±28 and 2515±22Ma. We conclude that these magmatic precursors are contemporaneous emplacements of two Neoarchean granitoid magmatic events, which were followed by regional granulite–facies metamorphism at ∼2.45Ga.The charnockitic plagioclase gneiss series and charnockite series exhibit similar ɛHf(t) values of −2.4 to +5.3 and −3.2 to +5.5, indicating either depleted mantle or juvenile crust affinity, and involvement of ancient crustal components. The charnockitic plagioclase gneiss series exhibit relatively low SiO2 contents (54.1–65.2%), with various MgO contents (1.93–5.36%), Mg# (0.36–0.55), and (La/Yb)N values (3.49–56.00); the charnockite series exhibit higher SiO2 contents (60.1–69.2%), with similar (La/Yb)N values (14.61–42.69), MgO contents (2.10–3.99%), and Mg# (0.29–0.52). These geochemical and zircon Lu–Hf isotopic features, together with chemical modeling, suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab-derived melts/fluids, and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases. The aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab, and it was strongly contaminated by mantle peridotites. These new data, combined with the previous studies of Neoarchean basement rocks in the Zunhua–Qinglong microblock in Eastern Hebei, suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back-arc-related tectonic environment at a convergent plate margin.