Charnockitic Gneisses Research Articles

Subduction to accretion in the southern Irumide Belt: Perspective from geochronology and petrology of granulites and charnockites in NW Mozambique

In the Mesoproterozoic Southern Irumide Belt, Tete Province, NW Mozambique, granulite-facies metavolcaniclastic rocks and orthogneisses from the Chidzolomondo Complex provide new constraints on the subduction and accretionary phases of the belt evolution. The complex comprises a metavolcano-sedimentary succession intruded by syn- to post-orogenic granitoids, charnockites and gabbros. The paragneisses exhibit a calc-alkaline chemical composition akin to intermediate magmatic rocks found in volcanic arcs. They are interpreted as products of a three-component mixture, consisting of pelite-aluminous, quartz, and reworked mafic volcanic rocks. A paragneiss sample contains detrital zircon with ages mainly between 1344 and 1098 Ma and εHf(t) values between −10 to +7, defining a maximum depositional age of 1121 ± 2 Ma. A charnockitic gneiss, dated at 1097 ± 3 Ma, and a metagabbro, dated at 1061 ± 3 Ma, respectively, represent the syn- and late-orogenic magmatic events. Both rocks exhibit juvenile εHf(t) signatures ranging from +1.5 to +4.5. The mineral assemblage of cordierite + garnet + orthopyroxene in metavolcaniclastic rocks, and plagioclase + hypersthene + diopside in the metagabbro, provide evidence of the HT-LP metamorphic conditions. Phase equilibria modeling of a paragneiss indicates that the peak metamorphic conditions reached approximately 785–795 °C and 3.7–4.2 kbar. By dating of zircon overgrowths in granulite-facies gneisses, we characterized the orogenic (M1) and thermal (M2) metamorphisms that affected the complex. The M1 event records the subduction-type and accretion metamorphism dated between 1092 and 1060 Ma. The M2 event, identified in the metagabbro sample close to the contact with a charnockite intrusion, yielded an age of 1023 ± 12 Ma for the thermal metamorphism. Intrusive charnockite and a gabbro record the post-orogenic magmatism with ages of 1026 ± 4 Ma and 1026 ± 12 Ma, respectively. The results provide a timeline for the last orogenic events of the Southern Irumide Belt, including the maximum depositional age of the Chidzolomondo Complex at ca. 1120 Ma, syn-orogenic magmatism and metamorphism until ca. 1060 Ma, and late- to post-orogenic magmatism and metamorphism between 1050 and 1020 Ma.

Neoarchean magmatism in the southern Scott and Raggatt Mountains, Napier Complex, east Antarctica

The Napier Complex of Enderby Land and Kemp Land is a unique part of the Eastern Antarctic Shield, as it preserves history of crustal growth extending from the Eo- to Neoarchean. It is composed mainly of enderbitic and charnockitic gneisses and granulites metamorphosed at ca. 2.5 Ga, and locally at ca. 2.8 Ga, under high- to ultra-high temperature conditions. These events have been well-characterized in the central and western parts of the complex; however, the south-southwestern region has been little studied. Samples from the southern Scott and Raggatt Mountains were selected for zircon U-Pb dating by Secondary Ion Mass Spectrometry and whole-rock geochemical analysis. Tonalitic gneiss from the Raggatt Mountains and trondhjemitic gneiss from the southern Scott Mountains have protolith ages of 2711 ± 5 Ma and 2726 ± 6 Ma, respectively. In contrast, tonalitic samples from the southern Scott Mountains have protolith ages of ca. 2539 ± 5, 2533 ± 5 and 2522 ± 9 Ma. The younger generation of magmatism overlaps the timing of metamorphic zircon growth between 2540 and 2440 Ma observed in this study. The geochemistry of ca. 2720 Ma gneisses indicates that magmatism was generated by medium-pressure melting (>1.5 GPa) of basaltic crust, whereas ca. 2530 Ma magmas were generated at lower pressures (<1.2 GPa). The ca. 2.7 Ga magmatism may be associated with continental crust formed at 2.8 Ga, or be newly formed crust that was assembled with older prior to or during a tectonometamorphic event at ca. 2.5 Ga. Tonalitic magmatism at 2.53 Ga during the first half of the 2585–2420 Ma metamorphic event may be a product of partial melting during high-grade metamorphism. Although protolith ages across the complex are still scarce, differences between regions raise the possibility that the complex contains crustal domains of different ages and origins that may not have been assembled until the ca. 2.5 Ga tectonothermal event.

Metamorphic Evolution of Calc-silicate Body Enclosed in Charnockitic Gneiss at West Ongul Island, Lützow-Holm Complex, East Antarctica

This paper provides a report of a study of a series of boudined bodies of mineralogically zoned calc-silicate bodies, some over 1 m thick, enclosed in granulite facies charnockitic gneisses on West Ongul Island, Lützow-Holm Complex, East Antarctica. Zircons extracted from the centre of one of the calc-silicate bodies indicate that they were metamorphosed around 532.3 ± 0.8 Ma and that the cores of these zircons were inherited from igneous rocks intruded 738.5 ± 1.8 Ma and possibly 605.0 ± 1.2 Ma. The meionite content of scapolite indicates that the peak metamorphic temperature reached a minimum of 790 °C. In the outermost rim of the calc-silicate scapolite is partially replaced by clinopyroxene, probably as a result of the influx of Mg bearing fluids from the surrounding charnockite gneiss during uplift and decompression. An influx of hydrous fluid resulted in the almost complete retrogressive alteration of plagioclase to sericite throughout the calc-silicates, with minor growth of amphibole after clinopyroxene and calcite after scapolite only in the innermost zone.

Monazite EPMA-CHIME dating of Sangmelima granulite and granitoid rocks in the Ntem Complex, Cameroon: Implications for Archean tectono-thermal evolution of NW Congo craton

Electron microprobe (EPMA) monazite Th–U – Pb ages have been determined from a suite of granulite and granitoids rocks of the Sangmelima granite-greenstone Belt (SGB) of the Ntem Complex in Cameroon or NW Congo Craton. The monazite-bearing rocks include charnockitic gneiss, migmatitic tonalite-trondhjemite-granodiorite (TTG) gneisses, granodiorite, monzogranite and a late pegmatite intrusion. The monazites of Sangmelima can be classified as monazite-Ce. Based on published ages and EPMA Th–U – Pb monazite age data from the SGB, the geodynamic evolution of the NW Congo Craton was refined through its tectonometamorphic and magmatic events. The major charnockitic and TTG magmatism event in Ntem Complex occurred between ~2950 and 2850 Ma. This Mesoarchean crystalline basement underwent two major ductile tectonothermal events M1 – D1 and M2 – D2. (1) The onset of M1 – D1 corresponds to the migmatization of TTG at 2843 Ma coeval with the emplacement of syn-kinematic granite dykes (monzogranites) at 2840–2825 Ma. The S1 regional penetrative foliation (D1) was progressively generated during the M1 metamorphic event, which led to the formation of associated hypersthene-garnet-bearing granulitic mineral assemblage that peaked at ~2830 Ma. (2) The M2 – D2 event between 2760 and 2740 Ma corresponds to the folding of S1 regional foliation by F2 folds and development of C2 shear zones synchronous with the anatexis event that generated the high-K granitoids at 2750 Ma. Late Neoarchean granitic pegmatite/aplite dikes and/or veins emplaced during the brittle deformation D3 and protracted cooling of the Ntem Complex between ~2700 and 2550 Ma. Our new EPMA monazite U–Th – Pb ages from the SGB highlight similar magmatic and tectonothermal events of the adjacent northern Gabonese domain and the Chaillu Massif in Gabon part of the NW Congo craton (ca. 3.0–2.5 Ga).

The U-Pb Zircon Geochronology (LA-ICP-MS) of Geological Processes in Granulites of Middle Bouh Area. Article 3. Rock association in the lower reaches of the Yatran river

LA-ICP-MS method was applied to investigate U-Pb and Lu-Hf isotope systematics of zircon crystals from charnockitic gneiss and biotite-garnet-hypersthene enderbite that occur in the lower reaches of the Yatran river (Yatran block of the Bouh river area). According to the obtained isotope data, charnockitic gneiss hosts three zircon populations. The oldest one is represented by three crystals that have isotope age between 3125 and 3300 Ma, and εHf values between –2.3 and –7.5. The next population is well-defined, it has an age of 2038±25 Ma and large variations of Hf isotope composition: 176Hf/177Hf — from 0.28122 to 0.28261, εHf — from –9.3 до 4.6. However, the ages of most of the analyzed zircons spread along the concordia between 2300 and 2800 Ma. All zircons in this population have a similar Hf isotope composition 176Hf/177Hf = 0.28072 to 0.28092, which does not depend on the age. It is characteristic that the oldest (with preserved U-Pb isotope systematics) crystals have positive or slightly negative εHf values. Most of the U-Pb isotope analyses of zircons from enderbite fall on the discordia line that has an upper interception age of 2029 ± 18 Ma. A small number of discordant grains have 207Pb/206Pb ages up to 2500 Ma. Hafnium isotope composition in zircons from enderbite varies widely: 176Hf/177Hf = 0.28131 to 0.28151, and εHf from –6.2 to 1.8.

English

The Ikare area is underlain by rocks of the migmatite-gneiss-quartzite complex and older granite lithologic groups and charnockites. Categorization of these rocks in the literature and published map is generalized and the rocks are undifferentiated. Though reported in few literature as an area with rocks composed of granulite facie grade, the nature of transition is not well documented. Field and petrographic studies were carried out to properly characterize the different rock types, evaluate the migmatization processes and elucidate the nature of amphibolite to granulite facie transition. Major rock types recognized and mapped includes grey gneiss, charnockitic gneiss, granite gneiss, pelitic gneiss, porphyritic granite and quartzite. Minor rock types are the mafic and ultramafic components which are made up of basic schist and amphibolites. The area suffered at least four episodes of deformation (D1, D2, D3 and D4) and at least three episodes of metamorphism (M1, M2 and M3). Petrographic studies further show that the mineral assemblage within the quartzo-feldspathic rocks changed from hydrous to anhydrous phases as one traverses from the southern part of the study area to the northern part. Reaction textures indicate that the transition zone between the amphibolite and granulite facie rocks is achieved by a prograde dehydration reaction while rehydration reactions took place during the waning retrograde stage. Anatexis and metamorphic differentiation were mainly responsible for the migmatization process in Ikare area while the charnockitization process was a product of dehydration reactions aided by reduction in water fugacity due to influx of CO2. Key words: Structure, migmatites, granulite facies, deformational episodes, metamorphism, charnockitic gneiss. &nbsp;

Streaming of saline fluids through Archean crust: Another view of charnockite-granite relations in southern India

The complementary roles of granites and rocks of the granulite facies have long been a key issue in models of the evolution of the continental crust. “Dehydration melting”, or fluid-absent melting of a lower crust containing H2O only in the small amounts present in biotite and amphibole, has raised problems of excessively high temperatures and restricted amounts of granite production, factors seemingly incapable of explaining voluminous bodies of granite like the Archean Closepet Granite of South India. The existence of incipient granulite-facies metamorphism (charnockite formation) and closely associated migmatization (melting) in 2.5 Ga-old gneisses in a quarry exposure in southern India and elsewhere, with structural, chemical and mineral-inclusion evidence of fluid action, has encouraged a wetter approach, in consideration of aqueous fluids for rock melting which maintain sufficiently low H2O activity for granulite-facies metamorphism.Existing experimental data at elevated T and P are sufficient to demonstrate that, at mid-crust pressures of 0.5–0.6 GPa and metamorphic temperatures above 700 °C, ascending immiscible CO2-rich and concentrated alkali chloride aqueous fluids in equilibrium with charnockitic (orthopyroxene-bearing) gneiss will inevitably begin to melt granitic rocks. The experimental data show that H2O activity is much higher (0.5–0.6) than previously portrayed for beginning granulite facies metamorphism (0.15–0.3). Possibilities for metasomatism of the deep crust are greatly enhanced over the ultra-dry models traditionally espoused. Streaming of ultrasaline fluids through continental crust could be a mechanism for the generation of the discrete mid-crust layer of migmatites suggested to characterize younger tectonometamorphic regions. The action of CO2-rich and hypersaline fluids in Late Archean metamorphism and magmatism could record the beginning of large-scale subduction of volatile-rich surficial materials.

High-grade metamorphism during Neoproterozoic to Early Palaeozoic Gondwana assembly, exemplified from the East African Orogen of northeastern Mozambique

Northeastern Mozambique exposes a deeply eroded high-grade segment of the East African Orogen, situated at the critical intersection of the Pan-African orogenic belts, showing widespread granulite facies lithologies. This study documents and characterizes metamorphism attributed to the multistage Neoproterozoic and Early Palaeozoic events, relating to a polyphase Pan-African history of outboard nappe stacking, nappe emplacement onto Mesoproterozoic crust, and compressional and extensional deformation along the Lurio belt: 1) A Neoproterozoic nappe stack (Cabo Delgado Nappe Complex) records high-pressure granulite facies: The highest peak conditions reach P = 1.45 GPa and T > 800 °C are in Grt-Cpx-Pl-bearing mafic granulites of the Xixano Complex. A post-peak isothermal decompressional P-T evolution is supported by garnet chemical zoning pattern, pseudsection modelling and garnet corona textures. 2) The underlying Mesoproterozoic gneiss complexes record amphibolite facies to intermediate-pressure granulite facies, the high-grade metamorphism is documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses in the southern part of the Unango and Marrupa complexes. This metamorphism is attributed to crustal thickening related to overriding of the nappe complex and shortening along the Lurio belt during the early Palaeozoic Kuunga orogeny. 3) The Ocua Complex coring the Lurio belt is a locus of Early Palaeozoic high-pressure metamorphism. A minimum of P = 0.79 ± 0.13 GPa at T = 760 ± 98 °C is constrained by Grt-Opx-Cpx-Pl-Qz assemblage in mafic granulite from the western limb while high-pressure granulite metamorphism was reached in the eastern part of the complex. The high-pressure granulite facies conditions is followed by isothermal decompression.

Petrological and geochemical evolution of the Central Gneissic Belt, Rengali Province, eastern India: Implications for the Neoarchean growth and orogenesis

This study focuses on the evolution of the Central Gneissic Belt of the Archean Rengali Province which evolved as a craton-margin orogenic belt. The Central Gneissic Belt is constituted of charnockite gneiss, migmatitic hornblende gneiss and granite gneiss often showing gradational contacts. While mafic granulite occurs as enclave within the charnockite gneiss, amphibolite and calc-silicate granofels enclaves are present within the granite gneiss. Mafic granulite shows peak metamorphic assemblage of garnet+clinopyroxene+plagioclase+quartz±orthopyroxene which was stabilized at 10.6±0.5kbar and 860±20°C. Charnockite gneiss with the peak assemblage of orthopyroxene+quartz+plagioclase+K-feldspar was metamorphosed at 792±48°C and 7.6±0.4kbar. Amphibolite and migmatitic hornblende gneiss contain hornblende along with plagioclase and garnet and these rocks were metamorphosed at 800±20°C, 8.5±0.2kbar and 695°C, 8kbar respectively. Later meta-dolerite dikes exhibit relic igneous textures which are slightly modified by greenschist-facies metamorphism. Charnockite gneiss, migmatitic hornblende gneiss and granite gneiss show similar trace and REE characteristics (moderate fractionation in terms of La and Yb, LREE enrichment and flat HREE pattern) implying the same protolith composition for these rock groups. Based on the field, petrographic and geochemical data, we propose that the protoliths for the charnockite gneiss, the migmatitic hornblende gneiss and the granite gneiss crystallized as fractionated magma in within-plate syncollisional setting during the ca. 2860–2780Ma orogeny at the Rengali Province.

U-Pb geochronology of the 2.0 Ga Itapecerica graphite-rich supracrustal succession in the São Francisco Craton: Tectonic matches with the North China Craton and paleogeographic inferences

LA-ICPMS U-Pb isotopic studies are conducted for key samples from the Itapecerica graphite-rich supracrustal succession and the crystalline basement in the southern São Francisco Craton (SFC). The detrital zircon analyses in paragneiss indicated a maximum deposition age of ∼2080Ma. The age spectra in the relative probability diagrams suggested the precursor basin was essentially fed with detritus from the accretionary complexes of the Mineiro belt (2470–2000Ma), which is the most preserved segment of the Minas Orogen. The basement rocks yielded zircon U-Pb crystallization ages between 2753±81 and 2679±61Ma, whereas the metamorphic overprint gave 2069±84Ma. One nearby charnockitic gneiss yielded a Sm-Nd garnet+whole rock linear array with an age of 2086±19Ma. The youngest ages estimate the timing of the regional prograde granulite facies metamorphism that also overprinted the Itapecerica rocks, given our upper intercept zircon U-Pb ages of 1997±20Ma and 1971±40Ma in paragneiss and quartzite, respectively. These ages also constrained the timing of the conversion of carbonaceous material into graphite in the Itapecerica mine – the second largest graphite mine in Brazil. The high-grade metamorphism was associated with the final docking of the Mineiro belt against the Neoarchean foreland. The Minas Orogen has geologic matches with the Jiao-Liao-Ji Orogen located in the present eastern margin of the North China Craton, where khondalites, UHT granulites, anatectic charnockites, as well as large graphite deposits are documented such as in the 1.97–1.88Ga Korean arc. The tectonic similarities between the two cratons support a relatively close position at the time frame. The dynamic scenario considers broadly coeval accretionary/collisional belts along the active margins of Archean blocks, the progressive closure of the Paleoproterozoic Ocean and final docking of the São Francisco/Congo paleocontinent against the active continental margin of the North China Craton. The work model allows new insights into the paleogeography of the core of supercontinent Columbia at ∼1.90Ga and the subsequent (1.78–1.50Ga) intraplate activity.

Petrology and U–Pb geochronology of zircon in a suite of charnockitic gneisses from parts of the Chotanagpur Granite Gneiss Complex (CGGC): evidence for the reworking of a Mesoproterozoic basement during the formation of the Rodinia supercontinent

Abstract Understanding the evolution of the Chotanagpur Granite Gneiss Complex (CGGC) of the East Indian Shield is crucial to decipher the role of the Indian Shield in the formation of the Rodinia supercontinent. The area around Deoghar–Dumka exposes a suite of granulite-facies orthogneisses (variably retrogressed to amphibole–biotite gneiss) that enclose remnants of Palaeoproterozoic metasedimentary and meta-igneous rocks. Results from mineral chemistry, laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) U–Pb dating of zircon and limited bulk-rock compositions of the studied rocks suggest that the magmatic protoliths of the felsic orthogneisses had A-type chemistry, and that these were emplaced at approximately 1450 Ma presumably in a continental rift setting. Intense deformation and metamorphism of the felsic rock culminated at approximately 9 kbar and 850°C along an apparent geothermal gradient of 26°C km −1 . These peak metamorphic conditions were successively followed by initially a steeply decompressive and then a weakly decompressive retrograde pressure–temperature path. The shape of the retrograde pressure–temperature path and the estimated geothermal gradient at the metamorphic peak are interpreted to be the products of continent–continent collision; U–Pb dates of metamorphic zircon overgrowths suggest an age of approximately 943 Ma for the collisional event. This study demonstrates that ‘Grenville-age’ orogenesis thoroughly reworked the approximately 1450 myr-old basement of the CGGC during the formation of the Rodinia supercontinent.

Zircon U-Pb ages and Hf isotopic systematics of charnockite gneisses from the Ediacaran–Cambrian high-grade metamorphic terranes, southern India: Constraints on crust formation, recycling, and Gondwana correlations

The Southern Granulite terrane, southern India, comprises a vast exposure of deep crust and forms a key region in the reconstruction of the Gondwana supercontinent. An E-W−trending crustal-scale shear zone, the Palghat-Cauvery suture zone system, which formed during the late Neoproterozoic−Cambrian (0.75−0.50 Ga), marks a prominent terrane boundary separating terranes with a predominantly late Neoarchean (ca. 2.5 Ga) regional granulite metamorphism to the north from those with an Ediacaran−Cambrian (0.63−0.50 Ga) regional granulite metamorphism to the south. Focusing on the younger granulite metamorphic domains, we present here new zircon U-Pb ages and Hf isotopic compositions for 11 charnockite orthogneisses from the Madurai, Trivandrum, and Nagercoil blocks and contribute to the resolution of the age of their magmatic protoliths. This study shows that the charnockite orthogneisses south of the Palghat-Cauvery suture zone relate to a minimum of four distinct episodes of felsic magmatism centered at: ca. 2.62−2.46 Ga, ca. 2.05−1.84 Ga, ca. 1.0−0.9 Ga, and ca. 0.80−0.76 Ga, pertaining to the Siderian, Orosirian, and Tonian Periods. Hafnium isotope analyses of zircon grains from the charnockite gneisses suggest that the protoliths of the ca. 2.05−1.98 Ga gneisses from the Trivandrum and Nagercoil blocks and the ca. 1.0−0.9 Ga gneisses along the southeastern Madurai block involved a significant juvenile magma component, while the protoliths of charnockite gneisses elsewhere in the Madurai block formed mainly through recycling of older crust up to ca. 3.2 Ga. A regional granulite-facies metamorphic imprint during the Ediacaran−Cambrian marked an advanced stage in the amalgamation of the Madurai, Trivandrum, and Nagercoil blocks into the East African orogen and its collision with the Dharwar craton.

The geochemistry of uranium occurrences and speciation in groundwater of Chinnar sub basin, South India

An attempt has been made in Chinnar sub basin of Dharmapuri district, South India to isolate the geochemistry of uranium occurrences in groundwater. The geology of the area is mainly of charnockite and granite gneiss. Groundwater samples were collected for two different seasons post and pre monsoon in two different litho units (granite gneiss and charnockite) and analysed for major, minor and uranium concentrations. Higher uranium (18.45 μg L−1) has been recorded during pre monsoon season in granite gneiss with increasing pH. The saturation index calculation for the groundwater isolated minerals like uaraninite, coffinite, haiweeite and soddyite to be precipitating and uranium oxides like UO2.25, UO2.25beta, UO2.33beta as oversaturated. The Eh-pH diagram attempted represents solubility of uraninite within the pH range of 6.0 to 8.0. The study isolate uranium in groundwater of the study area is controlled by the presence of (U4O9) uranium oxide.

Neoarchean tectonothermal imprints in the Rengali Province, eastern India and their implication on the growth of Singhbhum Craton: evidence from zircon U–Pb SHRIMP data

AbstractA detailed zircon U–Pb (SHRIMP) geochronological study of the amphibolite to granulite‐grade rocks of the Rengali Province of eastern India records the growth history of the southern margin of the Singhbhum Craton. Pelitic and mafic granulites from the gneissic belt exhibit contrasting styles of metamorphism. Zircon of the pelitic granulites from the eastern segment yields c. 3528–3064 Ma detrital ages. Charnockitic gneiss from the eastern segment has protolith age of 3058 ± 15 Ma while that from the central segment has protolith age of 2861 ± 30 Ma. The latter rock records high‐grade metamorphism at 2818 ± 15 Ma. Hornblende gneiss from the central sector has a protolith age of 2828 ± 9 Ma. Deformed leucogranite in the central and undeformed granitoid in the eastern segment were emplaced at 2807 ± 13 and 2809 ± 13 Ma respectively. The protolith of felsic gneiss from the central sector was emplaced at 2776 ± 24 Ma. Most of the zircon samples contain overgrowths of c. 2500 Ma, inferred to be the age of reworking of the gneissic belt. Our data suggest that the Rengali Province evolved as an orogenic belt in the Neoarchean time (c. 2800–2500 Ma) during southward growth of the Singhbhum Craton. These tectonothermal imprints at the margin of the Singhbhum Craton are possibly related to its inclusion within the supercontinent Ur.

Fluoride content of minerals in gneissic rocks at an area of endemic dental fluorosis in Sri Lanka: estimates from combined petrographic and electron microprobe analysis

A mass balance of the mineralogical sources of fluoride in charnockitic gneiss bedrock and regolith (weathered rock) in Sri Lanka has been undertaken, using optical petrography and Scanning Electron Microscopy (SEM) with energy dispersive X-ray spectrometry (EDX) to support development of a conceptual model of fluoride release to groundwater. Bedrock and regolith samples were collected in Polonnaruwa where there is a widespread occurrence of dental fluorosis attributed to excessive fluoride (up 5.25mg/L) in groundwater, while primary minerals of the gneissic bedrock are identified as the ultimate source of fluoride. Fluoride leaching related to long-term weathering of the bedrock and development of the regolith, control the occurrence of F in the groundwater system which is yet unresolved. In the study area, the charnockitic gneiss bedrock is mantled by a few meters thick regolith within which the groundwater table fluctuates seasonally. Mineral abundance and mineral fluoride content were estimated using petrographic and SEM analysis. Plagioclase, hornblende, pyroxene, K-feldspar, quartz, biotite and titanite (sphene) were the major minerals of the rock though the mineral proportions vary widely over the area. Apatite and magnetite were present as accessory minerals. Fluoride concentrations obtained from SEM-EDX analysis were combined with the volume percentage of minerals from petrography to estimate the total rock fluoride concentration. The order of fluoride abundance was apatite (highest), biotite, titanite, and hornblende. Plagioclase, K feldspar, pyroxene and magnetite did not have any noticeable fluoride concentrations. Loss of fluoride from the rock mass upon weathering was clearly evident and hornblende appeared to be the most significant fluoride releasing agent through weathering. Apatite, though present in accessory amounts, is distributed evenly in the rock and hence may be considered as a steady fluoride donor causing elevated and uniform fluoride distribution in groundwater. On the other hand, hornblende and biotite though of irregular occurrence (area wise), may cause high fluoride anomalies in groundwater in areas of high abundance.

Granulite-facies metamorphism of the Palaeoproterozoic – early Palaeozoic gneiss domains of NE Mozambique, East African Orogen

AbstractGranulite-facies metamorphism recorded in NE Mozambique is attributed to three main tectonothermal events, covering more than 1400 Ma from Palaeoproterozoic – early Palaeozoic time. (1) Usagaran–Ubendian high-grade metamorphism of Palaeoproterozoic age is documented in the Ponta Messuli Complex by Grt-Sil-Crd-bearing metapelites, estimated to pressure (P) 0.75 ± 0.08 GPa and temperature (T) 765 ± 96°C. The post-peak P-T path is characterized by decompression followed by near-isobaric cooling. (2) Irumidian medium- to high-pressure granulite-facies metamorphism is evident in the Unango and Marrupa complexes of late Mesoproterozoic – early Neoproterozoic age. High-pressure granulite-facies is documented by Grt-Cpx-Pl-Rt-bearing mafic granulites in the northwestern part of the Unango Complex, with peak conditions up to P = 1.5 GPa and T = 850°C. Medium-pressure granulite-facies conditions recording P of c. 1.15 GPa and T of 875°C are documented by Grt-Opx-Cpx-Pl assemblage in mafic granulites and charnockitic gneisses of the central part of the Unango Complex. (3) Tectonothermal activity during the Ediacaran–Cambrian Kuunga Orogeny is recorded in the Mesoproterozoic gneiss complexes as amphibolite facies to medium-pressure granulite-facies metamorphism. Granulite facies are documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses, reporting P = 0.99 ± 13 GPa at T = 738 ± 84°C in the Unango Complex and P = 0.92 ± 18 GPa at T = 841 ± 135°C in the Marrupa Complex. This metamorphism is attributed to crustal thickening related to overriding of the Cabo Delgado Nappe Complex, and shorthening along the Lurio Belt during the early Palaeozoic Kuunga Orogeny.

An Integrated Study to Assess the Groundwater Potential Zone Using Geospatial Tool in Salem District, South India

The present study reveals that the geospatial tool for delineating the groundwater potential zone in Salem district, Tamil Nadu, South India is quite useful. Assessing the ground water potential zone is very important for protecting and managing the groundwater system. The study area mostly consists of hard rock terrain especially in charnockite and fissile hornblende biotite Gneiss. There are four thematic layers were considered in this present study such as geology, geomorphology, drainage density, and lineament density. These four thematic layers were integrated for identify the groundwater potential zone. Therefore, five different groundwater potential zones were identified, which are very good, good, moderate, poor and very poor. These groundwater potential zones are covered by an area of 346 km2 , 2932 km2 , 666 km2 , 880 km2 and 406 km2 respectively. Finally, the present study concluded that the remote sensing and GIS is very fruitful and useful for the assessing the groundwater potential zones.

Evalauation of Groundwater Potential Zones using Electrical Resistivity Response and Lineament Pattern in Uppodai Sub Basin, Tambaraparani River, Tirunelveli District, Tamilnadu, India

The present attempt has been made to identify potential groundwater zones using electrical resistivity and lineament pattern in the dry watershed of Uppodai. The area comprises Archean hard rocks of charnockite and biotite gneiss. The major types of soil are red sandy soil and black cotton soils. Depth to the water table varies from 1 m to 10 m below the ground level. About 23 Vertical Electrical Soundings were carried out at different locations with AB/2 spreading up to 100 m by Schlumberger configuration. The measured apparent resistivity values were interpreted by curve matching technique using RESIST87 software. Lineament map was prepared to understand its influence on the ground water system. A, K,AK and KH are the curve types obtained in the study area. The resistivity values for first , second and third layers are varied from 3.6 Ωm to 256 Ωm; 65.7 Ωm to 2022.3 Ωm and 161.5 Ωm to 2500 Ωm respectively. The layer thickness of first and second layers observed with the variation of 0.7 m to 41.5 m and 8.3 m to 65.6 m. As the resistivity value observed with a wide variation, the resistivity value below100 Ωm of second layer has been conveniently taken for potential groundwater zones. The low resistivity with high second layer thickness zones is very limited and promising zones were observed at some patches in the present study area. Two sets of lineament pattern have identified with NW-SE and NE-SW directions. The density and length of lineaments were comparatively high with NW-SE pattern in the study area and not seems a potential aquifer at shallow depths.

Zircon from charnockite gneiss, charnockite, and leucosome of migmatite in the Nimnyr Block of the Aldan Shield

The microgeochemistry of zircon was studied in three samples: charnockite gneiss (1594), charnockite (1594a), and migmatite leucosome Lc4 (1594c). Prismatic (Zrn I) and oval (Zrn II) zircon morphotypes are distinguished in the first two samples. Most zircon grains consist of two-phase cores and overgrowth rims variable in thickness. The average weighted concordant U–Pb age of Zrn II cores from charnockite gneiss is 2436 ± 10 Ma. The concordant ages of Zrn I and Zrn II cores from charnockite are 2402 ± 16 Ma and 2453 ± 14 Ma, respectively. Some overgrowth rims are 1.9–2.1 Ga in age. In leucosome Lc4, all measured prismatic zircon crystals yielded a discordant age of 1942 ± 11 Ma (the upper intersection of discordia with concordia). These zircons are strongly altered and anomalously enriched in U and Th. Zrn I grains are enriched relative to Zrn II in REE, Li, Ca, Sr, Ba, Hf, Th, and U. Zrn I is considered to be a product of melt crystallization or subsolidus recrystallization in the presence of melt. Zrn II is relict or crystallizing from melt and then partly fused again. Zrn I from charnockite gneiss and especially from charnockite are markedly altered and have a more discordant age than Zrn II. This is probably related to concentration of fluid in the residual melt left after zircon crystallization.

Xes-Xen thermochronology of the Rayner metamorphic complex, Enderby Land (East Antarctica, Molodezhnaya Station Area)

The Xes-Xen dating of zircons from rocks of the Rayner Complex of the Enderby Land at the Molodezhnaya Station area (coast of the Alasheyev Bight) yielded age estimates of 550 ± 50 and 1040 ± 30 Ma. The metamorphic rocks of the Rayner Complex record two main events: first, the crystallization of the magmatic protoliths of charnockitic and enderbitic gneisses and, second, superimposed structural and metamorphic alterations under conditions transitional from the amphibolite to granulite facies (metamorphism manifested regionally in the rocks of the Rayner Complex). The most reliable Xes-Xen age estimates for magmatic zircons from the charnockitic and enderbitic gneisses correspond to the Grenville stage of the development of the Rayner Complex (∼1.0 Ga). The Xe isotopic systematics of metamorphic zircons reflect a pan-African stage in the evolution of the Rayner Complex (600–550 Ma). Pan-African events are reflected in the U-Xe isotope system in two cases: if metamorphic zircons crystallized at the same time (which probably resulted in the formation of a plateau in the Xes-Xen age spectrum) and if the initial isotopic systems were disturbed (which resulted in a decrease in apparent age toward low-temperature gas fractions). It is important that secondary alterations and a decrease in apparent ages to 600–550 Ma affected only those components (i.e., caused xenon release only from those traps) that were unstable under the maximum metamorphic temperatures and yielded Tcl values lower than 750°C (conditions transitional from the amphibolite to granulite facies). At a higher xenon retention, “primary” isotopic systems are preserved. Consequently, the age of metamorphism transitional between the amphibolite and granulite facies can be estimated at 600–550 Ma on the basis of Xes-Xen dating. In general, the results of our study indicate that the age of regional metamorphism of the Rayner complex at the Molodezhnaya area is approximately 600–550 Ma rather than ∼1.0 Ga, as was previously supposed.