Gneiss Complex Research Articles

Geochronology and chemostratigraphy of the 2.47–1.96 Ga rift-related volcano-sedimentary succession in the Karasjok Greenstone Belt, northern Norway, and its regional correlation within the Fennoscandian Shield

New geochronological data for the amphibolite-facies volcano-sedimentary Karasjok Greenstone Belt in northwestern Fennoscandia bracket its age between 2.47 and 1.96 Ga. The volcano-sedimentary units were deposited on the 3005 ± 24 to 2776 ± 69 Ma crystalline basement rocks of the Jergul Gneiss Complex. The lowermost Vuomegielas Formation contains rhyodacites dated at 2469 ± 12 Ma that were deposited in an intracratonic rift setting related to the Matachewan mantle plume event. The overlying Skuvanvárri Formation, correlative with the early Jatulian (c. 2.2 Ga) units on the Fennoscandian Shield, consists of siliciclastic sedimentary rocks deposited in coastal deltaic and shallow-marine settings. The overlying mid-Jatulian Gollebáiki Formation contains 2151 ± 5 Ma mafic intrusions and tholeiitic basalts related to a rifting event and breakup of the Superia supercraton that resulted in voluminous volcanic and shallow-marine deposits. The Ni-Cu-PGE-mineralized Porsvann intrusion, hosted by the Gollebáiki Formation, yielded an age of 2056 ± 10 Ma, which corresponds with other Ni-Cu-PGE-bearing intrusions on the Fennoscandian Shield. The uppermost Báhkilvárre Formation consists of basalts, komatiites, and andesitic lavas. A synvolcanic gabbro within the komatiites in the lower part of the formation yielded an age of 1984 ± 16 Ma. Andesites in the upper part of the same formation were dated at 1961 ± 1 Ma. They represent the final Paleoproterozoic magmatic event that affected large areas of the Fennoscandian Shield prior to the Lapland-Kola and Svecofennian orogenies. Carbon isotope values for carbonates of the Skuvanvárri and Gollebáiki formations range up to highly positive (+12.4 ‰) values, corresponding to the c. 2.22–2.06 Ga Lomagundi carbon isotope excursion. In contrast, carbonates from the Báhkilvárre Formation do not yield elevated δ13C values, indicating that it was deposited in the aftermath of the Lomagundi carbon isotope excursion. New geochronologic and chemostratigraphic data for the Karasjok Greenstone Belt support correlation with the early Paleoproterozoic sedimentary successions deposited on the eastern margin of the Superior craton and position the northern-northwestern margin of the Karelia-Kola craton against the eastern margin of the Superior craton between c. 2.5 and 2.05 Ga.

Structural Evolution of the Rocks of Western Chhotanagpur Gneissic Complex (CGC) in Balangi-Sanawal Area, Chhattisgarh

Structural Evolution of the Rocks of Western Chhotanagpur Gneissic Complex (CGC) in Balangi-Sanawal Area, Chhattisgarh

Wall Fracturing Versus Mechanical Instability as Competing Intrusion Mechanisms of Dikes: Insights From Laboratory Experiments

AbstractIgneous dike intrusion is a primary crust‐forming process at the Earth's plate boundaries. Understanding its mechanism is thus crucially important in lithospheric studies. Our present article combines experimental and field observations to investigate the problem of dike emplacement from a mechanical perspective. Scaled laboratory experiments were conducted by injecting immiscible liquids into visco‐elastic and visco‐elasto‐plastic host materials at varying volumetric flow rates (VFR = 0.100 to 1.670 ml s−1). Another set of experiments used different injecting liquid–host combinations to set their viscosity ratios (η*) at low (105), moderate (106), and high (109) values. These two lines of experiments allow us to recognize three principal mechanisms of liquid pathways: tensile fracturing of the host, wave instability at the liquid‐host interface, and coupled fracturing‐wave instability process. The three mechanisms give rise to a wide variation in intrusion geometry, ranging from planar structures with elliptical outlines to typical bulbous geometry, with intermediate patterns characterized by in‐plane and off‐plane wavy interfaces with the host. This study uses the experimental data to constrain the VFR conditions that determine the fracturing versus wave instability‐controlled mechanisms. It is also shown from the experiments that η* can significantly influence the evolution of three‐dimensional intrusive geometry. The Chotonagpur Granite Gneiss Complex in eastern India is chosen as a field study area to validate our laboratory findings using a 2D shape analysis of the intrusive boundaries in terms of their fractal dimensions (D) and skewness‐kurtosis estimates.

U–Pb geochronology and metamorphic history of gneissic rocks from Sarwar‐Junia Fault Zone, Rajasthan, NW India: Implications for the tectonothermal evolution of the Aravalli‐Delhi Mobile Belt

The gneisses of the Sarwar‐Junia Fault Zone (SJFZ), in the Mangalwar Gneissic Complex, a part of the Banded Gneissic Complex (BGC‐II) of northwest India recorded two phases of metamorphism. The pelitic gneiss from SJFZ mainly consists of biotite, sillimanite, garnet, quartz, K‐feldspar and plagioclase. On the other hand, the granitic gneiss from SJFZ mainly consists of quartz, plagioclase, K‐feldspar and biotite. From zircon and monazite geochronology, the protolith of pelitic gneiss was deposited on basement granitic gneiss and the metamorphism occurred at ca. 950 Ma affecting both the granitic gneiss and the pelitic gneiss. Estimation of the P‐T conditions of metamorphism and pseudosection modelling of the pelitic gneiss revealed that the peak stage metamorphic condition of M2 culminated at ~850°C and ~7 kbar pressure followed by a near‐isobaric cooling. Large garnet porphyroblasts in contact with retrograde biotite shows Fe‐Mg resetting close to the core region, which implies a slow cooling. Therefore, the possible exhumation during the subduction‐collision tectonics was very slow, if not completely ceased, at ca. 950 Ma. The shared tectonothermal history of the Sandmata Metamorphic Complex and Mangalwar Metamorphic Complex in the BGC‐II is now evident from the present study of the SJFZ, and confirms the presence of Grenvillian‐aged tectonothermal events in the eastern part of BGC‐II. The abovementioned near‐isobarically cooled and hydrated crust of the SJFZ records subsequent event(s) of shallow crustal brittle faulting as evident by the presence of pseudotachylyte veins and cataclastic fabric observed in the studied gneisses. Brittle faulting possibly occurred between ca. 905 Ma and ca. 750 Ma, as interpreted from the present geochronological data.

New Data on the Geological Structure and Precambrian Evolution of the Mountain Framework of the Denman Glacier Western Side (East Antarctica): First Paleoarchean Age for Plagiogneisses

The mountain framework of the Denman Glacier is a poorly explored area. However, it is a key region of East Antarctica in the context of studying the Precambrian geological history and geodynamic evolution of the Archean protocratons of the Antarctic Shield and in their comparison with the Archean crust-forming events of other regions of the Earth. Original U–Pb isotope geochronology data from zircons of metamorphic and intrusive rocks sampled from the western side of the Denman Glacier are reported. A geotectonic interpretation of the data was carried out. The Paleoarchean age of crystallization (3355 ± 5.4 Ma) of the magmatic protolith of tonalitic orthogneisses of the Davis granite–gneiss complex was obtained for the first time in this sector of Antarctica. In the Archean time interval, the Davis Paleoarchean protocraton was affected by multistage polymetamorphism in the intervals ~3100–3000 and 2900–2800 Ma ago. A crustal extension associated with the late stage is indicated by syntectonic intrusion of ultramafic dikes and pyroxenite sills (2827 ± 6 Ma). The time when granite veins and subalkaline granitoid plutons formed corresponds to the time of tectono-thermal Pan-African activization in the interval of 550–510 Ma typical for the Rayner Province of East Antarctica. The terrane of Davis craton is similar to the Paleo-Mesoarchean protocratons of East Antarctica, India, and Australia for the time of formation and the evolution of geodynamic processes.

Titanium and iron isotopic records of granitoid crust production in diverse Archean cratons

Archean granitoid provides critical clues on how Earth's felsic crust was established and its geodynamics evolved. In this study, we present Fe-Ti isotope and trace-element data for granitoids (mostly TTGs) from the ∼3.8-3.6 Ga Itsaq Gneiss Complex and ∼3.3 Ga East Pilbara Terrane, Pilbara Craton. TTGs from those localities and several other cratons follow the same Ti isotopic fractionation trend as modern calc-alkaline rocks. This similarity hides important petrogenetic differences as partial melting could have played a much more important role in establishing the felsic character of TTGs compared to modern granites of calc-alkaline affinity. Our thermodynamics-based modeling of isotopic fractionation shows that partial melting of hydrated metabasite can explain the Fe and Ti isotopic compositions of many TTGs but reworking of tonalite crust is needed to explain the most extreme compositions. The same Ti isotopic fractionation trends have been found now in several cratons (West Greenland, Slave, Pilbara, and Kaapvaal cratons) in granitoids formed in different geothermal gradients, meaning that it was likely a global signature of the Archean crust. Sediment provenance studies that use those TTG compositions and point to a primarily felsic crust in the Archean are therefore valid.

Petrology, geochemistry and U–Pb zircon geochronology of Fe–Ti oxide ore‐bearing mafic sill from Saltora‐Mejia area of Chotanagpur Granite Gneissic Complex, eastern India: Implication for late tectonic emplacement of mafic rock of E‐MORB nature in an orogenic belt

An east–west‐trending medium‐grained mafic sill containing co‐genetic Fe–Ti oxide ore lenses is found disposed within granite gneisses around Saltora‐Mejia area in the eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) of eastern India. CGGC is considered as a Proterozoic mobile belt as it witnessed multiple phases of deformation and high‐ grade metamorphism during 1.8–0.8 Ga. Occurrence of such Fe–Ti oxide ore‐bearing mafic sill is unique in the entire CGGC which is a vast Proterozoic orogenic belt and has witnessed many phases of voluminous mafic and felsic magmatisms. The mafic rock is of gabbronorite composition which contains plagioclase, clinopyroxene, orthopyroxene as major constituent primary minerals and amphibole as late magmatic mineral. The rock shows sub‐ophitic, intergranular, mosaic and poikilitic texture (defined by larger pargasitic grain). The gabbronorite shows iron enriched tholeiitic character, low Mg#, low abundances of Ni and Cr, slight enrichment in LILE, LREE and slight depletion in HFSE like Nb and Ti. The computed melt in equilibrium with the studied gabbronorite shows transitional orogenic to anorogenic, within‐plate and E‐MORB‐like geochemical character. In this study, the U–Pb zircon crystallization age (~960 Ma) of the Saltora‐Mejia gabbronorite is reported for the first time which coincides with the late tectonic stage of the most pervasive orogenic activity in the CGGC around 1.2–0.9 Ga. Transitional orogenic to anorogenic geochemical character, late tectonic evolution and other field and laboratory evidences together suggest evolution of the Saltora‐Mejia gabbronorite sill in a late tectonic extensional environment which might have been facilitated by delamination of a subducted plate and upwelling of asthenospheric mantle during the waning stage of a major orogeny in the CGGC.

Development of flanking structures in layered gneissic rock: Insights from numerical modelling

Flanking structures defined by dragged planar fabrics alongside of quartzo-feldspathic veins in gneissic rocks are very common. Varieties of mesoscale flanking structures were observed in parts of the Chhotonagpur Granite Gneissic Complex (CGGC), Eastern India. Alternate thin layering of quartzo-feldspathic and mafic minerals in gneissic rock causes mechanical anisotropy in rock during deformation. This study aims to explore specially the influence of mechanical anisotropy on the development of flanking structures. With the help of two –dimensional Finite Element modelling, we attempt to understand the effect of initial orientation of cross-cutting element (θ), kinematic vorticity number (Wk) and the anisotropy factor (δ) on the development of flanking structures. Deformed shape of cross-cutting element (RN), amount (LN) and nature (antithetic or synthetic) of offset of layers and pattern of drag (reverse or normal) are considered as important parameters for their nomenclature. As the final geometry of flanking structure does not only depend on the instantaneous deformational condition, we analyze the progressive development of flanking structures by varying the above parameters. Study reveals that mechanical anisotropy has strong influence on the geometry of flanking structures in layered gneissic rock, which depends on the initial orientation of long axis of cross-cutting element and its final position with respect to the orientation of principal stress axes and pole of anisotropy. The study also reveals that the flanking structures can be used as important tool for estimating the mechanical anisotropy in layered rock. Shape of cross-cutting element, amount of offset and the shifting of the object-tip points are the most important parameters for determining the anisotropy factor.

Mechanisms of Shear Band Formation in Heterogeneous Materials Under Compression: The Role of Pre‐Existing Mechanical Flaws

AbstractShear bands critically govern the shear failure processes and associated geophysical phenomena, for example, faulting, in Earth's crust. Earlier studies on homogeneous materials recognized temperature and strain rate as principal factors controlling the band growth. However, how inherent mechanical heterogeneities can influence their growth mechanisms and complex internal structures needs further investigations. The present article revisits this problem by combining field observations and laboratory experiments, supported by numerical simulations considering geological strain‐rates. Field studies in the Chotanagpur Granite Gneissic Complex of eastern India reveal varied types of macro‐scale shear zone localization. We conducted plane‐strain compression experiments on rock‐analogue models to investigate their origin. The experiments show that mechanical heterogeneities develop wide bands, localized preferentially in their neighborhood, unlike uniformly distributed, conjugate sets of closely spaced narrow bands in a homogeneous system. The wide bands eventually attain a composite structure, containing a core of densely packed band‐parallel sharp secondary bands, flanked by linear zones of closely spaced, narrow bands. We also demonstrate the effects of global strain‐rate on the band evolution in heterogeneous systems. Decreasing strain‐rates replace the composite bands by well‐defined homogeneous shear bands, containing a core of uniform shear, bordered by weakly sheared narrow zones, grading into almost undeformed walls. The experimental results are complemented with numerical models based on visco‐elasto‐plastic rheology, which establish appropriate scaling of the rock analogue to mid‐crustal deformations. This integrated approach finally leads us to conclude that inherent heterogeneities can result in large variations of shear band structures in geological terrains.

No evidence of supracrustal recycling in Si-O isotopes of Earth's oldest rocks 4 Ga ago.

Identifying the oldest evidence for the recycling of hydrated crust into magma on Earth is important because it is most effectively achieved by subduction. However, given the sparse geological record of early Earth, the timing of first supracrustal recycling is controversial. Silicon and oxygen isotopes have been used as indicators of crustal evolution on Archean igneous rocks and minerals to trace supracrustal recycling but with variable results. We present Si-O isotopes of Earth's oldest rocks [4.0 billion years ago (Ga)] from the Acasta Gneiss Complex, northwest Canada, obtained using multiple techniques applied to zircon, quartz, and whole rock samples. Undisturbed zircon is considered the most reliable recorder of primary Si signatures. By combining reliable Si isotope data from the Acasta samples with filtered data from Archean rocks globally, we observe that widespread evidence for a heavy Si signature is recorded since 3.8 Ga, marking the earliest record of surface silicon recycling.

Zircon within chromitite requires revision of the tectonic history of the Eoarchean Itsaq Gneiss complex, Greenland

The Ujaragssuit Nunât layered (UNL) unit in the Itsaq Gneiss Complex, west Greenland, has been considered to contain one of the oldest chromitites on Earth based on ∼ ca. 4.1 Ga Hadean whole rock Pt–Os model ages and ca. 3.81 Ga zircon U-Pb age of the surrounding orthogneiss. This study obtained zircon from the chromitite within this unit as well as granitoid sheets that intruded into the UNL unit. In–situ U–Pb–Hf–O isotope measurements were made on the zircons. Zircons from both the chromitite and the intrusive granitoids show concordant U–Pb ages of ca. 2.97–2.95 Ga. In contrast, Hf and O isotopic analyses indicate that zircons in the chromitites have a different origin from those in the intrusive granitoids. Zircons from granitoids yielded Th/U ratios higher than 0.2, initial Hf isotope ratios of 0.2805 – 0.2807 (i.e., initial εHf value of − 11 to − 5), and δ18O values of mostly 6.0‰–7.0‰, which are typical for felsic igneous rocks in Archean continental crust. The least altered zircons from a chromitite exhibited initial Hf isotope ratios of 0.28078 – 0.28084 (i.e., initial εHf value of − 1.1 to − 0.4), close the chondritic value at ca. 3.0 Ga and the depleted mantle at ca. 3.2 Ga. These zircons also have δ18O values of 4.2‰–6.1‰ which correspond to typical mantle values. The other chromitite zircons yielded Th/U ratios lower than 0.1, and Hf and O isotopic compositions ranging between the least altered zircons and the intrusive granitoid zircons. These results indicate that the zircons in the chromitites crystallized before or during the 2.97–2.95 Ga granitoid intrusion and most of the zircons were altered by subsequent metasomatism. Furthermore, the present results suggest that zircons in the chromitites originally had depleted Hf isotopic compositions at ca. 3.2–3.0 Ga. This can be explained by two different models of the evolution of the UNL unit. One is that if the UNL unit was formed at > 3.81 Ga as previously thought, with the zircons in the chromitites subsequently being precipitated by ca. 3.2–2.95 Ga during metamorphism or metasomatism. The other model is that the UNL unit itself was actually formed at ca. 3.2–3.0 Ga, with zircon in the chromitite representing the crystallisation age of the unit, which was then tectonically incorporated into the ca. 3.81 Ga orthogneiss prior to the 2.97–2.95 Ga granitoid intrusion event. In either case, our zircon analyses reveal significant evolutionary history prior to depleted mantle Hf model ages of 3.2–2.95 Ga. Revision of the geotectonic evolution of the UNL unit and the Itsaq Gneiss Complex is therefore required.

Sulfur isotope evidence from peridotite enclaves in southern West Greenland for recycling of surface material into Eoarchean depleted mantle domains

Eoarchean peridotite enclaves found in southern West Greenland's Itsaq Gneiss Complex (IGC) represent an important and valuable record of Earth's early geodynamic history. However, the origins of these rocks and the processes acting on them in the Eoarchean remain a subject of debate. Some researchers have proposed that these peridotites represent the oldest preserved pieces of Earth's mantle, while others have suggested that they represent ultramafic cumulates. Similarly, the geodynamic context in which they formed and were emplaced is subject to multiple interpretations. Some researchers argue that only vertical tectonic processes operated in the Eoarchean and others contend that these peridotites were embedded in the crust by horizontal tectonic processes. We present multiple sulfur isotope data from IGC peridotite enclaves, offering new insights into these rocks' origins and tectonic processes in the early Earth. Generally small but significant nonzero Δ33S values of 0.04 to 0.21‰ were detected in the studied peridotites. Peridotites with almost no petrographic signs of metasomatic alteration have on average slightly higher Δ33S values, whereas peridotites with clear petrographic and geochemical signatures of melt metasomatism were found to have slightly lower Δ33S values. These Δ33S values point to incorporation of surface-derived material of Archean age or older. Correlations between Δ33S values and previously published major and trace element data support the view that these peridotites were subject to hydrous melt depletion of incompatible elements, followed by variable melt re-enrichment. Notably, a distinct correlation also exists between previously published Hf isotope data and Δ33S values, indicating a depleted mantle source for melt percolating through the peridotites in the Eoarchean. The sulfur isotope data demonstrate that these processes included the introduction of surface-derived sulfur, pointing to an early onset of crustal recycling. These findings point to the existence of depleted mantle domains in the Hadean and suggest that crustal recycling processes operated during the Eoarchean or earlier.

Petrogenesis of high-Mg andesites from the Proterozoic Shillong Basin, Northeast India: evidence for continuation of the Central Indian Tectonic Zone to the Assam-Meghalaya Gneissic Complex and its implications for the Columbia supercontinent reconstruction

ABSTRACT This study provides comprehensive geochemical and petrological data for the newly discovered high-magnesium andesites (HMAs) from the Shillong Group of rocks in the Proterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex (AMGC). It is the first report of HMAs from the easternmost part of the Indian shield region. The AMGC holds immense geodynamic significance because this region has recorded magmatism related to Columbia, Rodinia, and Pangea supercontinental events. The present work discusses geochemical signatures of the Shillong HMAs characterized by calc-alkaline traits with high-K contents and significant concentrations of TiO₂ (0.54–0.76 wt %) higher than boninites (TiO2 < 0.5 wt %), and Mg# (53–56). They exhibit high LILE/HFSE and LREE/HREE ratios, which are distinctive features of magmas generated in subduction zones. Tectonic discrimination diagrams clearly suggest that the Shillong HMAs were generated in a back-arc basin regime. We argue that the Shillong HMAs were derived by 5% to 10% partial melting of a depleted lherzolite mantle source within the spinel – garnet transition zone. We also argue that the Shillong HMAs may have formed as a result of the interaction between subducted sediment-derived melts and mantle. The Shillong HMAs have undergone low- to medium-grade greenschist facies metamorphism characterized by significant mineral alteration, however, they still preserve bulk original alteration-resistant trace element compositions. From a regional perspective, the synchronous nature of origin and geotectonic setting, associated geochronological ages of formation, and regional structural trends suggest that the Shillong Basin is the easternmost continuation of the Mahakoshal Mobile Belt and specifically, the Bathani volcano-sedimentary sequence of the Central Indian Tectonic Zone (CITZ); and together they form the northern fragments of the same suture zone consisting of the CITZ, the Chotanagpur Granite Gneiss Complex, and the AMGC, which demarcates the northern and southern Indian blocks of the Greater Indian Landmass.

U–Pb SHRIMP zircon dating and geochemistry of metapelites from the Shillong Meghalaya Gneissic Complex, NE India: Implications for nature of protolith and tectonic setting

The Sonapahar area of Shillong Meghalaya Gneissic Complex (SMGC) is an extension of the Central India Tectonic Zone within the Precambrian Indian shield. The study area mainly consists of metapelites, basic granulites, granitic gneisses and high-grade gneisses as metamorphic imprints. U–Pb SHRIMP zircon dating yields a late Paleoproterozoic age of 1672 ± 6 Ma, which is age of the metapelites protolith. The petrochronology and geochemistry of metapelites and granitic gneiss basement rocks have been studied to learn more about the SMGC's tectonic environment, involvement in the supercontinent cycle, crustal development and geodynamic context. In the NCKFMASHTO system, phase equilibria modelling of metapelites shows peak metamorphic stage at P-T conditions of 8.25 kbar/ 820°C. Cordierite is formed during retrograde metamorphism during the decompression phases. The negative anomalies in Nb, Sr and Ti indicate that crustal sources played a significant role in magmatic emplacement. Geochemical data from metapelites indicate that protoliths were trachy-andesitic in nature, generated during the syn-orogenic environment by subduction tectonism. The SMGC experienced significant tectonic activity throughout the Paleoproterozoic age when the subduction tectonic setting coincided with orogenic activity. The presence of Paleoproterozoic age in the study area, as well as Southwestern Australia and East Antarctica correlates the Sonapahar granulite with the Columbia supercontinent history.

U-Pb Neoproterozoic age and petrogenesis of a calc-alkaline shoshonitic lamprophyre from Simdega area, Chhotanagpur Gneissic Complex (Eastern India): Implication for the evolution of the Central Indian Tectonic Zone and Rodinia tectonics

The age of emplacement and geochemistry of the lamprophyres are of tectonic significance as they have potential to unravel global scale geodynamic processes. We present petrology, U-Pb SHRIMP apatite and titanite ages and bulk-rock Sr-Nd isotope data for an unmetamorphosed and undeformed lamprophyre dyke from the Simdega area of the Chhotanagpur Gneissic Complex (CGC) which is a component of the E-W trending Central Indian Tectonic Zone (CITZ), India. The CITZ is a major intercontinental suture which separates the northern Indian and the southern Indian blocks whose polarity of their subduction is a contentious issue. The lamprophyre exhibits a strong porphyritic-panidiomorphic texture imparted by the megacrysts/phenocrysts of mica and amphibole with feldspar, apatite, titanite, zircon and opaques confined to the groundmass. Based on combined mineralogy and geochemistry, the lamprophyre is classified to be of calc-alkaline variety (minette) with shoshonitic affinities. Mg# (70.7–78.2) contents highlight the primitive melt character whereas incompatible trace element ratios exclude crustal contamination and are indistinguishable from those of the subduction-related global as well as Eastern Dharwar craton (southern India) calc-alkaline lamprophyres. U-Pb dating of apatite gave an emplacement age of 944 ± 82 Ma which is indistinguishable, within the error limits, from the U-Pb titanite age of 942.1 ± 5 Ma demonstrating a Neoproterozoic magmatic emplacement age of the lamprophyre synchronous with the Rodinia assembly. Bulk-rock 87Sr/86Srinitial (0.707239 and 0.708910) and ɛNdinitial (-8.9 to -8.3) highlights the involvement of an enriched mantle source. Calculated Paleoproterozoic model ages (Nd depleted mantle) of 2.1 Ga of the lamprophyre are indistinguishable from those of the co-spatial amphibolite dykes. Petrogenetic modeling involving rare earth elements reveals that the derivation of the lamprophyre magma from 2 to 3% partial melting of a mixed garnet (70%) and spinel (30%) lherzolitic mantle source with minor phlogopite. Our study highlights that the western part of the CGC was less affected, relative to the eastern part, by the M3 regional amphibolite grade metamorphic event (ca. 920-880 Ma) and also supports the geodynamic models involving northward-directed subduction of the Southern Indian block under the Northern Indian block.-

Geochemical constraints on the petrogenesis of mafic rocks (metadolerites) from the Proterozoic Shillong Basin, Northeast India: implications for growth of the Greater Indian Landmass

AbstractThe Paleo-Mesoproterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex is exposed in parts of Northeast India. The studied metadolerites are from the volcano-sedimentary sequence of Shillong Basin from the Borjuri area in the Mikir Massif. This episode of mafic magmatism can be correlated with the Columbia supercontinent formation and bears significance to its reconstruction. The present work discusses the field, petrography and geochemical characteristics of the metadolerites, which occur in close association with the quartzites of the Shillong Group of rocks (metasedimentary rocks of the Shillong Basin). Our data show distinctive characteristics of subduction-related magmatism exhibiting high LREE/HREE, large ion lithophile element/high field strength element ratios and pronounced negative Nb anomaly. Elemental ratios such as Zr/Ba (0.21–0.46), La/Nb (1.23–2.32) and Ba/Nb (30.08–56.90) point to a fluid-enriched lithospheric mantle source in a subduction regime. Metadolerites plot in the field of ‘back-arc basin basalts’ in tectonic discrimination diagrams reinforcing a subduction zone tectonic setting. The mafic rocks correspond to a 6–10 % partial melting of a mantle source incorporating spinel+garnet lherzolite. The metamorphic P-T of the metadolerites estimated from plagioclase-hornblende geothermobarometer (7–8 kbar, 664 °C) is indicative of amphibolite facies metamorphism in a medium P-T zone. Based on the comparative analysis of field observation, petrography, geochemistry and geological ages given by previous workers, we infer that the Shillong Basin represents a back-arc rift region and is the eastern continuation of the Bathani volcano-sedimentary sequence of the Chotanagpur Granite Gneiss Complex marking continuation of the Central Indian Tectonic Zone to the Mikir Massif.

U‐Th‐Pb and Trace Element Evaluation of Existing Titanite and Apatite LA‐ICP‐MS Reference Materials and Determination of 208Pb/232Th‐206Pb/238U Date Discordance in Archaean Accessory Phases

The last decade has seen a marked increase in U‐Th‐Pb petrochronological studies focused on titanite and apatite. This has motivated the need for detailed, phase‐specific method development and well‐characterised reference materials with a wide variety of ages and chemical compositions. This study presents new U‐Th‐Pb isotope and major‐, minor‐ and trace‐element mass fraction data for ten titanite and five apatite reference materials based on integrated EPMA, LA‐ICP‐MS and isotope‐dilution (ID) multi‐collector (MC)‐ICP‐MS characterisation. Cross‐comparison of EPMA and LA‐ICP‐MS data for the same titanite reference material suite demonstrates that careful selection of EPMA primary reference materials is necessary to minimise inaccuracies. We further identify a significant X‐ray interference when measuring low Ce abundances in Ti‐bearing phases and outline a method for its correction. New ID‐MC‐ICP‐MS and LA‐ICP‐MS data suggest tri‐concordance (defined as concordant ages for the 238U‐206Pb, 235U‐207Pb and 232Th‐208Pb decay systems) of the MKED‐1 titanite reference material and demonstrate the reproducibility of 208Pb/232Th measurements in some secondary titanite reference materials. Integration of 208Pb/232Th results with U‐Pb geochronology provides a meaningful tool to help interpret complex U‐Th‐Pb isotopic age spectra. We provide example applications of 208Pb/232Th LA‐ICP‐MS measurements toward interpreting Archaean titanite ages from the Acasta Gneiss Complex, Northwest Territories, Canada.

Formation of aluminous clinopyroxene-ilmenite-spinel symplectic assemblage in a regionally metamorphosed calc-silicate granulite from the Chotanagpur Granite Gneiss Complex, East Indian shield

Formation of aluminous clinopyroxene-ilmenite-spinel symplectic assemblage in a regionally metamorphosed calc-silicate granulite from the Chotanagpur Granite Gneiss Complex, East Indian shield

Groundwater quality enumeration and health risk in the extended part of Chhotanagpur granite gneiss complex of India.

The majority of people on the earth bank largely on groundwater to quench their thirst. In the era of rapid population growth, the over-exploitation of groundwater gives rise to water scarcity, and people find themselves in distress to manage safe drinking water. In this backdrop, the present study is carried out in the terrain of Pre-Cambrian igneous and high- to low-graded metamorphic rocks, to assess the groundwater potential zones (GWPZs) and evaluation of groundwater quality. The map of GWPZ is produced employing the multi-criteria decision-making model and geospatial technology. It unveils that around 29% area of the watershed enjoys good GWPZ, whereas around 43% area experiences low GWPZ. The overall accuracy of the simulated model is 92%. The water quality index indicates that 68% of water samples belong to excellent to good water quality. A significant proportion of water samples (24%) are found to be unsuitable for drinking, which may be due to groundwater contamination by the process of leaching of mineral-rich weathered rocks. The presence of fluoride (F-) beyond the maximum permissible limit (1.5mg L-1) of WHO is recorded among 18% samples of the watershed, where 24,963 souls including 3457 children aged between 0 and 6years lived and might have ingested F- through drinking water. Hence, the health risk of those people is quite high. Children are at a more non-carcinogenic health risk of F- than adults. The study also confirms no statistically significant difference (p ˃ 0.05) is observed between low and high GWPZ with respect to groundwater quality. The study recommends adopting a sustainable outlook to explore GWPZ, and an assessment of drinking water quality must be done before drinking.

Magneto-mineralogical characteristics of mafite-ultramafites of the Middle Bug River area and Holovaniv suture zone of the Ukrainian Shield (overview)

Mafites and ultramafites manifest in the magnetic field as significant magnetic anomalies due to the high content of magnetite, titanomagnetite and iron-magnesium silicates. Studying the mineralogy and magnetic properties of Archean rocks formed deep in the lower crust and uplifted to the surface allows to understanding the sources of magnetic anomalies. Such rocks are known in the Ukrainian shield, in particular in the Middle Bug River region and in the Holovaniv suture zone (HSZ). We consider mafic-ultramafic assemblages of the rocks of this region, their mineral composition and magnetic characteristics, manifestations in the magnetic field, and the distribution of magnetic minerals. Several mafic-ultramafic associations of different ages, composed of effusive, sedimentary-effusive and intrusive formations, are recognized for the area the Pobuzkiy ore mining region. Most of them were transformed intensively by metamorphism of granulite (to eclogite) facies and by intense tectonic and diaphoretic processes. The main volcanics and volcanogenic-sedimentary rocks belong to the Tivriv stratum of the Paleoarchean Dniester-Bug and Neoarchaean Bug series. Mantle rocks were the protolith of the Tivriv stratum, which are similar in composition to oceanic basalts. The Pavliv stratum is considered as a part of the Dniester-Bug series. It is composed mainly of two-pyroxene crystal schists (sometimes amphibolized, often with significant magnetite content (up to 10 %)), magnetite-orthopyroxene crystal schists, and bodies of ferruginous quartzites. Both series contain gneiss complexes, as well as bodies of basite-hyperbasites. Mafic rocks are mainly represented by hornblende-pyroxene crystal schists and amphibolites. Ultramafite and mafit-ultramafite intrusive bodies were mapped in the central and northern parts of the HSZ where they are presented by rocks of the hyperbasite and gabbro-peridotite formations. The Holovaniv block of the HSZ is spatially coincident with the Holovaniv gravitational maximum and magnetic anomalies, which are probably caused by the rooting of mafit-ultramafits from the upper mantle along deep fault zones. Magnetic sources with increased magnetization were identified within the district of Haivoron. They are associated with pyroxene schists, gneisses, coarse-grained pyroxene schists, and ferruginous quartzites. The high values of magnetic parameters of the rocks of the Haivoron-Zavallya region are explained by the presence of eulysites and magnetite-hypersthene crystal schists. Within the area of occurence of charnockite-enderbite rocks, the magnetic field with increased intensity and a large-mosaic structure is observed. Differentiation of the magnetic properties of the rocks of the upper part of the Earth’s crust, the shape and low power of the sources indicate their possible primary magmatic formation in the form of massifs and dykes with further metamorphic transformations. Magnetite is the main magnetic mineral of mafite-ultramafites according to thermomagnetic analysis and ore microscopy. Several generations of magnetite are observed. Early generations (reliably magmatic) are present in the dark-colored minerals as point inclusions and emulsion discharges along fissures (disintegration structures of solid solutions). The increase of the amount of magnetite in all types of rocks is associated with superimposed secondary transformations. Redistribution of iron occurs in the recrystallization zones with the formation of clusters of secondary coarse-grained magnetite. According to the hypothesis, the magnetite of ferruginous quartzites has metamorphic origin, while the origin of the magnetite of iron-carbonate and iron-siliceous formations is a controversial issue. It depends on the determination of the genesis of the original substance.