As a tectonic window into the Lesser Himachal Himalaya, India, a group of metasediments and gneissic rocks, known as the Jutogh Group and Wangtu Gneissic Complex (WGC), occurs near the Jhakri thrust to the west and Wangtu to the east. In the Jutogh Group, chlorite-mica schist, garnet-staurolite schist and sillimanite-schist develop successively. The formation of chemically zoned garnet, which destabilized low-temperature assemblages, is predicted to be at 550–650 °C and 0.8–0.9 GPa by phase equilibria modelling. The retrograde segment consists of exhumation and cooling, yielding a tight clockwise P–T path. Moreover, textural observations and in-situ U-Th-Pb chemical dating indicate that metasedimentary rocks contain Cambrian monazites. These monazites have ages that cluster around 500 Ma. The ƐNd[1.8Ga] of Jutogh rocks ranges from − 1.0 to -8.1, with depleted mantle-model ages between 3.07 and 2.25 Ga. The garnet core and its leachates yield an Sm-Nd isochron age of 472 Ma. Another Sm-Nd isochron age of 454 Ma is obtained from biotite, garnet rim, and garnet rim leachate. According to phase equilibrium modelling, Sm-Nd dating, and monazite geochronology, the Jutogh Group experienced metamorphism along the northeast margin of Gondwana during the Cambro-Ordovician accretion.

We report new Sm–Nd whole rock-mineral isochron ages of 2514 ± 13 Ma (MSWD = 0.79) and 2651 ± 95 Ma (MSWD = 7.4) from two east coast dykes (ECD) of Southern Granulite Terrain (SGT), India. The ages from the representative mafic dyke samples correspond to the time of intrusion of ECD into the eastern part of SGT, indicating the presence of an older Archean crust in SGT near the Pondicherry coast. The Sm–Nd ages obtained from the present study, along with geochronological information from Singhbhum Craton, suggest a magmatic linkage between SGT (including southern Dharwar Craton) and Singhbhum Craton during the Neoarchean period. The older ages obtained from the mafic dykes of the present study are comparable with the Sm–Nd ages of older mafic dykes from Nuggihalli green stone belt of Western Dharwar Craton (WDC), Pb–Pb ages of mafic dykes from Singhbhum Craton of India and the U–Pb ages from Pilbara and Kaapvaal cartons. These comparisons unlock a clue to Neoarchean (2.8–2.5 Ga) paleogeographic reconstructions of Pilbara, Kaapvaal, Singhbhum cratons, northern SGT (including southern Dharwar Craton) and also provide an opportunity for wide windows of research to be undertaken considering the dykes from SGT.

The Nidar ophiolite is one of the well-preserved and almost complete ophiolite sections of the Neo-Tethyan oceanic lithosphere, obducted along the continental margin between the Indian and the Eurasian plate. This ophiolite sequence is mostly dominated by ultramafic rocks, consisting of forearc-related refractory, mainly harzburgite, dunite, and serpentinite, with minor intrusions of lherzolite, chromitites, and pyroxenites. In this present study, detailed mineralogical, whole rock geochemistry (major oxides, trace elements, PGE), and Nd isotopic composition of mantle-derived peridotites have been carried out to constrain the petrogenesis and melt evolution. These peridotites are depleted in nature due to the low modal composition of clinopyroxene, high forsterite content in olivine, and wide variation in Cr# and bulk rock chemistry, indicating variable degree of partial melting. The spoon-shaped rare earth element (REE) patterns indicate metasomatism by fluids derived from a subducting slab enriched in light REEs. Geochemical composition of the studied peridotites rocks is marked by high ratio of Al2O3/TiO2, LILE-LREE enrichment, HFSE depletion, and spoon-shaped chondrite-normalized REE patterns and (La/Sm)(N) > 1 and (Gd/Yb)(N) < 1, indicates some involvement of boninitic mantle melts and validate a subduction initiation process. The total PGE of the peridotites (Sigma PGE = 33-337 ppb) is much more enriched than that of the primitive mantle and other ophiolite peridotites. The PGE distribution displays a concave upward pattern with higher PPGE/IPGE ratios (i.e., 0.11-1.45), suggesting that partial melting is not the only process for the evolution of the Nidar ophiolite peridotites. Enrichment of PPGE and incompatible elements (like LREE) and higher Pd/Ir ratio (0.69-8.26) indicates that these peridotites have undergone fluid/melt interaction in a supra-subduction zone (SSZ) tectonic setting. PGE concentrations of these depleted harzburgites and dunites, formed by partial melting of cpx-harzburgites in an SSZ that produced the boninitic-like melt. The enrichment of incompatible elements like the PPGE is mainly due to the circulation of fluids in the subduction zone, which leads to the PGE fractionation in mantle peridotites. Also, these peridotites have Nd-143/Nd-144 ratios (0.51148-0.51262) and epsilon Nd(t) (t = 140 Ma) values (i.e., +0.97 to -21.3), indicating derivation from depleted mantle sources within an intra-oceanic arc setting. The geochemical behavior exhibited by the Nidar ophiolite peridotites suggests the evolution of a highly depleted fore-arc mantle wedge significantly modified by various fluids and melts during subduction. The mineralogical, geochemical, and Nd isotopic composition of these peridotites and dunites mutually depict the diverse mantle compositions, suggesting insights into the interactions between the oceanic crust and mantle as well as associated geochemical cycling in an SSZ environment.

Magmatic Ni-Cu-platinum group element (PGE)-Te mineralization in the Gondpipri mafic-ultramafic layered intrusion of ca. 3323 +/- 74 Ma age, western Bastar craton, central India, is one of the most prospective explora-tion targets for magmatic sulfides in India. The Gondpipri layered intrusion is divided into two distinct groups of rocks based on their mineralization potential, which include (1) mineralized layered gabbro and pyroxenite and (2) a barren olivine gabbro intrusion. The host rocks show Cu + Ni concentrations up to 5,000 ppm with a Cu/Ni ratio <1 and all PGE values between 0.1 and 1.1 ppm. Mineralization occurs in two modes: type I min-eralization occurring as blebs, specks, and dissemination and type II mineralization occurring as stringers and minor veins. The geochemical data suggest that the parental magma of the host rock was generated at depths between spinel and garnet peridotite mantle source regions and subsequently modified by assimilation frac-tional crystallization (AFC) of the continental crust. High large ion lithophile elements, Th/Yb ratios of the stud-ied rocks, and Sm-Nd isotope studies are consistent with a depleted mantle source. The geochemical proxies such as Th versus Ba/Th and (Ta/La)PM versus (Hf/Sm)PM and higher Sr/Nd (2.21-82.58) ratios indicate involve-ment of fluid-related subduction metasomatism and enrichment processes in an island-arc tectonic setting.Mineral assemblages and textural relationships between platinum group minerals (PGMs) and base metal sulfides suggest that sulfide-silicate liquid immiscibility was brought about by the precipitation of magnetite/ Cr magnetite resulting in sulfide saturation in the melt by decreasing S solubility. Sulfur isotope compositions (delta 34S: 1.61-3.30 parts per thousand) and Sm-Nd geochemistry suggest that the sulfur was added in the tholeiitic magma by mag-matic process. Crustal contamination played a significant role in sulfide saturation and in bringing about PGE and Te, As, Bi, Sb, Se (TABS) mineralization. PGM-NiTeBi developed at relatively low temperatures, whereas moncheite (PtPd)Te2 and merenskyite (PdTe) were formed at 650 degrees C. The identification of Ni-Cu-PGM-Te in the margin of the western Bastar craton boosts deeper subsurface exploration.

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

This study reports on two lamprophyre dykes from the Rapuru area along the margin of the Eastern Dharwar Craton (EDC) and the Nellore Schist Belt (NSB). The Rapuru lamprophyre (RL) dykes are situated along the southern extension of the Prakassam Alkaline Province (PAP). The RL dykes are deformed, yet still preserve a porphyritic-panidiomorphic texture, with mica phenocrysts, and amphibole and feldspars in the groundmass. Geochemically, the RL dykes have a low Mg# (0.28-0.37), and Ni (30-60 ppm) and Cr (119-228 ppm) concentrations that indicate their evolved nature, such as for other reported lamprophyres from the PAP and EDC. This is further supported by Sr-Nd isotopic ratios that show an affinity towards a mid-ocean ridge basalt (MORB)-ocean island basalt (OIB)-like signature and a juvenile magmatic nature. The RL seems to have been affected by two major influences, namely, the primary source region, which is geochemically juvenile similar to the compositional field of enriched-MORB, and the continental lithosphere. Such magmas are known to have formed in a back-arc-basin environment. The initial Sr-87/Sr-86 ratio (c. 0.7012-0.7045) and initial eNd ratios (3.13-7.93) are in line with back-arc basin basalts recorded in other parts of the world. The field observations and bulk-rock Sr-Nd radiogenic isotope values in the present study support the Paleoproterozoic nature of the RL. This concurrence of juvenile radiogenic isotopes and fluid-related trace element compositions apparently suggest dehydration of a subducted-slab-triggered metasomatism of the overlying mantle wedge in a subduction-related geodynamic setting. Such intrusive lamprophyre rocks of older ages are limited in India as well as other parts of the world. The 2.1 and 1.8 Ga rocks are widely considered to represent the initial accretion and final break-up of an erstwhile Columbia supercontinent assembly. We argue that the RL were formed in the Paleoproterozoic during the waxing stages of the Columbia supercontinent assembly in a back-arc basin environment, most probably due to the low degree of partial melting of the asthenosphere-lithospheric interaction caused by the introduction of an influx of subduction components into the arc-back-arc basin system.

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

Neoarchean carbonate rocks of the Vanivilas Formation (2.7 Ga) occur extensively in greenstone belts of western Dharwar craton, are associated with banded Fe-Mn formations and clastic sediments including diamictites, preserve well-developed stromatolitic structures. We intend to understand the depositional environments of these carbonates, extent of oxygenation in shallow oceans and to examine the mantle and continent inputs to Neoarchean oceans. Depleted LREESN and slightly enriched HREESN patterns, Y/Ho > 28, La, Y and Eu spiking and varying Ce anomalies shown by most of the samples, are consistent with their marine origin. However, 4 samples exhibit negative Ce anomaly indicating presence of oxygen supporting the hypotheses of localised, protected, shallow marine oxygenation in redox stratified late Meso- to Neoarchean oceans. delta C-13 values are well preserved (mean = -0.27% (PDB)) similar to other Archean marine carbonates from other cratons and show no correlation with delta O-18 and Mn/Sr ratios. Whereas, the delta O-18 values (mean= -10.78%) show considerable depletion and can be attributed to later interactions with diagenetic/meteoric fluids. The mantle dominated, yet continental affected Neoarchean ocean chemistry is apparent from Sr-Nd isotope systematics. The lowest Sr-87/Sr-86(i) of carbonates, 0.7024 reflects ocean water Sr isotopic composition at 2.7 Ga, is higher than the model seawater evolution curve of Shields and Veizer, (2002). Similarly, other recent studies on Archean carbonates and barites reported higher Sr-87/Sr-86(i) values. This is due to the existence of Paleoarchean continental crust subjected to chemical weathering resulting in increased continental flux than previously thought of. Range of epsilon(Nd) values (-8.39 to 5.83) suggest that the carbonates had variable contributions of Nd from a long term depleted mantle through hydrothermal fluids, as well as, from 3.5 Ga old continental crust in the Dharwar craton. The carbonates were deposited in the Neoarchean ocean at variable depths within the shelf and those deposited at shallower and deeper environments are characterized respectively, by inputs derived from chemical weathering of >= 3.5 Ga old continental crust and mantle through hydrothermal fluids associated with submarine volcanic activity.

Neoarchean migmatized granodioritic gneisses and mafic enclaves from the Madras block of the Southern Granulite Terrain (SGT) were studied to understand their genetic relationship. The gneisses show calc-alkaline trend, more magnesian than tonalites, enrichment of LILE and LREE with HFSE depletion, and zero to slightly negative ɛNd values (t=2600 Ma) which indicate their precursors fractionated from sanukitoid magma generated by partial melting of hybridized mantle sources. Gabbroic magmas representing mafic enclaves with ɛNd values, –1.68 to +0.45, formed by partial melting of̄ fluid metasomatised mantle wedge and hybridized by interaction with granite magma. Underplating of these mafic magmas provided heat to trigger anatexis of the granodioritic arc-crust in the presence of H2O and formation of granite melts (leucosomes). The leucosomes with peritectic amphiboles have higher REE with prominent negative Eu anomaly, while quartzo-feldspathic leucosomes have lower REE, concave upward HREE and positive Eu anomaly. Fractionation and/or entrainment of amphibole, apatite, allanite, titanite and zircon controlled REE and other trace element abundances of the leucosomes. Thus, underplating of mafic magma caused migmatization, magma mixing and differentiation and transformation of the arc crust in the NE part of the Madras block which represents deeper parts of the eastern Dharwar craton.

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 similar to 500 Ma (published isotopic data). New petrological, geothermobarometric and P-T pseudosection studies show that dehydration-melting of amphibole (in mafic proto-lith) 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 degrees 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 degrees C. Followed by cooling through similar to 100 degrees 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.

Understanding the petrogenetic evolution of Archean gabbmic cumulates provides a window to the mantle geodynamics operated in the early Earth. Gabbmic cumulates are associated with various tectonic settings including convergent plate boundaries, mantle plumes, mid-ocean ridges, anorogenic, post-omgenic, continental rift settings and oceanic plateaus. The exposures of preserved gabbroic suites in the Archean are limited. The Mesoarchean Mayurbhanj Gabbro Anorthosite Complex (GAC) is a well-preserved mafic body situated along the eastern margin of the craton along with concomitant anorogenic Mayurbhanj granite. These rocks are devoid of deformational structures and/or mineral recrystallization. Based on their mineralogical characteristics, GAC rocks are mainly grouped into gabbronorite, leucogabbronorite, leucogabbm and gabbro. The minimal abun-dance of magmatic amphibole and the variation in the anorthite content (An(36)-An(84)) rules out the role of a hydrous mantle source in the evolution of these rocks. The studied samples display tholeiitic trend transitioning to talc-alkaline field, major and trace element data indicates the fractional crystallization of olivine and pyroxene. The trace element systematics of the Mayurbhanj GAC show LILE enrichment over HFSE depletion, with negative Nb-Ta and mixed Zr-Hf and Ti anomalies. The geochemical signatures such as elevated Th/Yb, Nb/Yb, variable Dy/Yb and Dy/Dy* and varying Gd/Yb-CN ratios suggest that parental magmas of the Mayurbhanj GAC originated from a heterogeneous depleted mantle source. The studied rocks' Sr-Nd isotopic systematics reveal that their parental magma has undergone assimilation by a Paleoarchean felsic crust. We argue that the 'arc-like' geochemical signatures displayed by these rocks resulted from crustal contamination and may not be due to subduction origin. Consolidating the field, geochemical and isotopic evidence suggests that the Mesoarchean Mayurbhanj GAC possibly evolved from an anhydrous heterogeneous mantle source in a non-compressive tectonic regime.

The timing of emplacement of the oldest supracrustal rocks represented by Sargur Group is significant in understanding crustal evolution processes in the western Dharwar craton. The age of ultramafic-mafic rocks of the Sargur Group is not well-constrained as the komatiitic and komatiitic basaltic rocks have always given unreliable ages due to larger errors attributed to their alteration. New Sm-Nd isotope data from relatively less-altered layered intrusive ultramafic-mafic rocks of Nuggihalli and Holenarsipur greenstone belts of Sargur Group, western Dharwar craton give a much precise age of 2934 ± 88 Ma. This is the youngest age obtained for the ultramafic-mafic rock complexes of Sargur Group and is indistinguishable from the oldest ages reported for the overlying Dharwar Supergroup rocks. Sargur Group rocks might merely represent older ultramafic equivalents of the Dharwar Supergroup. A clear temporal distinction does not exist to support the stratigraphic classification of these two groups separately. However, the sub-contemporaneous to older ages obtained for the tonalite-trondhjemite-granodiorite gneisses that surround these greenstone belts would imply that the narrow belts tucked within the gneisses are younger, and therefore, the greenstone belts, whose dismembered roots and parts are exposed alongside the gneisses, could represent an Archaean analogue of an ophiolite suite.

Carbonate rocks are a minor yet relevant component of Archean Dharwar craton. Exposures of Neoarchean carbonates (~2.7 Ga) serve as a valuable proxy to study the past ocean-atmospheric conditions. Samples from different parts of Vanivilas Formation, mostly dolomitic limestones and cherty dolomites with mica, graphite associations were studied. Most samples showed a substantial level of preservation of geochemical proxies. Depleted LREESN and slightly enriched HREESN patterns, superchondritic Y/Ho values, La, Eu and Y anomalies and lack of Ce anomaly in most of the samples indicated preservation of marine signatures. Presence of positive Eu anomaly in all the samples indicates the influence of hydrothermal fluids in Neoarchean oceans. The C isotopic compositions of most samples fall in the range of 0±0.5‰, with few samples show deviation in primary composition due to metamorphic decarbonation. Considerable depletion in δ18O values are observed and can be attributed to later interactions with fluids. The 87Sr/86Sri ratios of most of the samples are close to the estimated Sr isotopic composition of 2.7 Ga oceans, however, in some places, radiogenic values are encountered owing to post-depositional alterations.

This study reports, for the first time, Sr and Nd isotope ratios from the mafic rocks in the Manipur Ophiolite Complex (MOC), along with new elemental abundances to show the subduction zone influence. The initial Sr-87/Sr-86 ratios (for t = 127 Ma) range from 0.705230 to 0.709734. The initial Nd-143/Nd-144 and epsilon Nd-t (t = 127 Ma) range from 0.512611 to 0.512900 and +2.7 to +8.3, respectively. The high field strength element (HFSE) ratios vary widely, with Nb/Ta ranging from c. 3 to 18 and Zr/Hf ranging from 20 to 41, indicating fluid-rock interaction in the presence of rutile. The correlated variation in the Nd and Sr isotope ratios and the HFSEs, including TiO2, reflects the variation in the slab-derived fluids. The light rare earth element (LREE) enriched and flat patterns yielded by the mafic rocks are modelled by varying the degree of melting of the fluid-metasomatized mantle. The subsequent influx of the slab-derived fluid at a greater depth caused the re-melting of the previously depleted wedge to produce the LREE-depleted patterns. We propose that the geochemical variation recorded in the MOC rocks indicates the changing nature of fluid metasomatism of the mantle wedge across the subduction zone with time.

The Nuggihalli and Holenarsipur greenstone belts of the western Dharwar craton expose ultramafic–mafic rocks of the Mesoarchean. The rocks in these belts are geochemically considered as komatiites and komatiitic basalts with minor occurrences of tholeiitic and calc-alkaline basalts. The dominant ultramafic rocks of the Nuggihalli greenstone belt are layered and indicate fractionation processes at relatively shallower crustal levels. The Al-undepleted and Al-depleted signatures obtained could be attributed to magmatic differentiation processes and might be due to fractional crystallization of minerals such as hornblende and plagioclase, in addition to cumulus olivine and pyroxene. The chemical heterogeneity in the rocks of these greenstone belts might have therefore developed during the intrusion of the parental melts and their differentiation into a layered igneous complex. The differences in the lithological characteristics of the Holenarsipur and Nuggihalli greenstone belts can be explained by their different crustal levels of exposure. Presence of spinifex-textured komatiites need not necessarily imply that the sources have to be ultramafic and therefore of a deeper origin. This study indicates that the parental melts for unambiguous layered intrusive ultramafic–mafic complexes could be high-Mg basalts originating from relatively shallower levels. The probable geodynamic setting for the emplacement of the rocks of the two greenstone belts could be in a plume-modified mid-ocean ridge that was too thick and buoyant to be subducted, and the decompression-melted magma chamber developed igneous layering as the magma stalled in the lithosphere.

New geochemical and geochronological data are presented for a suite of metavolcanic rocks from Shimoga green-stone belt to understand their petrogenesis and to constrain the timing of final mafic magmatic activity in western Dharwar craton. We report an Sm-Nd isochron age of 2638 +/- 66 Ma for metabasaltic and metaandesitic rocks of the stratigraphically uppermost Medur Formation of Dharwar Supergroup exposed in the belt, which is the youngest age obtained yet for Neoarchean mafic magmatism in western Dharwar craton. The emplacement of the basalt-andesite-dacite-rhyolite suite of rocks took place in a magmatic arc culminating in the deposition of vast greywacke-argillites in a continental margin-type setting. The magmatic precursors of the metavolcanic rocks were generated by partial melting of depleted mantle by slab-derived fluids with variable input from the subducting slab. Assimilation-fractional crystallization processes may also have contributed to the magmas to a significant extent. The younger mafic lithologies of the Dharwar stratigraphy represent accretionary orogens and the date obtained in this study points towards the final stages for the assembly of western Dharwar craton by subduction processes during Neoarchean. The thickness and extent of the greywacke-argillite suite in SGB could be attributed to the protracted deposition during amalgamation of western Dharwar craton with a northern Archean continental crust that might underlie Deccan volcanic rocks. (C) 2019 Elsevier B.V. All rights reserved.

We present a combined mineralogical, chemical, and Sr-Nd isotopic study of hydrothermal alteration effects in near-surface Pleistocene rhyolite sampled at two distinct localities from the Los Azufres geothermal field (IAGF), Mexico. The alteration mineralogy of the near-surface rocks is dominated by silica polymorph minerals (cristobalite, tridymite, opal, and quartz), and kaolinite, showing an intense silicification in most altered samples. In some samples, alteration minerals berlinite, alunite, opal, and wollastonite, are also present. Sulfur (80-28,300 pg/g) and LOI (1.36-12.18%) contents were used to indicate the intensity of alteration for the LAGF rock samples. The changes were considered as significant when they exceeded the analytical errors. Two main types of chemical and isotopic effects were documented: (1) small loss of both SiO2 and alkalis represented by one pair of samples, which did not show significant changes in the most major elements, REEs, Nb and Ta negative anomalies, nor in the normalized multiple-element LILE/REE, LILE/HFSE, and REE/HFSE ratio parameters, but indicated significant increase in Al, Fe, Ba, Ga, and Pb; and (2) significant gain of SiO2 accompanied by loss of alkalis for the rest of the rock samples collected from a different site showed significant decrease of most major elements, all REE concentrations and normalized multiple-element ratio parameters as well as changes in numerous trace element concentrations. During this alteration, the size of the Nb and Ta negative anomalies became smaller as a result of the alteration of the second type. Both types of alteration showed significant changes in( 87)Sr/Sr-86, but generally not in Nd-143/Nd-144, probably related to the involvement of highly heterogeneous crust in terms of Sr-87/Sr-86. (C) 2018 Elsevier B.V. All rights reserved.

Major, trace and rare earth element (REE) abundances along with Nd isotopic ratios are measured in serpentinized peridotite samples from the Manipur Ophiolite Complex (MOC), Indo-Myanmar range (IMR). An attempt is made to resolve the conundrum caused by apparently contradictory petrogenetic models and tectonic settings reported in literature. Nd isotope ratios, reported for the first time, range from −7.6 to +10.4 in epsilon units in the whole-rock samples of serpentinized harzburgite and lherzolite. Variation of La/Yb with εNdt is consistent with the progressive addition of a subduction derived fluid to a depleted mantle source. Literature data from the mafic extrusive, intrusive and ultramafic rocks together with new data generated during this study can be explained by a two stage petrogenetic model. Non-modal dynamic melting in the mantle-wedge subsequent to the mixing of fluids derived from the subducted slab explains the observed range of elemental abundances in the MOC mafic and ultramafic rocks. This model also explains the observed variability of Nb and Th in the mafic rocks without invoking different sources for the different types of rocks. Taking into account field, petrographic and geochemical evidences, we propose, that MOC comprises a buoyant fore-arc-mantle-wedge system along with the subducted slab that was obducted during the terminal stage of subduction of the Neotethys below the Burmese plate.

The short-lived isotopic systematics of 146Sm-142Nd is a tracer of early silicate Earth differentiation events. Evidence for these events comes from anomalous 142Nd/144Nd, defined in terms of μ142Nd (μ142Nd=[{(142Nd/144Nd)sample/(142Nd/144Nd)standard}−1]×106) with respect to a terrestrial standard representing the modern accessible mantle. This requires measurement of accurate and highly precise 142Nd/144Nd, which is carried out by Thermal Ionisation Mass Spectrometry (TIMS). Since multiple factors affect the accuracy of the final results, we carried out a detailed investigation on the effect of various data acquisition, fractionation correction and normalization methods on the accuracy of 142Nd/144Nd determinations. Based on the analyses of Ames Nd standard using various combinations of the most commonly employed methods we observed that for a multi-dynamic mode of data acquisition, the power-normalised exponential law is the most appropriate method for mass fractionation correction. The time delays between successive sequences in a multi-dynamic mode had little effect on the final value of 142Nd/144Nd. The different standards have different 142Nd/144Nd ratios and therefore, their uses yield different μ142Nd values for the same sample. We extended this information to understand the two contradicting results from 1.48Ga alkaline rocks from Khariar, India, carried out on the same sample aliquots (Upadhyay et al. 2009; Roth et al. 2014b). A confirmation of 142Nd anomalies in such younger rocks is important because it could establish the longevity of early silicate differentiation signatures beyond Archean. Our experiment on freshly collected samples from the same outcrops, using identical analytical procedures, could not reproduce the results of Upadhyay et al. (2009). We did, however, observe slightly negative μ142Nd values with respect to Ames Nd, which became normal with respect to JNdi-1.