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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. [ABSTRACT FROM AUTHOR]

Bhanjada Bet igneous rock lies to the east of Pachham Island and west of Khadir Island, well within the Great Rann of Kutch, Gujarat, western India. It is a small isolated hillock made up largely of phonolite with patches of trachyte and traversed by a mafic dyke. The phonolite is composed dominantly of sanidine, nepheline, aegirine, Ti-amphibole and glass. Sanidine occurs as phenocryst and nepheline as microphenocryst. Groundmass is composed of smaller alkali feldspar, aegirine, Ti-amphibole and glass. The lath-shaped feldspar in the groundmass defines excellent flow texture. Major element chemical composition indicates occurrence of two groups of phonolite-low silica (around 53%) and high silica (around 58%). The phonolite has higher abundance of MgO (2.21–3.5%) over FeO (2.43–2.6%) and very high Mg# (65–69) suggesting its primitive nature. Trace element abundances suggest an overall enrichment in LILE and selected HFSE, highly fractionated LREE pattern. High Mg# of the Bhanjada bet phonolite along with low values of Zr and Hf compared to phonolites of basanite phonolite suite where phonolite is an evolved member, is noted. The lower Zr and Hf values hint towards a less evolved primary phonolitic magma rather than phonolite as late differentiates of basanitic–phonolite suite. Experimental studies indicate low degree partial melting of a metasomatised lherzolite mantle at a pressure around 1–1.5 GPa can produce primary phonolite magma. Mantle xenoliths, found in alkali basalt of Kutch basin, show evidence of carbonatite metasomatism of lithospheric mantle. Bhanjada Bet phonolites are associated with layered mafic complex of Nir Wandh and other magmatic rocks of Pachham Island. Ages of Nir Wandh magmatic rocks and magmatic rocks of Pachham Island coincide with ages of early Deccan magmatism. Age of phonolite is not yet known. From field association and petro-mineralogy, we propose that Bhanjada Bet phonolite represents alkali magmatism of early Deccan age in Western Deccan Province. [ABSTRACT FROM AUTHOR]

The evidence of a long‐lived Grenvillian orogeny around 1.0 Ga is preserved in the eastern part of India in the form of large‐scale syn‐collisional granitic activity and high‐grade metamorphism in the Chotanagpur Granite Gneissic Complex (CGGC). Geochemically, granitoid rocks of CGGC were classified as ferroan, per‐aluminous, and calc‐alkalic to alkali‐calcic S‐type granites which were generated from the anatexis of meta‐sedimentary protolith (khondalite) in a volcanic‐arc to syn‐collisional setting. Relatively high crystallization temperatures (800–850°C for garnetiferous granite gneiss, 770–830°C for megacrystic granite gneiss, and 639–725°C for pink granite) of these rocks were calculated by geothermometric calculations. The REE modelling testified that 40–45% partial melting of the khondalite in H2O undersaturated condition at around 5‐kbar pressure might have given rise to the parent magma of the garnetiferous granite gneiss (GGG). LA‐ICPMS zircon (U–Pb) dating yielded the crystallization ages of the megacrystic granite gneiss (MGG) and pink granite (PG) as ~1.0 Ga, and EPMA monazite (U–Th–total Pb) dating suggested that the GGG was metamorphosed during 1.0 Ga. The 1.0–0.90 Ga age of high‐grade metamorphism and anatexis in the CGGC may be correlated with that in the Rayner Complex–Eastern Ghats Belt in the Indo‐Antarctic sector, which is considered as the result of Indo‐Antarctic collision during the assembly of Rodinia. [ABSTRACT FROM AUTHOR]

Ultramafic and mafic rocks occur within a linear belt, trending nearly E-W along North Puruliya Shear Zone of the Chhotanagpur Gneissic Complex (CGC). These rocks are classified as gabbro, norite, gabbro-norite, dolerite, diorite, olivine-websterite and lherzolite. Mafic rocks (Group 1) often occur in association with ultramafic variants (Group 2) and sometimes in isolation. A genetic link has been established between these mafic and ultramafic rocks using disposition of ultramafic and mafic rocks in the outcrop, systematic variation in modal mineralogy, co-linearity of plots in biaxial chemical variation diagram. Chemical composition of biotite and clinopyroxene reveal calc-alkaline nature and arc signature in these mafic-ultramafic rocks and whole rock geochemical characters indicate similarity with arc magma in subduction zone setting. The high values of Mg no. (47-81) and Al O (5.5-17.9) of mafic rocks indicate primitive, aluminous nature of the parental melt and presence of amphibole and biotite indicate its hydrous nature. The parent mafic melt evolved through fractionation of olivine, spinel, clinopyroxene and plagioclase. The crystal cumulates gave rise to the ultramafic rocks and the associated mafic rocks formed from residual melt. Crustal contamination played an important role in magmatic evolution as evident from variation in abundance of Rb in different lithomembers. Mafic-ultramafic rocks of the present study have been compared with intra-cratonic layered complexes, mafic-ultramafic rocks of high grade terrain, Alaskan type ultramafic-mafic complex and ophiolites. It is observed that the ultramafic-mafic rocks of present study have similarity with Alaskan type complex. [ABSTRACT FROM AUTHOR]

Paharpur gabbroic intrusive is an arcuate body running east-west paralleling the foliation of Chhotanagpur Granite Gneiss which acts as country rock. The main gabbroic body is intruded by a number of dolerite dykes running north-south. It is composed of clinopyroxene (WoEnFs-WoEnFs, mg no. 72-82), plagioclase (An-An), hornblende (magnesian hornblende to ferro-tschermackite), orthopyroxene (En-En) and ilmenite. Hornblende occurs as large poikilitic grain and constitutes around 60% of the rock. Both gabbro and associated dolerite dykes, show relatively primitive character (mg no. 65-73). Primitive mantle-normalized and MORB-normalized spider diagrams indicate enrichment in Rb, Ba, Th, La, Sr and depletion in Nb, Zr, Y, Ti and Nd. The LILE enrichment and Nb, Ti, Zr, Y depletion suggest arc like geochemical signature for the gabbroic and doleritic rocks of Paharpur. Flat to slightly LREE fractionated pattern and variable degree of REE enrichment is observed. An early stage fractionation of clinopyroxene, plagioclase, orthopyroxene, ilmenite and late stage reaction of cumulate pile and evolved melt/hydrous fluid is suggested for magmatic evolution of gabbro. Associated dolerite dykes, which are geochemically similar to the gabbro, have tholeiitic with boninitic character. The mineralogical and chemical compositions of intrusive rocks also have some similarity with mafic rocks of ophiolite complex of subduction zone. [ABSTRACT FROM AUTHOR]

A N-S trending, lenticular, and dismembered ultramafic unit is found emplaced within the multiply deformed metapelites of the North Singhbhum Mobile Belt, belonging to the Singhbhum Craton, eastern India. Their occurrence in an orogenic belt, lack of evidences of intrusive activity and close proximity to the Singhbhum Shear Zone suggest emplacement by thrust control. These rocks have experienced extensive serpentinization which has obscured and obliterated some of the features of the protolith. Relict areas of the Bangriposi wehrlite rocks indicate that the protolith might have been a cumulate clinopyroxene-dunite/wehrlite from an oceanic magma chamber. Detailed petrography and mineralogy reveal numerous metasomatic features which are separate from the serpentinization event: (i)Very high Ni-content of the relict olivines which does not correlate with Fo contents, (ii) Presence of secondary clinopyroxene (Cpx2) rims around primary olivine and replacing primary clinopyroxene (Cpx1), which have major and trace element compositions similar to clinopyroxenes from diopsidites of crustal origin (iii) Occurrence of secondary calcic hornblendes as tiny blebs within the Cpx2 rims (iv) Occurrence of secondary Mg-chlorite as interstitial patches between olivine rimmed by Cpx2 and inter-mixed with antigorite in the matrix, (v) Modifications of original Cr-spinels to ferri-chromite cores rimmed by magnetite. These features indicate these rocks have experienced fluid-rock interaction at temperatures above the serpentinization process. Primary Cpx1 grains exhibit elevated LREE compositions and can either reflect effects of cryptic metasomatism or crystallization from an enriched melt. Both groups of clinopyroxenes exhibit LILE enrichment and HFSE depletion. P-T estimates from Cpx1 and Cpx2 yield values of ~1200 °C and 0.5–1.2 GPa. The calcic hornblendes provide more realistic estimates of ~1000 °C and ~ 1.0 GPa, which suggest that the Cpx2 + hornblende assemblage formed during high temperature fluid-rock interaction. Presence of interstitial chlorite, chlorite+antigorite in the matrix and Cr-spinel recrystallization suggests successively decreasing temperatures during the fluid-rock interaction episode. Modeled trace element characteristics of fluids which are supposed to be in equilibrium with Cpx2 show similarities with high-temperature, ore-forming basinal brines. Similar basinal brines formed Cu-U ores in the Singhbhum Shear Zone and was likely the metasomatic agent of the Bangriposi wehrlite rocks. These rocks were originally cumulate ultramafic units from an oceanic crustal magma chamber, which experienced prolonged fluid-rock interaction in a retrogressive regime, until its emplacement as a dismembered unit in an orogenic belt. • Dismembered serpentinized wehrlite found emplaced in mobile belt metasediments. • Evidence of prolonged high temperature fluid-rock interaction in retrogression • Secondary clinopyroxene chemically similar to diopsidites • Protolith was a cumulate ultramafic rock belonging to mid to lower oceanic crust. • Modeled metasomatic fluids similar to ore-forming basinal brines [ABSTRACT FROM AUTHOR]

Abstract: Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill. IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal demagnetizations of the Sadara sill yielded a mean ChRM direction as D=315.6°, I=−43.0° (α 95=9.78; k=25.38) and the corresponding VGP at 25°S; 114.6°E (dp/dm=6.58°/11.6°). The Sadara sill pole is significantly different from those of the Deccan (65Ma) and the Rajmahal Traps (118Ma) but is close to the Cretaceous poles of 85–91Ma rock units from southern India. This suggests a pre-Deccan age for the sill. [Copyright &y& Elsevier]

[Display omitted] • Structurally oriented massive lenses of Fe-Ti oxide ores within gabbronorite. • An unusual mode of occurrence, mineralogy and geochemistry of the ore bodies. • Fe-Ti oxide ore and gabbronorite formed from an enriched basaltic melt. • The parent melt generated by low degree partial melting of enriched mantle source. • Silicate fractionation increased Fe-Ti content and f O 2 of the residual melt. Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC), India occur as massive conformable lenses within a gabbronorite sill. The occurrence, lithological association, mineralogy, and geochemistry of the Fe-Ti oxide ore bodies are significantly different from the typical Fe-Ti oxide deposits associated with anorthosite complexes and mafic- ultramafic complexes. Textural evidence and mineral chemistry suggest mostly simultaneous fractional crystallization of the comprising minerals e.g. titanomagnetite, ilmenite and magnesio-hercynite. The temperature and f O 2 conditions of re-equilibration are calculated using the compositions of ilmenite and pre-exsolved titanomagnetite pairs. Extrapolation of the data along ulvospinel isopleths reveals the probable original magmatic conditions e.g. temperature ~ 900 °C and f O 2 ~ ΔFMQ + 2. The ore forming melt calculated by 'equilibrium distribution method' shows no crustal contamination and has a trace elemental pattern similar to typical enriched basaltic melts e.g. ferropicrites, alkali basalts, OIB and EMORB. High Ti and Zr contents of the melt suggest a possible within plate tectonic setting and enrichment in HFSEs and LREEs suggest that the parent melt could be generated from an enriched mantle source by low degree partial melting of garnet-peridotite layer of the upper mantle. The 'quantitative fractional crystallization modelling' indicates that early crystallization of olivine, pyroxenes, plagioclase and ilmenite from an enriched basaltic melt similar to EMORB (most fit melt) led to a condition of high oxygen fugacity and iron enrichment in the residual melt. The residual Fe-Ti rich melt moved to low pressure zones within the gabbronorite sill during late stage of D 2 deformation, and later crystallized in the form of present ore bodies. [ABSTRACT FROM AUTHOR]

The mafic dyke swarm, newer dolerite dykes (NDDs) intrudes the Archaean Singbhum granite of the Singhbhum craton, eastern India. The present investigation focuses on the petrography and geochemistry of 19 NNE–SSW to NE–SW trending NDDs in two sectors in the northern and south-western part of Bahalda town, Odisha, Singhbhum. Chondrite normalised rare earth element (REE) patterns show light REE (LREE) enrichment among majority of the 13 dykes while the remaining six dykes show a flat REE pattern. Critical analyses of some important trace element ratios like Ba/La, La/Sm, Nb/Y, Ba/Y, Sm/La, Th/La, La/Sm, Nb/Zr, Th/Zr, Hf/Sm, Ta/La and Gd/Yb indicate that the dolerite dykes originated from a heterogeneous spinel peridotite mantle source which was modified by fluids and melts in an arc/back arc setting. REE modelling of these dolerite dykes were attempted on LREE-enriched representative of NDD which shows that these dykes might have been generated by 5–25% partial melting of a modified spinel peridotite source which subsequently suffered around 30% fractional crystallisation of olivine, orthopyroxene and clinopyroxene. The reported age of ~2.75–2.8 Ma seems to be applicable for these dykes and this magmatism appears to be contemporaneous with major scale anorogenic granitic activity in the Singhbhum craton marking a major event of magmatic activity in eastern India. [ABSTRACT FROM AUTHOR]