Rootless calc-silicate folds in granite: An implication towards syn- to post-plutonic emplacement

Deformation of the Champaner Group of rocks that form a part of Southern Aravalli Mountain Belt, western India, occurred during the Grenville orogeny (ca. 1400–935 Ma). Two phases of deformation are recorded: \(\hbox {D}_{1}\), persistent throughout the group and characterised by westerly plunging tight isoclinal folds and \(\hbox {D}_{2}\), a localized phase of deformation associated with shortening of the earlier folds from the eastern margin. Both the phases of deformation are in association with the syn-tectonically emplaced Godhra granite. The present work records rootless calc-silicate folds in granite belonging to the older formation, located at the eastern fringe of the Champaner Group. Field evidences suggest superimposition of Type 2 interference pattern trending NE–SW over rootless Type 0 of varying trends from NW–SE to N–S. The superposed pattern obtained from the field study differs in terms of structural trends with the neighbouring Precambrian stratigraphic units. These stratigraphic units include the Champaner Group to which the study area belongs, the Kadana Formation of the Lunavada Group and Pre-Chamapaner Gneissic Complex. Rootless character of folds found within the study area imply syn-post plutonic emplacement of Godhra granite.     

Early In-orbit Performance of Scanning Sky Monitor Onboard AstroSat

We report the in-orbit performance of Scanning Sky Monitor (SSM) onboard AstroSat. The SSM operates in the energy range 2.5 to 10 keV and scans the sky to detect and locate transient X-ray sources. This information of any interesting phenomenon in the X-ray sky as observed by SSM is provided to the astronomical community for follow-up observations. Following the launch of AstroSat on 28th September, 2015, SSM was commissioned on October 12th, 2015. The first power ON of the instrument was with the standard X-ray source, Crab in the field-of-view. The first orbit data revealed the basic expected performance of one of the detectors of SSM, SSM1. Following this in the subsequent orbits, the other detectors were also powered ON to find them perform in good health. Quick checks of the data from the first few orbits revealed that the instrument performed with the expected angular resolution of 12’ \(\times \) 2.5\(^\circ \) and effective area in the energy range of interest. This paper discusses the instrument aspects along with few on-board results immediately after power ON.     

Structural pattern in the Precambrian rocks of Sonua-Lotapahar region,North Singhbhum,eastern India

In the western part of the North Singhbhum fold belt near Lotapahar and Sonua the remobilized basement block of Chakradharpur Gneiss is overlain by a metasedimentary assemblage consisting of quartz arenite, conglomerate, slate-phyllite, greywacke with volcanogenic material, volcaniclastic rocks and chert. The rock assemblage suggests an association of volcanism, turbidite deposition and debris flow in the basin. The grade of metamorphism is very low, the common metamorphic minerals being muscovite, chlorite, biotite and stilpnomelane. Three phases of deformation have affected the rocks. The principal D1 structure is a penetrative planar fabric, parallel to or at low angle to bedding. No D1 major fold is observed and the regional importance of this deformation is uncertain. The D2 deformation has given rise to a number of northerly plunging major folds on E-W axial planes. These have nearly reclined geometry and theL ₂lineation is mostly downdip on theS ₂surface, though some variation in pitch is observed. The morphology of D2 planar fabric varies from slaty cleavage/schistosity to crenulation cleavage and solution cleavage. D3 deformation is weak and has given rise to puckers and broad warps on schistosity and bedding. The D2 major folds south of Lotapahar are second order folds in the core of the Ongarbira syncline whose easterly closure is exposed east of the mapped area. Photogeological study suggests that the easterly and westerly closing folds together form a large synclinal sheath fold. There is a continuity of structures from north to south and no mylonite belt is present, though there is attenuation and disruption along the fold limbs. Therefore, the Singhbhum shear zone cannot be extended westwards in the present area. There is no evidence that in this area a discontinuity surface separates two orogenic belts of Archaean and Proterozoic age.     

Forsterite-diopside-spinel-liquid equilibria in the system CaO−MgO−Al2O3−SiO2 at 20 kbar and petrological applications

Liquidus phase relationships in the CaAlAl–SiO₆–Mg₂SiO₄–CaMgSi₂O₆–CaAlSi₂O₈ portion of the simplified basalt tetrahedron in the CaO–MgO–SiO₂–Al₂O₃ system have been experimentally determined at 20 kbar pressure. The fo+di _ss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+di _ss+ en_ss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.An algebraic method, relying on compositions of two successive liquids on a univariant curve and those of the crystalline phases in equilibrium with the respective liquids, is developed to identify the type of reaction that takes place along an isobarically univariant curve and to detect whether there is a temperature maximum on that curve. Use of this method for the di _ss+fo+sp+li univariant equilibria shows that a temperature maximum exists on this curve at the composition Fo₁₁Di₅₆An₃CaTs₃₀, very close to and slighthly to the SiO₂-rich side of the fo-di-CaTs join. The temperature along the univariant curve continuously decreases from the temperature maximum (1500°C) to the invariant point (1475°C) where the univariant curve is terminated by the appearance of e _ss as a member of the equilibrium assemblage. Along this part of the curve, a reaction relationship occurs according to the equation fo+li=di _ss+ sp. Compositions of di _ss in equilibrium with the liquids from the temperature maximum to the fo+di _ss+en_ss+ sp+li invariant point range from Di₆₆En₉CaTs₂₅ to Di₃₆En₄₀CaTs₂₄. Because of the reaction relationship of forsterite with liquid, fractional crystallization of a model alkalic basaltic liquid would cause liquids to move off the fo-di _ss-sp-li univariant curve onto the sp-di _ss divariant surface. Crystallization of di _ss and sp would then lead to silica enrichment of residual liquids. Thus at pressures below 30 kbar, at which pressure the Al₂O₃–CaSiO₃–MgSiO₃ plane becomes a new thermal divide cutting through both the tholeiitic and alkalic volumes, alkalic liquids will fractionate toward tholeiitic compositions without crossing a thermal divide. This relationship would be expected to persist at pressures down to about 4 kbar where a maximum on the fo-di-an-li boundary line causes a thermal divide near the fo-di-an plane. Strongly SiO₂-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO₂-undersaturated basalts (e.g. alkali olivine basalt) by fractional crystallization at 20 kbar. We also found that no gt primary phase volume cuts the wo-en-Al₂O₃ join at 20 kbar pressure. The wehrlite, the olivine clinopyroxenite, and the Al-augite group lherzolite xenoliths, containing highly aluminous clinopyroxenes (enriched in Ca-Tschermak), can be interpreted as crystal cumulates from alkalic basalts in the light of this experimental study. This is consistent with the mode of origin of these xenoliths proposed from petrographic, mineralogic, and geochemical studies.Abbreviations and notations di CaMgSi₂O₆ - fo Mg₂SiO₄ - an CaAl₂Si₂O₈ - CaTs CaAlAlSiO₆ - sp MgAl₂O₄ - en MgSiO₃ - wo CaSiO₃ - gt Ca₃Al₂Si₃O₁₂–Mg₃Al₂Si₃O₁₂ - qz SiO₂ - li Liquid - gl glass - ss Solid Solution - A An mxn matrix - X A column vector - kbar kilobar     

Gravity field and structures of the Rajmahal Hills: Example of the Paleo-Mesozoic continental margin in eastern India

A narrow strip of Gondwana basins separates the Rajmahal traps from the peninsular shield in eastern India. This part of the shield margin is associated with a conspicuous gravity high of 100 km wavelength and 48 mGal amplitude over an area of 25,000 km². Second order residual anomalies due to Gondwana sediments and traps are superposed on this wider gravity high. Gravity interpretation, partly constrained by seismic data, suggests that the wider high is caused by a denser metamorphic layer (amphibolite and granulite) up to 3.5 km thick. The metamorphic layer also extends below the eastern Rajmahal hills where the Gondwanas, traps and younger sediments have covered it. The Gondwanas are downfaulted against the shield edge and are preserved over an irregular basin floor whose deepest part underlies the eastern flank of the Rajmahal hills adjacent to the Bengal basin. It is inferred that the Gondwanas were deposited over a rifted and highly faulted shield margin that was intruded by the Rajmahal traps nearly 100 m.y. ago. High-grade metamorphism along the shield edge presumably preceded the continental rifting, perhaps occurring in the Precambrian as a part of the Eastern Ghats orogeny, along the east coast of India.     

Reappraisal of the structure of the Western Iron Ore Group, Singhbhum craton, eastern India: Implications for the exploration of BIF-hosted iron ore deposits

The middle to late Archean Iron Ore Group rocks occurring along the western margin (the Western Iron Ore basin) of the Singhbhum Granite massif in the Singhbhum craton were deformed during Iron Ore orogeny and are disposed in a horseshoe-shaped synclinal structure in the eastern part of the Indian shield. The Western Iron Ore basin hosts almost all the major high-grade iron ore deposits of eastern India. Contrary to the established view, present analysis emphasizes that the horseshoe fold in reality is a synclinorium consisting of a syncline–anticline fold pair which were later cross-folded along an east–west axis.

Structural analysis in the eastern anticline of the ‘horseshoe synclinorium’ suggests that the BIF hosting the high-grade iron ore bodies are disposed in three linear NNE–SSW trending belts, each showing an open synclinal geometry. Later cross folding produced development of widespread dome and basin pattern at the sub-horizontal hinge zones of these synclinal fold belts. The major iron ore deposits in the eastern anticline at the present level of erosion are preferentially localized within shallow elongated basinal structures only. The axis of the adjoining western syncline was similarly uplifted as partial culminations where cross-folded against E–W anticlinal axes. But here, the BIF-iron ore bodies are preferentially localized within elongated domal structures in contrast to the basinal sites in the adjacent eastern anticline. Such an inference based on structural analysis could probably be utilized as a potential tool for all future explorations, reserve estimation and recovery of the iron ore deposits in the terrain.     

Regional geoelectric structure beneath Deccan Volcanic Province of the Indian subcontinent using magnetotellurics

Understanding deep continental structure and the seismotectonics of Deccan trap covered region has attained greater importance in recent years. For imaging the deep crustal structure, magnetotelluric (MT) investigations have been carried out along three long profiles viz. Guhagarh–Sangole (GS), Sangole–Partur (SP), Edlabad–Khandwa (EK) and one short profile along Nanasi–Mokhad (NM). The results of GS, SP and NM profiles show that the traps lie directly over high resistive basement with thin inter-trappean sediments, where large thickness of sediments, of the order of 1.5–2.0 km, has been delineated along EK profile across Narmada–Son–Lineament zone. The basement is intersected by faults/fractures, which are clearly delineated as narrow steep conducting features at a few locations. The conducting features delineated along SP profile are also seen from the results of aeromagnetic anomalies. Towards the southern part of the profile, these features are spatially correlated with Kurduwadi rift proposed earlier from gravity studies. Apart from the Kurduwadi rift extending to deep crustal levels, the present study indicates additional conductive features in the basement. The variation in the resistivity along GS profile can be attributed to crustal block structure in Koyna region. Similar block structure is also seen along NM profile.Deccan trap thickness, based on various geophysical methods, varies gradually from 1.8 km towards west to 0.3 km towards the east. While this is the general trend, a sharp variation in the thickness of trap is observed near Koyna. The resistivity of the trap is more (150–200 Ω m) towards the west as compared to the east (50–60 Ω m) indicating more compact or denser nature for the basalt towards west. The upper crust is highly resistive (5000–10,000 Ω m), and the lower crust is moderately resistive (500–1000 Ω m). In the present study, seismotectonics of the region is discussed based on the regional geoelectrical structure with lateral variation in the resistivity of the basement and presence of anomalous conductors in the crust.