D. Srinivasa Sarma

Constraints on the Tectonic Setting of the Andaman Ophiolites, Bay of Bengal, India, from SHRIMP U-Pb Zircon Geochronology of Plagiogranite

The Andaman ophiolites are well exposed in the Andaman group of islands, which is part of the Sunda-Burmese double-chain arc system in the Bay of Bengal, India. Plagiogranites occurring on the eastern margin of the southern part of South Andaman Island appear as interstitial vermicular and micrographic intergrowths of quartz and plagioclase. They are tonalitic to trondhjemitic in composition, and their Rb, Yb, Ta, and Y abundances are characteristic of a volcanic-arc affinity. Sensitive high-resolution ion microprobe U-Pb dating of zircons from a plagiogranite within the Andaman ophiolite has yielded a weighted mean 206Pb/238U age of Ma, interpreted as the age of its crystallization. The subduction-related plagiogranite has intruded a gabbro unit of the Andaman ophiolites as well as extrusives of the East Coast Volcanics at this time. Since the Andaman ophiolitic rocks predate the plagiogranite, they cannot have been generated in the currently active Late Miocene Andaman-Java subduction zone and were most likely obducted onto the leading edge of the Eurasian continent at an earlier phase of subduction activity during early Cretaceous time.

Geology and geochemistry of arenite–quartzwacke from the Late Archaean Sandur schist belt—implications for provenance and accretion processes

Abstract Detailed geological, petrological and geochemical studies have been carried out on an arenite–quartzwacke suite of rocks constituting a part of the Late Archaean Sandur schist belt in Dharwar craton, southern India for understanding the nature of provenance for these sedimentary rocks. The arenite–quartzwacke consists of rounded to sub-rounded and angular fragments of monocrystalline–polycrystalline quartz, quartzite and chert embedded in a fine-grained matrix of quartz and sericite. While arenites are more siliceous (SiO 2 , 80–92 wt.%), the quartzwacke have relatively lower silica content (ca. 69–78 wt.%). The arenites and quartzwackes have CIA values ranging from 76 to 96 which suggest intense chemical weathering. This is further corroborated by the positive correlation between Al 2 O 3 and TiO 2 in both these rock types. The ACNK modeling of arenites and quartzwackes show evidence for addition of K 2 O during later metasomatic alteration. In the ACNKFM ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during K-metasomatism and the presence of mafic rocks in the source. The high concentration of HFSE such as Zr, Hf, Nb and Ta and the trace element ratios Th/Sc, La/Sc, Th/U and Ce/Th in the arenite–quartzwacke indicate a mixed provenance. The rare earth element modeling of quartzwackes considering tonalite, granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members. A composition comprising of 25% tonalite+25% granite+50% amphibolite in the provenance appears to match with the observed range of REE patterns of quartzwackes. The presence of higher proportions of granite in the provenance is evidenced by the large negative Eu anomalies in these sediments. Field evidence and structural discordance suggest that the arenite–quartzwacke suite is an allochthonous part of the Sandur schist belt.

Combined Experimental and Computational Study of the Gelation of Cyclohexane-Based Bis(acyl-semicarbazides) and the Multi-Stimuli-Responsive Properties of Their Gels

The current study reports the one-step synthesis and gelation properties of cyclohexane-based bis(acyl-semicarbazide) gelators with an additional -NH group incorporated into urea moieties and carrying hydrophobic chains of varying length (C8-C18). The gels exhibited thermoreversibility and could be tuned in the presence of anions at different concentrations in addition their the ultrasound-responsive nature, thus making them multi-stimuli-responsive. The combined experimental and computational study on these gels reveals that the balance between two noncovalent interactions, viz., hydrogen bonding between the amide groups in acyl-semicarbazide moieties and van der Waals forces between long hydrocarbon tails, is found to be the determining factor in the process of organogelation. A systematic increase in alkyl chain length leads to equilibrium between these two types of noncovalent forces that is manifested in the spectral and thermal properties of the gels. The H-bonding interactions dominated up to a certain chain length, and further increases in the alkyl chain length led to increased van der Waals interactions as observed by IR, XRD, and thermal studies. Computational calculations were carried out on dimer structures of C8-C18 to understand the variation in noncovalent forces responsible for aggregate formation in the gel state as a function of the alkyl chain length. The results indicate that both intermolecular and intramolecular hydrogen bonding stabilize the aggregate structures. Supramolecular aggregation in the gel state led to the viscoelastic nature of the gels, and the addition of anions led to the disruption of self-assembly, which was studied by rheology.

U–Pb dating of metamorphic monazite establishes a Pan-African age for tectonism in the Nallamalai Fold Belt, India

The Nallamalai Fold Belt comprises late Palaeoproterozoic to Mesoproterozoic sedimentary rocks deformed into a fold-and-thrust belt along the eastern side of Peninsular India. The age of thin-skinned thrusting, folding and low- to medium-grade metamorphism in the belt is unclear, with estimates ranging from Palaeoproterozoic to early Palaeozoic. A possible Pan-African age for thrusting has previously been inferred from Rb–Sr dating of muscovite in shear zones from the adjacent Krishna Province (501 – 474 Ma) but these structures are separated from the Nallamalai Fold Belt by a major thrust. Here, we present in situ U–Pb dating of metamorphic monazite within a low-grade metasedimentary rock in the Nallamalai Fold Belt at the Mangampeta barite mine. Our date of 531 ± 7 Ma for the monazite is the first direct evidence that west- to NW-directed nappe stacking, folding and low-grade metamorphism in the fold belt are related to Pan-African incorporation of India into the Gondwana supercontinent.

Palaeomagnetic Study on a 1765 Ma Dyke Swarm from Singhbhum Craton: Implications to the Paleogeographic Position of India

Columbia supercontinent during Paleoproterozoic era is still uncertain because of very few reliable, high-quality palaeomagnetic data with precise geochronology. Here we are reporting the results of a palaeomagnetic study on precisely dated 1765 Ma WNW-ESE trending dykes (which have an extension of several hundred kilometres) from Singhbhum Craton. Rock magnetic studies, including thermomagnetic curves, and the progressive acquisition of isothermal remanence conducted on selected samples, indicate that the dominant magnetic carrier is magnetite. Incremental alternating field (AF) demagnetization isolated high coercivity components directed to NNW/SSE with shallow inclinations from 10 sampling sites. The primary origin of the ChRM is supported by very high coercivities and unblocking temperatures of the grains preserving the characteristic remanence, consistency of palaeomagnetic data between sites despite being separated by large distances, lack of any major metamorphic event from the sampling area which might have heated rocks above their blocking temperature, and lack of signs of metamorphism from rock thin sections study. The WNW-ESE trending dykes yield a mean palaeomagnetic direction with a declination = 329.4 and an inclination = −26.6 (k = 21.5 ; A95 = 10.7). The pole position of Singhbhum craton at 1765 Ma is 43.4N, 308.7E (dp=6.3 and dm=11.6). The paleogeographic reconstruction was done, at ca. 1770 Ma, coupled with a correlation of geological features indicate a very clear spatial link between North China Craton and India during Paleoproterozoic. Ravi SHANKAR, Dr. D. Srinivasa SARMA, 2016. Palaeomagnetic Study on a 1765 Ma Dyke Swarm from Singhbhum Craton: Implications to the Paleogeographic Position of India. Acta Geologica Sinica (English Edition), 90(supp. 1): 44.

Trace, Rare-Earth Elements and C, O Isotope Systematics of Carbonate Rocks of Proterozoic Bhima Group, Eastern Dharwar Craton, India: Implications for the Source of Dissolved Components, Redox Condition and Biogeochemical Cycling of Mesoproterozoic Ocean

The Bhima basin is one of a series of Proterozoic basins that overlie the Archean Dharwar craton of South India. In the present study, we have systematically sampled the carbonate rocks from three stratigraphic horizons of Bhima Group and conducted geochemical and C–O isotopic studies in order to understand the source of dissolved components, redox condition and biogeochemical cycling of Mesoproterozoic Ocean. The presence of original microbial texture and Proterozoic marine like δ18O values (−6.38 to −7.17‰) indicate minimum diagenetic alteration. The carbonates have coherent REE + Y patterns and share the essential shale-normalised characteristics of well oxygenated, shallow ambient seawater, such as, (1) uniform heavy REE enrichment (Nd/YbSN = 0.43 ± 0.06), (2) consistent negative Ce anomalies (Ce/Ce* = 0.60 ± 0.05) and (3) superchondritic Y/Ho ratios (38.07 ± 3.17). The detailed geochemical modeling suggests (1) little influence ( 10%) of river/estuarine run-off to the ambient sea water and possibly minor input from oceanic hydrothermal sources. High positive values of δ13C (3.8‰) in the basal Shahabad carbonates indicate burial of a large mass-fraction of isotopically light organic carbon. The gradual up-section decrease to ~1‰ δ13C suggest transgression and mixing of isotopically heavy coastal water (~4‰) with global Dissolved Inorganic Carbon (DIC) reservoir (~0‰). The short term negative δ13C excursion of magnitude ~5‰ at the base is consistent with upwelling of Oxygen Minimum Zone during the transgression event. The wide variability of δ13C (5.15‰, −1.37 to +3.81‰ PDB) in carbonates indicate greater sensitivity of C-isotope system as a consequence of lower buffering capacity and shrinking size of DIC reservoir, which would indicate increased surface oxidation and release of oxygen to the atmosphere.

Geochemical characterization of the siliciclastic rocks of Chitravati Group, Cuddapah Supergroup: Implications for provenance and depositional environment

Petrological and geochemical studies have been carried out on Pulivendla and Gandikota Quartzite from Chitravati Group of Cuddapah Supergroup to decipher the provenance and depositional environment. Both the units are texturally mature with sub-rounded to well-rounded and moderately to well-sorted grains. Majority of the framework grains are quartz, in the form of monocrystalline quartz, followed by feldspars (K-feldspar and plagioclase), mica, rock fragments, heavy minerals, with minor proportion of the matrix and cement. Based on major element geochemical classification diagram, Pulivendla Quartzite is considered as quartz-arenite and arkose to sub-arkose, whereas Gandikota Quartzite falls in the field of lith-arenite and arkose to sub-arkose. Weathering indices like CIA, PIA, CIW, ICV, Th/U ratio and A–CN–K ternary diagram suggest moderate to intense chemical weathering of the source rocks of these quartzites. Whole rock geochemistry of quartzites indicate that they are primarily from the first-cycle sediments, along with some minor recycled components. Also their sources were mostly intermediate-felsic igneous rocks of Archean age. The tectonic discrimination plots, Th–Sc–Zr/10 of both these formations reflect active to passive continental margin setting. Chondrite-normalized rare earth element (REE) patterns, and various trace element ratios like Cr/Th, Th/Co, La/Sc and Th/Cr indicate dominantly felsic source with minor contribution from mafic source. Th/Sc ratios of Pulivendla and Gandikota Quartzite are in close proximity with average values of 2.83, 3.45 respectively, which is higher than AUCC (

SHRIMP zircon and titanite U-Pb ages, Lu-Hf isotope signatures and geochemical constraints for ∼2.56 Ga granitic magmatism in Western Dharwar Craton, Southern India: Evidence for short-lived Neoarchean episodic crustal growth?

\hbox {Th/Sc}=0.97