• K Gopalan

      Articles written in Journal of Earth System Science

    • The crustal record in Rajasthan

      K Gopalan A K Choudhary

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      Age determinations mostly by Rb/Sr whole rock isochrons of the Precambrian rocks of Rajasthan in northwest India are summarized and discussed. On present sampling and subject to its possible bias, the following conclusions can be made. The Untala Granite believed to be intrusive into the gneissic terrain (bgc) east of Udaipur has the oldest age, 2.95 b.y. yet measured for a granite in Rajasthan. This, coupled with the lead isochron age of 3.5 b.y. for detrital zircon from the Aravalli schists by Vinogradov and others extends the basement of Rajasthan well into the Archaean. The time equivalence of thebgc east of Udaipur with the Berach Granite dated only at 2.55 b.y. is not tenable. No satisfactory radiometric age control exists for the onset and duration of the Aravalli Supergroup, believed to be an early Proterozoic linear belt.

      Heron’s original Delhi Supergroup has recorded acid magmatism widely separate in space and time. The earliest activity between 1700 and 1500 m.y. is recorded mainly in the Alwar basin in northeastern Rajasthan while the younger activity between 850 and 750 m.y. is represented by the ‘Erinpura type’ granites in the central and southern Aravalli sector. This younger event not only has let its thermal overprinting on the older Alwar rocks but also marks the onset of emplacement of the Malani Igneous suite in the trans-Aravalli terrain. This raises the new possibility that the Delhi rocks of Heron represent atleast two chronologically independent sequences with varying geographical extent. The trans-Aravalli terrain is most probably floored by partly reworked, crystalline basement and developed along linear rift zones which acted as loci for high heat flow and igneous activity since about 800 m.y. ago.

    • Geology, geochemistry and geochronology of the Archaean Peninsular Gneiss around Gorur, Hassan District, Karnataka, India

      Y J Bhaskar Rao K Naha R Srinivasan K Gopalan

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      The Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking gneissosity parallel to the axial planes of a set of isoclinal folds (DhF1). These have been over printed by near-coaxial open folding (DhF12) and non-coaxial upright folding on almost N-S trend (DhF2). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically with DhF1 deformation.

      The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated REE patterns (CeN/YbN < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7).

      A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30 Ma with Sri of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sri = 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal residence history of less than a 100 Ma.

    • Rb-Sr age of the Sivamalai alkaline complex, Tamil Nadu

      T V Subba Rao B L Narayana K Gopalan

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      The Sivamalai alkaline complex comprises ferro-, pyroxene-hornblende- and nepheline-syenites. Field relations show that the nepheline syenites followed the emplacement of non-feldspathoidal syenites. Mineralogical data on the syenite suite have been reviewed. The Sivamalai alkaline rocks are not strongly enriched in rare-earth elements like most miaskites. Rb-Sr isotopic analyses of a suite of six samples from the various members of the complex define an isochron corresponding to an age of 623 ±21 Ma () and initial Sr ratio of 0.70376 ±14 (2σ). This is consistent with a model of fractional crystallization of a parent magma derived from an upper mantle source with apparently no isotopic evidence for more than one magma source for the complex. The Sivamalai alkaline complex represents a Pan-African alkaline magmatic event in the southern granulite terrane of Peninsular India.

    • Rare earth element geochemistry and Rb-Sr geochronology of Archaean stromatolitic cherts of the Dharwar craton, south India

      R Srinivasan G V C Pantulu K Gopalan

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      Stromatolites associated with cherty dolomites of the Vanivilaspura Formation of the Archaean Dharwar Supergroup show a morphology indicative of the deposition of the latter in a intertidal to subtidal environment. The cherts are moderately high in their Al/Al + Fe ratios but depleted in Fe2C3 and also most trace elements. Unlike most other Archaean cherts, the Vanivilaspur cherts exhibit significant negative Ce anomaly, which is interpreted to have resulted from contemporary manganese deposition. The Rb/Sr ratios in the cherts show a sufficient spread to define a linear correlation line in the Rb-Sr evolution diagram corresponding to an age of 2512 ± 159 Ma and initial Sr ratio of 0.7128 ± 0.0012 (2σ). While this age is strikingly close to that of regional metamorphism in the Dharwar craton, the initial ratio is distinctly higher than that of the associated volcanics. Acid leaching experiments on the cherts suggest that they may have been isotopically equilibrated on a mm to cm scale about 500 Ma later than the time of regional metamorphism.

    • Geochemistry and magnetostratigraphy of deccan flows at Anjar, Kutch

      A D Shukla N Bhandari Sheela Kusumgar P N Shukla Z G Ghevariya K Gopalan V Balaram

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      Chemical analysis of nine Deccan flow basalts at Anjar, Kutch, western India, indicates that all, except the uppermost flow F-9, are alkaline. In their major and trace element composition, the alkali basalts resemble Ocean island basalts (OIB). Similarities of many diagnostic trace element ratios (e.g. Sm/Nd, Ba/Nb,Y/Nb and Zr/Nb) are similar to those found in the Réunion Island basalts. The uppermost basalt is tholeiitic and chemically resembles the least contaminated Deccan basalt (Ambenali type). The Anjar basalts have iridium concentration ranging between 2 and 178 pg/g. Some of these values are higher by about an order of magnitude compared to the Ir concentration in other basalts of the Deccan. A synthesis of chemical, palaeomagnetic and geochronologic data enables us to construct a chemical and magnetic stratigraphy for these flows.

      The three flows below the iridium enriched intertrappean bed (IT III) show normal magnetic polarity whereas all except one of the upper basalts show reversed magnetic polarity. The sequence seems to have started in polarity zones 31N and probably continued up to 28R or 27R. The results presented here support the view that Deccan volcanism in Kutch occurred on a time span of a few million years.

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