• K Gopalan

      Articles written in Journal of Earth System Science

    • Rb-Sr age of Godhra and related granites, Gujarat, India

      K Gopalan J R Trivedi S S Merh P P Patel S G Patel

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      Rubidium and strontium determinations are reported for Godhra and geographically related granites from central Gujarat. The whole rock data define a Rb-Sr isochron corresponding to a common age of 955±20 m.y. and initial Sr ratio of 0·7130±0·001. This age is distinctly older than the age of 735 m.y. reported for the Erinpura suite of rocks from Mount Abu in western Rajasthan and from Idar in northern Gujarat. There are at least two generations of post-Delhi intrusive rocks in the Gujarat precambrian. Biotites associated with these granites have the same age as the whole-rocks within experimental error indicating the absence of significant metamorphic heating since the time of emplacement. It is significant that rocks of similar age occur in the Rajasthan Precambrian mainly in the axial zone of the Aravalli Mountains.

    • Rb−Sr age of Gaik granite, Ladakh batholith, northwest Himalaya

      J R Trivedi K Gopalan Kewal Kisharma K R Gupta V M Choubey

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      The Gaik Granite is a part of the Ladakh batholith outcropping between Gaik and Kiari in NW Himalaya. This is a pink porphyritic granite rich in biotite and poor in hornblende. Rb-Sr analyses have been made on six whole-rock samples of the Gaik Granite. Though the samples are poorly enriched in radiogenic Sr, they define a reliable isochron corresponding to an age of 235±13 (2σ) m.y. and initial87Sr/86Sr ratio of 0·7081±0·0004 (2σ). Biotite, plagioclase and potash feldspar fractions separated from two of the samples have yielded a much younger mineral isochron at 30±1·5 m.y. indicating a nearly complete redistribution of Sr isotopes between mineral phases at a time much later than the primary emplacement of the granite. The present results show that at least some components of the Ladakh batholith are of Permo-Triassic age. These rocks were isotopically re-equilibrated on a mineral scale during Upper Oligocene in response to the Himalayan orogeny.

    • Late Cretaceous mafic dykes in the Dharwar craton

      Anil Kumar Y J Bhaskar Rao V M Padma Kumari A M Dayal K Gopalan

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      Palaeomagnetic, geochemical and geochronological studies have been conducted on a set of dolerite dykes intruding the Peninsular gneisses near Huliyurdurga town, Karnataka, as a reconnaissance survey indicated a Cretaceous age for them. The dykes are mainly tholeiitic in composition with their 87Sr/86Sr ratios tightly clustered around 0·7045. Their palaeomagnetic data (Dm=329°,Im=−55°) and the corresponding palaeopole coordinates (λp = 34°S,Lp=108°E) are strikingly close to those of the Deccan Traps to the north. Whole rock K-Ar ages of these dykes ranging between 69 and 84 Ma are also similar to the range of K-Ar ages of the Deccan basalts. The chemical, palaeomagnetic and temporal coherence between the dykes and the Deccan basalts indicate that they may indeed be tectonically related events.

    • Granitoids around the Malanjkhand copper deposit: Types and age relationship

      M K Panigrahi A Mookherjee G V C Pantulu K Gopalan

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      On the basis of field relations, petrography and chemistry, three types of granitoids are recognized at Malanjkhand in and around the copper deposit over an area of about 200 km2. These are (i) a fine grained ‘leucogranite’ of restricted occurrence in the surrounding area (Gr-I); (ii) coarse-grained, grey in most parts, gneissose granitoid of regional extension (Gr-II); and (iii) the pink-feldspar bearing massive type hosting the mineralization with occasional representatives in the surrounding country (Gr-III). Gr-I comes out as a distinct entity on the basis of cross-cutting relation and mineralogical and chemical composition, the Rb-Sr whole rock isochron also giving a younger age than the other two groups irrespective of the regression model used. Gr-II comes out as the oldest unit but its age relationship with Gr-III cannot be established unequivocally. An uncorrelated error regression model establishes the age relationship as Gr-I<Gr-III<Gr-II, whereas a two-error regression model establishes temporal closeness between Gr-II and III.

    • The nature of the basement in the Archaean Dharwar craton of southern India and the age of the Peninsular Gneiss

      K Naha R Srinivasan K Gopalan GVC Pantulu M V Subba Rao A B Vrevsky Ye S Bogomolov

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      The Archaean Peninsular Gneiss of southern India is considered by a number of workers to be the basement upon which the Dharwar supracrustal rocks were deposited. However, the Peninsular Gneiss in its present state is a composite gneiss formed by synkinematic migmatization during successive episodes of folding (DhF1, DhF1a and DhF2) that affected the Dharwar supracrustal rocks. An even earlier phase of migmatization and deformation (DhF*) is evident from relict fabrics in small enclaves of gneissic tonalites and amphibolites within the Peninsular Gneiss. We consider these enclaves to represent the original basement for the Dharwar supracrustal rocks. Tonalitic pebbles in conglomerates of the Dharwar Supergroup confirm the inference that the supracrustal rocks were deposited on a gneissic basement.

      Whole rock Rb-Sr ages of gneisses showing only the DhF1 structures fall in the range of 3100–3200 Ma. Where the later deformation (DhF2) has been associated with considerable recrystallization, the Rb-Sr ages are between 2500 Ma and 2700 Ma. Significantly, a new Rb-Sr analysis of tonalitic gneiss pebbles in the Kaldurga conglomerate of the Dharwar sequence is consistent with an age of ∼2500 Ma and not that of 3300 Ma reported earlier by Venkatasubramanian and Narayanaswamy (1974). Pb-Pb ages based on direct evaporation of detrital zircon grains from the metasedimentary rocks of the Dharwar sequence fall into two groups, 3300–3100 Ma, and 2800–3000 Ma. Stratigraphic, structural, textural and geochronologic data, therefore, indicate that the Peninsular Gneiss of the Dharwar craton evolved over a protracted period of time ranging from > 3300 Ma to 2500 Ma.

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