Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

M. K. Pandit ¹, R. K. Srivastava ²

Affiliations
1 Department of Geology, University of Rajasthan, Jaipur 302 004, IN
2 Department of Geology, M.L.S. University, Udaipur 313 001, IN

Abstract

The northwestern fringe of the Deccan Flood Basalt sequence is characterized by isolated occurrences of sub-horizontal tholeiitic flows. Because the flow units in this region are not traceable over large distances, the petrographic and geochemical characters have been utilized to establish nine flow units of olivine tholeiite (OT), quartz tholeiite (QT) and low calcium quartz tholeiite (LCT). The flows show striking mineralogical and chemical similarity. Low-Mg numbers and moderate to high silica content indicate the derivative nature of the magma. The flow sequence has evolved through predominant plagioclase fractionation and less significant clinopyroxene separation from a non-primary olivine tholeiite melt from which olivine had been removed earlier. Cyclic variation in geochemical signatures and prominent chemical breaks within the flow sequence indicate a periodically replenished, open system, shallow level magma chamber with gabbroic fractionation.

Keywords

Crystallization Process, Deccan Basalts, Pratapgarh Region.

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H. C. Khanduri ¹, R. K. Srivastava ¹, V. K. S. Dave ¹

Affiliations
1 Department of Earth Sciences, University of Roorkee, Roorkee 247 672, IN

Abstract

Within the Dudhatoli Group in the southern limb of Dudhatoli synform four mappable units have been recognised viz., the Dudhatoli granitoids, Dudhatoli schist, Flaggy quartzite and the Manila phyllite. In the area under study two generations of minor folds are recognised. Statistical analysis of minor fold axes has been carried out. It is concluded that the axes of two fold generations are not randomly distributed, they have different mean plunge direction with 2 and 196 degree of freedom. The folds of first generation (F₁ have little or poor clustering while folds of second generation (F₂) have close clustering. It is also envisaged that F₁ and F₂ folds have been formed under different tectonic environment.

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R. K. Srivastava ¹, A. Maheshwari ², Rajani Upadhyaya ¹

Affiliations
1 Department of Geology, University of Rajasthan, Station Road, Udaipur 313 001, IN
2 Maharaja's College Campus, University of Rajasthan, Jaipur 302001, IN

Abstract

The felsic volcanics of Gurapratap Singh and Diri are mostly welded tuffs and belong to andesite-dacite-rhyolite association; they are a part of the Malani volcanic suite of Precambrian age. The volcanics are cut by rhyolite porphyry (comagmatic with volcanics) and a younger dolerite dyke. A few flows of ultrapotassic rhyolites are also found which do not conform to the main trend and presumably mark a different phase of magmatic activity.

The chemical data on 91 samples of different rock types and Harker diagrams reveal a systematic variation in all major elements with the progress of differentiation, suggesting that the entire sequence except the ultrapotassic rhyolite· is cogenetic and that the changes have been controlled by some sort of fractional crystallisation mechanism. The principal component analysis of the data reveals that in the earlier stages, the fractionating phases were soda-lime plagioclase and a mafic phase and in the later stages K-feldspar became a dominant phase. This is supported by phenocryst assemblages in these rocks. The suite also has very high inter-elemental correlation indicating that the rocks are cogenetic and a single process was responsible for their evolution. The mixing line calculation also confirms the operativeness of a single process.

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R. K. Srivastava ¹, A. Maheshwari ², Rajani Upadhyaya ¹

Affiliations
1 Department of Geology, University of Rajasthan, Station Road, Udaipur 313001, IN
2 Maharaja's College Campus, University of Rajasthan, Jaipur, IN

Abstract

Felsic volcanics of the Gurapratap Singh and Diri, belong to the Malani volcanic suite of Precambrian age. Major element chemistry of these volcanics shows systematic variations with progress of differentiation. The distribution and behaviour of trace elements in these rocks is in accord with their major element data and the distribution patterns are normal with no anomalous concentrations of the elements studied. With the progress of fractionation there is a smooth decrease in Sr and increase in Rb which along with Ba are the main trace contents of these rocks. Unlike Sr and Rb which have a linear distribution pattern, Ba shows a curvilinear trend, i.e., an initial increase from andesite to dacite and rhyodacite followed by a decrease amongst rhyolites. The behaviour of ratios like Ni/Co, Fe/Zn, Mg/Li and K/Rb agrees with a model of fractional crystallisation. Furthermore, the trace element abundances of the volcanics point to their crustal source. The work also shows that in felsic magmas Ba and Sr are sensitive indicators for tracing possible differentiation and that rhyolitic rocks depleted in Ba are enriched in Rb and that in last stages of differentiation Ba/Rb ratio shows rapid decrease.

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V. Chandrasekaran ¹, R. K. Srivastava ², M. P. Chawade ¹

Affiliations
1 Geological Survey of India, Jhalana Dungri, Jaipur 302004, IN
2 Department of Geology, University of Rajasthan, Udaipur, IN

Abstract

The alkaline suite of Sarnu-Dandali (comprising ultrabasic alkaline rocks, ijolite, foidal syenite, phonolite and carbonatite) is one of the few occurrences of carbonatite-nephelinite association reported in India. The suite is intrusive into the Lower Cretaceous sedimentaries. The rocks show a continuous gradation from alkali pyroxenite to feldspathic ijolite through micro-melteigite and ijolite, Foidal syenite and phonolite mark the end phase of ijolitic series and phonolites are the last to be emplaced. Carbonatites include sovitic alvikite and ferro-carbonatite varieties. The suite shows a progressive increase in Al₂O₃, Na₂O and K₂O and decrease in MgO, CaO, total Iron, MnO and TiO₂ with increasing SiO₂. Fractional crystallisation has played a dominant role in the evolution of the suite, the major fractional phases being pyroxene and iron oxide mineral.

Keywords

Alkaline Rocks, Geochemistry, Sarnu-Dandali, Rajasthan.

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V. Chandrasekaran ¹, R. K. Srivastava ²

Affiliations
1 Geological Survey of India, Operation TNKP, Madras 600032, IN
2 Department of Geology, M.S. University, Udaipur 313 001, IN

Abstract

The Sarnu-Dandali carbonatites of Western Rajasthan are some of the few occurrences of a typical carbonatit-nephelinite magmatic association known from India. They occur as dykes and dykelets of sovitic-alvikite and ferrocarbonatite showing distinct variation in their chemistry. The sovitic-alvikites have high contents of BaO (up to 3.42%) and SrO (up to 1 %). Fractionation trend is marked by decreasing concentration of SrO from sovitic-alvikite (1.00%) to ferrocarbonatite (0.06%). Trace element contents reveal pronounced preference of Ce and other rare earth elements for sovitic-alvikites. Their oxygen and carbon isotope values are that of typical primary igneous carbonatites. The chemical and isotopic data reveal that the Sarnu-Dandali carbonatites are magmatic, fractionated and have originated from a mantle-derived nephelinitic magma.

Keywords

Carbonatite, Geochemistry, Sarnu-Dandali, Rajasthan.

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V. Chandrasekaran ¹, R. k. Srivastava ²

Affiliations
1 Geological Survey of India, J.L. Nehru Road, Madras-600032, IN
2 Department of Geology, M. S. University, Udaipur-313001., IN

Abstract

Applying multivariate Linear Regression and Principal Component analyses, the petrogenetic aspects of the polyphase igneous rocks of the Malani Igneous Province, Western Rajasthan were dealt with and inferences arrived. The various rocks of the province belong to different magmatic activities unrelated to each other. Normal and peralkaline rhyolites of the province are not co-genetic. Siwana and Jalor granites are distinct and generated at different stages. The acid dykes represent the end phases of the Jalor granite activity, crystallising from the same melt. Fractional crystallisation has played a major role in the development of the mildly alkaline and hyperalkaline rock series. Pyroxene, iron oxide minerals, feldspathoid and alkali feldspar play a major role in the development of the alkaline rocks, whereas pyroxene, iron oxide minerals and plagioclase playa dominant role in the development of the mildly alkaline rocks. The rocks of different groups fall on different liquid lines of descent in the Latent vector variation diagrams.

Keywords

Correlation and Principal Component Analyses, Malani Igneous Province, Petrogenesis, Samu-Dandali, Rajasthan.

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S. S. Rathore ¹, T. R. Venkatesan ², R. K. Srivastava ³

Affiliations
1 Geochemistry Division, KDMIPE, ONGC, Dehradun-248 195, IN
2 Physical Research Laboratory, Ahmedabad - 380 009, IN
3 Geology Department, M.L. Sukhadia University, Udaipur - 313 001, IN

Abstract

Six whole rock samples have been dated by ⁴⁰Ar/³⁹Ar method to ascertain the emplacement time of the Mundwara Alkali Igneous Complex, Rajasthan, India. The dated samples include four from the Musala hill and one each from Mer and Toa hills of the complex. Out of these, only two syenite samples. viz. MR 86/4 and MR 86/7 from the Musala and Mer hills, respectively, have given plateaus at 64 Ma, while the remaining samples have exhibited saddle shaped spectra owing to the presence of excess argon. However, similar isochron ages of about 70 Ma obtained from a basalt (Musala hill) and a gabbro (Toa hill) by linear regression of all the data points might suggest initiation of igneolls activity at Mundwara around 70 Ma ago.

⁸⁷Sr/⁸⁶Sr ratios of this suite range from 0.70385 to 0.70551 indicating its derivation from a mantle source.

Keywords

Geochronology, Mundwara Alkali Igneous Complex, Rajasthan.

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Salil Agrawal ¹, R. K. Srivastava ²

Affiliations
1 Department of Geology, University of Rajasthan, Jaipur 302 004, IN
2 Department of Geology, M. L. Sukhadia University, Udaipur - 313 001, IN

Abstract

Cluster analysis based on major elements demonstrates existence of granite, granodiorite and tonalites as distinct natural sub-population in the Sendra granitoid suite. The peraluminous granite with limited variation in mineralogy and chemistry is characterised by higher total iron, K₂O, mol. A/CNK, K₂O/Na₂O, Rb, Ba, Li and lower Sr as compared to the granodiorites and tonalites of the suite. Different trends for tonalites. granodiorites and granites in variation diagrams coupled with lower DI values of the granites in comparison to some of the tonalites and granodiorites, disallow interpretation of their relationship in terms of closed system equilibrium crystal fractionation or rcstite unmixing.

Principal component analysis of the data negates the possibility of the geochemical variations being controlled by a single process. The presence of mafic microgranular enclaves (MME) in the granodiorite and tonalite suggest mixing of the felsic and mafic magmas. The homogeneous nature of the granite and the absence of tonalitic and granodioritic components in the Chang pluton are due to emplacement of unadulterated granitic melt generated in the lower crustal region by basaltic underplating. The granodiorite and tonalite with variable chemical composition, represent subsequent mixing of granitic melt with the underplating basaltic magma and different degrees of thermal and chemical disequilibrium in the hybrid magma.

Keywords

Geochemistry, Proterozoic Granitoid Suite, Delhi Fold Belt, Magma Mixing, Rajasthan.

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