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

S. K. Bhushan ¹, V. K. Chittora ¹

Affiliations
1 Geological Survey of India, WR, Jaipur - 302 004, IN

Abstract

Neoproterozoic Malani bimodal volcanism occurs in a cauldron subsidence structure of a shield volcano at Siwana. This collapse caldera has exposed 45 volcanic flows comprising six of basalt, 20 of rhyolite, alkali rhyolite and rhyodacite, 15 of trachyte, one of dacite and three of trachyandesite, composition. Major and trace element geochemistry of lavas shows a bimodal character in the mafic and felsic components. The lower felsic flows are alkaline and highly enriched in HFS elements while the upper flows are peraluminous in composition. Alkalinity has been reintroduced in the uppermost flows. Wide variation in incompatible trace element ratios indicates a nonfractional crystallization relationship between the mafic and felsic magma. Malani volcanism was triggered in an extensional tectonic setting where geotherm was raised by the repeated influx of basaltic magma. Crustal extension inhibited the bouyant rise of silicic magma.

Keywords

Petrology, Bimodal Volcanism, Cauldron Subsidence, Shield Volcano, Geochemistry, Malani Volcanism, Rajasthan.

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V. K. Chittora ¹, Anil Maheshwari ², Massimo Coltorti ³

Affiliations
1 III/124 G.S.I. Colony, Malviya Nagar, Jaipur 302 017, IN
2 Department of Geology, University of Rajasthan, Jaipur 302004, IN
3 Istituto di Mineralogia, Universita di Ferrara, 44100, Ferrara, IT

Abstract

The Malani volcanics exposed in a curvilinear pattern around Bhadrajan are characterised by the presence of eight flows of rhyolitic to dacitic composition and have chemical characteristics similar to within plate, A-type granitic rocks. The pyroclastic unit present at the base of each flow marks the beginning of next pulse of discharge. The chemistry of Bhadrajan volcanics suggests two distinct rhyolite groups. Group "A" is characterised by high Ca, Al, Na₂O/K₂O and low Rb, Th, La, Ce and Rb/Sr as compared to group "B" rhyolites. The field and chemical data suggest a heterogeneous nature of protolith for Bhadrajan volcanics. The partial, melting of igneous and siliceous metapelitic protolith could result in "A" and "B" group of rhyolites respectively.

Keywords

Geochemistry, Malani Rhyolites, Flow Stratigraphy, Rajasthan.

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S. K. Bhushan ¹, V. K. Chittora ¹

Affiliations
1 Geological Survey of India, Jaipur - 302 004, IN

Abstract

Two granitoid bodies each from Palaeo-, Meso- and Neo-Proterozoic periods have been studied for their chemical, petrological and source characteristics. The Proterozoic granitoids are megacrystic as well non-Megacrystic with variable effect by tectonometamorphic events. The Palaeo-Proterozoic granitoids are peraluminous, corundum normative with high proportion of biotite in their modal composition. They contain either orthoclase/Sanidine or andesine as feldspars and iron rich biotite. The Meso-Proterozoic granitoids have high silica with meta-to peraluminous composition. The feldspar chemistry indicates albite/Oligoclase composition whereas they exhibit high iron rich biotite and some being lepidomelane in composition. The Neo-Proterozoic granitoids are invariably porphyntic having higher silica content (>72 wt%), as compared to other granitoids and exhibit strong peraluminous affinity. The feldspar geothermometry indicate 1050°C for Palaeo-, 900-950°C for Meso- and 750°C for Neo-Proterozoic granitoids. The Proterozoic granitoids are orogenic, the degree of deformation varies from pristine, non-Foliated to gneissic and augen gneissic, mostly 'S' type, indicating a sedimentary source material for their generation. The biotite discriminant diagrams also support 'S' type collisional tectontic setting. The garnet-Biotite geothermometery for Palaeo-Proterozoic granitoids where these minerals co-Exist suggest equilibrium temperature of -540°C at 1 or 2 kb pressures. There is appreciable replacement in magnetites of Palaeo-Proterozoic granitoids by K₂O, SiO₂, Al₂O₃ and negligible in Meso- and Neo-Proterozoic rocks. There is gradual increase in SiO₂ and DI from Paleo- to Neo-Proterozoic granitoids. The basement complex, after attaining rigidity and cratonisation by the end Archaean igneous activity was rifted periodically during the Proterozoic times. Extensional events led to the formation of mafic dyke swarms, plutonic and volcanic activity and sedimentary platform covers. However, the aborted rifts were further effected by compressive tectonics giving rise to syn and late tectonic deformational signatures to the intrusive phases and their host rocks.

Keywords

Proterozoic Granitoids, Petrography, Modal Analysis, Geochemistry, Mineral Chemistry, Geothermometry, Rajasthan.

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