K. Rathna

Bio: K. Rathna is an academic researcher from Swami Ramanand Teerth Marathwada University. The author has contributed to research in topics: Basalt & Shonkinite. The author has an hindex of 4, co-authored 8 publications receiving 50 citations.

Density, Viscosity and Velocity (Ascent Rate) of Alkaline Magmas

Abstract: Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54–2.71 g/cc for shonkinite; 2.61–2.78 g/cc for lamprophyre; 2.66–2.74 g/cc for alkali basalt] and viscosities [3.11–3.39 Pa s for shonkinite; 3.01–3.28 Pa s for lamprophyre; 2.72–3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11–2.13 m/sec, 0.23–2.77 m/sec and 1.16–2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68–15.31 m/sec) > ultramafic-mafic magmas (3.81–4.30 m/sec) > intermediate-felsic magmas (1.76–2.56 m/sec). We propose that SiO2 content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.

Proterozoic Subduction-Related and Continental Rift-Zone Mafic Magmas from the Eastern Ghats Belt, SE India: Geochemical Characteristics and Mantle Sources

Abstract: Understanding the origin and growth of continental crust is a fundamental problem in geological sciences. Two distinct ways in which the continental crust grows include horizontal (subduction) and vertical (plume/extension) accretions. As the mantle reservoirs in these two tectonic settings are generated and/or modified by contrasting processes, the erupted melts offer clues on the nature of these divergent mantle sources. Trace element geochemistry is a robust tool to quantitatively model the mantle sources, melting mechanisms and relative roles of mantle and crust in the petrogenesis of magmatic rocks, which ultimately lead us to unravel the origin of continental crust. The present study portrays growth of the continental crust in the Proterozoic Eastern Ghats Belt, SE India. Mafic magmas within the Palaeoproterozoic Kondapalli-Kandra region illustrate subductionrelated island arc basalt-type geochemical signatures whereas alkali basaltic magmas in the Mesoproterozoic Prakasam continental rift-zone display ocean island basalt-type characteristics. Calculated mantle sources for subduction-zone and rift-related magmas display distinctly different geochemical traits. Mesoproterozoic gabbroic magmas in the Prakasam rift-zone exhibit geochemical signatures akin to the subduction-related mafic melts. This dichotomy of continental crust produced by intra-plate processes exhibiting plate-margin signatures advocates that we possibly have overestimated the proportion of continental crust generated above subduction zones.

Petrology of the Dykes of Ravipadu, Prakasam Province, Andhra Pradesh, India

Abstract: The doleritic and basaltic dykes of Ravipadu in the Prakasain Province traverse and extend beyond the gabbro pluton. The dolerites exhibit ophitic - subophitic, intergranular and equigranular textures. The basaltic dykes include lamprophyres (camptonites) and porphyritic basaltic dykes which display porphyritic/glomeroporphyritic and panidiomorphic textures. The chief rock forming minerals in these dykes are olivine, clinopyroxene and plagioclase with subordinate orthopyroxene, amphibole and biotite. The accessory minerals include opaques and green spinel. The dolerites and basaltic dykes show contrasting major dement compositions, especially with respect to alkalies and alkali/silica ratios, normative mineralogy and rare earth element concentrations and patterns. The dolerites, which contain modal and normative hypersthene, are tholeiitic and show iron enrichment as the differentiation proceeds. They show LREE enriched and HREE depleted patterns with positive Eu anomalies. Basaltic dykes in which modal and normative hypersthene is conspicuously absent are calc-alkaline to alkaline and are LREE enriched without any perceptible Eu anomalies. Petrogenetic modelIing suggests that the dolerites are probably replenished liquids which are geochemically similar to the parental magmas of the host gabbros, whereas the basaltic dykes are genetically unrelated to the dolerite dykes and to the host gabbros. The basaltic dykes are believed to have formed from the melts of an LREE enriched mantle and are emplaced in a rift setting, during an extensional regime.

Statistical evaluation of bivariate, ternary and discriminant function tectonomagmatic discrimination diagrams

Abstract: This work applies a statistical methodology involving the calculation of success rates to evaluate a total of 28 tectonomagmatic discrimination diagrams: four bivariate (Ti/Y-Zr/Y; Zr-Zr/Y; Ti/1000-V; and Nb/Y-Ti/Y); six ternary (Zr-3Y-Ti/1000; MgO-Al2O3-FeOt, Th-Ta-Hf/3; 10MnO-15P2O5-TiO2; Zr/4-Y-2Nb; and La/10-Nb/8-Y/15); and three old (Score1-Score2; F1-F2; and F2-F3) and three sets of new discriminant function diagrams (each set consisting of five DF1-DF2 type diagrams proposed during 2004-2008). I established and used extensive geochemical databases of Miocene to Recent fresh rocks from island arcs, back arcs, continental rifts, ocean-islands, and mid-ocean ridges. Rock and magma types were inferred from a SINCLAS computer program. Although some of the existing bivariate and ternary diagrams did provide some useful information, none was found to be totally satisfactory, because success rates for pure individual tectonic settings typically varied from very low (1.1-41.6%) to only moderately high values (63.6-78.1%) and seldom exceeded them. Additionally, only "combined" tectonic settings were discriminated, or numerous samples plotted in overlap regions designated for two or more tectonic settings or even in areas outside any field. Furthermore, these old diagrams are generally characterized by erroneous statistical basis of closure problems or constant sum constraints in compositional data and by subjective boundaries drawn by eye. All such diagrams, therefore, should be abandoned and replaced by the new sets of discriminant function diagrams proposed during 2004-2010. These diagrams, especially those of 2006-2010 based on the correct statistical methodology and the boundaries drawn from probabilities, showed very high success rates (mostly between 83.4% and 99.2%) for basic and ultrabasic rocks from four tectonic settings and should consequently be adopted as the best sets of tectonomagmatic discrimination diagrams at present available for this purpose. Three case studies from Turkey (Kula, Eastern Pontides, and Lycian-Tauride) were also provided to illustrate the use of two new sets of discriminant function diagrams (2006-2008). For the Kula area, both sets of major- and trace-element based diagrams provided results consistent with a rift setting. For the Pontides area, trace-element based diagrams suggested an arc setting to be more likely, according to both basic and intermediate rocks. For the Lycian ophiolites, however, only the major-element based set of diagrams could be applied, and because of alteration effects, the tectonic inference between an arc or a MORB setting could not be decisive. A newer set of immobile element based, highly successful diagrams currently under preparation (2010) should provide a complementary set to the existing diagrams (2006-2008) for a better application of this important geochemical tool. Further work on these lines is still necessary to propose discrimination diagrams for other types of magmas such as those of intermediate silica compositions.

Did the circum-Rodinia subduction trigger the Neoproterozoic rifting along the Congo–Kalahari Craton margin?

Abstract: Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko–Dom Feliciano–Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820–785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800–770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740–710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este–Coastal Terrane is interpreted as an axial part of a Neoproterozoic “Adamastor Rift”. Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Perez–Luis Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.