J. Ratnakar

Bio: J. Ratnakar is an academic researcher from Osmania University. The author has contributed to research in topics: Gabbro & Pluton. The author has an hindex of 4, co-authored 9 publications receiving 38 citations.

Petrogenesis of the Ravipadu Gabbro Pluton, Prakasam Province, Andhra Pradesh

Abstract: The Ravipadu gabbro pluton (RGP) is one of the basic intrusives occurring within the deep Precambrian crust composed of amphibolites and granulites in the Prakasam igneous province juxtaposed against the Eastern Ghats mobile belt and Cuddapah basin. The pluton is a post-kinematic lopolith emplaced either permissively into the preheated country rocks as crystal mushes or emplaced by convective stirring. The intrusion has induced very high temperature thermal metamorphism of the pelites occurring on the fringes of the pluton. Dolerite and basaltic dykes traverse the RGP. The RGP is composed chiefly of olivine gabbro, olivine norite, troctolite and anorthosite in the decreasing order of areal extent. Sporadically developed layering is recorded in olivine norites, which may represent quiescent zones of magma chamber. The rocks exhibit cumulate, poikilitic and sub-ophitic textures. Olivine, plagioclase, orthopyroxene and clinopyroxene are the essential minerals crystallised in that order, and their variable proportions and combinations have produced the observed lithology. Sub-solidus diffusion along olivine-plagioclase and Fe-Ti oxide-plagioclase interfaces has produced spectacular coronas. The composition of olivine ranges from Fo 56.70 to Fo 67.09 and lower Fo content is attributed to derivation of parental liquids from a more Fe-rich source than the normal mantle. The composition of plagioclase varies from An 55.31 to An 75.48 Orthopyroxene is bronzite showing restricted variation from En 68.82 Fs 29.37 to En 71.66 Fs 27.19 . Wollastonite component is low and erratic in orthopyroxenes. Clinopyroxene is Al-rich titanaugite and its end-member composition varies from En 41.95 Wo 44.45 Fs 15.23 to En 47.05 Wo 38.91 Fs 11.03 . llmenite composition ranges from Il 87.83 to II 89.09 with very little haematite component. The ilmenites represent re-equilibrated compositions. The Ravipadu gabbroic rocks are primarily hypersthene and olivine normative; they do not contain quartz or nepheline in the norm. Textures, mineral assemblages, and mineral- and whole-rock chemistry indicate that the Ravipadu gabbros could be formed by crystal-liquid fractionation of subalkaline tholeiitic magma under reducing (anhydrous) conditions. The tholeiitic nature is also re-affirmed by Fe-enrichment as the parental magma evolved. The chondrite normalized REE patterns show enriched LREE, convex downward MREE and almost flat HREE trends with positive Eu anomalies for all the rocks. Petrographic features and geochemical traits accord cumulate status to the rocks and they do not represent melt compositions. Parental melt REE concentrations are calculated by utilising exploratory geochemical inversion techniques. The calculated parental liquids display enriched LREE and flat HREE trends. The variation in the concentrations of REE calculated from different cumulate rocks represents progressive evolution of a parental magma by fractional crystallization. Petrogenetic modelling suggests that the parental liquids are derived by 5 to 10% melting of an LREE enriched komatiite/tholeiite source in the deep crust. This source could be an underplated Late Archaean crust and this deep crustal melting might have been induced by the heat released from a mantle plume.

Petrogenesis of Quartz-Bearing Syenite Occurring Within Nepheline Syenite of the Elchuru Alkaline Complex, Prakasam Province, Andhra Pradesh

Abstract: A quartz-bearing syenite (QS) dyke cuts across the nepheline syenite (NS) in the Elchuru alkaline complex. The QS is leucocratic, medium grained and hypidiomorphic in texture. It is composed of K-feldspar, albite, amphibole, biotite and quartz. The QS has higher amounts of SiO 2 Al 2 O 3 and Na 2 O and, lower CaO and K 2 O than the average quartz syenite. The QS is metaluminous and oversaturated-in character. It has higher concentrations of Ga, Zr, Hf, Ta, V, Cr and U and, lower Ba, Rb, Sr, Y, Nb, Zn, Pb, Sc, Co, Ni and Th than fhe NS . The QS exhibits LREE-emiched and HREE-depleted pattern with a conspicuous negative Eu anomaly. The normalised incompatible trace element abundance pattern of the QS shows spikes for Ba, Ta and Y. A petrochemical comparison of the QS and NS suggests that they are derived from a feldspathic residual liquid by liquid immiscibility under variable P H 2 O and P CO 2 conditions. This process mimics "branching differentiation" mechanism.

The Gabbros of Prakasam Alkaline Province, Andhra Pradesh, India

Abstract: The gabbro plutons of Boggulakonda, Settupalle, Purimetia, Pasupugallu and Chimakurti in the Prakasam alkaline province (PAP) in Andhra Pradesh occur in a narrow zone (east of the Cuddapah basin) within the Precambrian amphibolites and granitic gneisses. The plutons are emplaced into preheated country rocks as either crystal mushes and multiple intrusive pulses or convective stirring. The chief rock types of the plutons are gabbro, norite, anorthosite, troctolite and rare pyroxenite. The gabbros are of high-alumina olivine tholeiitic type, excepting those in Pasupugallu pIuton which are of alkali olivine basaltic type. In general, all the gabbros are derived by shallow melting within the mantle. The liquidus olivine temperatures (T° Iiq ) of gabbros (excepting a few cumulate rocks) range from 1102° to 1412° C. The Boggulakonda, Purimetla and Pasupugallu gabbros are formed by fractionation of olivine and plagioclase. The Settupalle gabbros represent melts which are derived by a process of "continuous me1ting"(?). The Chimakurti gabbros are derived by "adiabatic melting" in a rising diapir. All the plutons have experienced olivine fractionation prior to their emplacement and differentiated under PCO 2 /PH 2 O conditions.

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.

Petrology of the gabbro-diorite-syenite-granite complex of Chanduluru, Prakasam alkaline province, Andhra Pradesh, India

Abstract: The calc-alkaline complex of Chanduluru (CL) in the Prakasam alkaline province of Andhra Pradesh is composed of two distinct petrographic suites viz. a gabbro-diorite suite and a melasyenitegranite suite. The chief rock forming minerals of the complex are clinopyroxene, amphibole, biotite, plagioclase, K-feldspar and quartz; the accessory minerals include apatite, sphene, zircon, opaque oxide and garnet. Major, trace and rare earth element variations in the two suites of rocks suggest two distinct petrogenetic series. The two suites are derived from two different parental magmas of varying composition and source. A mantle derived gabbroic magma has yielded gabbro-diorite suite by fractional crystalliiation. The underplating of the gabbroic magma has led to partial melting of the LREE-enriched arnphibolite crust to yield a melasyenitic magma, which on fractional crystallization has yielded melasyenitegranite suite. The complex is emplaced in a within-plate-collision tectonic environment.

A Geochemical Classification for Feldspathic Igneous Rocks

Abstract: In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeO þMgO) ratio or Fe-index, the modified alkali^lime index, the aluminum-saturation index, the alkalinity index, and the feldspathoid silica-saturation index.The Fe-index distinguishes between melts that have undergone extensive iron enrichment during differentiation from those that have not. The transition from tholeiite to ferrobasalt allows us to extend this boundary to silica values as low as 48 wt %. We introduce the feldspathoid silica-saturation index, which, coupled with the alkalinity index, allows us to extend the geochemical classification to alkaline rocks. We show that most alkaline rocks are ferroan and that this probably reflects extensive fractional crystallization of olivine and pyroxene with minimal participation of Fe^Ti oxides. The expanded classification allows us to illustrate the geochemical and petrogenetic relationship of the plutonic rocks from ferroan granites to nepheline syenites that commonly occur in intracratonic environments. It also allows us to distinguish four families of feldspathic rocks: (1) magnesian rocks, which are exemplified by Caledonian and Cordilleran batholiths and are characterized by differentiation under oxidizing and relatively hydrous conditions; (2) ferroan rocks, which include fayalite granites, alkali granites, and nepheline syenites and are characterized by differentiation under reducing and relatively dry conditions; (3) leucogranites, which commonly form by crustal melting; (4) potassic and ultrapotassic rocks, which originate from mantle that has been enriched in K2O.

Crustal architecture and evolution of the Eastern Ghats Belt and adjacent regions of India

Abstract: Abstract Extending along the east coast of peninsular India, the Eastern Ghats expose a deep section through a composite orogenic belt that once formed part of the Proterozoic mobile belt system within East Antarctica and East India. The critical evaluation of the existing geological and isotopic data strongly suggests that this orogenic belt includes not only the granulite facies Eastern Ghats Belt but also the Nellore-Khammam Schist Belt and lower grade units at the southern margin of the Singhbhum Craton. The present authors propose its subdivision into four crustal provinces with widely different geological evolutions. The Rengali and Jeypore Provinces formed at the margin of the Bhandara Craton in the Late-Archaean. In the Krishna Province, volcanosedimentary rocks equivalent to the Cuddapah Supergroup accumulated, probably on the Dharwar Craton in the Palaeoproterozoic, and the major tectonometamorphic event took place between 1.67 and 1.55 Ga, subsequent to a short-lived igneous activity. The Eastern Ghats Province, which shows considerable similarities with the Rayner Province of East Antarctica, was strongly affected by pervasive deformation, high-grade metamorphism and crustal-derived magmatism between 1.1 and 0.9 Ga, which extensively modified the crustal structure of present eastern peninsular India. Neoproterozoic and Early Phanerozoic tectonothermal activities were largely restricted to pre-existing shear zones, but the present configuration of the composite orogenic belt may have been achieved only during the Pan-African Orogeny.

Proterozoic mountain building in Peninsular India: an analysis based primarily on alkaline rock distribution

Abstract: Peninsular India was assembled into a continental block c. 3 million km2 in area as a result of collisions throughout the length of a 4000 km long S-shaped mountain belt that was first recognized from the continuity of strike of highly deformed Proterozoic granulites and gneisses. More recently the recognition of a variety of tectonic indicators, including occurrences of ophiolitic slivers, Andean-margin type rocks, a collisional rift and a foreland basin, as well as many structural and isotopic age studies have helped to clarify the history of this Great Indian Proterozoic Fold Belt. We here complement those studies by considering the occurrence of deformed alkaline rocks and carbonatites (DARCs) in the Great Indian Proterozoic Fold Belt. One aim of this study is to test the recently published idea that DARCs result from the deformation of alkaline rocks and carbonatites (ARCs) originally intruded into intra-continental rifts and preserved on rifted continental margins. The suggestion is that ARCs from those margins are transformed into DARCs during continental, or arc–continental, collisions. If that idea is valid, DARCs lie on rifted continental margins and on coincident younger suture zones; they occur in places where ancient oceans have both opened and closed. Locating sutures within mountain belts has often proved difficult and has sometimes been controversial. If the new idea is valid, DARC distributions may help to reduce controversy. This paper concentrates on the Eastern Ghats Mobile Belt of Andhra Pradesh and Orissa, where alkaline rock occurrences are best known. Less complete information from Kerala, Tamil Nadu, Karnataka, West Bengal, Bihar and Rajasthan has enabled us to define a line of 47 unevenly distributed DARCs with individual outcrop lengths of between 30 m and 30 km that extends along the full 4000 km length of the Great Indian Proterozoic Fold Belt. Ocean opening along the rifted margins of the Archaean cratons of Peninsular India may have begun by c. 2.0 Ga and convergent plate margin phenomena have left records within the Great Indian Proterozoic Fold Belt and on the neighbouring cratons starting at c. 1.8 Ga. Final continental collisions were over by 0.55 Ga, perhaps having been completed at c. 0.75 Ga or at c. 1 Ga. Opening of an ocean at the Himalayan margin of India by c. 0.55 Ga removed an unknown length of the Great Indian Proterozoic Fold Belt. In the southernmost part of the Indian peninsula, a line of DARCs, interpreted here as marking a Great Indian Proterozoic Fold Belt suture, can be traced within the Southern Granulite Terrain almost to the Achankovil-Tenmala shear zone, which is interpreted as a strike-slip fault that also formed at c. 0.55 Ga.