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

V. Nagsai Sharma ¹, J. Ratnakar ¹

Affiliations
1 Department of Geology, Post-Graduate College of Science, Osmania University, Saifabad, Hyderabad-500 004, IN

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.

Keywords

Igneous Petrology, Gabbro, Diorite, Syenite, Granite, Partial Melting, Fractional Crystallization, Chanduluru Complex, Prakasam Alkaline Province, Andhra Pradesh.

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J. Ratnakar ¹, C. Leelanandam ²

Affiliations
1 Department of Geology, Post-Graduate College of Science, Osmania University, Saifabad, Hyderabad - 500 004, IN
2 Department of Geology, Osmania University, Hyderabad - 500 007, IN

Abstract

The ferrosyenites of Sivamalai (Tamil Nadu, South India) contains ortho- and clinopyroxenes in addition to fayalitic olivine, quartz, palgioclase, orthoclase perthite and opaques. The orthopyroxene occurs as (a) submicroscopic rim around fayalite, (b) exsolved (001) and (100) lamellae within clinopyroxene and (c) discrete blebs close to clinopyroxene. The composition of orthopyroxene among the analysed samples varies from eulite to orthoferrosilite and that of clinopyroxene from ferroaugite to ferrohedenbergite. The ortho- and clinopyroxenes individually exhibit systematic intraspecimen and intralintergranular variations of Fe and Mg, with a restricted range of Ca. The Fe/(Fe+Mg) ratios in the pyroxenes (0.966-0.722) indicate their consistency iron-rich nature, and the orthopyroxenes, in particular. are among a few extremely iron-rich ones reported.

The occurrence of orthopyroxene lamellae within clinopyroxenes of ferrosyenites of Sivamalai is a diagnostic feature of slow cooling of a tholeiitic magma. The two compositional trends defined by the pyroxenes in the quadrilateral suggest strongly fractionated nature of the liquid. The mineral assemblage fayalite+quartz+ferrosilite-rich orthopyroxene suggests a minimum P of 6 kb. The Sivamalai pyroxenes are considered to have formed at -740°C and 7 kb. These pyroxenes are presumably crystalllized from a Fe-rich acidic liquid under low fO₂, moderate to high P and increasing SiO₂.

Keywords

Pyroxenes, Ferrosyenites, Sivamalai, Tamil Nadu.

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K. Vijay Kumar ¹, J. Ratnakar ²

Affiliations
1 School of Earth Sciences, Swami Ramanand Teerth Marathwada University, Vishnupuri, Nanded 431603, IN
2 Department of Geology, Post-Graduate College of Science, Saifabad, Hyderabad-500004, IN

Abstract

In the Ravipadu gabbro pluton (RGP), Prakasam province, Andhra Pradesh, phlogopites occur as coronal rims along the interfaces between plagioclase-olivine, pIagioclase-ilmenite and rarely plagioclase-clinopyroxene, and as independent grains grown at the expense of intercumulus pyroxenes.

The changes observed in the chemical composition of phlogopites are in the contents of MgO, FeO¹ and TiO₂ SiO₂ and Al₂O₃ do not show much variation and K₂O is almost constant, Na₂O is low and CaO is absent. Calculated end members are rich in phlogopite, Ti-phlogopite and eastonite components.

The phlogopites faithfully record the progressive iron enrichment trend of the original phases and consequently that of tholeiitic parental magma. It is established that in a single phlogopite the Mg number, and Mg, Fe, and Ti contents are controlled by its habit, whereas in a series of rocks the phlogopite composition is steered by the composition of host rock. Concentrations of A1 and K in the Ravipadu phlogopites are independent of chemistry of minerals that are consumed and also evolutionary history of the parental magma but are a signature of "chemical collaboration" of interacting fluid with original mineralogy. Textural features and geochemical traits of the phlogopites suggest probable attainment of perfect equilibrium between the fluid and primary phases during the formation of phlogopites in the RGP.

Keywords

Geochemistry, Phlogopites, Gabbros, Prakasam Province, Andhra Pndesh.

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K. Vijaya Kumar ¹, J. Ratnakar ¹

Affiliations
1 Department of Geology, Post-Graduate ColIege of Science, Osmania University, Saifabad, Hyderabad - 500 004, IN

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 PCO₂/PH₂O conditions.

Keywords

Gabbros, Igneous Petrology, Prakasam District, Andhra Pradesh.

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J. Ratnakar ¹, C. Leelanandam ², R. Natarajan ³

Affiliations
1 Department of Geology, Post-Graduate College of Science, Osmania University, Safiabad, Hyderabad-500 004, IN
2 Department of Geology, Osmania University, Hyderabad-500 007, IN
3 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

The electron microprobe analysis of olivines from the ferrosyenites of Sivamalai shows that they are extremely rich in iron with Fa_96.4-Fa_93.9. The restricted range of Fe²⁺:Mg:Mn values and high Fe²⁺/Mg ratios amongst the analysed samples suggest that parental liquids underwent high degree of fractionation. Association of fayalitic olivine, orthopyroxene, inverted pigeonite and quartz in the ferrosyenites of Sivamalai strongly suggests their tholeiitic nature.

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J. Ratnakar ¹, D. V. Rama Krishna ¹, K. Vijaya Kumar ¹

Affiliations
1 Department of Geology Post-Graduate College of Science Osmania University, Saifabad, Hyderabad - 500 004, IN

Abstract

The lamprophyre (sannite) of Kellampalle (KL) occurs as a plug in tholeiitic gabbro. The KL has phenocrystic amphibole and biotite ± clinopyroxene and, groundmass amphibole, biotite and orthoclase perthite, with accessory apatite and opaques. The major, trace and rare earth element geochemistry of the KL suggests that it does not represent a directly crystallized product of a primary magma. The data suggests that a primary magma, as produced by low degrees of partial melting of metasomatised mantle; underwent olivine fractionation (prior to the emplacement) and subsequently differentiated into the cpx-bearing and cpx-free lamprophyre plug.

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J. Ratnakar ¹

Affiliations
1 Department of Geology, Post-Gractuate College of Science, Osmania University Saifabad, Hyderabad-500 004, IN

Abstract

No Abstract.

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K. Vijaya Kumar ¹, J. Ratnakar ²

Affiliations
1 School of Earth Sciences, Swami Ramanand Teerth Marathwada University, Vishnupuri, Nanded - 431 606, IN
2 Department of Geology, Postgraduate College of Science, Osmania University, Saifabad, Hyderabad - 500 004, IN

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.

Keywords

Petrology, Geochemistry, Petrogenesis, Magma underplating, Gabbros, Ravipadu, Andhra Pradesh.

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J. Ratnakar ¹

Affiliations
1 Department of Geology, Post-Graduate College of Science, Osmania University, Saifabad, Hyderabad - 500 004, IN

Abstract

No Abstract.

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K. Rathna ¹, K. Vijaya Kumar ¹, J. Ratnakar ²

Affiliations
1 School of Earth Sciences, Swami Ramanand Teerth Marathwada University, Vishnupuri, Nanded - 431 606, IN
2 Department of Geology, Postgraduate College of Science, Osmania University, Saifabad, Hyderabad - 500 004, IN

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.

Keywords

Igneous Petrology, Geochemistry, Dolerites, Basaltic Dykes, Lamprophyres, Prakasam Province, Andhra Pradesh.

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K. Krishna Reddy ¹, J. Ratnakar ², C. Leelanandam ³

Affiliations
1 Geological Survey of India, Bandlaguda, Hyderabad-500 660, IN
2 Department of Geology, Post-Graduate College of Science, Osmania University, Hyderabad-500 004, IN
3 Department of Geology, Osmania University, Hyderabad-500 007, IN

Abstract

The Proterozoic Uppalapadu alkaIine complex (1348±41 Ma) in the Prakasam province of Andhra Pradesh is predominantly composed of nepheline syenite, hornblende syenite and quartz syenite, all of which represent an alkaline series. The pluton is closely associated with tholeiitic gabbro and ferrosyenite representing a sub-alkaline series. Geochemical characters suggest that the rocks of the alkaline series were formed by the differentiation of the hydrous alkaline basaltic magma under high P_H2O, whereas those of the sub-alkaline series were formed by differentiation of an anhydrous olivine tholeiitic magma under low fO₂ (reducing) conditions. Quartz syenite (an offshoot in the alkaline series) is formed either by fractionation of K-feldspar, ferrohastingsite and/or biotite under high P_H2O conditions or by crustal contamination of critically saturated syenitic liquids.

Keywords

Petrochemistry, Proterozoic, Alkaline Complex, Andhra Pradesh.

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