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

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, K. Vijaya Kumar ¹

Affiliations
1 Department of Geology, Post-Graduate College of Science, Osmania University, Hyderabad-500004, IN

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 SiO₂ Al₂O₃ and Na₂O and, lower CaO and K₂O 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_H2O and P_CO2 conditions. This process mimics "branching differentiation" mechanism.

Keywords

Syenites, Petrogenesis, Andhra Pradesh.

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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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K. Vijaya Kumar ¹, C. Leelanandam ²

Affiliations
1 School of Earth Sciences, SRTM University, Nanded - 431 606, IN
2 House No. 12-13-205/1, Street No. 2, Tarnaka, Hyderabad - 500 017, IN

Abstract

The Proterozoic Eastern Ghats Belt (EGB) along the east coast of India exposes deep segments of the continental crust, and is considered as a deeply eroded remnant of the present-day Himalayan-type setting. The EGB registers crustal growth of the Indian plate on its eastern side. Precambrian sutures in this poly-metamorphosed and multiply-deformed belt are recognized by the presence of ophiolites (containing unequivocal sheeted dykes and plagiogranite), shear zones, ultrahigh-T metamorphism, massif-type anorthosites and more significantly, by the presence of deformed alkaline rocks and carbonatites (DARCs). The southern segment of the EGB, around Kondapalli (Andhra Pradesh), records the entire gamut of processes from Palaeoproterozoic rifting to Grenvillian/Pan-African collision. The first convergence event that operated between 2 and 1.6 Ga is yet to be identified in other parts of the EGB. The mafic and felsic granulites of Kondapalli exhibit trace element characteristics typical of subduction-related magmatic arcs and the Kondapalli Layered Complex (KLC) is construed as an arc-ischolar_main complex in the deeply eroded Eastern Ghats orogen.

We postulate that the evolution of the EGB involved two distinct episodes of convergence, both being initiated by the onset of continental rifting. In our favoured model, the first rifting event at ~ 2.0 Ga along the eastern margin of the Indian plate, initiated the formation of an Atlantic-type continental margin, which was later converted into an Andeantype continental margin by convergent plate motion at ~1.85 Ga. Final culmination by continent-continent collision was possibly completed by 1.55 Ga. The second episode of rifting between 1.5-1.35 Ga, along the eastern margin of the thickened arc crust, facilitated the emplacement of alkaline rocks and carbonatites (ARCs). The exact timing of the Mesoproterozoic basin inversion and conversion of ARCs into DARCs - either during Grenvillian or Pan-African orogeny - remains elusive.

Keywords

Rifting, Collision, Suture, Arc Magmatism, ARCs-DARCs, Eastern Ghats Belt, India.

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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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C. Leelanandam ¹, K. Vijaya Kumar ¹

Affiliations
1 School of Eartli Sciences, Swanni Ramanand Teerth Marathwada University, Nanded - 431 606, IN

Abstract

Tiie Kondapalli Layered Complex (KLC) is a minor mafic-ultramafic intrusion occurring in the charnockitic region of the Eastern Ghats Mobile Belt (EGMB). it consists dominantly of gabbroic and anorthositic rocks, with subordinate ultramafic rocks (orthopyroxenite, websterite, cliriopyroxenite, harzburgite and dunite), which are associated with chromitite. The components of the KLC are grouped into two series: (1) Chromitite-Ultramafic Rock Series (CURS), and (2) Gabbro-Anorthosite Layered Series (GALS); some mafic gabbros belong to CURS. A profound petrological and geochemical hiatus exists between the primitive CURS and evolved GALS, which constitute a regressive intrusive sequence, it is shown that the REE abundances and chondrite-normalized patterns of 27 rocks from the KLC are controlled by the dominant cumulus minerals and variable amounts of trapped melt. The cumulate anorthositic and gabbroic rocks of the GALS are classified into those showing (1) positive Eu anomaly (An I and Gb I), (2) no Eu anomaly (An II and Gb II), and (3) negative Eu anomaly (Gb III). The whole-rock REE compositions possibly record the evolution of the "associated" liquids. Calculated associated liquid compositions, when viewed in conjunction with the available mineralogical, petrological and geochemical data of CURS and GALS, strongly suggest a two-magma genesis for the Kondapalli Layered Complex.

Keywords

Rare Earth Elements, Cumulates, Associated Liquids, Petrogenesis, Kondapalli Layered Complex, Andhra Pradesh.

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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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