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

V. Madhavan ¹, J. Mallikharjuna Rao ², V. Balaram ², Ramesh Kumar ³

Affiliations
1 Department of Geology, Osmania University, Hyderabad-500 007, IN
2 National Geophysical Research Institute, Hyderabad-500 007, IN
3 Atomic Mineral Division, Civil Lines, Nagpur-440 001, IN

Abstract

Late stage intrusives, dominantly represented by lamprophyres, form a dyke swann in the Elchuru alkaline pluton of Andhra Pradesh; the other dykes of the swann which are non-lamprophyric include microshonkinite, nepheline syenite porphyry, basanite and olivine dolerite. The Elchuru lamprophyres and their plutonic equivalents, shonkinite and foidolite, consist of clinopyroxene, biotite. alkali feldspar and nepheline. Due to variation in the amount of nepheline, these lamprophyres, cover a wide range of rock types like minette, sannaite and tjosite. A plagioclase- bearing lamprophyre, camptonite, is also present though not abundantly.

The hallmark of the Elchuru lamprophyres is their transitional geochemical character-ranging between alkaline lamprophyres(AL) and calc-alkaline lamprophyres (CAL). From the present study it is surmised that these lamprophyres have crystallised from an LILE and LREE enriched, hydrous basic alkaline (mildly potassic) magma of a deep-seated (mantle?) origin. A petrogenetic scheme is presented covering the evolution of various rock units of the Elchuru alkaline pluton in general and the late stage intrusives (lamprophyres and other associated dykes) in particular.

Keywords

Lamprophyres, Dykeswarms, Elchuru, Andhra Pradesh.

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J. Mallikharjuna Rao ¹, S. Nirmal Charan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The diamond-rich Pipe-7 kimberlite found about l0km east of Lattavaram in Anantapur district, Andhra Pradesh in south India is an elongated dyke - like body, that extends over a length of 450 m and has a maximum width of 25 m. It is porphyritic in texture and contains sub-angular to rounded xenoliths of peridotite, garnet-clinopyroxene rocks, websterite, amphibolite, metabasic rocks and granite. Petrographic studies reveal that it is porphyritic in texture and contains euhedral to sub-hedral altered olivine. phlogopite, garnet, clinopyroxene (chrome diopside), chromite, perovskite and ilmenite which are the uneqvivocal primary liquitlus phases whereas serpentine, chlorite, phlogopite, calcite, amphibole, magetite, monticellite, clay minerals and others represent secondary alteration assemblages. Chrome-diopsides and garnets represent an important megacryst suite.

The Pipe-7 kimbeerlite has higher CaO and lower MgO, Al₂O₃, K₂O and iron contents compared to rhe other six known kimberlites in the vicinity. Fe/Mg ratio in coexisting garnets and clinopyroxenes, indicates the equilibration temperature for (a) the nodules to be 1040 - 1070°C and for (b) the megacrysts of garnet and clinapyroxene to be 1350°C respectively.

Pyrope garnets from titis kimberlite show a positive correlation between CaO and Cr₂O₃ and are similar in nature to those of the low calcium garnets reported from the diamondiferousSouth African kimberlites.

Keywords

Kimberlite, Petrography, Geochemistry, Anantapur Dist, Andhra Pradcsh.

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G. V. S. Poornachandra Rao ¹, J. Mallikharjuna Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The Bijawar Group of rocks unconformably overlie the 2555 Ma Bundelkhand Granite massif and contain in its lower sub-group the Kawar Volcanic Formation exposed near the Bhusor village (24° 39'N; 79° 51' 3O"E) SW of Panna.

The Kawar volcanics are fine grained, magnesium rich tholeiites, basalt to basaltic andesite in composition, with spinifex texture. The immobile trace elements suggest that these volcanics were emplaced in a continental environmental setting in rapid succesion with a low residence time. The REE pattern reveals that these rocks were derived from a depleted mantle source. A comparison of average chemical compositions of Mahakoshal and Gwalior basalts with a Kawar volcanics suggest that these were originated in different environmental conditions although they had previously believed to be the same.

Detailed AF and thermal demagnetization of 20 oriented block samples from 4 sites revealed a characteristic stable remanence in the Kawar volcanics with a declination of 254.9 and inclination of -64.0 (K = 70.75, α₉₅, = 8.32, N = 4). The corresponding VGP, located at p = 27.6° N, Lp = 129.3° E (δp = 6.29, δm = 13-26), appears to be approximately 2000 Ma when compared with other Indian Precambrian data. This age is in agreement with the stratigraphic position above the Bundelkhand granite massif.

Keywords

Kawar Volcanics, Geochemistry, Palaeomagnetisrn, Central India.

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V. Madhavan ¹, J. Mallikharjuna Rao ², M. Srinivas ³, R. Natarajan ², Ausaf Sayeed ³

Affiliations
1 Geology Department, Kakatiya University, Warangal - 506 009, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN
3 Geology Department, Osmania University, Hyderabad - 500 007, IN

Abstract

Rare bodies of syenites of alkaline or subalkaline character occur as intrusions in the rocks of the Cuddapah Supergroup within the Cuddapah basin. The alkali syenites (found at Racherla, Idamakallu and Giddalur) are mainly composed of riebeckite (arfvedsonite), ilmenite, orthoclase and the subalkaline syenites (located at Gundlapalle) comprise hedenbergite, and microcline-perthite. Accessory quartz is invariably present in both the syenites. These intrusive bodies display contrasting mineralogical and geochemical characteristics. The quartz- and hypersthenenormative subalkaline syenite has the attributes of a ferrosyenite and has been subjected to hydrothermal alteration which not only resulted in the formation of secondary minerals like nontronite, amphibole, biotite and magnetite- all at the expense of hedenbcrgite, but also partly converted the Fe⁺² into Fe ⁺³. These subalkaline syenites are characterized by an extreme paucity of Mg, has low to moderate contents of Ba, Sr, Rb, Zr and REE. The quartz, hypersthene and acmite- normative alkali syenites (lusitanite) on the other hand are abnormally enriched in Ti; Rb is low but Ba, Sr, Ce, Nb, Nd, Ta, Sm, La, Zr am also the ratios K/Rb, Ti/Zr, Ce/Y and HREE/LREE are conspicuously high.

The syenites of the Cuddapah basin, seem to have crystallized under contrasting conditions. While the alkali syenites crystallized under high PH₂O, the crystallization of subalkaline syenite took place under dry arid reducing conditions signifying a closed system. It is envisaged that the subalkaline ferrosyenite of Gundlapalle was formed by low-pressure crystal fractionation of a continental tholeiitic basaltic magma. The alkali syenites, on the other hand, are postulated to have crystallized from a mantle-derived, hydrous, alkaline magma enriched in Large Ion Lithophile Elements and Light Rare Earth Elements.

Keywords

Syenites, Igneous Petrology, Cuddapah Basin, Andhra Pradesh.

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J. Mallikharjuna Rao ¹, G. V. S. Poornachandra Rao ¹, K. P. Sarma ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Department of Geology, Guwahati University, Guwahati, IN

Abstract

The Archaean gneissic basement of Shillong plateau has been traversed by number of mafic dyke swarms. At least two suites of dykes are identified in the region represented by Proterozoic Khasi greenstone related dolerites and younger Cretaceous dolerite dykes in addition to mafic alkaline dykes. The older Khasi greenstone dolerites are altered and have undergone low-grade metamorphism compared to fresh Cretaceous dykes, which are well exposed in the West Garo Hills region. All the Khasi greenstone dolerites are tholeiite in composition and range from basalt to basaltic andesite in composition and show olivine or quartz normative character. Most of the dykes show continental nature of emplacement with some overlapping oceanic tectonic setting of origin. Petrochemical study suggests that they were derived from picrites that subsequently undergone low- pressure fractionation. Palaeomagnetic study of the older Khasi greenstone related dolerites show a direction of magnetization of Dm=17, Im=+57 (α₉₅= 23.34; K=31.5; N=24) with a palaeolatitude of 29.7°N to the Indian subcontinent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from K-T studies is in multi domain (MD) size and cation deficient (CD) domain states.

Keywords

Khasi Greenstone, Dykes, Mafic Magmatism, Shillong Plateau, Meghalaya.

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V. Madhavan ¹, K. David ¹, J. Mallikharjuna Rao ², N. V. Chalapathi Rao ³, M. Srinivas ⁴

Affiliations
1 Department of Geology, Kakatiya University, Warangal, A.P. - 506 009, IN
2 National Geophysical Research Institute, Hyderabad- 500 007, IN
3 Department of Applied Geochemistry, Osmania University, Hyderabad- 500 007, IN
4 Department of Geology, Osmania University, Hyderabad- 500 007, IN

Abstract

The mid-Proterozoic lamprophyric rocks broadly represented by kimberlites, lamproites and lamprophyres (sensu stricto) are emplaced in and adjacent to the intracratonic Cuddapah basin in the eastern Dharwar craton of India which was recently recognized as the Cuddapah (intrusive) province (CIP). While the kimberlite pipes and lamprophyre dykes are located outside but close to the western and eastern margins of the Cuddapah basin respectively, the lamproites are hosted within the basin. Lamprophyres occur in seven locations of the Prakasam district of Andhra Pradesh. EIchuru, where more than one hundred lamprophyre dykes are found, represents a prominent centre of lamprophyre magmatism. Purimetla comes next to Elchuru with a tally of twenty lamprophyre dykes. At other places lamprophyres are numerically insignificant. The CIP lamprophyres encompass different varieties and they cut across an equally wide-ranging variety of host rocks; these lamprophyres come under 1) feldspathoidal or non- feldspathoidal and 2) plagioclase- or orthoclase-bearing categories. With a single exception, all the lamprophyres are micaceous and distinctly alkaline. The genesis of CIP lamprophyres as a manifestation of alkaline magmatism is discussed.

Keywords

Petrology, Geochemistry, Lamprophyres, Cuddapah Province, Andhra Pradesh.

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