Geological Setting and Petrographic Diversity of the Lamproite Dykes at the Northern and North...

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INDIAN DYKES: Geochemistry, Geophysics and Geochronology Editors: Rajesh K. Srivastava, Ch. Sivaji and N. V. Chalapathi Rao © 2008, Narosa Publishing House Pvt. Ltd., New Delhi, India

Geological Setting and Petrographic Diversity of the Lamproite Dykes at the Northern and North Eastern Margin of the Cuddapah Basin, Southern India N. V. CHALAPATHI RAO1*, G. D. KAMDE 2, H. S. KALE 2 and A. DONGRE3 1Department of Geology, Banaras Hindu University, Varanasi 221 005, India 2Post Graduate Department of Geology, Nagpur University, Amaravati Road, Nagpur 440 001, India 3Post Graduate Department of Geology, Government Institute of Science, Aurangabad, India Abstract A suite of about thirty lamproites outcrop in the Krishna valley at the Northern and North-Eastern margin of the Cuddapah Basin, southern India. They occur as dykes as well as plugs and intrude the basement Archaean granite-gneissic rocks. These lamproites are characterised by the varying proportions of serpentinised olivine, phlogopite, clinopyroxene, amphibole, sanidine, titanite, leucite (?), apatite, perovskite, spinels and a number of secondary phases like talc, chlorite, carbonate and quartz and also exhibit remarkably diverse textures viz., flow, pyroclastic (tuffaceous), inequigranular, microporphyritc and equigranular. The available geochemical data favour involvement of a similar magma in the genesis of all these lamproites. Thus, the Krishna valley lamproites constitute an extremely rare example of their kind in the world exhibiting the well known phenomenon, exhibited mostly by lamprophyres, of heteromorphism wherein a magma of similar composition can give rise to petrographically diverse rock types. The observed petrographic diversity of Krishna lamproites is attributed to the differences in the volatile contents in their respective source regions and the subsequent low -pressure crystallisation of these magmas. Keywords: Lamproite, Texture, Petrography, Krishna valley, Southern India. Introduction Lamproites are extremely rare Mg-rich (MgO>5 wt.%), peralkaline [(K2O+Na2O)/Al2O3>1 (molar)], perpotassic [(K2O/Al2O3)>1 (molar)] and ultrapotassic [(K2O/Na2O>3 (molar)] igneous rocks characterised by one or more of exotic minerals such as leucite, K-richterite, Ti-phlogopite, diopsidic clinopyroxene, wadeite and priderite (Mitchell and Bergman, 1991). The Mid-Proterozoic Cuddapah basin and its environs in the Eastern Dharwar craton,

Indian Dykes; Editors: Rajesh K. Srivastava, Ch. Sivaji and N.V. Chalapathi Rao

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southern India, hosts one of the oldest yet dated lamproites in three distinct lamproitic fields at the (i) NE and Northern margin of the basin (Krishna lamproite field; KLF), (ii) NW margin of the basin (Ramadugu lamproite field; RLF) and (iii) within the Cuddapah basin (Cuddapah lamproites; CL) and are focus of intensified research in recent years (e.g. Reddy et al., 2003; Chalapathi Rao et al., 2004; Chalapathi Rao, 2007; Paul et al., 2007; Chakrabarti et al., 2007). It is well known that the Krishna valley at the NE and northern margin of the Cuddapah Basin has produced some of the world famous alluvial diamonds such as Koh-I-Noor, Pitt, Hope, Nizam etc but their primary source is still debated. Therefore, the study of the KLF lamproites is of considerable importance. The purpose of this paper is to add further information on their geological setting and petrographic diversity from the recent studies undertaken by the authors in KLF.

Figure 1: Geological map of the Krishna lamproite field (KLF) showing the location of lamproite

dykes (adapted from Reddy et al. 2003; Paul et al. 2007 and Chakrabarti et al. 2007). Mode of Occurrence of Lamproites About thirty lamproites occur (Fig. 1) in a widespread area of 150 sq km in the Krishna Valley at the northern and north-eastern margins of the Proterozoic Cuddapah Basin of

Lamproite Dykes of N & NE Margins of Cuddapah Basin: N.V. Chalapathi Rao and others

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southern India and collectively constitute Krishna lamproite field (KLF; Reddy et al., 2003). They occur mostly as dykes and intrude the basement Archaean granite-gneissic rocks near Vedadri, Nallabandlagudem, Pochampalle, Jayantipuram, Tirumalgiri, Jaggayyapeta, Balusupadu and Ramannapeta areas (Fig. 1). A lamproite plug was also reported from the Vedadri area (Reddy et al., 2003) but could not be located during this study. Lamproites of KLF occur close to the contact between the basement granitoid rocks and the sediments of the Cuddapah basin. Xenoliths of the sedimentaries within the KLF bodies is widespread and suggests that the lamproites to be younger than the Cuddapah and Kurnool sediments. Interestingly, this area is structurally very significant since evidences of post-Kurnool thrusting wherein older Cumbum (Cuddapah) phyllites are thrusted over younger Nargi (Kurnool) limestones (Natarajan and Rajagopalan Nair, 1977). The relationship between this thrusting event and its relationship, if any, with the lamproite emplacement is yet to be evaluated. Ramannapeta lamproite is the only yet dated lamproite from KLF and its age range from 1384± 18Ma (K-Ar; method; Chalapathi Rao et al., 1996) to 1224 ± 14 Ma (Rb-Sr method; Anil Kumar et al., 2001).

Lack of detailed description of the individual occurrences in KLF (Reddy et al., 2003) is less informative to workers interested in locating them (see Nambiar, 2003) as our own field experience in KLF has shown. Therefore, we present a detailed geological description of the individual lamproite occurrences sampled by us for the benefit of future workers.

Figure 2: Lamproites in KLF intrude the basement Archaean granite-gneiss essentially as narrow and

small dykes as at (A) Tirumalgiri and (B) Ramannapeta.


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Pochampalli Lamproite dykes are located in the granite hill 2.5 km west of Pochampalli village and 1.2 km NW of Gundabonipalem village in the south flank of hill 158. They extend in NW-SE with N40°W strike direction. The average length of dykes is about 20 m and average and width is about 1m. All the samples collected from the different locations in the dykes from the hill give a glistening appearance due to their rich micaceous nature Tirumalgiri Two parallel lamproite dykes are situated on a granitic hill 133, which is 1.2 km north of Tirumalgiri village, which in fact hosts a temple: (i) T1 in western flank which continues up to hill top and (ii) T2 in eastern flank across the slope (Fig. 2A). Both the dykes are well exposed and intrude the basement granite with strike direction N60°-70°W. The length of T1 is about 30m with varying width of 1-1.5 m where that of T2 is about 15 m with a width of 0.50 m. T1 shows greenish gray colour, slightly coarse-grained and contains occasional granite xenoliths. Weathering effect is also prominently seen on the surface. T2 is greenish gray in colour with similar texture but degree of weathering is absent. Nallabandlagudem The lamproite dyke of Nallabandlagudem is located south of temple hill (Hanuman temple), which is about 700 m E- SE of Nallabandagudem village on the way to Garikpadu. The body is found emplaced in between the dolerite dykes and the basement granite. The boulders of lamproite are seen covered with thick layer of calcrete at places. The lamproite dyke strikes N50°W and extends for about 600 m. The width of the lamproite dyke could not established because of its weathered nature and dolerite intervention. Ramannapeta Ramannapeta lamproitic body is located ~1 km S of Ramannapeta and occurs in close contact with the Archaean granites (Fig. 2B). The body forms boulders which can be traced for ~150 meters in a NW-SE direction. The body is grey coloured and contains a number of crustal xenoliths (chiefly granitoids) Balsupadu This lamproite dyke is located about 1 km S50°E of Balsupadu village. Almost E-W trending lamproite dyke is well exposed in the plain land on the western bank of irrigation canal. Boulders of lamproite are found detached from the original dyke and are found spread all over the dolerite dyke. The samples collected from the lamproite dyke are rich in calcite. The well-developed rhombohedral calcite crystal varies from 1 to 3 inch in size. The association of lamproite and dolerite leads to confusion because of very small extent of the dyke and


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intermixing of boulders. Its very narrow 0.20m to 0.50m in width and 8.0 m to 10 m in length. Jaggayyapeta Jaggayyapeta lamproite dyke is located 1.5 km, S20°W of Jaggayyapeta village & 800m NE of Balsupadu road in the open shrub east of Jaggayyapeta - Balsupadu road. The lamproite is found emplaced in granite/ granite gneisses. The general strike is almost N-S. The total length of dyke is about 30 m and width varies from 0.50 m to 1.0 m. Jayanthipuram This lamproite body is located in the granite hill, about 1.5 km S70°E of village Jayanthipuram and about 800 m east of Jayanthipuram-Vedadri road. The long & narrow lamproite dyke is intruded in granite rocks and is found tapped between two parallel dolerite dykes. The average strike of the lamproite dyke is N60°W. Lamproite is greenish gray in colour and fine grained and exhibit vesicular and amygdaloidal nature. The length of dyke is about 120 m and width varies from 0.30m to 0.50 m. Vedadri North This Lamproite dyke is well exposed, North of Vedadri village, on a hilltop, about 2.5 km NE of Vedadri temple hill & N20°E of village Vedadri. The dyke is closely associated with dolerite in the western portion of the East-West trending granite hill cutting across the latter. The lamproite dyke exposed at latitude 80°8’48” & longitude 16°49’55” with an average strike NW-SE and its dimensions are 20m long and 0.50 to 1 m in width. Small lamproite boulders are found in scattered manner mixed with the dolerite boulders and appearance of both of leads to confusion because of moderately weathered nature of dyke on the surface. Vedadri East This dyke is located about 150m North- NNE of the Vedadri North dyke in the hill flanks. It is about 3 km N40°E of Vedadri. It occurs at latitude 80°04’33” & longitude 16°49’48”. The lamproite dyke in found emplaced in granite country rock and the average strike of the dyke is N50°W with almost vertical dip. The lamproite rock is greenish gray in colour, fine grained. It shows prominent phenocrysts of olivine in the hand specimen also. The upper surface of the dyke is slightly weathered. Vedadri South The lamproite dyke is located all along the hilltop 2 km S 60° E of Vedadri temple and runs parallel to the dolerite dyke on the same granite hill. The lamproite dyke is bounded by latitude 80°08’39” and longitude 16°48’26”. It intrudes the granites in the immediate vicinity


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of the dolerite dyke. The general strike of the dyke is N40° E. It runs about 500 m long with average 0.50 to 1m width.

Figure 3 (A to D): Microphotographs of the Krishna lamproites displaying their diverse petrographic

features: (A) Olivine macrocrysts of varying sizes oriented along flow direction in Vedadri East lamproite (PPL). (B). Jayantipuram lamproite exhibiting pyroclastic (tuffaceous) texture dominated by lithic lapilli tuffs (CPL). (C) A singular large serpentinised olivine macrocryst surrounded by very fine grained groundmass in Vedadri North lamproite (CPL). (D) Typical inequigranular texture dominated by large macrocrysts of olivine in Nallabandlagudem lamproite. Note the phlogopite-rich groundmass (CPL). The width of all the microphotographs is ~ 3.5 mm.

Petrography Detailed petrographic and microprobe studies of the sampled KLF bodies were carried out. Serpentinised olivine, phlogopite, clinopyroxene, amphibole, sanidine, titanite, leucite (?), apatite, spinels and a number of secondary phases like talc, chlorite, carbonate and quartz constitute the mineralogy of the KLF bodies with their modal proportion varying from one occurrence to another. Fresh olivine and leucite are not found in any of the thin sections studied. Likewise, characteristic lamproitic phases such as priderite and wadeite are not


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observed. Trapezohedral pseudomorphs, believed to be of leucite (see Reddy et al., 2003; Paul et al., 2007) are noticed in many of the studied sections. Salient petrographic aspects of the studied lamproites are dealt here whereas results of the microprobe studies will be published elsewhere.

Based on the modal proportion of minerals, the KLF bodies can be broadly classified into three categories: (i) diopside-lamproite (e.g. Ramannapeta, Vedadri-South, Tirumalgiri, Jaggayyapeta) (ii) phlogopite-lamproite (e.g. Pochampalle, Balusupadu, Jayantipuram) and (iii) olivine-lamproite (e.g. Nallabandlagudem, Vedadri-East, Vedadri-North). Despite their similar mineralogical constituents, albeit in varying proportions, the KLF bodies exhibit remarkably diverse textural variations discussed below. Flow Texture Lamproite of Vedadri east display a characteristic flow texture wherein serpentised olivine macrocrysts of different sizes show preferential alignment along different flow laminae of alternate dark and light bands (Fig. 3A) made up of cryptocrystalline and glassy material. Polycrystalline aggregates of olivine are also seldom present. Titanite is found to be an important accessory. Lamproitic lava flows are reported from Leucite Hills and Smoky Butte (USA); West Kimberley (Australia) and Gaussberg (Antarctica) [Mitchell and Bergman, 1991]. Pyroclastic (Tuffaceous) Texture Lamproite at Jayantipuram exhibit pyroclastic (tuffaceous) texture dominated by lithic lapilli tuffs (Fig. 3B) and crystal-lithic tuffs with widespread serpentinised olivine macrocrysts, groundmass phlogopite, vesicles and glass. Lapilli of different sizes and shapes are found. Such pyroclastic textures are reported from the diamondiferous lamproites at Prairee creek (USA), Argyle (Western Australia) and Majhgawan (Central India) [e.g. Scott-Smith and Skinner, 1984; Scott-Smith, 1989]. Inequigranular (Porphyritic) Texture Some of the KLF lamproites exhibit porphyritic (inequigranular) texture wherein olivine macrocrysts surrounded by a finegrained matrix containing olivine, phlogopite, clinopyroxene, titanite and opaque oxides (Fig. 3C and D). At places, opaque minerals and perovskite are seen garlanding the macrocrysts (Fig. 3E). This texture is well developed in lamproites at Nallabandlagudem and Vedadri North. Micro-Porphyritc (Fine Grained) Texture Lamproites at Pochampalle and Balusupadu are, in general, fine grained, compared to the other bodies in KLF, containing only occasional macrocrysts. These lamproites are


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particularly rich in phlogopite and also contain clinopyroxene as an important phase (Fig. 3F and G).

Figure 3 (E to H): Microphotographs of the Krishna lamproites displaying their diverse petrographic

features: (E) Opaque oxides (mostly) and perovskite mantling the olivine macrocrysts as ‘garlands’ in Nallabandlagudem lamproite (PPL). (F) Acicular laths of clinopyroxene (diopside) in the groundmass of Pochampalle lamproite (PPL). (G) A portion of the Tirumalgiri lamproite dominated by the nearly equigranular habit with equant grains of phlogopite, leucite pseudomorphs and opaque minerals (PPL). (H) Olivine macrocrysts along with fine grained acicular laths of clinopyroxene interspersed with phlogopite, sphene and opaque oxides in Balusupadu lamproite (PPL). (L) The width of all the microphotographs is ~ 3.5 mm.

Equigranular Texture Some of the lamproites, as at Jaggayyapeta and Tirumalgiri, are dominated by the equigranular or nearly equigranular habit dominated by equant grains of phlogopite, leucite pseudomorphs and opaque minerals (Fig. 3H) at selected portions thereby imparting a ‘local’ equigranular appearance in the thin section.


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Figure 4: Chondrite-normalised (after Evinsen et al. 1978) REE distribution patterns of KLF

lamproites (this study). Discussion Existing geochemical models favour a common genesis for the lamproites of KLF even though contrasting opinions exist as to the presence (Chakrabarti et al., 2007) or absence (Paul et al., 2007) of garnet in their mantle source regions. A comprehensive geochemical study carried out by us on the Krishna lamproites (in preparation) indeed reveal their overall striking similarities. Their chondrite normalised rare earth element patterns are remarkably similar thereby implying similar processes and similar degrees of melting were involved in their genesis (Fig. 4). Despite their common geochemistry these bodies display contrasting petrographic characters involving diverse textural features ranging from flow texture to almost equigranular habit and highly variable modal mineralogy (above). Detailed literature search reveals that such varied petrographic features are seldom displayed by lamproites occurring in a single field and in this regard the KLF lamproites appear to constitute an extremely rare example in the world. They, thus, constitute an excellent example of heteromorphism wherein a bulk magma of singular composition can give rise to different rock types of varied modal mineralogy and diverse petrographic aspects (cf. Rock, 1991). In fact alkaline and ultramafic lamprophyres, rather than lamproites, are commonly well known to exhibit heteromorphism (e.g. Lanyon and Le Roex, 1995; Srivastava and Chalapathi Rao, 2007) and the implication of this observation has to be evaluated further. The variation in the modal mineralogy and petrography of Krishna lamproites can be attributed to the (i) differences in the volatile contents in their respective source regions and (ii) subsequent low pressure crystallisation of their magmas (e.g. Reddy et al., 2003; Chalapathi Rao et al., 2004; Mirnejaard and Bell, 2006).


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Geological Setting and Petrographic Diversity of the Lamproite Dykes at the Northern and North...

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