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

T. Ajit Kumar Reddy ¹

Affiliations
1 Geological Survey of India, Hyderabad 500001, IN

Abstract

Kimberlite and lamproite diatremes of late Proterozoic age (840 to 1020 m.y.) within the Archaean granites and gneisses (Peninsular Gneiss) around Vajrakarur in Andhra Pradesh show unusual mineralogy, mantle and crustal xenoliths and variable diamond distribution. Two of the pipe rocks correspond to kimberlite breccias with around 40% crustal xenoliths. Two pipe rocks are identical in mineralogy, xenolith content and chemistry to the classical kimberlites of Kimberley in South Africa. These four kimberlite pipes are diamond-bearing with xenocrysts and megacrysts of forsteritic olivine, pyrope-garnet, chrome diopside and magnesian ilmenite. Mantle xenoliths identified include peridotite and eclogite. The other two pipe rocks rich in phlogopite vary in petrography, mineral chemistry and whole rock geochemistry from classical kimberlites and show closer affinity to lamproites of West Australia, Spain and Arkansas. These rocks contain two generations of olivine, sanidine, phlogopite in a fine-grained matrix composed of phlogopite, olivine, perovskite, sanidine, clinopyroxene, richterite, serpentine, carbonates. melilite and possible leucite and glass. Rarity of pyrope garnet, chrome diopside from the' indicator' suite, xenoliths from 'mantle' suite and the absence of diamond are characteristic. The six kimberlites generally show high values of MgO, AI₂O₃, CaO, K₂O and Na₂O. The two olivine lamproites described are characterised by significantly high values of TiO2, Fe₂O₃, K₂O, Na₂O and P₂O₅•

The petrology, mineral chemistry and rock geochemistry of mica-rich kimberlite are found to be transitional between kimberlite and lamproite rocks and suggest a separate petrogenesis. possibly from a different area within the mantle. A shallower origin is indicated by the absence of kimberlite indicator minerals, mantle xenoliths and diamond.

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T. Ajit Kumar Reddy ¹, M. Sridhar ², S. Ravi ², V. Chakravarthi ², S. Neelakantam ²

Affiliations
1 Geological Survey of India, 3-5-170/G, Narayanguda, Hyderabad - 500 029, IN
2 Geological Survey of India, Hyderabad - 500 068, IN

Abstract

Search for primary source rocks of diamond in close proximity of the well known alluvial gravel workings for diamonds along the palaeochannels of the River Krishna, has resulted in the discovery of a lamproite field comprising several lamproite clan rocks in parts of Krishna and Nalgonda districts of Andhra Pradesh (South India). These rare rock types are defined in this work as 'lamproites', on the basis of their occurrence in a unique tectonic setting and containing one or more of the typomorphic minerals and having the requisite rock geochemical characters of members of the suite as a whole. These bodies with an exotic and diagnostic mineralogy, composition and textures occur in a complex tectonic setting with unique intrusive history. The discovery of this lamproite field is considered very significant from the point of diamond exploration as it is located very close to the famous Golconda Mines along the banks of the River Krishna which yielded many of the celebrated diamonds in the past. This field is christened "Krishna Lamproite Field", in view of the proximity of the lamproite occurrences to the Krishna River having the alluvial placer diamonds. These rocks are emplaced along the eastern margin of the Precambrian Dharwar Craton, at the contact between the northeastern periphery of the Proterozoic Cuddapah Basin and the Archaean crystalline basement rocks, in the form of dykes (usually in clusters) and plugged conduits. The emplacement is controlled by major faultslfractures, trending in NNW-SSE and WNW-ESE directions and occur closely associated with mafic (dolerite) dykes and followed the same channels for their emplacement. The lamproites contain glass, numerous vesicles, and xenoliths of the basement granite-gneiss, dolerite and probably platformal sediments of the Cuddapah Basin, besides exhibiting fragmental and pyroclastic textures supgestive of near-surface emplacement. Radiometric age determinations suggests emplacement of these lamproites took place around 1224±14 Ma, probably during the reactivation of major crustal lineaments coinciding with the uplift of the mid-Proterozoic Nallamalai sediments of the Cuddapah Basin.

Diopside, phlogopite, tetra-ferri phlogopite, richterite, sanidine, apatite and pseudomorphs of leucite, enstatite and olivine are the major primary constituent minerals present. Diopside-leucite-lamproite, phlogopite-leucitelamproite, olivine-lamproite and richterite-phlogopite-lamproites are the variants. The bodies are ultrapotassic (molar K₂O/Na₂O >10), and contain high TiO₂, P₂O₅, K₂O, MgO, Ba, Sr, Zr, Cr, Ni, La and Ce, pointing to crystallisation from typical lamproite magma. The presence of mantle peridotite xenoliths and xenocrystic minerals such as chromediopside, spinel and garnet indicate deep seated mantle derivation of the Krishna lamproites.

Keywords

Lamproites, Petrology And Geochemistry, Emplacement, Major Faults/Fractures, Krishna Lamproite Field, Andhra Pradesh.

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A. R. Nambiar ¹, T. Ajit Kumar Reddy ², M. Sridhar ², S. Ravi ², V. Chakravarthi ², S. Neelakantam ²

Affiliations
1 Geological Survey of India, Op.: Karnataka and Goa, Vasudha Bhavan, Bangalore - 560 078, IN
2 3-5-17Q/G, Narayanguda, Hyderabad - 500 029, IN

Abstract

No Abstract.

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