Informatics Publishing Limited

N. V. Vladykin ¹, S. G. Viladkar ², T. Miyazaki ³, V. Ram Mohan ⁴

Affiliations
1 Institute of Geochemistry SB RAS, Irkutsk-33, Box 4019, RU
2 Geology Department, St Xavier's College, Mumbai-400001, IN
3 Graduate School of Science and Technology, Niigata University, Niigata 950-2181, JP
4 Department of Geology, University of Madras, Chennai-600025, IN

Abstract

Benstonite-Ba-Sr carbonatites are found only in two localities in the world the Murun massif in Siberia and Jogipatti in Tamil Nadu, South India. The specific feature of these carbonatites is high BaO (20-35%) and SrO (up to 10% as in the Murun carbonatites) concentrations. These carbonatites jointly with calciocarbonatite varieties occur in caibonatite-alkaline complexes of Siberia and South India. However, carbonatites of South India and Siberia differ in terms of the chemical composition, especially the trace and rare earth element contents. In South India (Tamil Nadu area), carbonatites are found in three alkaline massits Samalpatti, Sevathur, and Elagiii. Benstonite carbonatites are found only in the Samalpatti massif as a small dyke. In the Murun massif carbonatites are separated from the silicate rocks and occur as a residual silicate-carbonate complex. It comprises microclinites, pyroxene-microclines, quartz-microcline- pyroxene-calcites, which contain calcite carbonatites and charoite rocks as schleiren and dykes. Benstonite carbonatites form a horizontal body of 40 m thick in the southwestern part of the complex. We examined and compared the geological and geochemical features of these two areas where Bensonite occurs in association with other carbonatites and discuss their origin. The Indian carbonatites crystallized as the plutonic phase while Siberian ones formed in the near-surface area and effused on the Earth's surface.

Keywords

Benstonite, Carbonatites, Geochemistry, Sevathur, Samalpatti, Tamil Nadu.

Full Text

     

M. Jayananda ¹, T. Miyazaki ², R. V. Gireesh ¹, N. Mahesha ¹, T. Kano ³

Affiliations
1 Department of Geology, Bangalore University, Bangalore - 560 056, IN
2 Institute for Earth Evolution, JAMSTEC, Yokosuka, JP
3 Department of Earth Sciences, Yamaguchi University, Yamaguchi, JP

Abstract

We present a first overview of the synplutonic mafic dykes (mafic injections) from the 2.56 – 2.52 Ga calcalkaline to potassic plutons in the Eastern Dharwar Craton (EDC). The host plutons comprise voluminous intrusive facies (dark grey clinopyroxene-amphibole rich monzodiorite and quartz monzonite, pinkish grey porphyritic monzogranite and grey granodiorite) located in the central part of individual pluton, whilst subordinate anatectic facies (light grey and pink granite) confined to the periphery. The enclaves found in the plutons include highly angular screens of xenoliths of the basement, rounded to pillowed mafic magmatic enclaves (MME) and most spectacular synplutonic mafic dykes. The similar textures of MME and adjoining synplutonic mafic dykes together with their spatial association and occasional transition of MME to dismembered synplutonic mafic dykes imply a genetic link between them. The synplutonic dykes occur in varying dimension ranging from a few centimeter width upto 200 meters width and are generally dismembered or disrupted and rarely continuous. Necking of dyke along its length and back veining of more leucocratic variant of the host is common feature. They show lobate as well as sharp contacts with chilled margins suggesting their injection during different stages of crystallization of host plutons in magma chamber. Local interaction, mixing and mingling processes are documented in all the studied crustal corridors in the EDC. The observed mixing, mingling, partial hybridization, MME and emplacement of synplutonic mafic dykes can be explained by four stage processes: (1) Mafic magma injected during very early stage of crystallization of host felsic magma, mixing of mafic and felsic host magma results in hybridization with occasional MME; (2) Mafic magma introduced slightly later, the viscosities of two magmas may be different and permit only mingling where by each component retain their identity; (3) When mafic magma injected into crystallizing granitic host magma with significant crystal content, the mafic magma is channeled into early fractures and form dismembered synplutonic mafic dykes and (4) Mafic injections enter into largely crystallized (>80% crystals) granitic host results in continuous dykes with sharp contacts. The origin of mafic magmas may be related to development of fractures to mantle depth during crystallization of host magmas which results in the decompression melting of mantle source. The resultant hot mafic melts with low viscosity rise rapidly into the crystallizing host magma chamber where they interact depending upon the crystallinity and viscosity of the host. These hot mafic injections locally cause reversal of crystallization of the felsic host and induce melting and resultant melts in turn penetrate the crystallizing mafic body as back veining. Field chronology indicates injection of mafic magmas is synchronous with emplacement of anatectic melts and slightly predates the 2.5 Ga metamorphic event which affected the whole Archaean crust. The injection of mafic magmas into the crystallizing host plutons forms the terminal Archaean magmatic event and spatially associated with reworking and cratonization of Archaean crust in the EDC.

Keywords

Synplutonic Mafic Dykes, Granite Plutons, Mafic Magmatic Enclaves, Magma Chamber, Eastern Dharwar Craton.

Full Text