Rb-Sr Geochronology, Nd-Sr Isotopes and Whole Rock Geochemistry of Yelagiri and Sevattur Syenites, Tamil Nadu, South India

Alkaline magmatism during the late Proterozoic is an important event in the northern part of the South Indian granulite terrain. A number of alkaline plutons comprising saturated syenite and ultramafic rocks often associated with carbonatite are found localized along NEHYPHEN;SW trending lineaments, which are considered as deep crustal fractures. Along one such lineament, the alkaline complexes of Yelagiri, Sevattur and Samalpatti have intruded into the country rocks comprising epidote hornblende gneiss. The isotope characteristics and geochemistry of Yelagiri and Sevattur plutons are examined in this paper. Whole rock Rbhyphen;Sr isochron ages of the Yelagiri and Sevattur syenites are 757±32 Ma and 756±11 Ma respectively. The close spatial relationship, similarities in age, mineralogical and geochemical characteristics of these plutons strongly suggest their close genetic relationship. The initial Sr and Nd isotope ratios of the Sevattur carbonatites suggest their derivation from an alkali metal and LREE enriched mantle source. However, the silicate rocks of the Yelagiri and Sevattur plutons have distinctly different isotopic characteristics from this enriched mantle source. Combined geochemical and isotopic characteristics of these silicate rocks indicate that silicate rocks of both plutons are derived independently from isotopically different sources from those of carbonatites. Moreover, comparison with the isotopic characteristics of Archean crustal rocks in South India indicates that the source regions of both silicate rocks are lowerhyphen;crustal portions, which are deeper than any other crustal portion exposed in South India, or isotopically metasomatized crustal portions by volatile influx from carbonatite.