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 par? 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 NE-SW trending lineaments, which are considered as deep crustal fractures. Along one such lineament, the al kaline complexes of Yelagiri, Sevattur and Samalpatti have intruded into th e country racks comprising epidote hornblende gneiss. The isotope character istics and geochemistry of Yelagiri and Sevattur plutons are examined in th is paper. Whole rock Rb-Sr isochron ages of the Yelagiri and Sevattur syeni tes are 757+/- 32 Ma and 756+/- 11 Ma respectively. The close spatial relat ionship, similarities in age, mineralogical and geochemical characteristics of these plutons strongly suggest their close genetic relationship. The in itial Sr and Nd isotope ratios of the Sevattur carbonatites suggest their d erivation from an alkali metal and LREE enriched mantle source. However, th e silicate rocks of the Yelagiri and Sevattur plutons have distinctly diffe rent isotopic characteristics from this enriched mantle source. Combined ge ochemical and isotopic characteristics of these silicate rocks indicate tha t silicate rocks of both plutons are derived independently from isotopicall y different sources from those of carbonatites. Moreover, comparison with t he isotopic characteristics of Archean crustal rocks in South India indicat es that the source regions of both silicate rocks are lower-crustal portion s, which are deeper than any other crustal portion exposed in South India, or isotopically metasomatized crustal portions by volatile influx from carb onatite.