Depleted and enriched mantle sources for Paleo- and Neoproterozoic carbonatites of southern India: Sr, Nd, C–O isotopic and geochemical constraints

Paleoproterozoic (Hogenakal) and Neoproterozoic (Samalpatti, Sevattur, Mulakkadu–Pakkanadu) carbonatites of Tamil Nadu, southern India, have been investigated for whole-rock geochemistry and Nd, Sr and C–O isotopes. These temporally distinct carbonatite complexes are located close to a tectonically active zone that marks the transition between cratonic non-charnockitic (low- to medium-grade) terrain to the north and the charnockitic mobile belt (granulite facies) to the south. The carbonatites are variably enriched in LREE; the Hogenakal carbonatites being extremely enriched, with the highest ∑REE among the data while the younger carbonatites show variable enrichment levels and broadly comparable REE patterns. The Hogenakal carbonatites have coherent and typically mantle C- and O-isotopic ratios (δ13CV-PDB∼−6‰ and δ18OV-SMOW∼8‰). The Neoproterozoic carbonatites are relatively enriched in 13C (δ13CV-PDB=−5.3‰ to −3.3‰) and also show a range of O-isotopic composition (δ18OV-SMOW=7.3‰ to 15.4‰; two samples ∼25‰). Higher δ18O values indicate variable degree of post-magmatic low-temperature alteration. The older carbonatites have marginally positive εNd (+0.54 and +1) and significantly low initial 87Sr/86Sr ratios (0.70161 and 0.70174) while younger carbonatites have rather low εNd (−16.5 to −6.23) and higher 87Sr/86Sr ratios (0.70486–0.70658). The Sr–Nd isotopic data are compatible with a depleted mantle source for the Hogenakal carbonatites and an EM-I-type enriched mantle component for the younger group. This is the first report of the existence of depleted mantle beneath the southern Indian continental crust. The stable isotopic ratios are interpreted as suggesting a depletion event (crustal extraction) in the south Indian subcontinental mantle ∼2.6 Ga ago. The depleted mantle was subsequently enriched by metasomatic fluids under the influence of the subducting Dharwar plate (sediments and modified oceanic crust).