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

Department of Geology, University of Rajasthan, Jaipur—302004, India; NEG LABISE Department of Geology, Federal University of Pernambuco, C.P. 7852, Recife-PE 50.732-970, Brazil; Department of Geology, Presidency College, Chennai—600 005, India; Institute of Geosciences, University of Brasilia, Brasilia—70910-900, Brazil
Chemical Geology (Impact Factor: 3.48). 09/2002; DOI: 10.1016/S0009-2541(02)00136-5

ABSTRACT 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).

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    ABSTRACT: The magmatic heritage of carbonatites can be identified on the basis of a combination of geological criteria such as, their mode of occurrence, the nature of associated igneous rocks, the presence of minerals of igneous origin, fenitization, characteristic trace element contents and isotopic composition. Late Proterozoic Samalpatti carbonatites were studied in view of these criteria, and were found to contain metamorphic minerals that normally form under thermal metamorphic conditions and which have unusual chemical compositions. A combination of criteria points clearly to a magmatic origin for these carbonatites. Field relations indicate that the dominant modes of intrusion of carbonatite into the encompassing pyroxenites and syenites include small dykes, veins, or lenses. The igneous nature of these carbonatites has been described elsewhere and chemically they are classified as calico-carbonatites. Currently, very little is known about the metamorphic textures and mineralogy observed in the Samalpatti carbonatites. In this study, several metamorphic minerals are reported including diopside, grossularite, vesuvianite, K-feldspar and wollastonite, and a hornfelsic texture is described. These mineral phases and texture characterize thermal metamorphism under low pressure and high temperature (LP-HT) metamorphic conditions (650°–750°C) or metasomatism aided by hot-fluid advection. The metamorphic nature of minerals reported is also confirmed by electron microprobe study. The Samalpatti carbonatite samples show much lower values of characteristic trace elements (P, Sr, Ba, Zr, Nb, Th, Y and REEs) than average concentrations for magmatic carbonatite. Stable isotopic (δ13C and δ18O) compositions of Samalpatti carbonatites do not fall in the primary igneous carbonatite (PIC) domain. The petrological and chemical signatures of these carbonatites suggest metasomatism in conjunction with fluid advection. Such a metasomatic process may drastically change the chemistry of the rocks in addition to enrichment of heavier stable isotopes. During this metasomatic process, characteristic elements would be dissolved in the high δ18O fluid, and together with Rayleigh fractionation would contribute to enhanced concentrations of 13C and 18O in Samalpatti carbonatites.
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