Age and geochemistry of the Newania dolomite carbonatites, India: implications for the source of primary carbonatite magma

摘要

The Newania carbonatite complex of India is one of the few dolomite-dominated carbonatites of the world. Intruding into Archean basement gneisses, the rocks of the complex have undergone limited diversification and are not associated with any alkaline silicate rock. Although the magmatic nature of the complex was generally accep ted, its age of emplacement had remained equivocal because of the disturbed nature of radioisotope systems. Many questions about the nature of its mantle source and mode of origin had remained unanswered because of lack of geochemical and isotopic data. Here, we present results of our effort to date the complex using ~(147)Sm-~(143)Nb, ~(207)Pb-~(206)Pb and ~(40)Ar- ~(39)Ar dating techniques We also present mineral chemistry, major and trace element geo chemistry and Sr-Nd isotopic ratio data for these carbon atites. Our age data reveal that the complex was emplaced at ~ 1,473 Ma and parts of it were affected by a thermal event at -904 Ma. The older ~(207)Pb-~(206)Pb ages reported here (~2.4 Ga) and by one earlier study (~2.3 Ga; Schleicher et al. Chem Geol 140:261-273, 1997) are deemed to be a result of heterogeneous incorporation of crustal Pb during the post-emplacement thermal event. The thermal event had little effect on many magmatic signa tures of these rocks, such as its dolomite-magnesite-ankerite-Cr-rich magnetite-magnesio-arfvedsonite-py-rochlore assemblage, mantle like δ~(13)C and δ~(18)O and typical carbonatitic trace element patterns. Newania carbonatites show fractional crystallization trend from high-Mg to high-Fe through high-Ca compositions. The least fractionated dolomite carbonatites of the complex possess very high Mg# (>80) and have similar major element oxide contents as that of primary carbonatite melts experimentally pro duced from peridotitic sources. In addition, lower rare earth element (and higher Sr) contents than a typical calcio-carbonatite and mantle like Nb/Ta ratios indicate that the primary magma for the complex was a magnesio-carbon-atite melt and that it was derived from a carbonate bearing mantle. The Sr-Nd isotopic data suggest that the primary magma originated from a metasomatized lithospheric mantle. Trace element modelling confirms such an infer ence and suggests that the source was a phlogopite bearing mantle, located within the garnet stability zone.