科研之友
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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 accepted, 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 Sm-147-Nd-143, Pb-207-Pb-206 and Ar-40-Ar-39 dating techniques. We also present mineral chemistry, major and trace element geochemistry and Sr-Nd isotopic ratio data for these carbonatites. Our age data reveal that the complex was emplaced at similar to 1,473 Ma and parts of it were affected by a thermal event at similar to 904 Ma. The older Pb-207-Pb-206 ages reported here (similar to 2.4 Ga) and by one earlier study (similar to 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 signatures of these rocks, such as its dolomite-magnesite-ankerite-Cr-rich magnetite-magnesio-arfvedsonite-pyrochlore assemblage, mantle like delta(13) C and delta O-18 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 produced from peridotitic sources. In addition, lower rare earth element (and higher Sr) contents than a typical calciocarbonatite and mantle like Nb/Ta ratios indicate that the primary magma for the complex was a magnesio-carbonatite 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 inference and suggests that the source was a phlogopite bearing mantle, located within the garnet stability zone.
- 出版日期2013-12
