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The main objective of this research project was to better constrain the petrogenesis of the Palaeozoic carbonatite intrusions of the Kola Peninsula (Russia) and to decipher their relations with associated rocks (ultramafic rocks and alkaline silicate rocks).
An integrated petrological-geochemical approach was developed using trace elements, radiogenic (Sr, Nd) and noble gases (He, Ne, Ar) isotopes. A typology of the Kola carbonatites has been established on the basis of their mineralogy (carbonate minerals, mafic and accessory minerals). Ultramafic rocks are early Mg-rich olivine and clinopyroxene cumulates fractionated from a strongly silica-undersaturated magma.
Cathodoluminescence (CL) investigations (spectra recorded from the 200 to 900 nm) have demonstrated that the main CL emission band in apatite is centred on 360 nm and is due to the Ce3+ ion. Moreover, new cathodoluminescent minerals (lorenzenite, burbankite, mosandrite) were identified in both carbonatites and associated rocks.
Geochemical and isotopic investigations have been focused on the Kovdor and Khibina intrusions as they show the most complete and representative suites of rocks. Few samples from other intrusions have been analysed for comparison. The multi-elements normalised abundance diagrams (spidergram) show the typical patterns of alkaline rocks. The HFS elements (Zr, Hf, Nb, Ta, U, Th, ...) distribution of the whole rocks is strongly controlled by accessory minerals (pyrochlore, perovskite, baddeleyite, zircon and titanite). The Kovdor calcite- and/or dolomite-bearing carbonatites have lower REE abundance's than the average carbonatite worldwide; this is interpreted as resulting from the fractionation of apatite and other REE-rich accessory minerals in the phoscorites. Sr and Nd isotopic data for most carbonatites (except Khibina) and ultramafic rocks (except some contaminated dunites) are very similar (87Sr/86Sr: 0.70310 to 0.70370; Nd:+1.6 to +5.2) suggesting a cogenetic origin: the magma originated from a slightly depleted mantle source-region comparable to the OIB-type source. Ijolites are more heterogeneous (87Sr/86Sr up to 0.7074), the y are variously contaminated. The ranges of He content and isotopic composition are quite large. 3He/4He ratios vary from radiogenic (=crustal) values (~108) down to values (6.4 104) lower than the average MORB value. These low values, together with Ne isotopic data and higher than atmosphere 40Ar/36Ar ratio point to a plume-related (less degassed) component in the parental magma of the Kola carbonatitic and alkaline suite of rocks.
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