Sr–Nd Isotopic Composition of the Guli Massif. Relationship between the Size of Alkaline-Carbonatite Intrusions and Their Isotopic Heterogeneity

The paper presents data on a representative collection of rocks from the Guli Massif, the largest alkaline-ultramafic carbonatite intrusion of the Maimecha–Kotuy Province and one of the largest worldwide. The Rb–Sr and Sm–Nd isotopic systems of rocks of the various intrusive phases of the pluton were analyzed by TIMS. The samples represented a wide spectrum of rocks from olivinites and melteigite–ijolites to melanephelinites and carbonatites. The Sr and Nd initial isotopic compositions of rocks of the Guli Massif suggest that their parental melts were derived from a mantle source moderately enriched relative to the convecting mantle. These enriched isotopic signatures suggest that the source of the parental melts contained an enriched component. At the same time, the Sr and Nd isotopic ratios of the rocks display narrow variation ranges : ( 87 Sr/ 86 Sr) 0 = 0.7031–0.7038 and ε Nd (250) = +3.9 to +5.4 and no systematic changes from earlier to later intrusive phases. This likely reflects the isotopic homogeneity of the magma source and a minimal contamination by the host rocks. Comparison of our data on the Guli Massif with those on other alkaline–carbonatite intrusions worldwide seems to point to relationships between the size of a magmatic body and the degree of isotopic heterogeneity of its rocks. The Guli Massif and the Khibiny and Lovozero complexes are the world’s largest alkaline-carbonatite intrusions and show the smallest dispersion of their Sr and Nd isotopic ratios, whereas smaller intrusions display a significantly wider scatter of isotopic characteristics of their rocks. It is proposed that the large magma volumes of the larger massifs maintained the stability of their composition and limited the effects of contamination on their isotopic systems.