Melt sources for alkaline carbonate-bearing rocks of the Terskiy Coast (Kola Alkaline Carbonatitic Province)

Abstract

We investigate relationships between melt sources for lamprophyres, kimberlites and other alkaline carbonate-rich melts by studying rocks of the Terskiy Coast (Northwest Russia) situated between the coeval Devonian Kola Alkaline Carbonatite (KACP) and Archangelsk Kimberlite Provinces (AKP). This study reports Sr-Nd-Pb isotope systematics of lamprophyres, carbonatites, turjaite and foidite dykes, Turij Mys massif turjaites, perovskite and garnet from the dykes, and the Ermakovskaya-7 kimberlite, the only kimberlite of the Terskiy Coast. Principal Components Analysis was used to classify the data by five isotopic variables into rock groups T1, T2 and T3. A factor analysis technique was then applied to new Sr-Nd-Pb data and the KACP literature data to portray the analyses as three clusters on a planar 2-dimentional array. The first of these clusters, T1 is characterized by high combined parameter f(Nd, Pb) > 0.5137 and moderately radiogenic (87Sr/86Sr)375. T1 rocks are mostly ultramafic and plot along the Kola Carbonatite Line in Sr-Nd space. Group T2 is recognized by the presence of a linear correlation between Sr isotope compositions and f(Nd, Pb); the extended span of εNd is combined with a moderately radiogenic (87Sr/86Sr)375, and this group includes the least radiogenic Pb compositions. T2 rocks are petrographically diverse, ranging from kimberlites to phonolites. T3 rocks form a cloud of points with elevated (87Sr/86Sr)375 > 0.7041 and widely varying f(Nd, Pb); they are mostly alkaline lamprophyres. All three groups of studied rocks record a contribution from the depleted mantle, while the mantle beneath the AKP kimberlites that was metasomatized just prior to eruption contributed to T1 rocks, and the Paleoproterozoic metasomatized lithospheric mantle contributed to T2 rocks. T3 rocks record 10% contamination by the lower crust, as recorded by the Sr-Nd isotopic shift between the whole rock and garnet compositions. A geographic position of Ermakovskaya-7, Zolotitsa and Mela kimberlites within the adjoining areas of the KACP and AKP stretched along the strike of the Lapland-Kola belt results in their distinct low ɛNd(375) signatures highlighting a local control on melt generation by the ancient metasomatized mantle. The repeated generation, over billions of years, of melts in spatially restricted areas explains the observed contribution of ancient fеnitized crust and metasomatized mantle to magmas from large alkaline or kimberlite provinces.