2013

Sr–Nd isotopic compositions and emplacement age and of the ultrabasic alkaline series of the Kola Province, Russia: A case study of the Afrikanda massif

 Andrei A. Arzamastsev¹, Fu-Yuan Wu²

 ¹ Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia

² Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

The Kola Peninsula is characterized by diverse alkaline magmatism, including alkaline ultramafic, carbonatite, “kimberlite” and agpaitic rocks, although many aspects of the ages of emplacement and petrogenesis remain unresolved. Rocks from the Afrikanda complex were selected for U–Pb age determination and Sr–Nd isotopic analyses using in situ ion probe and laser ablation techniques. Conclusions:

1) Perovskites from pyroxenite, calcite-bearing perovskite ore and ijolite–melteigite yield identical U–Pb ages of 377±6, 379±5– 385±5 and 376±5 Ma, respectively with an average value of 381±2 Ma, indicating that the different phases of the complex were contemporaneously emplaced at ~380 Ma;

2) Sr–Nd isotopic analyses of perovskite, apatite, titanite and calcite indicate that the complex was derived from depleted mantle. However, these data suggest that silicate and carbonatitic magmas are not related by simple crystal fractionation within a closed magmatic system. The silicate magma has an initial 87Sr/86Sr isotopic ratio of ~0.7034 to 0.7038 and ε_Nd(t)380 value of ~+2.0 to +4.9, whereas the carbonatitic magma was more primitive with the above values of ~0.7033 to 0.7035 and ε_Nd(t)380~+5.1 to +5.8, suggesting that either both magmas were derived from two distinct mantle sources or by contamination of a melts derived from a single source.

3) Perovskite U–Pb ages indicate that the Afrikanda complex was formed contemporaneously with other alkaline ultramafic, carbonatite, “kimberlitic” and agpaitic rocks in the area, their similar Sr–Nd isotopic compositions suggest that the Kola alkaline magmatism was probably controlled by mantle plume activity.

Financial support: Russian foundation for Basic Research (Grant 12-05-00244).