Isotope-geochemical (Sm–Nd, Rb–Sr, REE, HFSE) composition of the University foidolite-gabbro pluton, Kuznetsk Alatau ridge, Siberia

Abstract

The University foidolite-gabbro pluton is located among the Cambrian carbonate-volcanogenic deposits. Its composition is dominated by subalkaline and alkaline mafic rocks, broken by dikes of ijolite-urtites, nepheline and alkaline syenites. The chemical composition of igneous rocks is characterized by low silica contents (SiO2 = 41–49 wt. %), wide variations of alkalinity (Na2O + K2O = 3–19 wt. %; Na2O/K2O = 1.2–7.2 wt. %), low titanium content (TiO2 = 0.07–1.59 wt. %) and high alumina content (Al2O3 = 15–28 wt. %), which corresponds to the K-Na derivatives of the basic-alkaline formation. By their content of rare earth elements, alkaline rocks (104–246 ppm; La/Yb(n) = 5.79–12.73) are more differentiated derivatives than gabbro (94–111 ppm; La/Yb(n) = 6.87–6.95). All varieties are characterized by low concentrations of most highly charged elements (Nb, Ta, Zr, Hf, +Y), which in terms of accumulation are located between the basalts of oceanic islands and basalts of island arcs. The presence of negative Nb–Ta anomaly and the relative enrichment of Rb, Ba, Sr, and U indicate the probable interaction of plume material with previously formed accretionary complexes of subduction zones. The primary isotope 87Sr/86Sr ratio (~ 0.705–0.706) and a wide range of εNd (T) from +3.2 to +8.7 in the rocks also indicate a mantle-crustal nature and a complex geodynamic setting of the Paleozoic alkaline magmatism of the Kuznetsk Alatau. The obtained results of Sm–Nd dating suggest the formation of subalkaline gabbroids in the Early Paleozoic (494–491 ± 36 Ma), with the intrusion of dikes of alkaline rocks of the Middle Paleozoic age (394 ± 16 and 389 ± 37 Ma).