The Paleozoic-Aged University Foidolite-Gabbro Pluton of the Northeastern Part of the Kuznetsk Alatau Ridge, Siberia: Geochemical Characterization, Geochronology, Petrography and Geophysical Indication of Potential High-Grade Nepheline Ore

Abstract

Geological, geochemical and ground magnetic techniques are used to characterize the University alkaline-gabbroid pluton and crosscutting N-S trending alkaline dikes, located northeast of the Kuznetsk Alatau ridge, Siberia. Trace element concentrations and isotopic compositions of the igneous units were determined by XRF, ICP-MS and isotope analysis. The Sm-Nd age of subalkaline (melanogabbro, leucogabbro 494–491 Ma) intrusive phases and crosscutting alkaline dikes (plagioclase ijolite, analcime syenite 392–389 Ma) suggests two stages of activity, likely representing separate events. The subalkaline and alkaline rocks are characterized by low silicic acidity (SiO2 = 41–49 wt %), wide variations in alkalinity (Na2O + K2O = 3–19 wt %; Na2O/K2O = 1.2–7.2 wt %), high alumina content (Al2O3 = 15–28 wt %) and low titanium content (TiO2 = 0.07–1.59 wt %). The new trace element data for subalkaline rocks (∑REE 69–280 ppm; La/Yb 3.7–10.2) of the University pluton and also the crosscutting younger (390 Ma) alkaline dikes (∑REE 10–1567 ppm; La/Yb 0.7–17.8 ppm) both reflect an intermediate position between oceanic island basalts (OIBs) and island arc basalts (IABs). The presence of a negative Nb–Ta anomaly and the relative enrichment in Rb, Ba, Sr, and U indicate a probable interaction of mantle plume material with the lithospheric mantle beneath previously formed accretion complexes of subduction zones. The isotopic signatures of strontium (εSr(T) +3.13–+28.31) and neodymium (εNd(T) +3.2–+8.7) demonstrate the evolution of parental magmas from a plume source from moderately depleted PREMA mantle, whose derivatives underwent selective crustal contamination.