Geochronology, geochemistry, Sr–Nd isotope composition and petrogenesis of Early Devonian Uryup sub-alkalic and alkalic igneous rocks in Minusinsk trough (Altai–Sayan Large Igneous Province, Siberia)

Early Devonian magmatism in the Minusinsk trough of the Altai–Sayan Large Igneous province provides a critical record of regional geologic history. In this study, we determined the U–Pb baddeleyite isotope age, major and trace element geochemistry, and Sr–Nd isotope composition of igneous rocks in the Uryup sector of Minusinsk trough, with the aim of constraining their magmatic origin. The volcanic succession is divided, from base to top, into three Suites: (1) the Bazyr Suite, composed of basalts, trachybasalts, basaltic trachyandesites, transitional tephrites–trachybasalts, and phonotephrites; (2) the Beresh Suite, consisting of trachybasalts, basaltic trachyandesites, phonotephrites, tephriphonolites, and feldspathoid ijolites; and (3) the Ashpan Suite, comprising basalts, trachybasalts, and basaltic trachyandesites. The Early Devonian age of these Suites, established in previous geological and palaeontological studies, is confirmed here by new U–Pb geochronological data on baddeleyite obtained by ID–TIMS. Rare-element concentrations in all mafic rocks, regardless of alkalinity, vary widely, showing features typical of both oceanic island basalts and island arc basalts. Compared with sub-alkalic counterparts, alkalic mafic rocks contain abundant nepheline phenocrysts (up to 30 wt.%) and are enriched in Na2O, Al2O3, Rb, Ba, Th, U, K, Nb, REE, and radiogenic Sr, while exhibiting similar and only slightly variable radiogenic Nd values. We propose a model in which all Uryup sector rocks were derived from a single sub-alkalic basaltic parental melt generated in the asthenospheric mantle by the influence of a mantle plume on fragments of ancient supra-subduction lithospheric mantle. Geochemical and isotopic variations are inconsistent with a simple comagmatic relationship between sub-alkalic and alkalic rocks or with fractional crystallization as the sole controlling process. Bazyr sub-alkalic basaltic magmas experienced simultaneous fractional crystallization and contamination by silicate crust. Beresh phonotephrites, tephriphonolites, and feldspathoid ijolites formed from differentiated sub-alkalic melts through carbonate contamination, possibly within the continental crust.