Sources of Melts of Early Neoproterozoic–middle Paleozoic Granitoids in West Sayan

This study summarizes petrochemical, geochemical, and isotope (Sm–Nd and Rb–Sr) data on granitoid and gabbroid complexes in West Sayan, which formed in different geodynamic settings (island arc, accretion–collision, transform–shear, and active continental margin) between the Late Vendian and Late Devonian. The results provide constraints on the physicochemical conditions of the generation of parental melts, the nature of magma sources, and their temporal evolution throughout successive geodynamic stages. The study has shown a regular decrease in ɛNd values and an increase in (87Sr/86Sr)0 values in Vendian–early Paleozoic intrusive complexes from early to later geodynamic stages: island arc (545–524 Ma), ɛNd = +7.5 to +4.9, (87Sr/86Sr)0 = 0.7042–0.7056; accretion–collision (505–460 Ma), ɛNd = +6.9 to +0.6, (87Sr/86Sr)0 = 0.7036–0.7041; transform–shear (437–433 Ma), ɛNd = +8.4 to +7.2, (87Sr/86Sr)0 = 0.7031–0.7057; and active continental margin (425–370 Ma), ɛNd = +5.7 to +1.0, (87Sr/86Sr)0 = 0.7035–0.7103. These changes reflect the declining contribution of mantle sources (of island arc and oceanic types) and the increasing contribution of ancient crustal sources (metamorphic complexes, turbidites, and flysch-type sediments) to the generation of parental melts.