Sources and geodynamic environments of formation of Vendian-Early Paleozoic magmatic complexes in the Daribi range, Western Mongolia

Abstract

Rock complexes composing the Daribi Range were produced in Late Vendian, Early Cambrian, and Early Paleozoic suprasubduction systems. All of the studied mafic and ultramafic magmatic mantle rocks (the post-Vendian ophiolite complex, Early Cambrian pillow basalts, and Early Paleozoic picrobasalts of the sill-dike complex) have geochemical characteristics typical of early evolutionary episodes of island arcs: low LILE concentrations, horizontal REE patterns or patterns close to those of N-MORB, and HFSE minima. The magmas were derived from depleted mantle sources of variable isotopic composition with ɛNd(T) from +2.5 to +10. The Early Paleozoic rocks of the sill-dike complex were likely produced by a complicated interaction of melts derived from different sources. The rocks of group 1 resulted from the mixing of low-K picrite and tonalite melts. The picrite melts with ɛNd(T) from +6 to +8 were melted out of garnet lherzolite in the mantle wedge. The tonalite melts with ɛNd(T) = −3 seem to have been formed by the partial melting of mafic oceanic rocks of a subducted slab or the bottom of an island arc. The trondhjemite melts of group 2 with ɛNd(T) varying from 2.5 to 7.5 could be formed via the melting of subducted metapelites or amphibolites with low sulfide concentrations. Massifs of sodic Early Paleozoic granites also occur elsewhere in western Mongolia, Tuva, and the Altai territory. The generation of sodic silicic melts was likely a common process in supra-subduction systems in CAFB. The potassic granites (group 4) could be formed by the melting of subducted pelites or by the fractionation of mantle magmas. The genesis of the basaltic andesites (group 5) was likely related to Mesozoic-Cenozoic intraplate processes.