Mesozoic Alkaline Syenite Porphyries of the Lamproite Series in the Gold-Bearing South Uguy Zone (Western Aldan): a Petrological Study

The study presents petrographic, geochronological, and isotopic-geochemical data on the Mesozoic alkaline pyroxene–amphibole syenite porphyries in the Uguy graben, western Aldan Shield, East Siberia. These syenites represent the earliest (~145 Ma) and the most mafic group of alkaline magmatic rocks associated with gold mineralization in the South Uguy gold-bearing zone. The syenite porphyries show ultrapotassic, agpaitic geochemical signatures, in line with a phonolitic composition. Mafic microgranular enclaves (MMEs) within the syenite porphyries exhibit the most primitive tephryphonolite composition, thus represent remnants of early melt. Analysis of the phenocryst assemblages (clinopyroxene + amphibole + alkaline feldspar + fluorapatite) enables the reconstruction of the multi-stage evolution of the syenitic melt, spanning depths from the lower crust to a shallow magma reservoir. Based on petrographic and mineral thermobarometry data, it was established that the early phenocryst assemblages (clinopyroxene + fluorapatite + leucite?), which was in equilibrium with the MMEs, crystallized at Т = 1200°С and Р = 10–15 kbar. Two-stage numerical modeling of the system “parental magma = cumulate + residual meltˮ by a least-square mass-balance calculation established that syenitic melt likely formed through the crystal fractionation of more primitive parent melt close in composition to Early Cretaceous lamproites of the Murun massif located 100 km north of the study area. Sr-Nd isotopic composition of the syenite porphyries and lamproites indicates that the alkaline magmas originated from a metasomatized lithospheric mantle source. This source experienced two-stage enrichment in incompatible elements: in the Paleoproterozoic via intraplate and supra-subduction processes and in the Mesozoic during tectonic evolution of the Mongol-Okhotsk Orogenic Belt.