New data on the age of the Guli intrusion and implications for the relationships between alkaline magmatism in the Maymecha-Kotuy province and the Siberian superplume: U-Th-Pb isotopic systematics

The Siberian flood basalts (SFB) make up one of the world’s largest known magmatic province and cover a vast area of the northern Siberian platform. The northeastern flank of the SFB province is occupied by the Maymecha-Kotuy ultramafic-alkaline complex consisting of enormous volumes of alkaline lavas, numerous dikes, and the Guli massif comprising many smaller alkaline plutons. The genetic relationship between SFB and the Maymecha-Kotuy complex continues to be a matter of considerable debate. Despite the fact that the rocks of both blocks have almost the same age, close to the Permian-Triassic boundary, there are still major uncertainties in the relative succession of emplacement and the source material for each lithology present. On the basis of previous petrological, geochemical, and isotopic results, this study attempted to put tighter limits on possible relationships between SFB and alkaline rocks.A U-Pb whole-rock age of 250 ± 9 Ma was obtained for the Guli rocks, which is consistent with known dates for the SFB. The Guli rocks have Pb isotopic compositions that lie in the lower part of the OIB field, whereas the dunites and carbonatites plot outside OIB toward the MORB array, suggesting a more depleted source than that for SFB. The combined Pb, Sr, and Nd isotope systematics for SFB and Guli alkaline rocks allow identification of several distinct source components. One component is a major contributor to many Guli rocks and has low 87Sr/86Sr ratios (0.7031 to 0.7038), high ɛNd values (+5.35 to + 3.97), and a relatively unradiogenic Pb composition (206Pb/204Pb = 17.88−18.31; 207Pb/204Pb = 15.38−15.46, and 208Pb/204Pb = 37.33−37.70), which is attributed to a depleted mantle source (MORB-type). The second component, which represents the major portion of SFB, displays fairly homogeneous chemistry and isotope compositions with 87Sr/86Sr of 0.7046–0.7052, ɛNd from 0 to + 2.5, and mean Pb isotopic characteristics of 206Pb/204Pb = 18.3, 207Pb/204Pb = 15.5, and 208Pb/204Pb = 38.0. This component, a major contributor to SFB, is presumably a relatively primitive lower-mantle plume with near-chondritic signatures. Minor contamination with upper and lower crust is invoked as the third and fourth components to explain the isotopic characteristics of some of SFB and high-silica Guli rocks. Finally, the metasomatic event associated with the Siberian superplume activity contributed as the fifth component and caused the unusual enrichment in REE and related elements in some of SFB and Guli rocks.