Late Permian–Early Triassic Kamensky dyke belt, NW margin of Siberian platform: Two stages of the Siberian superplume event

Abstract

A large (900 km long by 50-60 km wide) Early Triassic dyke swarm intruded along the northern margin of the Siberian platform in the Yenisei-Khatanga trough, crossing the Noril’sk, Kamensky and Maimecha-Kotui provinces. The dykes trace major structural lineaments, including the up to 140 km Dyumtaley intrusion, and forming swarms in several regions in the Kamensky province, with the highest concentration in the Bolsheavamsky volcanic basin. Compositionally, the dykes range from subalkaline to alkali-basic to alkali-ultramafic, and include dolerites, trachydolerites, syenites, minettes, lamprophyres (camptonite, spessartite, vosgesite), augitites, melanephelinites, alnoites, limburgites, alkaline picrites, lamproites and maimechites.

Multi-phase (up to five), symmetric ‘dyke in dyke’ intrusions show sharp differences in compositions, with ultrabasic picritic olivine porphyries in the center and alkaline borders close to olivine clinopyroxenites. The latest phases are silica-rich lamprophyres and rare lamproites. Bulk rock compositions show strong variation trends due to the segregation of olivine, magnetite and pyroxene, similar to that recorded in the subalkaline basaltic dyumtaley layered intrusion in Taimyr (Vernikovskaya et al., 2010; Vernikovsky et al., 2003). However, each separate dyke cluster reveals an individual compositional range. Silica and ⁸⁶Sr/⁸⁷Sr ratios are correlated for the young dykes, reflecting a process of melt-crust interaction. Pressure-Temperature conditions for the most Mg-rich dykes reveal an ascending trend from garnet-facies to the Moho (subalkaline). Silica-rich varieties originate within the crust. Meimechites and lavas sequences in the Noril’sk volcanics create a 150°C hotter branch of estimated geotherm for bulk rock compositions.

Trends in Ti enrichment reflect differentiation-self-contamination processes near the Moho with varying fO2. The evolution of subalkaline to alkaline basalts, and in general the lamprophyres, feature a rise in La/Ybn ratios and LILE and HFSE components, correlating to decreasing degrees of melting and calculated depths of origin and metasomatic source enrichment. The HFSE minima for subalkaline basalts are not characteristic for the glassy dykes. Multiphase dykes formed during tectonic fracturing by magmas intruded from polybaric magmatic chambers. The dyke swarm represents two stages of the Siberian superplume, intruding across the northern part of Siberian plateau at 256-247 and 243-231 Ma, as revealed by 40Ar/39Ar ages. Prevailing later volcanics show fluid (CO₂, H₂, F, Cl and CH₂) and silica enrichment from the crust took place in the intermediated magmatic chambers and melt feeders.