Abstract

The Borisikha ophiolite, composed of serpentinites and amphibolites, is an extension of the Isakovka ophiolite belt in the near-Yenisei part of the Yenisei Ridge. It is a slightly disturbed ∼3.5 km thick fragment of the ophiolite association; its crustal part is composed of metamorphosed wehrlite, troctolite, and gabbro cumulates. The cumulate composition and the geochemical features, which do not show any suprasubductional overprinting, indicate that the ophiolite is a normal oceanic lithosphere. Zircons extracted from pegmatoid amphibolite show sectorial magmatic zoning, low U contents, and magmatic Th/U values. The zircon age determined on a SHRIMP-II mass spectrometer is 682±13 Ma, which corresponds to the time of formation of oceanic crust. In the second half of the Late Riphean, the oceanic lithosphere that included the Borisikha ophiolite was subducted beneath the margin of the Siberian continent. At the same time, rifting, formation of half-grabens, and alkaline magmatism took place at the rear of the Yenisei Ridge. Similar events took place in southeastern East Sayan. Correlation between the coeval Late Baikalian structures along the southern and western margins of the Siberian Platform calls for special study.

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