MAGMATISM OF THE OMOLON CRATONAL TERRANE (<i>northeastern Russia</i>): GEOCHRONOLOGY, GEOCHEMISTRY, AND GEODYNAMICS

A long history of diverse magmatism of the Omolon Cratonal Terrane (OM) is traced from the Paleoproterozoic to the Late Miocene. The new isotope-geochronological (U-Pb, 40Ar/39Ar) dating and geochemical data allowed us to reliably identify nine main episodes and geodynamic settings of magmatism, as well as to discuss in detail potential sources of detrital zircon populations in sedimentary rocks of northeastern Russia. The earliest manifestations of fleck gneisses of the anorogenic Upper Оmolon complex with an age of about 1.9 Ga reflect the rifting and the beginning of destruction of the Precambrian basement which has the age of about 3.2 Ga. The ongoing rifting of the OM is traced in the Riphean, when swarms of dikes and small bodies of the gabbro-diabase Strelkin complex were intruded. In the Silurian (433–425 Ma), in oblique shears and local extension zones of the OM, plutonic syenite magmas of the Anmandykan and granitoids of the Abkit complexes (ENd from –23 to –10) are intruded. The largest manifestations of plutonic and volcanic magmatism were recorded in the OM in the Late Devonian (from 375 ± 3 to 356 ± 4 Ma), when calc-alkaline suprasubduction volcano-plutonic complexes (Bulun hypabyssal and Kedon volcanic) were formed. The Nd isotopic composition of Devonian magmas (ENd from –20 to –6), with an extremely low content of radiogenic lead isotopes (206Pb/204Pb = 17.2–15.7) indicates their melting from a mature crustal protolith in the OM basement. Early and Middle Jurassic rifting locally manifested on the OM, which are reflected by the Tummin trachybasalt and Omolon essexite-teschenite complexes. The Early Cretaceous (144–133 Ma, ENd from +7 to 0) Namyndykan and Egdegkych granodiorite-monzonite plutonic complexes form suprasubduction zones of an island-arc nature on the northeastern margin of the OM. In the Santonian-Campanian time (~ 85–77 Ma), the Early Cretaceous extension zone on the Omolon massif was healed by the Kongin and Viktorin calc-alkaline complexes of the Okhotsk-Chukotka volcanic-plutonic belt. The final stage of magmatism in the OM is represented by intraplate alkali-basaltic volcanism in the interval from 9 to 7 ± 1 Ma. Synchronization in time of the manifestation of the most voluminous Late Devonian suprasubduction magmatism of the OM and basaltic LIP-magmatism of the Vilyui rift in Siberian craton was established. Based on this and comparison with detrital zircon populations from Paleozoic sandstones of northern Siberia, we reconstruct that the OM broke off from the northeastern part of the Siberian craton during the Silurian to Devonian and migrated southeast (in modern coordinates). A vast area of crustal extension and thinning in the Alazeya zone during the Jurassic-Cretaceous time has been reconstructed.