THE MOST ANCIENT OPHIOLITE OF THE CENTRAL ASIAN FOLD BELT: U-PB AND PB-PB ZIRCON AGES FOR THE DUNZHUGUR COMPLEX, EASTERN SAYAN, SIBERIA, AND GEODYNAMIC IMPLICATIONS

Abstract

Ophiolitic rocks with a zircon age of ∼1020 Ma occur in the Dunzhugur complex of East Sayan, Siberia, and are part of a Neoproterozoic to early Palaeozoic segment of the Central Asian fold belt. The most spectacular suite is exposed along the Oka and Bokson rivers, where a complete ophiolite sequence with mantle tectonites, a layered sequence composed of dunite, wehrlite, and pyroxenite, a gabbro section, a sheeted diabase dyke complex and basaltic pillow lavas are exposed. Petrologic and geochemical data suggest that all members of the ophiolite originally belonged to the same cogenetic mafic–ultramafic crustal section and support a supra-subduction zone setting in a fore-arc rifting environment for its origin. Two multigrain zircon size fractions from a plagiogranite are both slightly discordant but yielded a combined mean 207Pb/206Pb age of 1020±10 Ma. Evaporation of three additional fractions of three to four grains each from the same sample produced a mean 207Pb/207Pb age of 1019.9±0.7 Ma that we consider to most closely reflect the time of igneous crystallization of the plagiogranite. This is the oldest ophiolite so far dated from the Central Asian fold belt. The southern margin of the Siberian craton and the palaeo-Asian ocean were established at the end of the Mesoproterozoic, at least 1000 Ma ago. During the time interval 1000–570 Ma, one or several large ocean basins existed between Baltica, Siberia, Kazakhstan, Tarim and northern China, and these blocks are therefore unlikely to have been part of the supercontinent Rodinia. Rifting, initiation of subduction, and marginal basin formation began prior to 1000 Ma and continued through 570 Ma. The Dunzhugur ophiolite of Eastern Sayan provides evidence for the early opening of the palaeo-Asian ocean not later than 1000 Ma ago.