Isotope Sm‒Nd Characteristics of the Juvenile Crust in the Central Mongol–Okhotsk Orogenic Belt

Abstract

In the Phanerozoic, the production of continental crust was associated with convergent geodynamic settings [1] and, therefore, with intracontinental orogenic belts developed after the closure of paleooceans. Evaluation of the proportions of the juvenile crust versus older recycled material is a key parameter for understanding how the crust in orogens formed and evolved. The Mongol–Okhotsk orogenic belt is one of the major structural parts of Central Asia. Its central (Transbaikalian) part contains well-reserved fragments of an island-arc system, as well as the entire spectrum of accretionary prism formations, allowing us to evaluate the proportions of juvenile versus recycled sedimentary material in the crust of this orogen. In this part of the belt, the most representative fragments of the juvenile crust are volcanogenic–sedimentary sequences of the Late Paleozoic Kamenka and Urtui formations. The former is an indicator of a subduction zone along the northwestern margin of the Mongol–Okhotsk Ocean, with a dip beneath the Siberian Paleocontinent, whereas the latter marks the subduction zone along the southeastern margin, with a dip beneath the Argun superterrane. The entire spectrum of compositions for volcanism of the Kamenka Formation, from basalts to rhyolites, is characterized by positive values of ɛNd(254 Ma) = +1.4–(+3.8) and by TNd(DM) = 896–920 Ma. Basalts of the Urtui Formation also have positive values of ɛNd(350 Ma) = +1.7–(+6.0) at TNd(DM) = 773–939 Ma. This gives us grounds to evaluate the characteristics of the juvenile crust of the Mongol–Okhotsk orogenic belt: it is characterized by positive ɛNd(t) and by model ages TNd(DM) < 1000 Ma. The composition of metasedimentary rocks from the accretionary wedge of the Mongol–Okhotsk orogenic belt is considerably dominated by older recycled crustal material, the likely source of which is Late Riphean rock complexes of the Argun superterrane.