Nd Isotopic Composition of the Continental Crustal Rocks and Its Relations with Geological Structures of the Southern Surroundings of the Mongol–Okhotsk Foldbelt

The continental crust along the southern edge of the Mongol–Okhotsk Foldbelt (MOFB) has been zoned according to the Nd isotopic composition of the magmatic rocks. Three terranes have been identified: Middle Gobi, Erendaba, and Idermeg. These terranes differ in the Nd isotopic composition of the granitoids and felsic volcanics and in their geological histories. The Middle Gobi terrane has a Precambrian basement and Mesoproterozoic isotopic model ages of its igneous rocks (TNd(DM-2st) > 1 Ga). The Erendaba terrane formed in the Early Paleozoic, incorporating recycled material with Mesoproterozoic isotopic signatures. The rocks of the Idermeg terrane possess predominantly Neoproterozoic isotopic characteristics [TNd(DM-2st) < 1 Ga]. The Mongol–Okhotsk Foldbelt formed via the accretion of these terranes to the Siberian continent at different time. The Middle Gobi terrane collided with Siberia at the beginning of the Triassic, forming the Khentei segment of the MOFB. The composite Erendaba–Idermeg terrane accreted to Siberia in the Middle Jurassic, completing the formation of the Jargantu segment of the MOFB. It can therefore be concluded that the Mongol–Okhotsk basin was a large embayment of the Paleo-Asian Ocean.