LOWER PALEOZOIC OCEANIC-CRUST IN MONGOLIAN CALEDONIDES - SM-ND ISOTOPE AND TRACE-ELEMENT DATA

Mongolian ophiolites range in age from Early Proterozoic to Middle Paleozoic with maximum of ophiolite production in Late Riphean to Early Cambrain time. One of the best examples is the Bayan-Khongor complex which comprise serpentinized peridotite, cumulate sequence, sheeted dike unit and metavolcanics. Sm-Nd mineral isochron obtained from upper gabbro yields the age of 569 Ma and initial Nd ratio + 11.9. Chemical data for gabbros, dikes and metavolcanics reveal their oceanic nature. Both isotopic and geochemical signatures of the Bayan-Khongor ophiolite are consistent with its origin in the plume-related spreading ridge segment of the Paleoasiatic oceanic basin.