THE SM-ND ISOTOPIC SYSTEMATICS OF OPHIOLITES IN THE OZERNAYA ZONE (MONGOLIA)

Abstract

The Sm-Nd isochron age was defined for rocks from various associations which make up part of the ophiolite assemblage of the Ozernaya zone of the early Caledonides. The age of gabbroids and tholeiite basic rocks of the silica-volcanic sequence in the Seriin-Nuru Range, which were formed under the conditions of an open marine basin, is 527 ± 43 Ma and εNd(T) = +6.7. The Geriin-Nuru volcanics are associated with graywackes and were, most likely, generated in an island arc setting. Their age was found to be 522 ±13 Ma, εNd(T) = +8.0. The time of the regional metamorphic reworking of the ophiolites was determined on the basis of metamorphic minerals from rocks of the island-arc affinity of the Khalzan-Buregtei-Ula mountains and defined to be 487 ± 6 Ma with εNd(T) = +7.8. The obtained data indicate the age similarity between MORE and island-arc ophiolites, and regionally metamorphosed rocks. They mark the final stage of the early Caledonian paleoocean with rapid change and lateral conjugation of various geodynamic settings.