Fochin Zhang

Zircon ages and geochemical data of the Biluutiin ovoo ophiolite: implication for the tectonic evolution of South Mongolia

The early Palaeozoic tectonic evolution of South Mongolia is not well constrained due to the limited exposure of early Palaeozoic rocks in the area and the scarcity of both geochemical and geochronological data. In order to help rectify this situation, we have conducted detailed studies on the Biluutiin ovoo ophiolite in South Mongolia to provide constraints on the tectonic evolution of the region during this period. The Biluutiin ovoo ophiolite consists of ultramafic rocks, mylonitic gabbro, basalt, tuff sandstone, plagiogranite, calcite, and chert. Gabbro and plagiogranite samples from the ophiolitic complex yielded SHRIMP zircon ages of 525 ± 5 Ma and 503 ± 6 Ma, respectively. Biluutiin ovoo ophiolitic basalts display LREE and LILE enrichment and strong HFSE depletion, indicating that the ophiolite is supra-subduction zone (SSZ) type. Plagiogranite with adakite-like geochemical compositions suggests that palaeo-ocean subduction occurred in South Mongolia during Cambrian time. Intruding granite yielded a SHRIMP zircon age of 353 ± 2 Ma, indicating that the ophiolite was emplaced before early Carboniferous time. Identification of the Cambrian ophiolitic complex and the occurrence of Cambrian adakites indicate that southern Mongolia underwent a period of active volcanism during the Cambrian. The Cambrian formations are likely correlated to the early Palaeozoic subduction-accretion belt of Western Mongolia.