Geochemistry and K-Ar age of the mafic rocks at the Uubulan area, Khangai-Daur belt, Central Asian Orogenic belt, central Mongolia

This paper describes the whole-rock geochemical composition, clinopyroxene (Cpx) chemical composition, and the hornblende K-Ar age of the mafic rocks in the Carboniferous accretionary complex of the Khangai-Daur belt, Central Asian Orogenic belt, exposed at the Uubulan area of Mongolia. The geochemical nature of the mafic rocks, enriched in large ion lithophile elements and light rare-earth elements compared with high field-strength elements (HFSE) and heavy rareearth elements, suggest that they are alkaline basalts. The (La/Yb)cn vs. HFSE diagrams show that the samples are oceanic island basalts. Therefore, the mafic rocks are concluded to be oceanic-island alkaline basalts. The comparatively high concentrations of Al, Ti, Na, and Ca in the Cpx of the mafic rocks strongly support this view. The mafic rocks just beside the Upper Silurian radiolarian chert give a hornblende K-Ar age of 412.7±8.6 Ma. The fact that Upper Silurian chert strongly suggests that the oceanic-island alkaline basalts are conformably underlain by the radiolarian chert formed at the pelagic region as a base of oceanic plate stratigraphy.