The Central Asian Orogenic Belt (CAOB) is the core geologic unit to understand the tectonic history of Eurasian continent. Although the Mesozoic volcanic rocks in the South Mongolia are the key factor in revealing the later stage of the CAOB formation, very few attempt on its geochemistry have been made. This study gives the basic geochemical information of the volcanic rocks of the Bombookhoi Formation, which has been assigned to the “Upper Cretaceous,” of the South Mongolia, and discusses its tectonic setting. The volcanic rocks of the study area are divided into the tholeiitic basalt (SiO2: 46–49 wt%, FeO*/MgO: 6.0–9.0, Mg#: 17–23) and calc-alkaline andesite (SiO2: 54–57 wt%, FeO*/MgO: 1.6–2.6, Mg#: 40–52). Although the both has a signature of volcanic arc environment such as distinctive Nb negative anomaly in the MORB-normalized multi-element concentration diagram, these has probably been derived from the different sources of magma each other judging from FeO*/MgO ratio, Mg#, and others. It is considered that, by the Jurassic, the South Mongolia block and Siberian craton/North China block had already been amalgamated each other to complete the core part of the “East Asia,” and neither oceanic plate subduction nor arc volcanism is assumed to have occurred in the South Mongolia at the Late Cretaceous. Therefore, age-confirmation of this formation, such as by isotopic dating and others, should be required further. Then, if the volcanic rocks of the Bombookhoi Formation are truly Upper Cretaceous, re-consideration for the basic tectonic scheme around the South Mongolia block at Late Cretaceous will be needed.
KHISHIGBADAM et al. (Mon,) studied this question.