Abstract
Mt. Ryuzan (elevation 1, 362m) represents the oldest edifice in the Zao volcanic group of NE Japan. It consists of thick piles of lavas and pyroclastic rocks, and has been subjected to volcanic eruption or land slide activity which produced mud-flow (debris flow) deposits. Both K-Ar ages and paleomagnetic data of these lavas indicate that the volcano formed at about 1 million years ago. The analyzed rocks from Mt. Ryuzan fall within the SiO2 range of 47.4-60.3% (water free basis) and basalt is the predominant rock type. Alkali contents, especially K2O, are very low in those basalts, compared with rocks forming the main (Central) Zao volcano. Chemically, rocks of Mt. Ryuzan belong to the tholeiitic series (TH) of Miyashiro (1974). It is noteworthy that low-K tholeiitic basalts occur at the early stage of the formation of Zao volcano.
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