| Summary: | <p>The time and mechanism of shifting from Paleogene-Neogene magmatic belts in the Southern Mountains of Java into Recent volcanic belts is still poorly understood. New geochronological data of intrusive and volcanic rocks are therefore collected from transitional areas to reveal new understanding regarding with this evolution. Eight (8) samples of shallow intrusive and volcanic rocks are collected from Borobudur, Selogiri (Central Java) and south of Lawu volcano, Ponorogo, and Ngebel Lake, Wilis volcano complex (East Java). These samples are dated on separated minerals (i.e., hornblende and/or plagioclase) using K- Ar method. Mineral separation works were done at the Laboratory of Economic Geology, Kyushu University, while K-Ar dating was done at Laboratory of Geochronology, Okayama University of Science, Japan.<br />
The results of K-Ar dating show that three rock samples from Lawu (040828-04 and 040829-09) and Wilis volcanic complex (040829-13) are very young (Quaternary). Sample number 040828-04 represent old Lawu volcanic center (dome or shallow intrusive complex), while sample 040829-09 represent a Pleistocene (0.298 Ma) laharic deposit from Lawu. Sample number 040829-13 represents a Quaternary explosive center at the Wilis Volcano complex. The dating results for the rest samples reveal three volcanic centers of Upper Miocene age located at the transition zone between Quaternary belt and Tertiary one (i.e., the Southern Mountains area). These volcanic centers are located at Borobudur (12.4 ±
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