Abstract
The Miocene Miyanotani composite dike, exposed in the central part of Kii Peninsula, southwest Japan, was examined to infer the processes of magma mixing and dike emplacement. This composite dike consists mainly of marginal basaltic andesite and central rhyolite, with an intervening andesitic facies. The central rhyolite contains irregularly shaped mafic enclaves, particularly along its margins. Based on field occurrences, petrography, and bulk-rock compositions, the following processes of magma mixing and dike emplacement are inferred: (1) the injection of mafic magma into a felsic magma chamber during cooling, and further mafic dike emplacement; (2) magma mixing in the felsic magma chamber to form intermediate magma; and (3) the injection of felsic and intermediate magma into a previously emplaced mafic dike to form a composite dike. Many composite dikes, including the Miyanotani dike, develop along arcuate faults and pyroclastic dikes or conduits at Odai caldera. The ages of the composite dikes and conduits indicate that they were emplaced contemporaneously. In addition, the pyroclastic dikes include juvenile fragments with mafic and intermediate compositions. These findings strongly suggest a genetic relationship between the composite dikes and the formation of Odai caldera. Many of the irregularly shaped granite enclaves included within the felsic center and andesite of the Miyanotani dike, which show micrographic texture indicative of emplacement at shallow levels in the crust, may have been derived from the felsic magma chamber during cooling, from which the Odai caldera eruption was fed. Therefore, the granite enclaves may yield important information in understanding the relationship between composite dike formation and the caldera eruption.
