PETROGENETIC EVOLUTION OF A TRANSITIONAL THOLEIITIC-CALC-ALKALINE SERIES - TOWADA VOLCANO, JAPAN

Detailed petrological and geochemical data of stratigraphically well-d efined tholeiitic and calc-alkaline rocks from Towada, Japan, are used to evaluate the system's evolution over similar to 0.2 myr. Available data suggest that Towada progressed from several small independent ma gma chambers to one large chamber, illustrated by systematic changes i n mineral composition, isotopic and trace element ratios. This is attr ibuted to variations in intracrustal processes in the shallow chambers , with the following evolutionary sequence divisible into four main un its: (I) the Precaldera Stage: consisting of several basaltic to dacit ic lava cones, independently fed by tholeiitic and calc-alkaline magma chambers. These are characterized by Periodic replenishment, fraction ation and magma mixing, with some crustal assimilation during late sta ges, increasing the Sr-87/Sr-86 ratio at constant SiO2. (2) The Calder a Stage: characterized by large volume tholeiitic and calc-alkaline an desitic to dacitic pyroclastic eruptions, associated with caldera coll apse. Geochemical data indicate that this Stage resulted from some of the precaldera chambers amalgamating Magma in this enlarged chamber un derwent mixing, continued fractional crystallization and assimilation. (3) The Ninokura Stage: characterized by primitive tholeiitic basalti c lavas and scoria deposits, which underwent fractional crystallizatio n plus minor crustal assimilation. (4) The Goshikiiwa Pyroclastic Depo sits: consisting of evolved calc-alkaline +/- tholeiitic magmas, which underwent assimilation and fractional crystallization.