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Petrogenetic Evolution of a Transitional Tholeiitic—Calc-alkaline Series: Towada Volcano, Japan

1995; Oxford University Press; Linguagem: Inglês

10.1093/oxfordjournals.petrology.a037266

1460-2415

A. G. HUNTER, S. Blake,

High-pressure geophysics and materials

Detailed petrological and geochemical data of stratigraphically well-defined tholeiitic and calc-alkaline rocks from Towada, Japan, are used to evaluate the system's evolution over ∼0⋅2 myr. Available data suggest that Towada progressed from several small independent magma chambers to one large chamber, illustrated by systematic changes in mineral composition, isotopic and trace element ratios. This is attributed to variations in intracrustal processes in the shallow chambers, with the following evolutionary sequence divisible into four main units: the Precaldera Stage: consisting of several basaltic to dacitic lava cones, independently fed by tholeiitic and calc-alkaline magma chambers. These are characterized by periodic replenishment, fractionation and magma mixing, with some undergoing crustal assimilation during late stages, increasing the 87Sr/86Sr ratio at constant SiO2. The Caldera Stage: characterized by large volume tholeiitic and calc-alkaline andesitic to dacitic pyroclastic eruptions, associated with caldera collapse. Geochemical data indicate that this stage resulted from some of the precaldera chambers amalgamating. Magma in this enlarged chamber underwent mixing, continued fractional crystallization and assimilation. The Ninokura Stage: characterized by primitive tholeiitic basaltic lavas and scoria deposits, which underwent fractional crystallization plus minor crustal assimilation. The Goshikiiwa Pyroclastic Deposits: consisting of evolved calc-alkaline±tholeiitic magmas, which underwent assimilation and fractional crystallization.