Intracrustal Controls on the Coexistence of Tholeiitic and Calc-alkaline Magma Series at Aso Volcano, SW Japan

Abstract

Aso caldera (SW Japan) was formed incrementally over a period of ∼300 kyr, by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits (Aso 1–4). These units are collectively referred to as the Aso Pyroclastic Flow Deposit (APFD) in which tholeiitic and calc-alkaline magmas have been erupted in close temporal and spatial proximity. Detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between Aso 1 and 4 as the caldera-forming stage evolved, changing from a predominantly tholeiitic to a calc-alkaline signature. The mineralogical and geochemical characteristics of Aso 1–4 suggest that magmatic evolution within each unit is controlled by fractional crystallization, associated with magma mixing ± crustal assimilation. By altering the fractional crystallization mineral assemblage, as well as the relative amounts of fractional crystallization, magma mixing and crustal assimilation, the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced. This suggests that the coexistence of tholeiitic and calc-alkaline magma series can be attributed to shallow-level intracrustal processes.