2016 年 61 巻 2 号 p. 429-448
We conducted detailed stratigraphic, petrographic and petrochemical investigations on the Benri subunit which is the products by one of the caldera-forming eruption cycles of Aso-4 pyroclastic flow, Aso volcano, central Kyushu, Japan. The Benri subunit is limitedly distributed in the outer side of the northwestern part of Aso volcano. It is mainly composed of scoria, pumice and banded pumice for juvenile fragments, and andesitic lithic fragments (accessory) set in a matrix. The Benri subunit is divided into seven units (Unit1∼Unit 7), based on the amount and shape of essential fragments, and on the characteristics of lithic fragments and matrix. As a whole, the upward transition of deposits from pumice-rich (non-welded) to scoria-rich (weakly-welded), and to pumice-rich again was recognized in the units. Petrographic observations and mineral chemistries, particularly for wide range of the anorthite content of plagioclase (An40-90) in most scoriae, pumices and banded pumices, provide evidences for magma mixing with different amounts of mafic and felsic components. Pumices in some units are characterized by low anorthite content of plagioclase (An30-50) which are similar to those in the silicic eruption products at the initial stage of the Aso-4 cycle. Whole-rock major and some trace element contents of scoriae, pumices and banded pumices show linear variation trends between two assumed (mafic and felsic) end members. These results suggest the magma mixing as an important process for producing the magmas of Benri subunit.
These observations and results suggest that, before the eruption producing the Benri unit, compositionally zoned (stratified) magma layers had been formed between mafic and felsic end member magma layers in the magma chamber. The eruption sequence of the composite magmas is assumed to have changed as from felsic magma dominant to mafic magma dominant, and then felsic magma increasing again.