The Volcanic Rocks and their Alteration in the Nukabira Area, Central Hokkaido, Japan (part 1)

Abstract

The Nukabira Area, Central Hokkaido is situated in the westernmost part of the " Kitami Green Tuff Region" and consists of Miocene lake deposits associated with basaltic pillow breccias, dykes and andesitic, rhyolitic tuff breccias. The Miocene sediments and volcanics are divided into the Horoka Formation and the Taushubetsu Formation, the former of which is intruded by a sheet of hornblende-pyroxene andesite (some 400m thick) and dykes of quartz porphyry. Two kinds of basaltic pillow breccias can be distinguished (1) isolated-pillow breccias which contain unbroken pillows widely separated from one another by a tuffaceous matrix and (2) broken-pillow breccias consisting of disaggregated pillow fragments in a tuffaceous matrix. The hornblendepyroxene andesite is large sheet intruding the Horoka Formation. On the basis of mineral assemblages of zeolites and clay minerals, these volcanic rocks are divided into following three zones; I zone montmorillonite-mordenite-(heulandite) IIa zone chlorite/montmorillonite mixed layer mineral-montmorillonite-prehnite-laumontite zone lib zone cholorite-prehnite-laumontite zone Clay minerals, such as montmorillonite and chlorite/montmorillonite mixed layer mineral, and laumontite and mordenite fill amygdales concentrically, and replace phenocrysts and groundmass, etc, Prehnite, epidote, calcite and quartz occurr between these minerals and as veins. It seems to be that the main alteration has been due to hydrothermal solutions in the diagenetic process and corresponds to zeolite-prehnite pumpellyite facies.