Petrology of the Tertiary basalts of Hamamasu, Hokkaido

Various kind of basalts of the Pliocene age are developed in the western extremity of the so-called “Kabato mountainland” in central Hokkaido. In this area, voluminous andesitic lavas and pyroclastic rocks are predominant in Miocene to Pliocene age, and normal sedimentary rocks are very limited in distribution. The basalts occur as lava flows, covering the Miocene and Pliocene formations, or as dikes, cutting them. The basalts can be divided into the following three types on the basis of the petrographic characters; A type: Olivine basalt (IIIb, IIIb→c) Augite-olivine basalt (IVb, IVb→c) B type: Olivine basalt (IIIc) Augite-olivine basalt (IVc) Augite basalt (Xc) Olivine-augite basaltic andesite (IVc) C type: Olivine basalt (IIIa→d, IIId) Olivine-augite basalt (IVa→d, IVd) Among them, C type basalts noteworthy include holocrystalline xenoliths and xenocrysts of plagioclase, quartz and hornblende. The present study clarified that the basalts of this area are presumably generated from the alkali basalt magma; A and B type basalts are derived solely by the fractional crystallization, whereas C type basalts which compose the majority of the basalts, are produced under the influences of contamination of the magma with salic rocks of the basement.