Note on rock-forming minerals (1) Chemical compositions of Ca-amplibole in the late Tertiary volcanic rocks from the joetsu district

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The volcanic and intrusive rocks of the Umikawa, Nanbayama and Yoneyama in late Pliocene are distributed in the northern part of Fossa Magna region, Niigata Prefecture. In the Umikawa pyroclastics, pyroxene and olivine basalts are common, and andesite is accompanied in subordinate amount. The majority of volcanic rocks from the Yoneyama are mainly two-pyroxene andesite and olivine basalt with small amounts of hornblende, clinopyroxene-orthopyroxene. The Nanbayama intrusive massif is composed of the porphyrite and porphyry. In these volcanic and intrusive rocks, amphibole occurs as phenocryst with opacitized rims. Hornblende gabbroic inclusions occur in calc-alkali andesites of the Yoneyama and Umikawa. The amphiboles from the Yoneyama and Nanbayama are magnesiohornblende to pargasite, on the other hand, the amphibole in the Umikawa pyroclastics is pargasite. Amphiboles in the volcanic rocks in the present study are near the tremolite-pargasite join. When comparing the amphiboles from the three occurrences, K contents in amphibole from Umikawa pyroclastic rock are higher than those from the Nanbayama intrusive and Yoneyama volcanic rocks. K_2O contents of the bulk composition are the highest in the Yoneyama volcanic rocks. In this case, the amphibole is not affected with the bulk composition.
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