Toyama breccia pipe, which outcrops in the area of Kamihikawa dam about 8 km east from Enzan, Yamanashi Prefecture, shows features of diatreme, intruded at about 6 Ma into Tokuwa granitic body. Mineralogy and distribution of alteration minerals in breccia pipe were investigated, using samples collected in the tunnel of Kamihikawa dam, by X-ray powder diffraction (XRD) and scanning electron microscopy equipped with an energy-dispersive type spectrometer (SEM-EDX). Spatial and temporal characters of hydrothermal alteration in the Toyama breccia pipe have been addressed in the present study together with clay mineralogy, comparing with those reported from similar breccia pipes and clay deposits distributed in the southern Fosa Magna region.
Four types of alteration including weathering were identified by XRD and SEM-EDX analyses: 1) hydrothermal alteration with mainly epidote and chlorite formation, similar to so-called propyritization, 2) hydrothermal alteration associated with pyrophyllite, kaolinite, illite, and mixed layer minerals, 3) weathering characterized by vermiculite, smectite, kaolin, and goethite formation, and 4) hydrothermal alteration with zeolite minerals. The four types of alteration which occurred at different stages, in turn, from 1) to 4) were superimposed each other along radial fractures developed inside the breccia pipe. Igneous and hydrothermal activities related to the formation of diatreme and alteration of Type 2) in Toyama that occurred at 6–3 Ma are noteworthy as events intimately linked with other activities occurred during Miocene-Pliocene ages in the southern Fosa Magna.
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