The high temperature hot springs such as in Yunomine and Tosenji and the hydrothermal alteration zones of the Hongu area are distributed in the southern part of Kii peninsula, though Quaternary volcanoes which can be their potential heat sources are not distributed in the area. This research has aimed to understand the thermal history of the area by using several dating methods. Thermoluminescence (TL) dating method was applied according to the distance from dykes or gushing out point of the hot springs which thought to be the center of alteration. The results show that the alteration age of Yunomine alteration zone is younger as it approaches the gushing out point of hot spring. No clear tendency was observed in other places. These results suggest that the alteration ages near Yunomine hot spring are controlled by the distance from the gushing out point. However, no clear tendency was observed at other hot springs between the alteration ages and the distance from the center of alteration when two or more activities of alteration have occurred. The results of the TL, Fission Track (FT) and K-Ar dating show that after the high temperature hydrothermal alteration (until tens of millions of years ago), relatively low temperature alteration (from at least hundreds of thousands of to tens of thousands of years ago) occurred in Hongu and Totsukawa area.
Petrological features of gravels of serpentinized peridotites in the Nagahama Formation of the upper Kazusa Group (middle Pleistocene), distributed in the central Boso Peninsula, are examined in detail. Studied peridotites are serpentinized to various degrees, but their protoliths were inferred from textures and relic minerals; harzburgites frequently containing plagioclase are dominant, and lherzolites (with plagioclase) and dunites are subordinate. Forsterite (Fo) contents of olivine are high, around 91, and Cr/(Cr + Al) ratios of chromian spinel are intermediate, mostly from 0.4 to 0.6. The geological and petrological characteristics of the serpentinite gravels indicate that they were derived from the Mineoka Belt (Circum-Izu Massif Serpentine Belt), southern Boso Peninsula. Serpentinite gravels in several strata, including the Nagahama Formation, from the early Miocene to Recent in the Boso Peninsula, show very similar petrological features (particularly chemical composition of chromian spinels), so the gravels were possibly derived from mantle peridotites of a common origin. Almost homogeneous peridotite bodies have continued to be uplifted at the fore-arc region of the Honshu arc (the Mineoka Belt) throughout the time after protrusion near/at the trench in the early Miocene. Northward streams were active in the central part of the Boso Peninsula due to uplift of the southern part at the time of sedimentation of the Nagahama Formation. The ‘Kamogawa Graben’, which topographically divides the Mineoka Belt and the northern area now, had not been developed in the middle Pleistocene.