The Torikabuto Volcano is located at a northeastern end of the "Central Upheaval Zone" in the North Fossa-magna region, central Japan. Most of the volcanic rocks of the Torikabuto Volcano belong to the calc-alkaline rock series, and are composed of basaltic andesite, dacite lavas, and volcaniclastics. Based on their stratigraphic data, volcanic activities in the Torikabuto Volcano are divided into four stages; Maekura Stage, Stage , Stage , and Stage. The rocks at the Maekura Stage are pyroxene-andesite pyroclastic and lava flows. The rocks at the Stage are characterized by large amounts of pyroclastic flows, which are composed of the rocks ranging from basaltic andesite to dacite. The lava flows ranging from basaltic andesite to dacite are widely developed at Stage. A small quantity of dacitic pyroclastic flows was formed at the Stage. The volcanic rocks at the Stages and contain gabbroic inclusions and in some places they exhibit the layered structure. The K-Ar age dating data indicate that the age of the rocks at the Maekura Stage is. Ma and the age of those at the Stage is between .-. Ma Kaneko et al.,. Base on the examination of the petrographical, mineralogical, wholerock geochemical, and Sr-Nd isotopic characteristic of the volcanic rocks from the Torikabuto Volcano, we draws following conclusions. Magma mixing is recognized from field observation, petrology of the volcanic rocks and mineralogy of plagioclase after the Stage. However, fractional crystallization processes of the calc-alkaline magma mainly formed the calc-alkaline volcanic rocks from the Torikabuto Volcano. The fractionated minerals are plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides and hornblende. This is supported by the results from mass-balance calculation. The Sr-Nd isotopic ratios of volcanic rocks and inclusions from the Torikabuto Volcano, have the same values. This indicates that the volcanic rocks and inclusions formed from the same magma. Accordingly, the magma mixing process that is recognized from the volcanic rocks of the Torikabuto Volcano is the "internal mixing" and the inclusions are cumulated rocks.

岩石鉱物科学 41, 103 121, 2012 北部フォッサマグナ，関田山脈 志久見川流域に分布する 前期更新世火山岩類の岩石学的特徴 -ソレアイト系列火山岩類の形成過程-Petrogenesis of the early Pleistocene volcanic rocks in the Sekita Mountains and the Shikumi river basin, North Fossa Magna, Central Japan -DiŠerentiation process of tholeiitic basaltic magma-The Sekita Mountains and Torikabuto Volcano District lies at the northeastern end of the Central Upheaval Zone located within the North Fossa Magna region. In this area, early Pleistocene volcanic rocks ranging basaltic to dacitic compositions are widely distributed. These volcanic rocks are subdivided into four distinct groups, based on their geological and geochemical characteristics, the Sekita Mountains (SM), the Shikumi River Basin (SRB), the Kenashi (KN) volcano and the Torikabuto (TK) volcano.In this paper we report petrographical, mineralogical, whole rock geochemical, and Srand Ndisotopic characteristics of the SM and SRB volcanic rocks. The SM and SRB volcanic rocks are divided into group 1 and group 2, and these volcanic rocks are composed mainly of basalt, andesite and dacite belonging to the tholeiitic rock series. The ages of SM and SRB rocks range from ～2 Ma to 1.3 Ma. Our data can concluded as the followings. 1) The SM and SRB volcanic rocks were formed by fractional crystallization processes of tholeiitic magma, involving plagioclase, clinopyroxene, orthopyroxene, and Fe Ti oxides. 2) The Srand Nd isotopic compositions of the SM and SRB tholeiitic volcanic rocks are nearly identical to that of the TK calc-alkaline volcanic rocks, suggesting a genetic link between the SM, SRB and TK rocks. 3) The tholeiitic primary magmas for the SM and SRB volcanic rocks and the calc-alkaline primary magmas for the TK rocks would be formed by partial melting processes of the mantle source materials having similar Srand Nd isotopic composition. We interpret that the tholeiitic rocks are products of crystal diŠerentiation at middle crustal level (～5 kbar), while the calc-alkaline rocks are derived by fractional crystallization at shallower level (2 3 kbar).

丹沢 大磯地域に分布する火山岩類の K Ar 年代と 南部フォッサマグナ地域における鮮新世火山フロントの西方移動 K Ar ages of volcanic rocks in the Tanzawa Oiso area, central Japan: Implication for westward migration of the Pliocene volcanic front in the southern Fossa Magna region 森 慎一(Shin'ichi MORI) 山下 浩之(Hiroyuki YAMASHITA)  有馬 眞(Makoto ARIMA)   藤岡換太郎(Kantaro FUJIOKA)    We determined K Ar ages of the late Miocene to Pliocene andesitic to basaltic dykes and lavas distributed in the Tanzawa Oiso area in the southern Fossa Magna region, central Japan. Our data indicate two distinct periods of volcanic activity in this region; one took place during the middle Miocene (about 15 Ma) and another during the late Miocene to Pliocene (7.5 4.2 Ma). Combined the present results with the age data reported previously from volcanic rocks in the southern Fossa Magna region, the present study suggests that the volcanic front in this region during 8 to 4 Ma was located at about 40 km east of the present-day volcanic front. The volcanic front began to migrate westward to the present-day location at about 4 Ma. The westward migration of volcanic front was probably associated with an abrupt change of lateral motion of the Philippine Sea Plate from north-northwestward to northwestward occurred between 4 and 2 Ma.