,+ /,1, Susumu N, Ikuo K and Junichi N

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1 J. Hot Spring Sci. /3 +*- +++,** ,+ /,1, High Temperature Hot Springs and Their Geological Structures of the Central and Southern Part of Kinki District, Japan ( + ) From the Results of the Geological and Geophysical Survey around Shirahama and Arima Hot Springs Susumu N, Ikuo K and Junichi N ISHIMURA ATSURA ISHIDA Abstract Some hot springs having high temperature and high concentration of chemical components are well known at the middle and southern part of Kinki District, but these are no related to any volcanism. It is discussed partly how to originate these hot springs in Kii Peninsula. The report is divided into three parts that ( + ) and (,) are discussed on the origin of hot springs at Nanki-Shirahama and Arima, and (-) are getting our idea on the origin of these hot springs along the outer zone of Southwest Japan Arc. It is assumed that Nanki-Shirahama and Arima hot springs (i) are gushed out mainly with bubble of CO,, (ii) are contain some elements of mantle, and (iii) rise up along the crushed zone of the active faults through the neck of igneous intrusion. The report ( + ) is given the gravity survey curried out at these hot springs areas. It has been cleared the evidence of the necks of acidic igneous intrusion. Key words : Nanki-Shirahama hot spring, Arima hot spring, acidic intrusion, Bouguer anomaly, dehydration of subducted slab. + 0*02-*/ +. + NPO Think-tank Kyoto Institute of Natural History, Kinnki-chiho Hatsumei Center, Yoshida-Kawaramachi +., Sakyo-ku, Kyoto 0*02-*/, Japan. 103

2 +, : + - He 0* -* km,***,**+ 0* km -* km +31. ; Nakamura and Maeda, +30+ ; Sakai and Matsubaya, +311 ; Sano and Wakita, +32-,**0,**2, Tatsumi, +323 ; +33/ ;,*** 0* km +,*** Fig. + Fig., -1.,,. + MPa,** MPa 104

3 /3,**3 + Fig. + Schematic diagram of super-critical H, O derived from subducted slab (solid arrows) beneath the forearc region, that slab should be dragged downward on the slab by the subduction of oceanic lithosphere (Nishimura,,***). Amphibole (AM) and chlorite (CHL) in the dragged hydrous peridotite layer decompose to release H, O at around ++* km just beneath a volcanic front in normal, phlogopite (PH) decomposes at around +/* km, and lawsonite Peridotite dehydrated at around,/* -** km beneath the backarc side of volcanic arc, in normal subduction region. When the front of migration of H, O from the hydrated peridotite reaches the region with the solidus temperature of hydrous peridotite. Partial melting takes place to produce initial magmas. Successive addition of H, O to the region expands the partially molten zone upwards in which the degree of partial melting increases upwards. From the top of the partially molten zone a mantle diapir starts rising. The mantle diapirs must go through the region with temperatures higher than +,-** and stop rising to release primary magmas. It should be stressed that the solidus temperature at hydrous peridotite ( +,*** at depth shallower than +** km) cannot be attained in normal subduction zone. It follows that no magma can be produced by partial melting of hydrous column beneath the forarc region. +. km,*** Fig., Constitutional diagram of H, O. Phase diagram of water

4 Fig. - Tensional and compressional field around Shirahama hot spring on reconsideration upon geological survey Fig. - -, *,.,,/** +,**. Fig.. GPS +** D *1.,1 mgal 3+23 BM 3+23 : ,0 mgal 106

/3,**3 + Fig.. Bouguer anomaly map around Shirahama town. : measurement station. line : iso-bouguer anomaly line, each *,. mgal. X and Y figures are shown by the plane orthogonal coordinates.

5 /3,**3 + Fig.. Bouguer anomaly map around Shirahama town. : measurement station. line : iso-bouguer anomaly line, each *,. mgal. X and Y figures are shown by the plane orthogonal coordinates. g BA g f +301,. g 312*-+. 2/ + *.**/,1223/ Sin f*.****,-.0, Sin f gal * Atm Atm *. 21 *.****30/ H mgal H m FA dgdt *.-*20 mgalm FA *.-*20 H mgal TC +321 /* m /* m GKTC31/* - /* km,. 01 gcm BG BG, pgrh G - r,01. gcm BA BAgg FAAtmTCBG * * Fig.. Fig.. 107

$% $%/ 0123456789:9;< =>? @1A -.$))3# BC A DE#89:9;< %/ 0&FGH 3*./20 IJK& +.0*0-,0 mgalῌkm LM# 6789:9;<%NOῌPQ=R% ST1UV3W Fig. / XY- $%$ ) Z[ \]E#)$^X_`a bca 5. 1* de%f %g c$%a DE#_`a%h\1\ij kxtl1-.$

6 Fig. / Residual Bouguer anomaly map around Shirahama hot spring area. Line : iso-residual Bouguer anomaly line, each *.+ mgal. A low-density intrusion.is shown in mesh area.!"# $% &'(% )*+,-.% $%/ :9;< -.$))3# BC A DE#89:9;< %/ 0&FGH 3*./20 IJK& +.0*0-,0 mgalῌkm LM# 6789:9;<%NOῌPQ=R% ST1UV3W Fig. / XY- $%$ ) Z[ \]E#)$^X_`a bca 5. 1* de%f %g c$%a DE#_`a%h\1\ij kxtl1-. mn opq +33/ Fig. 0.ῌ ῑ ῐ.ῌ+ ῑ ῐ ῐ ΐῒ rs 108 %t XuvW& wxy Fig. 0 Iso-temperature map of hot springs at Shirahama. Over 1* hot springs are srarouding a low-density intrusion.

Atago-yama, Tenjin-yama and Teppo-yama. Fig. 2 Bouguer anomaly map in Arima hot springs area.

7 /3,**3 ῌ + Fig. 1 The distribution of Kinsen (Golden color hot springs) at Arima with some igneous stocks of Atago-yama, Tenjin-yama and Teppo-yama. Fig. 2 Bouguer anomaly map in Arima hot springs area. : measurement station. line : iso-bouger anomaly line, each *.+ mgal. A low-density intrusion is shown in mesh area. X and Y ﬁgures are shown by the plane orthogonal coordinates. 109

8 ,**3, He Ra, Rn - 0* km,**1 Fig. 1 -., Fig. 2 Fig. 2 - tension field Fig. 2 / 0* km Fig. 3 ;,**1 0* km Fig. 3 Depth contour map of the upper boundary of the Philippine Sea Slab with interals of +*,* km (modified from Hirose et al.,,**1). 110

9 /3,** / : +*. p : Urban Kubota, -2.,.3,**1 : Double-Difference Tomography -, 0* +,*.,**. : CMC +1,,/0 p., +321 : KS-++* -+.* ,**3 : /3,. -/ :.- +/, km Nakamura, H. and Maeda, K. ( +30+ ) : Thermal waters and hydrothermal activities in Arima hot spring area, Hyogo Prefecture. +, ,*** :.3,*1,+0.,**+ : /+ 32 +*1.,**0 : /0 - +/.,**2 : 0+ Sakai, H. and Matsubaya, O. ( +311) : Stable isotopic studies of Japanese geothermal systems. Geothermics, /, 31 +,.. -. Sano, Y. and Wakita, K. ( +32-) : Geographic distributions of the He/ He ratios in Japan : Implications for arc tectonics and incipient magmatism. J. Geophys. Res., 3.,.031.1* : 1* ++* +, : - No. -1,*** : -,, p., Tatsumi, Y. ( +323) : Migration of fluid phases and genesis of basalt magmas. J. Geophys. Res., 3.,.031.1*1. +33/ : +20,**. : 111

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