Weathering on Granitic Rocks in Imagane, Hokkaido

Weathering on granitic rocks in Imagane, Hokkaido

著者 SHIRAHATA Hiroshi, SUZUKI Tetsuo journal or Memoirs of the Muroran Institute of publication title Technology. Science and engineering volume 9 number 1 page range 255-267 year 1976-12-18 URL http://hdl.handle.net/10258/3649 Weathering on granitic rocks in Imagane ，Hokkaido

日iroshi Shirahata and Tetsuo Suzuki

Abstract Abstract

W 巴athering effects on granitic rocks have been investigated. Relatively small acid plutonic bodies ， which are are distributed in Imagane ，a district in southwestern Hokkaido ，are hornblende biotite granodiorites ， which are consid 巴red to be formed in the Cretaceous Field time. observations have disclosed that the weathering feature and and processes of the granodiorites have been affected markedly by wate r. Especially ，Masa was frequently found found around the junctions of water streams. The degre 巴s of weathering on a profile of a rock are di vided into five five macroscopic classes : fresh ，slight weathering ， moderate weathering ， high weathering and Masa. Follow ing ing the field observ 且tions ， the mineralogical and geochernical changes d巴veloped on weath 巴ring profiles were investigated. investigated. Th 巴 relative rnineral stabilities ， from the least stable to the most ， are : plagioclase ，biotite and hornblend 巴， K-feldspar and quartz as expected. A gradual depletion in calsium and sodium ，steep depletion in magnesium magnesium and dispersive variation in pot ぉ sium during weathering are presented di 且grammatically. A new weathering weathering index ， mole (CaO 十 Na 20 + K 20)/mole Al ，03 ，is proposed. It is a good indicator of chemical weathering weathering just as Reiche's WP l. Both indices ，moreover ， were directly related with bulk density of the granodi granodi ori tes studi ed

Introduction

The granitic rocks distributed in particular parts of southwestern Hokkaido have been studied studied petrographically and petrochemically by Suzuki and Nemoto (1935) ，Suzuki (1957) ， Sato and Shirahata (1965) ，and Tonosaki (1967). However ， unfortunately ， no systematic investigations investigations on the weathering of them have been promoted. Landslides Landslides triggered by heavy rainfalls have occurred frequently in weathered granitic regions regions (Koide ， 1968a) : for instance ， in July 1964 the eastern part of Shimane Prefecture was damaged heavily by numerous landslides. It is worthy to note that these areas are granitic regions regions consisted mainly of rocks which have been completely or almost transformed tu Masa (Koide ，1968b ; Khono et al. ， 1968). Rock weathering must be one of the essential factors in landslide landslide disasters ，and also weathering reactions would be considered to reflect the changes of of chemical and mineralogical compositions and the changes in physical properties. Therefore studies studies on the weathering of granitic rocks may be of importance to the geologists as well as to to the civil engineers Geochemical and mineralogical researches of weathering on granitic rocks in the southwestern part of Hokkaido were started several years ago. Some mineralogical alterations and chemical variations during weathering processes in the granodiorites in Imagane district are presented presented in the following sections.

Outline of geology

Imagane district situated on the eastern part of Setana-gun ， roughly central area of

(255) (255) 256 Hiroshi Shirahata and Tetsuo Suzuki

Kb. Kunnui Form DlEETmnCEDep Lp. 区司医ru The The numbers show St. 霊童 Setana Form Liparite Hd. 1:::::::1 Palaeozoic F the the points of sampl 巴 Hf. Hf. Hornfels ow.zzz pzratsuna An 医23 ~ collection Andesit 巴 Gr. 区司 G 肌 tic rock Wk. 皿阻 Yakumo Form

Fig.l Fig.l Geological map of Imagane district

southwestern southwestern Hokkaido ， is noted of the granitic intrusions. These granitic rocks there are are the basements of Neogene Tertiary systems ， which are divided into Kun-nui ， Yakumo ，Kuromatsunai and Setana formations in ascending order (Matsui et al. ，1955). Three principal principal granitic masses are located in northwestern part of Imagane : the northern side of Imagane ， the upper part of the Toshibetsu river and a part between them ， as shown in a geological geological map (Fig.1). The granitic masses intrude discordantly into the pre-Tertiary

(256) (256) Weatheriog on granitic rocks in Imagane ，Hokkaido 257

Plate Plate A

Outcrop showing the granitic rocks from

fr 巴sh to weathered par t.

The clayey zone found in the fresh host host rock.

Plate Plate B

Outcrop showing Masa zone.

The larg 巴 round 巴d blocks ar 巴 the cor 巴stones escaped weathering.

(257) (257) 258 258 Hiroshi Shirahata and Tetsuo Suzuki

formations formations that are composed of dominant black slate and gray coloured greywacke accompanied with chert and limestone ，and are also covered unconformably with Neogene deposits. deposits. Pre-Tertiary formations have been considered as the Palaeozoic (Matsui et al. ， 1955). 1955). They have been widely affected by metamorphism and contact aureoles in particular are are observed around the granitic bodies. The typicallocation of the contact aureole was found at at the granitic body of the northern side of Imagane. Lithology of each plutonic rock shows a distinct distinct difference in mineralogical and petrochemical compositions ， and besides that essential essential mineral grains in marginal parts of every mass become fine r. Since the report of Kawano and Ueda (1966) ， acid plutonic activity in Imagane district has been considered to be an event in the late stage of the lower Cretaceous

Field Field description and collection

In In order to study the weathering of granitic rocks ， as given in a map ， the samples were collected collected from twelve weathering profiles ， in each of which there appeared clearly a series of weathering weathering degree from fresh to Masa. These profiles observed on the outcrops would certainly certainly be indicated in situ alteration. Samplings were made at roadcuts and recent excavations ， and also fresh and weathered rocks were collected according to the following scale scale of weathering 1 ) Fresh ; A rock specimen shows no macroscopic change in constituent rninerals and and stands against hittings with a hamme r. 2) Slight weathering ;The rock still keeps its solidity ， but a few reddish brown spots are are found on its surface 3) Moderate weathering ; The rock is easily broken by 4 or 5 hittings with a rock hamme r. It contains oxidized iron and plagioclase ，a major mineral in granitic rocks ， becomes tarnished. 4 ) High weathering ;The rock's surface is nearly covered with iron rust ， and appears porous. porous. The rock can be broken by only one hammering . 5 ) Masa ; The rock can be broken apart with hands but the original granite fabric remains ，which suggests that it is in situ weathering Although ， roughly speaking ， the degree of weathering increases gradually from fresh to Masa ，a remarkable feature of the weathering degrees is that they do not always occur in a regular regular orde r. Photoplate A. shows a discordant weathering appearance . It appeared that a gritty gritty clay zone in the middle of the picture is the result of violent weathering along a joint or or fracture in the fresh granitic bloc k. The boundary between unweathered host rock and clayey clayey zone is quite sharp ， in spite of the presence of watery zone. It is observed generally that that the granitic rocks around the junctions of water streams are altered strongly ，and frequently frequently becomes to Masa. This most weathered part ，Masa zone ，consists mainly of crossed crossed clay bands of several centimeters in thickness and friable sands of quartz ，feldspars and and mica in between them the former represent joints and fractures in the pas t. Com paratively paratively fresh corestones are found frequently in these Masa (Photo B .). The weathering feature feature indicate that the original structure has been well preserved in Masa.

(258 ) Weathering on granitic rocks in !magane. Hokkaido 259

Tab!e 1 Chemical analyses of frseh and weathered rocks

75062208 75062208 75062212 75062210 75062213 75062211 75062101 75062105

Si0 2 65.89 65.54 65 . 90 65.21 68.00 6 1. 74 62.43 Ti0 2 0.66 0.63 0.62 0.60 0.45 0 . 77 0.73 A l， 03 13 . 97 15.25 14.75 15.32 14 . 02 16.18 15.88 Fe 2 03 1. 32 1. 36 1. 44 1. 73 2.87 1. 26 1. 56 FeO 3.85 3.42 3.33 3.02 0.82 4.77 4.44 MnO 0.11 0.11 0.10 0.10 0.11 0 . 11 0.11 MgO 2.38 2.02 2.06 2 . 01 1. 76 3.04 3.00 CaO 4.35 4.09 4.04 3.66 2.56 5 .34 5.27 Na 2 0 3.05 2.80 2.63 2.35 2.07 2.65 2.66 K 2 0 2.90 3.18 3.58 3.41 2. 56 2.76 2.35 P 2 0. 0.37 0.16 0.21 0.11 0.13 0.12 0.17 H 2 0+ 1. 18 1. 25 1. 03 1. 52 2.19 1. 27 1. 31 H 2 0 0.18 0.29 0.28 0 . 64 2.62 0.23 0.38 Total Total 100 . 21 100.10 99.97 99.68 100.16 100.24 100.29 II II III 75062102 75062102 75062103 75062104 75062119 75062122 75062123 75062120 75062121 61. 92 61. 80 61. 10 68 .80 69 . 25 69.53 69.54 70.02 0.76 0.76 0.69 0.70 0.36 0. 32 0.30 0.30 0.28 15.53 15.53 15.75 15.26 15.23 15.10 15.25 15 . 14 15.11 1. 60 2.63 3.66 0 . 49 0.84 1. 56 2.4 5 2.03 4.66 4.66 3.55 2.47 2. 85 2.55 1. 62 0.86 0.76 0.11 0.11 0.10 0.08 0.10 0.09 0.07 0.05 0.05 3.14 3.14 3.07 2.97 1. 13 1. 10 0 . 96 0 . 85 0.79 4.99 4.99 4.60 3.88 3 .19 3.13 2.62 2. 34 2.33 2.58 2.58 2.45 2.16 3.18 3.20 3.10 3.00 2.99 2.90 2.90 2.60 2.58 3.76 3.39 3.48 3. 93 3.78 0.14 0.14 0.11 0.12 0.09 0.09 0.09 0.08 0.08 1. 42 1. 83 2.62 0.87 0 .84 1. 15 1. 19 1. 21 0.40 0.40 0.82 1. 73 0.15 0.21 0.34 0.46 0.59 100 . 15 100 .00 99.3 3 100.20 100.11 100.07 100 . 19 100.02

1 : Hornblende biotite granodiorite 5.4km northern part of Tanekawa 75062208 fresh 75062212 75062212 slight weathering 75062210 moderate weathering 75062213 75062213 high weathering 75062211 75062211 Masa II II : Hornblende biotite granodiorite 3.2km northern part of Pirika (marginal (marginal part of the granitic mass) 75062101 75062101 fresh 75062105 75062105 slight weathering 75062102 75062102 moderate weathering 75062103 75062103 high weathering 75062104 75062104 Masa m :Hornblende biotite granodiorite 3.7km northern part of Pirika 75062119 75062119 fre sh 75062122 75062122 slight weathering 75062123 75062123 moderate weathering 75062120 high weathering 75062121 75062121 Masa

(259 ) 260 260 Hiroshi Shirahata 且nd Tetsuo Suzuki

Petrography 、

In In general ，granitic bodies in Imagane district are light gray ，medium-grained hornblende biotite biotite granodiorites. Their texture are hypidiomorphic granula r. In a thin section of the fresh sample sample plagioclase is subhedral ， usually polysyntheticaliy twinned and at the same time weakly zoned crystals ， while potash feldspars are wholly allotridiomorphic and fill the inter spaces spaces between the surrounding minerals ; there is only a trace of kaoline in potash feldspar and also a small amount of sericite is found here and there in plagioclase. The amount of hornblende hornblende contained in granodiorites found along the Toshibetsu river ， the northern side of Pirika ，is slightly smaller than those in granitic bodies of other parts of Imagane. Particularly ， this this modal composition in a sample from Chaya-gawa is very (Sato small and Shirahata ，1965). The biotite ，a major ferromagnesian mineral present ，is of a subhedral ，sometimes idiomorphic slender slender shape. Even in a sample of relatively fresh rock ，it is often altered to the chlorite along cleavage cleavage planes and fractures ，and is somewhat stained with iron oxides. In the early phases of of weathering sequence ，sericitization and kaolinization occur on some parts of the plagioclase surface. surface. These reactions take place initially around calcic cores ， cleavages and cracks. A little amount of kaoline is observed on K-feldspar surface and a slightly larger amount of chlorite is produced produced on the surfaces of both hornblende and biotite. The next stage of alteration is characterized characterized by plagioclase with prominent sericite and kaoline ， potash feldspar showing gradational gradational enrichment of kaoline ，and ferromagnesians with a considerable amount of chlorite chlorite and iron oxides. In the late and final stages of weathering ， almost all plagioclase is transformed transformed to aggregations dominantly of kaoline clay and micaceous minerals. A great deal of of kaoline clay and some micaceous minerals are found in K-feldspa r. Hornblende and biotite are are converted nearly perfectly to chlorite and other clay minerals ，therefore they sometimes show pseudomorph and/or completely irregular appearance in shapes under the microscope. lron-oxide lron-oxide network or veinlet derived from the dissolution of mafic minerals are spread on the the highly weathered rock surfaces. On the other hand ，quartz ， being the most resistant mineral ， has no considerable changes during weathering ， though in the final weathering phase it it becomes smaller and fragmental crysta l. A trace of calcite was developed in the thin section ion of the granodiorite from the llpper side of the Toshibetsu rive r. It is not clear whether this this is a product of the weathering or the hydrothermal alteration. Relative mineral stabili ties ties in granodiorite found on the basis of microscopic observations ，from least to most stable ， are are :plagioclase ， biotite and hornblende ， potash feldspar ，and qllartz. This stability order agrees agrees well with the previous investigations by Harris and Adams (1966) ，and Jin-nai (1973).

Chemical variation

Sample of various weathering stages ， 1-2Kg each ，were crushed to less than 60 mesh in size ，and a quarter of each was grinded finely in a agate motor a~d analyzed chemically by the ordinary ordinary wet method except for calsium ，magnesium ，sodium and potassium (Shirahata ， 1972). 1972). Alkaline and alkaline earth metals were determined by the atomic absorption spectrophotometry photometry (Terashima ， 1970) using a spectrophotometer Hitachi model 508. The results are

(260) (260) We 丘thering on granitic rocks in Imagane ， Hokkaido 261

400 ト

300 300 300~

200 ト 200

0 。 75062208 75062208 75062212 75062210 75062213 75062211 汚O位町1 75062105 7506 目02 750621ω 汚 06 到04 fresh fresh slighl mod le high Mosa fresh sli gi'沈 mo 事問le. tictl M国 α 軒。 weαIh 町 ing wealhering we α甘官曲、 g weathering weathering weα.Ih 町 I"g we 叫 hering Fig.2 Fig.2 Chemical variations during weathering Fig.3 Chemical variations during weath 巴nng

o.勺斗・一一一一一一一 1:8i 2 2:TiO" -'~~-2 -司自ーーーーラ-----"~~2-3:Al~O_ _・_._ -'---'-~''''''''2v3:Fe"Q 一一 --1:810 2 …一一一 2Ti0 2 一一一ー -3:A1 2U ラ一一 -4:Fe 2uラ山ラ:FeO _.._.._.._..6: Mn O -----7:MgO ____8:CaO ラ:FeO 一一一一 6: Mn O -----7:MgQ ----_8:0 a. 0 U 一一一 -9~Na ..， O -一一 -LO: K..:， O 一一一一一-ll:P...O~ -__同・供 12:H"O+ 一一 -..9:Na 20 一一一 -.lOd'2 0 一一一 -1l :P2 5 一一・ 12:H 20+ 2" ~-"''''''''2 ラ H 0 ー---13:H 0- ー・ー『町・ 13: 2 - 2 listed listed in Table l. From the table ，it is seen that the ferric iron and water contents of every mass enhance with increasing weathering ，whereas the amounts of alkaline and alkaline earth elements reduce especially ， decreases in lime and soda are clearly observed. When the individual individual granodiorite have been weathered from fresh to Masa ， the chemical variations in them are given diagramatically in Figs. 2 to 4. Assuming that the aluminum content has remained constant throughout weathering (Harrison ， 1934 ; Goldish ，1938 ; Brewer ， 1955 ; and Nam and Taneda ，1974) ， these variations were calculated in the following way. When the contents contents of the individual elements in the each fresh rock are set to be 100 ， their relative variations ，gain or loss ， in each weathering stages have been computed and then these con centrations centrations except A1 20 3 were multiplied by the following correction factor :

Al 20 3 in fresh rock

Al 20 3 in any weathered roc k. As shown in figures CaO ，Na 20 and FeO in every profile tend to decrease with increas ing ing weathering ，while H 20 (十)， H 20 (一) and Fe 203 tend to increase with weathering. No silica silica content in each figures shows considerable change. The rocks analyzed show little or slight slight enhancement in potassium content during most stages ，but the potassium reduces sometimes in the final stage ，Masa. There are no remarkable change in the manganese and phosphorus contents ，though slight reduction in the titanium and magnesium are observed in

(261) (261) 262 262 Hiroshi Shirahata and Tetsuo Suzuki all all analyzed suites. Reiche (1943) devised a weathering potential index (WP I) which denotes degrees of chemical chemical weathering. This index is defined as 100mole (K 20 十 Na 20 十 CaO + MgO - H 20) WPI = mole (Si0 2 + Al 20 3 + Fe 203 十 CaO + MgO + Na 20 + K 20) The values of WPI are generally reduced as weathering increases. The relationship between the the WPI and the individual elements in a series of weathering rocks in Imagane are given in Figs. Figs. 5 to 9. The total H 20 ，Na20 ，CaO ，MgO and FeO contents in three weathering suites depend almost directly on the values of WPI ，whereas K 20 for all suites are scattered on the diagram These chemical characteristics have been noticed not only in the granitic rocks from Imagane district but in those from Okushiri island (Shirahata ，1975). Jin-nai and Mukaiyama (1973) (1973) asserted that there was a close relation between the total H 20 and WPI for granitic rocks rocks from Iizuka ，Kyuragi and Yasu in the northern part of Kyushu ，and they also pointed out the the significant correlations for CaO and Na 2 0. Weathering process for granitic rocks is a history of the dissolution and alteration in their constituent constituent minerals and ，consequently ， chemical changes during weathering would be related related closely to mineral breakdown. From the result of microscopic observations the most unstable unstable mineral for the weathering was plagioclase. In the incipient stage ，plagioclase con- 旬 irill-- 同〆一一=一一一一九//一一一一一一一唱/一白/日}一一一一一一早//一三一一一寸 /JT//J/ 400

/F WPI ////一・ノ一レ/十斗辛キ一.///'一 15 300 dj.//rJ//////F ¥・・、¥1丸、・司ロ¥、 10 10 ¥ k¥¥可.、¥↑ /////二日可 ¥!¥¥一、 LU¥ ・、¥¥、一 5 ¥斗 ¥F¥、¥一¥、 1世 H 0 //一。 2 マミふ¥ 2 A 、失¥6 吋ロ同州 ¥J J/ J 一三¥ ，-宮一一~九晶、円『ノ一/一司四 5

~F ι E ヨ一ゴ咽 Fh-¥z-2~h 一一一二一 t唱苧一?、¥々乞ぐ〈、 U 2MW6 -10 品 i l 中

←一一ー--1 5 -15 0 7506 封筒汚O段官2 75062123 汚 062 世O 汚 06 割引 fresl 、 51 ゆ>1 m 。品開 te hi 昔、 M 倍。 W 四 thering weatheriig W' 酎謝，.". ing Fig.5 Relation between WPI and Total H 20 Fig.4 Fig.4 Chemical variations during weathering Solid Solid circle ; 3.7km northern part of Pirika. --1:510 ------2: 目 O -3:Al ..， O 4::&'e ..， 0 一 2 2 一一一 2 ラ一一一・・ 2"3 -一一日 -5:FeO ー戸- 6: Mn O -一一一一 ?:MgO 一一一 8:CaO Opened circle ; 3.2km northern part of Pirika. _..-..-9:Na':)O _..-..-9:Na':)O _._._._lO;~O 一一一一ll:P...O 申ーーー -12:H...O~ト品 2" ~~"''''''''2v5 一一一日 :H 20- Square 5; .4 km northern part of Tanekawa

(262) (262) LへTeathering on granitic rocks in Imagane ， Hokkaido 263

WPI WPI

15 15 / 15 ///P / / oei 10 10 /ぷ /g 10 コ ‘.・・ 7 /1 〆戸町// 5 σ 5 / /プ / / / CaO ・'1. Na ρ。l。。。 M/// 〆T一~ι ，，，，， 6 4 ，，，，，定-w ，，，，， ED /白唱 J //ιv Jbv -10 -10 -10

ー15 ー15

Fig.6 Relation betw 巴en WPI and CaO Fig.7 Relation between WPI and Na 20 The symboles are the same as Fig. 5 The symboles are the same as Fig. 5

WPI WPI

15 15 ll+ / 15 Illid-- / /ゐ //，/ 10 10 /n c:fr 、 10 10 ロ lip--∞ ノィγ:/ ハ

5 ・/， J/YO/ ・r ロ / 5 J jl / ・r fe'- / / ・ rJ' Fe- O ・'1. 。ムチ → MgO "io 4 6 。 //正〆 E3 / 4 ;' "どい〆/ -5 自 10 〆ー10 ー15 ー15

Fig.8 Relation between WPI and FeO The symboles are the same as Fig.5 Fig.9 Relation b巴tween WPI 旦nd MgO

The symbol 巴s are the same as Fig. 5

(263) (263) 264 264 日iroshi Shirahata and Tetsuo Suzuki

WPI

15 15 。 10 トロ 10 Jd Jd pdb

5十 5 '1:!・ Zず mole(CaO+Na20+K20) . 。 moleA l203 。 Bulk density 0.5 0.5 /七1- 0 15 20 2. 2 24 26 2.8 10

担 5卜 / -5 / 白ー10 トヌL -10

ー15~ ー15

町10le (CaO +Na ，O 十K ，O) Fig.ll Relation between WPI and Bulk Fig.l0 Fig.l0 Relation between WPI and mole mole Al ，0 3 densi ty The symboles are the same The symboles are th 巴 sam 巴 as Fig. 5 as Fig. 5

tained tained sericite. On the contrary ， prominent mole(Caa+Na20+K20) kaoline kaoline clay is found on the plagioclase surface mol 崎 Al203 in in the late stage of weathering. K- feldspar is gradually gradually replaced by kaoline as weathering 1. 5 proceeds. proceeds. Those facts must c; orrespond directly directly to depletions of calsium and sodium ，

two main constituents of plagioclase. Ca los O' ロ process process would be given by the following equa 1.0 o ~ tion tion (Grant ， 1963)

2CaA1 2Si 20 s + 4C0 2 + 6H 20 一→Al.Si.0 1o (OH)s + 2Ca (HC0 3)2 A study by Kitano et al. (1967) supports the 0.51 臥Jlkdg 渇 ity fact fact that calsium ions have been carried away 20 22 24 26 2B 10

in in water in the form of bicarbonate. They also Fig.12 Relation between __rIl ole (CaO+Na 2 0 十 K ，O) mole mole Al ， 03 suggested suggested that values Ca 2 +(actually ，Ca 2 + 十 and Bulk density Mg 2 +) and HCO~- in water can be utilized as The symboles are the same as Fig. 5 indicators indicators of rock weathering. Sodium would be resolved the solution ， as shown in the following following reactions (Thompson ，1972).

2NaA1Si 30 s 十 2日+sozn 十 9H20S01n. ニ A1 2 Si 20 5 (OH). 十 2Na+ soln. 十 4Si (OH) 4S oln ・ albite albite kaolinite On the other hand ，it was observed that there was an irregular variation in potassium which is is a mobile cation in K-feldspa r. This appears to be connected with sericitization in the feld

(264) (264) w 巴athering on granitic rocks in Imagane ，Hokkaido 265

Table Table 2 Chemical weathering indices and physical properties 。 f fresh and weathered rocks from Imagane WPI Mole ratio Bulk density Shore hardness 75062208 9.68 1. 15 2.72 83.0 75062212 8.03 1. 02 2.68 77 .1 75062210 9.00 1. 05 2.69 74.4 75062213 4.83 0.93 2.54 45.5 75062211 -8.15 。.77 75062101 11.10 1. 05 2.68 75.1 75062105 9.89 1. 04 2.72 6 1. 5 75062102 9.65 1. 06 2.73 74.7 75062103 5.49 0.97 2.51 20.8 75062104 2.61 0.88 75062119 8.11 0.99 2.69 80.7 75062122 7.61 0.97 2.66 84.3 75062123 5.07 0.89 2.59 67.2 75062120 4.16 0.89 2.38 24.2 75062121 3.36 0.87 spars. spars. A change in magnesium content would mainly depend upon the degree of the chloriti zation zation in hornblende and biotite. Because some of ferrous iron coming out of mafic minerals is fixed ，it follows that the loss of iron during weathering in each profiles is not always definite.

Miyashiro Miyashiro and Kushiro (1 975) asserted that ratio mole (CaO + Na 20 + K 20)/moleA1 20 3 computed from chemical compositions in igneous rocks was characterized essentially by their mineral mineral assembleges. Especially ， in the fresh acid plutonic rocks the ratio should be almost 1， and then ， corresponding to the transition from fresh rock to Masa ， the index should be reduced reduced to zero. Because ， as mentioned above ， calsium and sodium are released and washed away by water during weathering and also potassium is carried away in the final stage. Calculated Calculated mole ratios for a series of weathering rocks are tabulated in Table 2， together with their their WPI ， bulk densities and Shore.hardnesses ，and relations between them are presented in Figs. Figs. 10 to 12. Thus ratio this is shown to be a reasonable index as a measure of weathering degree degree

Summary

In In order to study the weathering of granitic rocks ， acid plutonic masses in Imagane were selected. selected. Field observations support that the of role water in weathering is to regulate the weathering weathering degree and feature. Mineralogical changes during weathering in hornblende biotite biotite granodiorites were almost the same microscopically as in those in other humid c1 imates (Harris (Harris and Adams ， 1966 ; Jin.nai ， 1973). Relative mineral stabilities ，from the least stable to the the most ，were plagio c1 ase ，biotite and hornblende ， potash feldspar and quartz. Due to alteration alteration of constituent minerals ， chemical variations take place in each weathering suite Assuming that alumina is constant during weathering ，ferric iron and water are enriched fai r1 y， but calsium ，sodium and ferrous iron reduce gradually. Changes in silica and potassium is is not definite. Depletions in manganese and phosphorous are not necessarily definite. It It was ascertained that Reiche's WPI was usefull as a measure of chemical weathering degree. degree. Simila r1 y the ratio mole (CaO 十 Na 20 十 K 20)/moleA1 20 3 would be regarded as a good index of weathering degree. Moreover ， both indices were directly related to bulk density of of the granodiorites studied here

(265) (265) 266 266 日iroshi Shirahata and Tetsuo Suzuki

Acknowledgments. --~ We are gratefull to D r. H. Sato for his continual encourage ments and advices. Thanks are also due to D r. ]. Sato of Hokkaido University ，whose support has helped us in the investigation. The authors wish to thank D r. M. Kitamura of Hokkaido University ， for kindly reading the manuscript (Received May. 22 ，1976)

References References Brewer ， R.， 1964 ， Fabric and mineral analysis of soils ， John Wiley & Sons ，New York Goldish ， S. S. ，1938 ，A study in rock w 巴athering ，Jour. Geology ，v. 46 ， p. 17 ~58 Grant ， H. W. ，1963 ，W 巴athering of Stone mountain granite ， Eleventh Nat. Con f. Clays and Clay minerals ，p

65~73. Harris ，R. C. and Adams ，J. A. S. ，1966 ， Geochemical and min 巴ralogical studies on the weathering of granitic

rocks ，A m. ]our. Sci. ，v. 264 ，p. 146~173 Harrison ，J. B. ，1934 ，The katamorphism of igneous rocks under humid tropical conditions ，Harpenden ， Imp. Bu r. Soi l. Sci Jin-nai ， K.， 1973 ，On the weathering process of the granites in Mikawa district ， Aichi Prefecture ， Reports of

the the Gov 巴rnment Industrial Research Institute ，Kyushu ，No. lO， p. 582~591 (in Japanese with English abstract). abstract). Jin-n 且i，K. and Mukaiyama ， H.， 1973 ，On variation of some characteristic of granitic rocks through weathering For For utilization of the weathered granitic rocks as industrial resources 一一一， Min. G 巴ology ，v. 23 ，p

411 411 ~423 (in Jap 呂nese with English abstract).

Kawano ， Y. and Ueda ，Y. ，1966 ，K 十 Ar dating on the igneous rocks in Japan (IV) -~ Grani tic rocks in north ←

eastern eastern Japan -~ Jour. Jap. Ass. Mine r. Petro l. Econ. G 巴01.， v. 56 ，p. 41~55 (in Japanese with English abstract) abstract) Khono ， Y.， Namba ，S. ，Takiguchi ， K.， Kurotori ， T.， Arimitsu ，K. and Kobayashi ，C. ，1968 ， Roles of topography ， soil soil and forest in the landslides of aw 巴且 thered granite area ， Reports of Cooperative Research for

Disaster Disaster Prevention ， No. 14 ， p. 77 ~ 112 ， Nat. Res. Cen. Dis. Prev. ，Tokyo (in J呂panese with English abstract) abstract) Kitano ，Y. ，Kato ， K.， Kanamori ，S. ，Kanamori ， N. and Yoshioka ， R.， 1964 ， Rockslides resulting from the

geochemical geochemical weath 巴ring of parent materials ，Disaster Prevention Research Institute Annuals ， No. 1OA，

p. p. 557 ~587 (in Japanese with English abstract). Koid 巴， H. ，1968a ， Landslides in granitic r巴gions based on geological classification with special reference to

dev 呂station in granitic regions ， Reports of Cooperative Research for Disaster Prevention ， No. 14 ，p. 5

22 ，Na t. R 巴s. Cen. Dis. Prev. ，Tokyo (i n Japanese).

，1968b ， Landslide disasters in mountainous regions in the Shimane prefecture ，ibid. ， No. 14 ，p. 23~ 30. 30. (in Japanese) Matsui ，K. ，Doi ，S. ，Tak 巴da ，H. ，Uozumi ，S. ，Fujie ， T.， Akib 且， T. ，Yoshimura ，N. ，Yamaguchi ，S. ，Obara ， T. and Odakiri ， T.， 1955 ，R 巴port on the surveys of geology and ore deposit in the northeastern part of Imagane ， Hokkaido ，Geological Survey of Hokkaido ，Sapporo ， Japan pp. 56 (in ]apanese) Miyashiro ，A. and Kushiro ，1.， 1975 ， Ganseki gaku 11 ，Kyo-ritsu Shippan ，Tokyo ， pp. 171 (in Japanese) Nam ，K. and Tan 巴da ，S. ，1974 ，Relative movility of major 巴l巴ments in decomposition of granitic and basaltic rocks rocks in Fukuoka. Japan ，Chigaku kenkyu ，v. 25 ， p. 257 ~ 267 (in Japanese).

Reiche ，P. ，1943 ， Graphic repres 巴ntation of chemical weathering ，Jou r. Sed. Petrology ， v. 13 ，p. 58~68 Sato ， H. and Shirahata ，H. ，1965 ， Geochemical studies of the granitic rocks from southwestern Hokkaido ，Japan ，

Jour. Jour. ]ap. Ass. Min. Pet. Econ. Geo 1.， v. 54 ， p. 14~22 (in Japanese with English abstract). Shirahata ， H.， 1972 ， Chemical analysis of the geochemical standards JG-1 and JB- l， Me m. Muroran Inst. Tech.

(266) (266) Weathering Weathering on granitic rocks in Imagane ， Hokkaido 267

Science Science and Engineering ，v. 7， p. 201-209 (in ]apanese with English abstract). Shirahata ，H. ，1975 ，Weathering on the granitic rocks in Okushiri island ， Presented at the ]oint Annual Meeting of the Mineralogical Society of ]apan ， the Society of Mi ning Geologists of ]apan and the ]apanese ]apanese Association of Mineralogists ，Petrologists and Economic Geologists ， October 1975 ，abstracts p. 69. 69. (in ]apanese). Suzuki ，].叩 d Nemoto ， T.， 1935 ，The chemical composition of the granitic rocks in ]apan ，]ou r. Fac. Sci Hokkaido Hokkaido Univ. ，Se r. IV. ，p. 1-48

Suzuki ，Y. ，1957 ，On the granitic rocks in Hokkaido ，]our. Fac. Sci. Hokkaido Univ. ，Se r. IV ，p. 519 ー 536

Terashima ，S. ，1970 ， Determination of sodium ，potassium ，magnesium ，calsium ，mang 加 ese and iron in the silicate silicate rocks and terrestrial water by atomic absorption spectrophotometry ，Bul l. Geo l. Serv. ]apan ，v 21 ， p. 693-707 (in ]apanese with English abstract). Thompson ， A. B. ，1972 ，The instability of feldspar in metamorphism ，The feldspars ， Manchester Univ. Press ， New York ， p. 645-672. Tonosaki ，Y. ，1967 ，On the chemical composition of the granitic rocks in Hokkaido ，]apan ，Professor Hidekata Shibata Shibata Memorial Volume ，p. 103-112 (in ]apanese with English abstract).

(267) (267)