Bulletin of the Geological Survey of Japan，vo1．38（1），p．7－12，1987 550．361 539．122 552．3．4（593－17） Pre且i醜naryst眼dyonheatgenerationfromthegmniticrocks innorthemThailand Kiyoo KAwADA＊，Masakatsu SAsADA＊＊and Hiroshi KANAYA＊＊＊ KAwADA，K．，SAsADA，M．and KANAYA，H．（1987） Preliminary study on heat genera－ tion from the granitic rocks in northem Thailand。B％JJ．G60」．S％7∂．∫の伽，voL38 （1），P．7－12． Abstract＝Radioactive elements，Th，U and K are abundant in the Mesozoic granitic rocks of northern Thailand．The heat generation in the eastern granite chain includ－ ing San Kamphaeng，Mae Chan and Fang geothermal fields ranges from10，0×10－13 to16．4×10－13cal／cm3・s．This corresponds to N－S trending high heat flow anomaly greater than1．5HFU．The granitic rocks could make a major contribution to the heat flow from this area．The situation of heat generation is more complicated in the central crystalline massif where Mae Chaem and Pa Pae hot springs＆re located，be－ cause of the heterogeneity of the contents of the radioactive elements in the granitic and metamorphic rocks． from the five major geothermal He1（1s，that is， 1．亙睡r翻uct置on San Kamphaeng，Mae Chan，Fang，Pa Pae and Mae Chaem（Fig．1）．Radioactive miner－ Many hot springs are present in northem als of zircon，monazite and allanite have been Thailand，where Mesozoic granitic rocks are identi丘ed under the microscope． widely distributed．Some of the hot springs Hot sp：rings are located in the Paleozoic are aSsociated with the margin of the gra－ sedimentary and volcanic rocks at San nitic plutons（BARR et a1．，1979），an（1some of Kamphaeng． Granitic rocks a．re distributed them appeartobe correlative with the fracture widely on the east of the manifestation area． system in the granitic region （SAsADA et aL， They consist of biotite granite and muscovite this issue）．The granitic rocks in Thailand biotite granite，both of which are character－ are generally abundant in radioactive ele－ ized by phenocrystic potassium feldspar．Pris－ ments（↑HIENPRAsERT et al．，1980；IsmHARA matic zircon and granular monazite make a and MocHIzuKI，19801WAπANANIKoRN et a1．， Pleochroic halo up to O．02mm wide in biotite。 1981）．In order to estimate the heat source of Some zircon crystals are zonally arranged the hot springs，we preliminarily determined in biotite． the heat generation from the granitic rocks Mae Chan hot spring is located on the in and aroun（1some geothermal fields in welljointed porphyritic homblende biotite no耽hern Thaiぬnd．． granite，which is altered near the hot water discharge zone．There are three hot springs 2．Sam夢且es along Huai Pong Nam River20kilometers southwest of Mae Chan．Homblende biotite Twenty eight granitic rocks and seven granite is distributed both sides of the river． metamorphic rocks were collected mainly Euhedr＆1allanite and prismatic zircon make a pleochroic halo，when they are in con－ ＊Geology Department，Geological Suryey of tact with biotite in the porphyritic granite． Japan，presently at DRICO Co． Granitic rocks are partly mylonitized in ＊＊Geothermal Research Department，Geological Survey of Japan Fang geotherma1且eld．Metamorphic rocks ＊＊＊Geophysics Department，Geological Survey of are exposed to the north of the manifestation Japan area．All the granitic rocks from the three 7 □胴 B％〃e枷oμh6G60Jo9づ6αIS％7吻oμの礁70乙38，No．1 ヒ 980 99。 1000 『 I l Legend ．へ ／ 監・臨・…㎞ 藤論， 20。一 欝謁鰻㌧、，ノ，、、鰍購蕊 ズム重♪聴驚！ll ノ ズロロ ドの 女 （ノ 魚翻1醤llノ 》 ．ノ 19。一 ／ 。＋＋＋ 輪猛』轍 曳 ，＋ll，s鷺嚇灘 k 十 、 十 ノ 十 ／o｝ ■ 十 ’》 ．熔 ＼ 十 ～．ノヘ． 十 十 十 ぐ 十 十 ＼ 十 十 ＼ o l 十 1 ＼’ 182一一 ←＿ 十 グ ） 1の 十 ・ト ピ び ＼ ＼ O 50 lOO km ＼． ＼． ＼ Fig．1 Distribution of granitic and metamorphic rocks，high heat region少and locality of samples in northern Thailan（1． High heat flow anomaly greater than L5HFU is based on THIENPRAsERT and RAKsAsKuLwoNG （1984）． areas described above belong to the eastern biotite granite。They intrude paragneiss and granite chain of Triassic age（BRAuN et a1．， other metamorphic rocks concordantly．A 1976）。 predominant radioactive mineral is zircon． Pa Pae geothermal field is located at the But its content is variable especially in the margin of the granitic，pluton on the northem metamorphic rocks． e耳d of the central crystalline complex．（Fig．1）． Graniticrocksconsistofwellfoliatedporphyrit－ 3．A髄＆亘yt互ea亘騨oced1澱e歪or T臨，U a翻K ic biotite granite and minor fine－grained granite． About two kilograms of granitic and meta－ Mae Chaem hot spring is situated at the morphic rocks were crushed under40－50 middle part of the central crystalline complex meshes，and500grams of them were packed composed of foliated granitic rocks and Pre－ into cylindrical polypropylene containers． cambrian metamorphic rocks．Most granitic Each container was store（1at least two weeks rocks near the hot spring are well foliated before counting in order to attain a complete 8 ∬顔9脚7伽％〃0〃zg名αn批70C々S（K側痂伽ム） Th，ppm 診 100 り 1’ ． ミ 7 β 建 O Q Q ヤ ⑱ Q 0 0 慰 0 0 50 ③ ⑱卜 ⑱ O O㊥ ○ ○ ） ○ O O o ＠ o O 勝OO O ⑱ Q O Q O O ○ ⑱ Q O O 50 U，ppm Fig．2 Uranium versus thorium contents． Solid circle：granitic r㏄ks fr㎝αthe eastem granite chainl solid diamond：those from the central crystalline complex；solid triangle：metamorphic r㏄ks from northem Thailandl open circle： granitic r㏄ksズrom the Peninsular Thailand （IsHIHARA and MocHIzuKI，1980）． equilibrium of radon－222． Determination of Th，U and K were mainly 4．A黙亘yt且ca亘res翻ts performe（1by7－spectrometry on1000chamel analyzer with5×5in．，Nal（T1）crystal detec－ Results of the analysis for radioactive ele－ tor and partially on4000channel analyzer with ments are listed on Table 1．Generally coaxial Ge detector comparing the counts the granitic rocks in northem Thailand are with those of the standard samples．New abundant in Th，U and K．The variation range Brunswick Laboratory（N．B．L．）analyzed ores of Th and U of the granitic rocks from the were used as calibration standards to deter． central crystalline complex is larger than that mine the contents of Th an（i U in these from the eastem granite chain（Fig．2）．The rocks．Potassium bromide grading superpure Th／U ratio ranges from3to5in most gra－ reagent was used for determination of potas－ nitic rocks．The granitic rocks from northem sium content in rocks． Thailand generally show higher Th／U ratio 9 「 B％〃e枷o方h6GooJo9勿αJS％7∂6ッo∬ψ伽．yo乙38，No．1 旨 ヒ Table1 Contents of radioactive elements in the granitic and metamorphic roks of northem l Thailan（1 Sample Nos． Loc．Nos． ThUTh／UK Rock l （ppm） （ppm） （％） （Granitic rocks） SAN KAMPHAENG KK－83121801 1 35，4 9．6 3．7 4．06 Porphyritic biot重te granite KK－83121802 1 33，5 10．3 3．3 4．26 Porphyritic biotite granite MS－82Y2401 1 35．6 9，5 3．7 4．23 Porphyritic biotite granite l MS－82Y2402 1 33．2 12．3 2．8 4．28 Porphyritic biotite gr＆n壷te MS－82Z O301 1 41．8 14．7 2，8 4．64 Porphyritic biotite granite 1T－SK－1 1 34．9 9．3 3．8 3．66 Porphyritic biotite granite 1T－SK－5 1 34．0 10．8 3．1 4．14 Porphyrit孟c biotite granite KK－SK－3 2 22．0 25．2 0。9 4．38 Muscovite biotite granite KK－SK－3B 2 33．1 8。9 3．7 3．56 Porphyritic biotite granite KK－CM2 2 34．4 7．7 4．4 3．68 P・rphyhticbi・titegranit亀 MS－82Y2307 2 31．8 9．1 3．5 3．63 Porphyritic biotite granite Average（n＝11） 33、6 11．6 2，9 4．05 MAE CHAN KK－MCN－2 3 46．9 12．6 3，7 3．40 Porphyritic hornblen（le biotite granite MS－82ZO101 3 37．4 8，8 4。2 4．62 Porphyritic hornblen（ie b重otite granite（altered） MS－82ZOIO2 3 44．7 11．0 4．0 3．88 Porphyritic hornblen（ie biotite granite MS－82Y3001 4 58．1 16．3 3，6 4，09 Hornblen（1e biotite granite Average（n＝4） 46．8 12。2 3．8 4．00 FANG KK－FANG－2 27．6 6．5 4．2 3．30 Mylonite MS－82Y2501 15．1 1L4 1。3 4．31 Mylonite 1T－FANG－1 19．9 6．5 3．1 3．60 Foliated biotite granite 1T－FANG－4 32。4 8。9 3．6 3．67 Biot至te granite Average（n＝4） 23、8 8．3 2．9 3．72 PA PAE KK－PP－1 6 35．7 7．0 5．1 2．45 Foliated muscovite－bg biotite tonalite MS－82Y2703A 6 35．8 8，0 4、5 3．08 Fo1孟ated biotite granite Average（n＝2） 35．8 7．5 4．8 2．77 DOI SUTHEP KK－83122101 7 10．4 11．5 0，9 3．82 Gamet－bg muscovite biotite granite DOI INTHANON MS－82ZO701 8 34．6 10．6 3，3 4．81 Muscovite biotite granite MAE CHAEM KK－HOD－1 10 9．4 7．9 1．2 2．44 Foliated biotite granodiorit£ MS－82Y2601 10 87．7 12．5 7．0 4．76 Foliate（i biotite granite 1T－MCM－1 10 49．2 13．1 3，8 4．82 F・liatedbi・titegrani㌻e KK－TP－2 11 3．8 3．9 1．0 1。31 Biotite muscovite granodiorite MS－82Y2602A 11 26．5 5．9 4．5 3。25 Fohated biotite granodiorite Average（n＝一5） 35．3 8．7 4．0 3．32 （Metamorphic rocks） KK－FANG－1 5 17．5 3．3 5。3 2．14 Orthogneiss MS－82Y2702C 6 26。4 10．4 2．5 2．93 ParagneiSS KK－83122102 7 16．7 11．8 1．4 2．76 Paragneiss KK－83122501 9 46，6 12．4 3．8 3．57 Paragneiss KK－83122502 9 29．8 6．8 4．4 3．82 Paragneiss KK－MCM－2 10 120．0 12、1 9．9 4．46 Paragneiss KK－HOD－2 10 18．5 3．4 5．4 1．66 Calcareousgneiss Average（n＝7） 39．4 8．6 4。6 3．05 ∬6α渉96銘07αあo％〃o刀¢8壁7η％ゑあc700々s（Kα卿α4α（3オα1．） than those from the Peninsular Thailand sHIMA for giving us several granite samples （Fig．2）． for analysis，and Mr，0．MATsuBAYAsHI for reviewing the eaτ1y vgrsion of the manuscript。 5．Heatge聾er厩ion Re£ere恥ces The average of the heat generation of28 granite samples is calculated to be13，0×10－13 BARR S．M．，RATANAsTHIEN，B．，BREEN，D．，RAMING－ cal／cm3・s using conversion factors of Roy et woNG，T．and SERTsRIvAMT，S．（1979）Hot a1．（1968）．The heat generation of the gra－ spring and geothermal gradients in nitic rocks from the Peninsular Thailand is northern Thailand。G60渉h67郷づos，vol．8，p． 16．6×10－13cal／cm3・s on an average（lsHIHARA 85－95． and MocHlzuKI，1981）．These values from BRAuN，E．von，BEsANG，C．，EBERLE，W。，HARRE． Thailand are higher than the average heat W．，KREuzER，H。，LEz，H．，MuLLER，P． generation of granite due・to radioactive（iecay， and WENDT，1．（1976）Radiometric age de． 7．0×10－13cal／cm3・s （KAPPELMEYER an（1 termination of graintes in the northem HAENEL，1974）。 Thailand．G60」．乃．，B21，p．171－204．