Bulletin of the Geological Survey of Japan，vo1．38（1），p．13－20，1987 550．93552．323550．361（593－17）

NewK－Ar我ges倉囎the：L蹴聖我ngb麗謝，欝賄he膿丁恥ai且騰d

Masakatsu SAsADA＊，Benjavun RATANAsTHIEN＊＊ and Praphat SoPoNPoNGPIPAT＊＊＊

SAsADA，M．，RATANAsrHIEN，B．and SoPoNPoNGPIPAT，P．（1987） New K－Ar ages from the Lampang bas窺1t，northem Thailand．B％Jl．Gθo’．S％7∂．∫のαn，voL38（1），p．13－20．

Ab謝act＝Two s縦mples of the Lampang basalts were collected from the southem Mae Moh basin，northe燃Thailand．They are aphyric basalts consisting of plagioclase，01i－ vine，clinopyroxene，叩aque minerals，and interstitial glass．Chemically both are of basanite composition．Their K－Ar ages may give the lower limit of the formation age， because of the Ar loss due to the hydration．But one of the ages，0．6±0，2Ma，is pre・ sumably not greatly deviated from the formation age．This indicates that the Lampang basalt is one of the youngest igneous activity in Southeast Asia。It may be indicative of high heat potential in the crust or the upper mantle under the northem part of Thai－ land．

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Late Cenozoic basalts are wi（1ely distributed The Lampang basalt underlies the southern in Southeast Asia（Fig，1），’and occur locally part of the Mae Moh Basin（Fig．2）．It con－ in northem Tailand．Many hot springs also sists of several flows and locally pyroclastic occur in no丘hem Thailand （e，g．BARR et al．， （1ebris associated with vents （BARR，and 1979）， but the surface distribution of the MAcDoNALD，1981）． One of the samples basaltic rocks is not correlative with the hot （82ZO401）for the present study was collected springs．Neve姓heless，the basalts．are the from a pillow basalt cropping out along the only known evidence of recent igneous activi－ railroad cutting at 18。15！30”N an（i99。39！00！！ ty． E．The pillows range from O．5to l m in This paper presents K－Ar ages of two ra（1ius （Plate I－1） and display radial cracks samples from the Lampang basalt，collected and concentric zones of vesicles at the from the southem Mae Moh Basin，northem margins．Inter－pillow breccia is also present． Thailand（Fig．2）．TheauthorsthankDr．S．M． The other sample（82ZO402）was collected from BARR，Acadia University，Canada for critical a massive basalt How overlying gravel de－ reading of the manuscript，and Mr．K．Uto， posits（Plate I－2）at18。07！32”N and99。37！52” Geological SurveyofJapan for helpful sugges－ E． tion．Theyalsothank Messrs．K．TuRALTAM－ KuL，V．PAIJITPRAPAPAN an（1C．CHAPHYEN， 3． Sam欝且e岨escr亘昼》t亘o聡 Department of Mineral Resources Thailand for their aid in昼eld菖tu（1ies． The texture of basalt82ZO401is aphyric and hypocrystalline． Principal constituents are plagioclase， 01ivine and clinopyroxene． ＊Geothermal Research Department，Geological Eresh glass occurs interstitially． Plagioclase Survey of Japan，Tsukuba，Ibaraki305，Japan is．of．andesine composition，and occurs as ＊＊Department of Geological Sciences，Chiang euhedral prismatic crystals up to O．5mm Mai University，Chiang Mai，Thailand． ＊＊＊Geological Survey Division，Department of long．Dendritic growth is typically present Mineral Resources，Bangkok10400，Thailand， at contacts with glass（Fig．3）．01ivine occurs 一13一 碗1伽・プ渉h6G・・」・92・α」S％遡・勲卿，7・」・38・N・・1

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Fig．1 Distribution of Late Cenozoic basalts in Southeast Asia after Geologic Map ofAsia and the FarEast（ECAFE，1971）。

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v y 〉 〉 〉 V 〉 V ヤノ レ Legend 〉0〉〉〉 団 Bosq I t （Lαte Cenozolc） TertiQry sedimentGry rocks □ ond AIluvium Mesozoic sedimentαry ond 國 volcqnic rocks

× Sαmple locαlity

Fig。2 Distribution of the Lampang basalt and sample localities． Geology is based on the geological map of northem Tailand，scale1：500，000compiled by Geolo・ gical Survey Division，Department of Mineral Resources of Tailan（1（1982）．

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Fig． 3 Photomicrograph of sample82ZO401．Dendritic growth is observed on the surface of plagioclase， Abbreviations＝P：p1αgioclase，O：olivine，G：glass，Scale：0．1mm，plane polahzed light．

petrographica11y similar to82ZO401except that some silicate an（10paque minerals have 働○ 阯甑監l lattice4ike dendritic forms． O⑲ O 1’ ○ ／ Chemical analyses and CIPW nomative ○ ／ O ／ mineralogies are given in Table1．Both ／ 氣 醐蝋肌闘圃鵬 samples contain normative nepheline．They 冨5 ／ ！ 十 are classified as basanite（YoDER andTILLEY， ／ ！ 〇 ／ ／ 1 帽N 1962），and are chemically similar to several 認 ／ 1 ／ ！ ／ previously analy・zed samples of Lampang ／ ／ ／ ！ 一開OLEll－IC basalt（BARR and MAcDoNALD，1978）（Fig，4）． 1 ！ 1 1 4．K－Ar ages

Whole－rock K－Ar ages of samples82ZO401 and82ZO402were determined by Teledyne 45 50 55 60 Sio2 Isotopes，and yielded ages of O．8±0．3Ma and O。6±0．2Ma，respectively（Table2）．Previous Fig． 4 Total alkalis，plotted against SiO2for the K－Ar age determinations of samples from the Lampang basalt．Open circles are present data，and solid circles are the data from Lampang basalt gave ages less than O．2Ma， BARR and MAcDoNALD （1978，1981）． but those data are not reliable because of Dashed lines represent boundaries of argon loss（BARR et a1．，1976）． The present Japlanese basalt types（KuNo，1966）． samples are apparently fresh，but they have been somewhat affected by alteration，judg』 as euhedral to subhedral crys惚1s up to O．5 ing from H20（十）contents（Table1）．Ob－ mm in size． It locally contains minute o－ sidi＆n with H20（十）contents higher than l wt pa（lue minerals and glass inclusions． Clino・ ％has probably been affected by ra（liogenic pyroxene forms prismatic crystals up to O。05 Ar loss（KANEoKA，1971），and hydration of mm in len齢h．Scattered vesicles are lined glass in the Lampang basalt samples pre－ by carbonate minerals．Sample82ZO402is sumably also affects the K－Ar ages，especially

一15一 「 l

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Table1 Chemic＆komposit沁鷺鋤雛qPW Table2K－Ar＆蓼es o｛the Lampang basalt E norms魚r thεLa顯欝綿懸搬総焦 Sample蕪were an泌y欝縫糎癒総 愚m獄驚 K 船A罫r総「 Atm40Ar Age Tokyo（沁畿1猷nd．M絵鷺夏鳩嵐1獄気無撫讐繊＜〉欝y。 ，No敷 儲〉（1韮0一％礁ヂ鉛P／9） （％） （Ma）

Sample No． 82ZO401 82漁402 82ZO40！鷺，69 0．008 7．9 0．8±0．3 ！ 鴛，7｛） 0、009 8．5 Sio2 47．31 46，70 82ZO402 2．74 0．007 10．1 0．6±0．2 Tio2 2．焉 2．41 2．76 0．007 10．3 Al203 置7。2＄ 墨7．儀8 1．75 2、39 Theconstantsforag礁む翫塞憾麓緬鎚are Fe203 えβ＝4．96×10－10yr－1， FeO 馨．47 窃．n λεニ0．581×10舳loyガi翫瓢豊蝋＝・1．167x10－4 MnO 0．17 o．！8 atomperatomofnatur急麺醜総腕m． MgO 6．77 7．04 7．58 7．78 CaO 1旦ow，鋤d some煮（》w欝癖vα！le the gravel de一 4．02 Na20 3，74 茎x欝量t欝 α）鷺t撫業捻量難慧 鷺縦烈y P菰1eolithic pebble K20 3．04 3，23 数雌欝（i3AR蕪繊操．，19お）、The K－Ar age of P205 0．78 0．83 泌撒ple82Z艇（／2，鴛o搬撫now overlying grave1 H20（＋） 2．37 1．17 畿登osits，重鷺《）難蚤謝蹴with the paleomagnetic H2α一） 0．34 0．28 and archeological evi（1ence． CO2 0．08 0．37 Late Cenozoic basaltic activity began about Tota1 99。83 99．69 13Ma ago in Southeast Asia，and some basalts appear to be younger than3．5Ma or 18．53 19．50 （BARR and MAcDoNALD，1981）．The present ab 20．02 16．02 age determination indicates that the Lam－ pang basalt is one of the youngest in South－ an 22．06 19．71 east Asia． ne 6．68 10．13 4．81 5．93 ぎ」・・ 5。 亘朧夢且量cat藍o亙亘 3．14 4．13 1．34 1．32 Although the hot spring distribution dose 9．98 9。65 not correlate with the basalt，some of the 蓋 4．70 3．40 high heat flow anomalies appear to be cor・ 、mt 2．62 3．53 relative with the basaltic activity．High heat i1 4．21 4．68 now（more than2．O HFU）has been measured 1．91 2．00 ap around Phrae（THIENPRAsERT and RAKsASKUL－ Total 100．00 100，00 woNG，1984），where the Lampang and Den Chai basalts are distributed．The age of the Abbreviations for normative minerals； or：orthoclase， ＆b：albite， an：anorthite， latter basa至t is5．64Ma（BARR and MAcDo－ ne：nepheline， WO：WOllaStOnite， NALD，1981）．Atthe westem marginofKhorat en：enstaもite， fs：ferrOS皿iもe， plateauaNNE－tren（1inghigh heatflowanoma－ di：diopside， fo：forsterite， fa：fayalite， mt：magnetite， iL ilmenite， ap：apatite． 1y also occurs（THIENPRAsERT and RAKsAsKuL－ woNG，1984），and basaltic rocks are distrib－ ute（i at Khok Samrong on the south of the of the pillow basalt with H20（十）content of anomaly．The age of one of the basaltic 2．37％． Hence both samples may give a rocks is11．29Ma（BARR and MAcDoNALD， minimum age of crystallization．However， 1982）． the H20（十）of sample82ZO402is only slightly In continental regions， Cenozoic basalts over l wt％and hence the deviation from the are typically associated with high heat fiow crystallization age is presunlably nOt great． anomalies．For example，at Urach ge6ther－ Most Hows of the Lampang basalt show mal Held，West Gemany（HAENEL，1976； normal magnetic polarity，except the lower MAussNEsT，1982），a high temperature anoma一

一16一 K一ルα96s戸o〃3加吻砺9δαs読（Sαsαの6砲」．）

1yislocatedatthe centerofa number ofnephe－ Dept．Mineral Resources of Thailand． linite pipes whose K－Ar ages are30－11Ma HAENEL，R．（1976）Mδglichkeiten der Nutzung （LIPPoLT et a1．，1973）．The Cenozoic basalts Geothermischer Energie in der Bundesre－ are also regionally associated with high heat publik Deutschland．Report，MLfB Han－ How anomalies in the Basin and Range in nover，Achive No．70409． North America（LEEMAN and RoGERs，1970） ed．（1982）The Urach geothermla project、 and in eastem Australia （SAAs， 19641 Schweizerbartysche Verlagsbuchhandlung， MoRGAN，1968）．This is considered to be due Stuttgart．41g P． to thermally anomalous upper mantle and KANEoKA，1．（1971） The effect of hydration on the lower crust （LEEMAN and RoGERs， 1970； KIAr ages of volcanic rocks，Eα7∫h PJ伽θ’． HAENEL1982）． Although the deep crystal Soz’．，乙6甜．，vol．14，P．216－220． stmcture in northem Thailand is not known， KuNo，H．（1966）Lateral variation of basalt magma the presence of late Cenozoic basalt including type across continental margins and is－ the Lampang basalt，is probably indicative of land arcs。 B％〃。 1VoJoα％oZ．， vol．92， P．1069－ high heat potential in the crust or the upPer 1142． mantle under the northern part of Thailand． LEEMAN，W．P．and RoGERs，J．」．W．（1970）Late Cenozoic alkaliっlivine basalts of the Basin・

Re貫ere恥ces Range Province． US∠4． Co窺名 M初6名αよ Po蕗oJ．，vol．25，P．1－24． BARR，S，M．，MAcDoNALD，A．S．，HAILE，N．S．and LIPPLoT，H．J．，ToDT，W．and BARANYI，1．（1973）．K－ REYNoLDs，P。H，（1976）Paleomagnetism Ar ages of basaltic rocks from the Urach and age of the Lampang basalt（northem volcanic district，SW Germany．Foガsoh7。 Thailand） and age of the underlying ルf勿z67．vo1．50，Beih3，p．101－102． pebble tools．ノloz6名 G601．Soo．Thα〃ごzπ4，vol． MAussENEsr，0．（1982）The volcanic phenomena 2，p．1－10。 in the Urach・・region，in the Urach Geo・ and （1978） Geochemistry and thermal Project．ed．by Haenel，R．，Schwei・ petrogenesis of late Cenozoic alkaline zerbart，sch Verlagsbuchhandlung， Stutt・ basalts of Thailand。 OooJ．Soo。ルfα勿吻 gart，p．157－160。 Bz6〃．，vo1．10，P．25－52． MoRGAN，W．R．（1968）The geology and petrology an（玉 一 （1981） Geochemistry and of Cenozoic basaltic rocks in the Cooktown geochronology of late Cenozoic basalts of Area，No抗h Queensland．10％7．OooJ．Soo． Southeast Asia．Oooム Soo．z4郷θ吻召B％〃．， ．4z6s渉7α」宛，vol．15，P．65－78． pt I， vo1．92， P．508－512 and pt II of the SAAs，J．H．（1964） Heat fiow values from eastern Bull．voL92，p．10694142， Australia．10％7．Gθo望）hys．Rθs．，vol．69，p． ，RATANAsATHIEN，B．，BREEN，D．，RAMING－ 3889－3893． woNG，T．and SERTsRlvANIT，S．（1979） Hot THIENPRAsERT，A．and RAKsAsKuLwoNG，M．（1984） springs and geothermal gradients in Heat flow in no丘hern Thailand．T60オo％o・ northern Thailand． G60渉h6鰯zづos， voL86， ρhツsz。os，vo1．103，P．217－233。 p．85－95． YoDER，H．S．Jr．and TILLEY，C．E．（1962） Origin of ECAFE（1971） Geological map of Asia and Far East， basaltic magmas：An experimental study scale1：5，000，000．Paris，ECAFE． of natura王and synthetic rocks systems． Geological Survey Division（1982）Geological map ノひ％γ．Pε≠影o♂．，vo1．3，P．342－532． of northem Thailand，scale 1：500，000．

一17一 B％IZ・枷oβhθσ60Jo9勿αJS％彫ッ・μ吻％，70乙38，N・・1

タイ北部ランパン玄武岩の新しいK－Ar年代値

笹田政克・B・ラタナスティエン・P・ソポンポングピパト

要 旨 メモ盆地南部から採取したランパン玄武岩2試料にっき，化学分析を行うとともにK－Ar年代を求め た．これらはともに斜長石，かんらん石，単斜輝石，不透明鉱物及び間隙充填状のガラスからなる無 斑晶質のペイサナィトである．これらは0．8±0．3Ma，0．6±0．2MaのK－Ar年代を示すが，ガラスの 水和によるArの散逸が考えられるので，厳密にはこれらの年代は形成年代の下限を与えているといえ る。しかし後者では岩石中の水の含有量が少ないので，形成年代から大きな隔りはないものと考えられ る．ランパン玄武岩は東南アジァの大陸地域における最も若い火成活動の1っであるといえる．この若 いアルカリ玄武岩の噴出がみられるタイ北部では，地殻及び上部マントルの熱的ポテンシャルは高いか もしれない．

（受付：1986年7月10日1受理：1986年10月27目）

一18一 β認Oool．翫7”．1砂㈱，γoJ．38，No，1 PJ諺θ1

1 0utcrop of pillow basalt（sample82ZO401）in the southern Mae Moh Basin、

懸麗

臨鯛蝋

蝋鯛繍

2 0utcrop of basalt（sample82ZO402）overlying the gravel deposit in the southem Mae Moh Basin。