newk-ar我ges倉囎the:l蹴聖我ngb麗謝,欝賄he … survey division,department of...
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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.
且. 亘ntroductio聾 2。 Loea且亘ty&醜{温oce既rre恥e¢
Late Cenozoic basalts are wi(1ely distributed
in Southeast Asia(Fig,1),’and occur locally
in northem Tailand.Many hot springs alsooccur in no丘hem Thailand (e,g.BARR et al.,
1979), but the surface distribution of the
basaltic rocks is not correlative with the hot
springs.Neve姓heless,the basalts.are theonly known evidence of recent igneous activi-
ty.
This paper presents K-Ar ages of two
samples from the Lampang basalt,collected
from the southem Mae Moh Basin,northem
Thailand(Fig.2).TheauthorsthankDr.S.M.
BARR,Acadia University,Canada for critical
reading of the manuscript,and Mr.K.Uto,
Geological SurveyofJapan for helpful sugges-
tion.Theyalsothank Messrs.K.TuRALTAM-KuL,V.PAIJITPRAPAPAN an(1C.CHAPHYEN,Department of Mineral Resources Thailandfor their aid in昼eld菖tu(1ies.
*Geothermal Research Department,Geological Survey of Japan,Tsukuba,Ibaraki305,Japan
**Department of Geological Sciences,Chiang Mai University,Chiang Mai,Thailand.
***Geological Survey Division,Department of
Mineral Resources,Bangkok10400,Thailand,
The Lampang basalt underlies the southern
part of the Mae Moh Basin(Fig.2).It con-
sists of several flows and locally pyroclastic
(1ebris associated with vents (BARR,and
MAcDoNALD,1981). One of the samples(82ZO401)for the present study was collected
from a pillow basalt cropping out along the
railroad cutting at 18。15!30”N an(i99。39!00!!
E.The pillows range from O.5to l m in
ra(1ius (Plate I-1) and display radial cracks
and concentric zones of vesicles at themargins.Inter-pillow breccia is also present.
The other sample(82ZO402)was collected from
a massive basalt How overlying gravel de-
posits(Plate I-2)at18。07!32”N and99。37!52”E.
3. Sam欝且e岨escr亘昼》t亘o聡
The texture of basalt82ZO401is aphyricand hypocrystalline. Principal constituents
are plagioclase, 01ivine and clinopyroxene.
Eresh glass occurs interstitially. Plagioclase
is.of.andesine composition,and occurs aseuhedral prismatic crystals up to O.5mm
long.Dendritic growth is typically present
at contacts with glass(Fig.3).01ivine occurs
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碗1伽・プ渉h6G・・」・92・α」S%遡・勲卿,7・」・38・N・・1
q O
〆ず 風爺d議錨盆醐 Sea
O
㌔匙
る
◎o
、
も 00
ノ!辱、・
戸馨轡s・電
\ θr 、 、
㌧ 礪肌鮒髄 \・ \
♂’穐.㍗〃、皇..畢麟\\ ㈱
繍◆一’一~
“~ &、. 蕊¢o \ 1. 。 \ 伊 D ” ./1 {
“ /
。〆 0 0 8
も
、
o
! 噺つ
レ・、F
G
ゆ ∂ o 亀
”
嚢職鑑
の§。Oも o
D8
⑰
O
4⑪⑪㎞
一一一一
1翻o匪
2⑰o閥
且⑪o
980
Fig.1 Distribution of Late Cenozoic basalts in Southeast Asia after Geologic
Map ofAsia and the FarEast(ECAFE,1971)。
LGmpong O
MqeMohiiiii妻…::
Bqsip
v y 〉 〉 〉 V 〉 V ヤノ レ 〉0〉〉〉
団
□
國×
Legend
Bosq I t (Lαte Cenozolc)
TertiQry sedimentGryond AIluvium
Mesozoic sedimentαryvolcqnic rocks
Sαmple locαlity
rocks
ond
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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K一ル卿s〃o郷Lα卿伽9δαsα1!(sαsα4副α」。)
Fig.
麟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.
氣冨5十
〇
N帽認
働○ 阯甑監l O⑲ O 1’ ○ / ○ /O /
/ 醐蝋肌闘圃鵬 / ! / ! 1 / / / 1 / / ! / /
/
1 1 1!
! 一開OLEll-IC
petrographica11y similar to82ZO401exceptthat some silicate an(10paque minerals have
lattice4ike dendritic forms.
Chemical analyses and CIPW nomative
mineralogies are given in Table1.Both
samples contain normative nepheline.Theyare classified as basanite(YoDER andTILLEY,
1962),and are chemically similar to several
previously analy・zed samples of Lampangbasalt(BARR and MAcDoNALD,1978)(Fig,4).
4.K-Ar ages
Fig.
45 50 55 60 Sio2
4 Total alkalis,plotted against SiO2for the
Lampang basalt.Open circles are present
data,and solid circles are the data from
BARR and MAcDoNALD (1978,1981). Dashed lines represent boundaries of Japlanese basalt types(KuNo,1966).
as euhedral to subhedral crys惚1s up to O.5
mm in size. It locally contains minute o-
pa(lue minerals and glass inclusions. Clino・
pyroxene forms prismatic crystals up to O。05
mm in len齢h.Scattered vesicles are lined
by carbonate minerals.Sample82ZO402is
Whole-rock K-Ar ages of samples82ZO401
and82ZO402were determined by TeledyneIsotopes,and yielded ages of O.8±0.3Ma and
O。6±0.2Ma,respectively(Table2).Previous
K-Ar age determinations of samples from the
Lampang basalt gave ages less than O.2Ma,
but those data are not reliable because of
argon loss(BARR et a1.,1976). The present
samples are apparently fresh,but they havebeen somewhat affected by alteration,judg』
ing from H20(十)contents(Table1).Ob-
sidi&n with H20(十)contents higher than l wt
%has probably been affected by ra(liogenic
Ar loss(KANEoKA,1971),and hydration of
glass in the Lampang basalt samples pre-
sumably also affects the K-Ar ages,especially
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「 l
E
!
Table1
β%〃6渉」妬oプ泌ho OooJo9廊αl S%7z/6ツoプ74ρα%,Vrol.38,ノ〉lo。1
Chemic&komposit沁鷺鋤雛qPWnorms魚r thεLa顯欝綿懸搬総焦Sample蕪were an泌y欝縫糎癒総Tokyo(沁畿1猷nd.M絵鷺夏鳩嵐1獄気無撫讐繊<〉欝y。
Table2K-Ar&蓼es o{the Lampang basalt
愚m獄驚 K 船A罫r総「 Atm40Ar Age,No敷 儲〉(1韮0一%礁ヂ鉛P/9) (%) (Ma)
Sample No. 82ZO401 82漁402
Sio2
Tio2
Al203
Fe203
FeO
MnOMgOCaONa20K20P205
H20(+)
H2α一)
CO2
47.31
2.焉
置7。2$
1.75
馨.47
0.17
6.77
7.58
3,74
3.04
0.78
2.37
0.34
0.08
46,70
2.41
墨7.儀8
2、39
窃.n
o.!8
7.04
7.78
4.02
3,23
0.83
1.17
0.28
0.37
82ZO40!鷺,69
鴛,7{)
82ZO402 2.74
2.76
0.008
0、009
0.007
0.007
7.9
8.5
10.1
10.3
0.8±0.3
0.6±0.2
Theconstantsforag礁む翫塞憾麓緬鎚areえβ=4.96×10-10yr-1,
λεニ0.581×10舳loyガi翫瓢豊蝋=・1.167x10-4
atomperatomofnatur急麺醜総腕m.
Tota1 99。83 99.69
or
ab
an
ne
ぎ」・・
蓋、mt
i1
ap
18.53
20.02
22.06
6.68
4.81
3.14
1.34
9.98
4.70
2.62
4.21
1.91
19.50
16.02
19.71
10.13
5.93
4.13
1.32
9。65
3.40
3.53
4.68
2.00
1旦ow,鋤d some煮(》w欝癖vα!le the gravel de一
茎x欝量t欝 α)鷺t撫業捻量難慧 鷺縦烈y P菰1eolithic pebble
数雌欝(i3AR蕪繊操.,19お)、The K-Ar age of
泌撒ple82Z艇(/2,鴛o搬撫now overlying grave1
畿登osits,重鷺《)難蚤謝蹴with the paleomagnetic
and archeological evi(1ence.
Late Cenozoic basaltic activity began about
13Ma ago in Southeast Asia,and some
basalts appear to be younger than3.5Ma
(BARR and MAcDoNALD,1981).The presentage determination indicates that the Lam-
pang basalt is one of the youngest in South-
east Asia.
5。 亘朧夢且量cat藍o亙亘
Total 100.00 100,00
Abbreviations for normative minerals;
or:orthoclase,
ne:nepheline,
en:enstaもite,
di:diopside,
mt:magnetite,
&b:albite, an:anorthite,
WO:WOllaStOnite,fs:ferrOS皿iもe,
fo:forsterite, fa:fayalite,
iL ilmenite, ap:apatite.
of the pillow basalt with H20(十)content of
2.37%. Hence both samples may give a
minimum age of crystallization.However,the H20(十)of sample82ZO402is only slightly
over l wt%and hence the deviation from the
crystallization age is presunlably nOt great.
Most Hows of the Lampang basalt show
normal magnetic polarity,except the lower
Although the hot spring distribution dose
not correlate with the basalt,some of the
high heat flow anomalies appear to be cor・
relative with the basaltic activity.High heat
now(more than2.O HFU)has been measured
around Phrae(THIENPRAsERT and RAKsASKUL-
woNG,1984),where the Lampang and DenChai basalts are distributed.The age of the
latter basa至t is5.64Ma(BARR and MAcDo-
NALD,1981).Atthe westem marginofKhorat
plateauaNNE-tren(1inghigh heatflowanoma-1y also occurs(THIENPRAsERT and RAKsAsKuL-
woNG,1984),and basaltic rocks are distrib-
ute(i at Khok Samrong on the south of the
anomaly.The age of one of the basaltic
rocks is11.29Ma(BARR and MAcDoNALD,1982).
In continental regions, Cenozoic basalts
are typically associated with high heat fiow
anomalies.For example,at Urach ge6ther-
mal Held,West Gemany(HAENEL,1976;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-
linite pipes whose K-Ar ages are30-11Ma
(LIPPoLT et a1.,1973).The Cenozoic basalts
are also regionally associated with high heat
How anomalies in the Basin and Range in
North America(LEEMAN and RoGERs,1970)
and in eastem Australia (SAAs, 19641
MoRGAN,1968).This is considered to be due
to thermally anomalous upper mantle and
lower crust (LEEMAN and RoGERs, 1970;
HAENEL1982). Although the deep crystal
stmcture in northem Thailand is not known,
the presence of late Cenozoic basalt including
the Lampang basalt,is probably indicative of
high heat potential in the crust or the upPer
mantle under the northern part of Thailand.
Re貫ere恥ces
BARR,S,M.,MAcDoNALD,A.S.,HAILE,N.S.and
REYNoLDs,P。H,(1976)Paleomagnetism
and age of the Lampang basalt(northem
Thailand) and age of the underlying
pebble tools.ノloz6名 G601.Soo.Thα〃ごzπ4,vol.
2,p.1-10。
and (1978)
petrogenesis of late
basalts of Thailand。
Bz6〃.,vo1.10,P.25-52.
an(玉 一 (1981)
Geochemistry and Cenozoic alkaline
OooJ.Soo。ルfα勿吻
Geochemistry and geochronology of late Cenozoic basalts of
Southeast Asia.Oooム Soo.z4郷θ吻召B%〃.,
pt I, vo1.92, P.508-512 and pt II of the
Bull.voL92,p.10694142,
,RATANAsATHIEN,B.,BREEN,D.,RAMING-
woNG,T.and SERTsRlvANIT,S.(1979) Hot
springs and geothermal gradients in
northern Thailand. G60渉h6鰯zづos, voL86,
p.85-95.
ECAFE(1971) Geological map of Asia and Far East,
scale1:5,000,000.Paris,ECAFE.
Geological Survey Division(1982)Geological map
of northem Thailand,scale 1:500,000.
Dept.Mineral Resources of Thailand.
HAENEL,R.(1976)Mδglichkeiten der Nutzung
Geothermischer Energie in der Bundesre-
publik Deutschland.Report,MLfB Han-
nover,Achive No.70409.
ed.(1982)The Urach geothermla project、
Schweizerbartysche Verlagsbuchhandlung,
Stuttgart.41g P.
KANEoKA,1.(1971) The effect of hydration on the
KIAr ages of volcanic rocks,Eα7∫h PJ伽θ’.
Soz’.,乙6甜.,vol.14,P.216-220.
KuNo,H.(1966)Lateral variation of basalt magma
type across continental margins and is-
land arcs。 B%〃。 1VoJoα%oZ., vol.92, P.1069-
1142.
LEEMAN,W.P.and RoGERs,J.」.W.(1970)Late Cenozoic alkaliっlivine basalts of the Basin・
Range Province. US∠4. Co窺名 M初6名αよ
Po蕗oJ.,vol.25,P.1-24.
LIPPLoT,H.J.,ToDT,W.and BARANYI,1.(1973).K-
Ar ages of basaltic rocks from the Urach
volcanic district,SW Germany.Foガsoh7。
ルf勿z67.vo1.50,Beih3,p.101-102.
MAussENEsr,0.(1982)The volcanic phenomena
in the Urach・・region,in the Urach Geo・
thermal Project.ed.by Haenel,R.,Schwei・
zerbart,sch Verlagsbuchhandlung, Stutt・
gart,p.157-160。
MoRGAN,W.R.(1968)The geology and petrology
of Cenozoic basaltic rocks in the Cooktown
Area,No抗h Queensland.10%7.OooJ.Soo.
.4z6s渉7α」宛,vol.15,P.65-78.
SAAs,J.H.(1964) Heat fiow values from eastern
Australia.10%7.Gθo望)hys.Rθs.,vol.69,p.
3889-3893.
THIENPRAsERT,A.and RAKsAsKuLwoNG,M.(1984)
Heat flow in no丘hern Thailand.T60オo%o・
ρhツsz。os,vo1.103,P.217-233。
YoDER,H.S.Jr.and TILLEY,C.E.(1962) Origin of
basaltic magmas:An experimental study
of natura王and synthetic rocks systems.
ノひ%γ.Pε≠影o♂.,vo1.3,P.342-532.
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