Upload
others
View
6
Download
0
Embed Size (px)
Citation preview
Bulletin of the Geological Survey of Japan,Vo1.45(2),1).51-61,1994
The且麓h⑪且⑪gy認聰《蚤s愈置盟¢t服置e o董窺Meso2ゆ且¢
se耐童朧£囎愈我ry一且9醜£⑪眠s我sse且甑恥舳9㊧眠漁eaも韻亘
愈he M聡s騰㎜,B我gh⑪蛋曲童⑬且麓e,
聾棚恥er魍B幽踊st鵬:P慮醜蹴
Jan Mohammad MENGAL#,:Katsumi KIMuRA*,
Muhammed Rehanul Haq SIDDIQuI繹,Satoru KoJIMA**,
Takahito NAKA繹,Mali:k Sikander BAKHT淵孝and:Kotaro:KAMADA***
MENGAL Jan Mohammad,KIMuRA Katsumi,SIDDIQul Muhammed Rehanul Haq,KoJIMA Satoru,NAKA Takahito,BAKHT Malik Sikander and KAMADA Kotaro(1994)The
lithology and stnlcture of a Mesozoic sedimentary-igneous assemblage beneath the
Muslim Bagh Ophiolite,northem Balochistan,Pakistan_8%JJ.Goo云S%7び.血卿η,voL
45(2),p.51-61,7figs.,1table.
Abs色raet:AMesozoicsedimentary-igneousassemblagenamedtheBaghComplexbeneaththeMuslim Bagh Ophiolite is widely distributed in the Muslim Bagh area,westem Pakistan。
Onthebasis oflithologyand age,the BaghComplexis divisible into seven lithologicunits:1)
serpentinite melange,2)mudstone melange,3)basalt-chert unit(Upper Jurassic to
Cretaceous),4)hyaloclastite-mudstone unit(Cretaceous),5)ultramafic-mafic rock unit,6)
upper sedimentary rock unit(Jurassic)and7)10wer sedimentary rock unit(Triassic)。These
units are bounded by layer-parallel thnlst faults and stacked against each otheL With this
assemblage,the Bagh Complex can be distinguished from the sedimentary cover sequence on
the Indian subcontinent.The lithologic features suggest that the Bagh Complex consists of
rock assemblages which were initially deposited in the extensive area which ranges from the
continental margin of the Indian subcontinent to the Neo-Tethys ocean。
且.亘鵡胸心uet量o醜
The Himalayan orogenic belt is the largest
convergent zone,which have resulted from colli-
sion between the Eurasia and Indian continental
plates about50Ma(Powe11,1979).In thewestem Pakistan,oblique collision of the Indian
subcontinent with the Afghan block resulted in
孝Geological Survey of Pakistan,Quetta,PakistIm
繹Geoscience Laboratory,Geological Survey of Pakista皿,
Islamaba(1,Pakistan
岸繹Geological Survey of Pakistan,Islamabad,Pakistan
*Geology Department,Geological S皿vey of Japan
**Department of Earth and Planetary Sciences,Nagoya
University,Nagoya,Japan
***Department ofEducation,Hirosaki University,Hirosaki,
Japan
the development of the Sulaiman and Kirthar
fold-thrustbelt(Yeats and Lawrence,1984).The
westem margin of the fo1(i-thrust belt is marked
by the discontinuous occurrence of several huge
ophiolitebodies inBela,MuslimBagh,Zhob,andWaziristan(Fig.1)(Asrarullahθ渉α」.19791De-
Jong and Subhani,19791Ahmad and Abbas,1979).These ophiolite bodies occur as thnlst
sheets overlying the sedimentary cover of the
Indian subcontinent(Ahmad and Abbas,19791
DeJongand Subhani,1979).The emplacement of
the ophiolites occurred in the Paleocene or
earliest Eocene time(Allemenn,1979).These
ophiolite and associated sedimentary-igneous
Keywords:Triassic,Jurassic,Cretaceous,oP1亘olite,melange,
chert,basalt,Bagh complex,Neo-Tethys,Muslim Bagh,
Pa短stan
一51
B%ZZ8∫勿z(~プ云hεG60Zo9∫oαJS%7z2⑳ρプノ4ψα%,Vb及45,.〈砂.2
Afghan
650E
block
and sedimentary
cover
30。N
ロ フぴヒ お い
耳............肇law肝一
≡彗’:…:…:…:…:…:…:…:…:ヌ……糞辮
肇……:戴轍’
麟諜iiii,恥1び,一,
灘難1灘8一町
δ
250N IndianOcean .罫a脚噂li:
!♂ !倒”垂ら lndianshield
!曽andunf・ldedI
sedimentary cover
逸匪ヨ・phi・lite
Fig.1 Schematic tectonic map Pakistan. (modified from
Rana,1982;DeJong,1982)
of westemKazmi and
assemblage offer significant evidence toreconstruct the paleoenvironments of a passive
margin of the Indian subcontinent and the Neo-
Tethys ocean.There are,however,a few
geologic studies.Among these ophiolites,the
Muslim Bagh Ophiolite is one of the best studied
ophiolites(Ahmad and Abbas,1979),and it is
underlain by the sedimentary-igneousassemblage regar(1ed as an large remnant of an
emplaced Neo-Tethyan sedimentary sequence(Otsuki6如1.,1989;Kimuraαα1.,1992).
In this paper,we call the sedimentary-
igneous assemblage beneath the Muslim Bagh
Ophiolite the Bagh Complex after Bagh village
located south of the Saplai Tor Ghar(Fig.2).
This work is the result of cooperative field
research in1991by members of GeologicalSurvey of Pakistan and the Japanese party.This
project was financially supported by Japan Inter-
national Cooperation Agency(JICA).Detailed
radiolarian fossil ages of the Bagh Complex is
reported in the same volume by Kojimaαα」.
(1993).Radiometric ages an(i geochemistry of
igneous rocks of the Bagh Complex and Muslim
Bagh Ophiolite have been preliminarily reported
by Sawada6地」.(1992),and their details will be
published later.The purpose of this paper is to
desc1’ibe the lithologic divisions and their
lithofacies oftheBagh Complexbased ondetailed
field survey in the Muslim Bagh area,and to
discuss the environment ofdeposition ofthe com一
萎
婁
…
$き嚢
藝
30’50’N プー一、・N〆67。50’E
鍔牟憾
! 鱗 ¥Fig,4離.
Zhob VaIley
痔,ノ 勲
瀞’〉il;9・ ・、9臼Σl
ii毒 ・・ ang Tor Ghar:
βP。40’N
E
二1,●……i….:麟
ナl
l_}一
}、覧
r、
68●0~E
》鴨メ
蕊
MusIm a h O hio巨te
、ノ
、こ『竃一辱
、、躍:
ク・、
∴盆・’ 一ム=
30●50’N
酬8‘1
器ノ
ノ Malor thrust
or reverse fau陸
9.
〃!
□Q旧temary
隔MuslimBagh・phidite
翻BaghC・mplex
目Calcare・usZ・ne (Jurβssict・PaIe・cene sedimer践ary cover on
o thelndiansubconヒinent)
Saplai Tor Gha
A .・β蔵.聴
・齪::9『1こ’
驚講灘
ノ ・0〃∂
ア2/醗
〃 ’
.〃 ¢5・ q『/
’,麟
・● 、か .ノ〆
欝をノ
ケ㌘籍“一3α4αN
等一し_一k2wa卜喝餅と 1些編・蓼肇c.ノ!,・
67。50’E 6800’E 68010’E
Fig.2 Geologic map of the Muslim Bagh area(after from H:unting Survey Corporation,1961;
Ahmad and Abbas,1979;Otsuki6麺」.,1989and this study).
一52一
LithoIogic units of Bagh CompIex
醗麟懸麗騒團臨
Serpentinite meIange(Bsm)
Mudstone melange(Bmm)
U[tramafic-mafic rock unit(Bum)
Basalt-chert unit(Bbc)
Hyaloclastite-mudstone unit(Bhm)
Upper sedimentary rockunit(Bus)
Lower sedimentaryrockunit(Bls)
Saplai Tor Gh須r
雛藏・,獅2ケB灘、黙膿臨
..郷講。β10’
ζ,ζ識醸}昏.. 二.・〆
1きき辮鐸ノグllき灘き Bum
!グ
湾!
30。40’N
/ * 1
N
Mus髄m ag Ophio髄te l
68。0’E
一一一一ゴーハ\一冒/\ 銭’BISD、o・。●●.
傘魅艶
・Bum
ぺ じぺ
箋墜叢畿講
・一ノ’、 、、、馨hm ロ ヤ ヒノ ヤちヤヤ
ノ♪’11一ロ!、暖、〆ノ \一・、、
警蝿凝麟縢’、 Bhm
暢1\㍗㍉憶一...『、.一甲一一’一一.一一ロー一灘層ロ
Manda ’
~一趨●、撃…蟻・
Kohai麟
一蝕一_一一 C
0 5km\、
Calcareous Zonea 68。0’E b
MusIim Bag Bagh Complex Ophiolite
N Waz・S尋kaiGhar3。。。A BsmPaslaiManda澄 Bhm
m・phi・litey iBbc/’ノIBumB饗!!勇kBus..、、、Bus、BIs
/!/二/1/、・ノノ〃/!!!〆’物錫z駿二二蚤暴ミ1緑藝雛、.,ろ
2000 ノ ・ γノBhm・y’\7二物・!’!ノりう’
./
←upwarddirecti・h6fbedding .〃/・dip・fstrata
Gwa卜Bagh Fault
Bh勢Bus Bus㌧ ‘, 戸護!覧 ,民
Calcareous
zone
SE
C
3000 m
!ノ!’置、’!2000
Fig.3 Geologicmap oftheBaghComplexintheeastempartoftheMuslimBagharea.WGlWazoSakai Ghar
Thε励oJo剛伽4s翻吻名6げα盈so鰯6so伽醐αη一ゆ60%sαssθ励Z卿(Mθ卿」吻Z.)
plex。Our study area is located in the Muslim
Bagh area,northwestem Ba.10chistan,westem
Pakistan(Figs.1and2).
2・馳ev童o鵬st賊y
Hunting Sur▽ey Corporation (1961) publi-
shed a series of geologic maps on a scale of
1:253,440including the Muslim Bagh area under
the Colombo Plan Project an(l first clarified
outline of the geology of the Muslim Bagh area.
Detailed studies on the Muslim Bagh Ophiolite
andtheassociated“melange’y havebeenreported
recently.Ahmad and Abbas(1979)and Gansser
(1979)pointed out that the“melangeシ’zone
beneath the Muslim Bagh Ophiolite includes
slivers ofmetamorphic rocks and ophiolite rocks,
and considered that it was formed by shear in
association with obduction of the ophiolite.
Otsuki 6≠ α差 (1989) presented that the
“melange,,zone includes not only・melange unit,
but also acoherent sedimentary-igneousunit con-
sisting ofTriassic sedimenta㎎rocks and igneous
rocks,and considered that these rocks were
deposited in the trough basin resulting from the
breakup of the northern margin of the Indian sub-
continent。These ages were determined only
based on megafossils including Halobia of
Triassictime。However,theoccurrenceoftheUp-per Jurassic to Lower Cretaceous radiolarians in
the“melange少,zone are noted in the apPend致
section of the volume edited by Japanese-
Pakistani Research Group (1989). Theseradiolarian ages are not consistent with the
stratigraphic division offered by Otsukiθ!σ義
(1989)』Kimura6地」.(1992)preliminary divi(1ed
the“Melange”zone into seven lithologic units
base(i on lithologies and ages,an(1indicated that
the“Melange”zone is characterized by a variety
of Mesozoic sedimentary-igneous rock succes-
sions.This paper follows the lithologic division
presented by Kimuraαα1.(1992).
3。 Ge盟ε麗岨9εo且ogy
The study area around Muslim Bagh is
located in the northem margin of the:E-W
trending Sulaiman Range,and is occupied by the
Muslim Bagh Ophiolite,the Bagh Complex,and
the Jurassic to Paleocene sedimentary cover of
the Indian subcontinent(calle(1Calcareous Zone
hereafter)(Fig.2)。These rocks comprise a fold-
thmst belt consisting mainly of Permian to
Eocene platform deposits on the Indian subconti-
nent(6,g。,Kazmi and Rana,1982;Humayon8渉
α乙,1991).The Muslim Bagh Ophiolite thrusted
over the Bagh Complex,which is in fault contact
with the sedimentary cover to the south.The
boundary fault between the Bagh Complex and
thesedimentarycovercanbeclearlytrace(1intheMuslim Bagh area(Fig.2)and is assumed to be
extended toward west near Gwal based on the
topographic features(Fig.1;Otsuki6」σ」.,1989).
The fault is named the Gwa1-Bagh Fault after
Gwal town and Bagh village in this paper.
The Muslim Bagh Ophiolite is made up of
ultramafic tectonites,ultramafic and mafic
cumulates and a sheeted dike 33 complex(Ahmad andAbbas,1979).The Bagh Complexis
a sedimentary-igneous assemblage consisting of
Triassic to Jurassic sedimentary rocks,and
Jurassic to Cretaceous basic igneous rocks with
Cretace・uspelagict・hemipelagicsedimentsandsmall amomts of melange.
To the north of the Zhob valley,Eocene to
Mi・cene且ysch-1ikec・arsesedimenta壇・cksare
widely distributed and extend to the Makran
range in southwest Pakistan(Fig.1).Eocene
transgressive marine beds of the flysch belt un-
confom・ablyoverlieinpartthe・phioliteb。diesin-
cluding the Muslim Bagh Ophiolite and Mesozoic
sedimentaryrocks(Hunting Survey Corporation,
1961;AIlemann,1979).The strata of the flysch
beltareinte印retedt・havebeendep・sitedinthe
narrow zones between the Indian subcontinentan(1the Afghan block:s after the emplacement of
the ophiolites on the Indian subcontinent(De-
Jong,1982).
4・ B題9恥Co醜昼頭ex
4.1(沁t昼i醜e
In the Muslim Bagh area,the Muslim Bagh
Ophiolite occupies mainly two mountain areas
such as the Saplai Tor Ghar and the Jang Tor
Ghar。The Bagh Complex is distributed around
these mountains,and is in fault contact with the
overlying Muslim Bagh Ophiolite(Fig.2).The
fault boundary is commonly outlined by serpen-
tinite melange(Figs.3an(14),rarely by slivers of
一53一
i…1
B%」観伽働θG6晦吻IS%7鰐げ血卿,嗣45,抽.2
Ja バ G
&
0
WSW D
30碧
2000
1000
Qphi
b
ノ
購……,,
●’9.’v鳩。篤。潔’・.
灘灘三1…….,.
齢雛講…鯉㊤、
譲難i灘・・1,…
ゆ◎ ●o
TorA
a r
壼 \’ ヤ 蹴, ’璽
霧…獣
縫麟翻盤釧灘難霧鱗
2km 淵。● !イ ,’ , !ノ 1
ノ じゆ ココ ド
1 ・・。...磐鱒。詔 N
1圏 ・◎’亀’●唇 Saplai
l慧 ’鼠欝丁・rGharlΣ 。.。。鵬・,藝・Bsm〉
、 ◎o●oヤ じロ ゆ
べ¥、Bmm灘レド1 ヤ コゆ のヤ
\、 ・・.”も \、灘4
、》一’9・、 一 ぎ 鵠’・、 撫員ll、頂 ζ’ βa的 … O樋oilte
1 \ !! \
li夕/ Bsm、 Bls 1’、、 Bmm 、
1!
1!つ
!ノタ!//聾く論¥Bls斜Ophi◎lite!ーつ //つ/// !!!ノ・ ■ →’1畳I l
-!1
!////二雛繍麟ll’、1 聯蝉、いへl
Bls
Fig.4 Geologic map of the Bagh Complex in the
westem part of the Muslim Bagh area.
:Legends are the same with those of Fig.3。
EENE
3常00
2000
1000
metamorphic rocks at the northwest side of the
Jang Tor Ghar(Gansser,1979;Otsuki6渉α1.,
1989).WehaveresearchedtheBaghComplexinthe areas aromd the Marzaghan Nara(Westem
part)and aroun(1Bagh-Kazhaba (Eastern part)
(Fig.2)within the Muslim Bagh area and the
geologicmaps ofthe two areas are showninFigs.
4and3,respectively. The Bagh complex is divisible into seven
1ithologic units based on lithologic assemblage,
stmcture,and ages:1)serpentinite melange(Bsm),2)mudstone melange(Bmm),3)basalt-
chert unit(Bbc),4)hyaloclastite-mudstone unit
(Bhm),5)ultramafic-mafic rock unit(Bum),6)
10wer sedimentary rock unit(Bls)and7)upper
sedimentary rock unit(Bus)(Table1).Eachunit is bounded by layer-parallel thrust faults.
4。2L亘tho且ogy
1) Serpentinite melange(Bsm)occurs at
thebottomoftheMuslimBaghOphiolite,andalsoalong the margin of ultramafic rock slices in the
Bagh Complex.Serpentinite melange includes
various types of blocks in scaly serpentinite
matrix (Fig.5).Blocks consist mainly of
ultramafic-mafic rocks and metamorphic rocks
such as amphibolite,gamet-homblende schist,
and greenschist, intercalating basalt, chert,
1imestone, and shale. Judging from thelithologies,it is estimated that ultramafic-mafic
rocks were derived from the Muslim BaghOphiolite,basalt and sedimentary rocks from the
basalt-chert unit.
2),Mudstone melange(Bmm)occurs near
the fault boundary between the ophiolite and
Bagh complex east of Ghazgai Nara,and also
along faults bounding the ultramafic unit at
Kazhaba (Fig.3).The mudstone melangeinclu(1es various types of blocks surrounded by
scaly mudstone(Fig.5).These blocks consist of
basalt,radiolarian chert,foliated limestone,
interbedde(11imestone and shale,and massive
limestone.Basalt,chertan(11imestonearecorrela-
tive to those of the basalt-chert unit, and
interbedded limestone and shale is similar in
lithology to Triassic sedimentary rocks in the
Bagh Complex. 亀
3) Ultramafic-mafic rock unit(Bum)con-
sists of ultramafic cumulate and mafic cumulate,
and conspicuously forms higherridgesthan other
units in the Bagh Complex.Typical tectonic
slices,consisting mainly of ultramafic cumulate,
occur discontinuously along the boundary fault
between the Bbc and Bhm,extending fromKazhabato east of Bagh.The margin ofthis slice
is commonlyremarkedbymelangemits(Fig.3).The lithology and occurrence of this unit suggest
that this unit is a tectonic slice derived from the
Muslim Bagh Ophiolite.There are no age data
from the Bum.
一54一
丁距6」髭hoJogy‘z%4sか門%o孟%名召6ゾ‘zハ44so20ゼ0364づ卸z6%た耽ソー歪8ワz60%sαss6秒zZ》云α8i召 (ハ42%8とzJθ渉αZ.)
Block-in-m飢rix texture in any scale
〆
一 ん _ ㈱
/ ’、’、1、二、1、、
ノ\ ノ
(
/
Se甲entinite melange unit
Se甲entinite matriX
blocks
Mudstone melange unit
mudstone matnxblocks
blocks
amphibolite,gamet-homblendite,greenshcist((1erive(1from high TIP metamorphic rocks)
ultramafic-mafic rocks(derived from ophiolites)
basalt,chert,limestone,shale(derived f士om Bbc)
interbe(lde(11imestone&shale,massive limestone((1erive(1from Bls and Bus)
Fig.5 Schematic descriptions of melange units in the Bagh Complex。
4) Basalt-che丘unit(Bbc)is characterized
by thick basalt rocks with bedded chert,micritic
limestone and hemipelagic mudstone.The basalt
rocks are tholeiitic(Sawadaαα」.,1992),and
occur as pillow lava with minor amounts of
massive lava and volcanic breccia.The pi玉10w
lava consists mostly of closed-packe(1pillows,
ranging in diameter from10cm to l m,with
some intercalations of limestone.The bedded
chert is reddish brown in color and rich in
radiolarian fossils,forming altemating beds of2
to15cm-thick chert layer and fine mudstone
layer ofup to10cmthick.Thisunitis so severely
sheare(I and faulted that the original sedimentary
sequence is almost broken.Some outcrops,however,show that the pillow lava is conform-
ably covered by30to50m-thick limestone-chert sequence. In contrast, there is no
evidence that the pelagic sediments are conform-
ably covered by the basaltic rocks.In ad(1ition,
the basalt-chert unit is marked by thmst-relate(i
repetition of the vertical sequence of basalt,
1imestone and chert in ascending order.Accord-
ingly,the original stratigraphic succession of the
basaltic-chert unit can be reconstructed欲s the
basaltic rocks in the lower part,and the
1imestone-chertintheupperpartas shown inFig.
6.Siliceousshale,2to10mthick,cropsoutcloseto bedded chert.Although the stratigraphic rela-
tionship of these rocks is not clear,bed(1ed chert
is likely to be conformably covered by siliceous
shale as shown in Fig.6.The chert-1imestone
interval ranges in age from Early Cretaceous to
Turonian on the basis of ra(1iolarian fossil ages
from chert(Kolimaαα」.,1994).
5) Hyaloclastite-mudstoneunit(Bhm)con-
sists of thick basalt volcanic rocks interbe(1(1ed
with siliceous mudstone and micritic limestone.
This unit is less sheared compared to the basalt-
chert unit so that the sedimentary sequence can
be sαfely reconstructedαs shown in Fig.6.The
lower subunit consists of siliceous mudstone and
limestone,which are frequenctly intmded by
basalt and dolerite.These intrusions are similar
in lithology with volcanic rocks of the middle
subunit.The middle subunit consists of volcanic
rocks,intercalating small amounts of limestone
and siliceous shale.The volcanic rocks consist of
basanite,alkali basalt,and trachybasalt(Sawada
α α」。, 1992) an(1 0ccur as hyaloclastite and
rework:edvolcanicbrecciawith minor amounts of
pillow Iava and lava sheets(Fig.6).They are
一55一
B%JJ6渉勿z(~プ孟h6G60Jo9づαzJS%7∂のノ(ゾノ4ゆσ犯,VbZ.45,.〈1ひ.2
100 m
1簸il購ii
Basalt-chert Unit
500 m
upper
part
middle
灘議pa,t
lower
part
Hyaloclastite-
mudstone Unit
匿ヨ1
囮2
皿3
囲4
國5
麗6
國・
羅8
國9
Fig.6 Schematic reconstructed stratigraphy of the basalt-chert unit and the hyaloclastite-mudstone
unit in the Bagh Complex.1:bedded siliceous shale,2:siliceous shale,3:bedded chert,
4:interpillow limestone,5:1imestone,6:hyaloclastite,7:1ava sheet,8:pillow lava,9:sill an(1
dike,6-9:basic rocks.
characterize(1by the development of vesicles and
the existence of phenocryst of pyroxene of
several mm to2cm in diameter.The uppersubunit is composed of siliceous shale with
limestone. Early Cretaceous radiolarianassemblages were collecte(1 from siliceous
mudstone of the lower subunit(Kojimaαα1.,
1994).K-Ar ages of amphibole and biotite
collected from volcanic rocks range from68to
81Ma(Sawada6緬1.,1992).Hence,the appro-
ximate age of this unit can be Cretaceous.
6) The Lower sedimentary rock unit(Bls)
consists ofinterbedded limestone and shale in the
upper part,and fissile greenish gray shale in the
lowerpart(Fig.7).Thethickness is some500m.
A limestone bed of interbedded limestone an(1
shale is characterized by sedimentary structures
such as parallel and cross laminations,grading
もand current marks inclu(1ing groove and flute
marks.The interbedded limestone and shale
yield Triassic伽励毎fossils from manylocalities
(Otsuki¢渉α1.,1989 an(1this study).Figure 3
indicates locα1ities of伽励毎which were newly
found in this study.The fissile shale yields:L,ate
Triassic Ammonite and Triassic radiolarian
assemblage(Kojima6施」.,1994).These fossil
data show that this unit is approximately of
Triassic age.
7) The Upper se(1imentaryrockunit(Bus)
consists of siliceous shale,bedded micritic
limestone,fissile shale and calcareous sandstone
with conglomerate.The thickness is some500m
(Fig.7).The siliceous shale collected from two
localities at north of Kohai yields Early Jurassic
and Late Jurassic radiolarian assemblage fossils
(Kojimaαα」.,1994).An ammonite showing
一56一
丁距6」露hoZogy‘z銘4sかz60渉%名oρプαM(3so20づo s6(万%zθπたπy一づ87zθoz6sαss6ηの伽8iO (.~レ陀7z8iα」ααZ.)
Upper sedimentary rock unit(Bus)
01あ
の
o』
5『
〃 〃 〃 〃
〃 〃 〃
〃 〃 〃 〃
o o ● o ●
●
一 一 一
GazgaiNara
6
Kazhaba
一 一 一
〃 〃 〃 〃
〃 〃 〃
〃 〃 〃 〃
〃 〃 〃
〃 〃 〃 4
ケ 〃 〃
一 一 一
North of Kohai
aPP・
oのの邸
’ニ
ト
Lower sedimentary rock unit(Bls)
公
East of Bagh
6
South ofKazhaba
aPP・
團團國冒目囹回回
COnglOmerate
sandstone
i nterbedded
Iimestone&shaie
bedded si[iceous shaie
shale
Radiolarian
Ammonite
Ha’0δ’a
Fig。7 Geologic column of the sedimentary rock units in the Bagh Complex.
Jurassic age was collected around the Gazgai
Nara,although it w3s a dropped stone。These
fossi1(1ata show that Bus is almost of J皿assic
age・ 4。3 Geo亘09亘e s愈r胆et盟re
The Bagh Complex is clearly separated from
Jurassic to Paleocene calcareous rocks to the
south by the Gwa1-Bagh Fault(Fig.2).This
fault,with a50to200m-thick shear zone in the
hanging wa11,generally strikes ENE and dips
steeplytothenorthintheMuslimBagharea.TheBagh Complex is also in fault contact with the
overlyingMuslimBaghOphiolite(Figs.3and4).
The boun(lary fault between the Bagh Complex
and the ophiolite have approximately the same
attitude as the underlying Bagh Complex。
Each lithologic unit of the Bagh Complex is
separated by layer-parallel faults.These』fault
zones sometimes contain serpentinite melange or
mudstone melange.Bedding within each unit is
commonly right-way-up,although there are few
recumbentfolds.Inthe:Eastemarea(Fig.3),the
BaghComplexgenerallystrikesENE-WSWtoE-Wanddipstothenorth,andhoweverintheareafrom Kazhaba to Kohai,strata are folded to form
ENE-WSW trendi阜g upright folds with a
一57一
雛1
β%」彪ガ%ρプ渉hθG60-09づoαJS%廻の7げノ4ψσ%,’Vひよ45,ハわ.2
Table1 Lithologic units ofthe Bagh Complex in the Muslim Bagh area and these remarks。Absolute ages are
from the time scale published by Harland6’磁(1989).Radiolarian ages and K-Ar radiometric ages
are based on Kojimaθ彪」.(1994)and Sawada6如」.(1992),respectively.
LITHOLOGIC UNITS Abb. LITHOFACIES AGE
SERPENTINITE MELANGE Bsm b1。cksinscalyse甲entinitemaるrixchert blocks:early Cretaceous to
Turonian(Rd)
MODSTONE MELANGE B;nm blocks in scaly mudstone matrixchert blocks:Ben・iasiIm to Turonian
(Rd)
ULTRAMAFIC-MAFIC UNIT Bum ultramafic cumulate and mafic cumulate
BASALT-CHERT UNIT Bbc
th・1eiiticわasaltr・cks・verlainby
limestone,be(1ded chert and siliceous
shale
chert:Be1Tiansian to Turonian(Rd)
HYALOCLASTINE-
MUDSTONE UNITBhm
alkaline volcanic rocks interbe(ided with
siliceous shale and limestone
alkaline rocks:68to81Ma(K-Ar)
shale of the lower part:Early
Cretaceous(Rd)
UPPER SEDIMENTARY ROCKUNIT
Bussiliceous shale,micritic limestone with
SandStOne and COng10merateJurassic to Cretaceous(?) (R(1)
LOWERSEDIMENTARYROCKUNIT
Blsinterbedded limestone an〔1shale and
siliceous shaleTriassic(Rd,MF)
Abb.:Abbreviation,R(1:radiolarian age,K-Arl radiometric age of K-Ar
MF:mega fossils including施Zo励and Ammonite.
萎
轄き
馨難き華
蒙
wavelength of500to1500m.A largescale tight
recumbent fold,which are refolded by upright
fo1(1s,is foun(1south of Kazhaba(Fig.3).In the
Westem area(Fig.4),strata form a N-S tren-
ding upright anticlinal fold,plunging southward.
The boundary fault between the ophiolite to the
east and the serpentinite melange to the west
strikes N-S and (1ips steeply,being consist6nt
withtheattitudeoftheBaghComplexasshowninaENE-WSWprofilesection(Fig.4).Accordingto Otsukiαα」.(1989)and Gansser(1979),at the
northwestem side of the Jan Tor Ghar,both the
bomdary fault and the Bagh Complex are fo1(ied
to form an antifomal structure.
As pointed out by Ahmad and Abbas(1979)
an(10tsukiαごzZ.(1989),the attitude of the boun-
dary fault close to be(1(ling in the Bagh Complex
suggests that the boundary fault was originally
nearly horizontal and was later folded(luring the
deformation event also responsib1βfor folding of
the Bagh Complex.The intemal structure of the
ophiolite is,however,crosscut by the boundary
fault(Ahamd and Abbas,1979).
5。Te伽醜sig讃ie蹴eeo實血e B鼠ghCo㈱且ex
The Bagh Complex contains various types of
lithologic units as described in the previous sec-
tion(Table1).Each lithologic unit forms an
individual thnlst sheet.We will describe their
depositional environments base(i on thelithofacies and age.Serpentinite melange and
mudstone melange are characterized by various
types of blocks in scaly matrix.As mentioned
before,the characteristic features of lithologies
indicate that these blockls,except for metamor-
phic rocks such as garnet-hornblende schist an(1
greenschist,were possibly derive(1from the sur-
rounding rock mits such as the Muslim Bagh
Ophiolite and the coherent lithologic units of the
Bagh Complex based on these lithofacies.The
origin of metamorphic rocks is not clear.
Melanges occur only along the margin of the
Muslim Bagh Ophiolite and the ultramafic slices
possibly derived from the ophiolite.The above
mentioned occurrence and the development of
scaly fabric of the melanges suggest that they
were formed by shear in association with the
emplacement of the Muslim Bagh Ophiolite.
一58一
ThθZ露hoJo度ソごz%4s〃%o≠%名召ρプαハ4ゑsogoづo s84づηzθηたz勿ソー歪9%ooz63αss6〃3わ」σ8i8 (ハ4吻z8iα」θ≠αよ)
The previous studies such as Ahmad and
Abbas(1979)and Otsuki6麺乙(1989)regardedthe basalt-chert unit as an upper member of the
Muslim Bagh Ophiolite.The reconstructe(istratigraphy of the basalt-chert unit in this study
shows that basalt rocks are overlain by Early
Cretaceous to Turonian pelagic radiolarian
bedded chert,followed by the deposition of
younger hemipelagic mudstone。In contrast,the
:K-AragesfromtheMuslimBaghOphioliteareof67to82Ma(Sawada6地1.,1992).The age dif-
ference clearly indicates that the basalt-chert unit
is not a member of the Muslim Bagh Ophiolite.
The hyaloclastite-mudstone unit is of
Cretaceous age based on radiolarian fossils
(Kolimaαα1.,1994)and K-Ar ages(Sawadaα
α」.,1992).It consists of alkaline rocks interbed-
ded with radiolarian siliceous shale an(1micritic
limestone.This association of sediments and
alkaline igneous rocks shows that alkaline rock:s
were empted in the hemipelagic environment.
As for sedimentary rocks in the sedimentary
units of the Bagh Complex,these are characteri-
zedbymore abundanthemipelagicsediments andmuch less shallow-marine sediment than the
calcareous sediment on the Indian subco血tinent
to the south.These features show that sedimen-
tary rock units of the Bagh Complex were
depositedinthemoredistalpartoftheIndiansub-continent.
The above mentioned characteristic
lithofacies of the Bagh Complex clearly
distinguish the Bagh Complex from the
CalcareousZonetothesouth.Wethinkthattheserock assemblages of the Bagh Complex were
possibly initiated during Triassic to Cretaceous
time in the area which extends from the con-
tinental margin of the Indian subcontinent over
the Neo-Tethys ocean,and then were thmst-
stacked during obduction of the Muslim Bagh
Ophiolite over the Indian subcontinent。
Aek齢w且e魂gem錨s:We thank S。H。Gauharand T. Shirahase (Geoscience Laboratory,
Geological Survey of Pakistan)for logistic sup-
po耽s during the field survey in1991,S。G.Abbas
(Geological Survey of Pakistan,Quetta) for
guiding in the field sur▽ey and helpful discu-
ssions,K.Hoshino(Hiroshima Univ.)and Y.
Sawada(Shimane University)for helpful discu一
ssions and supports inthe field survey.Wewish
to thank Y.Okimura(Hiroshima University),T.
Tokuoka(Shimane University),M.Okamura(Kochi University)and K.Otsuki(Tohoku Uni-
versity) for usuful suggestions an(1encourage-
ment.
Re£ere亙亘ees
Ahmad,Z.and Abbas,S.G。(1979)The Muslim
BaghOphiolite.動:A.Farahand K A。De-
Jong (eds.) G804yつzごz勉づos ρプ 」Rπ々ゑsたzπ,
Geo1.Suw.Pakistam,Quetta,p.243-249.
Allemam,F.(1979) Time of emplacement ofthe
Zhob valley Ophiolites and Bela
Ophiolites, Baluchistan (prelimina可
report).動:A.Farah and K A。DeJong
(eds.) Gθo吻%伽¢∫os げP励ゑs如%,GeoL
Surv.Pakistan,Quetta,p。215-242.
Asran111ah,Ahmad,Z.and Abbas,S,G.(1979)
Ophiolites in Pakistan:An introduction。
動:A.Farah and K.A.DeJong(eds.)
Gooみπα郷づos げ勲彦お如%,Geo1。Surv・
Pakistan,Quetta,p.181-192。
DeJong,K.A.(1982) Tectonics of the Persian
Gulf,Gulf of Oman,and southem
Pakistanregion.肱A.E.M.NaimandF。
G.Stehli (eds.) Th6 00αz%Bσs勿¢sα%4
惚堰伽s,vo1.6,The Indian Ocean,p。
315-351.
and Subhani,A.M.(1979)Note onthe
Bela Ophiolites,with special reference to
the.Kanararea.動:A.FarahandK.A.De-
Jong (eds.) Gθ04ソ㎜卿∫α; (ゾ Pα庖s如犯,
Geo1.Surv.Pakistan,Quetta,p.263-269。
Gansser,A.(1979) Recomaissance visit to the
ophiolites in Baluchistan and the
Himalaya.動:A.Farah and K。A。DeJong
(eds.) G60み郷郷づos qプ勘雇s如%,Geo1。
Surv.Pakistan,Quetta,p.193-213.
H:arland,W.B.,Armstrong,R.:L.,Cox,A。,Craig,
L.E.,Smith,A.G.and Smith,D。G.
(1989) A geologictime scale1989。Cam-
bridge University Press,Cambridge.
Humayon,M.,Li皿e,R.J.and Lawrence,R D・
一59一
嚢
華
B%JJoあ7zqプ云h6σ80Jo9づαzJS%7”¢yqプノ4ψαη,Vbよ45,ハ1ひ.2
(1991) Structural interpretation of the
eastem Sulaiman foldbelt and foredeep,
Pakistan.Tectonics,vo1.10,p.299-324.
H:unting Sur▽ey Corl)oration (1961) Recon-
naissance Geology of Part of West
Pakistan.published for the Govemment
ofPakistan bythe Govemment ofCanada,
Toronto,550p.
Japanese-Pakistan Research Group (1989)
Preliminary report of general survey on
the geology of Baluchistan.動:Y.
Okimura and A.N.Fatmi(eds.)71協o勉os
伽z4S64」%zθ%たzあo多z(ゾ渉hθ1カ40-E%微燈批τ%Co〃
1づ伽9.P勧召βo%伽ηR69伽㈱の云S動一
ノZ%6%06 0% 云h6 Mづ%θ名πZ 1)8zノθZoφ挽θ7z云s づ%
勲肋孟朋,Hiroshima University,p.1-31.
Kazmi,A。H.and Rana,R.A.(1982) Tectonic
map of Pakistan,scale1:2,000,000,Geol.
Sulv.Pakistan,Quetta,
Kimura,K.,Hoshino少K.,Menga1,」.M.,Siddiqui,
M.R H。,Kojima,S。,Naka,T.andBa㎞t,
M。S。(1992)Geology of the Muslim
Bagh Ophiolite and associated Bagh Com-
plex, in northwestem Baluchistan,
Pakistan. 14δs伽鷹孟s ρゾ ε卿ψ03づz6郷 o犯
1五卿ぜz勿ごz% 060Jogy,Shづ㎜%召 ’92,ノ4ρごz%,
P.23.
Kojima,S。,Naka,T.,Kimura,K.,Menga1,」.M.,
Siddiqui少M。R.H.and Bakht,M.S.
(1994)Mesozoic rediolarians from the
Bagh Complex in the Muslim Bagh area,
Pakistan:Theirsignificance inreconstruct-
ing the geologic history of ophiolites
along the Neo-Tethys suture zone.B%JZ.
GθoZ.S%7z7.ノ4ψα%,vo1.45,P.63-97。
Otsuki,K.,Hoshino,K.,Anwar,M.,Menga1,J.
M.シBrohi,1.A.,Fatmi,A.N.and
Okimura,Y.(1989) Breakup of Gond・
wanaland and emplacement of ophiolitic
complex in Muslim Bagh area of
Baluchistan,Pakistan.動:Y.Okimuraand
A.N.Fatmi(eds.)丁協o勉os伽4S64加6%一
たzガoつ¢(ゾ渉ho1勿40帥Ez脳とzs批z%CoJあ4勿¢g PZσ云6
Bo%%44冗ソR69づo%ごηz4づ菰s1姻%θ%080η ≠hθ
価%卿」1)θ∂θJo卿傭s づ銘 P鷹s伽,
Hiroshima University,p.33-57.
Powe11,C.McA.(1979)A speculative tectonic
history of Pakistan and surroundings=
Some constraints from the Indian Ocean.
動:A.Farah and K.A.DeJong(eds.)
Gθoみ郷勉∫os げP罐ゑs孟伽,GeoL Surv.
Pakistan,Quetta,p.5-24。
Sawada,Y.,Haq,M.R.,Khan,S.R.and Aziz,A。
(1992)Mesozoic igneous activity in the
/
Yeats,R.S.and Lawrence,R.D
Muslim Bagh area,Pakistan -with
special reference to hot spot magmatism
related to the break-up of Gond-
wanaland一.z4δS飽0孟Sげ助吻OSづ襯0%
1五吻‘z云‘zツごz% GθoZogy,Sh吻z‘z%6 ’92,ノ4i飯z犯,
P.42.
(1984) Tec・
tonics of the Himalayan thrust belt in nor-
them Pakistan.肋:B.U.Huq and J.D.
Milliman (eds.)!吻吻0060Jo劇佛4
006σ%09猶とψh夕qプノ1名ヒzゐ伽薙 Sθα‘刀¢4CoαsホごzJ
勲烈sホ朋,Van Nostrand Reinhold,New
York,P.177-198.
一60一
丁捌露ho⑳姻s翻o云膨吻惚30goσosθ伽6箆雄晦%θo%s硲so励嬢(M碗弼吻孟)
パキスタン西部,モスリムバーグオフィオライトの下位の中生代堆積岩・
火成岩コンプレックスの岩相・構造およびその重要性
」.M:。Me聡ga丑・木村克己・M。R.H.S越面哩腫・小嶋 智
中 孝仁・M.S.Ba紬t・鎌田耕太郎
要 旨
イソドーユーラシア大陸の衝突境界にそって,オフィオライトがインド大陸上に衝上している.それ
らのうち,西縁境界に相当するオフィオライト岩体の一つがパキスタソ西部,モスリムバーグ地域に分
布する.このモスリムバーグオフィオライト(MBO)の直下に断層を境にして,中生界の堆積岩・火成
岩コンプレックス(バーグコンプレックス)が分布する.バーグコソプレックスの南側にはイソド大陸上
に堆積した浅海成中生界(石灰岩地帯)が広く分布し,摺曲・スラスト帯を構成する.バーグコソプレッ
クスと石灰岩地帯とは層相が異なり,両者は東西走行のグワールーバー断層で境されている.本研究は
パキスタン地質調査所との共同プロジェクトであり,論文ではバーグコンプレヅクスの地質を記載し,
ネオテーチスの古地理復元におけるその重要性を指摘する.堆積岩の地質年代及び火成岩の年代・化学
組成はそれぞれ小嶋ほか(本号),沢田ほか(1992)に基づく.バーグコソプレックスは7つの岩相ユニ
ットに区分される.①蛇紋岩メランジュ(Bsm):オフィオライト・変成岩,及びBbcから由来したブ
ロックと蛇紋岩基質からなる.②泥岩メランジュ(Bmm):おもにBbcとBls由来のブロックと泥岩基
質からなる.③玄武岩一チャートユニット(Bbc):ソレアイト質の枕状玄武岩とそれに重なる白亜紀前期からTuronianにかけての層状チャート,そし七半遠洋性泥岩からなる復元層序を有す。④ハイアロ
クラスタイトー泥岩ユニット(Bhm):半遠洋性泥岩(下部は白亜紀前期を示す)とアルカリ火山岩(68-
81Ma)からなる.⑤堆積岩ユニット:石灰質泥岩・砂岩ないし珪質泥岩からなる三畳系(Bls)とジュラ
系(Bus)からなる.⑥超塩基性一塩基性岩ユニット(Bum):オフィオライト由来の岩石からなる.
Bsm及びBmmのメラソジュはオフィオライトの直下,ないしBumの境界にそって産出すること,
勢断構造が発達していることから,オフィオライトのオブダクトに関連して形成された構造性メランジ
ュと考えられる.BbcはMBOとは年代が異なり,MBOの一員ではない.堆積岩ユニットのBls・
Busは,断層で境されているが,堆積岩の岩相の類似性は,三畳紀からジュラ紀にいたる一連の堆積体
をなしていたことを示唆する.これらは浅海成相の石灰岩地帯の同時代の地層と異なり,層厚が約
1000mと薄く,かつ珪質堆積物に富む半遠洋性層の発達を特徴としており,インド大陸の縁辺にあっ
たと推定される.
バーグコンプレックスについての以上の層相の特徴と石灰岩地帯の地層との比較に基づくと,バーグ
コソプレックスを構成する主要な堆積岩と火山岩は,インドとユーラシア大陸の衝突にともなって消失
したネオテーチスからインド大陸縁辺で形成されたと考えられる.
(受付:1993年6月8日;受理:1993年11月19日)
一61一
醐