称号及び氏名

博士（理学）

学位授与の日付

平成２０年６月３０日

論文

名Yousif Osman Mohammad「Petrology of ultramafic and related rocks along Iraqi Zagros Thrust Zone（イラク・ザグロス衝上断層帯に分布する超苦鉄質岩類よび付随する岩石類の岩石学的研究）」

論文審査委員

主査

前川寛和

副査

田中智

副査

小川英夫

副査

石井和彦

論文要旨
Four main ultramafic rock bodies are distributed along the Iraqi Zagros Thrust Zone (IZTZ). They are
Penjwin, Mawat, Pauza, and Qalander ultramafic bodies in the order from southeast to northeast (Fig. 1). Theyare typical lithological markers of the suture zone between Arabian and Iranian plates. Mineralogical andpetrological investigations of the IZTZ peridotites indicate that they are mantle tectonite, rather than cumulate
or replacive rock. They are mainly depleted harzburgite and dunite along with restricted occurrence of spinellherzolite to the Pauza ultramafic body, and consist of olivine, diopside, enstatite, spinel, serpentine polymorphs,amphibole, and chromian chlorite. In dunite and harzburgite, chromian spinel occurs as a coarse subhedral toeuhedral grain. Chromian spinel in dunite and harzburgite commonly has Cr-rich core and Cr-poor rim. Rarely
Chromian spinel in harzburgite shows peculiar zoning of Cr-rich core and Al-rich rim. The lherzolite contains
aluminous spinel which occurs as fine subhedral to anhedral grains with lobate boundaries.
The core compositions of chromian spinels in both Mawat and Penjwin peridotites [Cr#(=Cr/(Cr+Al)) >0.6] correlate with those of type III alpine peridotite. The chromian spinel core composition in Pauza comprises
both type I alpine peridotite for lherzolite (Cr# = 0.36 - 0.40) and type III alpine peridotite for dunite (Cr# =0.78 - 0.82) and harzburgite (Cr# = 0.58 - 0.6). Gradual increasing of TiO2 and gradual decreasing of Al2O3 inchromian spinels with increasing thedegree of partial melting in peridotite,

serpentinite

suggesting that these ultramafic bodies

Pauza
ultramafic
rocks

Qalanderare fragments of suprasubduction zone

38˚

36˚

Mawat
ophiolite
Penjwinophiolite

residual mantle peridotites.
The peridotites of the IZTZ show

34˚

many petrologic and geochemicalsignatures of partial melting. On the32˚

scale of the entire complex, the mantle
rocks range from lherzolite throughout
harzburgite to dunite. These petrologicalvariations are associated with systematicchanges in mineral compositions

30˚

200 km40˚42˚

44˚46˚

48˚50˚

52˚54˚Fig. 1. Main tectonic units of Iraq and adjacent territories, showing
the location of the main ultramafic bodies.

100

especially spinel and olivine. The Cr# of spinel
vs. Fo diagram of the Pauza peridotites show

40 %Parmelting becomes harzburgite at about 30 %

4020 %

peridotites with 30 % of partial melting
becomes dunite at about 40 % melting.

extensive. The transition from partly toextensively serpentinized rock is best observedin the Penjwin peridotites where the color ofperidotites change from chocolate-brown ofpartly serpentinized samples to the yellowishgreen of extensively serpentinized samples.

Fract

ional

10 %

20

The degree of serpentinization of peridotites
along the IZTZ ranges from incipient toAbyssal peridotite

30 %geltin

Cr#60mtial2). Harzburgite in both the Mawat and Penjwin

SSZ peridotite

80

that a fertile lherzolite 10-15 % of partial
melting and dunite at about 40 % melting (Fig.

OSMAFMM

096949290

Fo88

crystallization

Passive margin
peridotite

868482

Fig. 2. Cr-number of spinel vs. Fo content of olivine in ultramafic rocks
from the IZTZ. Fields for spinels occurring in abyssal (and oceanridge), oceanic SSZ and passive margin peridotites are after Dickand Bullen (1984) and Pearce et al. (2000). OSMA field is after Arai(1994). Gray boxes, colorless boxes, and colorless triangles arefrom the Pauza, Mawat, and Penjwin peridotites, respectively.

This color variation between partly and extensively serpentinized samples cannot be noticed in the isolated
serpentinite bodies in Mawat and Qalander areas. For these occurrences, the best evidence of this transitionis the development of “kernel” texture, which is more common for the Mawat serpentinized peridotite. The
kernel texture consists of extensively serpentinized green colored zone forming a rind around rectangular to
trapezoidal block of dun-colored partly serpentinized rock.
The serpentinization process of peridotite along the IZTZ having both criteria of isochemical andallochemical processes. The formation of metasomatic rocks, rodingite and albitite in the Penjwin peridotite
body involves loss and gain of CaO, SiO2, and Na2O between serpentinized peridotite and country rock,indicating that the serpentinization of the Penjwin peridotites is constant-volume process. However, theintensive fracturing in the massive serpentinite as well as the occurrence of kernel pattern requires an increaseof the volume of ultramafic body, suggesting that serpentinizations of Mawat and Qalander peridotites are ofconstant-chemical process.
For the first time, albitite was found in the IZTZ near the village of Mlakawa, 60 km northeast ofSulaimani City, Kurdistan region, northeastern Iraq. It occurs as a white pod within the massive tectonized andserpentinized part of Penjwin ophiolite sequence. The preserved texture and mineralogical, petrological, andgeochemical data from the core of the albitite pod suggest that protolith of Mlakawa albitite is plagiogranite. It
has undergone rodingitization and blackwall formation along its rim.
The occurrence of barium aluminoslicate (celsian), cymrite, barium muscovite, and a high Na 2O
concentration (11 wt. %) of albitite suggests that barium-sodium-rich fluid was involved during the albitization
process of plagiogranite. Evidence of the progressive albitization includes the metasomatic replacement of Caplagioclase to albite and grossular, celsian to cymrite, replacement of tremolite by edenite, and new formationof sheaf-like barium muscovite. The presence of analcime and multiple generations of chlorite suggest thatthe albitite protolith was accompanied by chloritization and retrograde metamorphism before and after thealbitization process.
Ca-amphibole thermobarometry (Fig. 3) and modes of occurrence of polygenetic metamorphic titanite,along with the occurrence of strontium apatite and cymrite, suggest that the albitization of plagiogranite
occurred at < 650 °C and 1.5 GPa.
Three types of metamorphic titanite have been noted in albitite. Type I metamorphic titanite occurs as
subhedral to anhedral fine-grained disseminated crystals within chlorite that was formed during chloritization ofbiotite as a result of low-temperature

0.5

2.5

(T = 330 – 340 °C) ocean-floor

15

metamorphism or rodingitization
2of plagiogranite under a reducing

and intermediate TiO2 (Av: 34.7 wt.

P (GPa).%), high FeOtotal (Av: 0.89 wt. %),1.51%). Type II metamorphic titanite
occurs as a thin rim around ilmenite

0.5

and has high Al 2 O 3 (Av: 4.8 wt.
%), intermediate FeOtotal (Av: 0.6wt. %), and low TiO2 (Av: 33.7 wt.
%); it represents a reaction productbetween grossular and ilmenite,0nts

nte

environment. It is characterized byintermediate Al 2 O 3 (Av: 3.61 wtRimTransitional zoneCore1.0l 2OAco31.5

TiO2 contentsAlbitization

102.0+++++ ++ +++ + +++ + + +
5.0+++ +++++ ++++++ + +Rodingitization+ ++ +++++++ +
400

500

600

700

2.5

3.0
3.5

4.0

800

900

1000

1100

T (˚C)
Fig. 3. Chemical compositions of amphiboles in Mlakawa albitite plotted on a diagramwith isoplethes of Al2O3 and TiO2 of clacic amphibole (after Ernst and Liu, 1998).

which resulted from an oxidizing
high pressure–high temperature (PMinerals paragenesis= 1.4 – 1.6 GPa and T > 750°C)metamorphic event involving
plagiogranite. Type III metamorphic

Plagiogranitetitanite occurs as very coarse, highly

Pre-serpentinization
stage

fractured grains up to 0.5 mm,with inclusions of ilmenite, andFresh ultramafic rocksurrounded by albitic plagioclase,
analcime, and chlorite. The titanite
is characterized by low Al2O3 (Av:1.23 wt. %), low FeOtotal (Av: 0.30

Serpentinization
Rodingitization
Plagiogranite
core
Plagiogranite

Rodingite

wt. %). It formed during extensive

Serpentinization stage

titanitization of ilmenite by aLow P/Tmoderate pressure and temperature

Massive serpentinite

Rodingite +Blackwall rim

Albitite

Rodingite

Albitite core

Many researchers working ontitanite conclude that the iron isAlbitization stage

trivalent and occupies the Ti site.

High P/T
Shear serpentinite

Fe in the titanite structure is directly
controlled by oxygen fugacity duringmetamorphic titanite formation. Inconditions below QMF, the Fe in
the titanite structure is divalent andOlivine
Pyroxene
Cr-spinel

Serpentinization

AlbiteOligoclase
Allanite
ZirconApatiteIlmenite
GrossularPrehnite
TremoliteActinolite
ChloriteType II titaniteCelsian
Grossular

Lizardite
Chrysotile
Cr-chlorite
magnetite
relict -olivine
Talc
Tremolite

Serpentinization

Albitization

conditions, as a result of albitization

This study suggests that the state of

Oligoclase
Andesine
BiotiteAllanite
ZirconApatiteIlmenite

Albitization(P < 1.6 GPa and T< 750°C)
of Ca-plagioclase in plagiogranite.

Peridotite

Rodingitization

wt. %), and high TiO 2 (Av: 36.98

reaction with Ca- plagioclase during

Plagiogranite

Rodingite +Blackwall rim

AlbiteAllanite
ZirconApatiteSr-apatite
CymriteEdeniteDiopside
Bariummuscovite
Type I titanite
Type III titaniteIlmenite
GrossularPrehnite
ChloriteType II titanite
Celsian

Antigorite
Lizardite
Chrysotile
Cr-chlorite
magnetite
Carbonate

Fig. 4. Time-paragenesis diagram showing the relation between albitization,serpentinization, and rodingitization.

occupies the Ca site. In conditions above QMF, the Fe is trivalent and occupies the Ti site
Serpentinization, rodingitization, and albitization processes observed in the Mlakawa albitite arecomplementary processes (Fig. 4). Each process has a specific effect during a particular time in the evolutionof the ultramafic-plagiogranite part of the Penjwin ophiolite sequence, leading to the formation of the Mlakawa
albitite. Both serpentinization of peridotite and rodingitization of plagiogranite represent low-temperature andlow-pressure conditions during the oceanic and the subduction stages before collision of the Arabian plate with
the Iranian plate. In contrast, the albitization of plagiogranite represents high-pressure and high-temperature
conditions at the collisional stage of the Arabian plate with the Iranian plate during the Late Cretaceous period.

審査結果の要旨
本研究は、イラク・ザグロス衝上断層帯に分布する超苦鉄質岩類およびそれに付随する岩石類を対象とし、その岩石学的・鉱物学的特徴を解析し、それらの成因を解明した。
イラク・ザグロス衝上断層帯は、アラブプレートとイランプレートの境界部に位置し、大陸衝突の現場として第一級の研究価値をもつ。本研究が対象とする超苦鉄質岩類は、元来、衝突帯深部に位置していたマントル物質が地殻変動により地表まで上昇してきたもので、その研究を通じて、衝突帯深部の温度圧力条件をはじめとする、さまざまな地球科学的現象を理解することが可能となる。
本研究では、衝上断層帯に分布する４つの超苦鉄質岩体について、その鉱物共生、形成条件を解析した。主要構成鉱物であるスピネルとカンラン石について系統的な組成変化があることをみいだし、超苦鉄質岩類がマントルの部分溶融（メルトの抜き去り）を経て形成されたことを示した。その上で、全体の 10-15％部分溶融したレルゾライトと 30-40％部分溶融したダナイト〜ハルツバー
ジャイトを区別し、それぞれが海嶺起源、島弧起源である可能性を指摘した。単純な鉱物組合せをもつ超苦鉄質岩から得られる情報量には限りがあるため、本研究では、調査地域の超苦鉄質岩体に含まれる優白質の曹長岩、ロジン岩にも注目し、その詳細な解析を行った。その結果、それらが Srアパタイト、Ba- 白雲母など高圧変成作用に特徴的な鉱物を含む斜長石花こう岩起源の変成岩であることを明らかにした。さらに、曹長岩に含まれるチタナイトに注目し、その産状、化学組成を詳細に検討を行い、蛇紋岩化作用、ロジンジャイト化作用、アルバイト化作用の相互関係を明らかにした。
イラク国内におけるイラク・ザグロス衝上断層帯は、第一級の衝突体としてその研究価値が非常に高いにもかかわらず、治安上の問題から岩石学的研究はほとんど行われてこなかった。衝突体深部における高圧変成作用の発見をはじめとする本研究の一連の成果により、アラブプレートとイランプレートの衝突前後の地殻〜マントル深部の地学現象の理解が大きく進展したことに疑いの余地はない。岩石試料の観察から具体的な研究のターゲットを選定・研究テーマを設定し、短期間で問題を解決、成果を論文としてまとめる能力には非凡なものがあり、博士論文からも、今後のさらなる発展が期待できることが読み取れる。
本委員会は、本論文の審査ならびに最終試験の結果に基づき、博士（理学）の学位を授与する事を適当と認める。