Emplacement da ofiolita de Semail

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Semail ophiolite emplacement: a consensual model?
Conference Paper · January 2020
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International Conference on Ophiolites and the Oceanic Lithosphere: Results of the Oman 
Drilling Project and Related Research 
12-14th January, 2020, Sultan Qaboos University, Muscat, Sultanate of Oman 
 
 
 1 
Semail ophiolite emplacement: a consensual model? 
 
 
Françoise Boudier and Adolphe Nicolas 
Geosciences Montpellier, Univ.Montpellier2 & CNRS, 34095 Montpellier, France 
 
 
The Semail ophiolite, preserved for now from 
major orogenic events related to Neotethyan 
Ocean closure, provides in the Oman 
mountains, a unique record of a pre-collision 
system, and motivated largest interest in 
Geosciences community. 
Initiated in the early eighties [1] and [2] a 
considerable controversy has affected the 
interpretation of Semail ophiolite assigned to 
represent a piece of oceanic lithosphere 
detached at a fast-spreading ridge margin, or 
an arc-related lithosphere as supported by 
trace elements signature. Extended to the 
genesis of ophiolite in general, the second 
model took the lead, illustrating the soft 
power of Geochemistry. 
Having largely contributed to the fast-
spreading ridge model, we like to point that 
either model is restricted to the oceanic stage 
of ophiolite evolution, in which the 
obduction-related records are overlooked. 
Integrating the tectonometamorphic evolution 
of the foreland belt during late Mesozoic, and 
looking for subduction-related contamination 
of the ophiolitic mantle, reveals the 
complexity of the ocean-continent system 
leading to obduction of the ophiolite. 
Referring to recent literature, we point here to 
some key aspects. 
 
Early contamination of the ophiolite by 
magmas of continental origin.		
A	 calk-alkaline	 suite	 evolving	 from	 grano-
diorite	 to	 mica-bearing	 granites	 intruding	
the	 mantle	 section,	 are	 exceptionally	
developed	in	the	Emirates	massifs	(Fig.	1).	
They	 are	 also	 represented	 occasionally	 in	
the	Omani	part	of	 the	ophiolite.	Numerous	
excellent	 exposures	 in	 the	 Emirates	 show	
that	 granitoids	 intrude	 the	 mantle	 fabric	
with	some	reactive	margins	(Fig.	1),	or	are	
cross-cut	 by	 diabase	 dikes,	 dating	 the	
granite	 injections	 during	 cooling	 of	 the	
mantle	 section.	 Dated	 91-93	 Ma	 in	 Khor	
Fakkan	 massif	 [3]	 and	 95	 Ma	 in	 Haylayn	
massif	 [4],	 the	granitic	 intrusions	compare	
with	 age	 of	 detachment	 (high-T	
metamorphic	soles),	95-96	Ma		[4].	
Close	 relationships	 of	 spreading	 and	
detachment,	based	on	new	precise	ages.	
A	 series	 of	 precise	 U-Pb	 dating	 on	 single-
grain	zircon	from	gabbro	and	trondjhemite	
[4]	 has	 constrained	 the	 age	 of	 accretion	
between	96.5	and	95.5	from	north	to	south	
along-strike	 the	 Semail	 ophiolite.	 This	
north-south	 shift	 is	 expressed	 similarly	 in	
the	 ages	 of	 detachment,	 96-96.5	 Ma	 in	
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MASAFI
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FAKKAN
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Figure 1. Granitic intrusions (red label) 
in Khor Fakkan massif from Emirates
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granitic intrusions
 
International Conference on Ophiolites and the Oceanic Lithosphere: Results of the Oman 
Drilling Project and Related Research 
12-14th January, 2020, Sultan Qaboos University, Muscat, Sultanate of Oman 
 
 
 2 
northern	 metamorphic	 sole	 of	 Sumeini,	
94.5-95	 Ma	 in	 southern	 Wadi	 Tayin	
exposure. As an achievement, series of 
measurements in Sumeini window document 
an overlapping age of the metamorphic sole 
with that of accretion-related magmatism, 
suggesting that first ages of detachment are 
coeval of late accretion stages [4]. 
 
Instability	 of	 the	 Arabian	 margin	 during	
Late	Jurassic-Cretaceous	times.		
The	 sedimentary	 record	 of the thrust-
repeated allochtonous Hawasina nappes point 
to deposition in a continental marginal basin 
(Hamrat Duru), open to Neotethyan Ocean 
spreading since Triassic.	 Following a period 
of extension during Triassic-Jurassic times, a 
foundering of the continental margin recorded 
at the inner margin of Hamrat Duru basin 
occurs during Titonian-Berriasian (~140 Ma). 
This event is marked by a ~300km southward 
retreat of the continental slope [5]. The next 
tectonic event is the opening of the Muti 
Basin during Cenomanian-Turonian (91 Ma), 
an intrashelf trough coeval of the advancing 
ophiolitic nappe [6].	
	
Cretaceous tectonics. 
Tectonometamorphic evolution of the 
Arabian continental margin is recorded in 
both domal structures of Jebel Ahhdar 
(anchizone to greenschist) and Saih Hatat 
(greenschist to eclogite-amphibolite). 
NE trending stretching lineations represent an 
ubiquitous marker of the shield margin 
deformation (Fig. 2), assumed to relate to a 
Cretaceous event in Jebel Akhdar [6] and [7]. 
In Saih Hatat, NE trending lineations, 
recorded as well in the high-pressure rocks 
and retrograde greenschists, mark a 
structuration evolving during Cretaceous (120 
to 90 Ma) [8], and its NE vergence is assumed 
to represent the progressive exhumation of the 
subducted continent-ocean margin underneath 
the continent [9]. NE-SW transport direction 
is similarly recorded in the kinematics of 
greenschist part from the metamorphic sole of 
the southern ophiolite [7]. 
 
Toward a convergent-margin model. 
Integrating the listed criteriaopens to a model 
of obduction of a hot oceanic lithosphere 
detached in closeness to the continental 
margin, itself marked by instability and 
development of HP metamorphism during the 
hundred million years pending the oceanic 
detachment of the ophiolite. 
 
EMIRATES 
OPHIOLITE
 NORTHERN
 & CENTRAL 
 OPHIOLITE
SOUTHERN
OPHIOLITE
ARABIAN PLATFORM
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ophiolite detachment at 96 Ma 
ophiolite detachment
 at 95 Ma 
HP rocks exhumation
 110-80 Ma ? 
granite intrusions
 at 93 Ma 
Figure 3. Convergent plate model 
for northern and southern Semail ophiolite 
lineation in continental margin
S A I H H
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Fanjah
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lineation in metamorphic sole (greenschist)
Figure 2. Upper Cretaceous tectonics in shield 
 and ophiolite. (red, in LT mantle)
 
International Conference on Ophiolites and the Oceanic Lithosphere: Results of the Oman 
Drilling Project and Related Research 
12-14th January, 2020, Sultan Qaboos University, Muscat, Sultanate of Oman 
 
 
 3 
 
Looking for a present-day analogue point to a 
ridge-trench collision environment at a 
convergent margin of the type America-
Pacific (Fig. 3). Such margin produces a large 
panel of lava geochemical signatures evolving 
from MORB to SSZ [10] that fit the 
variability of Semail effusive compositions. 
As documented from recent studies, the ridge-
trench collision results in a slab window, and 
the possibility of detachment and obduction 
of hot lithosphere on the continental margin. 
The America-Pacific analogue implies a 
continent-oriented subduction of the Tethyan 
plate beneath Arabia margin during 
Cretaceous times (Fig. 3). The ridge-trench 
hybrid model accounts for SSZ geochemical 
signatures and does not eliminate possible 
genesis of the ophiolite at a mid-ocean ridge 
[10]. It provides an explanation of the along-
strike variability recorded over the 500km 
exposure of the Semail ophiolite [7]. 
 
References 
[1] Coleman, R.G., 1981. Jour. Geoph. Res., 
86, 2497-2508. 
[2] Pearce, J.A. et al., 1981. Phil. Trans. 
Royal Soc. London, 300, 299-317. 
[3] Styles, M.T. et al., 2006. The geology and 
geophysics of the United Arab Emirates. 
[4] Rioux, M. et al., 2016. Earth Planet. Sc. 
Lett., 451, 185-195. 
[5] Bechennec, F. et al., 1990. Geol. Soc. 
London, spec. publ., 49, 213-223. 
[6] Le Métour, J. et al., 1990. Geol. Soc. 
London, spec. publ., 49, 327-339. 
[7] Boudier, F. & Nicolas, A., 2018. 
Tectonics,37,https://doi.org/10.1029/2018TC005099 
[8] Gray, D.L. et al., 2004. Earth Planet. Sc. 
Lett., 222(2), 407-422. 
[9] Gregory, R.T. et al., 1998. Tectonics, 
17(5), 657-670. 
[10] Sturm, M.E. et al., 2000. Geol. Soc. 
Amer. Spec. Pap., 349, 13-20. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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