insights on deep, accretionary subduction processes from the sistan ophiolitic “mélange”...
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Lithos 156–159 (2013) 139–158
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Insights on deep, accretionary subduction processes from the Sistan ophiolitic“mélange” (Eastern Iran)
S. Angiboust a,e,⁎, P. Agard a, J.C.M. De Hoog b, J. Omrani c, A. Plunder d
a ISTEP, Université Pierre et Marie Curie, UMR CNRS 7193, F-75005, Paris, Franceb School of GeoSciences, Edinburgh, United Kingdomc Geological Survey of Iran, Tehran, Irand Laboratoire de Géologie, UMR CNRS 8538, Ecole Normale Supérieure, Paris, Francee Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Potsdam, Germany
⁎ Corresponding author at: GFZ German Research Ce3.1, Lithosphere Dynamics, Telegrafenberg, C 220,Tel.: +49 331 288 1363; fax: +49 331 288 1370.
E-mail addresses: [email protected], sam(S. Angiboust), [email protected] (P. Agard), cee(J.C.M. De Hoog), [email protected] (J. Omrani), plu(A. Plunder).
0024-4937/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.lithos.2012.11.007
a b s t r a c t
a r t i c l e i n f oArticle history:Received 2 June 2012Accepted 3 November 2012Available online 13 November 2012
Keywords:OphiolitesMetamorphismSubductionThermobarometrySerpentinites geochemistrySistan
The Sistan ophiolitic belt, formed by the closure of the N–S trending Sistan Ocean during late Cretaceoustimes, comprises several branches and basins across a 100×700 km area along the Iran–Afghanistan border.One of these, the Ratuk complex, exposes disrupted HP ophiolitic blocks from a paleo-subduction complexgenerally interpreted as a tectonic “mélange”.In order to better understand its overall structure and evaluate the degree of mixing within this mélange, anextensive set of serpentinized peridotites, mafic rocks and metasediments was collected in the Sulabest area(Ratuk complex). A detailed geological and structural map of the Sulabest area is herein provided, in whichthree main units (the Western, Upper and Eclogitic Units) separated by relatively sharp tectonic contactswere identified. The latter two of these slices exhibit metamorphic evidence for burial along the same HP–LTgradient (up to blueschist and eclogite facies, respectively). Sharp differences in peak metamorphic conditionsand retrograde parageneses nevertheless suggest that they followed two distinct P–T trajectories. Geochemicalsignatures of ultramafic rocks indicate an abyssal origin for the non-metamorphic Western Unit while the pres-ence of mantlewedge serpentinites is inferred for some samples from the high-pressure units. The differences inpeak temperatures (between 520 and 650 °C) and the geochemical heterogeneity of mafic rocks suggest thattectonic mixing occurred (only) within the high-pressure units, possibly within the hydrated mantle wedge.Our results show that this portion of the Sistan ophiolitic belt did not form, as earlier proposed, by chaotictectonic “mélange” (i.e. where small tectonic blocks with distinct P–T histories are mixed in a mechanicallyweak matrix). We instead propose that this segment of the ophiolitic belt formed via accretionary processesdeep in the subduction zone, whereby distinct slices with different P–T histories were tectonically juxtaposedat ca. 20 km depth during exhumation.
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1. Introduction
Many fossil convergent systems are characterized by the presenceof a subduction-related metamorphic belt, whose structure resultsfrom protracted tectono-metamorphic processes during both burialand subsequent exhumation (e.g. Agard et al., 2009; Ernst, 2003;Garcia-Casco et al., 2006; Platt, 1993). Documenting deep structuralandmetamorphic processes is not only critical to better constrain exhu-mation dynamics or the deep geochemical cycle but also to shed lighton the location and genesis of seismic events presently recorded along
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subduction zones (e.g. Angiboust et al., 2012b; Oncken and ANCORPworking group, 2003).
Accretionary complexes formed in subduction zones are oftenmodeled as wedge-shaped bodies in which weak sedimentary mate-rial is accreted via a decollement surface in front of a rigid buttress(e.g. Cloos, 1982; Platt, 1986). Internal structures unraveled fromfield-based studies appear generally complex, however, with eithertectonic discontinuities or blocks leading to jumps in recorded meta-morphic grade (e.g. Plunder et al., 2012; Sedlock, 1988; Wallis, 1998).In addition, deep tectonic contacts may be obscured in the fieldby tectonic reactivation and overprinting, leading to ambiguities asto which primary tectonic features (i.e., tectonic slices or mélanges)formed at peak burial.
The combination of structural observations with petrological andgeochemical data, in particular high-end thermobarometric methods,is thus needed to identify coherent tectonic slices with distinct meta-morphic histories (such as in the Western Alps HP belt; Angiboust
140 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
et al., 2009, 2012a) or define trajectories of blocks and matrix in“mélange” settings. This approach is particularly important in orderto determine whether the observed tectonic structural complexityis primary (e.g., olistostrome complexes; Wakabayashi, 2011 andreferences therein) or inherited from tectonicmixing along the subduc-tion interface (e.g. Federico et al., 2007).
We herein focus on the Sistan ophiolitic domain (Eastern Iran) asa classical tectonic “mélange” (Tirrul et al., 1983). This ophioliticdomain was recently inferred to have formed by the mixing ofblueschist and eclogite blocks and lenses in amechanically weakma-trix during subduction of the Sistan Ocean (Fotoohi Rad et al., 2005),such as for the Franciscan complex (Coleman and Lanphere, 1971;Tsujimori et al., 2006).
Through detailed mapping, metamorphic petrology and P–T esti-mates tied to geochemical data, our aim is to better document tec-tonic mixing and accretionary processes in the Sistan suture zonein order to provide further constraints ondeep tectonic processes takingplace along subduction interface as well as on regional geodynamics.
2. Geology of the Sistan ophiolitic belt
The Sistan suture zone extends over 1000 km from north to southalong the Iran–Afghanistan border and belongs to the Alpino-Himalayan orogenic cycle associated with the closure of the Neotethysduring Jurassic and Cretaceous times (Agard et al., 2007, 2011; Dewey,1977). This suture resulted from the closure of a N–S extension ofthe Neotethys (Fig. 1, inset) located between the Lut continental blockto the west and the Afghan block to the east from Early Cretaceousto Paleocene times (~120–55 Ma; Agard et al., 2011; Babazadeh andDe Wever, 2004; Delaloye and Desmons, 1980; Tirrul et al., 1983 andreferences therein).
East-dipping subduction (Tirrul et al., 1983) is thought to have ini-tiated during the Turonian–Maastrichtian through an intra-oceanicarc (Saccani et al., 2010). This led to the formation of an accretionarywedge and a (presumably mélange-type) dismembered ophioliticsequence, in which eclogites are found in a few places (Fotoohi Radet al., 2005). After closure, the Sistan region was affected by an in-tense Eo-Oligocene to early Miocene magmatic activity (Camp andGriffis, 1982; Maurizot, 1979). As for other parts of Iran, a late alkalineactivity is found along some major dextral strike-slip faults fromthe late Miocene onwards (Walker et al., 2009). Unlike other Alpineor Himalayan sutures, no subducted continental crustal material hasyet been reported within the Sistan belt. Similarly, the absence ofany significant collision and the lack of a Barrovian overprint permit-ted a relatively good record of the structure and the metamorphic his-tory of this accretionary wedge.
According to Tirrul et al. (1983), the Sistan suture zone can bedivided in two main units: the Neh–Ratuk complex (accretionarywedge “mélange”) to the west and the Sefidabeh fore-arc basin. TheRatuk complex is formed by an anticlinoriummade of Upper Cretaceousflyschs and by an “ophiolitic domain”, deformed during Campanian–Maastrichtian times (Tirrul et al., 1983). The latter is dominantlycomposed of flyschs and tuffs, together with varied ophiolitic materialcomprising peridotites, serpentinites, orthopyroxenites, gabbros, ba-salts and cherts (Geological Survey of Iran, 1990). These lithologiesgenerally occur as variably-sized blocks, from decimeter- to kilome-ter-scale (Fig. 1), within a sedimentary or volcano-sedimentary matrix(Maurizot, 1979; Tirrul et al., 1983).
Although the smallest blocks may be olistolithic in nature,kilometer-sized slices are possibly of tectonic origin. These extremelyheterogeneous and very deformed bodies were interpreted as formedby a tectonic “mélange”, despite the report of 30-km long sub-unitsseparated by tectonic contacts (Fotoohi Rad et al., 2005). Blocks aredominantly made of ultramafics, metabasaltic rocks and metachertsmetamorphosed during late Cretaceous times (Bröcker et al., 2010;Fotoohi Rad et al., 2005, 2009), now juxtaposed as greenschist-facies
metabasalts, epidote–amphibolites, blueschists and eclogites. Maxi-mum depth P–T estimates for these eclogites suggest a burial downto 70 km depth (19–23 kbar) along a relatively cold gradient of ca.8 °C/km (Fotoohi Rad et al., 2005).
3. Structural mapping of the Sulabest area
We focused our investigations on one of the sampling areas ofFotoohi-Rad et al. (2005; Sulabest). The studied area is a 2.5×1 kmzone located 5 km to the southeast of Sulabest village (Figs. 1 and 2),where detailed mapping was performed with a resolution of ~2 m.In this area, the ophiolite is apparently composed of a series of blocks(Fig. 3; tuffs, serpentinized peridotites and mafic rocks) embeddedwithin a serpentinite or tuffaceous matrix and locally cross-cut bysubvertical dykes of granitic composition (Fig. 2). For the sake of clarity,we herein focus on the central part of the study area where the combi-nation of detailed structural and petrological data permitted the identi-fication of at least three structurally independent tectonic units inthe area, as shown in Figs. 2and 4.
(1) The Western unit is dominantly composed of chert (orradiolarite) blocks and mafic tuffs embedded within a partlyserpentinized peridotite matrix. The very rare mafic blockslying within this unit indicate that the Western Unit did notexperience high-pressure (HP) metamorphism (absence ofglaucophane, omphacite or garnet). The absence of HP rocksand the widespread presence of cherts (i.e., > 40 vol.%) werethe diagnostic criteria to establish the boundaries of theWesternUnit. Most of the blocks from theWestern unit are aligned alongthe NW–SE direction, which is the dominant fabric directionin that portion of the Sistan suture zone (Figs. 2 and 3a). The con-tact between the Western Unit and the Upper Unit is sharplydefined in the field (Fig. 2).
(2) The Upper Unit is characterized by a much greater abundance oftuffaceous material (mafic tuffs and schists; Fig. 2). The rocksfrom the Upper Unit underwent a pervasive, stronger deforma-tion than theWesternUnit, leading to the formation of amarked,flat-lying foliation and to large-scale boudinage of more compe-tent lithologies. Stretching mineral lineations, dispersed be-tween N140 and N010, strike N160 on average (Fig. 2). Shearsenses are scarce and deformation is dominated by flattening.TheUpperUnit is also characterized by the presence of a stronglyserpentinized peridotite matrix (especially in the southern partof the study area; Fig. 2). Meter- to hectometer-sized blocksfrom this unit are made of amphibolitized metabasites and/ormetatuffs and blue amphibole-bearing quartzites dispersedwithin the Upper Unit matrix (Fig. 3b). These blocks werestretched, rounded and flattened along the regional stretchingdirection (N160°) during the deformation stage which affectedthe whole suture.
(3) The Eclogitic Unit lies in the middle of the study area as tectonicwindows underneath the metatuffs from the Upper Unit(Figs. 2 and 4a). It is characterized by the presence of meterto decameter-sized rounded blocks of amphibolitizedmetabasitesembedded within a pervasively serpentinized matrix. Metaso-matic talc-schist and tremolite-schist rinds form locally at theboundary between blocks and matrix. The margins from themetabasaltic blocks also exhibit a paragenesis typical of theepidote–amphibolite facies (biotite, hornblende, epidote, albiteand titanite). Importantly, the cores of the blocks, exposed inquarries, generally preserve the prograde eclogite-facies para-genesis (garnet, omphacite, phengite and rutile). Even thoughexposures of the Eclogitic Unit are limited nometatuffaceousma-terial was observed inside this unit. Only one meta-radiolariticblock (showing a sedimentary breccia texture) has been observedwithin the Eclogitic Unit. The contact between the Upper Unit
Doroh
Tabas
Gurchang
Sulabest
Sediments & volcanics from Afghan block
Post-collisionnal volcanicsand intrusives
Peridotites and serpentinites
Mafic rocks and tuffs
Cherts and radiolarites
Sistan ophiolitic suture zone
10 km
Sampling locations fromFotoohi-rad et al. (2005)
Our mapping location
N 3
3° 0
0'32
° 30
'
E 60° 15' 60° 30'
Gazik
N
Figure 2
Flyschs and sandstones
NW Iran
Afg
han
Neo-Tethyanaccretionary complexes
500 km
Sistansuture zone
Lut
Fig. 1. Geological map of a branch of the Sistan suture zone (Ratuk complex) indicating ultramafic domains (dark green) and accreted ophiolitic material (blue, gray and pink). Starslocalize sampling areas from the work of Fotoohi Rad et al. (2005), one of the very rare studies on that geographical area.Modified after the geologicalmap of Geological Survey of Iran at 1/250,000. Inset shows the location of the Sistan suture zone at the regional scale (modified fromFotoohi Rad et al., 2005).
141S. Angiboust et al. / Lithos 156–159 (2013) 139–158
and the Eclogitic Unit is a 5 to 10 m-thick shear zone alongwhichrounded fragments from the Upper Unit were torn off and dis-persed by ductile deformation (Fig. 4b). The main schistosityis well-developed in the vicinity of the contact and parallelto the shear zone between the two units. However, the scarcity
of reliable kinematic criteria and the prevalence of rotationalblock-in-matrix deformation prevented identification of relativemovement between these two units. No evidence of brittle defor-mation (such as cataclastic flow) has been observed along thisshear zone.
?
N
Mafic tuffs Mafic schists
Upper Unit
Calcschists, marbles & siliciclastiques
Metabasites (Epidote blueschists to amphibolites)
Metabasites (amphibolites to eclogites)
Peridotites +/- serpentinized
Metabasites (Epidote blueschists/amphibolites)
Eclogitic Unit
Mafic tuffs
Serpentinites, talcschists
Granitic intrusives
Trails
Mafic samplementionned in text
Satellite view (Figure 3)
Cherts & radiolarites
Tuffs (blocks & matrix)
Western Unit
Metabasites
Peridotites +/- serpentinized
Sampling location
Main lineation
02
09
13
14
m8
g i
j
m2p
m
nm3p
p
m6p
m7p
m32
m41
m52
m58
m59
m
m1p
17
paleocene sandstones
Ultramafic samplementionned in text
m7p
ac
Su
Su
60° 20' 40" 60° 21' 00" 60° 21' 20"
32°
30' 4
0" 3
2° 3
0' 2
0"
300 m
E 60° 20' 20"
32°
31' 0
0"N
32°
30'
00"
Western Unit
Upper Unit
EclogiticUnit
Fig. 2. Detailed field geological map of the Sulabest area (indicated in Fig. 1), showing the three main units identified in the field, the main lithologies and the sampling locations.
N200 m
a b
mafic tuffsblocks serp.-rich
matrix
10 m
W E
Western Unit
Upper Unit
Eclo. Unit
Fig. 3. a. Google Earth© view of a portion of the study area (indicated in Fig. 2) illustrating the structural features observed and described in the text. The numerous trails werecreated when eclogitic blocks were mined. b. Field view of the central part of the Upper Unit, showing the presence of individual blocks (mainly metatuffs) in a matrix consistingof serpentinized peridotites or mafic schists.
142 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
Eclogiteblock Ecl.
MT
MT
Mafic tuffs
MTMT
SerpSerp
Eclogitic Unit
Upper Unit
shear zone
Serp10 m
W E
b
a
?
?West. Unit
Upper Unit
Eclo. Unit
Paleocene sandstones
Basal Unit
N 500 m
radiolarites and cherts
mafic tuffs
mafic tuffsserp.eclogites
peridotites & radiolarites1600
1700
z(m)
b
Fig. 4. a. Tri-dimensional block diagram illustrating the structural relationships between the main tectonic units identified in the field. b. Field view of the tectonic contact betweenthe Upper Unit (here mainly composed of mafic tuffs and mafic schists) and the Eclogitic Unit (mainly composed of serpentinite and eclogite blocks) in which rounded mafic tuffblocks (MT) are dispersed. The Basal Unit, clearly observed at the north of the study area underlying the Upper Unit may be correlated to similar lithologies (i.e. low-grade tuffs)observed south of the study area.
Table 1Occurrence, rock types and representative paragenesis observed in samples from this study.
Sample Unit Rock type Occurrence Paragenesis observed Geochemistry P–T estimates
Su-m52 Western Serpentinite Matrix Srp(Lz) XSu-17 Western Serpentinite Matrix Srp(Lz), Ol, Chr, Mag, Di, En XSu-m6p Western Serpentinite Matrix Srp XSu-m7p Western Serpentinite Matrix Srp XSu-m32 Eclogitic Serpentinite Matrix Srp(Atg), Chr, Mag XSu-02 Eclogitic Mafic eclogite Block Omp, Grt, Phg, Ep, Brs, Ab XSu-14 Eclogitic Mafic eclogite Block Omp, Grt, Phg, Bt, Brs, Mhb, Act, Ab, Chl X XSu-j Eclogitic Mafic eclogite Block Omp, Grt, Phg, Mkt, Ep, Ab XSu-m Eclogitic Mafic eclogite Block Omp, Grt, Phg, Bt, Ep, Ab X XSu-n Eclogitic Mafic eclogite Block Omp, Grt, Phg, Gln, Mkt, Bt, Ab, Ep X XSu-09 Eclogitic Mafic eclogite Block Omp, Phg, Act, Ab, Ep, Chl XSu-m3p Upper or Eclog. Serpentinite Matrix Srp(Atg), Chr, Mag XSu-m1p Upper Serpentinite Matrix Srp XSu-m2p Upper Serpentinite Matrix Srp(Atg), Mag, Di, Ti-Chn XSu-m8 Upper Mafic tuff Matrix Ab, Amp, Chl, Ep, Ttn, Qtz, Mag, Phg XSu-m41 Upper Mafic tuff Matrix Ab, Chl, Ep, Amp, Qtz, Mag, Phg XSu-m51 Upper Mafic schist Matrix Ab, Chl, Ep, Amp, QtzSu-m58 Upper Mafic schist Matrix Ab, Ep, Amp XSu-m59 Upper Mafic schist Matrix Ab, Ep, Amp XSu-g Upper Mafic tuff Block Ab, Amp, Ep, Chl, Phg, Qtz XSu-13 Upper Mafic tuff Block Ab, Ep, Amp, Chl XSu-p Upper Mafic schist Block Ab, Ep, Phg, Amp, Chl, Ab, Cal XSu-ac Upper Mafic tuff Block Ab, Ep, Phg, Amp X
143S. Angiboust et al. / Lithos 156–159 (2013) 139–158
Table 2Paragenesis observed in five samples (block and matrix) from the Upper Unit mafictuffs.
Nature UpperUnit
Paragenesisand relativemodalproportions
Average mineral composition
Samp.ref.
Amphibole Chl Ep Lwspseudo.
Block Su-g Ab(33)Amp(26)Ep(18)Chl(14)Ph(7)Qz(2)
Wnc, Brs,Mkt
Clc82Ame15Sud3 Ps23Zo77 Yes
Matrix Su-m51 Ab(32)Chl(22)Ep(17)Amph(16)Qz(13)
Act Clc80Ame15Sud5 Ps22Zo78 Yes
Matrix Su-m41 Ab(30)Chl(20)Ep(18)Amp(16)Qz(10)Ph(6)
Brs, Act Clc81Ame15Sud4 Ps28Zo72 No
Matrix Su-m8 Ab(47)Amp(20)Chl(18)Ep(6)Sph(4)Qz(2)Mag(2)Ph(1)
Gl, Fbrs,Act
Clc81Ame12Sud7 Ps25Zo75 ?
Block Su-p Ab(24)Ep(21)
Act, Mhb Clc79Ame6Sud15 Ps23Zo77 Yes
144 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
4. Petrography
In combination with the above structural observations, furtherpetrological and thermobarometric data were gathered to assesswhether the Sistan ophiolitic belt can be regarded as a mélangewith markedly different P–T histories between the blocks and thesurrounding matrix (Fotoohi Rad et al., 2005; Tirrul et al., 1983).One of the main challenges is obtaining accurate P–T constrains forthe meta-tuffaceous matrix of the Upper Unit, where paragenesesare very poor and strongly recrystallized during late exhumation tec-tonic processes. Mineral occurrences for the samples studied beloware given in Table 1.
4.1. Petrology of mafic rocks from the Upper Unit
The mineral paragenesis of the metatuffs is very homogeneousalong strike within the Upper Unit (for both the matrix and theblocks). Albite (ca. 30 vol.% on average; Ab98), epidote (~15 vol.%,Ps15–25) and chlorite (5–10 vol.%; Mg#=[0.5–0.6]) are the dominantmineral species (Fig. 5; Table 2). The foliation is underlined by am-phibole (10–30 vol.%), whose composition varies from actinolite toMg-hornblende (for calcic amphiboles) or from winchite to barroisite(for calcio-sodic amphiboles; Leake et al., 1997). Glaucophane is locallypreserved in the core of some barroisites, which are themselvesrecrystallized to actinolite or hornblende on the rims. Some domainscontain lozenge-shaped aggregates dominantly constituted by epidote(±chlorite±albite; Fig. 5c) and interpreted as pseudomorphs afterlawsonite. Phengite in textural equilibrium with the main fabric ex-hibits silica content generally comprised between 3.3 and 3.4 p.f.u.,with higher values (up to 3.42 Si p.f.u.) preserved in core domains.
Some levels are particularly enriched in magnetite (Fig. 5a) withvariable Ti contents (up to Usp50). Biotite is extremely rare withinUpper Unit samples and was only observed in one sample. Althoughno relict clinopyroxene was found within Upper Unit mafic tuffs,the coeval presence of lawsonite pseudomorphs and blue amphibolessuggests blueschist facies conditions for the prograde part of the path.
4.2. Petrology of mafic rocks from the Eclogitic Unit
Mafic eclogites are systematically retrogressed near their marginsby an amphibolite-facies paragenesis but occasionally exhibit well-preserved relics of the eclogite-facies peak conditions (Fig. 6; Table 3).Amphibolitization of the eclogites is characterized by (i) garnetbreakdown and crystallization of an epidote/hornblende/chlorite as-semblage (Fig. 6a, b), (ii) the breakdown of omphacite and its replace-ment by a symplectitic mixture of albite and sodi-calcic amphibole(Fig. 6c–e), and (iii) the breakdown of eclogite-facies phengite
AbAct
Chl
Mag
EpSu-m41 a Su-g
Ep
Chl
Ab
Fig. 5. a. Optical microscope (plain polarized light) view of a typical mafic tuff matrix (samp(Mag) coexist (black bar for scale: 500 μm). b. Optical microscope (plain polarized light) viamphiboles (mainly barroisite here — Brs, locally rimmed by hornblende — Hbl), chlorite anpolarized light) view of a mafic schist matrix (sample Su-m51) showing a lozenge-shapedafter lawsonite (black bar for scale: 500 μm).
(peak phengite has Si contents of 3.50–3.55 atoms p.f.u.) by biotite.Omphacite exhibits a marked chemical zoning characterized by a de-crease in ferric iron content from core to rim (Table 4). Lawsonitepseudomorphs, now replaced by epidote and paragonite, are frequentlyobserved within garnet cores and mantles (Fig. 6f). Garnet generallyshows a classical “alpine” zoning profile with Mn–Ca enriched coresand Mg–Fe enriched rims (Fig. 6g, h). Occasionally, oscillatory zoning(ca. 10 μm-wide) is observed within garnet rims (sample Su-02).
4.3. Petrology of serpentinites
The peridotites from the study area are generally stronglyserpentinized (over 90%). Those from the Western Unit constitute largeand weakly deformed bodies, whereas those from the Eclogitic Unit arestrongly schistose and wrap around eclogitic blocks. Serpentinites fromthe Western Unit (in which serpentine has been identified by Raman
b cSu-m51
Brs
HblEp
PhChl
Ep
Ab
le Su-m41) where epidote (Ep), actinote (Act), chlorite (Chl), albite (Ab) and magnetiteew of a mafic schist matrix (sample Su-g). The foliation is defined by the alignment ofd occasionally phengite (Ph) (black bar for scale: 500 μm). c. Optical microscope (plainaggregate consisting of epidote, chlorite and albite, and interpreted as a pseudomorph
Hbl Ab
TtnEp
a b
c
Ab
Grt
Ep
Hbl
Eclogite
Amphibolite
Mhb
Bt
5 mm
1 mm
1 cm
d e
f g h
Su-mSu-m Su-n
Su-n Su-n Su-n
Grt
1 mm
Mhb
Ab
Omp
100 µm 200 µm
Ps. Lws
Ps. Lws
GrtQz
Mn MgEp
Pg
Bt
MhbOmp
1 cm Am
phib
oliz
atio
n fr
ont
Fig. 6. a. Picture of an amphibolized block rim where large titanite crystals (Ttn) coexist with albite, hornblende and epidote. b. Picture of an amphibolized block rim showing largepseudomorphs after garnet consisting of hornblende and epidote. Note that garnet (Grt) is occasionally preserved in the middle of the pseudomorph. The surrounding matrix, re-placing former omphacite, is symplectitic and dominated by a Na-rich feldspar (Ab). c. Scanned thin section showing the transition between the amphibolized eclogite (top) and thefresh eclogitic matrix (bottom). Mhb: magnesio-hornblende, Bt: biotite. d. Optical microscope view (plain polarized light) of a magnesio-hornblende crystal overgrowing theeclogitic matrix (sample Su-m). Phengite is recrystallized by biotite. e. Optical microscope view (plain polarized light) of the eclogitic matrix replaced by a symplectitic assemblagedominated by Mg-hornblende and albite (sample Su-n). f. Back-scatter electron image of a garnet crystal from sample Su-n showing quadrangular assemblages consisting of epi-dote and paragonite (Pg) included within garnet mantle, and interpreted as pseudomorphs after lawsonite. Note that quartz (Qz) is also included within garnet rim. g and h. Elec-tron microprobe chemical maps (Mn left and Mg right) showing the chemical zoning of a garnet from sample Su-n. Hot colors point to higher concentrations.
145S. Angiboust et al. / Lithos 156–159 (2013) 139–158
spectroscopy as lizardite) best preserved their primary peridotiticparagenesis (Tables 1 and 5). These samples were unfortunatelytoo extensively serpentinized in order to determine the petrologicaffinity of the protolith (i.e. lherzolite or harzburgite). Lizardite gen-erally statically replaces olivine and pyroxene (sample Su-17; Fig. 7a, b).Clinopyroxene is rich in Al2O3 and Cr2O3 (3.6–7.0 and 1.2–2.7 wt.%,respectively), has Mg# of 0.89–0.93 and low TiO2 (0.08–0.19 wt.%).Orthopyroxene also has high Al2O3 and Cr2O3 (2.9–3.5 and 0.75–0.93 wt.%, respectively), has high CaO (1.0–4.0 wt.%) and has Mg# of0.91–0.92. Olivine is Fo-rich (90.7–91.1) and has b0.02 wt.% CaO.Chromian spinel (Cr# 0.38–0.40, Mg# 0.63–0.66) is locally preservedand has low TiO2 (0.01–0.14 wt.%).
Unlike the serpentinized peridotites from the Western Unit, theprimary paragenesis is almost completely obliterated in Eclogitic andUpper Unit serpentinites and now made of antigorite (identified, again,by Raman spectroscopy). Metamorphic clinopyroxene is almost purediopside with high Mg# (0.98). Relics of chromite (chromian spinel)
Table 3Synthesis of observed paragenesis in eclogite blocks from the Eclogitic Unit.
Sample Peak paragenesis Retrograde paragenesis Inclusion withingarnet mantles
Su-14 Grt Omp Ph Bi Brs Mhb Act Ab Chl Omp Ep Chl PhSu-m Grt Omp Ph Bi Ep Ab Omp Ep Chl PhSu-n Grt Omp Ph Gln(?)
LwsAmp(Ca–Na) Bi Ph Ab Ep Omp Gln Ep+Pg(Lws)
Su-02 Grt Omp Ph Brs Ep Ab Omp Ep Chl PhSu-j Grt Omp Ph Lws Mkt Ep Ab Omp Ep
are preserved locally, but are in general pervasively oxidized to mag-netite (Fig. 7c). This alteration pattern is relatively common withinultramafic rocks (De Hoog et al., 2009; Hattori and Guillot, 2007;Ulmer, 1974). Chromites have high Cr# (0.47–0.65) and low TiO2
(b0.11 wt.%). Titano-chondrodite [Mg2SiO4]2[Mg(OH)2,TiO2] was ob-served in association with antigorite in a vein from sample Su-m2p(Upper Unit) and has a high Mg# of 0.94, high MnO (0.9–1.1 wt.%)and Ti# ~0.45. Titano-clinohumite, more commonly reported in thistype of rocks (e.g., Aoki et al., 1976; Trommsdorff and Evans, 1980),wasnot found, nor breakdownproducts of clinohumite such as geikieliteand magnesian ilmenite.
Note that the ubiquitous presence of lizardite in the Western Unitsuggests Tb350 °C, whereas the crystallization of antigorite withinUpper and Eclogitic Units requires T>350–400 °C (Evans, 2010;O'Hanley and Wicks, 1995). The occurrence of Ti-chondrodite insteadof Ti-clinohumite suggests high-pressure low-temperature conditionsof less than 600 °C at 25 kbar (Engi and Lindsley, 1980). No secondarymetamorphic olivine was observed.
5. Thermobarometry
5.1. Upper Unit
5.1.1. Raman spectroscopy of carbonaceous matter (RSCM)The RSCM geothermometer of Beyssac et al. (2002), based on the
irreversible transformation of carbonaceous matter into graphite dur-ing increasing metamorphism, allows the determination of the maxi-mum temperature reached by a sample, in the range 330–650 °C
Table 4Nine representative EMP analysis of minerals from the Eclogitic Unit samples.
Mineral Omp Omp Grt Grt Ph Ep Bt Amp Amp
Sample Su-m Su-14 Su-14 Su-n Su-14 Su-m Su-14 Su-n Su-14
Analyse q68 q51 ai64 q111 p233 193-9 ah35 p105 ai65
Location Core Rim Core Rim Core Core Core Core Core
SiO2 55.07 55.27 37.44 37.55 52.88 37.92 37.53 58.01 49.88TiO2 0.00 0.01 0.24 0.00 n/a 0.00 0.78 0.00 0.25Al2O3 6.12 7.67 19.47 20.89 24.01 24.84 14.11 11.09 8.06FeO 11.16 4.42 10.04 31.76 3.14 11.13 19.30 9.72 14.72MnO 0.13 0.06 24.94 0.45 0.00 0.27 0.82 0.02 0.39MgO 7.61 10.51 0.68 3.73 4.10 0.01 12.62 10.36 11.13CaO 12.57 16.07 7.28 5.59 0.05 23.02 0.13 0.68 7.32Na2O 7.44 5.99 0.04 0.00 0.18 0.05 0.11 7.32 4.12K2O 0.00 0.03 0.00 0.05 10.53 0.00 9.41 0.01 0.21Sum 100.11 100.03 100.14 100.02 94.89 97.22 94.82 97.20 96.09
# (O,OH) 6 6 12 12 11 13 11 23 23
Si 1.98 1.96 3.02 3.00 3.55 3.08 2.89 7.97 7.30Ti 0.00 0.00 0.01 0.00 0.00 0.00 0.04 0.03 0.03Al 0.26 0.32 1.87 1.96 1.90 2.38 1.28 1.76 1.39Fe 0.29 0.13 0.68 2.12 0.18 0.76 1.24 1.12 1.77Mn 0.00 0.00 1.71 0.03 0.00 0.02 0.05 0.00 0.05Mg 0.41 0.56 0.08 0.44 0.41 0.00 1.45 2.12 2.43Ca 0.48 0.61 0.63 0.48 0.00 2.00 0.01 0.10 1.15Na 0.52 0.41 0.01 0.00 0.02 0.01 0.02 1.95 1.17K 0.00 0.00 0.00 0.00 0.90 0.00 0.92 0.00 0.04Compo. Jd24Aeg28Quad48 Jd26Aeg13Quad61 Grs20Prp3Alm19Sps55 Grs16Prp14Alm69Sps1 Cel20Fcel9Ms70Pg2 Ps24Zo76 XMg=0.53 Gln Brs
146 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
(see Appendix A for details on the method). The meta-tuffaceousstudied here are extremely poor in carbonaceous organic matterand only four samples provided reliable Tmax results. Temperature es-timates from the Upper Unit lithologies are relatively homogeneous,ranging from 459 °C to 490 °C (±10–15 °C; Table 6; Beyssac et al.,
Table 5Seven representative analyses of minerals from ultramafic samples.
Mineral Lz En Ol Di Ti-Chn Chr Mag
Sample Su-17 Su-17 Su-17 Su-m2p Su-m2p Su-m3p Su-m3p
Unit Western Western Western Upper Upper Ecl/Upper
Ecl/Upper
Analysis ad101 ad97 ad98 z13 z21 ac6 ac7SiO2 41.27 55.77 41.60 55.35 33.82 0.10 0.33TiO2 bdl 0.02 bdl 0.02 8.84 0.06 0.01Al2O3 0.44 3.28 0.02 bdl bdl 19.12 0.04Cr2O3 bdl 0.90 0.02 0.01 0.01 49.63 0.93Fe2O3 1.40 67.55FeO 5.96 4.90 8.73 0.59 5.44 19.56 30.66MnO 0.09 0.10 0.11 0.08 0.94 0.27 0.06MgO 37.51 31.35 49.58 18.16 47.21 10.11 0.37CaO 0.00 3.97 0.00 25.50 0.02 0.02 0.09Na2O 0.05 bdl 0.02 0.01 bdl bdl 0.03K2O 0.02 0.01 0.01 0.01 bdl 0.01 bdlH2O 12.25 2.8Sum 97.58 100.29 100.09 99.73 99.08 100.15 100.06
#(O,OH) 9 6 4 6 10 4 4
Si 2.005 1.927 1.011 2.004 2.037 0.003 0.012Ti 0.000 0.000 0.000 0.001 0.400 0.001 0.000Al 0.025 0.134 0.000 0.000 0.000 0.714 0.002Fe3+ 0.000 0.000 0.000 0.000 0.000 0.033 1.947Cr 0.000 0.025 0.000 0.000 0.000 1.243 0.028Fe2+ 0.242 0.142 0.177 0.018 0.274 0.518 0.982Mn 0.004 0.003 0.002 0.002 0.048 0.007 0.002Mg 2.717 1.615 1.796 0.980 4.239 0.478 0.021Ca 0.000 0.147 0.000 0.989 0.001 0.001 0.004Na 0.005 0.000 0.001 0.001 0.000 0.000 0.002K 0.001 0.000 0.000 0.000 0.000 0.000 0.000OH 3.970 1.125Mg# 91.8% 91.9% 91.0% 98.2% 93.9% 48.0% 2.1%
Fe2O3 of Chr and Mag, and H2O of Chn and Liz calculated from stoichiometry.
2004). Sample Su-20, sampled 4 km south of the study area in a tec-tonic unit showing many structural and petrological similarities withthe basal unit (shown in Fig. 4a), recorded a much lower temperature(377 °C). This observation again points to the existence of tectonicdiscontinuities within the regional structure.
5.1.2. THERMOCALC average P–T estimatesTHERMOCALC software v.3.3 was used with an updated version of
the Holland and Powell (1998) dataset to estimate P–T conditions ofthe samples studied here. Activities of mineral end-members werecalculated following Holland and Powell (1998). Only few samplespassed the SigFit statistical criterion (Holland and Powell, 1998) whentrying to equilibrate the observed mineral paragenesis from the mafictuffs (Table 7). Temperature estimates (450–530 °C) are neverthelessin excellent agreement with RSCM Tmax estimates and fall within theupper greenschist facies. Pressure estimates range from 5 to 6 kbar.
5.1.3. FeTi-oxide thermometrySome samples (such as Su-m8 and Su-m41) contain significant
amounts of syn-metamorphic Ti-magnetite grown along the mainfabric. Thermodynamic data on miscibility domains from Ghiorso(1997) in the binary system magnetite Fe3O4–ulvöspinel Fe2TiO4 dem-onstrate that the presence of Usp50 Ti-magnetite in our samples requiresa minimum temperature of 480 °C. This temperature agrees with thetwo previous methods, confirming that the mafic tuffs from the studyarea experienced at least 480 °C during their metamorphic evolution.
5.1.4. Pseudosection modelingIn order to obtain constraints on the P–T conditions for the para-
genesis considered here, we performed pseudosection modeling ofsample Su-m8 using the software PerpleX v.7 (Connolly, 1990). Weused the following set of activity models: amphibole (Dale et al.,2005), chlorite (Holland et al., 1998), omphacite (Green et al., 2007),feldspar (Newton et al., 1980) and garnet (Holland and Powell, 1998).Given the moderate proportion of pistachite in epidote and the pres-ence of magnetite (instead of hematite) in most meta-tuffaceous sam-ples, we believe that oxygen fugacity was relatively low and thereforechose to neglect ferric iron in solid solution models for that calculation.
200 µm
Su-m32
Chr
Mag
Atg
c
En
Lz
500 µm
a b
200 µm
Su-17
En
Ol
Lz
Su-17
Fig. 7. a. Optical microscope view (plain polarized light) of an enstatite crystal (En) pseudomorphically replaced along cleavages, fractures and rimmed by lizardite (Lz) (sample Su-17).b. Opticalmicroscope view(crossed polarized light) of a serpentinizedperidotite sample (Su-17) showing relics of olivine and enstatite. c. Back-scatter electron image of a chromian spinel(Chr), locally replaced by magnetite (Mag) (sample Su-m32).
147S. Angiboust et al. / Lithos 156–159 (2013) 139–158
These heterogeneous lithologies, which often exhibit a markedbanding, are not suited for whole-rock chemical analysis. We thereforethe chemical composition in situ by combining modal proportions(based on microscopic observation; Terry and Chilinger, 1955) andaveragedmineral chemical compositions from EPMA. Averaged min-eral modal proportions are the following: Ab (47), Amph (20), Chl (18),Ep (6), Ttn (4), Qz (2), Mgt (2), and Phg (1). The calculated bulk-rockcomposition in weight percent is: SiO2 (55.2), Al2O3 (16.2), FeO(10.2), MgO (4.9), CaO (4.1), and Na2O (6.6). We chose to neglectminor components such asMnO (0.2%) andK2O (0.4%) as a first approx-imation. TiO2 (mainly present in titanite within mafic tuffs) does notgenerally enter the solid solutions considered here and has thereforebeen also ignored.
The P–T estimates obtained for the high-temperature retrogradeassemblage considered here (amphibole, epidote, chlorite and albite)are between 450–500 °C and 5–8 kbar (Fig. 8a). The position of thehatched domain in Fig. 8a is constrained by the appearance of amphi-bole, epidote, chlorite and the lack of omphacite. Mineral volume pro-portions calculated at T=480 °C and P=8 kbar are: Gln (39), Ab (34),Brs (11), Zo (10), and Chl (7). A direct comparison between observedand modeled assemblage proportions is however impossible due tometamorphic recrystallization after the thermal peak (such as the for-mation of green amphibole and albite after glaucophane).
5.2. Eclogitic Unit
5.2.1. THERMOCALC average P–T estimatesBased on petrological and textural relationships,wedetermined peak
metamorphic conditions with the software THERMOCALC (see above)in average P–T mode for five metabasaltic samples from the EclogiticUnit (with the paragenesis Grt–Omp–Phg–Qtz±Ep±Lws with purewater in excess). Results are presented in Table 8. Equilibriumpressuresare relatively homogeneous (mainly 21–25 kbar), whereas tempera-tures show larger variation from 530 to 650 °C. A few estimates forthe prograde P–T path,which could be obtained by equilibrating phasesincluded within garnet cores (Grt–Omp–Phg–Lws–Qtz–Gln) for threesamples (Su-J, Su-m and Su-14), cluster around 450 °C±25 °C and16–19 kbar (i.e. along a cold prograde path of 8 °C/km).
Table 6Synthesis of RSCM geothermometer results on four samples from the study area(details on the method are given in Beyssac et al., 2002). n: number of spectra acquiredfor each sample; SD: 1σ standard deviation on temperature T.
Sample Tmax (°C) n R2 SD (T)
Su-m58 (matrix) 490 11 0.34 9.1Su-m59 (matrix) 459 12 0.41 14.6Su-13 (block) 480 12 0.36 14.3Su-20* (matrix) 377 14 0.59 12.4
NoequilibriumP–T conditions could be obtained for the amphibolite-facies overprint due to large P–T error bars and dismissed SigFit values.We therefore took an alternative approach using pseudosections toconstrain amphibolitization conditions.
5.2.2. Pseudosection modelingThe above petrological observations indicate that amphibolitization
of the block margins from the Eclogitic Unit was non-isochemical andassociated with intense fluid infiltration. Block cores generally contain1–3 vol.% of water-bearing minerals (which themselves contain2–5 wt.% H2O) so the mineral-bound water content is b0.1 wt.% H2O.In contrast, block margins contain 40–60 vol.% water-bearing minerals,which themselves contain 2–12 wt.% H2O. Hence, the bulk water con-tent of the block margins reaches 2–3 wt.% H2O.
To determine the amphibolitization conditions we used a blockmargin composition as input for pseudosection modeling (assumingthat the system reached thermodynamic equilibrium). For the select-ed sample (Su-14), we used the same bulk-rock calculation methodas outlined previously in the section devoted to the Upper Unit. Aver-aged mineral modal proportions for the block rim are the following:amph (44), ab (24), bt (17), omp (6), grt (3), rt (3), and chl (3).The amount of ferric iron in the bulk rock composition was derivedfrom ferric iron estimates from structural formulae for each mineral(clinopyroxene; garnet: Droop, 1987; and amphiboles: Leake et al.,1997). The chemical system considered here is NCKFMASHTO andthe bulk-rock composition obtained is: SiO2 (50.3), TiO2 (3.1), Al2O3
(12.1), FeO (11.5), Fe2O3 (0.93), MgO (8.2), CaO (5.8), Na2O (4.4),and K2O (1.7). The activity models used for this calculation are fromDiener et al. (2007) for amphibole, Holland et al. (1998) for chlorite,Holland et al. (1998) for phengite and epidote, Green et al. (2007)for omphacite, Newton et al. (1980) for feldspar, White et al. (2000)for garnet and Powell and Holland (1999) for biotite. Pure water isconsidered to be in excess. As a first approximation, we chose to ne-glect partial melting because (i) no evidence for partial melting wasobserved and (ii) activity models for melting processes at relativelyhigh pressure conditions are still uncertain (e.g., Hacker et al., 2011;Trooper et al., 2006).
Table 7THERMOCALC average P–T results obtained for two Upper Unit mafic tuff samples. N.Ind. Reac.: number of independent reactions considered to calculate the equilibrium.
Sample Equilibriumparagenesis
T (°C) P (kbar) DT DP SigFit N.Ind.Reac.
Su-m8(matrix)
Brs–Ep–Chl–Pg–Qz–Ab
513 °C 5.3 kbar 22 °C 0.6 kbar 1.01 5
Su-p(block)
MgHbl–Ep–Chl–Pg–Qz–Ab
476 °C 5.7 kbar 30 °C 0.8 kbar 1.26 5
amp omp lws qz amp omp lws pa qz
amp omp zo pa qz
amp omp zo ab qz
2 amp chl fsp qz
2 amp fsp pa qz
2 amp chl fsp2 amp
chl fsp ab
2 amp ompchl zo ab
2 ampzo ab
2 amp zo ab
q
2 amp fsp zo pa qz
15
T(°C)
P(k
bar)
10
7.5
5
12.5
400 500450 550
2
22
omp +
chl +
zo +
fsp +fsp +p +sp ++p p p
z amp z omp lws pa
qz
mp amp oomab qz
2 ampchl fsp ab
mpp
ab qz zo a
2 amp chl zo ab
RSCM Tmax
Su-m8
Ave. PT a5
7
9
11
13
T(°C)
P(k
bar)
500 600 700650550
0.540.
55
0.5
5
0.55
0.55
0.55
0.56
0.56
0.56
0.58
0.58
0.6
0.6
0.62
Xmg(bi)
0.56
0.7
0.7
1
1
1.12
1.12
1.16
1.16
1.16
1.2
1.2
1.2
1.2
1.25
1.25
1.25
1.3
1.3
XMg(bt)=0.53-0.54
Na(amp)=1.1-1.2
Thermal peak assemblage observed:Brs-Bt-Ab(95)-Rt+/-Omp
Na(
amp)
1
1.25
Su-14
mafic tuff eclogite-rim
55
6
0
amp bi ompfsp ilm mag
amp bi ompfsp qz ilm mag
amp bi ompfsp qz rt ilm magamp bi omp
fsp qz rt magamp bi omp
fsp qz rt
amp bi ompfsp ph qz rt
amp bi ompph qz rt
amp bi ompfsp grt qz rt
bi omp chlph qz rt
amp bi ompph pg qz rt
amp bi ompph ab qz rt
amp bi fspspn qz rt mag
amp bi ompep ab qz rt
amp bi ompfsp ilm rt mag
Best-fit A
REA
Melt b
Fig. 8. a. P–T pseudosection calculated for a mafic tuff sample (Su-m8) showing the field where the sample recrystallized (hatched domain), the average P–T estimates obtained forthis sample (dotted ellipse) and the maximum temperature obtained with the RSCM method on this sample (gray shaded rectangle). The white arrow represents the hypotheticalP–T path followed by this sample. b. P–T pseudosection calculated for an amphibolized block rim composition (sample Su-14). Dotted lines represent isopleths calculated for biotite(Mg#) and amphibole (Na p.f.u). The field corresponding to the observed assemblage is rimmed by a thick line. The area where the fit between chemical data and modeled isoplethcompositions is the best is shown by a gray shaded envelope. The black-dotted arrow represents the hypothetical P–T path obtained for this sample. The thick dotted-line repre-sents the wet solidus of hornblende-bearing metabasalt (after Green, 1982).
148 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
The stability field corresponding to the observed assemblage asso-ciated with amphibolitization (amphibole, biotite, omphacite, feld-spar, rutile and quartz) lies within the dotted-line domain in Fig. 8b.To better constrain the P–T conditions of amphibolitization, the iso-pleths of the Na content of amphibole (0.7–1.3 p.f.u.) and of theMg# of biotite (0.54–0.63) are also shown. The best fit between min-eral and model data for Na content in amphibole and for Mg# contentin biotite occurs at conditions between 11–14 kbar and 600–700 °C(Fig. 8b), for which the model predicts values close to those deter-mined by EMPA (i.e., barroisite with Na ~1.15–1.20 and biotite withMg# of 0.52–0.53; Table 4). Modal abundances (in vol.%) calculatedfor 680 °C and 13 kbar are as follows: amphibole (56), biotite (16), feld-spar (15), omphacite (7), quartz (4), and rutile (2).
P–T estimates for the Upper and Eclogitic Units are shown inFig. 9a, and compared to those of Fotoohi Rad et al. (2005) for the lat-ter (Fig. 9b). Note that P–T estimates for the amphibolitization of theEclogitic Unit are very close to or within the hydrous partial-meltingdomain (Green, 1982; Peacock et al., 1994). Even though our fieldobservations did not identify partial melting features produced dur-ing amphibolitization, we cannot exclude some incipient melting(i.e., a few percents at the most). Such a small amount would neitherdramatically affect phase relationships within the studied chemicalsystem nor these P–T estimates of amphibolitization.
6. Geochemical characterization of the Sulabest ophiolite
In order to provide geochemical constrains on the nature of Sulabestophioliticmaterial, threemeta-tuffaceous samples from the Upper Unit,six eclogite samples from the Eclogitic Unit and eight serpentinitesamples (from the Western, Upper/Eclogitic and Eclogitic Units)were analyzed for their whole-rock composition (Tables 9 and 10; seeAppendix A for analytical methods).
6.1. Geochemistry of mafic rocks
Mafic eclogites have lowSiO2 (48.2–52.6 wt.%),moderateMgO (4.5–7.6) and a wide range of CaO contents (5.7–13.1 wt.%). Mg# ranges
from 0.42 to 0.53. High TiO2 contents (0.9–1.7 wt.%) are consistentwith these being metamorphosed volcanic rocks and not cumulates, asthe latter typically have low TiO2 (Pearce, 1983; this is not the casefor Fe–Ti gabbros, but no iron enrichmentwas observed in our samples).
Data for incompatible trace elements resistant to weathering andmetamorphism reveals some significant variations within samplesfrom the Eclogite Unit. The two different hand samples from blockSu-n and sample Su-09 exhibit slightly depleted rare earth element(REE) profiles similar to N-MORB with (La/Yb)N of 0.5–0.7 and no Euor Ce anomalies (Fig. 10a). In contrast, Su-m and Su-14 eclogites showpronounced LREE enrichments with (La/Yb)N=2.6–6.9, in addition tosmall negative Eu and Ce anomalies (Eu/Eu*=0.8-0.9 and Ce/Ce*=0.5–0.7). Mafic tuffs from the Upper Unit (Su-m41, Su-g and Su-ac)show profiles slightly enriched in LREE ((La/Yb)N=1.0–2.1), whichare intermediate to those of mafic eclogites from the Eclogitic Unit.
N-MORB normalized trace element profiles further confirm theexistence of distinct geochemical affinities among the various units(Fig. 10b). Note that this diagram has been normalized to Yb contentsof the samples to minimize the influence of fractionation and meltingprocesses. Su-n and Su-09 with N-MORB-like REE patterns also haveHFSE (Nb, Zr, and Ti) contents typical of N-MORB. However, Ba, Rb,Th, U and Pb show strong enrichments, probably related to mobility ofthese elements during deformation and metamorphism. The LREE-enriched eclogites Su-m and Su-14 on the other side, have higher HFSEcontents than N-MORB-like eclogites, but nevertheless show a pro-nounced negative Nb anomaly as well as small Zr and Ti anomalies(Fig. 10b). Their compositions are close to those of island arc tholeiites(IAT) apart fromdistinctly lowBa, Rb and Sr contents. Themafic tuff sam-ples generally exhibit profiles that tend towards an E-MORB compositionas they aremore enriched in highly incompatible elements thanN-MORBeclogites but lack the negative HFSE anomalies of the enriched eclogites.
These trace element relationships are further detailed in Fig. 10c(after Pearce, 2008). Depleted eclogites plot at low Nb/Yb values con-sistent with an N-MORB composition, but have elevated Th/Yb ratiosand thus plot above the MORB–OIB array. This may be due to magma-crust interaction or to subduction modification, two processes that aredifficult to distinguish in this type of diagram (Pearce, 2008). The samples
Table 8THERMOCALC average P–T results obtained for five metabasitic samples from the Eclogitic Unit.
Sample Su-02 Su-14 Su-m Su-n Su-j
Garnet r145-3-rim p256-rim p193-102-rim r146-299-rim r37-4-rimPrp 0.036 0.006 0.006 0.022 0.011Grs 0.027 0.011 0.016 0.007 0.016Alm 0.08 0.14 0.12 0.2 0.11Omphacite r180-core p258-rim p193-19-rim t71-rim r57-inc.GrtDi 0.35 0.4 0.44 0.27 0.33Hd – 0.13 0.12 0.16 –
Jd 0.34 0.37 0.34 0.43 0.35Acm 0.26 0.093 0.072 0.13 0.27Phengite r139-matrix p233-matrix p221-matrix p127-core r42-inc. GrtMs 0.48 0.33 0.34 0.41 0.45Cel 0.115 0.23 0.22 0.19 0.17Fcel 0.027 0.1 0.09 0.024 0.024Pg 0.339 0.3 0.26 0.342 0.331Others Ep r169 cz 0.34 ep 0.63
Qz H2O Qz H2O Qz H2O Qz Lws H2O Qz Lws H2O
Representative calculation (aH2O=1)T (°C) 647 572 562 565 561SD (T) 30 44 44 17 13P (kbar) 24 21.5 21.9 25.1 23SD (P) 1 1.8 1.7 0.9 0.7Correl. −0.4 0.34 0.34 0.82 0.84SigFit 0.14 0.69 1.00 1.28 0.89N. Ind. Reac. 5 3 3 4 5End member eliminated – – – Hd –
Average resultT (°C)/ΔT 640/25 559/43.2 573/47 547/16 540/16.8SD (T)/Δ SD 23.6/8.6 15.3/17.2 25.1/3.54 19.8/1.9 23/3.01P (kbar)/Δ P 22.9/0.9 23.9/1.94 22.1/1.8 23.9/0.84 21.8/0.9SD (P)/Δ SD 0.6/0.2 3.25/0.6 0.82/0.2 1.4/0.13 1.9/0.15Correl./Δ corr. 0.01/0.64 −0.25/0.51 0.36/0.04 0.81/0.02 0.84/0.01Number of calc. 6 5 5 5 5
149S. Angiboust et al. / Lithos 156–159 (2013) 139–158
also show very strong enrichments in fluid indicators such as Pb, U,Ba, and Rb (Fig. 10), which would be consistent with either scenario.However, the lack of Nb depletion relative to La points to crustal con-tamination during incipient rifting at a continental margin as themost likely tectonic scenario. The enriched eclogites plotwithin the vol-canic arc field (Pearce and Peate, 1995) and may therefore representrock from an arc-proximal back-arc or a supra-subduction-zone (SSZ)setting. Mafic tuffs plot on the MORB–OIB array in-between N-MORBand E-MORB and show little evidence of either a subduction componentor crustal assimilation, but elevated La/Yb and Nb/Yb ratios suggest amoderately enriched mantle source or lower degrees of melting, suchas observed at the continent side of ocean–continent transition zones(Desmurs et al., 2002).
6.2. Geochemical constraints on serpentinized ultramafic rocks
Combined major element, trace element and mineral compositionof serpentinized ultramaficsmayprovide evidence for their geodynamicaffinity following the criteria reviewed by De Hoog et al. (2009) andDeschamps et al. (2012). TheWestern Unit serpentinites are closely as-sociated with radiolarian cherts (Section 3) and their oceanic origin isvery likely. Distinct compositions for the Upper and Eclogitic Unitsmay help to identify their tectonic setting, providing alteration effectsby high-pressure metamorphism are not too pronounced.
The Sistan serpentinites all have high Mg# (89.6–92.0) typical ofresidual peridotites. Those from the Western Unit are nearly constant(90.7–91.0) whereas those from Upper and Eclogitic Units are morevariable (89.6–92.0). They span a range of Al2O3 (0.30–1.77 wt.%) andCaO (0.05–2.94 wt.%) concentrations, but correlation between thesetwo parameters is poor, indicative of late modification rather than amelting trend. Serpentinites fall slightly below the terrestrial meltingarray in Al2O3/SiO2 vs. MgO/SiO2 space, typical for abyssal peridotitesdue to sea-floor weathering (Snow and Dick, 1995). Serpentinites of
Upper and Eclogitic Units are offset to lower MgO/SiO2 than thosefrom the Western Unit.
More significant differences between units are visible in incompat-ible trace-element patterns. Western Unit serpentinites show spoon-shaped REE patterns with slightly to highly depleted HREE and MREEbut slightly elevated LREE contents (Fig. 11a). The HREE–MREE patternsare consistent with those expected from 10 to 20% fractional melting ofa depleted MORB-source mantle. Such a scenario is supported by theexcellent correlation between TiO2 and HREE (not shown), which areall incompatible duringmelting. LREE contents were probably modifiedafter the melting event by small amounts of late-stage cryptic metaso-matism (cf. Niu, 2004) or during serpentinization (e.g., Paulick et al.,2006). The most depleted REE patterns are comparable to those fromdepleted abyssal peridotites (e.g., ODP Leg 209 Site 1274; Godard et al.,2008; Paulick et al., 2006). The lack of Eu and U anomalies often observedin abyssal serpentinites suggests rock-dominated low-temperature ser-pentinization as the main alteration stage (cf. Paulick et al., 2006).
Serpentinites from Upper and Eclogitic Units have rather differentpatterns with lower HREE contents, consistent with larger degrees ofmelt depletion of up to 25%, but at the same time much higher MREEand LREE contents. The slope of the LREE patterns is similar to thoseof enriched mafic melts; it is therefore possible that the protolithsof the serpentinites were modified during cryptic metasomatism in arising melting column as commonly observed for abyssal peridotites(e.g., Niu, 2004). However, similar LREE-enriched patterns have beenalso proposed for slab-derivedmelt and/or fluids in subduction settings(e.g., Pearce and Peate, 1995; Plank and Langmuir, 1998).
Additional constraints on the source of the metasomatic agentmay be derived from fluid-mobile elements (FME; Deschamps et al.,2011; Hattori and Guillot, 2007). Highly soluble elements such asAs, Sb, Pb and B are particularly enriched in Himalayan serpentinitesof presumably mantle wedge origin (Hattori and Guillot, 2007). Sim-ilar enrichments are observed in serpentinites from the Eclogitic Unit
Coes
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Juxtaposition at5-20km depth
Average P-Testimates from
Fotoohi-Rad et al., 2005
Upper Unit
Eclogitic U
nit
T (°C)
b
Fig. 9. a. Synthesis of average P–T estimates (ellipses) obtained with THERMOCALC for Upper and Eclogitic Units (each ellipse color refers to a distinct sample). Also shown in dottedline are the P–T conditions for amphibolitization of Eclogitic Unit (derived from pseudosection modeling on sample Su-14; Fig. 8b). Gray shaded rectangle represents the maximumtemperature conditions obtained by RSCM thermometry for Upper Unit. 1: prograde path; 2: P–T estimates for peak metamorphism of Eclogitic Unit suggesting the existence ofindependent blocks with distinct peak metamorphic histories; and 3: P–T estimates for the gathering of the two tectonic units studied here. The fields in the background are derivedfrom Angiboust et al. (2009). b. Comparison of obtained P–T paths from this study (thick gray arrows) with those obtained by Fotoohi Rad et al. (2005) (thin dark ellipses and thinarrows).
150 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
(and to a lesser extent in Upper Unit serpentinites) compared to thosefrom theWestern Unit (Fig. 11b). This enrichment in FMEmay thus re-veal a mantle wedge origin for the Eclogitic Unit. However, the patternsoverlap those of non-subduction related serpentinites (Hattori andGuillot, 2007) and their interpretation is also complicated by the influ-ence of the composition of subductedmaterial (particularly sediments).Caution is thus needed and so far no unequivocal signature can be at-tributed to subduction settings using these trends (Deschamps et al.,2012).
Data in support of a mantle wedge origin for the Eclogitic or UpperUnit comes from platinum-group elements (PGE). This suite of ele-ments provides information on the degree of partial melting undergoneby ultramafic rocks (Fig. 11c). Although Pdhas been found to be partiallymobile during serpentinization, other PGE still retain the signatureof the protolith (Luguet et al., 2003). Three samples exhibit relativelyflat patterns typical of residual peridotites. However, sample Su-m3p(Eclogitic or Upper Unit) shows a smooth depletion trend from Ru toPt, indicative of more extensive melt extraction. A strongly refractorycomposition for this sample was already indicated by its low YbN of0.15 (Fig. 11a, b) and the lowest TiO2 content of the serpentinite samples(0.004 wt.% compared to 0.007–0.037 wt.%). Such depleted rocks aretypically formed by extensive hydrous melting in the sub-arc mantle(Bizimis et al., 2000).
The composition of chromite, in particular its chromium numberCr# (Cr/Cr+Al), may also provide important information on the pet-rological affinity of peridotites (Dick and Bullen, 1984; Hattori andGuillot, 2007; Hellebrand et al., 2001). High Cr# values attest to the re-fractory character of peridotites and have been shown to correlate withYb contents of residual Cpx and the degree of melting (Hellebrand et al.,2001). Low TiO2 contents (b0.15 wt.%) indicate that chromite in theSistan serpentinites is part of the relic primary mineral assemblageand not related to melt infiltration (Dick and Bullen, 1984). Chromitesfrom sample Su-17 (Western Unit) have Cr# of 0.38–0.40, which agreeswith an oceanicmantle origin (Dick and Bullen, 1984). In contrast, chro-mites from sample Su-m32 have Cr# of 0.63–0.65, which is higherthan the abyssal peridotite field, including highly depleted harzburgites
from ODP Legs 1272 and 1274 (Cr# spinel=0.41–0.57; Moll et al.,2007), and typical of forearc or subarc mantle (Hattori and Guillot,2007). This sample also had the strongest melt depletion indicators(PGE, REE pattern, and TiO2). Therefore, the protolith of this samplealmost certainly has a mantle-wedge origin. Chromites from sampleSu-m3p (Eclogitic or Upper Unit) also have high Cr# (0.48) but do over-lap the high end of abyssal peridotites and thus do not permit to deter-mine decisively the tectonic affinity of host serpentinite protolith.
7. Discussion
7.1. Interpretation of P–T-geochemical data: three distinct units
Detailed, meter-scale mapping reveals that the study area comprisesthreemain tectonic units: a non-metamorphic (or very low-grade)West-ern Unit, a blueschist facies Upper Unit and an Eclogitic Unit. P–T resultsfor the Upper Unit (for both the dominant metatuffs and the minorserpentinites) inferred through four independent thermobarometricmethods point to retrograde thermal peak P–T conditions of 470–500 °C and 5–8 kbar. The nearly complete lack of prograde HP paragene-sis prevents obtaining quantitative estimates of prograde P–T conditions.Co-existing lawsonite pseudomorphs, glaucophane and relict phengite(with Si contents ranging from 3.3 to 3.42) nevertheless indicate thatthe Upper Unit metatuffs were buried down to lawsonite–blueschistfacies conditions, probably in the range 10–15 kbar at around 400 °C(Fig. 8a; e.g. Agard et al., 2001; Coggon and Holland, 2002). In contrast,the Eclogitic Unit preserves peak conditions of 550–600 °C and22–24 kbar. Both estimates lie along the same cold prograde burialpath earlier reported by Fotoohi Rad et al. (2005) for Sistan HP rocks(ca. 8 °C/km). Yet the two HP units underwent significantly differentP–T retrograde trajectories (Fig. 9a). The entire Eclogitic Unit underwentheating during exhumation (ca. 650 °C and 13 kbar)whereas themain-ly tuffaceous Upper Unit did not. Unfortunately, the portion of the P–Ttrajectory between peak pressure conditions and amphibolitization ofthe Eclogitic Unit (Fig. 9a) cannot be satisfactorily constrained due to
Table 9Bulk-rock major and trace-element data of mafic samples from Sulabest ophiolite. Two chemical analyses from two different hand specimens from the same tectonic block are givenfor both Su-m and Su-n.
Sample Su-m41 Su-g Su-ac Su-14 Su-m Su-m(2) Su-09 Su-n Su-n(2)
Type Tuff Tuff Tuff Enriched Enriched Enriched Depleted Depleted Depleted
Unit Upper Upper Upper Eclogite Eclogite Eclogite Eclogite Eclogite Eclogite
Major elements (XRF, wt.%)SiO2 48.96 49.19 49.32 49.27 48.43 48.17 52.61 51.59 51.61TiO2 1.96 1.68 1.56 1.86 1.65 1.02 0.99 0.89 0.99Al2O3 17.03 15.19 14.45 9.86 15.09 11.39 15.17 14.91 14.83Fe2O3 10.92 11.26 11.60 13.28 11.16 12.77 10.39 12.96 12.51MnO 0.17 0.21 0.19 0.41 0.20 1.10 0.10 0.16 0.14MgO 4.70 6.54 6.50 7.59 6.35 7.37 4.51 4.66 4.62CaO 6.74 9.97 8.37 12.48 9.94 13.07 5.73 6.12 5.99Na2O 4.49 3.67 4.58 4.50 3.60 3.75 6.95 6.69 6.80K2O 1.62 0.60 0.19 0.29 0.59 0.30 1.87 0.69 0.73P2O5 0.19 0.21 0.14 0.12 0.19 0.07 0.08 0.04 0.04LOI 2.85 0.93 2.33 n.a. −0.31 −0.33 0.83 0.65 0.44Total 99.62 99.44 99.22 99.65 96.90 98.68 99.23 99.36 98.70
Trace elements (XRF, ppm)Rb_XRF 26 12 3 8 7 8 46 17 18Sr_XRF 172 273 289 63 51 113 46 103 87Y_XRF 39 39 36 36 53 57 16 28 25Zr_XRF 172 127 108 119 139 120 48 50 51Nb_XRF 7.4 5.4 3.6 4.968 9.8 6.9 1.4 2.2 1.8
Trace elements (ICP-MS, ppm)Sc 39 41 37 n.a. 28 30 38 41 39TiO2 wt.% 1.99 1.69 1.58 n.a. 1.29 0.99 1.01 0.80 0.98V 228 291 292 344 287 281 424 386 398Cr 300 277 173 211 233 148 13 7 60Co 49 46 41 44 65 65 39 70 53Ni 104 98 61 148 228 167 17 19 17Cu 35 64 61 23 10 15 65 69 67Zn 102 95 102 121 96 74 109 98 103As 2.7 2.2 0.7 n.a. 3.2 2.1 5.5 1.4 0.1Rb 27 13 2 8 6 8 51 17 19Sr 180 290 303 63 52 116 51 104 93Y 40 40 35 36 53 57 16 28 24Zr 19 15 52 119 11 9 7 17 5Nb 5.3 3.7 2.7 5.0 6.2 5.0 1.0 1.0 1.0Sn 1.7 1.3 13.1 1.6 1.4 1.8 2.3 0.6 1.3Sb 0.26 0.21 0.10 0.11 0.13 0.12 0.05 0.05 0.06Cs 0.23 0.13 0.08 0.17 0.11 0.18 0.89 0.26 0.29Ba 131 50 69 53 27 46 280 81 87La 9.9 6.4 4.9 40.6 29.6 20.3 1.3 2.4 2.1Ce 23.1 17.1 14.4 47.3 35.1 30.2 3.8 6.8 5.9Pr 3.8 2.8 2.3 11.1 7.4 5.1 0.6 1.1 1.0Nd 18.5 14.1 12.1 48.6 32.3 22.8 3.6 6.1 5.4Sm 5.19 4.39 3.95 11.41 7.80 5.77 1.47 2.28 2.07Eu 1.71 1.46 1.41 2.96 2.02 1.73 0.60 0.89 0.90Gd 5.73 5.18 5.37 8.75 8.27 6.98 2.27 3.29 3.55Tb 1.02 0.98 0.96 1.03 1.38 1.41 0.41 0.66 0.66Dy 6.49 6.48 6.25 5.97 8.50 9.21 2.71 4.52 4.28Ho 1.33 1.37 1.34 1.27 1.80 1.88 0.60 0.97 0.91Er 3.71 3.86 3.93 3.83 5.31 5.33 1.81 2.71 2.60Tm 0.54 0.57 0.55 0.59 0.83 0.83 0.26 0.40 0.35Yb 3.19 3.41 3.58 4.07 5.33 5.34 1.69 2.41 2.20Lu 0.42 0.46 0.53 0.64 0.80 0.80 0.27 0.37 0.34Pb 1.84 2.24 0.98 1.26 1.61 2.50 1.77 3.43 2.58Th 0.35 0.37 0.49 3.45 2.79 2.67 0.21 0.33 0.35U 0.24 0.30 0.09 0.61 0.60 0.75 0.10 0.17 0.15
151S. Angiboust et al. / Lithos 156–159 (2013) 139–158
the lack of petrological evidence for post-peak recrystallization andtherefore remains hypothetical.
Tectonic contacts mapped in the field between the Upper andEclogitic Units, as well as between them and theWestern Unit (the lat-ter being devoid of high-pressure indicators), are further confirmedby geochemical data (Figs. 10, 11). Serpentinites from the WesternUnit derive from residual oceanic peridotites onto which radiolariteswere directly deposited, reminiscent of (Alpine type) slow-spreadingoceans where the oceanic mantle is directly exposed on the seafloor.
By contrast, the highly refractory character of serpentinites from theEclogitic Unit (as shown by high spinel Cr#, depleted PGE pattern, lowHREE contents and enrichment in FME and LREE; Fig. 11b, c, d) supportsa mantle wedge geochemical affinity for this unit. Evidence from addi-tional samples is nevertheless required before it can be satisfactorilygeneralized. The chemical affinity of Upper Unit ultramafics is moredifficult to assess as their characteristics are systematically intermediatebetween the two end-members identified above. Tectonic juxtaposi-tion may account for the presence of ultramafic massifs with different
Table 10Bulk-rock major and trace-element data of serpentinite samples from Sulabest ophiolite.
Sample Su-17 Su-m52 Su-m6p Su-m7p Su-m1p Su-m2p Su-m3p Su-m32
Unit Western Western Western Western Upper Upper Ecl/Upper Ecl
Major elementsSiO2 38.79 39.61 38.51 37.40 41.76 40.64 37.72 41.94TiO2 0.013 0.037 0.007 0.008 0.018 0.011 0.004 0.011Al2O3 1.26 1.77 0.71 0.71 1.03 0.67 0.30 1.11Cr2O3 0.49 0.38 0.32 0.30 0.42 0.30 0.22 0.46Fe2O3 7.33 7.43 7.74 7.75 6.36 7.51 6.99 8.52MnO 0.11 0.13 0.11 0.11 0.11 0.11 0.13 0.11MgO 37.27 36.65 38.48 39.17 36.99 37.85 36.27 37.02NiO 0.27 0.28 0.26 0.28 0.28 0.27 0.31 0.29CaO 1.29 0.17 0.96 0.86 0.76 0.05 2.94 0.05Na2O 0.023 0.004 0.015 0.011 0.008 0.002 0.009 0.003K2O 0.001 0.002 0.003 0.001 0.004 0.001 0.004 b0.001P2O5 0.010 b0.0001 0.0004 0.0001 0.0006 0.003 0.0005 0.002LOI 13.15 13.43 12.55 13.01 12.31 11.98 14.51 11.62Total 100.01 99.90 99.68 99.61 100.04 99.39 99.40 101.14
Trace elements (ppm)Li 2.8 1.1 0.9 0.7 0.3 0.5 0.8 0.8Sc 10 12 10 9 11 8 4 9V 47 55 37 32 34 35 16 41Co 97 104 100 104 108 97 110 105Cu 25 23 21 20 8 18 13 17Zn 42 62 42 41 48 35 32 60As 0.25 0.23 0.12 0.18 0.31 2.35 1.82 1.40Rb 0.21 0.09 0.13 0.08 0.12 0.08 0.09 0.07Sr 0.92 24.09 0.29 0.06 7.06 2.48 81.94 1.17Y 0.37 1.22 0.14 0.20 0.45 0.17 0.12 1.09Zr 1.72 0.13 0.04 0.01 0.36 0.10 0.27 0.95Nb 0.01 0.00 0.01 0.00 0.08 0.04 0.02 0.20Sn 0.04 0.03 0.57 0.36 0.23 0.06 0.21 0.05Sb 0.00 0.01 0.00 0.00 0.12 0.17 2.77 0.05Cs 0.07 0.02 0.03 0.02 0.01 0.01 0.03 0.07Ba 1.16 1.70 1.26 0.04 1.41 28.64 8.17 0.71
Trace elements (ppb)La 27 25 5.0 1.5 133 32 34 588Ce 47 46 8.7 3.3 243 66 63 1339Pr 5.4 7.5 1.4 0.4 28 7.7 10 170Nd 20 58 4.8 1.9 112 28 42 702Sm 4.5 46 1.7 1.6 30 5.4 11 157Eu 2.1 20 0.7 0.6 9 8.1 4.7 49Gd 14 94 4.4 5.7 43 8.6 14 167Tb 4.3 23 1.4 2.1 9 2.2 2.6 28Dy 45 187 16.1 22.8 69 22 18 181Ho 13 45 5.2 7.3 17 6.5 4.4 37Er 49 141 22.1 30.2 57 24 14 106Tm 10 24 4.7 5.9 9 5 2.6 16Yb 73 158 44.3 49.4 70 36 21 97Lu 13 25 8.0 9.0 12 6.9 3.9 14Pb 34 2377 11.9 4.5 510 916 357 1123Th 24 32 1.7 0.5 21 27 11 68U 2.8 2.2 0.6 0.2 14 7.1 2.4 25
Platinum-group elements (ppb)Os n.a. n.a. 4.8 4.9 3.1 n.a. 4.5 n.a.Ir n.a. n.a. 4.9 4.6 4.4 n.a. 3.6 n.a.Ru n.a. n.a. 7.1 7.6 6.8 n.a. 5.8 n.a.Rh n.a. n.a. 0.8 1.3 0.8 n.a. 0.6 n.a.Pt n.a. n.a. 10.3 7.5 6.9 n.a. 2.9 n.a.Pd n.a. n.a. 7.3 7.1 4.0 n.a. 1.8 n.a.
Major elements by XRF except TiO2, Cr2O3, NiO, Na2O, K2O and P2O5 by ICP-MS.
152 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
geodynamic affinities (i.e., mantle wedge for the Eclogitic Unit versusabyssal peridotites for the Western Unit), as hypothesized by Hattoriand Guillot (2007) for the Cuban ophiolite.
Metabasalts and metatuffs from the Sulabest ophiolite show het-erogeneous geochemical characteristics (from N-MORB to IAT withvariable amounts of crustal contamination), suggesting the presenceof a subducted oceanic basin, possibly undergoing the influence ofarc volcanism. This agrees with recent interpretations from Saccaniet al. (2010) for the Sistan ophiolitic material (see also Desmons
and Beccaluva, 1983). We note, however, that the evidence for SSZmaterial presented by Saccani et al. (2010) was rather ambiguous,as Cr# of Cr-spinel is less than 0.35 even in their most depletedharzburgite, REE patterns are similar to those observed for abyssal pe-ridotites (e.g., Paulick et al., 2006) and none of the mafic rocksshowed a SSZ signature. Nevertheless, our data strongly supports asupra-subduction zone setting for some of the rocks from the SistanEclogitic Unit and possibly the Upper Unit (Sections 6.1 and 6.2).The observed enriched-mantle signatures and evidence of crustal
1
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Su-09
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Fig. 10. a. REE profiles normalized to chondrite (McDonough and Sun, 1995) from mafic eclogites (Eclogitic Unit) and mafic tuffs (Upper Unit) samples. Enriched and depletedeclogites have distinctly different patterns. N-MORB, E-MORB (both Sun and McDonough, 1989) and island arc tholeiite (IAT, Batur Volcano, Indonesia; De Hoog, unpublished data)trace element profiles are shown for comparison. b. Trace element profiles normalized to N-MORB (Sun and McDonough, 1989) and Yb content for mafic eclogite (Eclogitic Unit) andmafic tuff (Upper Unit) samples. The right-hand panel is a simplified version of the left hand panel and excludes elements prone to weathering or metamorphism, which allows differ-ences between groups to be distinguished more easily. E-MORB and IAT: see under a. c. Tectonic discrimination diagram of Th/Yb vs. Nb/Yb for mafic eclogites and tuffs (after Pearce,2008). See text for explanation.
153S. Angiboust et al. / Lithos 156–159 (2013) 139–158
contamination point to a near-continent setting rather than an intra-oceanic arc setting.
7.2. Implications on subduction interface accretionary and mixingprocesses
The structural, petrological and geochemical data presented aboveprovide insights on the geodynamic history of the study area and onthe tectonic processes leading to the formation of the Sistan ophioliticbelt.
The tectonic slice forming the Eclogitic Unit corresponds to di-verse oceanic crustal material embedded in serpentinites that likelyderived from a serpentinized mantle wedge (Fig. 12a, b). This hypoth-esis is supported by several arguments such as (i) the presence ofblocks with different geochemical affinities (MORB and IAT), (ii) thepresence of highly refractory serpentinized mantle wrapping eclogiticblocks and (iii) heterogeneous peak burial temperatures of differentblocks (between 520 and 650 °C). We propose that this unit recordedrelatively short-scale (meters to tens ofmeters at themost)mixing pro-cesses between the subducting mafic crust and the overlying mantlewedge at depths of 65 to 80 km (Fig. 12b; see also Guillot et al., 2000,2001). The slightly different peak burial temperatures observed here
may reflect the fact that detached blocks crossed different isothermsduring burial and became mixed along the hydrated portion of themantlewedge (Fig. 12b). In the absence of subducted continental crustalmaterial in the Sistan subduction zone, mechanisms responsible forthe exhumation of the Eclogitic Unit along the subduction interfacemust have differed from Alpine or Himalayan case studies (e.g., Agardet al., 2009; Guillot et al., 2009; Lapen et al., 2007). Return-flow (asfirst proposed for the exhumation of the Tauern eclogites; England andHolland, 1979; see also Raimbourg et al., 2007) and/or buoyancy forces(e.g. Angiboust and Agard, 2010; Hermann et al., 2000; Schwartz et al.,2001) probably played a key role for the exhumation of these eclogiteembedded in a low density, serpentinite-rich matrix.
The Upper Unit is made of a thick accumulation of volcano-sedimentary material, locally intercalated with serpentinites, anddoes not containmuch truly basaltic material. Although the geochemicalaffinity of associated serpentinites could not be determined precisely,the homogeneous lithological facies and P–T conditions of themetatuffsand the absence of significant differences inmetamorphic facies betweenthe blocks and the serpentinites (i.e., blueschist facies, T>400 °C) sug-gest that the Upper Unit detached from the subducting slab at ca.40 km depth as a coherent tectonic slice (Fig. 12). The fact that theUpper Unit did not experience the amphibolitization stage observed for
0.001
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Marianaforearc
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Fe3+
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Su-17 (Western Unit)
Su-m3p (Upper or Eclo. Unit)
Su-m32 (Eclogitic Unit)
Him
alayas
Alps
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Eclogitic Unit
Upper orEclogitic Unit
m1p
m3p
Su-m52
Su-m6p
Su-m7p
Su-17
Su-m1p
Su-m2p
Su-m3p
Su-m32
Fig. 11. a. Chondrite-normalized REE patterns of serpentinites from the three tectonic units identified in the field. Also indicated are the melting curves for non-modal fractionalmelting of spinel peridotite with a DMM (depleted MORB mantle; Workman and Hart, 2005) composition; see De Hoog et al. (2009) for model details. Shaded fields represent com-pilation of average abyssal peridotite compositions of sections of mid-oceanic spreading ridges from Pacific and Indian oceans after Niu (2004) and strongly depleted harzburgites atODP Site 1274 of the Fifty-Twenty Fracture Zone on the Mid-Atlantic Ridge (Godard et al., 2008). Chondrite values from McDonough and Sun (1995). b. Concentrations offluid-mobile and immobile elements in serpentinized peridotites normalized to primitive mantle (modified after Hattori and Guillot, 2007). Plots of Western Alps (abyssalserpentinites) and Himalayas (mantle wedge signatures) are represented for comparison (Hattori and Guillot, 2007). c. Platinum-group elements (PGE) concentrations of fourserpentinites normalized to primitive mantle (McDonough and Sun, 1995). Gray field indicated as MORB represents field of MORB glass compositions from the KolbeinseyRidge (Rehkämper et al., 1999). d. Ternary trivalent cation plot of chromian spinel cores from three ultramafic rock samples. Fe3+ estimated from stoichiometry (see Table 5).Gray shaded fields after Hattori and Guillot (2007) except abyssal peridotites from Dick and Bullen (1984).
154 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
the Eclogitic Unit, and the absence of brittle deformation along thecontact between the two units allow to bracket the deformation eventresponsible for their tectonic juxtaposition at around 20 km depth(i.e., close to the P–T conditions of pervasive greenschist facies recrys-tallization recorded across the Upper Unit: ca. 500 °C, 6–7 kbar;Fig. 9). The pressure gap (~8–10 kbar) between these two units pointsto the existence of extensional, normal-sense movement betweenthe two units (with yet unknown direction and sense of shear), whileconvergent boundary conditions still prevailed, as is so often observedin exhumed terranes (e.g., for the Western Alps: Agard et al., 2009;Plunder et al., 2012; Fig. 12c).
The origin of the amphibolitization of the Eclogitic Unit is yet un-certain but could be related to stagnation and/or underplating along themantle wedge (e.g., New Caledonia: Fitzherbert et al., 2004; Oman:Agard et al., 2010) at ca. 13 kbar (i.e., 40 kmdepth) before exhumation.Further radiometric data would help to test this hypothesis. The associ-ated block-rim hydration may have been caused by free fluids releasedfrom the downgoing slab (possibly derived from metasediments, assuggested by the higher potassium contents of block rims and by therelative enrichment in U and Pb; Fig. 10).
This juxtaposition of the Eclogitic Unit below the Upper Unit, thoughpossiblymore complex thandrawn in Fig. 12a–c, is tentatively associatedwith underplating processes such as those occurring at the bottom ofaccretionary wedges (e.g. Platt, 1986). These dynamics, involving sharptectono-metamorphic discontinuities between units, resemble tectonicprocesses recently unraveled from petrological and structural studiesin the fossil accretionary wedge of the Western Alps (Angiboust et al.,
2011, 2012a, 2012b; Plunder et al., 2012), in Corsica (Molli et al., 2006)or in the Sanbagawa belt (e.g., Endo et al., 2012; Wallis, 1998). Thismodel is also supported by recent geophysical data suggesting theexistence of sediment underplating mechanisms occurring along thesubduction interface down to mantle depths in active oceanic subduc-tion zones (ca. 30 km; Calvert et al., 2011; Kimura et al., 2010).
Importantly, the structural, petrological and geochemical data shownhere demonstrate that the study area does not correspond to a chaotic,tectonic “mélange”, as suggested by Fotoohi Rad et al. (2005) or Tirrulet al. (1983) where blocks with different P–T trajectories are gatheredin a lower grade, mechanically weaker matrix. Indeed, not only did theso-called, previously hypothesized “matrix” of the eclogites (i.e., theUpper Unit) reach HP conditions, but also a mappable contact existsbetween them (Figs. 2, 4). By contrast with the large-scale tectonic“mélanges” formed along the subduction interface in some numericalmodels (i.e., Blanco-Quintero et al., 2011; Gerya et al., 2002; Malatestaet al., 2012) or proposed for some exhumed ophiolitic massifs (Federicoet al., 2007; Guillot et al., 2009; Shreve and Cloos, 1986 and referencestherein), tectonic mixing is herein restricted to one unit only (theEclogitic Unit). Given the relative homogeneity of the P–T paths acrossEclogitic Unit tectonic blocks, it is proposed that the tectonic mixingherein observed is relatively short-scale (probably hectometer-scale;Fig. 12b). In addition, we suggest that the block-in-matrix fabric ob-served in the Upper Unit (Fig. 3b) or in theWestern Unit (Fig. 4a) dom-inantly resulted from sedimentary processes (such as those occurring inolistostrome basins) rather than from large-scale tectonic mixing pro-cesses (see also Burg et al., in press; Festa et al., 2010; Wakabayashi,
Mantle wedge
Serpentinizedchannel
500°C
550°C
600°C
Oceanic crustBlocks
not to scale
Samples14,02,m,n
EclogiticUnit
a b
c
)mk(
htpeD
400 500 600
30
60
90
Upper Unit
Eclogitic U
nit
T (°C)
7°/km
d
a-b
c
Detachment of Upper Unit slice blueschists
SW NEc. 80 Ma (?)
Afghan block
Accretionary complex
Lut block
Sistan ocean
0
30
60
dep
th (
km)
hydration front
Underplating
Upper Unit
Eclogitic Unit
(Western Unit)0
30
60
dep
th (
km)
Progressive juxtaposition of tectonic slices
Formation and detachment of HP slices
c. 60 Ma (?)
Fig. 12. a. Geodynamic scenario showing the tectonic evolution of the Sistan ophiolitic belt. Slices constituting the Upper and Eclogitic Units detached from the downgoing plate atdifferent depths. b. Close-up view on the structure of the Eclogitic Unit showing the mixing of eclogitized basaltic blocks embedded within serpentinites from the mantle wedge thatformed by infiltration of fluids produced by the dehydration of the downgoing plate. c. Underplating processes occurring at relatively shallow depths, within the accretionary wedge(ca. 20 km, under greenschist facies) led to the underthrusting of Eclogitic Unit below the Upper Unit. The Western Unit has been probably thrusted when entering the accretionarywedge before any noticeable metamorphism. These successive events occurred before the Paleocene (that lies unconformably on the ophiolitic domain). d. Summary of P–T con-ditions for the two processes (a, b and c) mentioned above.
155S. Angiboust et al. / Lithos 156–159 (2013) 139–158
2011). Later shearing during slice emplacement probably increasedthe disruption of these lithologically heterogeneous units by boudinageprocesses (e.g., Wakabayashi, 2011).
Further geochronological constraints are now required for the differ-ent tectonic units identified in the field to better constrain exhumationvelocities. Tightly-clustered Rb/Sr ages (85±3 Ma) recently obtainedby Bröcker et al. (2010) on five amphibolites sampled along the Sistanophiolite tend to corroborate our hypothesis on the presence of relativelycoherent volumes with similar P–T–t paths. This study also mentions apeakmetamorphismage of 85.7±0.7 Ma,whichwould imply extremelyfast exhumation velocities for the Eclogitic Unit (on the order of severalcm/yr). Note that these authors dismissed, on the suspicion of excessargon, the earlier Ar/Ar ages of Fotoohi Rad et al. (2009), which weremainly in the range 124–116 Ma.
8. Conclusions
(i) The Sulabest portion of the Sistan suture zone comprises threemain tectonic units: the non-metamorphic (or very low-grade)Western Unit, the blueschist facies Upper Unit and the EclogiticUnit (which reached ca. 75 km depth during subduction). Thelatter two were subducted along a similar HP–LT gradient, yettectonically juxtaposed along the subduction interface only atrelatively shallow depths (ca. 20 km) on exhumation.
(ii) Our results showed that the Eclogitic Unit formed by complex,relatively short-scale tectonic mixing processes, whereby vari-ous eclogiticmafic blocksweremixed together, along the subduc-tion interface, with probablymantlewedge derived serpentinizedmaterial.
(iii) The Sistan ophiolite, at least in the Sulabest area, does not consti-tute a typical HP tectonic mélange as earlier proposed by FotoohiRad et al. (2005) or as for the Franciscan complex (Cloos, 1982)but rather corresponds to a complex fossil accretionary systemwhere tectonic slices with different lithological affinities weretectonically juxtaposed, possibly via underplating mechanisms.In any case, we stress that HP mixing is restricted to the EclogiticUnit in the Sulabest area.
(iv) This study suggests the existence of hectometer to kilometer-sized tectonic slices exhuming along the subduction interface.This provides important constraints for geophysical studies aimingat imaging subduction channel nature and tectonic processes(e.g., Collot et al., 2011).
(v) In the light of this study, we finally emphasize that multi-disciplinary approaches combining petrology, geochemistry andstructural data (comparing blocks and matrix chemical affinitiesand P–T histories) are essential to infer the existence of a HP“tectonic mélange” (especially in areaswith limited surface expo-sures) and that caution is probably needed for several “so-called”tectonic “mélanges”worldwide.
Acknowledgments
We gratefully acknowledge KeikoHattori (Ottawa U.) for PGE analy-sis and Nic Odling (Edinburgh U.) for XRF analysis. Benjamin Huet is ac-knowledged for insightful discussions on thermobarometry. Specialthanks are due to the Geological Survey of Iran and to J. Ghalamghoshin particular. We would like to thank the two reviewers F. Rossetti andF. Deschamps for insightful comments and careful reading of this
156 S. Angiboust et al. / Lithos 156–159 (2013) 139–158
manuscript, andMarco Scambelluri for editorial handling. S.A. acknowl-edges the Alexander Von Humboldt foundation for providing apost-doctoral fellowship which permitted the writing of this publica-tion. This work was funded by the DARIUS program and an EGIDE-PHCAlliance grant (19349F) to P.A. and C.J D.H.
Appendix A. Analytical procedures
Electronmicroprobe analyseswere performedwith a Cameca SX100(Camparis, Univ. Paris 6). Conventional analytical conditions wereadopted for spot analyses [15 kV, 10 nA, wavelength-dispersive spec-troscopy (WDS)mode], using Fe2O3 (Fe), MnTiO3 (Mn and Ti), diopside(Mg and Si), CaF2 (F), orthoclase (Al, K), anorthite (Ca) and albite (Na)as standards. Mineral abbreviations are given following Whitney andEvans (2010).
Raman spectroscopy was chiefly used on carbonaceous material(RSCM) to provide an estimate of the maximum temperature reachedby a given sample. RSCM was performed at the Geology laboratoryfrom the Ecole Normale Supérieure of Paris, following the proceduredescribed in Beyssac et al. (2002). Absolute uncertainties are on theorder of 50 °C but later studies demonstrated a much smaller internalreproducibility, on the order of 10–15 °C (Beyssac et al., 2004). Or-ganic matter is very rare within metatuffaceous material and only foursamples provided reliable Raman spectra (based on a sufficient numberof spectra per thin section).We also used Raman spectroscopy in order toidentify the nature of serpentinite minerals in thin section, focusing onthe 180–1100 cm−1 region. Analytical conditions were the following:3 analyses per sample with 100 s acquisition each, using a 514.5 nmargon laser (power: 20 mW) (see Groppo et al., 2006 for details on iden-tification criterions).
Bulk rock major elements and selected trace elements (Rb, Sr, Nb,Zr, and Y) were determined by XRF (Panalytical PW2404) in the EarthScience Department at University of Edinburgh. LOI was determinedafter ignition of 1 g of powdered sample to 1050 °C for 20 min. Majorelements were measured on lithium metaborate fused glass discs,whereas pressed pellets were used for trace elements.
Bulk rock trace elementswere determined by ICP-MSusing Thermo-Finnegan Element II HR-ICP-MS (Dept. of Earth Sciences, Oxford) andAgilent 7500ce collision cell ICP-MS (Dept. of Chemistry, Edinburgh).Powdered samples were digested of in a concentrated HF-HNO3 mix-ture for 48 h at 120 °C in closed Teflon vessels, dried down andredissolved in HNO3. Final dilution factor was 1000 for both maficand ultramafic samples. A 115In spikewas added for internal calibration,whereas quantification was performed using a working curve basedon multi-element standards. Reference materials BCR-2, BHVO-2, JB-1(mafic rocks) and UB-N, NIM-P, PCC (ultramafic rocks) were used tomonitor accuracy andwere generallywithin 5% of recommended values(GEOREM; http://georem.mpch-mainz.gwdg.de). Agreement betweenXRF and ICP-MS was excellent except for Zr, which was considerablylower for ICP-MS. A faint residue of rutile was visible in some eclogitesamples after HF digestion, but agreement between ICP-MS and XRFfor TiO2 was excellent for all samples but one. Hence, we attributedthe difference to incomplete dissolution of zircon in HF digestion andtherefore Zr and Hf from ICP-MS are not reported.
Platinum-group elements (PGE)were determined for 1.5 to 2 g sam-ples by a fire assay NiS pre-concentration-isotope dilution techniqueat the University of Ottawa, Canada. NiS beads were dissolved in 6 NHCl, after which the insoluble material in the beads (PGE) was dissolvedin concentrated HNO3 and measured by ICP-MS (HP 4500).
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