Late Cretaceous Forearc Ophiolites of Iraniar 745 1..4

Island Arc (2011) 20, 1–4

Pictorial Article

Late Cretaceous forearc ophiolites of Iraniar_745 1..4

HADI SHAFAII MOGHADAM1,* AND ROBERT J. STERN2 1School of Earth Sciences, Damghan University, Damghan, Iran (email: [email protected]), and 2Geosciences Department, University of Texas at Dallas, Richardson, TX75083-0688, USA

Ophiolites are on-land tracts of oceanic lithosphere impregnations, chromite pods, ultramafic cumulate and the tectonic setting that they form in is often sills (Fig. 3d), pegmatite gabbroic pockets/sills controversial. Reconstruction of the Neotethyan (Fig. 3b) and isotropic gabbro lenses. The crustal Ocean from Permian rifting to final closure in sequences of these ophiolites vary in thickness and Cenozoic time is largely based on the ophiolitic lithology but are mostly similar (Fig. 2). Outer belt sutures, such as the Late Cretaceous Ophiolite ophiolites are characterized by slices of sheeted Belt of Southwest Asia (LCOBSWA). The dikes (Fig. 3e) associated with pillowed and LCOBSWA stretches for 3000 km from Cyprus to massive lavas (Fig. 3f). Cenomanian to Campanian Oman through Iran and is remarkably coherent, pelagic limestones are found as thin layers between continuous, and contemporary but is only well the lavas or lie on top of ophiolite. Sheeted dikes and known at its terminations: Cyprus (94–90 Ma) and pillow lavas are common in IB (Fig. 3a,c). The Oman (95 Ma). The rest of the LCOBSWA, espe- typical IB (Fig. 2a) is overlain by pyroclastic rocks cially in the Zagros region of Iran, is more poorly of Turonian to Maastrichtian age with basaltic to known. The Zagros Fold-and-Thrust Belt formed dacitic sills and rhyolitic dikes, demonstrating that as an accretionary prism and has grown in concert the nascent Urumieh–Dokhtar Arc to the northeast with the continuing subduction of Arabia beneath developed slightly after ophiolite formation. Iran, which began about 20 Ma. The Zagros Ophio- To reconstruct the tectonic setting of the lite Belt lies along the northeast flank of the Zagros ophiolites we use whole-rock and mineral Zagros Fold-and-Thrust Belt (Fig. 1a). Zagros geochemical indicators for peridotite and lavas. ophiolites represent forearc lithosphere that Most Zagros ophiolite peridotites are very formed during a Late Cretaceous episode of sub- depleted and have abundances of CaO and Al2O3 duction initiation on the north side of the Neot- that plot in the forearc field (Fig. 4a) and have high ethys (Shafaii Moghadam et al. 2010). Cr# (= Cr/Cr + Al) spinel compositions that also Zagros ophiolites can be subdivided into the mostly plot in the forearc field (Fig. 4b). ‘Inner Zagros’ and ‘Outer Zagros’ Ophiolitic Belts, Zagros ophiolitic lavas mostly have nearly flat to separated by the Sanandaj–Sirjan metamorphic slightly light rare earth element (LREE)-depleted terrane, which we interpret as exhumed subducted patterns. Extended trace element (spider) dia- materials (Fig. 1b). The Outer Zagros Ophiolitic grams show typical supra-subduction geochemical Belt (OB) includes the Kermanshah, Neyriz and signatures: enrichments in large ion lithophile and Esfandagheh ophiolites, from northwest to south- fluid mobile incompatible trace elements and east, cropping out south of the Main Zagros Thrust depletions in high field strength elements (HFSE) Fault (Fig. 1a). The Inner Zagros Ophiolite Belt (Fig. 4c). Some lavas have low REE and very low (IB) lies along the southwest margin of the Central HFSE abundances, similar to boninites, generated Iranian Block, comprising the Nain, Dehshir, by melting of highly depleted (harzburgitic) Shahr-e-Babak, and Baft ophiolites. These ophio- mantle. Nearly all of the Zagros ophiolite lavas fall lites are disrupted by faulting but generalized litho- into island-arc tholeiitic and boninite fields on a spheric columns can be reconstructed based on field Ti–V diagram (Fig. 4d), similar to the depleted observations (Fig. 2). Mantle sequences of IB and Lasail lavas (V2 unit) of Oman. OB ophiolites are similar, comprising depleted In summary, all rock units of the Inner and harzburgites with diabasic–gabbroic dikes; melt Outer Zagros Ophiolitic Belts, from harzburgitic mantle to lavas, are characterized by strong supra- *Correspondence. subduction zone compositional features. The simi- Received 14 February 2010; accepted for publication 10 September 2010. larity of ages for igneous rocks and overlying © 2010 Blackwell Publishing Asia Pty Ltd doi:10.1111/j.1440-1738.2010.00745.x 2 H. S. Moghadam and R. J. Stern

(a) Main Zagros Thrust Zagros Fold-Thrust + Belt + + + + Urumieh-Dokhtar arc + Caspian Sea Khoy + Other Mesozoic + ophiolites + + Late Cretaceous Forearc Ophiolites + Sabzevar + Mashhad Kermanshah Tehran + 34° + N + G o lp + ay + eg + S an Birjand Z an + a + g an Nain ro d s aj F -S + o ir ld ja B - n + T Z h o r n Dehshir u e 30° st B + e Shahr-e-Babak lt + Balvard + + P Neyriz + e Baft + r + s + Iranshahr ia + + n Esfandagheh + + G u Band-Zeyarat lf 25 mm/yr A Fanuj- Maskutan 26° Semail Gulf of Oman 200 km ophiolite

50°E 54° 58° (b) SW ONCE-CONTINUOUS FOREARC OPHIOLITIC NAPPE NE Inner Zagros Imbricated Zone Outer Zagros Ophiolitic Belt Ophiolitic Belt (Dehshir) Zagros Simply Urumieh-Dokhtar Arc Folded Belt MZT

Persian Gulf <

< <

< < < <

< < <

< <

< < < <

< <

Iranian Arabian Continental Crust Continental Crust

Sanandaj-Sirjan Zone

Continental Crust Metamorphic Rocks < < <

< < < Magmatic Rocks Sedimentary Cover < < < Folding

Fig. 1 (a) Map showing the distribution of late Cretaceous Inner Zagros (IB) and Outer Zagros (OB) Ophiolitic Belt the location of the Urumieh–Dokhtar magmatic arc (Eocene–Quaternary), Zagros Fold–Thrust Belt and main Zagros thrust (MZT). (b) Schematic cross-section showing the relationships between the Outer and the Inner Zagros Ophiolitic Belts, the Zagros Thrust-Fold Belt, the Sanandaj-Sirjan Zone, and the Urumieh-Dokhtar Magmatic Arc.

(a) (b) Typical Inner Belt Ophiolite Typical Outer Belt Ophiolite (BAFT) (NEYRIZ) UK pelagic limestone Massive lava Pillow lava

Pyroclastic rocks Chert & UK p. limestone v

v v

v v v

P. limestone & chert v

Gabbroic-pyroxenitic v / / / v / / / / / Sheeted dike complex

Hyaloclastites / dike & sill +

+ + / +

/ +

/ / +

v /

/ + v

v -

v Tuffaceous SS Impregnated +

v v v v

/ -

= Coarse-grained gabbros

v v v v v v

- -

v v - v

v v v v -

v =

= -

& tuff - - peridotite + - Massive lava - - Plagiogranite - = = Rhyolitic dike Marble - -

Pillow lava

v . . . Wehrlite, pyroxenite

v . .

v ......

. =

v ......

. v ...... v . v v

v v

v = Basaltic to dacitic Highly depleted = v

v v cumulates =

Chromitite pod v v

sill harzburgite v = v v ...... Isotropic gabbro v ...... Isotropic gabbro v . . .. . v ...... Pyroxenitic sill ...... Chromitite pod Diabasic dike . . . .

+ Harzburgite Gabbroic dike .... v Melt pathways ..... Pegmatite gabbro . v ...... v ...... Residual dunite . . . . . Pegmatite gabbro v v Harzburgite Diabasic dike Pyroxenitic sill & dike

Fig. 2 Simpliﬁed psuedostratigraphic Amphibolitic sole Amphibolitic sole Mylonitic harzburgite lithospheric columns displaying generalized lithological successions in typical (a) inner (Baft), and (b) outer (Neyriz) Inner Belt Ophiolites Outer Belt Ophiolites Ophiolite Overlying pelagic sediments age Crust thickness (km) Ophiolite Overlying pelagic sediments age Crust thickness (km) belt ophiolites. The age of the overlying Nain Coniacian to Maastrichtian 1-2 pelagic sediments and crust thickness of Neyriz Cenomanian-Turonian to early Santonian 1-2 Dehshir Turonian to Maastrichtian 0.5-1 Kermanshah Turonian to Campanian 2-3 each ophiolite from IB and OB are pre- Shahr-e-Babak Turonian-Coniacian to Maastrichtian 1-2 Haji-Abad late Cretaceous, study in progres 2-3 sented for comparison. Baft Cenomanian?-Turonian to Maastrichtian 2-3

Fig. 3 Field photographs of inner and outer belt ophiolite components. (a) Sheeted dike complex in the Nain ophiolite (inner belt, IB). (b) Pegmatite gabbro sill within depleted harzburgite from the Nain ophiolite (IB). (c) Pillow lava sequence from the Baft ophiolite (IB). (d) Ultramaﬁc–maﬁc cumulate sills within the mantle sequence from the Neyriz ophiolite (outer belt, OB). (e) Contact between dikes in sheeted dike complex from the Kermanshah ophiolite (OB). (f) Outcrop of pillow lavas in the Haji–Abad ophiolite (OB). Author (Hadi Shafaii) for scale.

(a) (c)

Peridotite Whole Rock Composition PUM 100 Baft

4 Shahr-e-Babak FMM 10 Inner Belt ) k

c Abyssal peridotite o 3 r Nain k l

u 1 b s u o r d

y 2

h d n en a r Sample/N-MORB 0.1 ( T n Dehshir io % t

t e pl

w e D

3 le nt O 1 a

2 M l

A 0.01 Mariana Forearc peridotite

0 0 1 2 3 4 100 CaO wt% (anhydrous bulk rock) Outer Belt Inner Ophiolite Belt (11 WRS ; 77 Sp) Nain ophiolite (4 ; 40) 10 Haji-Abad Dehshir ophiolite (3 ; 20) Baft ophiolite (3 ; 12) 1 Neyriz Shahr-e-Babak (1 ; 5)

Outer Ophiolite Belt (15 WRS ; 110 Sp) Kermanshah Fig. 4 (a) Al2O3 vs CaO diagram Neyriz ophiolite (2 ; 96) Sample/N-MORB 0.1 Haji-Abad (12) (volatile free, normalized to 100% total) Kermanshah (1 ; 14) for Zagros forearc ophiolitic peridotites. 0.01 Semail ophiolite (Oman) The composition of primary upper Rb Th Nb K Ce Pr P Zr Ti Yb mantle (PUM) and fertile mid-oceanic (b) Ba U Ta La Pb Sr Nd Hf Y ridge basalt (MORB) mantle (FMM) are 1.0 Oman boninite (d) Increasing Compatibility also shown for comparison. WRS, whole rocks; Sp, spinel. (b) Cr# (=Cr/Cr + Al) Boninite Arc tholeiites 600 Ophiolitic lavas vs Mg# (=Mg/Mg + Fe) diagram (modi- ﬁed after Dick & Bullen 1984) for spinels 500 Oman V2 in Zagros forearc harzburgites. (c) MORB & BABB

) 400 N-MORB normalized multi-elements

p patterns for inner belt and outer belt ( 300 V Zagros ophiolitic lavas. Nb, Ta are

200 immobile critical elements for supra- Forearc peridotite Oman V1 subduction ophiolites; K, U, Pb, Sr are # r 0.5 100 C mobile elements. (d) Ti vs V diagram (after Shervais 1982) for inner belt (IB) 0 0 5 10 15 20 25 and outer belt (OB) mafic rocks. Most IB Ti (ppm)/1000 mafic rocks occupy island arc tholeiites Backarc peridotite Inner Zagros Ophiolitic Belt (IAT) and boninite (Ti/V < 10) fields, Abyssal peridotite Dehshir whereas more OB mafic rocks fall into Nain the MORB/back-arc basin (BABB) field. Shahr-e-Babak Fields for the V1 (lower, Geotimes unit) Baft and V2 (upper, Lasail unit) lavas of the 0 Outer Zagros Ophiolitic Belt Oman ophiolite are also shown for Haji-Abad comparison. Data for Dehshir ophiolite 1.0 0.5 0 Neyriz come from Shafaii Moghadam et al. Mg# Kermanshah (2010). sediments, and of geochemical compositions for REFERENCES both IB and OB Zagros ophiolites, along with simi- larity to other LCOBSWA ophiolites (in Cyprus, DICK H. J. B. & BULLEN T. 1984. Chromian spinel as a Syria, Turkey and Oman), and along with the loca- petrogenetic indicator in abyssal and Alpine-type peridotites and spatially associated lavas. Contribu- tion of IB and OB between the slightly younger tions to Mineralogy and Petrology 86, 54–76. Urumieh–Dokhtar magmatic arc and the Zagros SHAFAII MOGHADAM H., STERN R. J. & RAHGOSHAY M. accretionary prism, suggest that the entire ophio- 2010. The Dehshir ophiolite (Central Iran): lite belt formed as an approximately 3000-km-long geochemical constraints on the origin and evolution tract of forearc oceanic lithosphere fringing south- of the Inner Zagros Ophiolitic Belt. Geological ern Eurasia during a subduction initiation event. Society of America Bulletin 122, 1516–47. Financial support from the Research Council SHERVAIS J. W. 1982. Ti-V plots and the petrogenesis of Damghan University with the grant number of modern ophiolitic lavas. Earth and Planetary 88/GEO/64/127 is acknowledged. Science Letters 59, 101–18. © 2010 Blackwell Publishing Asia Pty Ltd