Late Cretaceous forearc ophiolites of Iran

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