U–Pb zircon ages, field geology and geochemistry of the Kermanshah ophiolite (Iran): From continental rifting at 79 Ma to oceanic core complex at ca. 36 Ma in the southern Neo-Tethys

Highlights

• The older part of Kermanshah ophiolite was generated by detachment faulting at 79 Ma.

• The younger part of ophiolite originated was from the oceanic core complex at 36 Ma.

• The Zagros ophiolite belt is part of an accretionary prism.

• Neo-Tethys Ocean may have closed after 36 Ma.

Abstract

The geodynamic evolution of the Zagros Mountains of Iran remains obscure. In particular, the time of formation of the Zagros ophiolites and the closure of the Neo-Tethys Ocean are highly controversial. Here we present new precise zircon U–Pb ages that show that the younger part (Sahneh–Kamyaran) of the Kermanshah ophiolite formed at 35.7 ± 0.5 Ma and the older part (Harsin) at 79.3 ± 0.9 Ma. Field relations and geochemical evidence show that the younger Sahneh–Kamyaran part is probably a fossil oceanic core complex, and the older Sahneh part is probably a continental-oceanic transition complex. Both the Sahneh–Kamyaran and Sahneh parts were later emplaced into an accretionary complex. We conclude and infer that the final closure time of the southern Neo-Tethys Ocean was after the Late Eocene. Our data and tectonic model have crucial implications for the geodynamic evolution of the Zagros region.

Introduction

The Zagros ophiolite zone, outcropping along the Main Zagros Thrust (MZT) records the geodynamic evolution of the Neo-Tethys Ocean between Arabia and Eurasia (Alavi, 1980, Berberian and King, 1981, Dercourt et al., 1986, Şengör et al., 1988, Alavi, 1994, Stampfli and Borel, 2002, Agard et al., 2005, Agard et al., 2011, Mouthereau et al., 2012) (Fig. 1a). The Zagros ophiolite zone, the so-called “Thrust Assemblage Sub-zone” (Stöcklin, 1968) or “the Crush zone (or high Zagros)” (Agard et al., 2005, Allahyari et al., 2014), consists tectonic slices composed of Mesozoic shelf limestones and radiolarites, ophiolitic remnants, Eocene volcanic rocks and flysch, Late Cretaceous sedimentary rocks, and pillow basalts and calc-alkaline andesites (Fig. 1b). The Zagros ophiolites, including the Sawlava, Kermanshah, Neyriz, and Baft ophiolites in Iran (Ghazi and Hassanipak, 1999, Sarkarinejad, 2005, Allahyari et al., 2010, Moghadam et al., 2013, Saccani et al., 2013, Whitechurch et al., 2013, Allahyari et al., 2014, Saccani et al., 2014) and the Penjween ophiolites in Iraq (Aswad et al., 2011) are considered to be equivalent to the Oman ophiolite, and were obducted onto the Arabian plate margin in the Campanian–Maastrichtian (Coleman, 1981, Ricou, 1994, Hacker et al., 1996, Agard et al., 2007, Agard et al., 2011).

However, the geodynamic evolution of the Zagros domain remains obscure. In particular, both the timing of ophiolite formation and the closure of the Neo-Tethys Ocean are still highly controversial. The Zagros ophiolites are thought to have formed in the Cretaceous mostly according to the ages of radiolarites. On a regional scale, radiolarites range in age from Late Triassic to Cretaceous, whereas in the Kermanshah area they are just Maastrichtian in age (Shahidi and Nazari, 1997). Unfortunately only limited radiometric age constraints are available for the Zagros ophiolites. A leucodiorite dyke in the Kermanshah ophiolite has a whole-rock K–Ar age of 86.3 ± 7.8 Ma (Delaloye and Desmons, 1980). In contrast, available K–Ar ages for the Neyriz ophiolite range from 77 ± 2.4 Ma to 104 ± 1.0 Ma (Lanphere and Pamić, 1983). A more reliable Ar–Ar date on amphibole from a foliated gabbro of the Neyriz ophiolite has a plateau age of 93.2 ± 2.5 Ma (Babaie et al., 2006), which is similar to the zircon U–Pb age of the Oman ophiolite (Tilton et al., 1981). Moreover, the time of closure of the Neo-Tethys Ocean is highly controversial, ranging from Late Cretaceous (Berberian and King, 1981) to Late Eocene–Oligocene (Jolivet and Faccenna, 2000, Agard et al., 2005, Vincent et al., 2005, Ballato et al., 2010), to the Miocene (Berberian and Berberian, 1981, Şengör, 1990) or to the uppermost Pliocene (Stöcklin, 1968).

The aim of this paper is to record new high quality zircon U–Pb ages, structural field relations, and geochemical data of the Kemanshah ophiolite in order to more precisely constrain the time of formation of the southern Neo-Tethys oceanic floor, and to present a new model for the geodynamic evolution of the Zagros.