The Penjween gabbro, northeastern Iraq, revealing a forearc hyperextension regime with a slow spreading ridge center in the Late Cretaceous

Abstract

In the Zagros suture zone of western Iran, mafic and ultramafic rocks, along with deep marine sediments, are exposed and known as the Zagros ophiolites. Among these, the Penjween ophiolite in northeastern Iraq is notable and it is predominantly composed of gabbroic and mantle peridotite (tectonite), and also features minor felsic rocks like plagiogranites and mafic members such as pillow lavas and sheeted dikes. The Penjween gabbro which is the subject of this research were compressed between mantle peridotite to the east and Jurassic-Cretaceous cherts and Bistun limestone to the west. The chemical composition of the gabbroic rocks indicates low contents of alkaline elements, with higher FeO^t and MgO contents and low K contents and tholeiitic affinity. The basaltic and fine-grained gabbro have slightly higher alkaline element contents and LREE/HREE ratios than the gabbroic rocks, with slightly higher ⁸⁷Sr/⁸⁶Sr ratios (0.7038–0.7074) and lower ¹⁴³Nd/¹⁴⁴Nd ratios (0.51294–0.51301) than the gabbroic rocks with ⁸⁷Sr/⁸⁶Sr (0.7036–0.7054) ¹⁴³Nd/¹⁴⁴Nd (0.51281–0.51366) ratios. The basaltic rocks have a close affinity with the MORB composition, while the gabbroic rocks have Ce negative anomalies consistent with sources of hydrated depleted mantle melting. Therefore, given the 107 Ma age of the arc magmatic rocks in northwest Iran and the 98–93 Ma age of the plagiogranite in the Penjween area, the Penjween gabbros are 15–10 Ma younger than the magmatic activity in the northern Sanandaj-Sirjan zone (N-SaSZ) in western Iran. The Neotethys subduction probably began beneath the N-SaSZ at approximately 110 Ma, with nascent oceanic crust near the trench zone. Subsequently, the nascent plate spreading ridge slowly developed, resulting in the formation of the Penjween ophiolite in a supra-subduction system. The extensional regime in the forearc basin allowed for the exhumation of a large volume of mantle peridotite in the oceanic crust as a core complex. This process is known as forearc hyperextension, which has occurred in supra-subduction zones.