The tectonic evolution of a Neo‐Tethyan (Eocene–Oligocene) island‐arc (Walash and Naopurdan groups) in the Kurdistan region of the Northeast Iraqi Zagros Suture Zone

Abstract

AbstractThe Walash and Naopurdan groups are incorporated into the lower allochthonous thrust sheet in the Iraqi Zagros Suture Zone (IZSZ). 40Ar–39Ar dates on magmatic feldspar separates from both Walash and Naopurdan volcanic rocks indicate an Eocene–Oligocene age (43.01 ± 0.15 to 24.31 ± 0.60 Ma). The Walash and Naopurdan groups form a thrust sheet that is structurally overlain by an upper allochthon of Cretaceous arc‐related rocks (106–92 Ma) now known as the Hasanbag igneous complex (formerly known as the Gemo–Qandil Group). The Walash and Naopurdan lower allochthon is thrust over the foreland basin Red Beds series. Volcanic and subvolcanic units in the Walash and Naopurdan groups were studied from the Mawat, Galalah–Choman, Leren, and Qalander–Sheikhan provinces. Most of these rocks are basaltic to andesitic for both the Naopurdan and Walash suites. The petrographic study shows that these rocks are affected by metamorphic alteration under greenschist facies conditions, but preserve primary porphyritic textures with some relict igneous plagioclase, pyroxene, and hornblende. The enrichments in LREE/HREE and high Th/Nb and Nb/Zr show that the Walash and Naopurdan rocks have distinct subduction‐related signatures: specifically island‐arc tholeiite for the Naopurdan and calc‐alkaline to alkaline for the Walash suites. Hence the Walash and Naopurdan suites are back‐arc and arc systems, respectively, that developed 43–24 Ma. Accordingly, the IZSZ contains a full record of Neo‐Tethys pre‐collision‐related volcanism in dual subduction settings, from the Early Cretaceous (Hasanbag igneous complex) to the Eocene–Oligocene (Walash–Naopurdan suites). Final continent–continent collision started when the last of the Neo‐Tethys Ocean was subducted beneath the Iranian continent, resulting in its collision with the Arabian Plate, probably during the Middle Miocene. This reinforces a continuity of events along the entire edge of the Arabian Plate from Turkey, through Iraq and Iran, and into Oman.