Recognition of Late Cretaceous Hasanbag ophiolite-arc rocks in the Kurdistan Region of the Iraqi Zagros suture zone: a missing link in the paleogeography of the closing Neotethys Ocean
The Hasanbag igneous complex is situated near Sidekan, 100 km northeast of Erbil city, Kurdistan Region, within the Iraqi Zagros suture zone. It forms part of an ophiolite-bearing terrane referred to as the "Upper Allochthon" or Gemo-Qandil Group. The Hasanbag igneous complex consists predominantly of calc-alkaline basaltic andesites to andesites cut by microgabbro and diorite dikes, which previously were interpreted as a part of the Eocene Walash volcanic group. However, 40Ar-39Ar dates on Hasanbag igneous complex kaersutite (magmatic hornblende) indicate an Albian-Cenomanian age (106-92 Ma). This reveals a previously unrecognized portion of the Late Cretaceous, Neotethyan ophiolite-arc complexes in the Iraqi Zagros suture zone that may well represent lateral equivalents of the ophiolite-arc sequences in Oman-Neyriz and Cyprus. Plagioclase in all rock types has been strongly albitized, such that vestiges of more calcic igneous plagioclase are rare. Other relict igneous silicates are kaersutite, augite (Wo44.46-En46.43-Fs9.1), and rarely anorthoclase. The whole-rock geochemical variation can be explained by fractionation of plagioclase, clinopyroxene, hornblende, magnetite, and apatite but with superimposed alteration. Nb/Yb-Th/Yb relationships show that all the samples fall within the compositional field of arc-related rocks and above the field of the mid-ocean-ridge basalt (MORB)-ocean island basalt (OIB) mantle array. Hasanbag igneous complex samples show enrichment relative to normal (N)-MORB in the large ion lithophile elements and depletion in the high field strength elements, with characteristic Nb-Ta troughs. Hence, they were generated from the mantle wedge within a suprasubduction-zone setting influenced by volatile components from the downgoing slab. The Hasanbag igneous complex shows similar geochemical trends to arc rocks from Neyriz in southwestern Iran. We interpret the Hasanbag igneous complex as a remnant of the Late Cretaceous ophiolite-arc system (named here the Hasanbag arc complex) that developed within the Neotethys Ocean and was subsequently accreted to the Arabian plate during the Late Cretaceous to Paleocene. This represents the first time in Iraq that Late Cretaceous ophiolite-arc complexes have been identified in the Iraqi Zagros collision zone. The Hasanbag arc complex is unequivocally separated from the lithologically similar but younger Eocene-Oligocene Walash-Naopurdan arc-backarc complex, in the same general area but in a structurally lower thrust slice.