Year

2012

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

The Hasanbag Arc Complex (HAC) is situated near Sidekan, 100 km northeast of Erbil city, Kurdistan region, within the Iraqi Zagros Suture Zone (IZSZ). It forms part of an ophiolite bearing terrane referred to as the ‘Upper Allochthon’ or Gemo-Qandil Group. The igneous complex is comprised of volcanic rocks of arc affinity cross-cut by dolerite and diorite dykes. The HAC consists predominantly of calc-alkaline basaltic andesites to andesites, which in the past have been interpreted as a part of the Eocene Walash volcanics. However, Ar-Ar dating of magmatic hornblende from the Hasanbag volcanic rocks indicate an Albian – Turonian age (106-92 Ma). The discovery of Late Cretaceous arc volcanism reveals an as yet unidentified episode of Late Cretaceous, Neo-Tethyan ophiolite-arc complex in the Iraqi Zagros Thrust Zone that may well represent lateral equivalents of the ophiolite-arc sequences in Oman-Neyriz and Cyprus. The petrography and mineral chemistry of the HAC shows that plagioclase, pyroxene, amphibole and iron oxides are the main phenocrysts. Albite forms the main composition of the subhedral laths and microlites in the matrix but primary oligoclase (An27) is also observed. All clinopyroxene samples are augite (Wo44.46-En46.43-Fs9.1) and amphibole is Ti-rich hornblende (kaersutite). Iron oxides are mostly ilmenite, magnetite and hematomagnetite, while apatite, sphene and zircon occur as accessory minerals. Arc spatial-chemical relationships indicate magmatic evolution of a calc-alkaline, island-arc suite with low-K characteristics. The geochemical variation in the rocks can be explained by fractionation of common mineral phases such as plagioclase, clinopyroxene, hornblende, magnetite and apatite. The subduction signature of volcanic and subvolcanic rocks of the HAC is confirmed by the Nb/Yb-Th/Yb diagram, which shows that all the samples fall within the compositional field of arc-related rocks, and above the field of the MORB-OIB mantle array. Their Nb/Yb > 3 suggests their mantle source had considerable subduction influence and the resulting magmas were affected by a significant contamination of lithospheric material from the pre-existing crustal component.They are generated from the mantle wedge within a supra-subduction zone setting and are influenced by slab-derived components. HAC samples show enrichment relative to N-MORB in the large-ion lithophile elements and depletion in the high field-strength elements with characteristic Nb troughs, confirming a subduction influence. The HAC shows similar geochemical trends to arc rocks from Neyriz in south western Iran. We interpret the HAC as a remnant of Late Cretaceous ophiolite/arc rocks that developed within the Neo-Tethys Ocean and were subsequently accreted to the Arabian Plate during the Late Cretaceous to Paleocene. This is the first time that Late Cretaceous ophiolite-arc complexes have been identified in the Iraqi Zagros collision zone and unequivocally separates the HAC from the similar, but younger, Eocene - Oligocene Walash-Naopurdan arc-backarc complex.

A comprehensive petrographic, geochronological, and geochemical study of a group of samples from the Walash-Naopurdan volcanic and subvolcanic rocks in four provinces, Mawat, Galalah-Choman, Leren (Sidekan) and Qalander-Sheikhan was carried out. The study area is within a lower allochthonous thrust sheet that occupies part of the Iraqi Zagros Suture Zone (IZSZ), northeast Iraq.. 40Ar-39Ar dates on three magmatic feldspar separates, one from Walash and two from Naopurdan volcanic rocks, indicate an Eocene-Oligocene age (43.01 ± 0.15 to 24.31 ± 0.60 Ma). The petrographic study shows the disappearance of some original textural and mineralogical characteristics, due to the superimposed conditions that prevailed during ocean-floor and subsequent hydrothermal alteration. This led to the appearance of secondary minerals that partially or completely replaced the original ones in both the Walash and Naopurdan volcanic rocks. The petrography and mineral chemistry confirm that all Walash-Naopurdan rocks are mafic to intermediate and show that plagioclase, pyroxene, and iron oxides are the main phenocrysts, whereas, apatite, sphene and zircon occur as accessory minerals. Variation of the less mobile elements (such as Ti, Zr, P, Y, Nb and REE) conforms that the majority of the rocks are basic in composition with basalt to andesitic basalt for Naopurdan samples and basalt to basaltic andesite, alkali basalt to trachytic andesite for Walash samples, and that they have subalkaline (tholeiite for Naopurdan and calc-alkaline to alkaline for Walash) affinities. Also these elements show that the studied rocks represent assemblage of island-arc tholeiite affinity (IAT) for Naopurdan and calc-alkaline to alkaline affinity (CAB-AK) for Walash. These rocks can be related to different degrees of partial melting. The magma of Walash alkaline and calc-alkaline rocks was generated by

The tectonic model proposed opening of the Neo-Tethys and separation of the Sanandaj-Sirjan microplate from the Arabian plate in the Permian to Triassic. During the Early Cretaceous a supra-subduction zone ophiolite/arc complex, represented by the Hasanbag rocks (106-92 Ma), developed in the Neo-Tethys Ocean and was subsequently obducted onto the Arabian continental margin during the Late Cretaceous to Paleocene. This collision initiated a new intra-oceanic subduction system in the Neo-Tethyan Ocean. The Walash Group includes calc-alkaline volcanic rocks with a distinct backarc geochemical signature (enrichments in highly incompatible elements relative to less incompatible elements (i.e. LREE/HREE and high Th/Nb, Nb/Zr), which commenced in the Eocene (43-34 Ma). The Naopurdan and limited Walash volcanic rocks are characterized by alkaline magmatism and were erupted during the final stage of intra Neo-Tethys oceanic subduction (33-24 Ma), before the final closure of Neo-Tethys. Development of the Walash-Naopurdan arc – back-arc system up until 33-24 million years ago indicates that arc collision with the Arabian continental passive margin must have occurred after the youngest age of volcanism, probably during the Early Miocene (23-16? Ma). Final continent-continent collision between the Iranian-Turkish and Arabian continents occurred when the final remnants of the Neo-Tethys were subducted beneath the Iranian-Turkish continent resulting in collision with the Arabian continent, probably during the Middle? Miocene.

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