(2013). 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. Island Arc, 22 (1), 104-125.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
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.
Accessory chrome spinels are scattered throughout the serpentinite masses in two allochthonous thrust sheets belonging to the Penjween–Walash sub-zone of the northwestern Zagros Suture Zone in Kurdistan. Based on field evidence, the serpentinites are divided into two groups: (1) highly sheared serpentinites (110–80 Ma), which occupy the lower contact of the ophiolitic massifs of the Upper Allochthonous sheet (Albian–Cenomanian age), and (2) ophiolitic mélange serpentinites of mixed ages (150 and 200 Ma) occurring along thrust faults on the base of the volcano-sedimentary segment (42–32 Ma) of the Lower Allochthonous sheet. The Cr-spinels of both groups show a wide range of YCr (Cr/(Cr + Al) atomic ratio) from 0.37 to 1.0, while the XMg (Mg/(Mg + Fe2+) atomic ratio) ranges from 0.0 to 0.75. Based on the Cr-spinel compositions of the entire dataset and in conjunction with back-scattered electron imaging, from core to rim, three spinel stages have been recognized: the residual mantle stage, a Cr-rich stage and a third stage showing a very narrow magnetite rim. These three stages are represented by primary Cr-spinel, pre-serpentinization metamorphosed spinel and syn- or post-serpentinization spinel, respectively. The chemical characteristics of primary (first-stage) Cr-spinels of both serpentinite groups indicate a tectonic affinity within a fore-arc setting of peridotite protoliths. The second stage indicates that Cr-spinels have undergone subsolidus re-equilibration as a result of solid–solid reaction during pre-serpentinization cooling of the host rock. Here the primary Cr-spinel compositions have been partly or completely obscured by metamorphism. During the third stage, the Cr-spinels have undergone solid–fluid re-equilibration during syn- or post-serpentinization processes. Both the second and third stages point to diachronous metamorphic paths resulting from continuous tectonic evolution influenced by either slow or fast uplift of mantle protoliths. In the fast metamorphic paths, the primary chrome spinels are flanked by a very narrow magnetite rim. The presence of two groups of distally separated serpentinites with different emplacement ages and fore-arc tectonic affinity could indicate that the closure of the Tethys Ocean culminated in two fortuitous subduction processes.
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