Contrasting settings of serpentinite bodies in the northwestern Zagros Suture Zone, Kurdistan Region, Iraq

Abstract

Protrusions and lenses of serpentinite–matrix mélanges occur at several places along the thrust faults of the Zagros Suture Zone. They separate the lower allochthonous thrust sheet, the ‘Lower Allochthon’ (i.e. Walash–Naopurdan nappe), of Paleocene–Eocene age from sediments of the Arabian platform and the upper thrust sheet of Mesozoic, ophiolite-bearing terranes termed the ‘Upper Allochthon’ (i.e. Gemo–Qandil nappe). The serpentinite–matrix mélanges occur mostly as stretched bodies (slices) on both sides of the Lower Allochthon (Hero, Halsho and Pushtashan (HHP) and Galalah, Qalander and Rayat (GQR)). Their overall chondrite-normalized rare earth element (REE) patterns form two main groups. Group One exhibits enrichment in the total REEs (> 1 × chondrite) whereas the Group Two pattern shows depletion (i.e. < 1 × chondrite). Bulk-rock MORB-normalized profiles of Group Two are almost flat in the MREE–HREE region with flattening profiles in the Gd–Lu range (> 3 times the MORB composition). In comparison with Group One, Group Two has extremely high REE content and displays variable depletions in the moderately incompatible high-field-strength elements (HFSEs) (Zr, Hf, Y) relative to their adjacent REEs. The REEs in the GQR serpentinite–matrix mélanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 × chondrite (i.e. < 0.01 to 1 × chondrite). The latter indicates that the hemipelagic sedimentary, melt-like components (i.e. high LREE, U/La, La/Sm and low Ba/Th) control the geochemical peculiarities of this type of serpentinite. The HHP serpentinite–matrix mélanges, however, are either equally divided between the two REE pattern groups (e.g. Hero, Halsho) or inclined towards Group One (e.g. Pushtashan). Contrary to GQR serpentinites, the variation in HHP serpentinite–matrix mélanges spans a compositional spectrum from U/La-rich to more Ba/Th-rich. Such ratio variations reflect the large variation in these two subducted sedimentary components (i.e. carbonate and hemipelagic sediment mix). The obvious differences in the trace element signatures of the GQR and HHP serpentinite–matrix mélanges might be related to plate tectonic parameters such as convergence rate, subduction age and thickness and type of subducted slab. It is more likely that the influx of subducted components to the mantle wedge relied heavily on the composition of the sedimentary inputs. These vary considerably with time from the relatively deepwater hemipelagic sediments (Qulqula Radiolarite Formation) to platform carbonate sediments (Balambo limestone). The trace element signatures of the GQR and HHP serpentinite–matrix mélanges might suggest multi-staging of the allochthonous sheet emplacement on the Arabian platform sediments.

The water and fluid-mobile element cycles during serpentinite subduction. A review

“…Oceanic serpentinites also inherit the REE compositions of their mantle protoliths. Figure 4 reports the trace-element compositions of numerous abyssal serpentinites from the large datasets by Deschamps et al (2013) and Peters et al (2017) normalized to the Primitive Mantle (PM, McDonough & Sun, 1995) (the dataset, in Table S1, includes analyses by Lagabrielle et al, 1992;Parkinson & Pearce, 1998;Viti & Mellini, 1998;Anselmi et al, 2000;Pearce et al, 2000;Guillot et al, 2001;Scambelluri et al, 2001b;Chalot-Prat et al, 2003;Garrido et al, 2005;Savov et al, 2005aSavov et al, , b, 2007Aldanmaz & Koprubasi, 2006;Marchesi et al, 2006Marchesi et al, , 2009Paulick et al, 2006;Hattori & Guillot, 2007;Delacour et al, 2008;Pereira et al, 2008;De Hoog et al, 2009;Deschamps et al, 2010Deschamps et al, , 2012Jöns et al, 2010;Saumur et al, 2010;Ulrich et al, 2010;Aziz et al, 2011;Blanco-Quintero et al, 2011;Augustin et al, 2012;Kodolányi et al, 2012;Boschi et al, 2013;Lafay et al, 2013;Andreani et al, 2014;Cannaò et al, 2015Cannaò et al, , 2016…”

Cr-spinel compositions in serpentinites and their implications for the petrotectonic history of the Zagros Suture Zone, Kurdistan Region, Iraq

“…This internal disruption of the stratigraphy of the latter is best considered as a broken formation (i.e. tectonic mélange) rather than a true mélange, which shows sporadic or chaotic blocks of exotic origin embedded in a sedimentary matrix (Aziz, Aswad & Koyi, 2011). Field observations west of Penjween town and at the tectonic contact between the Qulqula (Lower Pliensbachian-Turonian; Gharib & De Wever, 2010) and Balambo (Albian-Cenomanian age; formations show that the occurrence of blocks belonging to the latter in the former can easily be traced.…”

Origin of arc magmatic signature: A temperature-dependent process for trace element (re)-mobilization in subduction zones

“…S1b,c). Furthermore, these rocks are similar to some subducted serpentinites in Mesozoic ophiolites such as those of the Zagros suture zone in Iraq 38 and ophiolite complexes of Northwest Anatolia in Turkey 39 . Previous work (e.g.…”