Khalid Aswad

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

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 melange 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.

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

Protrusions and lenses of serpentinite–matrix melanges 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 melanges 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. 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 melanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 × chondrite (i.e.

Petrography and geochemistry of Jurassic sandstones, Western Desert, Iraq: implications on provenance and tectonic setting

A combined petrographic and geochemical study of the Jurassic Formations, Western Desert of Iraq was carried out to infer their tectonic setting, provenance history, and weathering in the source area. Texturally, these sandstones are immature, poorly sorted, and grain supported. Abundance of feldspars, especially plagioclase indicates rapid deposition of sediments from nearby source rocks. Using the geochemical classification diagram, the Jurassic sandstones are classified mainly as Fe-sand, quartzarenite, and sublitharenite, which is also supported by the petrographic study. The transition trace elements like Co, Ni, Cr, and Cu are lower in the Jurassic sandstones than upper continental crust (UCC) values. Plotting the present data on the provenance discriminating diagrams, most of Hussainiyat and Najmah sandstones fall within felsic (acidic) igneous provenance, and Amij and Muhaiwir sandstones mostly fall in the field of intermediate igneous provenance. The poor correlation between Cr and Ni (r = −0.110, number of samples n = 17) implies that these sandstones were derived from felsic source rocks. Tectonic setting discrimination diagrams based on major elements suggest passive margin and active continental margin. As indicated by chemical index of alteration (CIA) values for the Jurassic sandstones (averages 50 to 65), their source area underwent low to moderate degree of chemical weathering. The petrography and geochemistry results are consistent with a semihumid to semiarid climate for the deposition of these sandstones.

Tectono-stratigraphy and structure of the northwestern Zagros collision zone across the Iraq-Iran border

Tectono-stratigraphic units within the Zagros Oroge n in northeast Iraq (foreland) and northwest Iran ( hinterland) are correlated to provide an integrated map along t he collision zone. Access to this part of Iraq duri ng the past four decades has been limited due to geopolitical s ituation. Structural cross-sections across the Zagr os Suture Zone in this area reveal the relationships between the tectonic terranes of various ages and different origins. Terranes of oceanic affinity have accreted onto the Arabian plate during collision-accretion events th at started in the Late Cretaceous. The collision resulted in clos ure of the Neotethys Ocean and the construction of a structurally complex suture zone. Jurassic-Cretaceo us deep ocean radiolarites of the Qulqula-Kermansha h terrane and ophiolitic melange serpentinites were s tructurally accreted against the Arabian passive ma rgin during an ophiolite arc-continent collision event in the L ate Cretaceous. The overthrust radiolarites and oph iolitic melange terranes initiated the development of the f oreland basin overlapping flysch (turbidites) and m olasse assemblages on the now active Arabian margin. Eocene-Oligocene volcano-sedimentary rocks of the WalashNaopurdan-Kamyaran terrane developed as an intra-oceanic island-arc within the intervening Neotethys O cean. They now structurally overlie the older ophiolite m elange and radiolarite terranes as a result of cont inued convergence onto the margin of the Arabian Plate. T hese younger thrust sheets and nappes have been tra nsported over the Miocene molasse unit of the Tertiary Red B eds in the flexural foreland basin and covered the Late Cretaceous accretionary complex terranes and forela nd basin assemblages. The Qulqula-Kermanshah terrane is exposed in a tectonic window in the northeastern pa rt of the mapped area indicating that the Late Cret aceous accreted terranes occur below the Walash-Naopurdan-Kamyaran thrust sheet. A Late Cretaceous ophiolitebearing terrane named the “Upper Allochthon” (i.e. Gemo-Qandil nappe, 97-118 Ma) was emplaced by younger thrust sheets over the Eocene-Oligocene Walash-Naopurdan-Kamyaran terrane by out-of-sequence thrusting. Mesozoic metamorphic rocks from the hinterland, including volcanic and intrusive rocks of the active I ranian continental margin (Sanandaj-Sirjan zone), were emplaced during continent-continent collision and occu r in the youngest nappes and klippes along the Zagros Suture Zone.