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- Aswad, Khalid J. A., et al.
(författare)
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Cr-spinel compositions in serpentinites, and their implications for the petrotectonic history of the Zagros Suture Zone, Kurdistan Region, Iraq
- 2011
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 148:5-6, s. 802-818
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Tidskriftsartikel (refereegranskat)abstract
- 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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- Aziz, Nabaz R. H., et al.
(författare)
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Contrasting Settings of Serpentinite Bodies in Northwestern Zagros Suture Zone, Kurdistan Region, Iraq
- 2011
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 148:5-6, s. 819-837
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Tidskriftsartikel (refereegranskat)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.
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