Characteristics of the amphibolite rocks of Penjween area, Kurdistan Region, northeast Iraq: Genetic implication and association with Penjween Ophiolite Complexes

Abstract

Amphibolite rocks are found in Penjween ophiolite complexes within the Iraqi Zagros Thrust Zone, northeast Iraq. They appear in a discontinuous outcrops as pods and lenses in a sharp contact with peridotite and serpentinite rocks. Amphibole and plagioclase are the main mineral constituents with minor amount of clinpyroxene and quartz, Fe-Ti-oxides, titanite, apatite, zircon as accessory phases. Two mineral assemblages are recognized in these amphibolites;(1) amphibole+ plagioclase+ clinopyroxene+ iron oxides +titanite ± quartz± apatite± zircon;(2) amphibole+ plagioclase +iron oxide+ titanite± quartz± apatite± zircon, with chlorite, epidote and actinolite as secondary mineral phases. These amphibolites show different textures as granoblastic, granonematobalstic, porphyroblastic, and poikiloblastic. The amphiboles are calcic (Ca>1 apuf) and are of two types; Mg- hornblende and tschermakite. They are characterized by: SiO₂ (38.83-47.48%), Al₂O₃ (7.97-16.02%), TiO₂ (0.28-3.04%), MgO (12.03-16.38%), Cao (11.01-12.46%), FeO (8.55-13.4%) and Mg*(0.62-0.76). Plagioclase composition ranges between oligoclase (An_23.4-Ab_75.9) and albite (An_1.7-Ab_97.9). Geothermobarometry based on TiO₂-Al₂O₃ isopleths of calcic amphibole show that both Mg-hornblende and tschermakite  have P range (1.5-2.5 GPa) and T range (550-700°C) for Mg-hornblende and (700-900°C) for tschermakite, which are within amphibolite facies grade. Geochemical characteristics of these amphibolites indicate their igneous origin of tholeiitic basalt affinity with sub-alkaline basalt and andesite protoliths that are formed by fractionation of clinopyroxene, plagioclase and Fe-Ti- oxides. Primitive mantle- normalized trace elements diagram show similarity with subduction zone setting with striking variable enrichment of LILE, depletion of HFSE and HREE, and negative Nb-Ta anomalies. Chondrite normalized-REE diagram show LREE enrichment (La/Sm)_N= 3.295, (La/Yb)_N= 3.919 indicating the existence of garnet as residual phase in the source mantle. Tectonic discrimination diagrams based on immobile elements suggest island arc tholeiite, specifically back-arc basin basalt setting with 5-25% partial melting. The negative Nb-Ta anomalies, high Th and Ba/Yb, and La/Nb< 5, all confirm the back-arc basin setting or supra-subduction zone environment. This is consistent with the proposed idea that these amphibolites are genetically related to Penjween ophiolite and represent oceanic crustal rocks and sea-floor sediments that were detached and emplaced by mantle rocks of the ophiolite onto the Arabian Plate margin during Late Cretaceous. The processes of detachment and emplacement cause metamorphism of the oceanic crustal rocks reaching amphibolite facies grade.

Key words: Amphibolite, Penjween ophiolite, SSZ