Petrography and Geochemistry of Gabbroic Rock from the Penjwin Ophiolite, Kurdistan Region, Northeastern Iraq

The gabbroic rocks as a part of Zagros ophiolite are exposed in northeastern Iraq, Penjwin area. These rocks with granular to ophitic textures are widely distributed without metamorphic halos. The main minerals are plagioclase (An 90-99 ), olivine, clinopyroxene (Wo 27-47 En 45-64 Fs 8-14) and orthopyroxene (Wo 2 En 78 Fs 20) respectively based on the abundances. The major elements show a broad range of compositional variations, with SiO 2 (46.2–50.9 wt. %), and low concentrations Na 2 O (0.15–0.62 wt. %), K 2 O (0.01–0.03 wt. %) and TiO 2 (0.06–0.2) and high concentrations, Al 2 O 3 (6.4–19.75 wt. %), total Fe 2 O 3 (6.29–11.6 wt. %), MgO (9.63–24.5 wt. %), CaO (8.02–18 wt. %) and low alkali contents (Na 2 O + K 2 O = 0.16–0.65 wt. %). On Ti-V diagram, all of the gabbroic samples have Ti/V less than 10 and consequently fall in the low Ti-Island arc tholeiitic. Whole rocks chemistry shows a depletion of High field strength elements in comparison with the primitive mantle with an arched upward rare earth elements pattern, characterized by light rare earth elements depletion (La N/Sm N = 0.05–0.8) and enrichment in the High field strength elements. Whole rocks chemistry, mineral paragenesis and chemistry of these rocks are more consistent with tholeiitic magma series. Based on our findings in this research, the primary magma has been produced from the depleted mantle with a high degree of partial melting.


Introduction
The Zagros orogenic belt is located in the central Alpine-Himalayan orogenic system, this orogenic belt is estimated to be around 2000 km long, this was recorded depend on the geophysical and geochemical data (Alavi, 1994;Agard et al., 2005;Mohajjel and Ferguson, 2014;Shafaii Moghadam and Stern, 2015;Moghadam et al., 2019;Azizi and Moineraziri, 2009).Several magmatic activities have been recorded along Zagros orogenic belt including intra-oceanic arc trench systems during the late Mesozoic and the Paleogene.This led to formation of many ophiolites on the boundary between the Arabian and the Eurasian continental plates.The Iraqi Zagros ophiolites from southeast to the northwest is Penjwin, Mawat, Bulfat, Pushtashan and Hasanbag (Ali et al., 2019).The Penjwin ophiolite complex is part of the Penjwin Walash Zone in Iraq, this zone is about ~16 km long and ~5km wide (Mahammod,1978) that trends northwest-southeast parallel to the regional structures and continues into the Iranian Zagros segment and consists of three thrust sheets: the structurally lowest Naopurdan, the middle Walash and the upper Qandil (Ali et al., 2012, 2013, 2014, 2016, 2019, Aswad et al., 2011, 2013, Mohammad et al., 2014, 2017).The age 93.8± 0.7 Ma of Penjwin ophiolite complex has been determined by Abdulla (2015) and Aziz et al. (2021) by using zircon U-Pb dating of plagiogranite rock.The Penjwin ophiolite in Iraq is a remnant of the late Cretaceous suprasubduction zone (SSZ) oceanic lithosphere that was developed in Southern Neotethys (Aswad, 1999, Abdulla, 2015, Ali et al., 2019).Gabbroic rocks are the main components of the Penjwin ophiolite complex.Study this type of rocks is crucial to determine the petrogenesis and evolution of ophiolites (Moore et al., 1971;Coogan et al., 2000;Furnes et al., 2020).Study area is located in Iraqi Zagros Thrust Zone that extends northwest-southeast from eastern Turkey through northern and northeastern Iraqi-Iranian border into northern Oman (Jassim and Goff, 2006;Moghadam and Stern, 2011, Ali et al., 2012, 2013, 2014, 2016, 2019, Znad et al., 2020, Aziz et al., 2021), which is also considered as a part of the main Zagros Orogenic Belt (Fig. 1a).This orogenic belt is part of the Alpine-Himalayan Orogenic Belt which is one of the most comprehensive ophiolite depositories in earth's history.(Alavi, 1994;Talbot and Alavi, 1996;Stampfli and Borel, 2002;Ali et al., 2012Ali et al., , 2013Ali et al., , 2014Ali et al., , 2016Ali et al., , 2019)).Moghadam and Stern (2011) was divided these ophiolites into two groups (Fig. 1b).The first group is located between the Sanandaj-Sirjan Zone and Central Iran consists of Khoy-Nain-Dehshir -Baft and is called Inner Zagros Ophiolitic Belt (Fig. 1a), whereas the second group is located between the Sanandaj-Sirjan and Zagros thrust zone consists of Hasanbag-Pushtashan-Bulfat-Mawat-Penjween-Kermanshah-Neyriz-Haji Abad and is called Outer Zagros Ophiolitic Belt (Fig. 1b) (Moghadam and Stern, 2011;Ali et al., 2019).Due to lack of information about the gabbroic rocks in the Penjwin ophiolite complex, apart from only two papers by (Al-Hassan, 1975and 1987, Al-Hassan and Hubbard, 1985).In this paper we present new field observations, petrographic, and geochemistry data from gabbroic rocks in the Penjwin ophiolite complex to interpret the genesis of these gabbroic rocks within the Penjwin ophiolite complex, which can provide an important key to understanding the genesis and emplacement of Tethyan ophiolites.

Geological Setting
The Penjwin ophiolite complex is situated in the northeastern of Iraqi Zagros Thrust Zone of about 60 kilometers east of Sulaimaniya city and southwest of Penjwin town about 4 kilometers, Kurdistan region between Latitudes (35˚35ʹ-35˚38ʹ N) and Longitudes (45˚52ʹ -46˚00ʹ E) (Figs. 1 and 2).The Penjwin igneous complex is one of five ophiolites found along the Iraqi Zagros thrust zone, it covers a large area estimated at about (35 km 2 ) on the Iraqi side while the majority is extended to the Iranian side (Mahmmod, 1978;Al Hassan, 1982;Mohammad, 2008, Ali et al., 2019).This ophiolite is located between Red Bed and/or the Qulqula Formation and Gimo-Qandil Group (Figs. 1b and 2) and surrounded from the east by a Gimo-Qandil unit which is mostly consists of phyllite and calc-schist, whereas from the west by the Red Bed Series which consists of Cenozoic clastic by a high-angle reverse fault with the age of Miocene-Pliocene (Al Hassan and Hubbard, 1985;Jassim and Goff, 2006).According to Al-Hassan and Habbard, (1985) the Penjwin igneous complex contains mantle sequences (peridotite include harzburgite, dunite and lherzolite) on the bottom and oceanic crustal sequences accumulate gabbros, dykes of diorite and volcanic rocks on the top.In contrast, and depending on Ali et al., (2019) and Mohammad et al., (2020) and the current study fieldwork, it is very clear that the stratigraphy sequence of the Penjwin ophiolite complex from the top is ultramafic (peridotite include harzburgite, dunite and lherzolite) rocks overlain the gabbroic rocks (lower layered gabbro and upper gabbros) and the sheeted dykes in bottom.The conclusion of the geological of the Penjween ophiolite can be observed in new a geological map for the study area (Fig. 2).It also supported by a stratigraphic column and cross-section of the Penjwin ophiolite complex modified by (Ali et al., 2019) (Figs. 2 and  3).According to (Aswad, (1999) this ophiolite located in oceanic supra-subduction ophiolite.

Electron Microprobe Mineral Analyses
Mineral chemistry was determined using a Scanning Electron Microscopy (SEM) in the Geometallurgy Laboratory Freiberg, Technische Universitat Bergakademie Freiberg, based on a scanning electron microscope SEM Quanta 650 -FEG -MLA (Division of Economic Geology and Petrology, Institute of Mineralogy, TU Bergakademie Freiberg, Germany), equipped with Bruker Dual X -Flash spectrometer for energy dispersive spectrometry (EDS) analyses were applied to complete rock thin sections.Details of the analytical protocol and the instrumental conditions can be found in (Schulz et al., 2020).

Major Element and Trace Elements Analyses
Whole-rock geochemistry (major, trace and rare earth elements) and Loss-on-ignition were analyzed in the Australian Laboratory Services (ALS) Laboratory Group SL Camas Spain, by ICP-MS, ICP-AES and with the Lithium Borate fusion method as a whole rock package encoding (ME-ICP06), (ME-MS 81D), (OA-GRA05) and (TOT-ICP06).

Petrography and mineral chemistry
Petrographic studies were undertaken on 20 thin sections of Penjwin gabbroic rocks.The Penjwin gabbroic rocks are mostly fresh and medium to coarse grained.Its essential mineral composition includes15-60% plagioclase, 1-25% olivine, 30-70% clinopyroxene, orthopyroxene 1-12%, and 1-2% amphiboles and iron oxides 2-10 %, while the secondary minerals are secondary amphibole (mostly tremolite and actinolite), chlorite and sericite.The rocks show dominantly intergranular, ophitic, subophitic and poikilitic textures (Fig. 4).Plagioclase is mostly Ca rich (anorthite and Bytownite) (An90-99) (Table 1 and Fig. 5A).Olivine is the most abundant mafic minerals in the lower gabbroic part and is rounded to sub-rounded with a composition of Fo78.Unusually, olivine minerals are fresh in most gabbroic rocks, but some olivine crystals show alteration (Fig. 4B and Table 2).Orthopyroxene of enstatite composition Wo2 En78 Fs20, formed between the plagioclase and olivine, while clinopyroxene is mostly augite and diopside Wo27-47 En 45-64 Fs8-14 (Figs. 4A, B, C, E, 5B and Table 3).Exsolution lamellae of clinopyroxene are observed in the orthopyroxene (Fig. 4 E).In addition, the presence of pigeonite (FG6-Pxmat4, FG6-Pxmat3) in an igneous rock thus provides evidence for the crystallization temperature of the magma, and hence indirectly for the water content of that magma.Amphibole is one of the primary minerals that may be present in a small amount in gabbroic rocks.However, most of them are secondary amphibole minerals with a small amount of primary amphibole (Mg-hornblende) (Figs.4E, 5C and Table 5).Fe-Ti oxides comprise 1-10 % of the olivine gabbro samples.Ilmenite is the most common oxide and forms irregular interstitial grains or inclusions in olivine and amphibole.Ti magnetite occurs along the cleavages and cracks of olivine (Figs.4F, 5D and Table 4).

Petrogensis of Penjwin Gabbroic Rocks
Cr vs Y diagram indicates depleted mantle source show that amount of Y is nearly constant with variation in Cr; this is a typical feature of depleted mantle source (Shinjo et al., 2000).In addition, Fig. 7 A and B shows a simple equilibrium partial melting diagram adapted to model primary melt composition from a simple mantle lherzolite having about 2000 ppm Cr (Pearce 1982;Malpas et al., 1994).The Penjwin gabbroic rocks, showing partial melting trends (~30% to ~60%) of upper depleted (Probably a lherzolitic) mantle source and fractional crystallization paths (A) for all lower layered gabbroic rocks and fractional crystallization paths (B) for all upper gabbroic rocks (see Fig. 7).The Parallel-Subparallel and flat REE patterns of the Penjwin gabbroic rocks may indicate that they have originated from the same depleted mantle source which may dominated by harzburgite and dunite with local occurrence of lherzolite.All Penjwin gabbroic rocks show depleted in HFSEs in comparison with primitive mantle and display an arched upward REE pattern, characterized by LREE depletion (LaN/SmN = 0.05-0.8)and enrichment in MREE and HRE (Fig. 7C and D).Moreover, the tholeiitic-transitional gabbro is REE-and HFS-depleted relative to primitive mantle, indicating derivation from melting of a refractory mantle peridotite source, possible spinel lherzolite (Bonev and Stampfli, 2009).All gabbroic samples have relatively high LILE/HFSE ratios and negative Ta-Nb-Ti and Ce anomalies, typical for subduction zone-related settings (Fig. 7 C and D).Furthermore, the depletion in high field strength elements relative to fluid-mobile elements ( negative and Nb anomalies comparing to the neighbouring incompatible elements) in all Penjwin gabbroic samples on a primitive-normalized plot (Fig. 7D), which are common in arc settings and are usually attributed to subduction enrichment and fluid metasomatism processes in subduction zones, Pearce and Peate, (1995) or it is explained by retention of these elements in the residual mantle source during partial melting (Pearce, 1982;Wilson, 1989).Negative Ti anomalies in the patterns for all studied samples are due to early crystallization of Fe-Ti oxides which buffers the Ti concentration or the former removal of Fe-Ti phases, ( due to the fractionation of ilmenite and titanomagnetite; cf Ozgenc and Ilbeyli (2009) which is consistent with the petrographic evidences (Fig. 4 F).The geochemical evidences show that Penjwin gabbroic rocks may be formed in the early stages of intraoceanic young supra-subduction zone (SSZ) at a Palaeo-ridge axis or close to it which led to contemporaneous eruptions in a fore-arc setting of island arc tholeiitic basalts (Fig. 6B, C, D).

Conclusions
Our finding in this research show the gabbroic rocks in Penjwin ophiolite complex is a part of Zagros ophiolite and it would be more consistent with the other parts of ophiolite members such as Pillow lava, deeply basin sediments such as chert, radiolarite, layered gabbro, sheeted dikes and mantle taconites with mainly harzburgite composition.These sequences have a clear correlation with the typical ophiolite members in the middle oceanic ridge and/or supra-subduction zone ophiolite.These rocks continued to the Iran side in Marivan (Rezaei et al., 2021) and Baneh (Azizi et al., 2019) area with 40-37Ma radiometric ages.Therefore, new detailed age dating of gabbroic rocks in the Penjwin area is needed to make their relation to the Zagros ophiolite and or syn-to post collision magmatism in the Zagros suture zone.

Fig. 1 .
Fig.1.Geological maps of the Zagros thrust zone along the Iraq-Iran border, showing the location and tectonic subdivision of the study area (after Ali et al., 2019)

Fig. 2 .
Fig.2.Geologic map of Penjwin Ophiolite complex showing the locations of the gabbroic samples

Fig. 5 .
Fig. 5. (A) Compositions of plagioclase from Penjwin gabbroic rocks plotted on the Ab-An-Or diagram; (B) Composition of pyroxenes from Penjwin gabbroic rocks in the Wo-En-Fs diagram; (C) Composition diagrams of amphiboles from Penjwin gabbroic rocks; (D) Composition diagrams of iron oxides from Penjwin gabbroic rocks

Fig. 7 .
Fig.7.(A) Cr vs Y log plot for the Penjwin gabbroic rocks, showing partial melting trends of a lherzolitic mantle source and fractional crystallization paths for all the gabbros (modified after Malpas et al., 1994); (B) showing fractional crystallization paths for all the gabbros (C) chondrite normalized and (D) Primitive normalized rare-earth plot ofSun and McDonough (1989)

Table 1 .
Selective representative of electron microprobe analyses of plagioclase for gabbroic rock of Penjwin ophiolite complex

Table 2 .
Selective representative electron microprobe analyses of olivine for gabbroic rock of Penjwin ophiolite complex

Table 3 .
Selective representative of electron microprobe of pyroxene for gabbroic rock of Penjwin ophiolite complex

Table 4 .
Selective representative of electron microprobe analyses of amphibole for gabbroic rock of Penjwin ophiolite complex

Table 5 .
Selective representative of electron microprobe analyses of iron oxide for gabbroic rock of Penjwin ophiolite complex.