Detrital Chromian Spinel Geochemistry from Miocene Red Bed Sandstones of Northeastern Iraq: Implication for Provenance and Tectonic Setting

Abstract


Introduction
Cr-spinel can serve as reliable petrological indicators due to their narrow range of petrotectonic environments, as evidenced by their Cr#= Cr/(Cr+Al) and Mg# = Mg/(Mg+Fe+2) ratios.The chemically stable nature of Cr-spinel allows it to be preserved in deeply buried, ancient terrigenous successions.This has been noted by Morton (1985) and Mange and Maurer (1992).As a result, detrital spinel group minerals, particularly chromian spinels, have unique occurrences and geotectonic implications.Detrital Cr-spinels are frequently observed in exposed regions of ophiolites.Therefore, the analysis of detrital Cr-spinels in suture zones and fore-arc basins is a valuable tool for identifying the provenance and paleogeographic reconstructions of ophiolite sources.It is also useful for tracing the mafic and ultramafic sources of clastic wedges that come from collisional orogens.This has been discussed in various studies (Hiscott, 1984;Pober, 1988;Cookenboo, 1997;Najman and Garzanti, 2000;Luar-Oberiter et al., 2012;Hu et al., 2014).The interpretation of source rocks for the Red Beds Series in Iraq was conducted utilizing trace and rare earth elements, as reported by Muatasam et al. (2015) and Hassan et al. (2015).Mira (2008), Ismail et al. (2010) and Mirza and Rashid (2019) conducted research on the extensive and widespread occurrence of Chromitites in the Iraqi Zagros Suture Zone.The fundamental composition of Chromite may be indicative of the crystallisation process from boninitic magma.In particular, the Chromitites found in the Mawat ophiolite are classified as supra-subduction type.The aforementioned studies were insufficient for the purpose of interpreting the source rock and palaeogeographic reconstructions of the Red Bed Series located in the Iraqi Zagros Imbricate Zone.The objective of the research is to utilize detrital Cr-spinel compositions obtained from the Late Cenozoic molasses (Red Bed Series) to rebuild the provenance and tectonic setting and its consequences for the pre-collision phase of lithic orogeny in the Zagros Suture Zone.

Geological Background
The Iraqi Zagros Imbricate Zone is encompassed by the High Zagros Reverse Fault and the Zagros Thrust Front from the northeast and southwest, respectively, as illustrated in Fig. 1.In this zone, the anticlines are comprised of a sedimentary sequence of parautochthonous plate margin sediments of the Cretaceous marine carbonates (Qamchuqa, Balambo, and Kometan formations).While the synclines are embracing the Upper Cretaceous foreland soft sediments such as flysch Tanjero, Aqra, and Shiranish formations in addition to the molasses sequence of the Red Bed Series.The studied areas are severely tectonically disrupted by Late Miocene superimposed tectonic events (i.e., folding, gravity sliding, and wrench displacement) (Aswad, 1999).For example, in the Mawat area, the Late Cenozoic molasses (Miocene Red Bed Series) is overlain by the mafic-ultramafic rock bearing Mawat nappes.The Red Bed Series stretches as a narrow northwest-southeast belt that surrounds the thrust sheets of the Zagros Suture Zone near and parallel to the Iranian border (Fig. 1).The series uncomfortably overlies the obducted Neo-Tethyan units and the shelf carbonates.
The Series was vertically divided into four units by Al-Mehaidi (1975).Unit 1 predominantly comprises red mudstone and siltstone, whereas Unit 2 (middle part) primarily consists of sandstone with minor intercalations of red siltstone.Unit 3 constitutes the upper portion and is comprised of conglomerate, which is overlain by a layer of red mudstone, siltstone, and sandstone (Unit 4) with a thickness of 100-120 meters.Karim et al. (2008) reported that the Red Bed Series was deposited within the coastal region of the Zagros Foreland Basin during the Palaeogene-Eocene period.The present study was conducted in the Mawat-Chwarta region, situated in the northern and northeastern vicinity of Sura-Qalat Village, located between Chwarta and Mawat towns, approximately 45 km northwest of Sulaimani City.The sandstone unit (Unit 2) in this region is prominently exposed and has a thickness of approximately 500 meters.It is easily recognisable due to its distinct overall red colour.Generally, the sandstone beds are 1-5 m thick and are intermixed with brown claystone and siltstone.The contact between the Red Beds and the Tanjero Formation is unconformable.This contact is described as sharp in some areas and transitional in others by Karim et al. (2011).The Resting of blocks of sandstone on scrap slop of the unit is the most noticeable feature along the outcrops of this unit (Fig. 2).The area under investigation denotes the northeastern boundary of the Arabian Plate.During the Late Cretaceous period, the Early Cretaceous platform underwent a transformation into a foreland basin.The observed transformation can be ascribed to two possible causes: the ophiolite obduction process (Buday, 1980;Buday and Jassim, 1987;and Jassim and Goff, 2006), or the convergence of the Iranian and Arabian plates (Aziz et al., 2013).

Materials and Methods
The Chwarta-Mawat region exhibits a distribution of clastic sedimentary rocks within the Red Bed succession, as depicted in Fig. 2. Ten samples of the sandstones weighing around 10 kg were collected near Sura-Qalat and Tagaran villages.The conventional techniques as described by Carver (1971) were employed to separate heavy minerals, such as spinels, from sandstone.Approximately 100 grains from each sample were extracted and embedded in epoxy resin, followed by polishing and carbon coating.The Cr-spinels were manually chosen through a process of hand-picking, subsequently embedded in epoxy, and polished to facilitate analysis.The present study conducted major element analyses of Crspinel in ten samples through the utilization of the Zeiss EVO MA10 scanning electron microscope (SEM) in conjunction with an Oxford Instruments 80 mm 2 energy-dispersive X-ray spectroscopy (EDS) detector equipped with Aztec software.
The analyses were performed at Original Analytical Ltd., located in Welshpool, United Kingdom.The study involved capturing high-resolution backscattered electron (BSE) images of individual target grains, followed by conducting spot energy-dispersive X-ray spectroscopy (EDS) measurements to determine the concentration of major elements.The major element analyses were performed with beam conditions of 20 kV, 2nA, a fixed working distance, and measurements calibrated to in-house standards.Analyses near grain cores and rims, rather than in micro-veins or changed zones, were chosen to avoid inaccurate results.For each sample, a compilation PDF of grain images and spectra is provided, along with a data table of major element results (Table 1).

Spinel Textures
Sandstone samples from the Red Bed Series in the Sura-Qalat region include disseminated detrital Chrome spinels that are shaped as anhedral to subhedral and/or irregular grains.Along fractures and grain boundaries, some of these grains have been brecciated, fractured, and changed (Plate1C and D).The alteration processes have led to the formation of magnetite rims or mantles on the pre-existing spinel grains, as depicted in Plate1 F. Additionally, numerous inclusions containing olivine minerals have been observed, as illustrated in Plate1B.Metamorphosed Cr-spinels display compositional variations, either continuous or discontinuous, from their core to rim, as observed through BSE imaging.Primary Crspinel grains typically have no traces of abrasion on their surfaces (Plate1A).Certain grains exhibit corrosive characteristics, including minute etch pits and elongated grooves (Plate1D).The petrographic analysis and backscattered electron imaging of the detrital Cr-spinels present in the Red Bed sandstone reveal primarily the following textures: 1) a protogranular texture with grain boundaries that range from linear to curvilinear (as depicted in Plate1E); 2) a spongy texture (Plate1G) characterized by a symplectite lobe enclosing a spinel porphyroclast composed of chlorite and vermicular Cr-rich spinel (ferritchromite) (as depicted in Plate1G and F), and 3) an ameoboidal texture where the magnetite rims develop irregularly away from the Cr-spinel rims and merge with the magnetite (as illustrated in Plate1 H).

Spinel Geochemistry
The composition of Cr-spinel, particularly with regards to its content of TiO2, Al2O3, Cr2O3, FeO, and MgO, is of great significance in comprehending its crystallization, recrystallization, and alteration histories.The Red Bed Series exhibits variations in the Al2O3 and TiO2 concentrations of the Cr-spinels, ranging from 11.05 to 25.45 and 0.01 to 0.23 wt.% respectively, as presented in Table 3.The Red Bed's Cr-spinel is characterized by a low TiO2 content, which suggests incomplete transformation to ferritchromite due to an abrupt decrease in Mg and Al levels and an accompanying increase in Fe and, to some extent, Ti concentrations.The investigated spinel goes through three distinct phases.Spinels are Al-rich in the residual stage; they also have a low Cr# ([Cr/(Cr+Al)] = 0.34) and a high Al2O3 content (37.83 wt%).The Cr# of the spinels in the second stage is 59.21; the third stage is distinguished by a minimal outer layer of magnetite.The Cr# versus Mg# plot shows that the detrital Cr-spinels of the present study correspond to different types due to their varying Cr content and TiO2 contents (Fig. 3).Most of the samples fall within the chromite and magnesiochromite types, which are similar to the Zagros Thrust Zone chromian spinels from northeast Iraq (Fig. 4A).Lenaz et al. (2000) defined that the distinction between peridotitic and volcanic spinels can be achieved by analyzing their TiO2 content and FeO/Fe2O3 ratio.The spinels derived from peridotitic sources exhibit a TiO2 content of less than 0.2 wt.% and a FeO/Fe2O3 ratio greater than 3 wt.%.
Conversely, spinels originating from basaltic magmas display a TiO2 content exceeding 0.2 wt.% and a FeO/Fe2O3 ratio lower than 4 wt.%.The compositional characteristics of Mantle spinels from various tectonic environments can be distinguished by analyzing the Al2O3 vs. TiO2 (in wt.%) discrimination diagram, as proposed by Kamenetsky et al. (2001).Detrital peridotitic spinels of the Red Bed Series are located in the supra-subduction zone field (Fig. 4A), with the exception of a few samples that are within the field of MORB peridotite.Detrital chromian spinels are found in mafic and ultramafic host rocks in a variety of tectonic settings.As a result, the chemical composition of the spinels reveals detailed information on the origins of the rocks from which they were derived.Using Cr/(Cr+Al) vs. Mg/(Mg+Fe +2 ) (Pober and Faupl, 1988), paleogeographic reconstructions of the host rocks of the detrital chromian spinels within the Red Bed were achieved.According to the data, harzburgites and lherzolites are the only possible precursors to the detrital chromian spinels observed in the Red bed (Fig. 4B).(Augé, 1987;Ahmed and Arai, 2002).according to Pober and Faupl (1988).

Provenance and paleogeographic relations
In order to determine the provenance of the sediment source areas that have supplied the Zagros foreland basin during the Palaeogene, we employed the discrimination diagram 100 (Cr/Cr+Al) vs. 100 (Mg/Mg+Fe) in spinels (Fig. 5).According to Fig. 5, the analyzed samples exhibit tectonic affinities that are predominantly situated in the forearc environment of peridotite protoliths, with the exception of a small number of samples that exhibit boninite affinity.The aforementioned statement suggests that the Red Beds were supplied with detritus from a variety of source rocks, which is indicative of the uplift and unroofing history of pre-existing rocks.This phenomenon was likely a result of intense orogenic movements during the deposition period.Zagros Ophiolite peridotite spinel compositions plot in the fore-arc field and tend to be rich in Cr # (Mirza, 2008;Moghadam & Stern, 2011).The conclusion drawn from the observed Cr #.Spinel compositions of the samples being studied is that they bear similarities to the peridotites found in the Zagros Ophiolite.This similarity serves to validate the proposition that the source region of the Red Beds was most likely comprised of mafic-ultramafic rocks and dominated by boninitic magmas.Dick & Bullen (1984).Cr # v. Mg # diagram fore-arc setting (Dick & Bullen, 1984;Arai, 1994;Parkinson & Pearce, 1998) and boninites (Arai, 1994;Barnes and Roeder, 2001).
The detritus of ophiolitic origin within the clastic unit of the Red Bed, with a relatively high amount of Cr-spinels in the heavy mineral fraction, was derived mostly from the nearby Mawat nappes, which eventually rested on the former.Through the progressive removal of the Late Miocene tectonic domains and the gradual elimination of the Late Miocene tectonic domains, the pre-collision tectonic process gave rise to a narrow trough non-marine basin that emerged in conjunction with the outcrops of Neo-Tethys ophiolitic rocks.The presence of ophiolitic detritus chromian spinels in Cenozoic molasses has presumably been interpreted to indicate the presence of mafic and ultramafic rocks in source regions.Based on the presented evidence, it can be inferred that the detrital Cr-spinels within the red bed in the studied area derived predominantly from these mafic-ultramafic rocks of the Mawat-Penjween complexes.These complexes (Mawat-Penjween) are recognized as the largest and most complete ophiolitic sequences.The Mawat-Penjween complexes were uncovered near the above-mentioned sedimentary basin following the deposition of Late Cenozoic molasses (Red Bed Series).

Conclusions
Geochemistry data and Back-scattered electron image analysis of the detrital Cr-spinel within the clastic unit of the Palaeogene Red Bed Series provides the most suitable archive to study the evolution and the indication of source areas that have supplied the Zagros foreland basin in the north Zagros Thrust Zone with regards to the transition from passive margin to the accretionary complex terrene-flexural foreland basins setting.Freshness (angular to subrounded morphology) and metamorphic detrital Crspinel grains (Cr-spinel grains were cordoned off by a magnetite rim) were identified in the Red Bed Series sandstones of the Chwarta-Mawat area, along with a variety of textures, such as fractured, curvilinear grain boundaries, spongy, and ameoboidal.The observed textures are indicative of dissolution, grooving, and corrosion along the borders, which are attributed to the synergistic impact of mechanical abrasion and chemical engraving during transportation.The geochemical results obtained in the present study demonstrate a high concentration of Cr and a relatively low TiO2, suggesting a similarity to the chromian spinels found in the recognized source rocks located in the Zagros Ophiolite terrains of NE Iraq.They are partially derived from forearc and boninite peridotites in a suprasubduction zone and infant boninite environment, as well as an arc-setting ultramafic terrain.The tectonic setting of the Miocene red beds demonstrates indications of primary derivation of detritus from the Cretaceous Mawat Ophiolite Complex.Additionally, the findings imply that the Paleocene Walash-Naopurdan Series served as additional source through their deposition during the closure of the Neo-Tethys Ocean in Miocene.

Fig. 1 .
Fig. 1.Geological map of NE of Iraq showing the location of the study areas (modified fromKoshnaw et al., 2018)

Table 1 .
Representative EMPA of detrital Cr-spinels from the Red Bed Series (in wt.%)