Trace and Rare Earth Elements Geochemistry of the Mudstone Rocks from the Injana Formation: Implications for Provenance and Paleoclimate

Abstract


1-Introduction
The main source of information about the paleoenvironmental conditions of the earth's surface is the clastic sedimentary rocks, which may preserve the remains of eroded source rocks for a long time and provides evidences about the composition and time to which the source rocks were exposed (Armstrong-Altrim et al., 2015).The chemical composition of sedimentary rocks is strongly influenced by factors such as the chemical constituents of the source area, the energy of the transport medium and the increasing distance away from the source, the climatic, weathering and redox conditions during sedimentation (Moradi et al., 2016).The geochemistry of mudrocks has been widely used to infer the paleowathering, paleoclimate, provenance, and tectonic setting of the source area (Bhatia, 1983;Roser and Korsch, 1986;Verma andArmstrong -Altrin, 2013, 2016).In the past, little attention has been paid to the studies of trace elements and rare earth elements (REE) (Peng et al., 2001).Nevertheless, it is important to explain the paleodepositional environment, weathering intensity, and other geological processes (Tuttle et al., 2009;Moller and Dulski, 2010;Moosavirad et al., 2011).
Most studies of the Injana Formation were focused on the sedimentology, mineralogy, and depositional environment (Bellen et al.1959;Yakta,1976;Al-Sammarai, 1978;Buday, 1980;Al-Juboury,1994;Mahdi, 2006).Very few studies dealt with trace and REE of the formation.Therefore, this study aims to discuss the trace and REE of the Injana mudstone to infer the paleoclimate and the provenance.

Geological Setting
During Late Miocene most of the shelf units were uplifted as a result of the collision between the Iranian and Anatolian plates with the Arabian Plate.The uplifted area was subjected to intense erosion, and the products of erosion were deposited in the nearby molasse basin (Jassim and Goff, 2006).The Injana Formation represents the beginning of molasses deposits formed during the Alpine Orogeny as a result of the collision of the Arabian plate with the Iranian and Anatolian plates (Beydoun, 1993).In Iraq, it is exposed from the northern and middle parts in the Low Folded Zone and Jazira Area of the Mesopotamia Foredeep, as well as in the Mesopotamia Plain (Fouad, 2012).The type section of the formation in Iraq in the Injana area is on the northeastern edge of Jabal Hamrin where the thickness of the formation there is 620 m.Injana Formation extents to Iran (Upper Fars or Aghajari Formation) (James and Wynd, 1965), Turkey (Siirt series) (Brinkmann,1976) and in Syria (Upper Fars) (Ejel and Abdul Rahim, 1974).The depositional environment of this formation changed progressively from the lagoon into the lacustrine then into the fluviolacustrine environment (Sissakian, 1992).Its lower and upper contact is conformable with the Fat'ha and Mukdadya formations, respectively (Sissakian,1992).The mudstone samples of this study were collected from three locations.Mirawa (Lat/Long-36 o 24 ' 46.5 '' N:44 o 20 ' 47.5 '' E), Degala (Lat/Log-36 o 07 ' 32.6 '' N:44 o 26 ' 46 '' E) and Najaf (Lat/Log-31 o 59 ' 22 '' N;44 o 18 ' 26.8 '' E).The total thickness of the formation at these locations is 168, 133, and 29m, respectively.The lower contact with Fat'ha Formation at the two first locations (Mirawa and Degala) is gradational and determined on the first appearance of the gypsum layer.The upper contact with Mukdadia Formation is also gradational and determined by the first appearance of the pebbly sandstone bed.At the Najaf section, the lower contact is not exposed, whereas the upper contact with Dibbdiba Formation is gradational and determined on the first appearance of a pebbly sandstone bed.Late Miocene-Pliocene Injana Formation is widely distributed in Iraq, it is of great importance in terms of economic and raw materials (Al-Rawi et al., 1992).It represents clastic sediment deposits in a fluviatile environment.Because of its wide distribution, it was extensively studied.
The samples of this study were collected from three sections distributed in northern and southern Iraq.These sections are Mirawa, Degla, and Najaf.The first two sections are located in the High Folded Zone of an Unstable Shelf, while the third section (Najaf) is located in the Mesopotamia Zone of the Stable Shelf of Iraq (Fig. 1).

Materials and Methods
Geochemical analysis of mudstone was performed on 23 mudstone samples distributed on the three locations (8 samples from the Degala section (named D), 9 samples from the Mirawa section (named M), and 6 samples from the Najaf section (named NA).Measurements of trace elements and rare earth elements (REEs) concentrations of the mudstone samples were performed using inductively coupled plasma with mass spectrometry (ICP-MS) technique at Acme labs/ Vancouver, BC Canada V6P 6E5.The trace element concentrations were normalized to the upper continental crust (UCC) and the REE were also normalized to the chondrites and UCC values.

Trace Elements
Trace element concentrations are presented in Table 1.In general, the average concentrations of the trace elements which were more than 100 ppm include Ba (12.7-936.4ppm), Sr (100.9-303.7 ppm), Ti (90-570 ppm), Cr (34.8-224 ppm), Mn (453-1060 ppm), Ni (20-321.9ppm) and Cu (24.4-282.8ppm), while the concentrations of another element were less than 100 ppm.Generally, the trace elements in the mudstone of the Injana Formation which their concentration is lower than upper continental crust (UUC) namely are Sc , V, Zn, Mo, Cs, Ba, Pb, U, Be, Ti, Cr, Ga, Rb, Sr, Zr, Hf, Sn, and Th .While the trace elements which their concentration is higher than the upper continental crust (UUC) are Li, Mn, Co, Ni, Cu, Cd, and Bi .
As shown in Fig. 2, most of the large ion lithophile elements (LILE) (such as Rb) and high field strength elements (HFSE) (such as U, Hf, and Zr) are depleted compared to UCC, whereas, the compatible elements (such as Ni and V) are enriched (Table 1 and Fig. 2).

Rare Earth Elements
The rare earth elements (REE) concentrations are presented in Table 2.Although the absolute concentrations of the REE are variable in the Injana mudstone samples in the three sections, their UCC and chondrite-normalized patterns are similar (Figs. 3 and 4).The total content of rare earth elements (ƩREE) shows significant variability between 42.8 to 82.01ppm, with an average of 26.81 ppm.The ƩREE in mudstone samples are lower than the ƩREE contents of the UCC.This may be due to carbonate dilution.Mudstone and sandstone of the Injana Formation are highly enriched with carbonate fragments from the older formations (Al-Rawi et al., 1992).
All samples have negligible Eu anomaly (The Eu/Eu* varies from 0.62 to 0.87, with an average of 0.73).The concentrations of the light REE (LREE) (LREE=La to Eu) are generally higher than the heavy REE (HREE) (HREE= Gd to Lu).The LREE/HREE ratio is between 0.76 to 8.37 (average 6.59) (Table 2).
As shown in Fig. 3, which represents chondrite-normalized REE diagrams, the REE pattern distribution shows enriched with LREE and flat HREE distribution.All the REEs in the studied mudstone samples have identical features, while in the Najaf section, the HREE (especially Lu) are enriched.The ratios Eu/Eu*and Ce/Ce* were measured according to the following equations: Eu/Eu*=2(Eu)CN/(Sm)CN +(Gd)CN, Ce/Ce*=2(Ce)CN/(La)CN+(Pr)CN (Taylor and McLennan , 1985).As the symbol (CN) refers to the chondrite values.The Ce/Ce* anomaly varies from 0.67 to 0.97 (average 0.91).The normalized ratio (La)N/(Yb)N varies between 9.10 to 14.75 (average 10.99).The ratio (La)N /(Sm)N is between 2.08 to 3.38, with an average of 2.70 (Table 2).While the (Gd)N/(Yb)N ratio varies between 2.42 to 4.17, with an average of 3.01 (Table 2).As shown in the normalization with UCC (Fig. 4A-C), all the mudstone samples generally have similar features in comparison with chondritenormalized.The ratio sample/ UCC of samples D1, M3, and N2A1 = <1; therefore, their curve is below the line.Other samples have sample/ UCC >1 therefore, they are located above the line.

General Features
Enrichment Factors (EF) have been applied globally to describe the degree of element enrichment in sediments (Ding et al., 2018).EF is defined as the ratio of the element concentration of the sample in the mudstone to the concentration in the upper continental crust (UCC) using the following equation (EF)Sample =(Element)Sample /(Element)Ucc.As shown in Table 3 and Fig .5, some elements are strongly enriched such as Ni (7.26), Cu (2.93), Gd (1.44), La (1.52), Bi (1.70), Mn (1.24) and Co (1.98).The enrichment of these elements is attributed to the accumulation of organic matter or minerals containing these elements (for example, clay minerals) (Fu et al., 2011).The Injana Formation was deposited under oxic conditions (as will discussed later in the redox conditions section), therefore, the association, and enrichment, of thse elements with the organic matter is not expected.The enrichment of these elements is most likely due to the accumulation of detrital particles, especially mafic rocks (see provenance section below).On the other hand, the elements Be, Ti, Cr, Ga, Rb, Zr, Mo, Sn, Cs, Ba, Hf, Th, and U are depleted, where they have EF values <0.5.The consumption may be due to weathering of mudstone (Eskenazy, 2009;Fu et al., 2011) or these elements are present with low concentrations in the source rocks (provenance section below).As for the elements Sc, V, Zn, Sr, and Pb have shown slight enrichment.Generally, the trace element and REE concentrations in the mudstone are higher than in the upper continental crust.The chondrite-normalized REE patterns of the samples of this study showing slightly enrichment with LREE relative to the HREE with negative Eu anomaly (Fig .3).   (Taylor and McLennan,1985).

Paleoclimate
Paleoclimatic information can be provided using the graphical plot between the Sr/Cu versus Ga/Rb ratio (Roy and Roser,2013;Awan et al., 2020).The main indicator of the paleoclimate is Sr/Cu ratio.Generally, the high Sr/Cu ratio (> 5) indicates a hot-arid climate area, whereas the ratio between 1.3 and 5 indicates warm and humid paleoclimate (Ding et al. 2018;Wei and Algeo, 2020).The studied samples of the Injana Formation have Sr/Cu values between 0.79 to 6.63, with an average of 3.47 (Table 4).These values suggest warm and humid paleoclimate.Generally, Rb is one of the elements most closely related to mudstone (e.g clay minerals illite) under cold and arid climate (Al-Juboury et al., 2021).Consequently, the sediments that have a low value of Ga/Rb indicate the cold and dry paleoclimate (Al-Juboury et al., 2021).Ga is extremely enriched in mudstone (e.g clay minerals kaolinite), inferring warm and humid conditions (Ding et al., 2018).The warm and humid conditions at time of deposition of Injana Formation are also indicated from the common kaolinite and chlorite distribution in the Injana Formation (Al-Juboury, 2009).The Ga/Rb ratio of the studied samples of Injana Formation varies between 0.34 to 0.87, with an average of 0.50 (Table . 4).Fine sediments have low Sr/Cu ratio and high Ga/Rb ratio in warm and humid conditions (Ding et al. 2018).According to the graphical plot between Sr/Cu versus Ga/Rb (Fig. 6), most of the Injana samples fall in the warm humid field.These data suggest a warm and humid climate during the deposition of the mudstone of the Injana Formation.

Provenance
The nature of the source rocks controlled the mineralogical and chemical composition of the finegrained sediments (Cullers, 2000;Dai et al., 2016).Geochemical data are widely used to describe the source sediments and the source area (Roser and Korsch,1986;Cullers, 2000;Dai et al., 2016;Armstrong-Alrtin et al., 2021).Most of the major elements (such as Mg, Na, Ca, K) are highly soluble in water, therefore, they are unreliable for provenance studies.Due to the low mobility of the high field strength (HFSE) trace elements such as Th, Ti, Al, Sc, Zr, and Y; and a short residence time and the immobility of REEs during sedimentary processes, they are widely used as provenance proxies (Taylor and McLennan, 1985;Cullers, 2000).
The discriminant diagrams La/Sc versus Co/Th, Th/Sc versus Cr/Th, Th/Cr versus La/Cr and La/Sc versus Th/Sc are useful tools to provide the composition of the source area (Table 4).In the current study, the Cr is well correlated with Al, indicating it is of detrital origin and not influenced by the redox state.The average La/Th value of the Injana mudstone is 3.43 (Table 4).The average value of La/Sc and Co/Th of the Injana mudstone are 1.86 and 5.71 respectively (Table 4).Based on the La/Sc versus Co/Th plot (Fig. 7), the studied mudstone samples are between andesite and basalt fields.The average value of Th/Sc and Cr/Th of the Injana mudstone are 0.54 and 27.03 respectively (Table 4).Based on the Th/Sc versus Cr/Th plot (Fig. 8), the mudstone samples are close to the field between intermediate and mafic source fields.In addition, the average value of Th/Cr, La/Cr,La/Sc and Th/Sc of the studied samples are 0.05, 0.15, 1.86 and 0.54 respectively (Tables 4 and 5).Based on the Th/Cr versus La/Cr and La/Sc versus Th/Sc plot (Figs.9and 10), the mudstone samples are close to the field of mafic source.
Moreover, the sediments derived from mafic, less differentiated sources in active tectonic settings commonly have low Th/Sc (1.0) and high Sm/Nd ratio; and those derived from upper continental crust in passive tectonic setting will have high Th/Sc (>1.0) and low Sm/Nd ratios (McLennan and Hemming, 1992;Bankole et al., 2020).The low Th/Sc ratio (<0.75) and high Sm/Nd ratio (>0.2) of the Injana mudstone samples (Table 4) supports mafic provenance and active tectonic setting of the source area.(Taylor and McLennan,1985) **PAAS: Post Archean Australian Shale (Taylor and McLennan,1985) Fig. 7. La/Scversus Co/Th diagram for the mudstone of the Injana Formation (after Mclennan et al., 1993).The ratios (Gd/Yb)CN, LREE/HREE, and Eu anomaly (Eu/Eu*) were also used to infer sources of sedimentary rocks.The felsic source rocks have high LREE/HREE, low (Gd/Yb) CN and negative Eu anomaly, whereas the mafic source rocks are characterized by high (Gd/Yb)CN, low LREE/HREE ratios, and lack of Eu anomaly (Cullers, 1995).The Injana mudstone displays a high LREE/HREE ratio (average 6.78), a high Gd/YbCN ratio (average 2.98), and negligible negative Eu anomaly (0.73) (Table 2), which means that it was mainly derived from intermediate-mafic rock.Mafic source rocks for the Injana mudstones are also supported by very low concentrations of the incompatible elements, such as U, Th, Zr, Nb and Ta associated with high concentrations of the compatible elements such as Ni and Cu (Table 3).

Redox Condition
The concentration, mobilization, and precipitation of some elements such as Cr, V, Mn, Co, Ni, Mo, U, and Th are controlled by redox conditions of the depositional environment; therefore, they can be used as redox proxies (Guo et al., 2007).Most of these trace elements have a positive relationship with Al, indicating their detrital origin, therefore, they cannot be used as paleoredox proxies.However, U is not well correlated with Al.U +6 is mobile under oxic conditions and precipitates as U +4 under reducing conditions; whereas Th has one valence state and is immobile.Therefore, the U/Th ratio is widely used to decipher the redox conditions of the depositional environment.The U/Th values are between 0.06-0.18 in Mirawa and Degala samples and slightly higher in the Najaf samples (0.17-0.84), indicating oxic depositional environment.The oxic depositional environment of the Injana mudstones is supported by the Ce/Ce* values (average 0.89).

Conclusions
The main conclusions of this study are: • The Sr/Cu and Ga/Rb ratio indicate that these mudstones were deposited under humid and warm paleoclimate.

Fig. 1 .
Fig.1.Tectonic map showing the location of the study locations (after Fouad,2015)

Fig. 5 .
Fig.5.Enrichment Factors (EF) diagrams of the trace elementa of the mudstone from the Injana Formation
The mudstones of the Injana Formation have high concentrations of Ni and Cu and low concentrations of most of the incompatible elements such as Zr, Ta, Nb, U and Th.The chondritenormalized REE patterns of the samples showing slightly enrichment with LREE relative to the HREE with negative Eu anomaly.These features and the discriminant diagram La/Sc versus Co/Th, Th/Sc versus Cr/Th, Th/Cr versus La/Cr and La/Sc versus Th/Sc suggest that these mudstones were derived from intermediate-mafic source rocks.• The U/Th and the Ce/Ce* values indicate that the mudstones of the Injana Formation were deposited under oxic conditions.

Table 1 .
Trace elements concentration (in ppm) and Al (in wt.%) of Late Miocene mudstone from the Injana Formation

Table 2 .
Rare earth element concentration (ppm) of Late Miocene mudstone from Injana Formation

Table 3 .
Enrichment Factors (EF) values of the trace element of Mudstone from Injana Formation.Data from

Table 4 .
Provenance and paleoclimate elements ratios of Late Miocene Mudstone frome Injana Formation.Data from Table1 and Table2

Table 5 .
Elemental ratios of the Injana mudstone compared to the range values of sediments derived from mafic and felsic rocks, and upper continental crust