Zamaniyan, E., Khanehbad, M., Moussavi-Harami, R., Mahboubi, A., 2019. Geo chem is try of shales of the Qadir Mem ber (Nayband For ma tion, Up per Tri as sic), East Cen tral Iran (Tabas Block): im pli ca tions for prov e nance and palaeogeography.
Geo log i cal Quar terly, 63 (3): 603–618, doi: 10.7306/gq.1491 As so ci ate ed i tor: Tomasz Bajda
The Up per Tri as sic shale of the Qadir Mem ber of the Nayband For ma tion, East Cen tral Iran has been ana lysed geochemically to eval u ate prov e nance and palaeo ge ogra phy. The Qadir Mem ber in the Parvadeh Coal Mine sec tion is 450 metres thick, and in cludes sand stone, shale, coal, siltstone, and fossiliferous lime stone. XRD anal y sis of shale sam ples from the Qadir Mem ber largely in di cated the pres ence of illite and chlorite, with small amounts of kaolinite and mont mo ril lo nite. On bi nary and tri an gu lar di a grams the data sug gests an in ter me di ate ig ne ous source rock for these shales. Plot ting the geo - chem i cal data on bi nary di a grams also in di cates the tec tonic set ting of an ac tive con ti nen tal mar gin, per haps re flect ing the Early Cim mer ian tec tonic event with Neothetys subduction un der the Iran Plate, and col li sion of the Iran Plate with Turan dur - ing the Late Tri as sic. The Chem i cal In dex of Al ter ation (CIA) and Plagioclase In dex of Al ter ation (PIA) val ues for shale from the Qadir Mem ber of Nayband For ma tion vary from 74.04 to 80.54 (av er age 78.02) and 84.31 to 91.85 (av er age 87.81), re - spec tively, in di cat ing mod er ate to high chem i cal al ter ation in the source area and sug gest ing a semi-hu mid cli mate dur ing de po si tion. The geo chem i cal data and palaeo geo graphi cal mod els in di cate that the Qadir Mem ber shale was de pos ited on an ac tive mar gin in a shore line to tran si tional-ma rine set ting.
Key words: Tri as sic, Nayband For ma tion, Tabas, prov e nance, geo chem is try.
INTRODUCTION
In sed i men tary ge ol ogy, the term “prov e nance” is used to con sider all the fac tors re lated to the pro duc tion of sed i ments from source to sink. Many re search ers have in ves ti gated how tec ton ics, and prov e nance re late to the com po si tion of siliciclastic rocks (Dickinson and Suczek, 1979; Dickinson, 1985; Bhatia and Crook, 1986; Rieser et al., 2005; Flem ing et al., 2016). Prov e nance anal y sis may be used to iden tify the source area(s) and as sem blages of par ent-rocks from ini tial ero sion of the source rock to fi nal burial of the sed i ments (Weltje, 2002; Weltje and von Eynatten, 2004; Khanehbad et al., 2012a). In gen eral, the com po si tion of siliciclastic rocks de - pends on the types of source rocks, palaeoclimate and to pog ra - phy (Jin et al., 2006; Armas et al., 2014; Mahavaraju, 2015). It is pos si ble to in ter pret the prov e nance of siliciclastic rocks by us - ing pe trog ra phy and geo chem i cal data via discriminant di a - grams (Khanehbad et al., 2012b; Sabbagh Bajestani et al.,
2018). Geo chem i cal prov e nance stud ies are par tic u larly ap pli - ca ble to shales, be cause of their ho mo ge ne ity be fore de po si - tion, their post-depositional impermeability and their higher abun dance of trace el e ments rel a tive to sand stones (Cullers, 2000; Hessler and Lower, 2006; DaPeng et al., 2012).
In this study, the prov e nance of siliciclastic rocks of the Qadir Mem ber of the Nayband For ma tion in East Cen tral Iran (Tabas Block) is in ter preted and in ves ti gated through geo - chem i cal anal y sis. Geo chem i cal data from shale is used to de - ter mine prov e nance (in ter pret ing source rock, tec tonic set ting, palaeoweathering, and de ter min ing palaeogeography). This study al lows for a better un der stand ing of the palaeotectonic set ting and palaeo ge ogra phy of pe riod in East Cen tral Iran in the Tri as sic.
GEOLOGY OF THE STUDY AREA
Dur ing Perm ian times, the Cen tral-East Ira nian Micro - continent (CEIM) be came de tached from Gond wana and moved to wards Eur asia. The en su ing col li sion caused the clo - sure of the Palaeotethys Ocean and a se ries of events re lated to tec tonic up lift, magmatism, and meta mor phism tra di tion ally termed the Early Cim mer ian orog eny (Aghanabati, 2006;
Wilmsen et al., 2009a). Fol low ing this orog eny, there was sea
* Cor re spond ing au thor, e-mail: mkhanehbad@ferdowsi.um.ac.ir Re ceived: October 14, 2018; ac cepted: July11, 2019; first pub lished on line: October 3, 2019
level rise and the Nayband For ma tion (Late Tri as sic), with a thick ness of 3000 m, was de pos ited in Cen tral Iran’s sed i men - tary struc tural zone. In the Late Tri as sic (Norian–Rhaetian), the Nayband For ma tion was dis trib uted over a large area of cen tral and east ern Iran (Seyed-Emami, 2003); its mea sured type sec - tion is placed south of the Nayband Moun tain to the west of Nayband vil lage (Aghanabati, 2006). As re gards Iran’s struc - tural di vi sions (Aghanabati, 2006; Malekzadeh Shafaroudi et al., 2015), the study area is lo cated in the cen tral part of the Cen tral-East Ira nian Microcontinent (CEIM). The Tabas Block is an intra-con ti nen tal de pres sion and a part of the CEIM (Aghanabati, 2006) with a vol a tile geo log i cal his tory mak ing it one of the most com plex geo log i cal units in Iran.
The Qadir Mem ber of the Nayband For ma tion in the Parvadeh Coal Mine sec tion (33°00’21”N and 56°48’40”E), lo - cated about 80 km south of Tabas, has been mea sured and stud ied in the Nayband sed i men tary struc tural sub-block (sub - di vi sions of the Tabas Block; Fig. 1). This for ma tion is ex posed ex ten sively across the Tabas Block; one of the best ex po sures is lo cated in the Parvadeh Coal Mine (PCM) zone, south of Tabas. The Nayband For ma tion on the south ern flank of the cen tral anticline of the PCM (termed the Sand wich anticline among lo cal ge ol o gists) is 450 m thick (Figs. 2 and 3) and lithologically con sists of shale and sand stone with in ter ca la tions of fossiliferous lime stone and coal lay ers. Its lower bound ary is un ex posed in this sec tion while the up per bound ary is disconformable against the coarse-grained strata of the Ab-Haji
For ma tion (white sand stones and red-brown con glom er ates – Early Ju ras sic; Fig. 4).
SAMPLES AND METHODS
In study ing the siliciclastic rocks of the Qadir Mem ber in Parvadeh Coal Mine to the south of Tabas, fol low ing sam pling and field stud ies, 10 sam ples of shale were se lected to be analysed by XRF for de ter min ing ma jor and trace el e ments and 3 sam ples of shale were se lected for XRD anal y sis to iden tify the clay min er als. The shale sam ples were an a lysed by East Am e thyst An a lyt i cal Ge ol ogy Lab us ing an X-ray flu o res cence de vice (Philips PW 1480 X-ray spec trom e ter). Ten shale ex po - sure sam ples with min i mal car bon ate ce ment were cleaned and crushed to pow der in ag ate mor tars for ma jor and trace el e ment anal y sis. XRD anal y sis was also per formed to iden tify clay min - er als, by Geo log i cal Sur vey and Min eral Ex plo ra tions, North - west Branch (Tabriz cen ter), us ing a Siemens D-5000 ma chine.
To in ves ti gate micro struc tures and to de ter mine the semi-quan - ti ta tive com po si tion of clay min er als in the Qadir Mem ber de - pos its, a Scan ning Elec tron Mi cro scope (SEM, LEO 1450VP) was used, in the cen tral lab o ra tory of the Ferdowsi Uni ver sity of Mashhad, equipped with an En ergy Dispersive X-ray (EDX) de - tec tor. Fresh sur face sam ples were in ves ti gated us ing a scan - ning elec tron mi cro scope (SEM). For this pur pose, small fresh sam ples were bro ken and then were coated by Au-Pd sput ter prior to ex am i na tion.
Fig. 1A – struc tural map of the Cen tral-East Ira nian Microcontinent and its crustal blocks (mod i fied from Malekzadeh Shafaroudi et al., 2015); B – lo ca tion of the sec tion stud ied (yel low as ter isk) to the south of Tabas
RESULTS
CLAY MINERALOGY
XRD was used to iden tify clay min er als within the fine-grained de pos its of the Qadir Mem ber. Anal y sis of three shale sam ples show pres ence of illite and chlorite with small amounts of kaolinite and mont mo ril lo nite (Fig. 5). Illite and chlorite were also iden ti fied in SEM stud ies (Fig. 6).
CHEMICAL COMPOSITION
The data of ma jor and trace el e ment anal y ses of shale from the Qadir Mem ber are out lined be low and listed in Ta bles 1 and 2.
Re sults show that av er ages of ox ides are SiO2 59.97%, Al2O3 17.70%, Na2O 1.69%, MgO 2.35%, K2O 2.84%, TiO2
0.87%, MnO 0.06%, CaO 0.35%, P2O5 0.11% and Fe2O3
6.47%. The ra tios of SiO2/Al2O3 (av er age 3.23%), K2O/Na2O (av er age 1.73%), K2O+Na2O (av er age 4.44%), Chem i cal In dex of Al ter ation (CIA: av er age 78.04), Plagioclase In dex of Al ter - ation (PIA: av er age 87.81) and In dex of Compositional Vari abil - ity (ICV: av er age 0.77) re gard ing the shales were cal cu lated and are shown in Ta ble 1.
The sil ica pres ent may re flect the pres ence of quartz, chert, feld spar and clay min er als. Val ues of Al2O3 and K2O might be as - so ci ated with the pres ence of K-feld spar (microcline), mica, and clay min er als. The Na2O is largely as so ci ated with K-feld spar (al - bite). TiO2 is ex ten sively as so ci ated with rutile, opaque min er als, and vol ca nic rocks. Fe2O3 may be as so ci ated with iron ox ide, heavy min er als, and he ma tite ce ment. MgO and CaO could be as so ci ated with car bon ate min er als (Khanehbad et al., 2012a), with MgO re flect ing the pres ence of do lo mite ce ment and CaO largely as so ci ated with cal cite ce ment and skel e tal frag ments.
The ma jor el e ments of Qadir Mem ber shale may be com pared with the av er age chem i cal com po si tion of the Up per Con ti nen tal Crust (UCC; Tay lor and McLennan, 1985; Fig. 7). Here, CaO is much lower than that of UCC while con cen tra tions of most other ma jor el e ments in the shales are gen er ally sim i lar to the mean com po si tion of UCC. The low con cen tra tions of CaO may ei ther in di cate a lack of orig i nal car bon ate min er als or de ple tion in Ca
dur ing diagenesis (Salehi et al., 2014). As Al2O3 re mains gen er - ally un change able dur ing al ter ation, meta mor phism, and diagenesis, it can be ap plied as a fac tor for com par i son with the other ma jor el e ments (Getaneh, 2002; Akarish and El-Gohary, 2008). In the sam ples stud ied, the amount of Al2O3 was com - pared with other ma jor ox ides (Fig. 8). The Fe2O3, K2O, MgO, Na2O and TiO2 con tents show a pos i tive re la tion ship with Al2O3. This re flects the pres ence of clay min er als, es pe cially alu mi no - sili cate (Pettijohn et al., 1987; Das et al., 2006; Khanehbad et al., 2012b). The pos i tive re la tion ship of K2O with Al2O3 sug gests the pres ence of po tas sium-rich min er als (Jin et al., 2006). By con - trast, val ues of CaO, SiO2, MnO and P2O5 show a neg a tive re la - tion ship with Al2O3. The neg a tive cor re la tion of SiO2 with most ma jor el e ments (for ex am ple with Al2O3) is due to most sil ica be - ing se ques tered in quartz, as in di cated by Osman (1996). This could be be cause of the pres ence of more quartz phases and a high tex tural and compositional ma tu rity of the sam ples stud ied (Bayat-Goll and Hosseini Barzi, 2011; Salehi et al., 2014). The re verse re la tion ship with CaO may be re lated to a cal cite diagenetic ce ment and sec ond ary al ter ation (Das et al., 2006).
The K2O/Na2O bi nary di a grams against SiO2/Al2O3
(Wronkiewicz and Condie, 1987) for the shale in the Qadir Mem - ber sug gest that it lies within the range of Phanerozoic shale com po si tion (Fig. 9). Trace el e ments de ter mined in this study re - veals a dom i nance of Rb, Ba, Sr, Th, Cr, Y, Zr, Nb, V, Co, Cu and Ni (Ta ble 2). In Fig ure 10, Ni, V, Rb and Y are pos i tively cor re - lated with Al2O3. The re sults also show that trace el e ment av er - ages (ppm) are Rb 174, Ba 68, Sr 165.2, Th 13, Cr 86.1, Y 68.5, Zr 288.4, Nb 17.2, V 112.1, Co 16.3, Cu 87.7, Ni 72.6.
DISCUSSION
SOURCE ROCK
Many re search ers have stud ied the re la tion ship be tween the chem i cal com po si tion of siliciclastic rocks and the source area (Khanehbad et al., 2012b; Salehi et al., 2014; Valiani and Rezaee, 2014; Zaid, 2015). The geo chem is try of shale is more ap pli ca ble than the pe trol ogy of sand stone for in ter pre ta tion of the source rocks (Cullers, 2000). The ra tio of Al2O3 to TiO2 in Fig. 2. Sim pli fied geo log i cal map of the area
(mod i fied af ter Moussavi-Harami and Ghaemi, 2006; Saidi et al., 2004)
Fig. 3. Lithostratigraphic col umn of the Qadir Mem ber of the Nayband For ma tion in the area stud ied, lo ca tion of sam ples col lected for XRF and XRD are shown
Fig. 4. Field pho to graph of the Qadir Mem ber sec tion of the Nayband For ma tion (Up per Tri as sic) in the Parvadeh Coal Mine area
Sam ple no.
P 42 P 82 P 158.5 P 190.5 P 218 P 281 P 308 P 393 P 413.2 P 434 Av er age UCC
SiO2 61.50 57.05 56.55 55.56 60.74 56.01 57.02 67.01 65.77 62.53 59.97 66
Al2O3 18.00 17.85 17.13 21.38 16.95 19.93 18.74 15.29 15.68 16.10 17.7 15.2
Na2O 2.58 1.86 1.98 1.44 1.50 1.97 1.43 1.45 1.33 1.39 1.69 3.9
MgO 1.17 3.26 3.44 2.41 2.80 2.50 2.77 1.87 1.60 1.76 2.35 2.2
K2O 2.83 2.77 2.96 3.89 2.70 3.56 3.36 2.00 2.16 2.26 2.84 3.4
TiO2 1.02 0.81 0.90 0.91 0.85 0.92 0.98 0.75 0.87 0.76 0.87 0.5
CaO 0.10 0.16 0.47 0.11 1.15 0.01 0.18 0.86 0.30 0.24 0.35 4.2
Fe2O3 4.36 6.08 7.16 7.14 6.29 5.10 6.70 5.37 7.34 9.19 6.47 5.04
MnO 0.04 0.07 0.07 0.05 0.07 0.03 0.08 0.07 0.09 0.10 0.06 –
P2O5 0.11 0.13 0.11 0.08 0.12 0.07 0.14 0.14 0.12 0.12 0.11 –
SO3 0.14 0.06 0.22 0.02 0.00 0.12 0.04 0.06 0.00 0.03 0.6 –
LOI 6.50 9.11 8.03 6.66 6.41 8.79 8.22 4.79 4.52 5.25 6.82 –
PIA 84.98 88.18 85.25 91.85 84.31 89.2 90.52 85.19 89.24 89.46 87.81 –
CIA 75.56 78.84 74.02 79.71 76 78.24 79.03 78.01 80.53 80.54 78.04 –
ICV 0.61 0.79 0.93 0.7 0.85 0.65 0.77 0.75 0.81 0.92 0.77 –
LOI – loss of ig ni tion; PIA – Plagioclase In dex Al ter ation; CIA – Chem i cal In dex of Al ter ation; ICV – In dex of Compositional Vari abil ity T a b l e 1 Ma jor el e ment (ox ide wt.%) com po si tion of shale from the Qadir Mem ber of Nayband For ma tion
Fig. 5. X-ray dif frac tion pat tern of sam ples of Qadir Mem ber shale
A – sam ple P 1; B – sam ple P 42; C – P 308; k – the graph was taken at 550°C by po tas sium chlo ride; kt – peak dis tinc tion of kaolinite from chlorite; mg and mge – peak dis tinc tion of chlorite from mont mo ril lo nite
Fig. 6. SEM pic tures of Qadir Mem ber shale
A – illite with sheet struc ture; B – EDX anal y sis of A; C – chlorite (as ter isk) with nee dle struc ture; D – EDX anal y sis of C
Sam ple no.
P 42 P 82 P 158.5 P 190.5 P 218 P 281 P 308 P 393 P 413.2 P 434 Av er age
Rb 180 157 186 259 153 245 200 117 123 120 174
Ba 4 95 62 77 98 63 47 88 49 97 68
Sr 216 139 141 139 148 261 159 144 168 137 165.2
Th 11 11 14 16 11 13 21 13 9 11 13
Cr 95 85 93 107 84 103 88 65 75 66 86.1
Y 77 67 73 78 64 83 79 49 67 48 68.5
Zr 342 289 269 251 297 259 254 295 323 305 288.4
Nb 9 9 17 17 13 15 18 12 46 16 17.2
V 113 110 119 146 102 133 128 83 98 89 112.1
Co 13 17 18 19 14 13 19 12 17 21 16.3
Cu 114 N 99 74 54 107 89 150 82 108 87.7
Ni 66 80 75 93 70 77 94 58 46 67 72.6
T a b l e 2 Trace el e ment (ppm) com po si tion of shale from the Qadir Mem ber
Fig. 7. Nor mal iza tion of ma jor ox ides of Qadir Mem ber shale in com par i son with Up per Con ti nen tal Crust
(UCC; Tay lor and McLennan, 1985)
Fig. 8. Changes of ma jor ox ides ver sus Al2O3 in the Qadir Mem ber shales
Fig. 9. K2O/Na2O ver sus SiO2/Al2O3 plot for shale of the Qadir Mem ber Fields for the Archean Greenstone Shale and Phanerozoic shale af ter
Wronkiewicz and Condie (1987)
Fig. 10. Dis crim i na tion plots Al2O3 and trace el e ments (Ni, V, Rb, and Y) for shale of Qadir Mem ber
shales is a very good in di ca tor of shale source rock com pared to the other ma jor el e ments (Paikaray et al., 2008). Plot ting data re gard ing Al2O3 ver sus TiO2 on the Scheiber (1992) di a gram shows that these shales are sit u ated in the range of granodiorite ig ne ous source rock (Fig. 11A). Us ing the Paikaray et al. (2008) di a gram, of Zr against TiO2, our data show that the shale of the Qadir Mem ber plots as an in ter me di ate ig ne ous source rock (Fig. 11B). Plot ting the data on the Hayeshi et al.
(1997) ter nary di a gram, these shales are mainly lo cated in the range of an de site to dacite, which sug gests an in ter me di ate ig - ne ous source rock (Fig. 11C). In the Hayeshi et al. (1997) di a - grams, of (FeO+MgO), (CaO+Na2O+K2O), and (Al2O3), the dis - per sion of the shale data from the Qadir Mem ber is in the range of illite and chlorite (Fig. 11D). The in fer ence of illite and chlorite from the ma jor el e ments ox ide data also cor re sponds with the data ob tained from XRD anal y sis (Fig. 5). On the Na2O–K2O di - a gram (Fig. 12A), the Qadir Mem ber shale data plots in the quartz-rich field. Floyd and Leveridge (1987) plot ted K2O ver sus Rb to dis tin guish sed i ments de rived from acidic to in ter me di ate rocks from those de rived from ba sic rocks. Most of the shale of the Qadir Mem ber stud ied lies in the field of acid and in ter me di - ate com po si tions (Fig. 12B).
El e vated val ues of Cr >150 ppm and Ni >100 ppm are sug - ges tive of ultra mafic rocks in the source re gion (Garver et al., 1996). By com par i son, Cr in these shales var ies from 65 to Fig. 11A – ra tio of Al2O3 ma jor ox ides com pared with TiO2 (Schieber, 1992); B – TiO2 (wt.%) ver sus Zr (ppm) bivariate di a gram (Paikaray et al., 2008); C – SiO2/20–Na2O+K2O–MgO+TiO2+FeO ter nary di a gram (Hayeshi et al., 1997); D – plot ting the data on the
FeO+MgO–CaO+Na2O+K2O–Al2O3 ter nary di a grams (Hayeshi et al., 1997)
Fig. 12A – anal y sis of shales ac cord ing to the rich ness of quartz of the Qadir Mem ber (af ter Crook, 1974); B – K2O ver sus Rb di a - gram of the shale of the Qadir Mem ber (fields af ter Floyd and Leveridge, 1987)
107 ppm (av er age 86.1 ppm) and Ni con cen tra tion var ies from 46 to 94 ppm (av er age 72.6 ppm). This makes the pres ence of sig nif i cant mafic and ultra mafic rocks in the source area most un likely. Also, these shales show en riched val ues of Zr, Sr and Rb and de ple tion in Nb, Y, Cr and Ni. Fel sic rocks com monly have lower Y and Nb than mafic rocks (Humphreys et al., 1995).
This sug gests that al most all of these sam ples were de rived from plutonic and in ter me di ate ig ne ous rocks. In gen eral, based on the avail able data, an in ter me di ate ig ne ous source rock is sug gested as a source for the shale of the Qadir Mem ber.
TECTONIC SETTING
The chem i cal com po si tion of sed i men tary rocks can give clues not only about re work ing, re cy cling and weath er ing con di - tions, but also on the tec tonic set ting of their depositional ba sin.
For this pur pose, geo chem i cal dis crim i na tion di a grams have been de vel oped and used (e.g., Bhatia 1983; Bhatia and Crook 1986; Roser and Korsch, 1986, 1988; Verma and Armstrong-Altrin, 2013), in clud ing in Iran (Moosavirad et al., 2011; Khanehbad et al., 2012b; Jafarzadeh et al., 2014; Salehi et al., 2014, 2018).
Plot ting SiO2 against K2O/Na2O is com monly used to iden - tify the tec tonic set ting of shales (Bhatia, 1983; Roser and Korsch, 1988). Ac cord ing to the bi nary di a grams of Roser and Korsch (1988), the sam ples of shale in Qadir Mem ber have been largely de pos ited on an ac tive con ti nen tal mar gin
(Fig. 13A). This in ferred tec tonic ac tiv ity may cor re spond to the Early Cim mer ian Orog eny at the bound ary of Mid dle and Late Tri as sic time (the bound ary be tween Shotori For ma tion and the Nayband For ma tion), con cur rent with the orogenic phase of the Early Cim mer ian in Cen tral-East Ira nian Microcontinent (CEIM;
for ex am ple, Stöcklin, 1974; Stampfli and Borel, 2002; Fürsich et al., 2005; Fürsich et al., 2009; Wilmsen et al., 2009a, b). The re sults of our study can be cor re lated with palaeotectonic maps of the Tri as sic pe riod of the world (Golonka, 2004).
SOURCE AREA WEATHERING
Fac tors such as source rock com po si tion, du ra tion of al ter - ation, cli ma tic and tec tonic con di tions af fect the chem i cal al ter - ation of the source rock (Moosavirad et al., 2011). The min er al - ogy and chem i cal com po si tion of terrigenous sed i ments are con trolled be many fac tors in clud ing source area com po si tion and may be af fected by al ter ation in the source area and by the tec tonic set ting (Bauluz et al., 2000).
Geo chem i cal in di ces us ing the ma jor el e ment com po si tion of silicilastic de pos its is fur ther more used to in fer weath er ing and con se quently palaeoclimate con di tions. Among the most com monly em ployed in di ces are the Chem i cal In dex of Al ter - ation (CIA) af ter Nesbitt and Young (1982) and the Plagioclase In dex of Al ter ation (PIA) af ter Fedo et al. (1995).
The Chem i cal In dex of Al ter ation (CIA) may be ob tained by the fol low ing equa tion.
Fig. 13A – SiO2 ver sus K2O/Na2O dis crim i na tion plot of Roser and Korsch (1988); B – A-CN-K ter nary di a gram for shales of the Qadir mem ber (Paikaray et al., 2008; A – Al2O3, CN – CaO+Na2O, K – K2O); C – A-K–C–N ter nary di a gram of mo lec u lar pro por tions of A-K – Al2O3-K2O, C – CaO, N – Na2O (An – an or thite, By – bytownite, La – lab ra dor ite, Ad – an de sine, Og – oligoclase, Ab – al bite; Fedo et al., 1995); D – ICV against ICA dual di a gram (Cox et al., 1995; Pot ter et al., 2005) sug gest ing an in ter me di ate ig ne ous rock source
lo cated at 25–30°N. In this palaeo-sub trop i cal suc ces sion, a semi-hu mid cli mate pre vailed dur ing the de po si tion of these shales. The al ter ation pro cess, in a A-CN-K di a gram, may be ob tained by the al ter ation pro file method and also by ther mo dy - namic es ti ma tions (Nesbitt and Young, 1984). In this di a gram, the ini tial stages of al ter ation form a par al lel with the A-CN side be cause so dium and po tas sium ions are re moved by de stroy - ing feld spars (plagioclase) in the ini tial stages of al ter ation; with the con tin u a tion of K-feld spar al ter ation, po tas sium ions de - crease and the al ter ation pro cess is shifted to the Al2O3 com po - si tion (Paikaray et al., 2008). How ever, all shale sam ples plot near the Al2O3-K2O join, in di cat ing a high rate of al ter ation in the source area (Fig. 13B).
The Plagioclase In dex of Al ter ation (PIA; Fedo et al., 1995) was cal cu lated based on the fol low ing equa tion:
PIA = (AL2O3-K2O)/(Al2O3-K2O) + CaO+K2O ´ 100
The PIA value for the shales in the Qadir Mem ber var ies be - tween 84.31 to 91.85 with an av er age of 78.81 (Ta ble 1). These high val ues in di cate high plagioclase al ter ation in the source area (Alvarez and Roser, 2007; Ogala et al., 2014). On the Fedo et al. (1995) di a gram, sam ples are mostly lo cated on the right or in the mid dle of the di a gram and the plagioclase com po - si tion en com passes al bite, oligoclase and an de sine (Fig. 13C), which have turned into clay min er als such as illite and chlorite dur ing the chem i cal al ter ation.
Also, the re la tion be tween al ter ation and the source rock com po si tion can be eval u ated based on the in dex of compositional vari abil ity (ICV) and CIA (Cox et al., 1995; Pot ter et al., 2005). These in di ces can be used for iden ti fi ca tion of types of source rock (Valiani and Rezaee, 2014). The equa tion is the fol low ing:
ICV = (Fe2O3+K2O+Na2O+CaO+MgO+TiO2)/Al2O3
The ICV in dex in di cates the de gree of ma tu rity of fine aluminosilicatic ma te ri als de liv ered to the depositional ba sin.
An ICV of >1 in di cates im ma ture shales with a high per cent age of sil i cate min er als (with out clay min er als), while, more ma ture clayey rocks with abun dant clay min er als proper have lower ICV val ues (Cox et al., 1995; Moosavirad et al., 2011). The av - er age ICV value ob tained for shale of the Qadir mem ber is 0.77, which shows all these shales are ma ture rocks as so ci ated with abun dant clay min er als such as chlorite and illite. Con sid er ing the ICV vs. CIA di a grams (Lee, 2002) sug gests that the shale of Qadir Mem ber, with this rate of al ter ation, has orig i nated from in ter me di ate ig ne ous rocks (Fig. 13D).
(ªengör et al., 1988; Alavi et al., 1997; Seyed-Emami, 2003;
Golonka, 2004). The sed i men tary cy cle of the Late Tri as sic to the Mid dle Ju ras sic hap pened be tween two tec tonic events:
Early Cim mer ian and Mid dle Cim mer ian (Fürsich et al., 2009).
Extensional tec ton ics is doc u mented in Cen tral and East-Cen tral Iran dur ing the Late Tri as sic to Late Ju ras sic fol - low ing the Early Cim mer ian tec tonic event and Late Tri as sic Neotethyan subduction (Fürsich et al., 2003; Fürsich et al., 2005a; Seyed-Emami et al., 2004; Wilmsen et al., 2010;
Sheikholeslami, 2016). Dur ing the Norian (Nayband For ma tion as the first for ma tion in the Shemshak Group of Cen tral Iran), there were coastal and ma rine con di tions with the ac cu mu la tion of 1000 m of strata, while in the Rhaetian, the Qadir Mem ber was de pos ited in tran si tional-ma rine con di tions (Shahrabi, 1999). Dur ing the Late Tri as sic (Norian–Rhaetian) there was the be gin ning of subduction of the Neotethys oce anic crust be - neath the Iran Plate at the south ern mar gin of Iran Plate; this event re duced com pres sion of the Iran Plate with sub se quent for ma tion (Nayband Ba sin) of the extensional bas ins and trans - gres sion of the sea onto the Iran Plate (Wilmsen et al., 2009b;
Fürsich et al., 2005). In that pe riod, Iran was lo cated be tween lat i tude 25–30° North (Fig. 14A; Bar rier and Vrielynck, 2008).
This sup ports our in ter pret ing of a semi-hu mid cli mate as dis - cussed above. Also, the on set of Neotethys oce anic crust subduction at the south west ern mar gin of the Iran Plate in Norian–Rhaetian times is well es tab lished (Arvin et al., 2007;
Sheikholeslami, 2016), in agree ment with the ac tive con ti nen tal mar gin set ting for the shale of the Qadir Mem ber sug gested by the Roser and Korsch (1988) di a grams. Neotethys subduction un der the Iran Plate and also col li sion of Iran with Eur asia and the Turan Plate at the end of Tri as sic Pe riod is con sis tent with this in ferred his tory (Fig. 14); pos si bly, shal low an gle of Neotethys subduction be neath SW Iran (Ghasemi and Tal bot, 2006) caused the lim ited vol ca nic ac tiv ity in the extensional ba - sin (such as the Tabas Block in CEIM).
CONCLUSIONS
Shales of the Qadir Mem ber in the Nayband For ma tion in East Cen tral Iran are com posed of quartz, illite and chlorite.
Anal y ses of the data ob tained from geo chem i cal in ves ti ga tions in di cates that the source rocks of siliciclastic de pos its in the Qadir Mem ber are in ter me di ate ig ne ous rocks. In dif fer ent chem i cal weath er ing in di ces such as the CIA and PIA, and in a A-CN-K ter nary di a gram, all shale sam ples of the Qadir Mem - ber plot par al lel to the A-K line, in di cat ing mod er ate to high chem i cal weath er ing in the source re gion. The geo chem i cal re -
Fig. 14A – palaeogeographic sketches; B – geodynamic mod els of the Late Tri as sic (Mid dle Norian–Rheatian) for the Early Cim mer ian Orog eny on the Iran Plate (mod i fied af ter Bar rier and Vrielynck 2008; Wilmsen et al., 2009b)
CEIM – Cen tral-East Ira nian Microcontinent; the study area is in di cated by a yel low as ter isk on A
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