Geo chem is try of siliciclastic rocks from the Shemshak Group (Up per Tri as sic–Mid dle Ju ras sic), north east ern Alborz, north ern Iran:
im pli ca tions for palaeoweathering, prov e nance, and tec tonic set ting
Azizollah TAHERI1, Mahdi JAFARZADEH1, *, John S. ARMSTRONG-ALTRIN2 and Seyed Reza MIRBAGHERI1
1 Shahrood Uni ver sity of Tech nol ogy, De part ment of Earth Sci ences, Shahrood, Iran
2 Universidad Nacional Autónoma de México, Unidad de Procesos Oceánicos y Costeros, Instituto de Ciencias del Mar y Limnología, Ciudad Universitaria, Ciudad de México 04510, México
Taheri, A., Jafarzadeh, M., Armstrong-Altrin, J.S., Mirbagheri, S.R., 2018. Geo chem is try of siliciclastic rocks from the Shemshak Group (Up per Tri as sic–Mid dle Ju ras sic), north east ern Alborz, north ern Iran: im pli ca tions for palaeoweathering, prov e nance, and tec tonic set ting. Geo log i cal Quar terly, 62 (3): 522–535, doi: 10.7306/gq.1433
Com bined petrographic and geo chem i cal data of the siliciclastic sed i men tary rocks from the Shemshak Group in the north - east ern Alborz Moun tains, north of Iran are de scribed, to gether with their im pli ca tions for palaeoweathering, their prov e - nance, and tec tonic set ting. Based on field ob ser va tions and modal com po si tion, the sand stones are clas si fied as litharenites. The chem i cal in dex of al ter ation (CIA) in di cated that the source ter rains un der went a mod er ate in ten sity of chem i cal weath er ing. The in dex of chem i cal vari a tion (ICV) val ues in di cated that the Shemshak Group rocks were im ma ture and re lated to a source area with an ac tive tec tonic re gime. Ma jor, trace and rare earth el e ment (REE) data sug gested the dom i na tion of mixed sed i men tary (re cy cled) and ig ne ous rocks in the source area of the Shemshak Group. Petrographic and geo chem i cal char ac ter is tics of Shemshak Group rocks sug gest an ac tive con ti nen tal mar gin (ACM), which cor re sponds to the col li sion of the Iran plate with the Turan plate.
Key words: Shemshak Group, geo chem is try, prov e nance, palaeoweathering, tec tonic set ting.
INTRODUCTION
The geo chem is try of siliciclastic rocks is strongly in flu - enced by var i ous fac tors in clud ing source rock com po si tion, tec tonic set ting, weath er ing, trans por ta tion mech a nism as well as depositional and diagenetic pro cesses (Suttner, 1974;
Roser and Korsch, 1986). Many stud ies have doc u mented that the geo chem i cal com po si tion of sand stones and shales pro vides sig nif i cant clues to as sess prov e nance, tec tonic set - ting and palaeoclimate of the source area (e.g., McLennan et al., 1993; Cullers, 2000; Armstrong-Altrin, 2009; Nagarajan et al., 2015, 2017; Armstrong-Altrin et al., 2017, 2018). The siliciclastic Shemshak Group (Up per Tri as sic–Mid dle Ju ras - sic) is one of the most ex ten sive lithostratigraphic units of the Iran Plate, wide spread across the north ern Alborz and East Cen tral Iran of the Ira nian plate (Tabas and North of Kerman;
Fürsich et al., 2009). Nu mer ous stud ies have doc u mented
lithostratigraphic, sedimentological and palaeontological as - pects of the Shem shak Group (e.g., Seyed-Emami et al., 2006; Fürsich et al., 2009; Shekarifard et al., 2009; Zanchi et al., 2009; Moosavirad et al., 2011, 2012; Abbassi and Madanipour, 2014; Salehi et al., 2017). These stud ies have shown that the Shemshak Group is com posed largely of siliciclastic rock, lo cally reach ing up to 4000 m in thick ness.
There are wide spread coal beds in this siliciclastic suc ces sion and its sed i men tary en vi ron ments range from prox i mal al lu vial fans to deep ma rine (Fürsich et al., 2005, 2009; Zanchi et al., 2009). From a palaeotectonic point of view, there are two un - con formi ties at the top and bot tom of Shem shak Group: baux - ite-laterite de pos its char ac ter ize the con tact with the un der ly - ing plat form car bon ates of the Elikah For ma tion; the con tact with the over ly ing plat form rep re sents a sharp change from siliciclastic rocks to Mid dle-Up per Ju ras sic car bon ate plat - form-ba sin rocks (Fürsich et al., 2009, 2017). Nev er the less, the prov e nance and source area con di tions of the Shemshak Group siliciclastic rocks in the Alborz Moun tain, in terms of pe - trog ra phy and geo chem is try of the sand stones and shales, have not pre vi ously been stud ied in de tail. This study ex am - ines the pe trog ra phy and geo chem is try of siliciclastic rocks of the Shemshak Group in the north east ern Alborz Moun tain and in ves ti gates the source rock char ac ter is tics, palaeo weathe - ring and tec tonic set ting of the source ter rain.
* Corresponding author, e-mail: m_jafarzadeh@shahroodut.ac.ir Received: February 28, 2018; accepted: May 23, 2018; first published online: October 24, 2018
GEOLOGICAL SETTING AND STRATIGRAPHY OF THE STUDY AREA
Alborz Moun tain in north ern Iran is a part of north ern Hi ma - la yan-Al pine range, lo cated around the south ern parts of the Cas pian Ba sin (Rad, 1986). The col li sion be tween the Iran and Turan plates (part of Eur asia) in the Late Tri as sic was mainly re spon si ble for the pres ent frame work of Alborz with an over all
>12 km thick ex posed Pre cam brian to Re cent sed i men tary suc - ces sion (Stocklin, 1968; Rad, 1986; Zanchi et al., 2009). The up per most Pre cam brian-Mid dle Tri as sic part of these sed i men - tary suc ces sions was de pos ited along a pas sive con ti nen tal mar gin set ting. Dur ing the Mid dle-Late Tri as sic, Eo-Cim mer ian oro gen esis de formed the suc ces sions and the terrigeneous Shemshak Group (Up per Tri as sic–Mid dle Ju ras sic) over lies this suc ces sion with an gu lar un con formity. The siliciclastic Shem shak Group is wide spread across large ar eas of the Iran plate es pe cially in north ern Alborz and East Cen tral Iran (Fürsich et al., 2009; Wilmsen et al., 2009; Fig. 1). Dur ing the end of the Mid dle Tri as sic, the Iran plate, which was orig i nally part of the north ern Gond wana, col lided with the Turan plate of Eur asia (e.g., Saidi et al., 1997; Stampfli and Borel, 2002). Af ter this Early Cim mer ian orog eny, the Palaeotethys Ocean be - tween the Gond wana and Eur asia was closed. This event re - sulted in the de po si tion of shal low-wa ter car bon ates (Mid dle
Tri as sic) of the Elika For ma tion in Alborz be ing re placed by siliciclastic sedimention of the Shemshak Group.
Assereto (1966) de fined the Shemshak For ma tion, the type area be ing near the Shemshak area, north of Teh ran sit u ated in the up per Ruteh Val ley. Be cause of its im mense thick ness in dif fer ent ar eas of Alborz and Cen tral Iran, its rank was changed to a Group (e.g., Corsin and Stampfli, 1977; Seyed-Emami, 2003; Fürsich et al., 2009). Corsin and Stampfli (1977) di vided the Shemshak Group of north east ern Alborz into three mem - bers: lower (Up per Tri as sic), mid dle (Lower Ju ras sic) and up per mem bers (Mid dle Ju ras sic) (Fig. 2). The Shemshak Group de - pos its are com monly re ferred to as ter res trial de pos its (flu vial and deltaic; Rad, 1986), how ever, based on the fos sil as sem - blages a few au thors as signed ma rine in ter vals that are re - ported from sev eral lo cal i ties (e.g., Fürsich et al., 2009). The Shemshak Group, in the east ern and north east ern Alborz Ba - sin, disconformably rests on the Elika For ma tion. A ferricrete palaeosol zone as so ci ated with this disconformity in di cates subaerial ex po sure of the Tri as sic rocks (Rad, 1986). The Shemshak Group is con form ably over lain by the Farsian For - ma tion, the con tact be ing marked by a dis tinct lithological change (Najafi-Hajipour, 2009; Fig. 2). The Farsian sec tion in the Ghoznavi–Farsian area, which is lo cated along the Shahrood–Azadshahr road, has been mea sured in this study (Fig. 1). In the study area, the thick ness of the Shemshak Group is ~1250 m (Fig. 3) and the base of the sec tion stud ied (lower Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz... 523
Fig. 1. Lo ca tion map show ing the dis tri bu tion of the Shemshak Group de pos its in the east ern Alborz Range of north ern Iran
mem ber) is rep re sented by sand stones with in ter ca la tion of shales and siltstones (Fig. 4A). This part is over lain by sand - stones and shales of the mid dle mem ber, ar ranged in thick en - ing up wards cy cles (Fig. 4B). The sand stones are fine to me - dium-grained, some parts show ing ero sional bases lo cally with clay chips. Sed i men tary struc tures in clude pla nar cross-strat i fi - ca tion and asym met ri cal rip ple marks (Fig. 4C). Plant re mains and root ho ri zons are prev a lent through out the suc ces sion. The strata are of ten ol ive-grey in col our. The up per mem ber of the Shemshak Group is char ac ter ized by mo not o nous, dark grey–ol ive -grey shale (Fig. 4D). The coal de pos its of the Shem - shak Group were de rived from this up per mem ber.
METHODS
Fifty sand stone and shale sam ples were col lected from the Farsian sec tion. Among them, 16 fresh sand stone sam ples were se lected for de tailed petrographic anal y sis and the Gazzi - -Dickinson method was used for the point-count ing anal y sis (Ingersoll et al., 1984). The strat egy for sam ple se lec tion was based on pe trog ra phy to choose the least al tered, fine- to me - dium grained sand-size sam ples, and to rep re sent the en tire suc ces sion of the Shemshak For ma tion. Nine teen sand stone and 6 shale sam ples were se lected to de ter mine their chem i cal com po si tion. Geo chem i cal anal y sis of crushed and pow dered sam ples was done at Acme An a lyt i cal Lab o ra to ries, Can ada.
Anal y sis pack age LF200 was cho sen due to its ex ten sive list of el e ments ana lysed. Ma jor ox ides and mi nor el e ments were ana lysed fol low ing a lith ium metaborate/tetraborate fu sion ICP-ES and REE were ana lysed by ICP mass spec trom e try.
Weight dif fer ence af ter ig ni tion at 1000°C was used to de ter - mine loss on ig ni tion (LOI). Each anal y sis was done in du pli cate with a reproducibility found to be <2%. Anal y ses of stan dard ma te ri als in di cate that the re sults are gen er ally ac cu rate to within ±10%.
RESULTS
PETROGRAPHY
The pe trog ra phy of the sand stones of the Shemshak Group at the Farsian sec tion was ana lysed to de fine the prov e nance and tec tonic set ting. De tri tal modal com po si tion of the sand - stones, re cal cu lated to per cent age, is re ported in Ap pen dix 1*.
The ma jor con stit u ents char ac ter ized in these sand stones are quartz (monocrystalline and polycrystalline), feld spars and rock frag ments. Among quartz grains, monocrystalline quartz (Qm) is higher than polycrystalline quartz. Monocrystalline quartz grains show straight to slightly undulose ex tinc tion (Fig. 5A, B).
The sand stones con tain mi nor amounts of feld spar grains in - clud ing K-feld spar and plagioclase, with plagioclase dom i nat ing over K-feld spars (Fig. 5B). The rock frag ments are dom i nantly sed i men tary (chert, shale and siltstone frag ments, re spec tively) and meta mor phic frag ments (phyllite and schist frag ments, re - spec tively; Fig. 5C, D).
GEOCHEMISTRY
Ma jor el e ment con cen tra tions. The ma jor el e ment con - cen tra tions of the Shemshak Group sand stones are re ported in Ap pen dix 2. The sand stones of the Farsian sec tion have mod - er ate SiO2 con tents rang ing from 45.4 to 82.5 wt.% (mean 67%;
Ap pen dix 2). The SiO2 con tents of shales from the Farsian sec - tion vary from 58.1 to 71.5 wt.% (mean 62.3%). Al2O3 con tent ranges from 4.9 to 10.9% (mean 7.3%) in the sand stones. In con trast, shale sam ples have higher Al2O3 con tents rang ing from 13.7 to 19.2% (mean 17.53%). Both sand stone and shale have sim i lar K2O con tents (mean 0.96% and re spec tively).
Chem i cal com po si tion of de tri tal sed i ments de pends on grain - Fig. 2. Lithostratigraphic col umn
of the Shemshak Group in north east ern Alborz (Corsin and Stampfli, 1977); not to scale
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1433
Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz... 525
Fig. 3. Lithostratigraphic col umn of the Shemshak Group in the Farsian out crop
Fig. 4. Field pho to graphs of Shemshak Group strata
A – lower bound ary of the Shemshak Group with the Elika For ma tion near the vil lage of Ghoznavi; B – thick en ing and coars en ing-up wards cy cles in sand stones and shales of the mid dle mem ber; C – pla nar cross-bed ding in sand stones of the mid dle mem ber of Shemshak Group; D – up per bound ary of the Shemshak Group with the Farsian For ma tion in south of the vil lage of Farsian
Fig. 5. Se lected thin-sec tion pho to mi cro graphs of de tri tal grains of sand stones from the Shemshak Group A – pho to mi cro graph show ing mainly monocrystalline quartz grains with slightly undulose ex tinc tion (red ar rows).
B – quartz grain with straight ex tinc tion (red ar row) and plagioclase (yel low ar row) grain with twinning; C – sed i - men tary chert (red ar rows) and meta mor phic rock frag ments (yel low ar row); D – shale frag ments (yel low ar rows) in sand stone with a car bon ate ce ment
Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz... 527
-size, with high Al2O3 con tent in fine-grained sands and high SiO2 in coarse-grained sands, which also is re flected in the min - er al og i cal com po si tion (Vi tal and Stattegger, 2000; Armstrong - -Altrin et al., 2013, 2015). Fe2O3 con tent in the sand stones ranges from 2.3 to 7.2% (mean 3.76%) in the Farsian sec tion.
By con trast, shale sam ples have slightly higher Fe2O3 con tents and vary from 3.1 to 8.7% (mean 6.0%). By com par i son with av - er age up per con ti nen tal crust (UCC; Tay lor and McLennan, 1985) the MnO and CaO con tents in the Farsian sand stones are slightly en riched, but TiO2, P2O5, MnO, Al2O3, Fe2O3 as well as Na2O, K2O are de pleted (Fig. 6). The shale sam ples show slightly de pleted Na2O and K2O con tents, whereas MnO is slightly en riched by com par i son with UCC. Based on the Herron (1988) di a gram, the sam ples ana lysed are clas si fied as shales, wackes and litharenites (Fig. 7), con sis tent with the petro -
graphic data (Fig. 5). In Fig ure 8, a lin ear trend from sand to mud is ob served, which is due to the grain -size vari a tions in the de tri tal sed i ments. How ever, Na2O, MnO and CaO show dif fer - ent pat terns. In the sand stones, Na2O is pos i tively cor re lated with Al2O3, while this cor re la tion is neg a tive for the shale sam - ples. In the sand stones and shales, MnO and CaO are neg a - tively cor re lated against Al2O3. The cor re la tion is not sig nif i cant be tween SiO2 and Al2O3 in the sand stone and shale in di cat ing that much of the SiO2 is pres ent as quartz grains (Armstrong - -Altrin et al., 2013, 2014). This ob ser va tion is con sis tent with the high con cen tra tion of SiO2 in the sand stones and shales (>63%; mean value 82 and 67%, re spec tively; Ap pen dix 2).
Trace and rare earth el e ment con cen tra tions. The trace and REE con cen tra tions of the Shemshak Group are listed in Ap pen dix 3 and 4, re spec tively. The UCC-nor mal ized trace el - e ment con cen tra tions are shown in Fig ure 9. In com par i son with av er age UCC, Sr, Rb and Ba in the Farsian sec tion is strongly de pleted. Other trace el e ments such as Nb, U, Th, Hf and Zr are slightly de pleted rel a tive to UCC. How ever, sam - ples SF80, SF45 and SF83 are en riched in Hf and Zr. The trace el e ments with val ues higher than 100 ppm are Zr (78–259 ppm in the sand stones and 174–306 ppm in the shales) and Sr (58–222 ppm in the sand stones and 56–123 ppm in the shales), while the con cen tra tion of other trace el e ments is <100 ppm. The con tent of to tal REE (?REE) var ies from 73 to 147 ppm in the sand stones (av er age = 93 ppm) and from 134 to 171 in the shales (av er age = 148.14 ppm). The con cen tra tion of light REE in the sand stones and shales (~8–15.5 and ~15.7–22 ppm, re spec tively) is higher than that of the heavy REEs (~8–15.5 and ~16–22 ppm, re - spec tively). The chondrite-nor mal ized REE pat terns of the Shemshak Group sand stones and shales are shown in Fig ure 10. The ?REE con tent var ies widely and all sam ples show LREE-en riched flat HREE pat terns. The Eu anom aly is neg a - tive and the mean value of Eu/Eu* is 0.71 in sand stones and 0.67 in shales of the Farsian sec tion (Fig. 10).
Fig. 6. Up per con ti nen tal crust nor mal ized ma jor-el e ment di a gram for the Shemshak Group sand stone and shale sam ples (Tay lor and McLennan, 1985)
Fig. 7. Geo chem i cal clas si fi ca tion of sand stone and shales of the Shemshak Group (Herron, 1988)
INTERPRETATION AND DISCUSSION
PALAEOWEATHERING AND SEDIMENT RECYCLING
Chem i cal weath er ing may mod ify the geo chem is try and min eral com po si tion of siliciclastic sed i ments dur ing trans port and de po si tion (Nesbitt and Young, 1982; McLennan et al., 1993; Ramos-Vázquez et al., 2017). The chem i cal in dex of al - ter ation (CIA; Nesbitt and Young, 1982) is a com mon method to eval u ate the re la tion ship be tween al kali and al ka line earth el e - ments, and the weath er ing his tory of siliciclastic rocks. Higher CIA val ues (80–100) re flect the re moval of la bile cat ions (Na+, K+ , Ca2+) rel a tive to re sid ual cat ions (Al+3) and are an in di ca tor of in tense weath er ing in the source ter rains, whereas lower val - ues (<60) rep re sent lesser weath er ing (Fedo et al., 1995). The CIA can be cal cu lated by an equa tion: CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)]×100. All the ox ides are in mo lec u lar pro - por tion and CaO* rep re sents Ca in sil i cate min er als. The CIA val ues in the Shemshak Group range from ~52 to 71 and ~58 to 81 in the sand stones and shales, re spec tively. These val ues in - di cate a mod er ate de gree of chem i cal weath er ing in the source ar eas. Suttner and Dutta (1986) pro posed a bivariate di a gram of SiO2 ver sus Al2O3 + K2O + Na2O to de ter mine the cli ma tic con di tion of the source area. This plot re vealed a high prob a bil - ity of a hu mid cli ma tic con di tion in the source area (Fig. 11).
Leach ing of Sr com pared to Rb is also an in di ca tor of in - tense chem i cal weath er ing and diagenesis (Nesbitt and Young, 1982). High Rb/Sr ra tios (>1) are in di ca tors of sed i ment re cy - cling and strong weath er ing (McLennan et al., 1993). The Rb/Sr
ra tios in Shemshak Group sand stones (~0.1–0.87; av er age = 0.37) and shales (~0.35–0.99; av er age = 0.66) are lower than the av er age PAAS (0.80; Tay lor and McLennan, 1985) sug - gest ing a sim ple re cy cling his tory. Re cy cling of sed i ments with strong and re pet i tive weath er ing un der ox i diz ing con di tions may in crease the frac tion ation of Th and U, and the ox i da tion of U+4 to U+6 leads to a loss of U, which con se quently causes an in - crease in the Th/U ra tio. The Th/U ra tios of the Shemshak Group sand stones (~1.9–5.5; mean = 3.6) and shales (~3.4–4.3; mean = 3.8) are sim i lar to the av er age value of UCC (3.8; McLennan and Tay lor, 1991), in di cat ing a mod er ate de - gree of chem i cal weath er ing in the source area and a sim ple re - cy cling his tory. Based on the geo chem i cal in di ca tors and plant re mains such as Dictyophyllum, Otozomites, Pterophyllum and Zamites, Najafi-Hajipour (2009) pro posed a sub trop i cal cli mate for the Shemshak Group.
The In dex of Compositional Vari abil ity (ICV; Cox et al., 1995) is com monly used by re search ers to eval u ate sed i ment ma tu rity (Armstrong-Altrin, 2009; Tapia-Fernandez et al., 2017;
Hernández-Hinojosa et al., 2018). The ICV can be cal cu lated based on the equa tion ICV= (CaO + K2O + Na2O + Fe2O3
t + MgO + MnO + TiO2)/Al2O3. The ICV val ues of the Shemshak Group range from 0.99 to 5.58 (mean = 2.40) and 0.79 to 1.56 (mean = 1.16) in the sand stones and shales, re spec tively higher than that of PAAS (~0.89) in di cat ing that these sam ples rep re sent first cy cle im ma ture sed i ments and are re lated to ar - eas with an ac tive tec tonic re gime (Cox et al., 1995). The mod - er ate weath er ing in the source area also re veals that the sed i - ments were prob a bly sup plied from a rap idly up lifted re gion.
Fig. 8. Bivariate di a grams for sand stone and shale sam ples of the Shemshak Group
Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz... 529
Fig. 9. Multi-el e ment spi der di a grams for sand stone and shale sam ples of the Shemshak Group, nor mal ized against av er age up per con ti nen tal crust (Tay lor and McLennan, 1985)
Fig. 10. Chondrite-nor mal ised REE pat terns for sand stone and shale sam ples of the Shemshak Group (Tay lor and McLennan, 1985)
PROVENANCE IMPLICATIONS
The modal com po si tion of the Shemshak Group sand stones in di cates the quartzolithic petrofacies, which sug gests that these sand stones are com posed of re cy cled sed i men tary ma te ri als.
The geo chem is try of clastic rocks is also widely used to in fer the com po si tion of the source rocks and the tec tonic set ting of the source area (Gabo et al., 2009; Verma and Armstrong -Altrin, 2013, 2016; Armstrong-Altrin et al., 2015; Basu, 2016; Verma et al., 2016). The ma jor el e ment based discriminant func tion di a - gram of Roser and Korsch (1988) was used to dis crim i nate the prov e nance of sand stone and shales of the Shemshak Group,
and this sug gests that the sed i ments were de rived from quartzose sed i men tary (fel sic) and in ter me di ate ig ne ous prov e - nances (andesitic) (Fig. 12A). In ad di tion, a bivariate di a gram of TiO2 ver sus Zr shows that the source rocks of the Shemshak sed i ments were more fel sic than in ter me di ate types (Hayashi et al., 1997; Fig. 12B). Trace el e ments in siliciclastic de pos its can be used to in ves ti gate the source rocks, be cause in com pat i ble el e ments (e.g., Th, U, Zr, Nb, Y) are en riched in fel sic rocks and com pat i ble el e ments (e.g., Ni, Co, Cr, V) are abun dant in mafic and ultra mafic rocks (Cullers, 2000; Armstrong-Altrin et al., 2004). How ever, a re cent study by Saha et al. (2010) shows that al though the trace el e ment con cen tra tions in siliciclastic sed i - ments are suit able to dis crim i nate fel sic and mafic rocks, a mafic source does not nec es sar ily im ply arc-der i va tion. Sed i men tary pro cesses such as weath er ing and diagenesis do not sig nif i - cantly af fect REE, Th and Sc con tents (Mongelli et al., 2006) though the rel a tive mo bil ity of LREE and HREE may vary with re - dox con di tions (Basu et al., 2016). A few trace el e ments such as Zr, Hf, and Ti are strongly in flu enced by frac tion ation of heavy min er als and hy drau lic sort ing (McLennan and Tay lor, 1991). A Zr/Sc-Th/Sc di a gram pro posed by McLennan et al. (1993) is used to in fer source rock com po si tion, and this in di cates fel sic and in ter me di ate source rocks for the Shemshak Group (Fig.
12C). A bivariate di a gram of La/Th–Hf is also used to dis crim i - nate source area types (Fig. 12D; Floyd and Leveridge, 1987).
On this di a gram (Fig. 12D), most shale sam ples plot in the mixed fel sic and mafic source fields. Chondrite-nor mal ized REE pat - terns were also used to re veal the source area com po si tion of the Shemshak sam ples (Fig. 10). Al most all sam ples have clear neg a tive Eu anom a lies, re flect ing their der i va tion from a fel sic prov e nance (e.g., Yang et al., 2012; Wang et al., 2018).
Fig. 11. SiO2 (wt.%) ver sus (Al2O3+K2O+Na2O) bivariate di a gram for sand stones of the Shemshak Group
(af ter Suttner and Dutta, 1986)
Fig. 12. Dis crim i na tion di a grams for sand stone and shale sam ples from the Shemshak Group A – prov e nance dis crim i na tion di a gram (Roser and Korsch, 1988); B – TiO2 ver sus Zr bivariate di a gram (Hayashi et al., 1997); C – Zr/Sc ver sus Th/Sc bivariate di a gram (McLennan et al., 1993);
D – La/Th ver sus Hf bivariate di a gram (Floyd and Leveridge, 1987)
TECTONIC SETTING
One of the main fac tors which con trols the type of de tri tal grains and geo chem i cal com po si tion of clastic sed i men tary rocks is the tec tonic set ting of the source area (Dickinson and Suczek, 1979; Roser and Korsch, 1986; Armstrong-Altrin, 2015), be cause rocks from var i ous tec tonic set tings have dif fer - ent min er al og i cal and chem i cal com po si tions (Bhatia and Crook, 1986; Verma and Armstrong-Altrin, 2013, 2016). Be - sides the close re la tion ship be tween sand com po si tion and tec - tonic set ting, quan ti ta tive de tri tal modes of sand stones are also used to iden tify the tec tonic set ting of an cient bas ins of the source ter ranes (e.g., Dickinson and Suczek, 1979; Basu, 2016). Tri an gu lar plots of Qm-F-Lt and Qt-F-L (Dickinson et al., 1983) sug gest that the Shemshak sand stones were de rived from lithic to tran si tional re cy cled sources (Fig. 13). Sand stones Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz... 531
Fig. 13. Prov e nance dis crim i na tion di a grams (af ter Dickinson et al., 1983)
A – Qm-F-Lt; B – Qt-F-L
Fig. 14. Tec tonic dis crim i na tion di a grams for the sand stone and shale sam ples from the Shemshak Group
A – dis crim i na tion di a gram based on ma jor el e ment com po si tion (af ter Roser and Korsch, 1986; ARC – oce anic is land arc; ACM – ac tive con ti nen - tal mar gin; PM – pas sive mar gin). B – ma jor el e ment based tec tonic dis - crim i na tion plots for the low-sil ica sam ples (af ter Verma and Arm strong -Altrin, 2013): DF1(Arc-Rift-Col)m1 = [–0.263 × In(TiO2/SiO2)adj] + [0.604 × In(Al2O3/SiO2)adj] + [–1.725 × In(Fe2O3t/SiO2)adj] + [0.660 × In(MnO/SiO2)adj] + [2.191 × In(MgO/SiO2)adj] + [0.144 × In(CaO/SiO2)adj] + [–1.304 × In(Na2O/SiO2)adj] + [0.054 × In(K2O/SiO2)adj] + [–0.330 × In(P2O5/SiO2)adj] + 1.588. DF2(Arc-Rit-Col)m1 = [–1.196 × In(TiO2/SiO2)adj] + [1.604 × In(Al2O3/SiO2)adj] + [0.303 × In(Fe2O3t/SiO2)adj] + [0.436 × In(MnO/SiO2)adj] + [0.838 × In(MgO/SiO2)adj] + [–0.407 × In(CaO/SiO2)adj] + {1.021 × In(Na2O/SiO2)adj] + [–1.706 × In(K2O/SiO2)adj] + [–0.126 × In(P2O5/SiO2)adj] – 1.068. C – ma jor el e ment tec tonic dis crim i na tion plot for the high-sil ica sam ples (Verma and Armstrong-Altrin, 2013):
DF1(Arc-Rift-Col)m2 = [0.608 × In(TiO2/SiO2)adj] + [–1.854 × In(Al2O3/SiO2)adj] + [0.299 × In(Fe2O3t/SiO2)adj] + [–0.550 × In(MnO/SiO2)adj] + [0.120 × In(MgO/SiO2)adj] + [0.194 × In(CaO/SiO2)adj ] + [–1.510 × In(Na2O/SiO2)adj] + [1.941 × In(K2O/SiO2)adj] + [0.003 × In(P2O5/SiO2)adj] – 0.294.
DF2(Arc-Rit-Col)m2 = [–0.554 × In(TiO2/SiO2)adj] + [–0.995 × In (Al2O3/SiO2)adj] + [1.765 × In(Fe2O3t/SiO2)adj] + [–1.391 × In(MnO/SiO2)adj] + [–1.034 × In(MgO/SiO2)adj] + [0.225 × In(CaO/SiO2)adj] + [0.713 × In(Na2O/SiO2)adj] + [0.330 × In(K2O/SiO2)adj] + [0.637 × In(P2O5/SiO2)adj]– 3.631
com posed of re cy cled sed i men tary ma te ri als have quartz and abun dant lithic frag ments of sed i men tary-metasedimentary com po si tion (Dickinson and Suczek, 1979). This quartzolithic petrofacies iden ti fied in sand stones of the Shemshak Group is also re ported in the north of Alborz Range (Zanchi et al., 2009) and Cen tral Iran (Salehi et al., 2017).
The dis crim i na tion di a grams of Bhatia (1983) and Roser and Korsch (1986) have been used com monly to de ci pher the tec tonic set ting of an cient sed i men tary bas ins (Nowrouzi et al., 2014; Jafarzadeh et al., 2014; Ogala et al., 2015; Vosoughi Moradi et al., 2016). In this study, we used three di a grams based on ma jor, trace and rare earth el e ments for de ci pher ing the tec tonic set ting of the source area. As shown on the widely used K2O/Na2O log a rith mic ra tio against SiO2 di a gram of Roser and Korsch (1986), most of the sam ples plot in the ac tive con ti - nen tal mar gin (ACM) field and six sam ples in clud ing a shale sam ple plot in the con ti nen tal is land arc field (CIA). Also three sam ples plot in the pas sive con ti nen tal mar gin field but near to the bound ary with the ac tive mar gin field (Fig. 14A). Arm - strong-Altrin and Verma (2005) eval u ated the tec tonic dis crim i - na tion di a grams of Roser and Korsch (1986) us ing Mio cene to Re cent de pos its and iden ti fied a low per cent age suc cess rate, which var ies from 31.5 to 52.3%. Re cently, based on the log-ra - tio trans for ma tion ra tio of ma jor ox ides, Verma and Armstrong - -Altrin (2013) pro posed multi-di men sional tec tonic dis crim i na -
tion di a grams for high sil ica (SiO2= 63–95%) and low sil ica (SiO2 = 35–63%) sam ples (Fig. 14B, C). On these dis crim i na - tion di a grams (Fig. 14B, C), al though few sand stone sam ples plot in the arc field most of them plot in the col li sion field. These re sults sug gest a collisional set ting for the Shemshak Group.
As men tioned pre vi ously, we com pared the geo chem i cal re - sults with petrographic data. We did not find any ev i dence for an is land arc set ting us ing petrographic data and so con sider that a col li sion set ting may be more rea son able for the Shemshak Group. Con sid er ing the palaeo ge ogra phy, the Shemshak Group is com monly re garded as the Cim mer ian fore land molasse pro duced by the Early Cim mer ian orog eny (col li sion be tween the Iran and Turan plates) dur ing the early Late Tri as - sic (Rad, 1986; Alavi, 1996; Wilmsen et al., 2009). Fürsich et al.
(2006) showed that the evo lu tion of the depo sitional ba sin of the Shemshak Group is com plex. Wilmsen et al. (2009) stated that ini tial Cim mer ian col li sion started with synorogenic pe riph eral fore land de po si tion in the Late Tri as sic while si mul ta neously the Lower Shemshak sed i ments were de pos ited (Fig. 15A). Chan - ge from the Elikah For ma tion (plat form car bon ates) to the Shemshak Group (con ti nen tal/ma rine sili ciclastic de pos its) oc - curred be tween the Carnian and the No rian (Zanchi et al., 2009). Neotethys subduction be low Iran shifted south in the Norian and rapid up lift of the Cimmerides oc curred af ter slab break-off around the Tri as sic-Ju ras sic boun d ary. This was fol -
Fig. 15. Struc tural evo lu tion of the Alborz Ba sin dur ing the de po si tion of Shemshak Group sed i ments in an ac tive con ti nen tal mar gin (A and B) and back arc set ting (C)
(mod i fied from Wilmsen et al., 2009)
lowed by the de po si tion of Li assic post -orogenic molasse (mid - dle Shemshak) sed i ments (Wilmsen et al., 2009; Fig. 15B).
Dur ing the Toarcian–Aalenian, the time of up per Shemshak Group de po si tion, Neotethys backarc rift ing cre ated a deep ma - rine ba sin and led to the de po si tion of or ganic-rich fine-grained siliciclastic sed i ments dur ing the Aalenian, which filled the young rifted ba sin (Fig. 15C). Co in ci dence of the extensional event with the Eo-Cim mer ian orog eny sug gests that it may have been an ef fect of the pe riph eral bulg ing of the fore land in - duced by the col li sion (Zanchi et al., 2009).
CONCLUSIONS
Modal and min eral com po si tions of Shemshak Group sand - stones in di cate that they are com posed dom i nantly of quartz (monocrystalline and polycrystalline), feld spars and rock frag - ments (sed i men tary and meta mor phic rock frag ments) and pro - vide ev i dence of dis tinc tive quartzolithic petrofacies. The quartz o lithic com po si tion of the sand stones re veals a re cy cled orogenic prov e nance. CIA val ues in di cate that the source area un der went a mod er ate grade of chem i cal weath er ing and the
SiO2 ver sus Al2O3 + K2O + Na2O di a gram in di cates a hu mid cli - mate in the source area. Var i ous prov e nance dis crim i na tion di - a grams based on ma jor el e ments show mixed re cy cled sed i - men tary (fel sic) and in ter me di ate (andesitic) ig ne ous rocks, which dom i nate in the source area. Trace and REE com po si - tions also sug gest mixed fel sic and in ter me di ate prov e nances for the Shemshak Group de tri tal de pos its. On the tec tonic dis - crim i na tion di a grams, a few sam ples plot in the arc field and most of the sand stone and shale sam ples plot in the col li sion field, while the petrographic study did not in di cate an is land arc set ting for the Shemshak sand stones. These re sults sug gest a collisional set ting for the Shemshak Group, which is con sis tent with the palaeogeographic evo lu tion of Alborz and Cen tral Iran dur ing the Mid dle Tri as sic to Ju ras sic in ter val (Wilmsen et al., 2009).
Ac knowl edge ments. We would like to thank the Ed i tor and the re view ers A. Basu, S.K. Verma, and an anon y mous re - viewer for their con struc tive com ments, which sig nif i cantly im - proved our pre sen ta tion. JSA is grate ful to the In sti tute of Ma - rine Sci ences and Lim nol ogy (ICML), UNAM, In ter nal pro ject (no. 616).
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Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic–Middle Jurassic), northeastern Alborz... 535
