Geo chem i cal and tec tonic sig nif i cance of Arbat al kali gab bro-monzonite-syenite in tru sions, Urumieh–Dokhtar Mag matic Arc, Iran
Abdolnaser FAZLNIA1, *
1 Urmia Uni ver sity, De part ment of Ge ol ogy, 57153-165 Urmia, Is lamic Re pub lic of Iran
Fazlnia, A., 2019. Geo chem i cal and tec tonic sig nif i cance of Arbat al kali gab bro-monzonite-syenite in tru sions, Urumieh–Dokhtar Mag matic Arc, Iran. Geo log i cal Quar terly, 63 (1): 16–29, doi: 10.7306/gq.1449
As so ci ate ed i tor: Tomasz Bajda
The Oligocene Arbat al kali in tru sions of the East ern Miandoab are lo cated in the northwest ern part of Iran and be long to the Urumieh–Dokhtar Mag matic Arc (UDMA). The in tru sions show a ring struc ture with gab bro-monzogabbro-monzodiorite (mafic units) on the edges, with monzonite-monzosyenite-syenite (fel sic units) grad u ally go ing to wards the cen tral parts. The tex tures in dif fer ent rock types are cu mu late, gran u lar and lam i nated. The high val ues of (La/Sm)n and (La/Yb)n, con tents of K, Rb and Cs (pos i tive anom a lies nor mal ized on the ba sis of the prim i tive man tle), low con cen tra tions of Hf, Nb, Zr and Ta (neg a tive anom a lies), and the changes in Th/Nb, Th/Ta, La/Nb and Ce/Pb ra tios along with the geo chem i cal and tec tonic set ting ev i dence ex hibit a subduction-mod i fied man tle or i gin for the for ma tion of these rocks. Ac cord ingly, the in tru sions were formed be tween the Cen tral Iran and the Ara bian plates as a re sult of the par tial melt ing of a man tle wedge at a syn-col - li sion or post-col li sion arc-re lated en vi ron ment. Our data sug gested that, af ter the end of the oblique Neotethys subduction and dur ing/af ter the con ti nen tal col li sion, the break-off or roll back of the Neotethys slab be neath west ern Iran, in the Oligocene, might have oc curred. Such a pro cess led to the change in the geo ther mal gra di ent of the man tle wedge be cause of the subduction flu ids, transtension, pres sure re duc tion along the SE-trending lat eral depth strike-slip fault zones in the up - per part of the man tle wedge, de com pres sion par tial melt ing at the man tle, and the re sult ing for ma tion of a mafic po tas - sium-rich melt. The mafic magma was in jected into crustal magma cham bers; prob a bly, the frac tional crys tal li za tion and par tial con tam i na tion oc curred with crustal com po nents, form ing the in ter me di ate and fel sic rocks in the in tru sions. Geo - chem i cal ev i dence re lated to the vari a tions in the ra tios of Th/Yb, Ta/Yb, Rb/Y, and Nb/Y and Harker vari a tion di a grams along with the spi der di a grams con firmed frac tional crys tal li za tion and par tial FC (frac tional crys tal li za tion) and AFC (as sim i - la tion and frac tional crys tal li za tion) in the in tru sions.
Key words: gab bro-monzonite-syenite com plex, geo chem is try, collisional zone, Neotethys subduction.
INTRODUCTION
Many syenite-monzonite rocks are as so ci ated with gab bro, diorite, monzogabbro, monzodiorite and appinite rocks, and are metaluminous to peralkaline in terms of their com po si tion (Murphy, 2013; Haldar and Tišljar, 2014). In many cases, they are also com monly as so ci ated with gran ites (Gill, 2010).
Syenites of ten oc cur in extensional set tings in as so ci a tion with peralkaline A-type granitoids (anorogenic), monzonites, and al - ka line or tholeiitic diorite-gab bro (e.g., monzogabbro). These en vi ron ments are as so ci ated with the con ti nen tal rifts (e.g., Green, 1992; Upton et al., 1996; Upton et al., 2003), hot spots (e.g., Upton et al., 2003; Bailey et al., 2006; Kogarko et al., 2006), back-arc ten sional bas ins, subduction-re lated set tings (e.g., Beard and Borgia, 1989; Ba con et al., 2007; Fazlnia and
Alizade, 2013; Murphy, 2013; Fazlnia, 2017), and con ti nen tal syn- or post-col li sion zones (Gualda and Vlach, 2007; Gill, 2010; Aghazadeh et al., 2010; Cas tro et al., 2013; Moreno et al., 2014, 2016). Geo graph ical ar eas, in which syenite-monzo - nite-gab bro com plexes oc cur, in clude lopolith in tru sions (Kogarko et al., 2006), con ti nen tal rift in tru sions (Green, 1992;
Upton et al., 1996, 2003), and subduction-re lated as sem blages (Arculus and Wills, 1980; Conrad and Kay, 1984; Beard and Borgia, 1989; Ba con et al., 2007).
In Iran, ma jor plutonic out crops as so ci ated with the Zagros orogenic belt have been formed dur ing Me so zoic and Ter tiary times (Berberian and King, 1981; Alavi, 1994). The Zagros orogenic belt, which is con sid ered part of the Al pine orogenic sys tem, con sists of three par al lel zones with the NW–SE trend.
These zones in clude (1) the Zagros Fold-Thrust Belt, (2) the Sanandaj–Sirjan Zone, and (3) the Urumieh–Dokhtar Mag matic Arc (UDMA; Alavi, 1994; Fig. 1). All three zones ex tend to the north-west of Iran.
Pet ro log i cal stud ies in north west ern Iran have in di cated that syenite-monzonite-gab bro as sem blages might have been cre - ated in the su pra-subduction zone (Aghazadeh et al., 2010) and/or post-col li sion set tings (gabbroic-syenitic-monzonitic in -
* E-mail: a.fazlnia@urmia.ac.ir and nfazlnia@yahoo.com
Re ceived: July 8, 2018; ac cepted: October 23, 2018; first pub lished on line: January 18, 2019
Geo chem i cal and tec tonic sig nif i cance of Arbat al kali gab bro-monzonite-syenite in tru sions, Urumieh–Dokhtar Mag matic Arc, Iran 17
Fig. 1. Sim pli fied geo log i cal map of east ern Miandoab (mod i fied af ter Rezaei et al., 2009);
the bot tom right of the fig ure is a sim pli fied map of Iran and lo ca tions of the Urumieh–Dokhtar Mag matic Arc and Sanandaj–Sirjan Zone (mod i fied af ter Stöcklin, 1968)
tru sions from the Alborz mag matic belt, NW Iran; Cas tro et al., 2013). There fore, the al kali rocks, such as syenite- monzonite-gab bro as sem blages in the north-west of Iran, were formed dur ing the fi nal stages of the Neotethys subduction be - neath Cen tral Iran. In ad di tion to the al kali in tru sive rocks, al kali vol ca nic rocks, such as tra chytes, trachybasalts and bas alts of the same ages, have been found through out north west ern Iran (Kheirkhah et al., 2009; Azizi et al., 2014). These rocks are also as so ci ated with the last stages of the Neotethys subduction be - neath Cen tral Iran, or with syn- to post-col li sion en vi ron ments be tween the Ara bian and Cen tral Iran plates.
The Arbat syenite-monzonite-gab bro com plex, in the East - ern Miandoab from the north ern UDMA of Iran, com prises part of the rocks ex posed in the Zagros orogenic belt. This study in - ves ti gated the geo chem i cal and tec tonic set ting of the rocks.
The study of these rocks could help to re con struct the last stages of the Neotethys subduction in the northwest ern re gion of the UDMA, Iran. The study area could be of great in ter est for the Alpino-Hi ma la yan orogenic belt. The study of these rocks could also help to un der stand the old tec tonic en vi ron ment in the cen tral parts of the belt.
GEOLOGICAL SETTING AND FIELD OBSERVATIONS
The UDMA is a subduction-re lated An dean-type mag matic arc com posed of tholeiitic, calc-al ka line, and K-rich al ka line in - tru sive and ex tru sive rocks, along with an ac tive mar gin of the Ira nian Plate, lo cated be tween the Sanandaj–Sirjan and Cen - tral Iran zones (Alavi, 1994; Shahabpour, 2007). The men - tioned li thol ogy is a re sult of the Neotethys subduction be neath Cen tral Iran (Berberian and King, 1981). The old est rocks in the UDMA are calc-al ka line in tru sive rocks that cut across the Up - per Ju ras sic for ma tions and are over lain un con form ably by the Lower Cre ta ceous fossiliferous lime stone. The Eocene Uru - mieh– Dokhtar Mag matic Arc con tains abun dant mafic to in ter - me di ate vol ca nic and in tru sive rocks, with calc-al ka line to shoshonitic chem is try (Berberian and King, 1981; Alavi, 1994;
Yeganehfar et al., 2013; Jafari et al., 2015; Hassanzadeh and Wernicke, 2016; Babazadeh et al., 2017; Jafari et al., 2018;
Fazlnia, 2018a). The youn gest rocks in the UDMA con sist mostly of lava flows and pyroclastics that be long to the Plio cene to Qua ter nary vol ca nic zones of adakite, al ka line and calc-al ka - line com po si tion (Berberian and Berberian, 1981; McInnes et al., 2003; Jahangiri, 2007; Omrani et al., 2008; Neill et al., 2013, 2015; Rasouli et al., 2016). All these for ma tions are re lated to the subduction of the north ern Neotethys (Nabavi, 1976). In con trast, Kheirkhah et al. (2009), Allen et al. (2013), McQuarrie and van Hinsbergen (2013), and Azizi et al. (2014) have con - cluded that the youn gest rocks in the UDMA were formed at the end of subduction. The Arbat in tru sions are lo cated 25 km east of Miandoab and in the north of UDMA (Fig. 1), ac cord ing to the struc tural di vi sions of Stöcklin (1968).
Eocene–Oligocene syenites and the as so ci ated rocks from UDMA and north-west Cen tral Iran have been stud ied (Ashrafi et al., 2009; Ferdowsi et al., 2015). These rocks are sat u rated or undersaturated in sil ica. In tru sions from the north west ern part of UDMA, NW Iran, are Si-sat u rated and undersaturated, and are al ka line-shoshonitic in terms of their com po si tion (Ashrafi et al., 2009; Tajbakhsh et al., 2012; Ferdowsi et al., 2015). These rocks oc curred in a syn- or post-col li sion arc-re - lated set ting, dur ing the Neotethys clo sure and the fi nal evo lu - tion of the Zagros Orog eny.
Var i ous stud ies (Aghazadeh et al., 2010; Cas tro et al., 2013) of gran ite-syenite-monzonite-gab bro in tru sions from NW Iran (north ern UDMA), Late Eocene–Oligocene in age, and from the west ern Alborz mag matic belt (a part of the Zagros orogen) in di cate that these rocks show sat u rated-un der sat u - rated shoshonitic and adakitic char ac ter is tics. These rocks were de rived by the melt ing of a metasomatized man tle in a post-col li sion arc-re lated set ting that re sulted from the Neotethys clo sure dur ing the fi nal evo lu tion of the Zagros Orog - eny.
Some Late Eocene to Late Mio cene al ka line and shoshonitic out crops in the NW part of UDMA have been ex - panded into lava flows and dykes with the com po si tions of mi - cro-monzonite, mi cro-syenite, lam pro phyre, basanite, and foid-bear ing rocks in NW Iran (Moayyed et al., 2008; Ashrafi et al., 2009; Shafaii Moghadam et al., 2014; Aghazadeh and Badrzadeh, 2015). These re search ers in di cated that the rocks were formed in a post-col li sion arc set ting re lated to a con ti nen - tal col li sion be tween Cen tral Iran and the Ara bian plates.
The Arbat in tru sions from the east ern part of Miandoab (Fig. 1) are mostly monzonitic and monzosyenitic rocks along with mafic parts, such as monzogabbro and monzodiorite, and with fel sic parts, like al kali syenite (Fig. 2). This area is part of UDMA, based on the struc tural-sed i men tary di vi sions of Iran (Stöcklin, 1968). The Arbat in tru sions are lo cated in the south - ern and south west ern parts of the pre vi ously stud ied syenites and the re lated rocks from the north west ern UDMA (Ashrafi et al., 2009; Aghazadeh et al., 2010; Cas tro et al., 2013; Ferdowsi et al., 2015). In ad di tion, these rocks are lo cated 100 km to the south-east of the Islami (Saray) Pen in sula (Shafaii Moghadam et al., 2014). They sug gested that the Late Mio cene Saray high-K mag mas were de rived to a small de gree from par tial melt ing of the subduction-metasomatized (sub con ti nen tal) lithospheric man tle source in a post-collisional set ting.
The study of ig ne ous in tru sions into the Cre ta ceous lime - stone, shale and re gional meta mor phic rocks can be of in ter est.
Around the vil lage of Arbat (Fig. 1), all types of ig ne ous rocks have in truded into the Eocene Fajan and Ziarat for ma tions.
These in tru sions meta mor phosed the for ma tions and were cov - ered by the Oligo-Mio cene for ma tions (Rezaei et al., 2009). The frag ments of Eocene vol ca nic rocks can be seen in the form of meta mor phosed xe no liths (hornfels) within these in tru sions.
There fore, the age of all study rocks could be be tween Early to Late Oligocene. How ever, Rezaei et al. (2009) of fered the Oligocene age for these rocks.
Many stud ied out crops are ob served in the ero sional and mound formed sets (Fig. 2A–C). Some parts of the mafic and fel sic out crops have been ex posed in a rub ble form of dif fer ent sizes (Fig. 2B, C) as a re sult of the on ion-skin weath er ing. All rock types are coarse-grained and some crys tals are more than 3 cm in size (Fig. 2A, bot tom right, 2E). Fresh out crops of the mafic rocks are seen in grey (Fig. 2D). The mafic and fel sic parts have nor mal con tact (Fig. 2A) and the ex is tence of the rounded fine-grained, more mafic cog nate en claves from the early stages of crys tal li za tion in the magma cham ber in side the fel sic parts (2E, bot tom right) show two dif fer ent crys tal line ac - cu mu la tions. These en claves were formed at the ini tial pluses of magma in jec tion; they have been sur rounded by the sub se - quent pluses of magma in jec tion (Didier and Bar bar ian, 1991).
In ad di tion, they may be the re sult of two dis tinct magma types (e.g., one crustal, and one man tle-de rived; see the dis cus sion sec tion). Coarse-grained dikes with a small thick ness from a few centi metres to 10 centi metres of fel sic parts in side mafic parts and of mafic parts in side fel sic part (Fig. 2E) in di cate that
Geo chem i cal and tec tonic sig nif i cance of Arbat al kali gab bro-monzonite-syenite in tru sions, Urumieh–Dokhtar Mag matic Arc, Iran 19
Fig. 2. Field ob ser va tions of mafic to fel sic al ka line in tru sions from the east of Miandoab
A – com mon syenite to gab bro out crops; B, C – syenite and gab bro turned to cob bles due to on ion-skin weath er ing; D – thick and coarse-grained in tru sions of gab bro and monzogabbro; E – coarse-grained syenite-monzonite dikes ins gab bro and monzogabbro, mafic microgranular en claves from the mafic parts in the fel sic parts are shown in bot tom right; F – frag ments of Eocene vol ca nic rocks in the form of hornfels within the in tru sions
all types of the rocks are of the same age. The ob ser va tion of the frag ments of the Eocene vol ca nic rocks with the hornfels struc ture (Fig. 2F) in side the Arbat in tru sions in di cates the pen - e tra tion of these masses in the Eocene rocks.
PETROGRAPHIC OBSERVATIONS
The syenites are gen er ally coarse-grained (the av er age grain size is 4 mm) and com posed mainly of al kali feld spar (80–100 vol.%) with the vari able de vel op ment of perthitic inter - growth tex tures, plagioclase (0–10 vol.%), and bi o tite and opaque min er als (0–5 vol.%; Fig. 3A). The rocks have mostly gran u lar and lam i nated tex tures, and long perthitic inter growth al kali feld spars oc cur with the su tured bound aries. There are no mag matic ini tial-hy drous min er als, such as am phi bole and bi o - tite, in these rocks.
The monzonites are gen er ally me dium-grained (with the av - er age grain size of 0.6 mm); they are com posed mainly of al kali feld spar (40–60 vol.%), plagioclase (20–45 vol.%), bi o tite (5–20 vol.%), clinopyroxene (2–5 vol.%), and opaque min er als (2–5 vol.%). Ac ces sory ap a tite and some times ti tan ite oc cur with small abun dances (Fig. 3B). The rocks have pri mar ily a gran u lar tex ture. Lam i nated tex tures and long perthitic inter - growth al kali feld spars also oc cur in the rocks.
The mafic fa cies (Fig. 2A, B, D, E) are mostly com posed of gab bro, monzogabbro and monzodiorite. The rocks are gen er - ally me dium-grained (the av er age grain size is 0.7 mm). They are com posed mainly of plagioclase (25–45 vol.%), al kali feld - spar (5–15 vol.%), bi o tite (5–15 vol.%), ol iv ine (5–20 vol.%), clinopyroxene (10–30 vol.%), and opaque min er als (5–15 vol.%) along with small abun dances of ap a tite and ti tan - ite (Fig. 3C, D). In some of these rocks, bi o tite crys tals fill the spaces be tween the grains (Fig. 3D). Euhedral fine-grained ap - a tite crys tals oc cur within the bi o tite. Their tex ture var ies from gran u lar to cu mu late and intercumulate; how ever, in some places, a poikilitic tex ture, in which rel a tively large crys tals of bi - o tite en close nu mer ous smaller crys tals of ol iv ine, clinopyroxene and plagioclase, is ob served (Fig. 3C, D). Most of the cog nate en claves (Fig. 2E) are of monzonite- monzodiorite com po si tion. With a sharp bound ary, they are finer-grained than their host rocks. The en claves oc cur in syenites and monzonites.
WHOLE-ROCK GEOCHEMISTRY
ANALYTICAL METHODS
Whole-rock sam ples (22) were dried at 60°C and sieved to 80 mesh. They were jaw-crushed to 70% pass ing 10 mesh (2 mm), of which 250 g aliquots were pul ver ized to 95% pass ing 150 mesh (100 mm) in a mild steel ring and puck mill. The chem i cal com po si tions of the sam ples were de ter mined at the GeoLab and ACME An a lyt i cal Lab o ra to ries Ltd., Van cou ver, Can ada. Aliquots of the 0.2 g sam ple ma te rial were weighed into a graph ite cru ci ble and mixed with 1.5 g of LiBO2/Li2B4O7
flux. The flux/sam ple charge was heated in a muf fle fur nace for 30 min at 980°C. The cooled bead was dis solved in 100 ml of 5% HNO3. An aliquot of the so lu tion was poured into a poly - propy lene test tube for mea sure ment. Cal i bra tion stan dards, ver i fi ca tion stan dards, and re-agent blanks were in cluded in the
sam ple se quence. The val ues of the ma jor and mi nor ox ides and trace el e ments were de ter mined by X-ray flu o res cence (XRF) and in duc tively cou pled plasma emis sion spec trom e try (ICP-ES). Rare earth el e ment (REE) con tents were de ter mined by the in duc tively cou pled plasma mass spec trom e try (ICP-MS). Loss on ig ni tion (LOI) was de ter mined by the weight loss of 1 g sam ple af ter heat ing at 950°C for 90 min. Ad di tion - ally, the geo chem i cal data were eval u ated by cor re la tion co ef fi - cient anal y sis, el e men tal ra tios, and mass change cal cu la tions.
The geo chem i cal re sults are pro vided in Appendicies 1 and 2*.
CHEMICAL CLASSIFICATION
The Arbat plutonic rocks (in the east ern Miandoab) were clas si fied us ing the to tal al ka lis SiO2 (wt.%) vs (K2O+Na2O wt.%) di a gram of Middlemost (1994; Fig. 4A). The mafic rocks were plot ted in a wide range rep re sented by gab bro to monzogabbro and monzodiorite fields, in ac cor dance with their petrographic fea tures. The fel sic rocks were plot ted in monzonite, quartz monzonite and syenite fields, in agree ment with their petrographic char ac ter is tics. All Arbat plutonic rocks lay within shoshonite-ultrapotassic to al ka line fields de fined by Peccerillo and Tylor (1976; Fig. 4B) and Müller et al. (1992;
Fig. 4C), and Middlemost (1994; Fig. 4A). These plutonic rocks dis play low to high potassic af fin ity with the K2O/Na2O ra tios rang ing from 0.6 to 1.9 (Ap pen dix 1). The fel sic rocks show af - fin ity with metaluminous to weakly peraluminous rocks in the di - a gram of Shand (1927; Fig. 4D).
GEOCHEMISTRY
The study sam ples had a wide range of ma jor and mi nor el - e ments trends vs SiO2 (Harker, 1909): SiO2 (48–69 wt.%), Al2O3 (12–20 wt.%), Fe2O3 (2–11 wt.%), MgO (0.2–12 wt.%), CaO (0.6–10 wt.%), K2O (1.4–8.2 wt.%), Co, V, Sc, Ba, Sr, Rb, Th, Zr, Nb, Y, and REE (rare earth el e ments), from mafic to fel - sic rocks (Appendicies 1–3).
De creas ing trends of FeO*, MgO and MnO ox ides, and of Co, Ni, Sc, and V with in creas ing sil ica re flected the con tri bu tion of these el e ments to the struc ture of ferro mag nesi an min er als, such as ol iv ine, clinopyroxene, bi o tite, ti tan ite, and iron ox ides in mafic units dur ing the early stages of mag matic crys tal li za - tion. The in creas ing and then de creas ing trends of TiO2 ox ide and Y ap peared in the con tri bu tions of the ox ide in the struc ture of ferro mag nesi an min er als, such as bi o tite and ti tan ite, in the in ter me di ate units. The low lev els of the ox ide in the mafic and fel sic units could be due to the low abun dances of the min er als.
The down ward trend of CaO could be due to its par tic i pa tion in the build ing of clinopyroxene and Ca-rich plagioclase in rocks with a more ba sic prop erty. The in creas ing and then de creas ing trend of Al2O3 dem on strated also that plagioclase and al kali feld spar were the im por tant ac cu mu la tion phases dur ing the for ma tion of the in ter me di ate units. The in creas ing trends of K2O and Na2O ox ides, and Ba, Rb, Th, Nb, Hf, and REE with en hanc ing sil ica ex hib ited the con tri bu tion of these el e ments to the struc ture of fel sic min er als, such as al kali feld spar and bi o - tite, and the ac ces sory min er als, such as zir con, ap a tite, and ti - tan ite, in the fel sic units dur ing the late stages of mag matic crys tal li za tion. The high rel a tive val ues of K2O and Na2O in the mafic rocks were the re sult of the high modal per cent ages of al - kali feld spar and bi o tite. These vari a tions in the ox ides and el e - ments in di cated that frac tional crys tal li za tion was an im por tant
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1449
Geo chem i cal and tec tonic sig nif i cance of Arbat al kali gab bro-monzonite-syenite in tru sions, Urumieh–Dokhtar Mag matic Arc, Iran 21
Fig. 3. Mi cro scopic im ages of syenitic to gabbroic out crops, east ern Miandoab
A – al kali syenite with large al kali feld spar min er als; B – monzonite with large al kali feld spar min er als and clinopyroxene, bi o tite, plagioclase, al kali feld spar, ti tan ite and ap a tite min er als; C, D – monzogabbro with abun dant crys tals of ol iv ine and clinopyroxene; E – monzogabbro with abun dant crys tals of bi o tite, ol iv ine and clinopyroxene along with plagioclase, al kali feld - spar, and opaque min er als; F – the bound ary of fine-grained cog nate en clave (con tain ing plagioclase, al kali feld spar, clinopyroxene, and bi o tite min er als) with monzonite; ab bre vi a tions af ter Kretz (1983): Ap – ap a tite, Bt – bi o tite, Cpx – clinopyroxene, Kfs – K-feld spar, Ol – ol iv ine, Pl – plagioclase feld spar, Ttn – ti tan ite
Fig. 4. De scrip tive di a grams for de ter min ing the na ture and clas si fi ca tion of the Arbat in tru sions, east of Miandoab A – sil ica ox ide vs to tal al ka line ox ide di a gram (Middlemost, 1994); B – sil ica ox ide vs po tas sium ox ide di a gram (Peccerillo and Tylor, 1976), discriminant line be tween shoshonitic and al kali potassic suites is af ter Calanchi et al. (2002); C – Ce/Yb vs Ta/Yb di a gram (Müller et al., 1992); D – dis crim i na tion di a gram of alu mi num changes (A/NK vs A/CNK) based on mo lar val ues (Shand, 1927); E – Na2O+K2O-CaO vs SiO2 di a gram af ter Frost et al. (2001); F – The SiO2 vs Fe-num ber (wt.%) di a gram af ter Frost et al. (2001), spot ted fields in (E) and (F) are gab bro, monzonite, syenite and quartz syenite from the Katerina Ring Com plex, south ern Si nai, Egypt (Moreno et al., 2014, 2016)
Geo chem i cal and tec tonic sig nif i cance of Arbat al kali gab bro-monzonite-syenite in tru sions, Urumieh–Dokhtar Mag matic Arc, Iran 23
fac tor in the for ma tion of Arbat in tru sions. The dif fer ent val ues of P2O5 were com ply ing with the var i ous modal per cent ages of ap a tite.
High field strength el e ments (HFSE), such as Nb, Ta, Hf, Zr, P and Ti, large-ion lithophile el e ments (LILE), such as Rb, K and Cs (ex cept for Sr and Ba), rare earth el e ments (REEs), and the (La/Yb)n and (La/Sm)n ra tios were in creased from the gabbroic to monzonitic and syenitic rocks (Figs. 5 and 6; Ap - pen dix 2). In con trast, the (Sm/Yb)n ra tio was al most con stant in all types of rocks. The slightly neg a tive-Eu anom aly was ob - served in all rock units. This dem on strated that plagioclase might not be an im por tant ac cu mu la tion phase dur ing the for - ma tion of these rocks, or plagioclase was a sta ble phase at the liquidus of par tial melt ing. An other pos si bil ity is that the source rock was poor in this el e ment. How ever, the low Eu con tents in the Arbat in tru sive rocks con firmed the last sce nario. With a re - view of the pat terns of the de creas ing and in creas ing val ues of the el e ments (Figs. 5 and 6), it could be con cluded that the or i - gin of all rocks was sim i lar, and the frac tional crys tal li za tion in the pa ren tal magma cre ated a com bined vari a tion from gab bro to syenite. The val ues of U and Th were high in all rocks of the in tru sions.
DISCUSSION
TECTONIC IMPLICATIONS
In most sam ples, the HFSE (Nb, Ta, Zr and Hf) along with P and some LILE (Rb, K and Cs) clearly in di cated neg a tive and pos i tive anom a lies in com par i son with the prim i tive man tle, re - spec tively (Fig. 5). These anom a lies oc curred in the subduction zones (e.g., Pearce et al., 1984; Pearce and Peate, 1995;
White, 2005; Gill, 2010). The HFSE with very low sol u bil ity could not be in tro duced from the subducted slab to the par tial met ing sit u a tion by the subduction flu ids (Brenan et al., 1994;
Pearce, 1996; White, 2005). The oc cur rence of the pre served min er als of the el e ments, such as rutile, ap a tite and ti tan ite, and their sta bil ity in the melt liquidus at the man tle wedge also could cause neg a tive anom a lies in some of the HFSE (Woodhead et al., 1993; White, 2005; Xiong et al., 2005). In ad di tion, the de - ple tion of these el e ments in the fel sic parts could in di cate the crust in ter fer ence in their for ma tion (Swain et al., 2008). Such anom a lies could also be seen in the melts cre ated from post-col li sion en vi ron ments, where there was a subduc - tion-mod i fied man tle source (Fig. 7) to gen er ate these rocks (Neill et al., 2013, 2015). There fore, small-vol ume mafic melts were cre ated as a re sult of the metasomatized man tle in a post-ini tial col li sion or break-off sce nario (Fig. 7). The al ka - line-shoshonitic-ultrapotassic char ac ter is tic of all sam ples (Fig. 4) com plied with the syn- or post-col li sion tec tonic set ting.
These con di tions likely oc curred in the man tle wedge of the Neotethys subduction un der Cen tral Iran dur ing the last stages of the subduction.
Dur ing Oligocene–Mio cene times, the end of the Neotethys subduction might have oc curred be neath Cen tral Iran in the north west ern part of Iran (Molinaro et al., 2005; Vergés et al., 2011; Shafiei Bafti and Mohajjel, 2015; Azizi et al., 2016;
Hassanzadeh and Wernicke, 2016; Fig. 7). Oblique subduction of the Neotethys and tec tonic pres sure from the Ara bian Plate to - wards the north-east with an an gle of about 55° dur ing and af ter the col li sion with Cen tral Iran might have built up shear stresses in the up per crust (McClay et al., 2004). The re duc tion in the lithostatic pres sure of the up per man tle caused the decom - pressive par tial melt ing in the man tle wedge form ing the Arbat mafic melts. The re sults of this study sug gested that a post-ini tial
col li sion might have led to the break-off of the Neo tethys slab be - neath the Arbat con ti nen tal crust in the Oligo cene. Per sis tence of oblique tec tonic stresses caused the right-lat eral strike-slip faults, which fa cil i tated the in jec tion of the mafic mag mas into the base of the lower crust (Fig. 7). The faults re sulted in slab break-off in the rem nants of the oce anic crust un der the col li sion zone of the Ara bian-Eur asian plates and the de com pres sion melt ing pro cess in the upwelling man tle due to the for ma tion of the pull-apart ba sin re sulted from the faults.
Fig. 5. The multi-el e ment spi der di a gram nor mal ized to the prim i tive man tle (Sun and McDonough, 1989) for the Arbat
syenite-monzonite-gab bro
In or der to pro vide new con straints on the or i gin and tectonomagmatic evo lu tion of the Arbat in tru sions from the UDMA, NW Iran (Ap pen dix 4), we have com pared the com po si - tions of syenite-monzonite-gab bro from the Arbat com plex to those from post-collisional Ediacaran fel sic-mafic rocks in the Katerina Ring Com plex (S Si nai, Egypt; Moreno et al., 2014, 2016). Geo chem i cal fields for com par i sons in the fig ure are based on the stud ies of Moreno et al. (2014, 2016). The com - par i sons showed that the Y/Nb, Th/Nb, Th/Ta, La/Nb and Ce/Pb ra tios of the Arbat in tru sions were mostly sim i lar to those of the con ti nen tal crust (CC), subduction-re lated mag matic suites and con ti nen tal arc (Sub and CA), mafic-fel sic rock
suites, and con ti nen tal con ver gent mar gin rocks and shosho - nites (Sh). In ad di tion, these rock types have, rel a tive to the prim i tive man tle, neg a tive Nb, Ta, Hf and Zr anom a lies (Fig. 5).
These char ac ter is tics along with en rich ment in LILEs are con - sis tent with a con ti nen tal arc, syn- or post ini tial-col li sion tec - tonic set ting.
MAGMA GENERATION
The study of many al ka line vol ca nic and in tru sive rocks from NW Iran in the UDMA show that all the rocks oc curred and de - vel oped in an arc-re lated set ting as a re sult of the syn-col li sion or post-col li sion be tween the Cen tral Iran and Arabic plates dur - ing or af ter the clo sure of the Neotethys (Moayyed et al., 2008;
Ashrafi et al., 2009; Aghazadeh et al., 2010; Cas tro et al., 2013;
Shafaii Moghadam et al., 2014; Aghazadeh and Badrzadeh, 2015). How ever, three sug ges tions can be pre sented for the or - i gin of the Arbat syenite-monzonite-gab bro from the East Miandoab:
– par tial melt ing in the subducted slab,
– par tial melt ing of the metasomatic man tle wedge in the su pra-subduction zone,
– par tial melt ing at the base of the lower con ti nen tal crust in the su pra-subduction zone for the for ma tion of fel sic potassic mag mas be cause of the ex po sure of hot man tle mag mas.
There was no high-sil ica adakite (see Bonin, 2007) in the study area at the same time of the for ma tion of the stud ied rocks. There fore, the first sce nario could not sup port the for ma - tion of the rocks.
At the time of the col li sion and af ter that, the slab break-off oc curred (Molinaro et al., 2005; Omrani et al., 2008; Neill et al., 2013, 2015; Fran¸ois et al., 2014a, b; Azizi et al., 2014; Shafaii Moghadam et al., 2014). Slab roll back and then par tial delamination could also have oc curred (Jahangiri, 2007; Azizi et al., 2014). Post-collisional re lax ation pro cess (Aghazadeh et al., 2011; Neill et al., 2015) in the subducted slab at the Oligocene–Mio cene might have hap pened too. Hence, the in - ter fer ence of the subducted slab in the pro duc tion of subductional flu ids and their trans por ta tion to the man tle wedge, as well as man tle rais ing from the slab break-off sit u a - tion or man tle rais ing due to the subducted slab roll back, were the melt ing pro cess fac tors at the man tle wedge. The gabbroic-monzogabbroic melts were formed in such a con di - tion. Geo chem i cal ev i dence in di cated that these rocks were formed by the par tial melt ing of a man tle spinel lherzolite in the wedge (Ap pen dix 5). These depths along with the syn-tec tonic or post-tec tonic con di tions in di cated that mafic mag mas could be formed due to post-col li sion up lift ing; hence, pres sure on the par tial melt ing lo ca tion in the man tle wedge was re duced (de - com pres sion melt ing). This pro cess along with the subduction- fluid in ter fer ence pro duced from the rem nants of subducted slab, the slab break-off in the subducted-slab rem nants, and the in jec tion of the deeper parts of the man tle to the man tle wedge re sulted in the par tial melt ing (Figs. 7 and 8A) and pro duc tion of the mafic melt rich in po tas sium.
Oblique subduction of the Neotethys be neath Cen tral Iran oc curred dur ing the Mid dle Tri as sic to Mid dle Ce no zoic (McClay et al., 2004; Molinaro et al., 2005; Agard et al., 2011;
Alaminia et al., 2013; Azizi et al., 2014; Neill et al., 2015;
Hassanzadeh and Wernicke, 2016; Jafari et al., 2018; Fazlnia, 2018a, b). These re search ers have ex plained that such subduction caused the for ma tion of the right-lat eral strike-slip faults to the crust depths on the west ern edge of Cen tral Iran (UDMA). In ad di tion, Fazlnia (2018b) sug gested that the Fig. 6. The REE spi der di a grams nor mal ized to the prim i tive
man tle (Sun and McDonough, 1989) for the Arbat syenite-monzonite-gab bro
Geo chem i cal and tec tonic sig nif i cance of Arbat al kali gab bro-monzonite-syenite in tru sions, Urumieh–Dokhtar Mag matic Arc, Iran 25
Sardasht mafic and fel sic in tru sions were formed as a re sult of the Neotethys slab break-off and transtension along the SE-trending lat eral strike-slip fault zones re lated to the oblique subduction of the Neotethys plate un der neath the east SSZ in the Early to Mid dle Eocene. In ad di tion to this sce nario, it is pos - si ble that sub-crustal pro cesses, such as lithospheric drip ping, could be ap pro pri ate for the for ma tion of these rocks. As a re - sult, these faults helped the man tle to up lift in the study area dur ing syn-col li sion, es pe cially for the post-ini tial col li sion of the Cen tral Iran and Ara bian plates. There fore, the pres sure re duc - tion pro cess hap pened prop erly, lead ing to par tial melt ing (de - com pres sion melt ing) at the man tle wedge and to the for ma tion of the mafic melt rich in po tas sium.
EVOLUTION OF MAGMATIC PROCESSES
The mafic melts prob a bly re sulted from a par tial melt ing pro cess of a man tle spinel lherzolite in the wedge (Ap pen dix 5).
The ultrapotassic-shoshonite-al ka line char ac ter is tics of the Arbat gab bro-monzogabbro sam ples (Fig. 4A–C) in di cated that the melts had re sulted from the low per cent ages of the par tial melt ing of the man tle source. Af ter the melt ing of the man tle, the melts were sep a rated from that and in jected into the crustal magma cham bers. Po tas sium-rich in ter me di ate and fel sic rocks in the cham bers were formed by the frac tional crys tal li za - tion pro cess (Fig. 8B). At these crustal magma cham bers, the frac tional crys tal li za tion (FC) along with as sim i la tion and frac - tional crys tal li za tion (AFC) might have caused the evo lu tion of
the in ter me di ate to fel sic parts (Fig. 8C). Geo chem i cal ev i - dence of spi der di a grams (Figs. 5 and 6) and Harker vari a tion di a grams (Ap pen dix 3) dis played the same or i gin for all rocks.
Frac tional crys tal li za tion was also prob a bly the most im por tant fac tor in the evo lu tion of the Arbat in tru sions. Some par a doxes in the dis tri bu tion pat terns of the el e ments in some sam ples re - sulted prob a bly from the AFC pro cess in the crustal cham bers.
The Th/Ta (1.7–8.4), Th/Nb (1.5–18) and Ce/Pb (0.16–0.30) ra tios in the Arbat syenites sup ported the neg li gi ble in volve ment of a crustal com po nent in the sources of the rocks (e.g., Moreno et al., 2014, 2016). There fore, the de creas ing and in creas ing val ues of the el e ments, such as HFSE and LILE, were prob a bly due to the in flux of flu ids as so ci ated with subduction. Our data sug gested that a syn- or post-col li sion tec tonic set ting might have led to the break-off of the Neotethys slab be neath the Arbat ac tive con ti nen tal col li sion in the Oligocene. There fore, it is pos si ble that changes in the Th/Nb, Th/Ta, La/Nb and Ce/Pb ra tios could be due to the pen e tra tion of asthenospheric mag mas from be low the slab break-off to the man tle wedge. Frac tional crys tal li za tion along with the con vec - tion pro cesses in the Arbat magma cham bers led to the oc cur - rence of lithological vari a tions in the out crops of the in tru sions and more mafic cog nate en claves (Fig. 2E). Yang et al. (2012), Carvalho et al. (2014), Litvinovsky et al. (2015) and Bao et al.
(2016) em pha sized that prob a ble parts crys tal lized ear lier, in the next pluses of magma in jec tion or dur ing con vec tive flowes within the magma cham ber, were scat tered as en claves.
Fig. 7. Tectonomagmatic evo lu tion of the Arbat in tru sions
Three di men sional pic ture of the evo lu tion dur ing Oligocene with the di rec tion of the Ara bian Plate to wards Cen - tral Iran (mod i fied from Fazlnia, 2018b); the fields on the Sanandaj–Sirjan Zone and Urmia–Dokhtar Mag matic Arc are the ori en ta tion of the in stan ta neous strain el lipse
CONCLUSIONS
The Arbat gab bro-monzonite-syenitic com plex from the east of Miandoab is metaluminous with al ka line-shosho - nite-ultrapotassic char ac ter is tics. These rocks were formed in a syn-col li sion or post-col li sion arc-re lated set ting in the Oligocene. The in tru sions evolved dur ing or af ter the con ti nen - tal col li sion at the NW edge of UDMA, Iran. The oblique subduction, clo sure of Neotethys, and then a con ti nen tal col li - sion be tween the Cen tral Iran and Ara bian plates caused the evo lu tion of the study area. The man tle wedge be neath the Arbat area en dured par tial melt ing and pro duc tion of the po tas - sium-rich mafic melts as a re sult of subduction-zone flu ids and metasomatic pro cesses, de com pres sion melt ing due to the ac -
tiv i ties of the depth faults, pen e tra tion of deep-man tle liq uids due to the slab break-off or slab roll back, and/or lithospheric drip ping and then par tial delamination in the subducted-slab rem nants. There fore, the Arbat mafic melts were prob a bly pro - duced in a subduction-mod i fied man tle source. These man tle wedge mag mas en dured frac tional crys tal li za tion (FC pro cess) and, prob a bly, con tam i na tion and as sim i la tion (AFC pro cess) to form po tas sium-rich in ter me di ate and fel sic rocks dur ing the in - jec tion at the crustal magma cham bers.
Ac knowl edg ments. Fi nan cial sup port from the Urmia Uni - ver sity (Iran) is grate fully ac knowl edged. The au thor thanks the Ed i tors of Geo log i cal Quar terly and the Re view ers of the manu - script, Dr. J. Majka and Dr. I. Neill, for their ef forts.
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