Geo chem is try and tec tonic set ting of the Chah-Bazargan sub-vol ca nic mafic dykes, south Sanandaj–Sirjan Zone (SSZ), 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., 2018. Geo chem is try and tec tonic set ting of the Chah-Bazargan sub-vol ca nic mafic dykes, south Sanandaj–Sirjan Zone (SSZ), Iran. Geo log i cal Quar terly, 62 (2): 447–458, doi: 10.7306/gq.1417
As so ci ate ed i tor: Stanis³aw Mikulski
The Chah-Bazargan sub-vol ca nic mafic dykes (trachybasalt and ba saltic trachyandesite) are lo cated in the south of the Sanandaj–Sirjan Zone (SSZ), Iran. The dyke min er al ogy mostly com prises am phi bole, clinopyroxene, ol iv ine, orthopyroxene, and plagioclase as pheno crysts and fine-grained plagioclase and some ferro mag nesi an min er als in the ma - trix. The rocks are al ka line and shoshonitic in com po si tion. The mafic melts re late to Neotethys subduction ac tiv ity be neath the south ern SSZ in the ~Eocene–Mio cene in ter val. Mark edly pos i tive Ba, U, K, Pb, and Sr and neg a tive HFSE (high field strength el e ments: Nb, Ta, Zr, Hf, P, and Ti) anom a lies dem on strate this subduction. The sub-vol ca nic mafic dykes were pro - duced from a metasomatized up per lithospheric man tle wedge at a depth con sis tent with the sta bil ity field of phlogopite-spinel (or -spinel/gar net) lherzolite. Geo chem i cal stud ies on the ba sis of the rare earth el e ments (REE) and HFSE, and large ion lithophile el e ments (LILE) dis play that the man tle wedge un der went de grees of par tial melt ing av er ag ing be tween 5 and 15% to form the Chah-Bazargan sub-vol ca nic mafic dykes. It is pos si ble that the chem i cal com po si tion of the rocks was changed due to frac tional crys tal li za tion and crustal con tam i na tion dur ing em place ment.
Key words: alkali sub-vol ca nic mafic dykes, the Chah-Bazargan in tru sions, phlogopite-spinel (or -spinel/gar net) lherzolite, subduction zone, Neotethys.
INTRODUCTION
Bas alts are erupted in a wide va ri ety of tec tonic en vi ron - ments on Earth (e.g., mid-ocean ridges, is land arcs, back-arc bas ins, con ti nen tal col li sion zones, intra-plate oce anic is lands, large ig ne ous prov inces and intra-con ti nen tal rifts), and col lec - tively they are found on the Earth’s sur face in greater vol ume than any other vol ca nic rock type (Best, 2003; Gill, 2010; Neil et al., 2013, 2015; Haldar and Tišljar, 2014). Var i ous sources and con di tions of melt ing, frac tion ation, and crys tal li za tion of melts in dif fer ent tec tonic set tings are re flected, to var i ous de grees, in the chem i cal com po si tion of ig ne ous rocks such as bas alts (e.g., Gill, 2010; Haldar and Tišljar, 2014; Frost and Frost, 2014;
Velikoslavinsky and Krylov, 2014). Sub-vol ca nic mafic melt ing can oc cur to re sult in dykes. Such dykes ex ist in many geo log i - cal tec tonic set tings of west Iran. Ba saltic rocks and mafic dykes from the Zagros orog eny have been re ported (e.g., Kheirkhah et al., 2009; Azizi et al., 2014; Yousefi et al., 2017).
West ern Iran can be di vided into a set of three par al lel NW–SE trending tec tonic zones (Alavi, 1994; Mouthereau et
al., 2012; Mohajjel and Fergusson, 2014), namely, the Zagros Fold-Thrust Belt (ZFTB), the Sanandaj–Sirjan zone (SSZ), and the Neo gene–Qua ter nary Urumieh–Dokhtar Mag matic Arc (UDMA; Fig. 1A, B). The Zagros is the larg est moun tain belt and the most ac tive collisional orogen as so ci ated with Ara - bia/Eur asia con ver gence. It be longs to the Al pine–Hi ma la yan orogenic sys tem that re sulted from the clo sure of the Neotethys Ocean dur ing the Ce no zoic (Mouthereau et al., 2012). The tec - tonic his tory of these zones as part of the Tethyan re gion has been sum ma rized by many au thors (Berberian and King, 1981;
Alavi, 1994; Omrani et al., 2008; Khadivi et al., 2012;
Mouthereau et al., 2012; Mohajjel and Fergusson, 2014). The SSZ jux ta posed var i ous meta mor phic and mag matic rocks that mainly formed in Me so zoic time. Dur ing the Pa leo zoic, the SSZ was part of north-east Gond wana land, sep a rated from the Eur - asian Plate by the Palaeo-Tethys Ocean (Golonka, 2004;
Mouthereau et al., 2012; Mohajjel and Fergusson, 2014).
From Early Perm ian to Early Tri as sic time, the SSZ was sit - u ated along the south ern mar gin of the Eur asian Plate, sep a - rated from north ern Gond wana land by the Neo-Tethyan Ocean (Mouthereau et al., 2012). Dur ing the Me so zoic, the oce anic crust of the Neotethys was subducted be neath the Eur asian plate (Golonka, 2004; Molinaro et al., 2005; Fazlnia et al., 2009, 2013; Agard et al., 2011; Mouthereau et al., 2012; Mohajjel and Fergusson, 2014; Mehdipour Ghazi and Moazzen, 2015;
Davoudian et al., 2016; Hassanzadeh and Wernicke, 2016;
Honarmand et al., 2017), and the SSZ oc cu pied the po si tion of
* E-mail: a.fazlnia@urmia.ac.ir and nfazlnia@yahoo.com Received: December 9, 2017; accepted: February 6, 2018; first published online: May 21, 2018
448 Abdolnaser Fazlnia
Fig. 1A – simplified geological map of northeasten Neyriz (modified after Sabzehei et al., 1992), Zircon SHRIMP U-Pb isotopic ages of the different rock types from the Chah-Bazargan intrusions are after Fazlnia et al. (2007, 2009, 2013); B – simplified
geological map of western Iran (modified after Stöcklin, 1968); C – cross-section showing position of dykes investigated
a mag matic arc (Berberian and Berberian, 1981; Agard et al., 2005, 2011; Mouthereau et al., 2012). The SSZ in the south - east ern part in cluded Pa leo zoic meta mor phic rocks with rel a - tively high-grade meta mor phism which in the Mid dle to Late Tri - as sic were de formed and meta mor phosed (Berberian, 1995;
Fazlnia et al., 2009; Sheikholeslami, 2015; Karimi and Tabatabaei Manesh, 2016; Hassanzadeh and Wernicke, 2016). The meta mor phic li thol ogy in the south ern SSZ mainly con sists of metapelites, metabasites, mar bles, and meta-ultra - mafic rocks (Berberian and King, 1981; Fazlnia et al., 2009;
Sheikholeslami, 2015; Hassanzadeh and Wernicke, 2016). The high est grade meta mor phic rocks in the south ern SSZ are of up per am phi bo lite fa cies (Berberian and King, 1981; Fazlnia et al., 2009).
Subduction-re lated magmatism in the UDMA was ac tive from the Late Ju ras sic to the pres ent (e.g., Berberian and King, 1981; Berberian et al., 1982) or the Late Cre ta ceous to the pres ent (e.g., Hosseini et al., 2017). The fi nal clo sure of the Neotethys and the col li sion of the Ara bian and Eur asian plates took place dur ing the Late Neo gene (Berberian and Berberian, 1981; Alavi, 1994; Golonka, 2004; Molinaro et al., 2005; Omrani et al., 2008; Agard et al., 2011; Mouthereau, 2011; Khadivi et al., 2012; Mouthereau et al., 2012; Mehdipour Ghazi and Moazzen, 2015; Hassanzadeh and Wernicke, 2016). Dur ing the same in ter val, the Zagros Fold-Thrust Belt formed as part of the Al pine–Hi ma la yan moun tain chain, ex tend ing about 2000 km from east ern Tur key to the Oman line in south ern Iran (Berberian and King, 1981; Alavi, 1994; Agard et al., 2005, 2011; Omrani et al., 2008; Mouthereau, 2011; Mouthereau et al., 2012; Mohajjel and Fergusson, 2014; Mehdipour Ghazi and Moazzen, 2015; Hassanzadeh and Wernicke, 2016).
The the Chah-Bazargan sub-vol ca nic mafic dykes from the south ern SSZ of Iran com prise part of the rocks ex posed in the Zagros Folf-Thrust Belt (Fig. 1). This study in ves ti gates the geo chem i cal and tec tonic set ting of these dykes. The study of these rocks can help to re con struct the last stages of Neotethys subduction in the south east ern re gion of the SSZ, Iran. The rocks formed at the end of subduction, so the pres ent study can es tab lish the re la tion ship be tween magmatism and the tec tonic en vi ron ment in the south ern part of the SSZ. This study ex am - ines the geo log i cal, petrographic, min er al og i cal and geo chem i - cal char ac ter is tics of the Chah-Bazargan sub-vol ca nic mafic dykes in or der to ex plain their petro gen esis and tec tonic en vi - ron ment. Also, the data con strain the or i gin and em place ment his tory of the in tru sions and con trib ute to our un der stand ing of the tectono-mag matic evo lu tion of the south ern SSZ.
GEOLOGICAL SETTING
The Chah-Bazargan in tru sions, lo cated north-east of Neyriz (Fig. 1), are part of the south ern SSZ. The in tru sions are com - posed of granodiorite, quartz diorite, gab bro, troctolite, anorthositic troctolite, an or tho site, wehrlite and clinopyroxenite.
These in tru sions were emplaced into the low-grade part of the Quri meta mor phic com plex, which con sists pri mar ily of metabasites and mar bles, in ter spersed with metapsammitic, meta-ultra mafic, and metapelitic lay ers. Some out crops of sed i - men tary and low-grade metasedimentary rocks can be found as im bri cate slices among the high-grade meta mor phic rocks be cause of late stage shear ing of the Zagros Fold-Thrust Belt dur ing the late Ce no zoic (Berberian and King, 1981; Sabzehei et al., 1992; Fazlnia et al., 2007; Sheikholeslami et al., 2008).
The peak pres sure and tem per a ture for the Quri meta mor phic com plex (Fig. 1A) are es ti mated to be 9.2 ± 1.2 kbar and 705 ± 40°C re sult ing from crustal thick en ing dur ing the Early
Cim mer ian Orog eny be tween 187 and 180 Ma (Fazlnia et al., 2007, 2009). This event oc curred as a re sult of the ini ti a tion of Neotethys subduction be neath cen tral Iran (Fazlnia et al., 2013).
All rock types of the meta mor phic com plex are strongly sheared and were thrust as im bri cate slices over the Neyriz ophiolite (Fig. 1B; Berberian and King, 1981; Shahabpour, 2005; Sheikholeslami et al., 2008; Omrani et al., 2008;
Shahbazi et al., 2010; Mouthereau et al., 2012; Mohajjel and Fergusson, 2014). The Neyriz ophiolite is part of the Late Cre ta - ceous ophiolite belt that de fines the south west ern edge of the NW-SE-trending SSZ, north east of the Zagros Fold-Thrust Belt (Stöcklin, 1968, 1977; Stoneley, 1981). The re sults of re cent stud ies and new ideas about the sig nif i cance of Late Cre ta - ceous Zagros ophiolites were given by Moghadam and Stern (2011).
Af ter sev eral mil lion years, the Chah-Bazargan in tru sions were in truded into the Quri meta mor phic com plex as a re sult of Neotethys subduction be neath cen tral Iran (Sheikholeslami, 2015) at 170.5 ± 1.9 Ma (Fazlnia et al., 2013). The tholeiitic melts of the in tru sions, re lated to the start of Neotethys subduction ac tiv ity be neath the south ern SSZ at
~170.5 ± 1.9 Ma, were pro duced from a metasomatized up per lithospheric man tle wedge at a depth con sis tent with the sta bil - ity field of spinel lherzolite (Fazlnia et al., 2013). The in tru sive rocks formed in the Chah-Bazargan in tru sions dis play marked neg a tive HFSE (Nb, P, Hf and Ti) and pos i tive Ba, Sr, and U anom a lies typ i cal of subduction-re lated mag mas (Fazlnia et al., 2013). The sub-vol ca nic mafic dykes are lo cated on the east ern edge of the Chah-Bazargan in tru sions (Fig. 1A, C).
Ad di tion ally, the west ern edge of the Quri meta mor phic com plex un der went Barrovian-type meta mor phism at 147.4 ± 0.76 Ma as a re sult of crustal thick en ing dur ing the ini ti - a tion of the Neo-Tethyan mid-ocean ridge subduction be neath cen tral Iran. The meta mor phic event of the gar net am phi bo lites oc curred at pres sures and tem per a tures be tween 7.5 and 9.5 kbar (at a depth of 25 to 32 km) and 680 and 720°C, re spec - tively, based on the Grt–Hbl and Hbl–Pl ther mom e ters and a Grt–Hbl–Pl–Qtz ba rom e ter (Fazlnia et al., 2009).
There are many Oligocene and Mio cene dykes with the same chem i cal com po si tion and min er al ogy near the study area as re ported by Moradian (1990), Hassanzadeh (1993) and Aftabi and Atapour (2000), such as the Shahrebabak, Rafsanjan, and Bardsir vol ca nic-subvolcanic rocks (Fig. 1B).
Also, based on the geo chem i cal com par i son of the sam ples stud ied with the mafic col li sion zone sam ples from the Turk - ish–Ira nian Pla teau (Neill et al., 2013, 2015), it can be con - cluded that the dykes stud ied prob a bly formed dur ing Late Eocene–Mio cene in ter val.
FIELD AND PETROGRAPHIC OBSERVATIONS
Rock ex po sures of the sub-vol ca nic mafic dykes in the study area are con sis tently of al kali ba salt in com po si tion.
Based on abun dances of the mafic min er als, the dykes are mafic rocks (Fig. 2). Dyke widths vary from sev eral metres to tens of metres. In all ex po sures, the bound aries be tween the dykes and coun try rocks (quartz-diorites and granodiorites of the Chah-Bazargan in tru sions) are sharp (Fig. 2A) and the min - er al og i cal com po si tions of each dyke from the chilled mar gins to ward the cen tre are the same (Fig. 3). Frag ments of the host rocks are pres ent in the dykes. Ol iv ine megacrysts along with am phi bole and pyroxene are vis i ble in the rock (Fig. 2B, C). The chilled mar gins of the dykes are very fine grained. Modal per - cent ages of the large min er als in crease to ward the cen tres of
the dykes. This in di cates that the dykes in truded into the cold coun try rocks. Many ol iv ine grains are al tered to ser pen tine.
Min eral as sem blages in the mafic dykes (Fig. 3) con sist mainly of am phi bole (10–12 vol.%), clinopyroxene (5–7 vol.%), plagioclase (4–6 vol.%), orthopyroxene (al tered to ser pen tine;
1–2 vol.%), and ol iv ine (2–3 vol.%) along with a very fine-grained ma trix and por phy ritic tex ture. The bound aries be - tween the am phi bole and clinopyroxene and ma trix are mostly sharp and smooth. They are fresh, with no re ac tion rims be - tween the min er als and the ma trix. The min er als are euhedral (Fig. 3A, B). Ser pen tine is pres ent in frame work of the ol iv ine and orthopyroxene (Fig. 3C, D).
Exsolution lamellae of clinopyroxene are pres ent along cleav ages of the serpentinized orthopyroxene (Fig. 3E). Ol iv ine has been al tered to ser pen tine (Fig. 3C). Re ac tion bound aries among dif fer ent crys tals of ol iv ine and orthopyroxene and the ma trix are gradational. Ad di tion ally, these min er als are subhedral or anhedral in shape. There fore, these min er als were not in equi lib rium with the ma trix dur ing in jec tion into the quartz-diorite-granodiorite. Also, some am phi bole grains show a sieve-like tex ture in the rims and growths of bi o tite along the cleav age of the min eral. There fore, a rapid de crease of the pres sure in the magma dur ing in jec tion in creased the fluid pres -
sure; hence, the ol iv ine and orthopypoxene were un sta ble and al tered to ser pen tine (Fig. 3C, D), while some am phi bole grains de vel oped sieve-like tex ture in the rims and growth of sec ond - ary lamellae of bi o tite along min eral cleav ages (Fig. 3E). Xeno - crysts of quartz with re ac tion mar gins are ob served in the ma trix of some spec i mens (Fig. 3F).
ROCK GEOCHEMISTRY
ANALYTICAL METHODS
The rock sam ples were pow dered in an ag ate mill. LOI (loss on ig ni tion) was de ter mined by heat ing pow ders at 1000°C for 2 hours. The de creased weights of the pow ders were then cal cu - lated. Ap pen di ces 1* and 2 list the chem i cal com po si tions of 8 rep re sen ta tive sam ples ob tained by ICP-MS (in duc tively cou - pled plasma-mass spec trom e try). The ma jor and trace el e - ments of sam ples were ana lysed with an ICP-MS in stru ment at the ALS Chemex Com pany of Can ada (website:
www.acmelab.com; Cer tif i cate of anal y sis: ANK13000382).
450 Abdolnaser Fazlnia
Fig. 2. Photographs of the mafic dykes
A – mafic dykes with a chilled margin; B – specimen of the mafic rocks with large crystals of amphibole (Amph) and clinopyroxene (Cpx);
C – specimen of the mafic rocks from the mafic dykes with large crystals of amphibole, pyroxene (Px) and olivine (Ol)
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1417
Fig. 3. Photographs of the mafic dykes
A, B – am phi bole (Amph) and clinopyroxene (Cpx) in a fine-grained ma trix, al tered ol iv ine (Ol) is pres ent in the am phi bole (PPL and XPL light, re spec tively); C – oc cur rence of am phi bole, clinopyroxene, and al tered ol iv ine or orthopyroxene (to ser pen tine, Srp) with fine-grained ma trix (PPL light); D – large grain of orthopyroxene al tered to ser pen tine along with exsolution lamellae of clinopyroxene (PPL light); E – am - phi bole grain with sieve-like tex ture in the rims and growth of sec ond ary lamellae of bi o tite (Bt) along cleav age planes of the min eral (PPL light); F – xeno crysts of quartz (Qtz) with re ac tive mar gins, Pl – plagioclase; ab bre vi a tions are af ter Kretz (1983)
GEOCHEMISTRY
Mafic dykes of the Chah-Bazargan in tru sions show lim ited ranges of SiO2, Al2O3, Fe2O3*, MgO, and CaO con tent due to lim ited modal per cent ages of mafic and fel sic min er als. All sam - ples dis play clear neg a tive cor re la tions be tween SiO2, MgO and Fe2O3 and ob vi ous pos i tive cor re la tions be tween SiO2, Al2O3 and Na2O (Ap pen dix 1; Fig. 4). The ob vi ous neg a tive and pos i tive cor re la tions be tween SiO2 and other ox ides dem on - strate that frac tional crys tal li za tion has pre sum ably been an im - por tant fac tor in the geo chem i cal evo lu tion of these rocks. Such a char ac ter is tic is sup ported by de creas ing and in creas ing of in - com pat i ble el e ments cor re lated with sil ica (Ap pen di ces 1 and 2).
All rock types of the dykes plot on the al ka line se ries (Fig. 5A). The com po si tions of the rocks are equiv a lent to trachybasalt and ba saltic trachyandesite on a TAS di a gram (Fig. 5A). The oc cur rence of clinopyroxene, orthopyroxene, and ol iv ine in some sam ples cor re lates with the al ka line se ries. Ad - di tion ally, sam ples of the dykes plot on the high-K calc-al ka line and shoshonite se ries in the K2O (wt.%) vs. SiO2 (wt.%) di a - gram (Fig. 5B). For com par i son, the sam ples stud ied are plot in al most the same fields as the mafic col li sion zone sam ples from the Turk ish–Ira nian Pla teau (Neill et al., 2013, 2015) and the Qua ter nary high-Nb bas alts from the north Sanandaj–Sirjan Zone, NW Iran (Azizi et al., 2014).
There are no sys tem atic changes be tween in crease of SiO2
and in crease or de crease of all trace el e ments and REE in the mafic dykes. El e ment ra tios, such as XMg, Lan/Ybn, Gdn/Ybn, Smn/Ybn, Eu*, and K/Ba de crease in re la tion with in creas ing SiO2 ( Ap pen di ces 1 and 2; Fig. 6). Con tents of K2O, Al2O3, and CaO, weakly pos i tive Sr (Fig. 6A) and very weak Eu (Fig. 6B) anom a lies in the mafic dykes sug gest that plagioclase con trib - uted only slightly or not at all dur ing the par tial melt ing of the source. Ad di tion ally, the Eu/Eu* ra tios of the mafic sam ples (av - er age 0.86) may have been de pend ent on con di tions. The av er - ages of the Lan/Ybn, Lan/Smn, and Gdn/Ybn ra tios in the gab bros are 8.05, 2.96, and 1.62, re spec tively. This sug gests that pre - sum ably the mafic rocks seg re gated from an evolved magma in the cham ber.
Neg a tive Nb and Ta anom a lies sug gest that rutile was a re - sid ual phase dur ing the gen er a tion of the mafic magma. Rutile, il men ite, and ap a tite were pos si ble re sid ual phases dur ing the gen er a tion of the mafic magma, and as a re sult, the mafic rocks are not strongly en riched in LREEs (light REE) and all sam ples show neg a tive Hf anom a lies. This pos si bil ity is sup ported by neg a tive P anom a lies in some of the mafic sam ples (Fig. 6A).
Neg a tive Zr anom a lies sug gest that zir con was prob a bly a re - sid ual re frac tory phase dur ing the gen er a tion of the mafic magma, or the source was ini tially poor in the el e ment.
Changes in the pat terns of dif fer ent el e ments in the rocks stud -
452 Abdolnaser Fazlnia
Fig. 4. Binary diagrams of major element oxides vs. SiO2
A – MgO vs. SiO2; B – Fe2O3* vs. SiO2; C – CaO vs. SiO2; D – Al2O3 vs. SiO2; E – Na2O vs. SiO2; F – K2O vs. SiO2
ied (Fig. 6A) are al most the same as those of the mafic col li sion zone sam ples from the Turk ish–Ira nian Pla teau (Neill et al., 2013, 2015) and the Qua ter nary high-Nb bas alts from the north Sanandaj–Sirjan Zone, NW Iran (Azizi et al., 2014).
Mod er ate La/Yb ra tios (Ap pen dix 2) and low con cen tra tions of HREE in the mafic dykes (Fig. 6A) sug gest that spinel was mostly a re sid ual re frac tory phase and that par tial melt ing was not im por tant in the source re gion. This is sup ported by the Sm/Yb vs. La/Sm ra tios (Ap pen dix 2; Fig. 7) of the mafic dykes from the Chah-Bazargan in tru sion. These mod er ate ra tios sug - gest that par tial melt ing oc curred in the sta bil ity field of spinel in the man tle source.
DISCUSSION
MAGMATIC PROCESSES
Pet ro graph i cal and geo chem i cal char ac ter is tics of the dykes stud ied sug gest that they may be clas si fied as an al ka line and shoshonite se ries (Fig. 5). They are en riched in al ka lis (Na2O + K2O), large ion lithophile el e ments (such as Rb, Ba, Sr, and K2O), and light rare earth el e ments (Lan/Ybn of 8.02), and fea tures of trace el e ment con cen tra tions are sim i lar to those of some bas alts (see tec tonic set ting). There fore, it is pos si ble that par tial melt ing pro cesses in the source of the Chah-Bazargan sub-vol ca nic trachy-ba salt and ba saltic trachy-an de site dykes are pre sum ably sim i lar to those of the some bas alts from the Turk ish–Ira nian Pla teau (Neill et al., 2013, 2015) and the Qua - ter nary high-Nb bas alts from the north Sanandaj–Sirjan Zone, NW Iran (Azizi et al., 2014).
Geo chem i cal melt ing mod els based on batch and Ray leigh melt ing mod el ing can be used to de ter mine pos si ble amounts of par tial melt ing in the migmatitic xe no liths. Based on Fig ure 8, melt ing could have oc curred as batch melt ing. There fore, Shaw’s equa tions (Shaw, 1970) can be used to rec og nize par - tial melt ing into the source. Melt ing was mod eled us ing the equi lib rium batch melt ing equa tion:
( )
C
C D F F
i i l 0
1
= 1
- +
where: i – the el e ment of in ter est, Ci
0 – the orig i nal con cen tra tion in the solid phase (and the con cen tra tion in the whole sys tem), Ci
l – the con cen tra tion in the liq uid (or melt), F – the melt frac tion (i.e., mass of melt/mass of sys tem) and D – the par ti tion co ef fi cient of the el e - ment of in ter est: D Ci C
s
= / il; Ci
s – the con cen tra tion re main ing in the solid.
This equa tion is ex tremely use ful in de scrib ing the rel a tive en rich ment or de ple tion of a trace el e ment in the liq uid as a func tion of the de gree of melt ing. Two ap prox i ma tions are of ten use ful. First con sider the case where D<<F. In this case Cil/Ci0 »1/F, that is, the en rich ment is in versely pro por tional to the de gree of melt ing. This is the case for highly in com pat i ble el e ments at all but the small est de grees of melt ing. Now con - sider the case where F ap proaches 0. In this case Cil/Ci0 »1/D, and the en rich ment is in versely pro por tional to the par ti tion co ef fi cient. Thus the max i mum en rich ment pos si - ble in a par tial melt is 1/D. For highly com pat i ble el e ments, which are those with large D such as Ni, the de ple tion in the melt is 1/D when F is small and is rel a tively in sen si tive to F.
The av er ages of the ma jor and trace el e ments of the Chah-Bazargan sub-vol ca nic trachy-ba salt and ba saltic trachyandesite dykes were used (Ap pen dix 2). Thus, the con - cen tra tions of the dif fer ent el e ments from the av er age ini tial protolith (Ci0; spinel peridotite from McDonough, 1991) and the mafic sam ples (Ci
l) were con sid ered (Ap pen dix 2).
Sim ple trace-el e ment mod el ing was used to test whether the Chah-Bazargan trachybasalt and ba saltic trachyandesite dykes bear com po si tions con sis tent with der i va tion by par tial melt ing at in the sta bil ity field of spinel in the man tle source.
Bulk par ti tion co ef fi cients were cal cu lated as sum ing 1%, 5%, 10%, 20%, 30%, 40%, and 50% par tial melt ing (Ap pen dix 3).
From the re sults, sum ma rized in Ap pen dix 3 and Fig ure 6, it is ap par ent that the be hav iour of all el e ments, such as the REEs (La to Lu), HFSE (Zr, Ta, Nb, U, Hf, Y, P, Ti), Ba, Pb, K, Fig. 5A – Na2O + K2O vs. SiO2 discrimination diagram with
field delineated after Middlemost (1994); B – K2O vs. SiO2
discrimination diagram (Rickwood, 1989)
All sam ples plot be tween the high-K calc-al ka line, al ka line, and shoshonite se ries. For com par i son, av er age sam ples of the mafic col li sion zone mag matic rocks from the Turk ish–Ira nian Pla teau (Neill et al., 2013, 2015) and the Qua ter nary high-Nb bas alts from the north Sanandaj–Sirjan Zone, NW Iran (Azizi et al., 2014) are shown
Rb, and Sr is largely sen si tive to source en rich ment, but in sen - si tive to the amounts of the main and ac ces sory min er als in the res i due (man tle source). There fore, the ma jor pro por tions of the min er als can not gen er ally be a cause of de ple tion or en rich - ment in the magma dur ing the par tial melt ing of the source.
Hence, par ti tion co ef fi cients de crease with in creas ing lev els of par tial melt ing in the source (Ap pen dix 3).
Con sid er ing more in com pat i ble el e ments such as REE and HFSE, and LILE in di cates that rel a tively mod est de grees (av er - age be tween 10 and 15%) of par tial melt ing are per mis si ble (Ap pen dix 3). Ad di tion ally, trace-el e ment mod el ing based on Sm/Yb vs. Sm (Fig. 8) di a gram shows that a spinel or spinel-gar net lherzolite man tle source has ex pe ri enced de - grees of par tial melt ing of ~5–15%.
Al ka line rocks gen er ally oc cur in three prin ci pal set tings (Win ter, 2014): (1) con ti nen tal rifts, (2) con ti nen tal and oce anic intraplate set tings with no clear tec tonic con trol, and (3) subduction zones, par tic u larly in back-arc set tings or in the wan ing stages of ac tiv ity. A num ber of pro cesses are thought to be in volved in the gen e sis of al kali mafic rocks, in clud ing: (1) con tam i na tion of ultrabasic magma with crustal ma te rial (Fitton and Upton, 1987; Srivastava and Chalapathi Rao, 2007); (2) ex treme dif fer en ti a tion of ba sic magma en riched in CO2 and H2O (Cur rie and Wil liams, 1993; Mitch ell et al., 1996; Win ter, 2014); and (3) low de gree par tial melt ing of metasomatized con ti nen tal lithospheric man tle (Thorpe, 1982; Mor ris and Pasteris, 1987; Rock, 1991). A few sam ples show ev i dence for crustal as sim i la tion (Fig. 3F). This ev i dence in cludes par tially
454 Abdolnaser Fazlnia
Fig. 6. Primitive mantle normalized multi-element and REE plots
A – normalized multi-element diagram for mafic dykes; B – normalized REE diagram for mafic dykes; normalization values after Sun and McDonough (1989); for comparison see Figure 5
di gested xeno crysts of quartz in the ba saltic trachyandesite dykes. The sam ples with phys i cal ev i dence for crustal as sim i la - tion (Fig. 3F) are plot ted in the crustal con tri bu tion field in Fig - ures 9A and 10A. In these pro cesses, it is likely that con tam i na - tion of crustal ma te ri als along with frac tional crys tal li za tion (as - sim i la tion frac tional crys tal li za tion, AFC; Fig. 4) played im por - tant roles in the for ma tion of the geo chem i cal fea tures of the mafic dykes stud ied (Figs. 9 and 10A), be cause the trace el e - ment con cen tra tions, such as Rb, K, Th, U, and Pb of the fi nal mag mas (Ap pen dix 2; Fig. 6A) are sug gested to be high and they are eas ily mod i fied by crustal con tam i na tion. Prob a bly, mag matic dif fer en ti a tion was also im por tant, con sid er ing that the dykes show wide ranges of REE pat terns (Fig. 6B).
As dis cussed above, the Chah-Bazargan sub-vol ca nic trachybasalt and ba saltic trachyandesite dykes are sug gested to have been de rived from the metasomatized up per man tle, form ing a ba sic magma en riched in CO2 and H2O. Ev i dence such as neg a tive Nb, Ta, Zr, Hf, P, and Ti and pos i tive Ba, U, K, Pb, and Sr anom a lies (Fig. 6A), the high-K calc-al ka line, al ka - line, and shoshonite se ries (Fig. 5), and changes in the con cen - tra tions of Th, Hf, and Ta (Fig. 10A) dem on strate that par tial melt ing of a metasomatized spinel lehrzolite oc curred in a typ i - cal subduction-re lated set ting. Then, the magma com po si tion was changed due to crustal con tam i na tion and mag matic dif fer - en ti a tion (FC and AFC) dur ing magma in jec tion and stoping in
the crustal cham ber. Plot ting the sam ples stud ied in the fields of the mafic col li sion zone sam ples from the Turk ish–Ira nian Pla - teau (Neill et al., 2013, 2015) and the Qua ter nary high-Nb bas - alts from the north Sanandaj–Sirjan Zone, NW Iran (Azizi et al., 2014), dem on strate that sim i lar pro cesses, such as FC and AFC, have led to the evo lu tion of the pa ren tal magma (Fig. 10A).
TECTONIC SETTING
Ages ob tained on the Chah-Bazargan gabbroic in tru sions by Fazlnia et al. (2013; 170.5 ± 1.9 Ma; see zir con U-Pb SHRIMP dat ing sec tion in Fazlnia et al., 2013) and the Chah-Bazargan leuco-quartz diorite-an or tho site-granodiorite batholiths (173 ± 6.1 Ma based on zir con U-Pb SHRIMP dat ing;
Fazlnia et al., 2007) showed that these in tru sions (Fig. 1A) are re lated to the early stages of Neotethys subduction be neath the south ern SSZ (also, see Shahabpour, 2005 more dis cus sion).
There fore, the south ern SSZ was an ac tive con ti nen tal mar gin be tween 175 and 168 Ma. Al ka line rocks are typ i cally in truded at a late stage in ig ne ous cen tres where they oc cur (Fitton and Upton, 1987; Mor ris and Pasteris, 1987; Rock, 1987; Win ter, 2014).
The dykes stud ied plot ted on the high-K calc-al ka line, shoshonite (Fig. 5B), and al ka line (Fig. 5A) se ries. Such com - po si tions are most com monly as so ci ated with late-orogenic transtension re lated to low de gree lithospheric man tle melt ing and typ i cally form thin and dis con tin u ous dykes (Fitton and Upton, 1987; Gill, 2010, Scarrow et al., 2011; Win ter, 2014).
The cen tral Ira nian al ka line, potassic calc-al ka line, and shoshonitic vol ca nic rocks (oc cur ring in the SSZ and UDMB) are part of the Neotethys subduction-re lated rocks in a con ti - nen tal arc set ting (Aftabi and Atapour, 1997, 2000). These rocks were in truded into the crust that is now cen tral Iran dur ing the Eocene–Qua ter nary (Berberian and King, 1981; Aftabi and Fig. 7. Sm/Yb vs. La/Sm plot showing theoretical melting
curves plotted along with the basic samples from the Chah-Bazargan mafic dykes
Frac tional and batch melt ing equa tions of Shaw (1970) were used to con struct the melt ing model. F – weight frac tion of melt pro duced (the equi lib rium batch melt ing equa tion listed above). Modal min er al ogy for the spinel- and gar net-peri dot ites are taken from Wil son (2007), and Ol0.66+Opx0.24+Cpx0.08+Spl0.02 and Ol0.63+Opx0.30+Cpx0.02+Grt0.05, re spec tively. The trace el e ment com po si tion of the spinel-peridotite (C0 val ues) is the av er age com po si tion of spinel peridotite xe no liths in young (Mio cene) al ka line bas alts of the Thrace re gion, NW Tur key (af ter Keskin, 2005), while that of gar net peridotite is from Frey (1980). Par ti tion co ef fi cient (Kds or D; the equi lib rium batch melt ing equa tion listed above) be tween the ba saltic melts and min er als given in the in set are com piled from Irving and Frey (1978), Fujimaki et al.
(1984), McKenzie and O’Nions (1991) and Rollinson (1993). The bulk par ti tion co ef fi cient (D0) of each el e ment has been cal cu lated for gar - net and spinel peridotite source rock com po si tions by tak ing the modal min er al ogy of these end mem bers into con sid er ation. The co - ef fi cients are given in the in set
Fig. 8. Plot of Sm/Yb vs. Sm for the mafic dykes in the Chah-Bazargan intrusions
Man tle ar ray (heavy line) de fined by de pleted MORB man tle (DM, McKenzie and O’Nions, 1991) and prim i tive man tle (PM, Sun and McDonough, 1989). Melt ing curves for spinel lherzolite (Ol53+Opx27+Cpx17+Sp11) and gar net peridotite (Ol60+Opx20+Cpx10+Gt10) with both DM and PM com po si tions are af ter Aldanmaz et al. (2000). Num bers along these lines rep re - sent the de gree of par tial melt ing. For com par i son see Fig ure 5
Atapour, 1997, 2000; Aftabi, 1999). Aftabi and Atapour (2000) in ferred that these rocks had been de rived from an al ka line po - tas sium-rich magma, gen er ated by melt ing of a phlogopite-bear ing subducting plate. Based on the neg a tive Nb, Ta, Zr, Hf, P, and Ti and pos i tive Ba, U, K, Pb, and Sr anom a lies, and the Th/Yb vs. Ta/Yb pat tern (Fig. 10B), it is pos - si ble that the Chah-Bazargan sub-vol ca nic trachybasalt and ba saltic trachyandesite dykes were formed in a metasomatized phlogopite-spinel (or -spinel/gar net) lehrzolite within a typ i cal of subduction-re lated or syn-collisional set ting.
The age of the dykes stud ied is pre sum ably ~Eocene–Mio - cene, and can be con nected with the Mid dle–Late Mio cene (~15–10 Ma) break-off of the north-dip ping south ern Neotethys oce anic slab be neath the Bitlis and Zagros su tures (Keskin, 2003; Allen and Armstrong, 2008; Zor, 2008; van Hunen and Allen, 2011; Neil et al., 2013, 2015; McQuarrie and van Hinsbergen, 2013; Skolbeltsyn et al., 2014; Azizi et al., 2014) in a syn-collisional geodynamic en vi ron ment. Fi nally, the geo - chem i cal data in di cate that the dykes were formed in subduction-re lated or syn-collisional set ting, though Neil et al.
(2013, 2015) noted also that “such com po si tions are most com -
monly as so ci ated with late-orogenic transtension”. It is pos si ble that the Chah-Bazargan sub-vol ca nic trachybasalt and ba saltic trachyandesite dykes were de rived from a subduction-mod i fied source dur ing orogenic transtension. Fi nally, the plot ting of the sam ples stud ied with the mafic col li sion zone sam ples from the Turk ish–Ira nian Pla teau (Neill et al., 2013, 2015) and the Qua - ter nary high-Nb bas alts from the north Sanandaj–Sirjan Zone, NW Iran (Azizi et al., 2014) in Fig ure 10B, shows that they are prob a bly formed as a re sult of the same tec tonic en vi ron ment.
CONCLUSIONS
The Chah-Bazargan trachybasalt and ba saltic trachyandesite melts re lated to Neotethys subduction ac tiv ity be neath the south ern SSZ ap prox i mately in the Eocene–Mio - cene in ter val, were pro duced from a metasomatized up per lithospheric man tle wedge at a depth con sis tent with the sta bil - ity field of phlogopite-spinel (or -spinel/gar net) lherzolite. The rocks stud ied formed in the Chah-Bazargan in tru sions and dis - play marked neg a tive HFSE (Nb, Ta, Zr, Hf, P, and Ti) and pos i - tive Ba, U, K, Pb, and Sr anom a lies typ i cal of subduction-re -
456 Abdolnaser Fazlnia
Fig. 9A – plot of Nb/Th vs. Th for the mafic dykes; B – plot of Ba/Nb vs. La/Nb for the rocks
Values of N-MORB, MORB, OIB and primitive mantle are from Sun and McDonough (1989); values for the lower crust and
continental crust are from Wedepohl (1995)
Fig. 10. Tectonic setting discrimination diagrams A – three an gle plot of Th-Hf/3-Ta (Wood, 1980) for the mafic dykes, crustal con tri bu tion is af ter Krmíèek (2010); B – plot of Th/Yb vs.
Ta/Yb (Wil son, 2007) for the rocks, all sam ples plot in an ac tive con - ti nen tal mar gin; for com par i son see Fig ure 5; PM – prim i tive man tle, S, C and W – subduction, col li sion, and within plate set tings, re spec - tively, F – magma frac tion ation
lated mag mas. Dur ing em place ment, they un der went mag - matic trans for ma tions and frac tional crys tal li za tion in an open sys tem, which was resupplied fre quently, and by these pro - cesses formed the dykes. Sim ple trace-el e ment mod el ing based on the REE, HFSE and LILE show that the Chah-Bazargan sub-vol ca nic mafic dykes were de rived as a re - sult of de grees of par tial melt ing av er ag ing be tween 5 and 15%.
The melts were eas ily mod i fied by crustal con tam i na tion and frac tional crys tal li za tion. These are the causes of the het er o ge -
ne ity ob served, such as in the REE, in some parts of the Chah-Bazargan sub-vol ca nic mafic dykes.
Ac knowl edge ments. Fi nan cial sup port from the Ira nian Min is try of Sci ence, Re search and Tech nol ogy and from the Urmia Uni ver sity (Iran) are grate fully ac knowl edged. The au - thor wishes to thank the anon y mous re view ers of the pa per, for their help.
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458 Abdolnaser Fazlnia
