Min eral chem is try and thermobarometry of plutonic, meta mor phic and anatectic rocks from the Tueyserkan area (Hamedan, Iran)
Ali A. SEPAHI1, *, Kazem BORZOEI1 and Seddigheh SALAMI1
1 De part ment of Ge ol ogy, Bu-Ali Sina Uni ver sity, Hamedan, Iran
Sepahi A. A., Borzoei K., Sa lami S. (2013) Min eral chem is try and thermobarometry of plutonic, meta mor phic and anatectic rocks from the Tueyserkan area (Hamedan, Iran). Geo log i cal Quar terly, 57 (3): 515–526, doi: 10.7306/gq.1108
The study area is a part of the NW to SE strik ing Sanandaj–Sirjan meta mor phic belt in west ern Iran. The Alvand Pluton, con - sist ing of rocks that range in com po si tion from gab bro to gran ite, is the ma jor mag matic rock com plex of this area. Gabbroic rocks in clude ol iv ine gab bro, gab bro norite, norite and gab bro. Rocks around the Alvand Pluton were sub jected to dif fer ent P–T con di tions due to polymetamorphism. Com mon meta mor phic rocks are meta-pelites, but some meta-psammites, meta-basites and meta-car bon ates also oc cur. Slates, phyllites, schists, migmatites and hornfelses are ma jor rock units of meta-pelites in the meta mor phic se quence. Based on min eral chem is try, the high est tem per a ture of crystallisation (1300°C) was de ter mined for the ol iv ine gab bros, and the low est tem per a ture (950°C) was cal cu lated for the hornblende-bear ing gab - bros. Clinopyroxene–plagioclase ba rom e try sug gests that pres sures near 5 to 6 kbars pre vailed dur ing the crystallisation of the var i ous mafic rocks. P–T es ti mates yield max i mum tem per a tures of 700–750°C at 5–6 kbars for the high-grade meta mor - phic rocks from the meta mor phic au re ole around the pluton. These re sults in di cate that the heat re leased from the Alvand Pluton (T = 950–1300°C), which in truded the meta mor phic rocks at mid dle and up per crustal lev els, was suf fi cient to cause par tial melt ing lead ing to for ma tion of the metatexites, diatexites and restite-rich S-type gran ites. Dur ing this pro cess, part of the deep-seated gab bro-dioritic rocks were trans ported to higher crustal lev els by vis cous, en clave- and crys tal-rich gra nitic mag mas of the par tial melt ing zone.
Key words: Alvand Pluton, Hamedan, Iran, min eral chem is try, Sanandaj–Sirjan zone, thermobarometry
INTRODUCTION
The study area is lo cated in the north ern Sanandaj–Sirjan zone, west ern Iran. The Sanandaj–Sirjan zone is part of the Al - pine-Hi ma la yan orogenic sys tem ex tend ing from NW Iran and west Tur key to SE Iran. Ba sic petrographic stud ies on plutonic and meta mor phic rocks of the Hamedan re gion of the north ern Sanandaj–Sirjan zone have been pub lished in the pa pers in Per sian (Zareiyan et al., 1971–1974). Ad di tional stud ies of pe - trog ra phy and petro gen esis of plutonic and meta mor phic rocks are dis cussed in some pub li ca tions and M.Sc. and Ph.D. the - ses (Irani, 1993; Sadeghian, 1994; Baharifar, 1997, 2004;
Sepahi, 1999, 2008; Badrzadeh, 2002; Sepahi et al., 2004, 2009; Shahbazi, 2010; Shahbazi et al., 2010; Tork, 2011;
Borzouei, 2012; Saki et al., 2012). How ever, the min eral chem - is try and thermobarometry of an in ter est ing plutonic and meta - mor phic se quence from the area north of Tueyserkan have not been stud ied in de tail so far. There fore, we de cided to fo cus our work on var i ous in dex min er als from mafic plutonic rocks and their meta mor phic au re ole from the Tueyserkan area in or der to es ti mate the P–T con di tions of magmatism and meta mor - phism. The ma jor goal was to de ter mine wether the heat pro -
vided by the Alvand Pluton was suf fi cient to cause high tem per - a ture meta mor phism and anatexis of the metapelites in the re - gion. This was ac com plished by us ing var i ous pub lished thermobarometric cal i bra tions, e.g., gar net–bi o tite and gar - net–cor di er ite ther mom e ters and gar net–plagioclase–mus co - vite–bi o tite (GPMB), gar net–alu mi no sili cate–quartz–plagio - clase (GASP) and gar net–bi o tite–plagioclase–quartz (GBPQ) ba rom e ters (Ferry and Spear, 1978; Perchuk et al., 1985;
Aranovich et al., 1988; Hoisch, 1990; Hol land and Powell, 1990;
Berman, 1991; Dasgupta et al., 1991; Bhattacharya et al., 1992; Kleemann and Reinhardt, 1994; Holdaway, 2000; Henry et al., 2005; Powell and Hol land, 2008).
GEOLOGICAL SETTING
The Sanandaj–Sirjan zone, or Zagros im bri cate zone of the Zagros Orogen, Iran (Alavi, 1994, 2004) com prises a meta mor - phic belt of low- to high-grade re gional and con tact meta mor - phic rocks that have been in truded by mafic, in ter me di ate and fel sic mag mas (Fig. 1). Ma jor magmatism in the Sanan - daj–Sirjan zone oc curred dur ing the Me so zoic (e.g., Baharifar, 1997, 2004; Sepahi, 1999; Rashidnejad-Omran et al., 2002;
Sheikholeslami et al., 2003; Sepahi et al., 2004; Ahmadi-Khalaji et al., 2007). Al to gether, the crystallisation ages of the ma jor plutons range from Me so zoic to Paleogene, ~200 to ~40 Ma (Valizadeh and Cantagrel, 1975; Braud,1987; Masoudi, 1997;
Baharifar, 2004; Ahmadi-Khalaji et al., 2007; Arvin et al., 2007;
* Corresponding author, e-mail: sepahi@basu.ac.ir
Received: March 2, 2013; accepted: June 3, 2013; first published online: June 28, 2013
Shahbazi et al., 2010; Ahadnejad et al., 2011; Mahmoudi et al., 2011; Esna-Ashari et al., 2012). The time span of magmatism in the Sanandaj–Sirjan zone of the Zagros Orogen can be re - lated to dif fer ent stages of magmatism re sulted from open ing of an ocean, later subduction of the oce anic crust, and col li sion and post-col li sion (post-orogenic) stages of magmatism. Be - cause the plutonic rocks of the area stud ied are mid-Ju ras sic in age (Shahbazi et al., 2010), magmatism and meta mor phism of the re gion is at trib uted to subduction of the Neo-Tethys Ocean be neath the cen tral Ira nian mi cro-con ti nent and to the sub se - quent col li sion of the Afro-Ara bian con ti nent and Eur asia from Ju ras sic to Neo gene times (e.g., Baharifar, 1997, 2004;
Sepahi, 1999; Sepahi et al., 2004; Shahbazi et al., 2010).
Sepahi et al. (2004) have re ferred to an arc-type meta mor - phism fol lowed by a some what higher pres sure event.
Plutonic rocks of the Alvand Plutonic Com plex, as a ma jor plutonic body of the north west ern Sanandaj–Sirjan zone, have been di vided into three rock as so ci a tions (Sepahi, 2008): gab - bros–diorites–tonalites (GDT), por phy ritic granodiorites and gran ites and leucocratic granitoids. The GDT as so ci a tion is com posed of ol iv ine gab bro, gab bro, gab bro–norite, quartz gab bro, diorite, quartz diorite and tonalite. In the granodiorite–gran ite suite, por phy ritic monzogranite is the most fre quent rock type. Leucocratic granitoids are com posed of leucotonalites, leucogranodiorites and leucogranites. U-Pb dat - ing shows a Ju ras sic crystallisation age for the Alvand Pluton.
Gab bros formed at 166.5 ± 1.8Ma, gran ites at 163.9 ± 0.9Ma and 161.7 ± 0.6Ma, and leucocratic granitoids at 154.4 ± 1.3 and 153.3 ± 2.7Ma (Shahbazi et al., 2010). K-Ar cool ing ages be tween 90 and 63 Ma (for micas of the gra nitic rocks) were re - ported by Valizadeh and Cantagrel (1975). Rb-Sr ages be - Fig. 1. Sim pli fied map show ing the geo log i cal set ting of the Sanandaj–Sirjan zone, Hamedan re gion
and north of the Tueyserkan area
tween 89 and 68 Ma (for micas of the gra nitic rocks) were pub - lished by the same au thors. Braud (1987) has de ter mined a K-Ar cool ing age of 64 ± 2 Ma for the por phy ritic gran ite of the Alvand Pluton.
Meta mor phic rocks of the area are meta-pelites and mi nor meta-psammites, quartzites, meta-basites, calc schists and calc sil i cates. Meta-pelites in clude slate, phyllite, pelitic schist/migmatite and hornfels (Fig. 1). The meta mor phic rocks of the Tueyserkan area have been sub jected both re gional and con tact meta mor phism. Re gional meta mor phic zones of chlorite, bi o tite, gar net, an da lu site (chi asto lite), sillimanite and sillimanite–(±cor di er ite)–K-feld spar and con tact meta mor phic zones of cor di er ite (±an da lu site), cor di er ite–K-feld spar and sillimanite–K-feld spar are wide spread around the Alvand Pluton, es pe cially in the Tueyserkan area. A de tailed de scrip - tion of meta mor phic rocks and zones was un der taken dur ing ear lier re searches (e.g., Sepahi et al., 2004, 2009).
ANALYTICAL METHODS
About 100 thin sec tions from var i ous plutonic and meta mor - phic rocks were eval u ated for petrographic in ves ti ga tion. EPMA anal y ses of var i ous min er als (Ap pen di ces 1–11) were made at the IMPRC, Karaj, Iran, us ing a Cameca SX100 elec tron microprobe un der 15 kV ac cel er at ing volt age, 20 nA beam cur - rent and 52–0 µm beam di am e ter. AX (free down load from tjbh url: http://www.esc.cam.ac.uk/astaff/hol land/in dex.html) and THERMOCALC (free down load) soft wares were used for min - eral for mu lae cal cu la tions, P–T es ti ma tions and petro gen etic in ter pre ta tions.
FIELD OBSERVATIONS AND PETROGRAPHY
Con sid er ing that data on the field re la tions and pe trog ra phy of the plutonic and meta mor phic rocks of the re gion has been
pub lished pre vi ously (Sepahi et al., 2004, 2009; Sepahi, 2008, see the sec tion on the geo log i cal set ting, above), we here only briefly re fer to field ob ser va tions and pe trog ra phy of the se - lected rock sam ples (i.e., we do not pres ent com plete de scrip - tions of all of the rock types of the re gion).
PLUTONIC ROCKS
Var i ous plutonic rocks, rang ing from gab bro to gran ite, crop out in the Tueyserkan area. These were in truded by aplit ic and peg ma titic dikes. The gran ites con tain feld spar megacrysts in some field ex po sures. They are com posed of quartz, K-feld - spar, plagioclase and bi o tite (Fig. 2A); mus co vite and zir con are ac ces sory min er als. Some restitic xeno crysts of Al2SiO5-min er - als (an da lu site and sillimanite) and gar nets oc cur in these rocks. Mafic rocks in clude ol iv ine gab bro, gab bro and norite. Ol - iv ine gab bro has subhedral gran u lar to inter gra nu lar tex ture and is com posed of ol iv ine, clinopyroxene and plagioclase as ma jor min er als and orthopyroxene, am phi bole and bi o tite as mi nor min er als (Fig. 2B). Plagioclase–ol iv ine co ro nas oc cur oc ca sion - ally. Gab bro has inter gra nu lar, ophitic and sub-ophitic tex tures and is com posed of clinopyroxene and plagioclase as ma jor phases (Fig. 2C) and ol iv ine and hornblende as mi nor min er als.
Norite has subhedral gran u lar to sub-ophitic tex ture and is com - posed of orthopyroxene and plagioclase (Fig. 2D), mi nor ol iv ine and clinopyroxene and ac ces sory ap a tite.
METAMORPHIC ROCKS AND MIGMATITES
Hornfels and migmatite are the most com mon meta mor phic rocks in the area. Field ob ser va tions in di cate a grad ual change in the de gree of anatexis from metatexite through diatexite to gar net-bear ing gran ite in the area (Fig. 3) but the or der of rock units to wards the plutonic bod ies is not reg u lar in many out - crops.
In migmatites, par tial melt ing fronts were ini ti ated around var i ous porphyroblasts, es pe cially Al2SiO5 min er als and cor di -
Fig. 2. Pho to mi cro graphs of (A) porphyroid gran ite, (B) ol iv ine gab bro, (C) gab bro and (D) norite
Bt – biotite, Cpx – clinopyroxene, Hbl – hornblende, Kf – K-feldspar, Ol – olivine, Opx – ortopyroxene, Pl – plagioklase, Qtz – quartz
Fig. 3. Grad ual changes from metatexite to diatexite to gar net-bear ing gran ite while gar net ± an da lu site/sillimanite porphyroblasts have re mained metastable in the anatectic rocks From A to H de gree of par tial melt ing is in creas ing grad u ally in anatectic rocks; A, B – metatexite migmatite, C–E – diatexite migmatite, F – sillimanite–gar net-bear ing diatectic gran ite and G, H – gar - net-bear ing gran ite; length of scales is: 5 mm (A), 200 mm (B, G), 100 mm (C), 200 mm (D, E), 40 mm (F), di am e ter of coin is 15 mm (H)
er ite, and then mi grated to other parts of the rock. Dur ing anatexis, gar net and alu mi no sili cate (an da lu site, sillimanite) porphyroblasts re mained partly sta ble but floated and were re-dis trib uted in the vis cous mush when the par tial melt reached its crit i cal mov ing thresh old.
Among the hornfelses, gar net–staurolite–Al2SiO5 and an da - lu site–fib ro lite–cor di er ite–gar net-bear ing hornfels (Fig.4A–C) pre vail. The typ i cal min eral as sem blage of the first rock type is quartz + bi o tite + mus co vite + an da lu site/sillimanite (rarely kyan - ite) + staurolite + gar net + chlorite ± spinel, and for the sec ond rock type is quartz + bi o tite + mus co vite + K-feld spar + cor di er ite
± an da lu site (fib ro lite) ± spinel. Re place ment of an da lu site by spinel + cor di er ite co ex ist ing with bi o tite is com mon in some meta mor phic rocks (see also Saki, 2011). Cor di er ite-bear ing migmatites are wide spread but gar net–sillimanite-bear ing migmatites oc cur in some parts of the meta mor phic au re ole as well. The typ i cal min eral as sem blage of this migmatite is quartz + K-feld spar + plagioclase + bi o tite + gar net ± sillimanite/an da lu - site. Overgrowths of sodic plagioclase on K-feld spar oc cur in places (Fig. 4D). Other migmatite types are spinel–an da lu site migmatites, gar net–cor di er ite migmatites and cor di er ite–spinel migmatites. Cor di er ite + K-feld spar + sillimanite/an da lu site + bi - o tite + plagioclase + quartz ± gar net ± spinel is a typ i cal as sem - blage seen near the plutonic rocks in the Tueyserkan area. For com par i son, the min eral as sem blage of cor di er ite-bear ing migmatites closely re sem bles D-1 to D-4 as sem blages from the deep-seated Steinach au re ole of Oberpfalz, North-East Ba varia, Ger many re ported by Okrusch (1969, 1971).
MINERAL CHEMISTRY
EPMA anal y sis on var i ous min er als in clud ing ol iv ine, clinopyroxene, orthopyroxene, am phi bole, bi o tite and plagioclase in plutonic rocks and cor di er ite, gar net, plagioclase, bi o tite, chlorite, mus co vite, staurolite, Al2SiO5-min er als and spinel in meta mor phic rocks are shown in Ap pen di ces 1–11.
MINERAL CHEMISTRY OF THE PLUTONIC ROCKS
The re sults of EPMA anal y ses for ol iv ine crys tals from the gabbroic rocks are given in Ap pen dix 1. Chem i cal com po si tions of these crys tals lie in the crysolite field. EPMA anal y ses for clinopyroxene from the same rocks (Ap pen dix 2) plot close to the calcic clinopyroxene (augite) field. Chem i cal com po si tions of orthopyroxene crys tals (Ap pen dix 3) lie in the bronzite, hypersthene and ferrohypersthene fields. EPMA anal y ses of am phi bole crys tals (Ap pen dix 4) re veal a typ i cal calcic hornblende com po si tion. EPMA anal y ses of bi o tite (Ap pen - dix 5) lie in the eastonite–siderophyllite fields near the phlogopite–annite fields. Plagioclase crys tals (Ap pen dix 6) from var i ous mafic plutonic rocks lie in the an de sine, lab ra dor ite and bytownite fields.
MINERAL CHEMISTRY OF THE META-PELITIC METAMORPHIC ROCKS
EPMA anal y ses have been car ried out on cor di er ite, gar net, spinel, bi o tite, staurolite, white mica, Al2SiO5-min er als, chlorite, plagioclase and K-feld spar. Rep re sen ta tive re sults are shown in Ap pen di ces 7 to 11. Cor di er ite has in ter me di ate Mg# num ber (0.489–0.580; Ap pen dix 7, Mg# = Mg/(Mg + Fe + Mn)). Gar nets show zon ing in some cases but they are almandine-rich pyralspites with small amounts of grossular in their com po si - tions (XAlm – almandine con tent in gar net = 61.8–79.0%, Ap pen - dix 8). Staurolite crys tals are Fe-rich and their Mg num bers (Mg#) are 0.121–0.132. Al2SiO5 min er als have mi nor to trace amounts of Fe and Mg as inpurities (FeO – 0.00–1.12 wt.% and MgO – 0.02–0.29 wt.%). Spinel is Fe-rich, thus hercynitic with mi nor amounts of Cr, Mg and Mn in com po si tion. Fe/(Fe + Mg) in biotites is 0.57–0.68. Chem i cal com po si tions of bi o tite crys - tals (Ap pen dix 9) lie in the siderophyllite and annite fields. White mica com po si tions lie in the com mon mus co vite range but they also con tain mi nor amounts of Na2O (Ap pen dix 10) whereas chlorite com po si tions plot in the rhipidolite field. Chem i cal com - po si tions of plagioclase lie in the oligoclase–an de sine field (Ap - Fig. 4. Pho to mi cro graph of se lected meta mor phic rocks of the area stud ied
A–C – hornfelses with var i ous in dex min er als as in di cated by their ab bre vi a tions on pho tos, D – over growth of plagioclase on K-feld spar in migmatite; And – an da lu site, Crd – cor di er ite, Ky – kyan ite, Ms – mus co vite; other ex pla na tions as in Fig ure 2
pen dix 11). K-feld spar crys tals con tain up to 90 wt.% orthoclase com po nent.
THERMOBAROMETRY
In thermobarometric stud ies, we may de ter mine the P–T con di tions which rocks sus tained dur ing equi lib rium crystallisation. For this pur pose, petrographic stud ies and de ter - mi na tion of the chem i cal com po si tion of in di vid ual min er als by var i ous an a lyt i cal tech niques such as EPMA are es sen tial. Sev - eral geothermometers and geobarometers have been in tro - duced by ge ol o gists in re cent years. We have ex am ined one ther mo met ric (i.e., clinopyroxene–orthopyroxene ther mom e try) and one bar o met ric (clinopyroxene–plagioclase ba rom e try) method to ob tain P–T con di tions of crystallisation of the gabbroic rocks. Then, we cal cu lated P–T con di tions of the metapelitic meta mor phic rocks of the re gion us ing the min eral chem is try of the in dex min er als. In this re gard, it was im por tant to us to be sure that if the geothermobarometric re sults are in ac cor dance with the idea that the Alvand Pluton has been a pos si ble heat source for meta mor phism and anatexis in the re gion.
THERMOBAROMETRY OF THE MAFIC PLUTONIC ROCKS
Min eral chem is try of orthopyroxene, clinopyroxene, am phi - bole and plagioclase used to de ter mine the P–T con di tions dur - ing the crystallisation of the gabbroic rocks.
CLINOPYROXENEO–RTHOPYROXENE THERMOMETRY
As sum ing a pres sure of 5 kbar (see sec tion on ba rom e try, be low) the clinopyroxene–orthopyroxene ther mom e ter of Lindsley (1983) yields crystallisation tem per a tures of 1200–1300°C for the ol iv ine gab bro and 900–1100°C for the gab bro (Fig. 5).
CLINOPYROXENEP–LAGIOCLASE BAROMETRY
The ap pli ca tion of the bar o met ric equa tions of Mc Car thy and Patino-Douce (1998) for co ex ist ing clinopyroxene and plagioclase:
P = [5.066 ± 0.760 + ((1300 ± 800/T) – lnK)/(276 ± 16)]
where: T – tem per a ture ±2.5°C, P – pres sure [kbar], K = aAn(Pl)/aCats(Cpx), K re sults in pres sure es ti mates be tween 5.5 and 6.5 kbars, aAn and acats are an or thite ac tiv ity in plagioclase and Ca-tschermak ac tiv ity in Cpx, re spec tively.
P = {([6.330 ± 0.116] – lnK)/(301 ± 9)} ´ T [±1.0 kbar]
We have con sid ered the rim com po si tion of plagioclase crys tals for thermobarometric cal cu la tions where these are zoned.
THERMOBAROMETRY OF THE METAPELITIC ROCKS
Us ing the com po si tions of var i ous in dex min er als of the meta-pelitic rocks, ob tained from EPMA anal y ses, we em - ployed var i ous ther mo met ric and bar o met ric meth ods to con - strain the P–T con di tions dur ing meta mor phism.
Ti IN BIOTITE THERMOMETRY
Bi o tite is one of the host min er als for Ti in meta mor phic rocks, the con cen tra tion of which is con trolled by tem per a ture.
Ac cord ing to Henry and Guidotti (2002) and Henry et al., (2005), the fol low ing equa tion can be used to cal cu late tem per - a ture:
T = {[ln(Ti) – a – c(XMg)3]/b}0.333
where: a = –2.3594, b = 4.6482 ´ 10–9, c = –1.7283, XMg – Mg/(Mg+Fe)
On the ba sis of this equa tion, tem per a tures be tween 550 to 575°C for the gar net–staurolite hornfels and 675 to 725°C for the mesosome of the migmatitic rocks were ob tained (Fig. 6).
GARNET–BIOTITE AND GARNET–CORDIERITE THERMOMETERS
Ac cord ing to Ferry and Spear (1978), Fe2+ and Mg2+ ex - change be tween gar net and bi o tite is tem per a ture-de pend ent.
Fig. 5. Plot of com po si tion of orthopyroxene and clinopyroxene on a Di–Hd–En–Fs rect an gle
with solvus curves (Lindsley, 1983)
Fig. 6. Plot of amounts of Ti ver sus Mg/(Mg + Fe) for biotites of the con tact meta mor phic rocks
This cat ion ex change can be ex pressed by the fol low ing re ac tion:
Mg3Al2Si3O12 (pyrope) + KFe3AlSi3O10(OH)2 (annite)
= Fe3Al2Si3O12(almandine) + KMg3AlSi3O10(OH)2 (phlogopite).
The cal i bra tion by Kleemann and Reinhardt (1994) was used for tem per a ture cal cu la tion re lated to this cat ion ex change re ac tion. Tem per a tures be tween 558 and 688°C for hornfelses and 759–790°C for migmatitic rocks were ob tained, which are in ac cor dance with the ob served min er al og i cal as sem blages of these rocks. The gar net–cor di er ite ther mom e ter (e.g., Aranovich and Podlesskii, 1989; Dwivedi, 1996) gave tem per a - tures of 780 to 785°C, for migmatitic rocks (Ta ble 1).
BAROMETRY
Var i ous geobarometers such as gar net–plagioclase–mus co - vite–bi o tite (GPMB), gar net–alu mi no sili cate–quartz–pla gioclase (GASP) and gar net–bi o tite–plagioclase–quartz (GBPS or GPBQ) were em ployed to es ti mate the pres sure(s) of meta mor -
phism. In the GPMB (gar net–plagioclase–mus co vite–bi o tite) ba - rom e ter, the fol low ing re ac tions were con sid ered:
Grs + Prp + Ms = 3An + Phl Grs +Alm + Ms = 3An + Ann Alm + Phl = Prp + Ann
(Min eral name ab bre vi a tions in these equa tions are from Whit ney and Ev ans, 2010).
For the min er als from the Alvand coun try rocks the var i ous cal i bra tions of the GPMB ba rom e ter yield pres sures be tween of 4.5 to 6 kbars for hornfels and migmatite (Ta ble 2). In the GASP ba rom e ter the es sen tial ref er ence re ac tion is: Sill + Qtz + Grt = An. By us ing the chem i cal com po si tions of min er als in var i ous cal i bra tions for GASP, pres sure es ti ma tions for hornfels and migmatite are var i ous but lie in a range be tween 4.4 to 6.8 kbars (Ta ble 2).
The GBPQ ba rom e try is based on the fol low ing Mg- and Fe-model equi lib ria (e.g., Hoisch, 1990): Mg3Al2Si3O12 (pyrope) + 2Ca3Al2Si3O12 (grossular) + 3KMg2Al(Si2Al2)O10(OH)2 (eastonite) + 6SiO2(quartz) = 6CaAl2Si2O8 (an or thite) + 3KMg3AlSi3O10(OH)2
(phlogopite) and Fe3Al2Si3O12 (almandine) + 2Ca3Al2Si3O12
(grossular) + 3KFe2Al)Si2Al2)O10(OH)2 (siderophyllite) + 6SiO2
(quartz) = 6CaAl2Si2O8 (an or thite) + 3KFe3AlSi3O10(OH)2 (annite).
By us ing the chem i cal com po si tions of min er als in GBPQ, pres - sures es ti mated for hornfels and migmatite are be tween 4.9 to 5.5 kbars (Ta ble 2).
THERMOCALC
In this method, equi lib rium curves of var i ous pos si ble re ac - tions be tween min er als are used for es ti ma tion of P–T con di - tions. To achieve this goal the TC3.2 and AX pro grams were ap plied (see an a lyt i cal method sec tion above, for ad dress). The re sults of P–T es ti ma tion ob tained by the THERMOCALC pro - gram for var i ous rocks are shown in Fig ures 7–10. For the gar - net–staurolite hornfels tem per a tures of 620–650°C (at ~5 kbar, Figs. 7 and 8) were ob tained. The spinel–cor di er ite-bear ing sam ple yielded tem per a tures of 720–760°C (at 4.5–5.5 kbars, Fig. 9) and the gar net-cor di er ite-bear ing sam ple ~750°C (at 5.0–5.5 kbars, Fig. 10). The es ti mated tem per a tures for var i ous hornfelses are 560–630°C at pres sures of 4–5.5 kbars. These tem per a tures ex tend up to 700–800°C for migmatitic rocks.
Method Gar net–cor di er ite
ther mom e try Gar net–bi o tite ther mom e try Sam ple/
cal i bra tion
migmatite
(N31) migmatite
(N31) hornfels
(A5)
A 848 889 654
B 779 824 639
C 798 759 558
D 785 803 642
E 786 842 631
F 810 820 688
Gar net–bi o tite ther mom e ters: A – Thomp son (1976), B – Perchuk et al. (1985), C – Aranovich et al. (1988), D – Dasgupta et al. (1991), E – Holdaway (2000), F – Dwivedi et al. (2007); gar net–cor di er ite ther - mom e ters: A – Thomp son (1976), B – Bahttacharya et al. (1992), C – Perchuk (1991), D – Aranovich and Podlesskii (1989), E – Dwivedi (1996), F – Dwivedi et al. (1998)
T a b l e 1 Re sults of geothermometry for the gar net-bear ing hornfels
and migmatite us ing var i ous cal i bra tions
Method GASP GPMB GPBQ
Sam ple/
Cal i bra tion hornfels
(A5) migmatite
(N31) hornfels
(A5) hornfels
(A5) migmatite
(N31)
A 5.8 5.8 4.1 5.5 5.1
B 5.2 6.2 4.1 5.1 4.9
C 4.4 5.7 4.5 – –
D 6.6 6.8 4.8 – –
E 6.1 6.2 4.6 – –
GASP: A – Berman and Aranovich (1996), B – Hodges and Spear (1982), C – Ganguly et al. (1996), D – Koziol and New ton (1988), E – Hol land and Powell (1990); GPMB: A – Ghent and Stout (1981), B – Hol land and Powell (1995), C – Bhattacharya et al. (1992), D – Hodges and Crowley (1985) (Fe), E – Hodges and Crowley (1985) (Mg); GPBQ: A – Hoisch (1990) (Mg), B – Hoisch (1990) (Fe)
T a b l e 2 Re sults of geobarometry of the gar net-bear ing hornfels and migmatite
us ing var i ous cal i bra tions
Fig. 7. P–T con di tions ob tained by the THERMOCALC pro gram for the gar net–staurolite-bear ing rocks us ing the chem i cal
com po si tions of rims of gar net and staurolite crys tals 1 – Grs + 2And + Qz = 3An; 2 – 32Alm + 23And + 75Clc = 125Prp + 24Fst + 252H2O; 3 – 69Clc + 6Fst + 117Qz = 115Prp + 24Fst + 252H2O; 4 – 4Phl + 96Ky + 3Cel = 5Prp + 7Ms; 5 – Phl + 3Ame + 9Qz = 5Prp + Ms + 12H2O; 6 – 62Ame + 53Eas + 10Mst + 228H2O; 7 – 25Grs + 8Alm + 96And + 12H2O = 75An + Fst + Sil; 8 – Prp + 2Pg + 3Qz = 3An + 2Ab + H2O; 9 – 64Alm + 318And + 75Ame = 100Prp + 48Fst + 204H2O
Fig. 8. P–T con di tions ob tained by the THERMOCALC pro gram for the gar net–staurolite-bear ing rocks by us ing the chem i cal
com po si tions of cores of the gar net and staurolite crys tals 1 – 23Ann + 6Fst + 48Qz = 31Alm + 23Ms + 12H2O; 2 – 51Alm + 48Pg + 8Clim = 17Grs + 48Ab + 10Mst + 6H2O; 3 – 8Grs + 46Pg + 21Qz = 46Ab + 6Mst + 34H2O; 4 – 26Ab + 7Clc = 24Mst = 17Prp + 26Pg + 10H2O ;5 – Grs + 2Pg + 3Qz = 3An + 2Ab + 2H 2O; 6 – 23Grs + 6Fst + 48Qz = 8Alm + 69An + 12H 2O; 7 – Grs + 3Eas + 2Qz = Prp + 3An + 3Cel; 8 – 17Grs + 8Ame + 20Ms + 2Mst = 20Eas + 51An + 36H2O; 9 – 159An + 48Ame = 4Prp + 53Grs + 18Ms + 156H2O; 10 – 109Grs + 192Clc + 66St = 88Alm + 327An + 240Ams + 60H2O; 11 – Prp + 2Grs + 3Eas + 6Qz = 3Phl + 6An + 53H2O
Fig. 9. P–T con di tions ob tained by the THERMOCALC pro gram for rocks which have spinel–cor di er ite inter growths 1 – Spl + 5Phl + 10And = 3Crd + 3Eas; 2 – 15Her + Phl = 3Spl + Ann;
3 – 3Hc + 2Phl + 15Qz = 3Crd + 2Ann; 4 – 3Her + 16Phl + 30And = 9Crd + Ann + 15Eas; 5 – 2Ann + 6Pg + 15Qz = 3Fcrd + 6Ab + 2Sa + 8H2O; 6 – 3Fcrd + 6Eas + 21Qz = 6Cerd + 2Ann + 4Sa + 4H2O; 7 – 3Fcrd + 6Eas = 12Spl + 2Ann + 4Sa + 4H2O; 8 – 2Her + 5Qz = Fcrd;
9 – Ann + 3Sil = 3Hc + 3Qz + San + H2O; 10 – 2Phl + Ann + 15Pg = 6Hc + 2Phl + 15Sil + 15Ab + 15H2O
Fig. 10. P–T con di tions ob tained by the THERMOCALC pro - gram for the gar net–cor di er ite-bear ing migmatitic rocks 1 – 3Fcrd + 2Ame + 7Qz = 4Crd + 2Alm + 8H2O; 2 – 12Sil + Prp + 5Phl = 4Crd + 5Eas; 3 – 4Sil + 2Prp + 5Qz = 3Crd; 4 – 4Crd + 7Phl + 6Ame = 13Prp + 7Eas + 24H2O; 5 – 2Sa + 2Ann + 3Ame + 9Qz + 3Fcrd + 4Phl + 10H2O; 6 – 12Crd + 29Phl + 18Ame = 39Prp + 8Ann + 2Feas + 72H2O
DISCUSSION
We es ti mated tem per a tures for var i ous meta mor phic rocks near the gabbroic bod ies. Es ti mated tem per a tures for hornfelses are 560–630°C at pres sures of 4–5.5 kbars. Tem - per a tures ex tend up to 700–800°C for migmatitic rocks near the gabbroic plutonic rocks. These es ti mated P–T con di tions and the re sults of pre vi ous stud ies (e.g., Sepahi, 1999, 2008;
Baharifar, 2004; Sepahi et al., 2004, 2009) in di cate that the Alvand au re ole is a mesozonal deep-seated au re ole with con - di tions that are dif fer ent from those of shal low-level epizonal au re oles. It is ob vi ous that in this deep-seated con tact au re ole there is an in crease in tem per a ture to wards the ig ne ous con - tact, but the or der of meta mor phic zones have been com pli - cated due to youn ger tec tonic events in the re gion.
Our thermobarometric re sults are im por tant be cause there have been dis putes among ge ol o gists who stud ied par tial melt - ing in the Hamedan area and the re sult ing rocks such as anatectic migmatites (metatexites and diatexites) and restite-rich S-type gran ites in terms of the pos si ble source of the heat (e.g., Baharifar, 2004; Sepahi, 2008; Sepahi et al., 2009;
Saki et al., 2012). Sepahi (1999) re ported migmatites in the re - gion for the first time but he ar gued that the heat of the plutonic bod ies at the pres ent out crop level of the crust was not suf fi - cient to cause par tial melt ing in the con tact au re ole (Sepahi, 2008). This in ter pre ta tion was raised from field ob ser va tions that show that ma jor anatectic migmatites are lo cated near the gra nitic plutons (e.g., in the Simin Val ley south of Hamedan) but are ab sent near the con tact with the gabbroic rocks (e.g., near Cheshmeh-Ghassaban vil lage, west of Hamedan). Baharifar (2004) con sid ered that the heat of the gra nitic in tru sions could have been re spon si ble for anatexis in the metapelites. Here we to pos tu late the in ter pre ta tion that the heat for anatexis came from crystallising gabbroic melt be cause it seems rather un - likely that gra nitic in tru sions with an in tru sion tem per a ture be - tween 700 and 800°C would cause anatexis suf fi ciently in tense to pro duce wide spread anatectic migmatites in the Tueyserkan and ad ja cent ar eas of the Hamedan re gion.
The geo chron ol ogi cal or der of the mafic and fel sic plutonism (gabbroic in tru sions at 166.5 ± 1.8 Ma and for ma tion of S-type gran ite at 163.9 ± 0.9 Ma and 161.7 ± 0.6 Ma;
Shahbazi et al., 2010) in di cates an evo lu tion ary path sim i lar to that pro posed by Hyndman (1985) for magmatism and meta - mor phism in a con ti nen tal arc set ting. Ac cord ing to this model, hot flu ids re leased by de hy dra tion re ac tions from hy drous min - er als to gether with the heat of mafic mag mas (such as gab bros) are re spon si ble for meta mor phism, par tial melt ing and gen er a - tion of anatectic migmatites (metatexites and diatexites) and gra nitic mag mas (the S-type monzogranites of the study area) in the arc crust. In this model, the fel sic anatectic mag mas end up as high level plutons pierc ing through the meta mor phic pile of the arc en vi ron ment, and thereby post dat ing the re gional meta mor phism by a brief in ter val. Our es ti mated tem per a tures for mafic plutonic rocks (~1300°C) as pos si ble source of the heat, and the max i mum tem per a ture es ti mated for anatectic
melts (~750°C) are in ac cor dance with the model of plutonic-meta mor phic events sug gested by Hyndman (1985).
CONCLUSIONS
EPMA anal y ses of min er als from gabbroic rocks of the Alvand Pluton in di cate that ol iv ine is Fo71–72, clinopyroxene is al - most augite, am phi boles are calcic (hornblende–tremo - lite–actino lite), bi o tite is eastonite–siderophyllite and plagio - clase is an de sine–lab ra dor ite–bytownite in com po si tion. These min eral com po si tions in di cate a typ i cal sub-al ka line magma.
Crystallisation tem per a tures for these rocks were 900–1300°C.
These rocks crys tal lised at pres sures of around 5–6 kbars. In the meta mor phic rocks, cor di er ite has in ter me di ate Mg#, gar net is almandine-rich, spinel is Fe-rich (hercynitic), staurolite is Fe-rich, bi o tite is siderophyllite–annite and plagioclase is oligoclase–an de sine in com po si tion.
Our study re veals that the gabbroic in tru sions at deeper lev - els of the crust (up to 1300°C at >5 kbar equal to >15 km depth) could have acted as the es sen tial heat source for the in tense anatexis and for ma tion of the anatectic migmatites. Thus, the S-type gran ites are not con sid ered as the heat sup pli ers of the anatexis but as the fi nal prod ucts of an in tense par tial melt ing pro cess caused by sub stan tial advective heat ing by the gabbroic in tru sion. The sub se quent in tru sion of some anatectic mag mas to the up per lev els of the crust caused re heat ing in the meta mor phic rocks close by.
Ac cord ing to Sepahi (2008) and Sepahi et al. (2009) and the re sults of this study, plutonism and meta mor phism in the Hamedan re gion, es pe cially in the Tueyserkan area, oc curred in the mid dle crust (15–20 km) of a con ti nen tal mar gin which sub se quently evolved in a con ti nen tal col li sion en vi ron ment (i.e., col li sion zone of the Afro-Ara bian con ti nent and the cen tral Ira nian mi cro-con ti nent as a south ern part of the Eur asia super-con ti nent). The sig na tures of a later, slightly higher pres - sure event on pre vi ous high T-low P arc-type meta mor phism (Sepahi et al., 2004), can be at trib uted to this type of geo log i cal evo lu tion in the re gion.
Field ob ser va tions show that the ex posed vol umes of the gab bro-dioritic rocks are not ad e quate to cause wide spread migmatisation and pro duc tion of S-type gran ites in the Tueyserkan area. We there fore sug gest the ex is tence of larger vol umes of mafic in tru sions at deeper lev els, re spon si ble for anatexis and granitisation. Ac cord ing to field ob ser va tions, megablocks and en claves of such in tru sions have been trans - ported to higher lev els of the crust by restite-rich, crys tal-laden vis cous gra nitic in tru sions with their roots pos si bly lo cated in the par tial melt ing zone around mafic in tru sions at deeper lev els of the crust.
Ac knowl edge ments. We thank Prof. M. Moazzen and Prof. W. Siebel for their re views, cor rec tions and com ments on the manu script of the pa per.
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