Metasomatism and corundum formation in migmatites of Broujerd area, Sanandaj-Sirjan Zone, Iran

1-Introduction
Corundum (Al₂O₃) is one of the rare metamorphic minerals that its formation requires particular chemical condition (low SiO₂ and high Al₂O₃) in combination with high temperature (Simonet et al., 2008). Therefore, one of the most critical cases in the study of corundum-bearing rocks is the accurate determination of protolith and metamorphic conditions (Yakymchuk and Szilas, 2018).
In the Broujerd area, Berthier et al. (1974) reported the corundum in migmatites for the first time. Despite the many studies in metamorphic rocks, these corundum-bearing rocks never reported in later works, until (Ghaffari, 2010) and then  (Papi, 2015) again reported corundum-bearing rocks in the area and both concluded that they are a kind of migmatites, evidence of granulite facies metamorphism. Neither Berthier et al. (1974) nor Gaffari (2010) and Papi (2015) reported albitite rocks that are in direct relations with corundum-bearing rocks. Also differences between whole rock composition in migmatites and corundum-bearing rocks and its possible role in corundum formation, never discussed.
2-Materials and Methods
Different samples collected for petrographic studies and finally, five samples had chosen for whole rock analysis (Table 1). Bulk rock compositions of selected samples were determined using an X-ray fluorescence spectrometer at the Zarazma Co., Tehran, Iran. Chemical compositions of minerals were obtained using a JEOL W-EPMA JXA8900-R electron microprobe in the Institute of Earth Sciences, Academia Sinica, Taiwan, at an acceleration voltage of 15 kV, a current of 15 nA, and a beam size of 2 nm. (Droop, 1987). The method used for Recalculation of Fe as Fe3+ and Fe2+. All mineral abbreviations are from (Whitney and Evans, 2010).
3-General geology and field relations
Borujerd area located in Sanandaj-Sirjan Zone in the western part of Iran. Intrusive rocks, as main granitoids and rare norite, gabbro, and pegmatites, with NW – SE trend, injected along dominant schistosity in metapelites (Berthier et al., 1974), during middle Jurassic (175-170Ma; (Ahmadi-Khalaji et al., 2007; Mahmoudi et al., 2011). Contact metamorphism developed during magmatism, especially in the northern part of Broujerd batholith. Highest metamorphic grade restricted either to the northern part of granitoids or as large enclaves.
Migmatites in Ab-Bakhshan area, show one of the most significant outcrops and located between granitoids and hornfelses. Based on Fig. 1, albitites dikes injected in migmatites. In general, towards albitites, migmatites first start to metasomatized and formed metasomatized-migmatites, then corundum-bearing rocks will appear either in contact or inside albitites.
Migmatites are metatexite with the low-melt portion, generally with patchy structure. In petrographic studies, they include relatively uniform mineralogy: quartz, plagioclase and potassium feldspar are main minerals in the leucosomes with granoblastic texture, while biotite, cordierite, andalusite, sillimanites, spinel, and Fe-Ti oxides, are major minerals in mesosome, generally with lepidoblastic or grno-lepidoblastic texture. Their chemical composition (Table 1) with high Al₂O₃ and SiO₂, and low alkali and calcium show that they have the pelitic origin.
Metasomatized migmatites in the field are the same as migmatites, but under the microscope, andalusite and sillimanites start to replace by sericites, and biotites with chlorite. Near the corundum-bearing rocks (based on Figure 1) all andalusite and sillimanites completely replaced by sericites, all biotites with chlorites and also, feldspars altered. Comparing to migmatites, they have higher SiO₂ and less Al₂O₃ (Table 1).
Corundum-bearing rocks have different mineralogy. They mainly composed of corundum, chlorite, white mica (tin muscovites) and feldspar (albite), with rare rutile, ilmenite, and apatite. Albitites are mainly composed of albite (more than 80%) and quartz, sericites, potassium feldspars as minor minerals. Their chemical composition is entirely different from migmatites, with a low concentration of SiO₂ and higher Al₂O₃ and MgO (Table 1). Albitites are deformed and make dikes and small patches in migmatites. They mainly composed of albite with rare muscovite, quartz, and k-feldspar. Their chemical composition is following high–Na plagioclases (Table 1.
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. Schematic illustration of field relations between migmatites, corundum-bearing rocks and albitites (without scale)
 
 
Table 1. Major elements analysis of selected samples, based on Figure 1





Sample


BM-96.5


BM-96.8


BM-96.9


BM-96.102


BM-96.7




SiO₂


63.85


71.08


43.25


43.73


61.45




Al₂O₃


17.35


14.65


28.38


26.55


19.54




TiO₂


0.82


0.73


0.69


0.48


0.67




Fe₂O₃


7.49


3.6


4.57


6.94


2.02




MgO


2.2


1.95


8.7


8.68


2.75




MnO


0.16


0.05


0.05


0.07


 




CaO


0.64


0.8


0.54


0.92


0.51




K₂O


3.35


2.39


5.48


2.79


0.32




Na₂O


1.53


1.74


1.82


3.59


10.09




P₂O₅


0.22


0.1


0.06


0.09


0.16




LOI


2.31


2.91


6.46


6.09


2.48





 
4-Mineral chemistry
Selected mineral analysis from corundum-bearing rocks is represented in Table 2. Corundum crystals in a mica matrix, have a relatively pure chemical composition (Table 2) and their aluminum content is above 1.99 a.p.f.u, only iron is a unique element that it reaches up to 0.006 a.p.f.u. Chlorite is one of the most abundant minerals in corundum-bearing rocks, which forms a large part of the rock's matrix. Chlorites usually contain rutile and or ilmenite inclusions. Chlorite are Mg-rich (Mg/Mg+Fe from 0.75 to 0.76; Table 2). In white micas, iron and magnesium are low and close to a pure muscovite composition. Feldspars are sodic in chemical composition, in which the XAb is 0.99 (Table 2). In other words, feldspars are pure albite. Cordierite is a rare mineral between feldspars and chlorites, and due to petrographic similarities, it is difficult to detect under the microscope. They are Mg-rich (Mg/(Mg+Fe)=0.81; Table 2). Rutile as inclusions in chlorite are relatively pure, and their iron and manganese are deficient (Table 2). Ilmenite is also found in some parts in the chlorite or the rock matrix, and its chemical composition is close to the ideal ilmenite, with little impurities (Table 2).
 
Table 2. Chemical analysis of minerals in corundum-bearing rocks of Broujerd area. Only selected analyzes are provided





 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 


 




Mineral


Chl


Chl


 


Ms


Ms


 


Crn


Crn


 


Crd


Crd


 


Fsp


Fsp


 


Rt


Ilm




Point


1


2


 


1


2


 


1


2


 


1


2


 


1


2


 


 


 




SiO2


27.31


27.29


 


47.70


45.98


 


0.02


0.01


 


48.21


48.18


 


68.67


68.03


 


0.42


0.00




TiO2


0.05


0.06


 


0.09


0.00


 


0.01


0.05


 


0.00


0.00


 


0.00


0.00


 


98.78


53.97




Al2O3


21.50


22.51


 


34.86


37.72


 


99.82


99.71


 


33.04


33.12


 


19.42


19.41


 


0.25


0.00




Cr2O3


0.04


0.15


 


0.00


0.00


 


0.06


0.00


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00




Fe2O3


0.06


0.00


 


0.00


0.00


 


0.00


0.00


 


1.21


1.23


 


0.00


0.00


 


0.00


0.00




FeO


13.34


14.08


 


0.65


0.33


 


0.16


0.20


 


4.35


4.44


 


0.00


0.00


 


0.17


43.67




MnO


0.08


0.06


 


0.00


0.00


 


0.01


0.00


 


0.65


0.61


 


0.00


0.00


 


0.00


1.85




MgO


24.14


23.36


 


1.22


0.39


 


0.00


0.00


 


10.36


10.45


 


0.00


0.00


 


0.18


0.10




CaO


0.00


0.01


 


0.00


0.60


 


0.00


0.00


 


0.00


0.00


 


0.15


0.09


 


0.05


0.00




Na2O


0.04


0.00


 


0.61


1.04


 


0.00


0.00


 


0.00


0.00


 


11.23


11.58


 


0.00


0.00




K2O


0.00


0.00


 


9.76


9.22


 


0.00


0.00


 


0.00


0.00


 


0.05


0.04


 


0.00


0.00




Totals


86.56


87.52


 


94.89


95.28


 


100.08


99.97


 


97.82


98.03


 


99.52


99.15


 


99.85


99.59




Oxygens


14.00


14.00


 


11.00


11.00


 


3.00


3.00


 


18.00


18.00


 


8.00


8.00


 


2.00


3.00




Si


2.73


2.70


 


3.15


3.03


 


0.00


0.00


 


4.94


4.92


 


3.01


2.99


 


0.00


0.00




Ti


0.00


0.00


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00


 


0.99


1.02




Al


2.53


2.63


 


2.72


2.93


 


2.00


2.00


 


3.99


3.99


 


1.00


1.01


 


0.00


0.00




Cr


0.00


0.01


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00




Fe3


0.01


0.00


 


0.00


0.00


 


0.00


0.00


 


0.09


0.10


 


0.00


0.00


 


0.00


0.00




Fe2


1.12


1.17


 


0.04


0.02


 


0.00


0.00


 


0.37


0.38


 


0.00


0.00


 


0.00


0.92




Mn


0.01


0.01


 


0.00


0.00


 


0.00


0.00


 


0.06


0.05


 


0.00


0.00


 


0.00


0.04




Mg


3.60


3.45


 


0.12


0.04


 


0.00


0.00


 


1.58


1.59


 


0.00


0.00


 


0.00


0.00




Ca


0.00


0.00


 


0.00


0.04


 


0.00


0.00


 


0.00


0.00


 


0.01


0.00


 


0.00


0.00




Na


0.01


0.00


 


0.08


0.13


 


0.00


0.00


 


0.00


0.00


 


0.95


0.99


 


0.00


0.00




K


0.00


0.00


 


0.82


0.78


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00


 


0.00


0.00




Sum


10.00


9.97


 


6.93


6.96


 


2.00


2.00


 


11.02


11.03


 


4.97


4.99


 


1.00


1.98




XMg


0.76


0.75


 


 


 


 


 


 


 


0.79


0.79


 


 


 


 


 


 




Mg/(Mg+Fe)


0.76


0.75


 


 


 


 


 


 


 


0.81


0.81


 


 


 


 


 


 




Or


 


 


 


 


 


 


 


 


 


 


 


 


0.31


0.20


 


 


 




Ab


 


 


 


 


 


 


 


 


 


 


 


 


98.96


99.40


 


 


 




An


 


 


 


 


 


 


 


 


 


 


 


 


0.73


0.40


 


 


 





 
5- Phase equilibria in the corundum-bearing rocks
Phase equilibria for corundum-bearing rocks were modeled in the system Na₂O-K₂O- CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O- TiO₂ (Ti-NCKFMASH) using the THERIAK-DOMINO software v.03.01.12 (Capitani and Petrakakis, 2010) with the internally consistent thermodynamic dataset of (Holland and Powell, 1998). The activity models of (Baldwin et al., 2015) are used for feldspar, those of (White et al., 2002) for corundum, and (Coggon and Holland, 2002) for micas.  The water content is taken from the 'loss of ignition' (LOI) during XRF analyses, following (Sarkar and Schenk, 2014), although the effects of variation in the amount of water calculated. The fluid phase is assumed to be pure H₂O. The final calculated pseudo section is represented in Figure 2.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2. Calculated pseudosection for corundum-bearing rocks in Broujerd area (Sampled BM-96.102 in Table 1 and Fig. 1). Gray region is following the mineralogy of corundum-bearing rocks. Dashed red and green lines represent equal values of Mg/Mg+Fe for chlorite and cordierite, respectively and their intersection show 605 °C - 3.3 kb as T and P, based on the mineral analysis in Table 2
 
6- Conclusion
In the Ab-Bakhshan area, corundum-bearing rocks appeared as small patches, located in albitites or albitite-migmatite contact. Based on calculated pseudosection for migmatites, their composition is not suitable for corundum formation, even in high T. Using whole rock composition of corundum-bearing rocks, T and P estimated as 605 ℃ in 3.3 Kbar. Field relation between metasomatic rocks (albitites) and corundum-bearing rocks show that metasomatism was effective during corundum formation. The albitites occur most commonly in conjunction with other types of metasomatic rocks including scapolitised metagabbros and Mg-Al-rich lithologies such as orthoamphibole-cordierite schists (Engvik et al., 2014; Engvik et al., 2018). During Na metasomatism and albitite formation, Mg-Al rich fluids generated and cause chemical changes in migmatites, lead to appropriate whole rock composition for corundum formation. Na metasomatism could generate Mg-Al rich rocks, as corundum-bearing rocks of Broujerd area. So Mg-metasomatism and corundum formation either in migmatites or albitites, are consequences of Na-metasomatism in the area or are not evidence of very high T metamorphism.
 
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