متاسوماتیسم و تشکیل کرندوم در میگماتیت های منطقه بروجرد، زون سنندج – سیرجان، ایران

منطقه بروجرد در زون سنندج – سیرجان، متشکل از سنگ های دگرگونی ناحیه ای است که توسط توده های نفوذی ژوراسیک میانی تحت تاثیر قرار گرفته و دگرگونی مجاورتی در آنها ایجاد شده است. میگماتیت ها بالاترین درجه دگرگونی در هاله مجاورتی هستند. در ناحیه آب بخشان، سنگ های کرندوم دار بصورت بخش های کوچکی در داخل آلبتیتیت ها یا همبری آلبیتیت – میگماتیت دیده می شوند. سنگ های کرندوم دار شامل کرندوم، کردیریت، کلریت، آلبیت، ایلمنیت، روتیل، میکای سفید و آپاتیت هستند. بر اساس محاسبات پزودوسکشن برای سنگ کل، دما و فشار تشکیل این سنگ ها 605 درجه سانتیگراد در 3/3 کیلوبار برآورد شده است. روابط صحرایی بین سنگ های متاسوماتیک (آلبیتیت ها) و سنگ های کرندوم دار، نشان می دهد که متاسوماتیسم در طی تشکیل کرندوم موثر بوده است. در طی متاسوماتیسم na و تشکیل آلبیتیت، سیالات غنی از mgal تشکیل شده و منجر به متاسوماتیسم mg و تشکیل کرندوم در میگماتیت ها یا آلبیتیت ها شده اند.

metasomatism and corundum formation in migmatites of broujerd area, sanandaj-sirjan zone, iran

1introduction 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 corundumbearing 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 corundumbearing rocks never reported in later works, until (ghaffari, 2010) and then  (papi, 2015) again reported corundumbearing 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 corundumbearing rocks. also differences between whole rock composition in migmatites and corundumbearing rocks and its possible role in corundum formation, never discussed. 2materials 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 xray fluorescence spectrometer at the zarazma co., tehran, iran. chemical compositions of minerals were obtained using a jeol wepma jxa8900r 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). 3general geology and field relations borujerd area located in sanandajsirjan 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 (175170ma; (ahmadikhalaji 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 abbakhshan 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 metasomatizedmigmatites, then corundumbearing rocks will appear either in contact or inside albitites. migmatites are metatexite with the lowmelt 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 feti oxides, are major minerals in mesosome, generally with lepidoblastic or grnolepidoblastic 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 corundumbearing 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). corundumbearing 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 kfeldspar. their chemical composition is following high–na plagioclases (table 1.                           figure 1. schematic illustration of field relations between migmatites, corundumbearing rocks and albitites (without scale)     table 1. major elements analysis of selected samples, based on figure 1 sample bm96.5 bm96.8 bm96.9 bm96.102 bm96.7 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   4mineral chemistry selected mineral analysis from corundumbearing 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 corundumbearing rocks, which forms a large part of the rock’s matrix. chlorites usually contain rutile and or ilmenite inclusions. chlorite are mgrich (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 mgrich (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 corundumbearing 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 corundumbearing rocks phase equilibria for corundumbearing rocks were modeled in the system na2ok2o caofeomgoal2o3sio2h2o tio2 (tinckfmash) using the theriakdomino 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 corundumbearing rocks in broujerd area (sampled bm96.102 in table 1 and fig. 1). gray region is following the mineralogy of corundumbearing 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 abbakhshan area, corundumbearing rocks appeared as small patches, located in albitites or albititemigmatite contact. based on calculated pseudosection for migmatites, their composition is not suitable for corundum formation, even in high t. using whole rock composition of corundumbearing rocks, t and p estimated as 605 ℃ in 3.3 kbar. field relation between metasomatic rocks (albitites) and corundumbearing 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 mgalrich lithologies such as orthoamphibolecordierite schists (engvik et al., 2014; engvik et al., 2018). during na metasomatism and albitite formation, mgal rich fluids generated and cause chemical changes in 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