ارتباط فابریک‏‌های دگرریختی و ذوب‏‌بخشی در میگماتیت‏‌های پلیتی منطقه‌ همدان، پهنه‌ سنندج- سیرجان

در شمال‌باختری پهنه‌ سنندج- سیرجان و در مناطق همدان و تویسرکان، رخنمون‏‌هایی از سنگ‏‌های میگماتیتی حضور دارد. در پیدایش میگماتیت‏‌های منطقه، به‌ترتیب اهمیت فرایندهای ذوب‌بخشی، تفریق دگرگونی و تزریق نقش داشته‏‌اند. میگماتیت‏‌ها دگرریختی را در بزرگ مقیاس به‌صورت لوکوسوم‏‌‏‌های چین‏‌خورده و بودین‏‌شده و در ریز‏‌مقیاس به‌صورت دگرریختی درون‏‌بلوری (خاموشی موجی، ماکل دگرریختی) و میان‏‌بلوری (مهاجرت مرز دانه، برآمدگی ...) نشان می‏‌دهند. پیدایش لایه‏‌های لوکوسوم در سنگ، پس از اوج دگرگونی و فرایند ذوب‏‌بخشی رخ می‏‌دهد. ازاین‌رو، لوکوسوم‏‌‏‌های چین‏‌خورده و بودین‏‌شده نشان‌دهنده‌ فعالیت نیروهای زمین‌ساختی در شرایط اوج دگرگونی منطقه هستند. همزمانی فرایندهای زمین‌ساختی با اوج دگرگونی، چه‌بسا میگماتیت‏‌های همزمان با زمین‌ساخت را در محدوده‌ رخساره‌ آمفیبولیت بالایی تا گرانولیت زیرین پدید آورده است؛ اگرچه عملکرد فرایندهای زمین‌ساختی پس از سرد‏‌شدن و پیدایش فابریک‏‌های پس از زمین‌ساخت، شاید در محدوده‌ رخساره‌ آمفیبولیت زیرین تا رخساره‌ شیست‌سبز روی داده باشند. میگماتیت‏‌های همزمان با زمین‌ساخت در بخش‌های مختلفی از همدان، مانند سیمین، تویسرکان، چشمه‌قصابان و دره‏‌مرادبیگ دیده می‏‌شوند. این میگماتیت‏‌ها پس از تبلور نیز دچار دگرریختی‏‌های حاکم بر منطقه شده‏‌اند. با توجه به ساختارهایی مانند انواع بودین‏‌‏‌ها، چین‏‌های میگماتیتی، میگماتیت چین‏‌خورده و رگه‏‌ها، که نقش مهمی در پیدایش حالت دینامیکی میگماتیت‏‌ها داشته‏‌اند، تکامل رئولوژیک میگماتیت‏‌های منطقه‌ همدان را می‏‌توان به سه مرحله دسته‌بندی کرد: (1) دگرریختی به کمک ذوب، (2) دگرریختی حالت جامد بلافاصله پس از تبلور و (3) دگرریختی حالت جامد بعدی.

relationship between deformation structure and partial melting in pelitic migmatites of hamaden region, sanandaj-sirjan zone

the present paper describes some field characteristics of deformation in a high-strain and high temperature zone in migmatitic rocks, with several kilometers wide occurring in hamedan and toyserkan regions, nw of the sanandaj- sirjan zone. a number of workers believe deformation and metamorphism are closely related. we tried to establish relationships between deformation evidence and rheological contrasts in different parts of migmatites during the migmatization process. in recent years, several studies have been carried out regarding the deformation and tectonic events of the various rocks of hamedan region. however, this work is the first attempt to establish the relationship between deformation structure and partial melting in the migmatitic rocks of the study region. the obtained data can be used to interpret probable relationship between deformation and metamorphism and to determine the role of rheology of the various rocks in the region.regional geologythe sanandaj-sirjan zone (ssz) comprises a metamorphic belt of low- to high-grade regional and contact metamorphic rocks intruded by mafic, intermediate and felsic plutonic bodies. plutonic rocks of the alvand complex in the hamedan region, belonging to middle jurassic (shahbazi et al., 2010; mahmoudi et al., 2011; chiu et al., 2013), consisting of gabbro-diorite-tonalite association, granite-granodiorite porphyroid, and hololeucocratic granitoids (sepahi et al., 2019).  major metamorphic rocks events in the ssz occurred 160-170 ma (e.g., sepahi et al., 2019). the metamorphic rocks of the area with different composition and metamorphic grade, are mainly pelites with small bodies of psammites, quartzites, meta-basites, calc-pelites and calc-silicates. during partial melting, some minerals such as cordierite, aluminosilicate (andalusite, sillimanite) and garnet porphyroblasts, in spite of floated, partly remained stable and when the partial melt reached its critical moving threshold, they were re-distributed in the viscous mush (sepahi et al., 2009).analytical methodsdescription of rock units, structural analyses, accurate measurements of parameters (i.e. lineation, boudins, folds, veins) were carried out during field observation. 44 thin sections parallel to lineation and perpendicular to foliation were prepared at the section preparation laboratory of bu-ali sina university, and then their petrography and microstructures were examined and analyzed. the samples are taken from metatexite dominated migmatites as well as diatexite. mineral abbreviations are taken from whitney and evans (2010).petrographyfollowing pelites the most abundant rocks of the area, slate, phyllite, pelitic schist/migmatite and hornfels are common rocks of the region under study. the migmatite rocks evolved from the hornfelses (metatexite) and schists (diatexite) of the region. the mesosome of schistic migmatites with similar mineralogy with that of the schists, crosscut by abundant granitic pegmatites, aplites, as well as quartz veins. this zone is associated with partial melting and development of granitic leucosomes in migmatites. plagioclase-bearing leucosomes are predominant, but some contain k-feldspar as well. melanosomes are less developed and resemble the mesosomes in their mineralogy and texture, except for greater amounts of mafic minerals (i.e. biotite), and smaller amounts of felsic minerals. partial melting fronts were initiated around various porphyroblasts, especially al2sio5 minerals and cordierite, and migrated to other parts of the rocks. during diatexis, garnet and aluminosilicate (andalusite, sillimanite) porphyroblasts remained partly stable but floated and were re-distributed in the viscous mush when the partial melt reached its critical moving threshold.discussion and conclusionthe essential processes of migmatites formation, are partial melting, metamorphic differentiation and injection of granitic magma. leucosome layers appear following the peak of metamorphism and partial melting process. in the course of anatexis, with increasing melt fraction, migmatite strength progressively decreases. the boudinage of high temperature metamorphic rocks is significantly controlled by the evolving rheological contrasts between the leucosome (melt) and mesosome of migmatites. the deformation observed in migmatites, occurred on a large scale in the form of folded leucosome and boudinage, and on a microscale in the form of intracrystalline (undulose extinction, deformation twinning) and intercrystalline deformation (grain boundary migration, protrusion...). therefore, the folded and boudinated leucosome represent the activity of tectonic forces in the conditions of the peak transformation of the region. the simultaneous adaptation of tectonic processes with the peak of metamorphism created synmigmatites and probably occurred in the catazone segment. while the performance of tectonic processes after cooling and creating post-tectonic fabrics, possibly happened in the mesozone to epizone. syn to post-tectonic structures have recorded the relationship between the different phases of the involved melt and deformation in the solid state in the area. synmigmatites can be seen in different parts of hamedan, such as simin, toyserkan, cheshme ghasaban and morad-beik valley. deformation occurred following the melt crystallization in the investigated migmatites. according to the structures of boudin, folds and veins, which played an important role in development of dynamic state of migmatites, the rheological evolution of hamedan migmatites (simin and toyserkan regions) can be divided into three stages: (1) deformation during melting, (2) solid state deformation immediately after crystallization, and (3) subsequent solid-state deformation.