تحولات دگرگونی شیست‌های پلیتی فشاربالا واقع در شمال‌غرب شهر نهاوند از نواردگرگونی سنندج- سیرجان، ایران

ﺷﯿﺴﺖﻫﺎی ﭘﻠﯿﺘﯽ ﮐﻪ دارای ﻣﺠﻤﻮﻋﻪ ﮐﺎﻧﯿﺎﯾﯽ ﻓﺸﺎر ﺑﺎﻻ ﻫﺴﺘﻨﺪ، در ﺷﻤﺎلﻏﺮﺑﯽ ﺷﻬﺮ ﻧﻬﺎوﻧﺪ از ﻧﻮار دﮔﺮﮔﻮﻧﯽ ﺳﻨﻨﺪج ﺳﯿﺮﺟﺎن واﻗﻊ در ﻏﺮب اﯾﺮان ﯾﺎﻓﺖ ﺷﺪﻧﺪ. ﻣﺸﺎﻫﺪات ﭘﺘﺮوﻓﺎﺑﺮﯾﮑﯽ و ﻧﺘﺎﯾﺞ ﺗﺠﺰﯾﮥ ﺷﯿﻤﯿﺎﯾﯽ ﮐﺎﻧﯽﻫﺎ ﻧﺸﺎن ﻣﯽدﻫﺪ ﮐﻪ ﺷﯿﺴﺖ ﻫﺎی ﭘﻠﯿﺘﯽ ﻣﻨﻄﻘﻪ ﻧﻬﺎوﻧﺪ دو ﻣﺮﺣﻠﻪ دﮔﺮﮔﻮﻧﯽ را ﺛﺒﺖ ﮐﺮدهاﻧﺪ. ﻣﺮﺣﻠﮥ ﻗﺪﯾﻤﯽﺗﺮ )m1( ﺑﺎ ﺑﻘﺎﯾﺎی ﭘﯿﺮوﮐﺴﻦ اﻣﻔﺎﺳﯿﺘﯽ ﺗﺒﺪﯾﻞﺷﺪه ﺑﻪ ﮐﻠﺮﯾﺖ، ادﺧﺎلﻫﺎی ﮔﻠﻮﮐﻮﻓﺎﻧﯽ و ﻣﯿﮑﺎی ﺳﻔﯿﺪ ﻓﻨﮋﯾﺘﯽ واﻗﻊ در ﻣﺮﮐﺰ ﭘﻼژﯾﻮﮐﻼز آﻟﺒﯿﺘﯽ، ﺳﺎﺧﺘﺎر ﻣﻨﻄﻘﻪﺑﻨﺪی ﺷﯿﻤﯿﺎﯾﯽ در آﻣﻔﯿﺒﻮلﻫﺎ ﺑﺎ ﻣﺮﮐﺰ وﯾﻨﭽﯿﺘﯽ و در ﻣﯿﮑﺎی ﺳﻔﯿﺪ ﺑﺎ ﻣﺮﮐﺰ ﻓﻨﮋﯾﺘﯽ ﻣﺸﺨﺺ ﻣﯽﺷﻮد .اﯾﻦ در ﺣﺎﻟﯽ اﺳﺖ ﮐﻪ در ﻣﺮﺣﻠﮥ دوم دﮔﺮﮔﻮﻧﯽ )m2( آﻣﻔﯿﺒﻮلﻫﺎی ﺗﺮﻣﻮﻟﯿﺘﯽ، ﮐﻠﺮﯾﺖﻫﺎی از ﻧﻮع ﮐﻠﯿﻨﻮﮐﻠﺮ و ﻣﯿﮑﺎی ﺳﻔﯿﺪ از ﻧﻮع ﻣﻮﺳﮑﻮوﯾﺖ ﮐﻪ ﺗﺸﮑﯿﻞدﻫﻨﺪة ﺷﯿﺴﺘﻮزﯾﺘﻪ اﺻﻠﯽ ﻫﺴﺘﻨﺪ و ﯾﺎ در ﺣﺎﺷﯿﮥ ﻓﺸﺎرﺷﯽ ﭘﻼژﯾﻮﮐﻼزﻫﺎی آﻟﺒﯿﺘﯽ دﯾﺪه ﻣﯽﺷﻮﻧﺪ، ﺷﻨﺎﺧﺘﻪ ﻣﯽﺷﻮد. ﻣﺤﺎﺳﺒﻪ و ﺗﺨﻤﯿﻦ ﺷﺮاﯾﻂ ﻓﺸﺎر-دﻣﺎی ﻣﺮاﺣﻞ دﮔﺮﮔﻮﻧﯽ m1 و m2 ﺑﺎ اﺳﺘﻔﺎده از دﻣﺎ-ﻓﺸﺎرﺳﻨﺞﻫﺎی اﺳﺘﺎﻧﺪارد و ﻧﺮماﻓﺰار ﺗﺮﻣﻮﮐﺎﻟﮏ ﻧﺸﺎن ﻣﯽدﻫﺪ ﮐﻪ ﻣﺮﺣﻠﮥ m1 در ﺷﺮاﯾﻂ ﻣﯿﺎﻧﮕﯿﻦ p=15 /5kbar و t=575 ◦c و ﻣﺮﺣﻠﮥ m2 در ﺷﺮاﯾﻂ ﻣﯿﺎﻧﮕﯿﻦ p=4/5kbar و t=600◦c ﺗﺸﮑﯿﻞ ﺷﺪهاﻧﺪ. اﻟﮕﻮی ﻓﺸﺎر- دﻣﺎی ﺣﺎﺻﻞﺷﺪه ﻧﺸﺎن ﮔﺮ ﻣﺴﯿﺮی ﺳﺎﻋﺖﮔﺮد اﺳﺖ ﮐﻪ در آن دﮔﺮﮔﻮﻧﯽ ﻓﺸﺎر ﺑﺎﻻ )ﻣﺮﺣﻠﮥ m1( ﺑﺎ ﯾﮏ دﮔﺮﮔﻮﻧﯽ ﺑﺎزﮔﺸﺘﯽ ﻓﺸﺎر ﭘﺎﯾﯿﻦ )ﻣﺮﺣﻠﮥ m2( دﻧﺒﺎل ﻣﯽﺷﻮد. اﯾﻦ اﻟﮕﻮی ﻓﺸﺎر- دما در منطقه نهاوند می تواند بر اثر فرورانش صفحه اقیانوسی نئوتتیس به زیر صفحه ایران ایجاد شده باشد.  

metamorphic evolution of high-pressure pelitic schists from northwest of nahavand in the sanandaj-sirjan metamorphic belt, iran

pelitic schists containing high-pressure mineral assemblage were found from the sanandaj-sirjan metamorphic belt in northwest of nahavand city in western iran. textural observation as well as chemical analyses of minerals shows that pelitic schists of the nahavand area recorded two metamorphic stages. the earlier stage (m1) is testified by relicts of pyroxene, glaucophane and phengite inclusions in core of albitic plagioclase and winchite and phengite cores in chemically zoned amphibole and white mica, respectively. while the second stage (m2) is documented by tremolite, chlorite and muscovite in the main schistosity and in the pressure shadow of albitic plagioclase blasts. p-t estimation of m1 and m2 metamorphic stage by using calibrated thermobarometers and thermocalc software revealed an average pressure and temperature of 15.5 kbar and 575 ◦c for the m1 stage, whereas the m2 stage has been constrained at p= 4.5 kbar and t= 600 ◦c. the obtained p-t pattern show a clockwise path in which the high-pressure metamorphic stage of m1 continued by the retrograde low-pressure metamorphic stage of m2. this p-t path could be formed during subduction of neo-tethys oceanic plate under iranian plateau.