مطالعه شرايط تشكيل و پتانسيل گوهرشناسي كرديريت، كيانيت و آندالوزيت‌هاي موجود در هاله دگرگوني كمپلكس الوند، همدان، ايران

Study of the formation conditions and gemology potential of cordierite, kyanite and andalusites in the alteration halo of Alvand Batholith, Hamedan, Iran

ﻣﻨﻄﻘﻪ ﻣﻮرد ﻣﻄﺎﻟﻌﻪ در ﻫﺎﻟﻪ دﮔﺮﮔﻮﻧﯽ ﺑﺎﺗﻮﻟﯿﺖ اﻟﻮﻧﺪ، اﺳﺘﺎن ﻫﻤﺪان، واﻗﻊ ﺷﺪه اﺳﺖ. ﮐﺮدﯾﺮﯾﺖﻫﺎي ﻣﻮﺟﻮد درﻣﺰوﺳﻮم ﻣﯿﮕﻤﺎﺗﯿﺖﻫﺎي ﻣﻨﻄﻘﻪ ﺑﺴﯿﺎر درﺷﺖ ﺑﻠﻮر ﻣﯽﺑﺎﺷﻨﺪ و ﻫﻤﺮاه ﺑﺎ اﺳﭙﯿﻨﻞ )ﻟﻌﻞ( ﻫﺴﺘﻨﺪﮐﻪ ﺑﺎﻻ رﻓﺘﻦ دﻣﺎي دﮔﺮﮔﻮﻧﯽ و ﭘﺪﯾﺪة ذوب ﺑﺨﺸﯽ ﺳﺒﺐ رﺷﺪ آﻧﻬﺎ ﺷﺪه اﺳﺖ. ﻣﻘﺪار ﮐﺴﺮ ﻣﻮﻟﯽ ﻣﻨﯿﺰﯾﻢ )xMg( ﮐﺮدﯾﺮﯾﺖ ﺑﺎ اﻓﺰاﯾﺶ درﺟﻪ دﮔﺮﮔﻮﻧﯽ از زون ﻓﻠﺪﺳﭙﺎر ﭘﺘﺎﺳﯿﻢ ﮐﺮدﯾﺮﯾﺖ ﺗﺎ اﺳﭙﯿﻨﻞ ﮐﺮدﯾﺮﯾﺖ اﻓﺰاﯾﺶ ﻣﯽﯾﺎﺑﺪ ) 0/45 ﺗﺎ 0/62(. دﻣﺎ و ﻓﺸﺎر ﺗﺸﮑﯿﻞ ﮐﺮدﯾﺮﯾﺖﻫﺎ 750 درﺟﻪ ﺳﺎﻧﺘﯿﮕﺮاد و ﻓﺸﺎر آن ﻣﻌﺎدل 4 ﮐﯿﻠﻮﺑﺎر ﻣﯽﺑﺎﺷﺪ. ﮐﯿﺎﻧﯿﺖ ﺑﻪ ﻃﻮر ﻣﺸﺨﺺ ﺧﺎص رﮔﻪﻫﺎي ﮐﻮارﺗﺰي در ﻣﻨﻄﻘﻪ اﺳﺖ، اﻣﺎ در ﺳﻨﮓﻫﺎي دﮔﺮﮔﻮﻧﯽ ﻣﺠﺎورﺗﯽ و در رﮔﻪﻫﺎي ﻣﺠﺎور ﻧﯿﺰ دﯾﺪه ﻣﯽﺷﻮد. وﺟﻮد رﮔﻪﻫﺎي ﮐﯿﺎﻧﯿﺖ دار، ﻧﺸﺎن ﻣﯽدﻫﺪ ﮐﻪ ﮐﯿﺎﻧﯿﺖ ﻣﺤﺪود ﺑﻪ رﺧﺴﺎره ﻓﺸﺎر ﺑﺎﻻ ﻧﺒﻮده ﺑﻠﮑﻪ در اﺛﺮ ﻓﺸﺎرﻫﺎي ﻫﯿﺪروﺗﺮﻣﺎﻟﯽ ﺷﮑﻞ ﮔﺮﻓﺘﻪ اﺳﺖ. ﮐﯿﺎﺳﺘﻮﻟﯿﺖﻫﺎﯾﯽ ﺑﺎ ﻃﻮل 20ﺳﺎﻧﺘﯽ ﻣﺘﺮ و ﻋﺮض 4 ﺳﺎﻧﺘﯽ ﻣﺘﺮ در ﻣﻨﻄﻘﻪ ﺑﻮﺟﻮد آﻣﺪه اﻧﺪ ﮐﻪ ﮔﺴﺘﺮس اﺻﻠﯽ ﺷﯿﺴﺖﻫﺎي ﮐﯿﺎﺳﺘﻮﻟﯿﺖ دار در ﺣﻮاﻟﯽ زﻣﺎن آﺑﺎد اﺳﺖ، دﻣﺎ و ﻓﺸﺎر ﺗﺸﮑﯿﻞ اﯾﻦ ﺳﻨﮓﻫﺎ ﺑﺎ اﺳﺘﻔﺎده از ﺑﺮﻧﺎﻣﻪ ﺗﺮﻣﻮﮐﺎﻟﮏ ﺑﻪ ﺗﺮﺗﯿﺐ 628 درﺟﻪ ﺳﺎﻧﺘﯿﮕﺮاد و ﻓﺸﺎر در ﺣﺪود 4 ﮐﯿﻠﻮﺑﺎر ﻣﯽﺑﺎﺷﺪ. ﺑﺮ اﺳﺎس ﻣﻄﺎﻟﻌﺎت ﮔﻮﻫﺮ ﺷﻨﺎﺳﯽ اﻧﺠﺎم ﺷﺪه، ﮐﺮدﯾﺮﯾﺖﻫﺎ ﺑﺮاي ﺗﺮاشﻫﺎي ﮔﻨﺒﺪي و ﻓﺎﻧﺘﺰي ﻣﻨﺎﺳﺐ ﻫﺴﺘﻨﺪ، ﺳﻄﺢ ﺑﺎﻻي ﺷﺪت دﮔﺮﮔﻮﻧﯽ ﻣﺠﺎورﺗﯽ ﺳﺒﺐ ﺷﺪه اﯾﻦ ﮐﺎﻧﯽﻫﺎ در ﻃﯽ ﺑﺮش و ﺗﺮاش ﻣﻘﺎوم ﺑﺎﺷﻨﺪ. ﻫﻤﭽﻨﯿﻦ ﮐﯿﺎﻧﯿﺖﻫﺎ ﻣﻨﺎﺳﺐ ﺗﺮاش ﺿﻠﻊدار، ﮔﻨﺒﺪي، ﻓﺎﻧﺘﺰي ﻫﺴﺘﻨﺪ ﺑﺨﺎﻃﺮ اﯾﻨﮑﻪ ﻣﺎﻫﯿﺖ ﺷﻔﺎف دارﻧﺪ و در آﺧﺮ، ﮐﯿﺎﺳﺘﻮﻟﯿﺖﻫﺎ ﻣﯿﺰان ﺷﻔﺎﻓﯿﺖ، ﻧﯿﻤﻪ ﺷﻔﺎف ﺗﺎ اﭘﮏ را دارﻧﺪ و ﺑﺼﻮرت ﮐﺎﺑﻮﺷﻦ ﺑﺎﯾﺪ ﺗﺮاش داده ﺷﻮﻧﺪ. در ﻧﻬﺎﯾﺖ ﻧﻘﺸﻪ ﮔﻮﻫﺮ ﺷﻨﺎﺳﯽ ﻣﻨﻄﻘﻪ ﻫﻤﺪان ﺑﺮاي ﺑﺨﺶ ﻣﻮرد ﻣﻄﺎﻟﻌﻪ ﺗﻬﯿﻪ و ﺑﺮاي اوﻟﯿﻦ ﺑﺎر ﭘﺸﻨﻬﺎد ﺷﺪه اﺳﺖ

The study area is located in the metamorphic aureole of Alvand Complex Hamedan province. Structurally, the study area is placed in the Sanandaj-Sirjan metamorphic zone. The Zagros formed during the subduction of a Neo-Tethyan seaway and subsequent oblique collision of Afro-Arabia (Gondwana) with the Iranian microcontinent in the Late Cretaceous–early Tertiary (Alavi, 1994; Mohajjel and Fergusson, 2000). Crustal shortening associated with subduction and collision metamorphosed and deformed Late Palaeozoic and Mesozoic sedimentary rocks, including a sequence of pelitic, psammitic, mafic, calc-pelitic, and calc-silicate rocks near Hamadan. The tectonic evolution of the Sanandaj–Sirjan belt involved continental arc magmatism followed by collision. Mafic to intermediate plutonic bodies (olivine gabbro, gabbro, gabbro-norite, diorite, quartz diorite, and tonalite) are older than crustal derived granitic plutons in the region (Alvand Plutonic Complex), but all intrusions formed during Cretaceous-Tertiary subduction and collision (Baharifar et al., 2004). All metamorphic rocks in the area were regionally metamorphosed before the contact metamorphism up to greenschist to amphibolite facies underwent contact metamorphism due to the emplacement of the Alvand Complex. In the Hamadan metamorphic sequence, cordierite and K-feldspar, are only found near (<3 km from) the Alvand granite. The general trend of increasing metamorphic grade is towards the pluton, with a succession of isograds in metapelitic rocks: biotite-in, garnet-in, andalusite-in, staurolite-in, cordierite-in, K-feldspar-in and spinel-in. In some parts of the field area, the complete sequence occurs, but in other areas, some of the lower grade zones are missing. 2-Methodology In the selected sample, major element compositions of biotite, garnet, spinel, plagioclase, cordierite, and ilmenite were determined by wavelength-dispersive spectrometry using a Cameca SX100 microprobe. The operational conditions were 15 kV, 10-20 nA specimen current. The analytical spot diameter was set between 3 and 5 μm, keeping the same current conditions — natural and synthetic standards used for calibration. 3-Result and discussion The cordierites in the mesosome of the migmatites have coarse crystals and are accompanied by spinel (lal), which has formed due to the increasing of metamorphism temperature and partial melting processes. The molar fraction (Xmg) of cordierite increases with the increasing degree of metamorphism from the zone of K-feldspar-cordierite to spinel- cordierite (0.45 to .620). The approximate temperature for the cordierites in the mesosome of the migmatites is estimated to be 750 ° C, and the pressure is ~ 4 kb. Kyanite is specifically hosted by quartz veins in the region, also is found in the adjacent of metamorphosed rocks and in the vicinity of veins