بررسی چگونگی رخداد تورمالین در مجموعه ی دگرگونی شمال گلپایگان، پهنه ی سنندج-سیرجان

رخداد تورمالین در دو منطقه در شمال گلپایگان دیده می‏‌شود. در منطقه ی نخست در باختر مزرعه ی اوچستان، تورمالین درون توده ی متبلور به‌صورت نیمه‌شکل‏‌دار (gt)، در همبریِ آمفیبول‏‌شیست به‌صورت شکل‌دار (at) و در همبریِ میکا‏‌شیست به‌صورت اسفنجی (mt) دیده می‏‌شود. در منطقه ی دوم، در شمال روستای اسفاجرد، تورمالین درون مرمر (ct) است. تورمالین‏‌های نوع gt با ترکیب شورل fe# بالای 0.93 دارند و خاستگاه آنها ماگمایی است. این تورمالین‌ها با گرانیتوییدهای فقیر از li سازگار هستند. هر دو دسته تورمالین‏‌های نوع mt و تورمالین‏‌های نوع at به‌ترتیب با ترکیب دراویت و شورل- دراویت، خاستگاه گرمابی و سازگار با سنگ‏‌های متاپسامیت‏‌ها و متاپلیت‏‌های فقیر از ca دارند. تورمالین‏‌های نوع ct ترکیب دراویت دارند و با fe# کمتر از 0.6 با متاپسامیت‏‌ها و متاپلیت‏‌های فقیر از ca سازگاری دارند و با یک فاز اشباع از al همزیست هستند. حضور تورمالین نوع ct در کنار بیوتیت‏‌هایی با آثار خوردگی و حضور کانی های ثانویه در سنگ و همچنین، تفاوت در دماسنجی، نشان‌دهنده ی پیدایش تورمالین در دمای کمتر در اثر واکنش سیال‌های بور‏‌دار با بیوتیت است. در پیدایش این تورمالین‏‌ها سیال‌های ناشی از دگرگونی پیشرونده ی میکاشیست‏‌ها و سیال‌های تاخیری- گرمابی برخاسته از ماگمای آلکالی‌سینیت نقش داشته‌اند.

investigation on tourmaline occurrencein the metamorphic complex in the north of golpayegan, sanandaj-sirjan zone

the occurrence of tourmaline in the metamorphic complex of the north of golpayegan is observed within the granitoid mass and schists in contact with granitoid and marbles.speaking of abundance, tourmaline is less frequently a minor mineral found in granites and granitic pegmatites as well as in low- to high-grade metamorphic rocks (krmícek et al., 2020). the minerals of the tourmaline group can adjust their composition to adapt to varied settings, and therefore show a remarkable stability range in terms of pressure, temperature, fluid composition, and host rock composition (van hinsberg et al., 2011). since tourmaline displays a negligible intra-crystalline diffusion, it can record the physical and chemical conditions of its setting and preserve this information in geological chronicles. therefore, tourmaline accurately presents the composition of fluids and the melts from which it crystallized (marks et al., 2013). although tourmaline is common in many rocks, it is not common in metamorphosed carbonates (krmícek et al., 2020).a petrographical and geochemical investigation of tourmalines in the north of golpayegan was undertaken to know the formation mechanism of this mineral and to determine the differences between tourmalines crystallized at the contact, in granitoid and tourmaline in marbles.regional geologythe golpayegan metamorphic complex is located in the sanandaj-sirjan zone and the occurrence of tourmaline in this complex is observed in two locations. the first location is on the west side of ochestan farm where tourmaline is found in three forms:1) in the granitoid mass (gt); 2) in the amphibole schists (at) in contact with the granitoid mass, and 3) in the mica schists (mt) in contact with the granitoid mass. the second area is in the north of esfajerd where tourmaline occurs inside the marbles (ct).methodologywith the completion of field reconnaissance and preparation of thin sections, a petrographic study was fulfilled to determine the texture and mineralogy of the minerals, and then, some samples were selected for electron microprobe analysis.petrographythe tourmaline in granitoid mass (gt), appears as idiomorphic and coarse-grained without any inclusions. micaschists (mt) in contact with the granitoid mass are sieve-shaped or spongy. inside the amphibole schists (at) in contact with the granitoid mass, it is idiomorphic without any inclusions.tourmaline in marbles (ct) is fine-grained and blue, which can be observed around biotites with corrosion marks, indicating its reaction with fluid.tourmaline chemistrythe tourmalines in ochestan granite-pegmatite (gt) are of alkali tourmaline variety with schorl composition, enriched in aluminum, and points to the replacement of al in the y (r2) position. the substitution type of these tourmalines is al(nafe+2)-1 owing to the high amount of fe versus mg,the tourmalines in amphibole schist (at) are alkali tourmaline with schorl-dravite composition and the tourmalines in mica schist (mt) are alkali tourmaline with dravite composition, and both types are characterized by insignificant amounts of aluminum. this demonstrates the replacement of al in the position of y (r2) has not occurred. due to the change in mg and fe content, the at-type tourmalines benefit from both al(nafe+2)-1 and al(namg)-1 substitution varieties. however, the substitutions of mt-type tourmalines are mainly al(namg)-1 due to the high content of mg versus fe. indeed, fe+3al-1 substitution can be observed in both types of tourmaline.tourmalines in marble (ct) are of the alkaline type of dravite composition and rich in aluminum, which indicates the replacement of al in the y (r2) position. their substitution is al(namg)-1 due to the high content of mg versus fe.discussionthe composition of tourmaline in gt is of schorl type (fe/fe+mg= 0.89-0.91) and has an aluminum replacement in the y position. the composition of tourmaline in mt type is of dravite