شیمی کانی گارنت در پگماتیت ها و سنگهای دگرگونی منطقه همدان

محدوده همدان به‌طور عمده متشکل از سنگهای دگرگونی و پلوتونیک مزوزوئیک است. در این منطقه از غرب به شرق به‌ترتیب واحدهای متمایزی از توده‌های نفوذی، سنگهای دگرگونی مجاورتی و رگه‌های پگماتیتی و آپلیتی رخنمون دارند که در برخی مناطق این سنگها حاوی گارنت می‌باشند. بر اساس مطالعات انجام شده ترکیب اصلی گارنت در شیست‌های کمری و پگماتیت‌های ابرو از نوع آلماندین (گارنت غنی از آهن) و به مقدار کمتر اسپسارتین (گارنت غنی ازمنگنز) است که این دو نوع گارنت به گروه پیرالسپیت معروفند. الگوی منطقه‌بندی گارنت در سنگهای دگرگونی (شیست‌ها و هورنفلس‌ها) با سنگهای آذرین (پگماتیت) تغییرات متفاوتی از مرکز به حاشیه نشان می‌دهد. به‌طوری که ترکیب گارنت به‌ترتیب از مرکز به حاشیه در شیست‌ها از alm0.63, prp0.07, sps0.24, grs0.05 تا alm0.71, prp0.09, sps0.13, grs0.05در هورنفلس‌ها از alm 0.79, prp0.14, sps 0.06, grs0.07تا alm0.8, prp0.13, sps 0.05, grs0.01 و در پگماتیت‌ها، از alm0.73, prp0.015, sps0.24, grs0.07 تا alm0.7, prp0.011, sps0.28, grs0.00 تغییر می‌کند. شواهد پتروگرافی نظیر همراهی تورمالین با گارنت پگماتیت‌ها و آندالوزیت با گارنت سنگهای دگرگونی مجاورتی و شیمی‌کانی گارنت (افزایش mn در هسته گارنت شیست‌ها و کاهش آن در هسته گارنت پگماتیت، کاهش عنصر mg در مرکز گارنت پگماتیت و افزایش آن در هسته گارنت شیست‌ها، خطی بودن ca, al از حاشیه به مرکز در هورنفلس‌ها) و تقریباً 10 درصد گارنت پیروپ حاکی از منشا گرفتن گارنت موجود در شیست‌ها و هورنفلس‌ها از یک سیستم بسته (دگرگونی مجاورتی) و ماگمایی بودن آن در پگماتیت‌هاست.

Mineral chemistry of garnet in pegmatite and metamorphic rocks in the Hamedan area

Introduction The area of this study is located near Hamadan within the Sanandaj Sirjan tectonic zone. In the Hamadan area, consisting mainly of Mesozoic plutonic and metamorphic rocks, aplites and pegmatites locally contain garnets.(Baharifar et al., 2004, Amidi and Majidi, 1977 Torkian, 1995. Garnetbearing schists and hornfelses in the area are products of regional metamorphism shown by slate and phyllite (Baharifar, 2004). In this investigation the distribution of elements in garnet in different rock type was studied to determine their mineral types and conditions of formation. Garnet sles from igneous and metamorphic rocks were analyzed by electron microprobe (EMPA), the results of which are presented in this article. Materials and methods Thirtyfive sles were selected for thin section preparation and twenty thinpolished sections were prepared for mineralogical and microprobe analysis. Thin sections of garnetbearing igneous (pegmatite) and metamorphic rocks (schist and hornfels) were studied by polarizing microscope. Chemical analysis was performed on the garnets (38 points) using a Caimeca SX100 electron microprobe at an acceleration voltage of 15 kV and electric current of 15 nA in the Mineral Processing Research Center, Iran. Separation of iron (II) and Fe (III) was calculated by Droop’s method (1987) and the structural formulas of the garnets were calculated using 24 oxygens to determine the relative proportions of the endmembers using the mineral spreadsheet software of Preston and Still (2001).Results Based on the analyses, almandine (Fe Al garnet) and spessartine (Mn Al garnet) are the principal types of the (Kamari) metamorphic and (Abaro) pegmatitic garnets, that belong to the wellknown pyralspite garnet group. Chemical zoning patterns of the garnets in the metamorphic rocks (schists) differ from those in the igneous rocks (pegmatite), showing different compositions from core to rim. Petrographic evidence such as: coexisting tourmaline with pegmatite garnets and andalusite with schist garnets zoning in garnets (oscillatory zoning of Al in pegmatite garnet, Mn increasing in the cores of schist garnet contrasted with Mn decreasing in the cores of pegmatite garnets the decrease of Mg in the cores of pegmatite garnets, contrasted with the increase of this element in the cores of schist garnets and the linear trends of Al and Ca in hornfels garnets) Pyrope garnet composition in schist indicates a closed system for garnet formation condition in schist and a magmatic source for pegmatites. The compositions of garnets from schists change from Alm0.63, Prp0.07, Sps0.24, Grs0.05 in the cores, to Alm0.71, Prp0.09, Sps0.13, Grs0.05 in the rims. Garnets from pegmatites show a change from Alm0.73, Prp0.015, Sps0.24, Grs0.07 in the cores, to Alm0.71, Prp0.011, Sps0.28, Grs0.00 in the rims. Garnets from hornfelses showed changes from Alm 0.79, Prp0.14, Sps 0.06, Grs0.07 in the cores to Alm 0.8, Prp0.13, Sps 0.05, Grs0.01 in the rims. DiscussionThe percent of almandine and spessartine in the garnets of the schists and pegmatites are higher than that of garnets in the hornfelses. Almandine and spessartine in the pegmatite garnets from core to rim show a completely reversed trend. In the schist garnets from core to rim, the almandine trend is decreasing outward– increasing inward, while the spessartine trend is increasing – decreasing. In the hornfels garnets no specific trend could be determined, there is no zoning. This difference in trend between pegmatite garnets from that in schist garnets and hornfels garnets shows differences in their origin. Texture homogenization, rich in potassium, metaluminous to peraluminous magma of Hamedan granitoid intrusion (Aliani et al., 2012), Peraluminous biotite in this intrusion and lack of garnet zoning show that garnet pegmatites have been formed directly from granitic melt crystallization.References Aliani, F., Maanijou, M., Sabouri, Z. and Sepahi, A.A., 2012. Petrology, geochemistry and geotectonic enviroment of the Alvand Intrusive complex, Hamedan, Iran. Chemie der Erde Geochemistry, 72(4): 363–383.Amidi, M. and Majidi, B., 1977. Geological Map of Hamadan, scale 1: 250,000. Geological Survey of Iran. Baharifar, A.A. 2004. Petrology of metamorphic rocks in the Hamedan area, Ph.D. Thesis, Tarbiat Moallem University, Tehran, Tehran, Iran, 218 pp. (in Persian with English abstract)Baharifar, A., Moinevaziri, H., Bellon, H. and Pique, A., 2004. The crystalline complexes of Hamadan (SanandajSirjan zone, western Iran): metasedimentary Mesozoic sequences affected by Late Cretaceous tectonometamorphic and plutonic events. Comptes Rendus Geoscience, 336(16): 14431452.Droop, G.T.R., 1987. A general equation for estimating Fe3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria. Mineralogical Magazine, 51(4): 431435.