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

Document Type : Research Article

Authors

• Ahmad Ahmadi Khalaji
• Zahra Tahmasbi

Lorestan

10.22067/econg.v7i2.31041

Abstract

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. Garnet-bearing 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 samples from igneous and metamorphic rocks were analyzed by electron microprobe (EMPA), the results of which are presented in this article.

Materials and methods
Thirty-five samples were selected for thin section preparation and twenty thin-polished sections were prepared for mineralogical and microprobe analysis. Thin sections of garnet-bearing 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 end-members 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 well-known 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: co-existing 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.

Discussion
The 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 (Sanandaj-Sirjan zone, western Iran): metasedimentary Mesozoic sequences affected by Late Cretaceous tectono-metamorphic and plutonic events. Comptes Rendus Geoscience, 336(16): 1443-1452.

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): 431-435.

Keywords

• Hamedan
• metamorphic
• garnet
• zoning

20.1001.1.20087306.1394.7.2.4.8

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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.

Atherton, M.P., 1968. The variety in garnet biotite and chlorite composite in medium grade pelitic rocks from the Dalradian, Scotland, with particular reference to zonation in garnet. Contribution to Mineralogy and Petrology, 18(4): 347-371.

Atherton, M.P. and Edmunds, W.M., 1966. An electron microprobe study of some zoned garnets from metamorphic rocks. Earth and Planetary of Science Letters, 1(4): 185-193.

Baharifar, A. A., 1997. New perspective on petrogenesis of the regional metamorphic rocks of Hamedan area, Iran. M.Sc. Thesis, Tarbiat Moallem University of Tehran, Tehran, Iran, 170 pp. (in Persian with English abstract)

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 (Sanandaj-Sirjan zone, western Iran): metasedimentary Mesozoic sequences affected by Late Cretaceous tectono-metamorphic and plutonic events. Comptes Rendus Geoscience, 336(16): 1443-1452.

Baldwin, J.R. and Konrring, O.V., 1983. Compositional range of Mn-garnet in zoned granitic pegmatites. Canadian Mineralogist, 21(4): 683-688.

Blackburn, W.H., 1969. Zoned and unzoned garnets from the Grenville Gneisses around Gananorue Ontario. Canadian Mineralogist, 9(5): 691-698.

Borzoei, K., 2001. The study of contact metamorphic rocks in the Sarabi area (Tuiserkan). M.Sc thesis, Bu Ali Sina University, Hamadan, Iran, 118 pp. (in Persian with English abstract)

Carlson, W.D., 1989. The significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization. Contributions to Mineralogy and Petrology, 103(1): 1-24.

Deer, W.A., Howie, R.A. and Sussman, J., 1981. An interdiction to the rock forming minerals. Longman group limited, London, 528 pp.

Deer, W.A., Howie, R.A. and Sussman, J., 1997. Rock-forming minerals, volume 1A: Orthosilicates. Geological Society, London, 919 pp.

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): 431-435.

Einaudi, M.T., Burt, D.M., 1982. Introduction terminology, classification and composition of skarn deposits. Economic Geology, 77(4): 745-754.

Evans, B.W., 1966. Microprobe study of zoning in eclogite. Geological Society of America, Special paper, 87: 54-60.

Fu-yuan, W., Sun, D. and Wilde, S., 2004. A Jurassic garnet-bearing granitic pluton from NE China showing tetrad patterns. Journal of Asian Earth Science, 23(5): 731-744.

Grant, J.A. and Weiblen, P.W., 1971. Retrograde zoning in garnet near the end sillimanite isograde. Ameriacan Journal of Science, 270(4): 281-296.

Green, T.H., 1976. Experimental generation of cordierite or garnet bearing granitic liquids from a pelitic composition. Geology, 4(2): 85-88.

Green, T.H.H., 1977. Garnet in liquid silicic liquids and its possible use as a P-T indicator. Contributions to Mineralogy and Petrology, 65(1): 59-67.

Green, T.H. and Ringwood, A.E., 1968. Origin of garnet phenocrysts in calc-alkaline rocks. Contributions to Mineralogy and Petrology, 18(2): 163-174.

Harrison, T.N., 1988. Magmatic garnets in the Cairngorm granite. Mineralogical Magazine, 52(4): 659-670.

Hollister, L.S., 1996. Garnet zoning an interpretation based on the Rayleigh fractionation model. Science, 154(3757): 1647-1651.

Huckenholz, H.G. and Fehr, K.T., 1982. Stability relationship of grossolar+ quartz + wollastonite + anorthite II, the effect of grandite+hydrograndite solid solution. Neues Jahrbuch Fur Mineralogie abhandlungen, 145(1): 1-33.

Hwang, S.L., Shen, P., Yui, T.F. and Chu, H.T., 2003. On the mechanism of resorption zoning in metamorphic garnet. Journal of Metamorphic Geology, 21(8): 761-769.

Hwang, S.L., Yui, T.F., Chu, H.T. and Shen, P., 2001. Submicron polyphase inclusions in garnet from the Tananao metamorphic complex, Taiwan, a key to unraveling otherwise unrecognized metamorphic events. Journal of Metamprphic Geology, 19(5): 601-607.

Jamtveit, B., 1991. Oscillatory zonation patterns in hydrothermal grossular-andradite garnet, nonlinear behavior in regions of immiscibility. American Mineralogist, 76(7-8): 1319-1327.

Jamtveit, B., Wogelius, R.A. and Fraser, D.G., 1993. Zonation patterns of skarn garnets, Records of hydrothermal system evolution. Geology, 21(2): 113-116.

Kertz, R., 1983. Symbols for rock-forming minerals. American Mineralogist, 68(1-2): 277-279.

Kistler, R.W., Ghent, E.D. and O’Neil, J.R., 1981. Petrogenesis of garnet two-mica granites of northeastern Nevada. Journal of Geophysical Research, 86(B11): 10591-10606.

London, D., 2008. Pegmatites. Canadian Mineralogist, Special Publication, 10: 347 pp.

Masoudi, F., Mehrabi, B. and Farazdel, F., 2005. Type of Garnet Zoning in Skarns of Ghohroud Intrusion, south of Kashan. Iranian Journal of Crystallography and Mineralogy, 13(1), 43-60. (in Persian with English abstract)

Miller, C.F. and Stoddard, E.F., 1981. The role of manganese in the paragenesis of magmatic garnet: An example from the Old Woman- Piute Range, California. Journal of Geology, 89(2): 233-246.

Miyashiro, A., 1955. Pyrlaspite garnets in volcanic rock. Geological society of Japan, 61(721): 363-470.

Owen, J.V. and Marr, R.A., 1990. Contrasting garnet parageneses in a composite Grenvillian granitoid pluton, Newfoundland. Mineralogical Magazine, 54(3): 367-380.

Plank, T., 1987. Magmatic garnets from the Cardigan pluton and the Acadian thermal event in southwest New Hampshire. American Mineralogist, 72(7-8): 681-688.

Preston, J. and Still, J., 2001. Mineral chemistry Spreadsheet V 15. Electronic internet publication, www.earth.ox.ac.uk/~davewa/pt/tools/formula.xls

Sepahi, A.A., 2007. A detailed study of morphology and chemistry of garnet crystals with suggestion of new subdivisions: data from pelitic schists, hornfelses and aplites of Hamadan region. Iranian Journal of Science and Technology, 31(A3): 281-289.

Sepahvand, F., 2013. Mineralogical and geochemical comparison of Alvand and Samen granitoid complexes. M.Sc thesis, Bu Ali Sina University, Hamadan, Iran, 178 pp. (in Persian with English abstract)

Shabani, Z., 2012. Investigation of geochemical characteristics tourmalines in the Alvand with a view of the boron in the economic geology cource. M.Sc thesis, Islamic Azad University, Khorramabad branch, Iran, 98 pp. (in Persian with English abstract)

Shahbazi, H., Siebel, W., Pourmoafee, M., Ghorbani, M., Sepahi, A.A., Shang, C.K. and Vousoughi Abedini, M., 2010. Geochemistry and U–Pb zircon geochronology of the Alvand plutonic complex in Sanandaj–Sirjan Zone (Iran): new evidence for Jurassic magmatism. Journal of Asian Earth Sciences, 39(6): 668–683.

Torkian, A., 1995. The Study of petrography and petrology of Alvand pegmatites (Hamadan). M.Sc thesis, Tehran University, Tehran, Iran, 172 pp. (in Persian with English abstract)

Tracy, R.J., 1982. Compositional zoning and inclusions in metamorphic mineral. Reviews in Mineralogy and Geochemistry, 10(1): 355-397.

Tracy, R.J., Miller, S.J., Solberg, T.N. and Waldron, K.A., 1992. Documentation of diffusional modification of garnet growth zoning in am-phibolite facies garnets. Geological Society of America Abstracts with Programs, 24: 219.

Vance, D. and Holland, T., 1993. A detailed isotopic and petrological study of single garnet from the Gastts schist, Vermont. Contribution to Mineralogy and Petrology, 114(1): 101-118.

Webber, K., Simmons, W., Falster, A. and Foord, E., 1999. Cooling rates and crystallization dynamics of shallow level pegmatites-aplite dikes, San Diego country. American Mineralogist, 84(5-6): 708-717.

Whithworth, M.P., 1992. Petrogenetic implications of garnets associated with lithium pegmatites from SE Ireland. Mineralogical Magazine, 56(1): 75-83.

Yardley, W.D., 1981. An interdiction to metamorphic petrology. Longman Scientific and Technical, London, 288 pp.

Yardley, B.W.D., Rochelle, C.A., Barnicoat, A.C. and Lioyd, G.E., 1991. Oscillatory zoning in metamorphic minerals, An indicator of infiltration metasomatism. Mineralogical Magazine, 55(3): 357-365.