Mineral chemistry and geothemobarometry of mantle harzburgites in the Eastern Metamorphic Complex of Khoy ophiolite -NW Iran

Document Type : Research Article

Author

• Morovvat Faridazad

Sahand University of Technology

10.22067/econg.v8i2.37949

Abstract

Introduction
Khoy ophiolite at the global scale is in the middle part of the Alp-Himalaya orogenic belt and it is extended over 3900 Km2 which indicates remnant Neotethys oceanic lithosphere in the Mesozoic era (Kananian et al., 2010). In this paper, in addition to a review of previous investigations about Khoy ophiolite, we will try to determine the nature and kind of minerals, origin and partial melting rate as well as the equilibrium pressure and temperature of harzburgites from the Eastern Metamorphic Complex of Khoy ophiolite.

Materials and methods
Thin sections microscopy studies were carried out following field investigations. EPMA analysis was carried out with using a Superprobe JEOL, JXA 8200 Microprobe unit at the state of WDS and under condition of 15kv accelerating voltage, 10nA current beam, 1µm beam diameter and collection of natural and synthetic standards for calibration.

Results
The study area is located at the NW of Iran and north of the Khoy city in the west Azarbaijan province. This area is part of the ophiolitic complex of NW Iran and belongs to its Eastern Metamorphic Complex. This metamorphic zone has large tectonically segments of the metamorphic ophiolites which mainly include serpentinized peridotites with associated metagabbros. There are three types of peridotitic rocks in this area which are: Lherzolites, harzburgites and dunites. Lherzolites are composed of olivine (60-70%), orthopyroxene (10-30%) and clinopyroxene (~10-20%) with minor amounts (~2%) of Cr-spinel mineral. Harzburgites are composed of olivine (70-80%), orthopyroxene (10-20%) and clinopyroxene (~5%) with minor amounts (~2%) of Cr-spinel mineral. Dunites are composed of olivine (90-95%), orthopyroxene (5-10%) with minor amounts (~1-2%) of Cr-spinel mineral. Composition range of olivines is between Fo89.46 Fa10.37 to Fo89.86 Fa10.0 as well as NiO content range is 018-046 (wt %). The calculated Mg# of olivines is 0.90 and the composition of olivines in Fo-Fa diagram is plotted in forsterite field. The end-members composition of clinopyroxenes is between En44.159 Wo46.910 Fs4.323 Ac1.459 to En46.803 Wo49.589 Fs4.786 Ac2.081. The calculated Mg# of clinopyroxenes is 0.91. The composition of clinopyroxenes in En-Wo-Fs (Morimoto et al., 1988) diagram is plotted on a diopside field. The plot of clinopyroxenes on Na versus Cr diagram (Kornprobst et al., 1981) indicates that the studied harzburgits are formed in the oceanic setting. Moreover, the Mg# versus Al2O3 in the clinopyroxenes is an indication of their relation to the Abyssal peridotites (Johnson et al., 1990). The end-members composition of orthopyroxenes is between En86.022 Wo2.491 Fs9.368 to En87.314 Wo6.719 Fs10.474. The calculated Mg# of orthopyroxenes is 0.90. The composition of orthopyroxenes in En-Wo-Fs diagram is plotted on an enstatite field. The plot of orthopyroxenes on Mg# versus Al2O3 diagram indicates their relation to Abyssal peridotites (Johnson et al., 1990). Compositions of spinels in the studied harzburgites indicates that they are high-Al type with Mg# and Cr# 0.67-0.72 and 0.19-0.26, respectively. Their TiO2 content are 0.01-0.11(Wt %) and Al2O3 content are 44.009-49.894 (Wt %). In Cr# versus Mg# diagram, spinels are plotted on the Abyssal peridotites field and indicate that the host peridotite has experienced 10-15% partial melting. In addition, using the equation F%=10Ln (Cr/Cr+Al) spinel+24) (Hellebrand et al., 2001) 7.6-10.4 (average 9.7%), the partial melting degree was obtained that means a slow spreading rate for the study area. Using the Cr# versus Mg# diagram, alpine I-type, abyssal and back arc setting for the studied harzburgites are determined. PTMAFIC Software (Soto and Soto, 1995) is used for orthopyroxene (single pyroxene), clinopyroxene-olivine, and orthopyroxene-clinoproxene thermometry. Based on these thermometers, overall temperatures of 1100 ±100 °C are estimated for the equilibrium stage of the minerals. For pressure estimation, the Cr content in clinopyroxene (Nimis and Taylor, 2000) barometer is used. Using this calibration and temperatures from single pyroxene thermometer a pressure of ~22±2.4 Kbar for equilibrium clinopyroxene and associated minerals in the studied harzburgites was estimated.

Discussion
Mineral chemistry studies indicate that these harzburgites may be related to oceanic settings. Moreover, the high Mg# in orthopyroxenes and clinopyroxenes and the high Fo% in olivines are indications of their tectonite origin. Calculation of partial melting degree using spinels compositions indicate that they are experienced 7.6-10.4 partial melting. In this regard, they are consistent with the partial melting degree in the Atlantic and Indian oceans. The spreading rate studies indicate that harzburgites are produced in the region with slow spreading rate. Their tectonic setting is more consistent with MOR peridotites. Based on geothemobarometry studies an overall ~1100±100°C temperature and ~22±2.4Kbar pressure are estimated which are consistent with mantle spinel lherzolite facies.

Acknowledgements
The author would like to thank the reviewers for the constructive comments which greatly contributed to the improvement of the manuscript.

References
Hellebrand, E., Snow, J.E., Dick, H.J.B., and Hofmann, A.W., 2001. Coupled major and trace elements as indicators of the extent of melting in mid-ocean ridge peridotites. Nature, 410(6829): 677–681.
Johnson, K.T.M., Dick, H.J.B., and Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. Journal of Geophysical Research, 95(B3): 2661-2678.
Kananian, A., Ataei, M., Mirmohammadi, M., and Emamalipour, A., 2010. Petrography, mineral chemistry and genesis of Aland and Gheshlagh Chromite deposits, Khoy ophiolite (NW of Iran). Iranian Journal of Crystallography and Mineralogy, 18(3): 369-380 (in Persian with English abstract).
Kornprobst, J., Ohnenstetter, D., Ohnenstetter, M., Ohnenstetter, M., 1981. Na and Cr contents in Cpx from peridotites: a possible discriminant between subcontinental and sub-oceanic mantle. Earth and Planetary Science Letters, 53(2): 241-254.
Morimoto, N., Fabries, J., Ferguson, A.K., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Aoki, K., and Gottardi, D., 1988. Nomenclature of pyroxenes. American Mineralogist, 73(1-2): 1123-1133.
Nimis, P., and Taylor, W.R., 2000. Single clinopyroxene thermobarometry for garnet peridotites. Part I. Calibration and testing of Cr-in-Cpx barometer and an enstatite-Cpx thermometer. Contributions to Mineralogy and Petrology, 139(5): 541–554.
Soto, J.I., and Soto, V.M., 1995. PTMAFIC: software package for thermometry, barometry and activity calculations in mafic rocks using IBM-compatible computer. Computers and Geosciences, 21(5): 619-652.

Keywords

• harzburgite
• Mineral chemistry
• partial melting
• Geothermobarometry
• Khoy ophiolite

20.1001.1.20087306.1395.8.2.6.7

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References

Allan, J.F., 1994. Cr-spinel in depleted basalts from the Lau basin back-arc: petrogenetic history from Mg–Fe crystal–liquid exchange. In: J. Hawkins, L. Parson and J.F. Allan (Editors), Proceedings of the Ocean Drilling Program, Scientific Results, Texas, pp. 565–583.

Arai, S., 1994. Compositional variation of olivine-chromian spinel in Mg-rich magmas as a guide to their residual spinel peridotites. Journal of Volcanology and Geothermal Research, 59(4): 279-293.

Azizi, H., 2001. Petrography, petrology and geochemistry of metamorphic rocks of Khoy. Ph.D. Thesis, University of Tarbiat Moalem, Tehran, Iran, 253 pp (in Persian with English abstract).

Azizi, H., Moinvaziri, H., Mohajjel, M., and Yagobpoor, A., 2006. PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for cretaceous-tertiary continental collision. Journal of Asian Earth Sciences, 27(1): 1-9.

Barker, A.J., 1990. Introduction to metamorphic textures and microstructures. Chapman and Hall, London, 162 pp.

Bloomer, S.H., and Fisher, R.L., 1987. Petrology and geochemistry of igneous rocks from the Tonga trench – a non-accreting plate boundary. Journal of Geology, 95(4): 469–495.

Bloomer, S.H., and Hawkins, J.W., 1983. Gabbroic and ultramafic rocks from the Mariana trench: an island arc ophiolite. In: D.E. Hayes (Editor), The Tectonics and Geologic Evolution of Southeast Asian Seas and Islands. American Geophysical Union, Washington D.C., pp. 294–317.

Bonatti, E., and Michael, P.J., 1989. Mantle peridotites from continental rifts to ocean basins to subduction zones. Earth and Planetary Science Letters, 91(3-4): 297-311.

Boudier, F., Nicolas, A., and Bouchez, J.L., 1982. Kinematics of oceanic thrusting and subduction from basal section of ophiolites. Nature, 296(5860): 825-828.

Dick, H.J.B., and Bullen, T., 1984. Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas. Contributions to Mineralogy and Petrology, 86(1): 54-76.

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(361): 431-435.

Faridazad, M., 2010. Petrology and petrography of metamorphosed ultramafic and mafic rocks in the Khoy ophiolitic complex (NW Iran). Ph.D. Thesis, University of Tabriz, Tabriz, Iran, 215 pp (in Persian with English abstract).

Hassanipak, A., and Ghazi, M., 2000. Petrology, geochemistry and tectonic setting of the Khoy ophiolite, North West Iran: implications for Tethyan tectonics. Journal of Asian Earth Sciences, 18(1): 109-121.

Hellebrand, E., Snow, J.E., Dick, H.J.B., and Hofmann, A.W., 2001. Coupled major and trace elements as indicators of the extent of melting in mid-ocean ridge peridotites. Nature, 410(6829): 677–681.

Hirose, K., and Kawamoto, T., 1995. Hydrous partial melting of lherzolite at 1GPa: The effect of H2O on the genesis of basaltic magmas. Earth and planetary Science letters, 133(3-4): 463-473.

Ishii, T., Robinson, P.T., Maekawa, H. and Fiske, R., 1992. Petrological studies of peridotites from diapiric Serpentinite Seamounts in the Izu- Ogasawara-Mariana fore arc, leg 125. In: P. Fryer, J. Pearce and L.B. Stokking (Editors), Proceedings of the Ocean Drilling Project, Leg 125. Scientific Results, Texas, pp. 445-485.

Johnson, K.T.M., Dick, H.J.B., and Shimizu, N., 1990. Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. Journal of Geophysical Research, 95(B3): 2661-2678.

Kamenetsky, V., Crawford, A.J., and Meffre, S., 2001. Factors controlling chemistry of magmatic spinel: an empirical study of associated olivine, Cr-spinel and melt inclusions from primitive rocks. Journal of Petrology, 42(4): 655–671.

Kananian, A., Ataei, M., Mirmohammadi, M., and Emamalipour, A., 2010. Petrography, mineral chemistry and genesis of Aland and Gheshlagh Chromite deposits, Khoy ophiolite (NW of Iran). Iranian Journal of Crystallography and Mineralogy, 18(3): 369-380 (in Persian with English abstract).

Khalatbari-jafari, M., Juteau, T., Bellon, H., Whitechurch, H., cotton, J., and Emami, H., 2004. New geological, geochronological and geochemical investigation on the Khoy ophiolites and related formations, NW Iran. Journal of Asian Earth Sciences, 23(4): 507-535.

Khalatbari-jafari, M., Juteau, T., and Cotton, J., 2006. Petrological and geochemical study of the late cretaceous ophiolite of Khoy (NW Iran), and related geological formations. Journal of Asian Earth Sciences, 27(4): 465-502.

Kornprobst, J., Ohnenstetter, D., Ohnenstetter, M., Ohnenstetter, M., 1981. Na and Cr contents in Cpx from peridotites: a possible discriminant between subcontinental and sub-oceanic mantle. Earth and Planetary Science Letters, 53(2): 241-254.

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

Miyashiro, A., 1975. Classification, characteristics and origin of ophiolites. Journal of Geology, 83(2): 249–281.

Monnier, C., Girardeau, J., Maury, R., and Cotton, J., 1995. Back-arc basin origin for the East Sulawesi ophiolite (eastern Indonesia). Geology, 23(9): 851–854.

Monsef, I., Rahgoshay, M., Mohajjel, M., and Shafaii Moghadam, H., 2010. Peridotites from the Khoy Ophiolitic Complex, NW Iran: Evidence of mantle dynamics in a supra-subduction-zone context. Journal of Asian Earth Sciences, 38(3-4): 105–120.

Morimoto, N., Fabries, J., Ferguson, A.K., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Aoki, K., and Gottardi, D., 1988. Nomenclature of pyroxenes. American Mineralogist, 73(1-2): 1123-1133.

Nimis, P., and Taylor, W.R., 2000. Single clinopyroxene thermobarometry for garnet peridotites. Part I. Calibration and testing of Cr-in-Cpx barometer and an enstatite-Cpx thermometer. Contributions to Mineralogy and Petrology, 139(5): 541–554.

Niu, Y., and Batiza, R., 1991. An empirical method for calculating melt compositions produced beneath mid-ocean ridges: application for axis and off-axis (seamounts) melting. Journal of Geophysical Research, 96(B13): 21753–21777.

Niu, Y., and Hekinian, R., 1997. Spreading-rate dependence of the extent of mantle melting beneath ocean ridges. Nature, 385(6614): 326–329.

Parkinson, I.J., and Pearce, J.A., 1998. Peridotites from the Izu-Bonin-Mariana Fore arc (ODP Leg125): evidence for mantle melting and melt–mantle interaction in a supra-subduction zone setting. Journal of Petrology, 39(9): 1577–1618.

Passagno, E.A., Ghazi, M., Kariminia, M., Duncan, R.A., and Hassanipak, A.A., 2005. Tectonostratigraphy of the Khoy Complex, northwestern Iran. Stratigraphy, 1(2): 49-63.

Passchier, C.W., and Trow, R.A., 1996. Microtectonics. Springer, Berlin, 289 pp.

Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25(4): 956-983.

Powell, M. and Powell, R., 1974. An olivine-clinopyroxene geothermometer. Contributions to Mineralogy and Petrology, 48(4): 249-263.

Radfar, J., and Amini, B., 1999. Geological Map of Khoy, scale 1:100,000. Geological Survey of Iran.

Richard, L.R., 1995. MinPet: Mineralogical and Petrological Data Processing System, Version 2.02. MinPet Geological Software, Quebec.

Soto, J.I., and Soto, V.M., 1995. PTMAFIC: software package for thermometry, barometry and activity calculations in mafic rocks using IBM-compatible computer. Computers and Geosciences, 21(5): 619-652.

Tamura, A., and Arai, S., 2006. Harzburgite–dunite–orthopyroxenite suite as a record of supra-subduction zone setting for the Oman ophiolite mantle. Lithos, 90(1-2): 43–56.

Witt-Eickschen, G., Seck, H.A., 1991. Solubility of Ca and Al in orthopyroxene from spinel peridotite: an improved version of an empirical geothermometer. Contributions to Mineralogy and Petrology, 106 (4): 431- 439.

Wood, B.J. and Banno, S., 1973. Garnet-orthopyroxene and orthopyroxene-clinopyroxene relationships in simple and complex systems. Contributions to Mineralogy and Petrology, 42(2): 109-124.