Journal of Economic Geology (Feb 2017)
Mineral chemistry and geothemobarometry of mantle harzburgites in the Eastern Metamorphic Complex of Khoy ophiolite -NW Iran
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. 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