Skeletal, microspinifex and harrisite textures in the Upper Paleozoic komatiitic sills and lavas from the Mashhad-Virani Complex, NE Iran

Document Type : Research Article

Authors

• Mohsen Mobasheri ¹
• Habibollah Ghasemi ¹
• Behnam Rahimi ²
• Mojtaba Rostami Hussory ¹

¹ Shahrood University of Technology

² Ferdowsi University of Mashhad

10.22067/econg.v11i2.66386

Abstract

Introduction
The Mashhad- Virani complex has been sandwiched between the collided Lut block and Turan plate. This complex is composed of the following four units: 1) ultramafic-mafic rocks, 2) metamorphosed sedimentary rocks, 3) pyroclastic rocks and 4) Mashhad’s granitoids including quartz-diorite, tonalite, granodiorite and monzogranite (interpreted as granitoids formed in an arc regime during the subduction of the Paleo-Tethys Ocean under the Turan Plate by Karimpour et. al., 2011). The association rocks in the Mashhad-Virani complex have experienced varying degrees of hydrothermal alteration and regional metamorphism. These rocks are typically metamorphosed in lower to upper green-schist facies, but rarely to pyroxene hornfels facies along the contacts with the Mashhad granitoids.
Researchers have challenging ideas on the nature of these rocks. Firstly, Majidi (1981) reported the komatiitic nature of these rocks. However, most of the geologists believed that these rocks are a part of an ideal ophiolitic sequence (Alavi, 1979; Fazel-Valipour, 2002). However, some geological studies have provided strong evidence that contradicts the ophiolite nature of these ultramafic- mafic rocks. Detailed studies of this research show that according to the petrological issues, field relationships, textures and internal stratigraphy, these rocks are not only an ophiolitic sequence but are also an ultramafic- mafic volcanics precisely named komatiite. In this complex, although the contact of the ultramafic rocks with the adjacent sediments is not visible in the majority of cases due to the coverage of Quaternary sediments and tectonic processes. However, this contact is partly preserved in the Khurshid Park and Zuh peak where there is some evidence of ultramafic lava eruption on the sediments. In these places, sediments in the border with komatiitic rocks has been clearly baked. They also have very interesting skeletal, microspinifex, and harrisite textures. These observations suggest that the ultramafic rocks in the Mashhad-Virani complex are ultramafic volcanic flows.
 
Materials and Methods
Field studies have been carried out in more than twenty cross sections in the southwest-northwest of Mashhad. More than 400 thin and polished sections were made from rock samples and studied in the petrography laboratory of the Faculty of Earth Sciences at the Shahrood University of Technology. Moreover, after detailed petrography studies, five samples with the least alteration were selected for preparing polished thin sections. Major element analyses on selected minerals (amphibole, plagioclase, pyroxene and olivine) were performed on a JEOL EPMA JXA-8900R electron microprobe at the Institute of Earth Sciences, Academia Sinica, Taiwan. Analytical conditions included an accelerating voltage of 15 kV, a beam current with 2μm diameter of 12nA and counting times of 10s on peaks and 5s on the background. For calibration of all elements, a set of mineral and synthetic standards has been used.
Discussion
The Mashhad-Virani complex includes an assemblage of ultramafic-mafic rocks with approximate length of 32km along the western side of the city of Mashhad. This complex consists of dunite, ortho- meso and crescumulate (harrisite) wherlite, clinopyroxenite, cumulative and noncumulative amphibole gabbro and differentiated- undifferentiated komatiitic flows. These komatiites have been shown with a wide range of textures such as random acicular pyroxene, hopper and chevron olivine, hopper pyroxene, skeletal olivine, skeletal pyroxene, micrographic intergrowth of plagioclase and clinopyroxene, dendritic pyroxene, olivine harrisitic, olivine orthocumulate, olivine mesocumulate, and olivine adcumulate textures. The rate of cooling and thermal gradient in the volcanic rocks along with super-saturation, exsolution of volatiles and magma mixing in the sub-volcanic rocks are the most important controlling factors in creation of these disequilibrium textures.
Amphibole gabbro sills are one of the main magmatic units of the upper parts of the lower horizons in the Mashhad-Virani complex. After detailed petrographical studies, five samples were analyzed for mineral chemistry measurements. In this study, only the composition of clinopyroxenes has been used for thermobarometry studies. Based on the obtained results, the clinopyroxenes are in the range of Ca-Mg-Fe sub-types in the Q-J diagram and in the diopside to augite fields on the Wo-En-Fs ternary diagram (Morimoto et. al., 1988). The results of the thermo-barometeric calculations by single clinopyroxene method indicate mean temperature of 1222°C and pressure of 2.4 kb that are in concord with the dyke and sill forms of gabbroic outcrops and also are very close to the crystallization temperatures of these magma types.
 
Results
Skeletal, spinifex and harrisite textures are the first unequivocal evidences reported from komatiitic sills and lava flows in the Mashhad-Virani Complex. These rocks are a part of the upper Paleozoic volcano-sedimentary sequence with approximately 32km length with NW-SE trend in the South and Southwest of Mashhad. This complex consists of dunite, ortho- meso and crescumulate (harrisite) wherlite, clinopyroxenite, cumulative and noncumulative amphibole gabbro and differentiated- undifferentiated komatiite flows. Application of the thermobarometry calculations on the single clinopyroxene from the amphibole gabbros (average pressure of 2.4 kb and average temperature of 1222 °C) are highly acceptable and consistent with the field and petrographic evidences.
 
Acknowledgments
The authors are most grateful to the vice chancellor of research at the Shahrood University of Technology for their financial and spiritual supports of this research.
 
References
Alavi, M., 1979. The Virani ophiolite complex and surrounding rocks. Geologisch Rundschau, 68(1): 334–341.
Fazel-valipour, M.E., 2002. Petrography, petrology of mafic-ultramafic rocks of Mashhad. Ph.D. Thesis, Islamic Azad University, Science and Research Branch, Tehran, Iran, 174 pp. (in Persian)
Karimpour, M.H., Farmer, G.L. and Stern, C.R., 2011. Rb–Sr and Sm–Nd Isotopic Compositions, U-Pb Age and Petrogenesis of Khajeh Mourad Paleo-Tethys Leucogranite, Mashhad, Iran. Scientific Quarterly Journal Geosciences, 20(80): 171–182. (in Persian)
Majidi, B., 1981. The ultrabasic lava flows of Mashhad, NE Iran. Geological Magazine, 118(1): 49–58.
Morimoto, N., Fabrise, J., Ferguson, A., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Akoi, K. and Gottardi G., 1988. Nomenclature of pyroxenes. American Mineralogist, 173(9–10): 1123–1133.

Keywords

• Skeletal
• Spinifex
• Harrisitic
• Komatiite
• Mashhad-Virani
• Upper Paleozoic

20.1001.1.20087306.1398.11.2.3.5

---

References

Alavi, M., 1979. The Virani ophiolite complex and surrounding rocks. Geologisch Rundschau, 68(1): 334–341.

Arndt, N.T., 1994. Archean komatiites. In: K.C. Condie (Editor), Archean crustal evolution. Elsevier, Amsterdam, pp. 11–44.

Arndt, N.T. and Fowler, A., 2004. Textures in komatiites and variolitic basalts. In: P.G. Eriksson, W. Altermann, D.R. Nelson, W.U. Mueller and O. Catuneanu (Editors), The Precambrian earth: tempos and events. Elsevier, Amsterdam, pp. 298–311.

Arndt, N.T. and Lesher C.M., 2004. Komatiite. In: D. Selle, R. Cocks and I. Plimer (Editors), Encyclopedia of geology. Elsevier, Amsterdam, pp. 260–268.

Arndt, N.T., Lesher, C.M. and Barnes, S.J., 2008. Komatiite. Cambridge University Press, London, 467 pp.

Brooks, C. and Hart, S.R., 1974. On the significance of komatiite. Geology, 2(2): 107–110.

Donaldson, C.H., 1974. Olivine crystal types in harrisitic rocks of the Rhum pluton and in Archean spinifex rocks. Geological Society of America Bulletin, 85(11): 1721–1726.

Donaldson, C.H., 1976. An experimental study of olivine morphology. Contributions to Mineralogy and Petrology, 57(2): 187–213.

Donaldson, C.H., 1982. Spinifex-textured komatiites: a review of textures, compositions and layering. In: N.T. Arndt and E.G. Nisbett (Editors), Komatiites. Allen and Unwin, London, pp. 213–244.

Faure, F., Arndt, N. and Libourel, G., 2006. Formation of spinifex texture in komatiites: an experimental study. Journal of Petrology, 47(8): 1591–1610.

Faure, F., Schiano, P., Trolliard, G., Nicollet, C. and Soulestin, B., 2007. Textural evolution of polyhedral olivine experiencing rapid cooling rates. Contributions to Mineralogy and Petrology, 153(4): 405–416.

Fazel-valipour, M.E., 2002. Petrography, petrology of mafic-ultramafic rocks of Mashhad. Ph.D. Thesis, Islamic Azad University, Science and Research Branch, Tehran, Iran, 174 pp. (in Persian)

Ghasemi, H., Juteau, T., Bellon, H., Sabzehei, M., Whitechurch, H. and Ricou, L.E., 2002. The mafic-ultramafic complex of Sikhoran (Central Iran): A polygenetic ophiolite complex. Comptes Rendus Geoscience, 334(6):431–438.

Ghasemi, H., Sabzehei, M. and Juteau, T., 1998. Geological character of Sikhoran mafic-ultramafic complex, southeast Iran. Geosciences, 7(29–30): 32–45. (in Persian)

Ghasemi, H., Sabzehei, M., Juteau, T., Bellon, H. and Emami, M.H., 2004. Radiometric age of the mafic parts and metamorphic hosts of Sikhoran ultramafic-mafic complex, southeastern Iran. Geosciences, 12(51–52): 58–67. (in Persian)

Ghasemi, H., Sabzehei, M., Juteau, T., Bellon, H., Rastad, E. and Emami, M.H., 2001. Petrogenesis of Sikhoran ultramafic-mafic complex, southeast Iran. Geosciences, 10(39–40): 46–69. (in Persian)

Hill, R.E.T., Barnes, S.J., Gole, M.J. and Dowling, S.E., 1995. The volcanology of komatiites as deduced from field relationships in the Norseman-Wiluna greenstone belt, western Australia. Lithos, 34(1–3):159–188.

Hill, R.E.T., Gole, M.J. and Barnes, S.J., 1988. Physical volcanology of komatiites: A field guide to the komatiites between Kalgoorlie and Wiluna, Eastern Goldfields Province, Yilgarn Block, Western Australia. Geological Society of Australia, Perth, 74 pp.

Hort, M., 1998. Abrupt change in magma liquidus temperature because of volatile loss or magma mixing: effects on nucleation, crystal growth and thermal history of the magma. Journal of Petrology, 39(5): 1064–1076.

Huppert, H.E. and Sparks, R.S.J., 1980. The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense ultrabasic magma. Contributions to Mineralogy and Petrology, 75(3): 279–289.

Karimi-Moghadam, A., 1997. Petrology of basic and ultrabasic rocks, southwest to northwest of Mashhad. M.Sc. Thesis, Tarbiat Moalem University, Tehran, Iran, 143 pp. (in Persian)

Karimpour, M.H., Farmer, G.L. and Stern, C.R., 2011. Rb–Sr and Sm–Nd Isotopic Compositions, U-Pb Age and Petrogenesis of Khajeh Mourad Paleo-Tethys Leucogranite, Mashhad, Iran. Scientific Quarterly Journal Geosciences, 20(80): 171–182. (in Persian)

Karimpour, M.H., Malekzadeh Shafaroudi, A. and Saadat, S., 2014. Mineralogy, geochemistry, genesis, and industrial application of silica in Arefi area, south of Mashhad. Journal of Economic Geology, 6(2): 259–276. (in Persian with English abstract)

Lofgren, G., 1974. An experimental study of plagioclase crystal morphology: isothermal crystallization. American Journal of Science, 274(3): 243–273.

Lofgren, G.E. and Donaldson, C.H., 1975. Curved branching crystals and differentiation in comb-layered rocks. Contributions to Mineralogy and Petrology, 49(4): 309–319.

Majidi, B., 1981. The ultrabasic lava flows of Mashhad, NE Iran. Geological Magazine, 118(1): 49–58.

Moore, A.G., Cas, R.A.F., Beresford, S.W. and Stone, M., 2000. Geology of an Archaean metakomatiite succession, Tramways, Kambalda Ni province, western Australia: assessing the extent to which volcanic facies architecture and flow emplacement mechanisms can be reconstructed. Australian Journal of Earth Sciences, 47(4): 659–673.

Morimoto, N., Fabrise, J., Ferguson, A., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Akoi, K. and Gottardi G., 1988. Nomenclature of pyroxenes. American Mineralogist, 173(9–10): 1123–1133.

Poldervaart, A.R.I.E. and Taubeneck, W.H., 1960. Layered intrusions. 21th International Geological Congress, University of Copenhagen, Copenhagen, Denmark.

Putirka, K.D., 2008. Thermometers and barometers for volcanic systems. Reviews in Mineralogy and Geochemistry, 69(1): 61–120.‏

Ruttner, A.W., 1991. Geology of the Aghdarband area (Kopet Dagh, NE-Iran). Abhandlungen der Geologischen Bundesanstalt, 38: 7–79.

Sabzehei, M., 1994. Differentiation of ultramafic magmas: constraint from layered ultramafic- mafic lava of Iranian aulacogeosynclines. 13th Annual Meeting, Geological Survey and Mineral Explorations, Tehran, Iran.

Sabzehei, M., 2016. Komatiite magma: it's role in the genesis of magmatic sequence of ophiolites: constraint from Iranian ophiolites. 2nd International Geosciences Congress, Geological Survey and Mineral Explorations, Tehran, Iran.

Sabzehei, M. and Pourlatifi, A., 1995. Ophiolitic magma and its role on petrogenesis of Iranian ophiolites: constraint from ultramafic-mafic differentiated lavas of komatiitic affinity. 14th Annual Meeting, Geological Survey and Mineral Explorations, Tehran, Iran.

Shore, M. and Fowler, A.D., 1999. The origin of spinifex texture in komatiites. Nature, 397(8): 691–694.

Siivola, J. and Schmid, R., 2007. List of Mineral Abbreviations, http://www.bgs.ac.uk/scmr/products.html

Theriault, R.D. and Fowler, A.D., 1995. Harrisitic textures in the Centre Hill complex, Munro Township, Ontario: product of diffusion limited growth. Mineralogy and Petrology, 54(1): 35–44.

Wager, L.R. and Brown, G.M., 1968. Layered Igneous Rocks. Edinburgh: Oliver and Boyd, London, 588 pp.

Wager, L.R., Brown, G.M. and Wadsworth, W.J., 1960. Types of igneous cumulates. Journal of Petrology, 1(1): 73–85.

Yavuz, F., 2013. WinPyrox: A Windows program for pyroxene calculation classification and thermobarometry. American Mineralogist, 98(7): 1338–1359.‏