Clinopyroxene application in petrogenesis identification of volcanic rocks associated with salt domes from Shurab (Southeast Qom)

زمین شناسی اقتصادی

Article 2, Volume 8, Issue 1 - Serial Number 14, June 2016, Pages 21-38   

Document Type: Research Article

DOI: 10.22067/econg.v8i1.29480

Authors

Somayeh Falahaty^* ; Moussa Noghreyan; Mortaza Sharifi; Ghodrat Torabi; Homayon Safaei; Mohammad Ali Mackizadeh

Isfahan

Abstract

Introduction
The study area is located in the Shurab area that is about 50 Km Southeast of Qom. Volcanic rocks of the Shurab area have basaltic composition that is associated with salt and marl units. Igneous rocks of the Shurab area have not been comprehensively studied thus far.
Clinopyroxene composition of volcanic rocks, and especially the phenocrysts show Magma chemistry and can help to identify magma series (Lebas, 1962; Verhooge, 1962; Kushiro, 1960, Leterrier et al., 1982), tectonic setting (Leterrier et al., 1982; Nisbet and Pearce, 1977) as well as temperature formation and pressure of rock formation. Some geologists have estimated temperature of clinopyroxene formation by clinopyroxene composition (Adams and Bishop, 1986) and clinopyroxene-olivine couple. So, clinopyroxene is used in this study in order to identify magma series, tectonic setting, plus the temperature and pressure of volcanic rocks of the Shurab.

Material and method
Clinopyroxene analyses were conducted by wavelength-dispersive EPMA (JEOL JXA-8800R) at the Cooperative Centre of Kanazawa University (Japan). The analyses were performed under an accelerating voltage of 15 kV and a beam current of 20 nA. The ZAF program was used for data corrections. Natural and synthetic minerals of known composition were used as standards. The Fe3+ content in minerals was estimated by Droop method (Droop, 1987).

Discussion
In the Shurab area, the volcanic rocks area with basaltic composition are located 50 km Southeast of Qom. Their age is the early Oligocene and they are associated with the salty marl units of the Lower Red Formation (LRF). The hand specimens of the studied rocks look green. These rocks are intergranular, microlitic, porphyric, vitrophyric and amygdaloidal and they consist of olivine, pyroxene and plagioclase. Accessory minerals contain sphene, apatite and opaque.
According to Wo-En-Fs diagram (Morimoto, 1988), clinopyroxenes indicate diopside composition.
Clinopyroxenes are representatives of magma composition and they are usually used for identifying magma series. There are several diagrams that are used for this purpose as follows.
1 - Al2O3 – SiO2 diagram (Lebas, 1962):
According to this diagram, the studied clinopyroxenes were plotted in sub-alkaline field.
2 - Al2O3 – TiO2 diagram (Lebas, 1962):
In this diagram, the studied clinopyroxenes show to be calcalkaline.
Some diagrams that are used for determining tectonic setting according to clinopyroxene composition are as follows:
1 - F1 – F2 diagram (Nisbet and Pearce, 1977):
Based on this diagram, the Shurab clinopyroxenes rocks lie between volcanic arc and mid ocean ridge basalt fields.
2 - Al2O3 - SiO2 diagram (Lebas, 1962):
Using this diagram, the studied clinopyroxenes are located in the sub-alkaline field.
Some methods that are used for determining temperature formation and pressure of clinopyroxene are as follows:
1 – Kretz method (Kretz, 1994):
Using this method, temperature formation of clinopyroxene is about 1200- 1250oC.
2 – Soesoo method (Soesoo, 1997):
Using this method, pressure formation of clinopyroxene is about 6-10 Kb.
In Al+2Ti+Cr against Na+Al diagram, Clinopyroxenes are located above the Fe+3=0 line (Schweitzer et al, 1979). This case and abundant hematite and magnetite in the Shurab area rocks confirm that oxygene fugasity is high. Based on Helz diagram (Helz, 1973), the content of magma water during clinopyroxene formation is about 2-5 percent.

Results
Using various methods, the temperature and pressure of clinopyroxene formation are about 1200 oC and 6-10 Kb, respectively. Clinopyroxene composition and the abundant hematite and magnetite in the studied rocks confirm that oxygene fugasity is high. According to Helz diagram, the amount of water is about 2-5 percent.
Additionally, the parent magma of the studied area rocks is calc alkaline and tectonic setting is subduction-related based on the clinopyroxene composition.

Acknowledgment
The authors thank the University of Isfahan for its financial supports.

Reference
Adams, G.E. and Bishop, F.C., 1986. The olivine- clinopyroxene geobar- ometer: experimental results in the CaO- FeO- MgO- SiO2 system. Contributions to Mineralogy and Petrology, 94(2): 230-237.
Droop, G.T.R., 1987. A general equation for estimating Fe3+ in ferromagnesian silicates and oxides from microprobe analysis, using stoichiometric criteria. Mineralogical Magazine, 51(361): 431-437.
Helz, R.T., 1973. Phase relations of basalts in their melting range at PH2O= 5 kb as a function of oxygen fugacity. Journal of Petrolology, 17(2): 139-193.
Kretz, R., 1994. Metamorphic Crystallization. Chichester and New York, New York, 530 pp.
Kushiro, I., 1960. Si- AI relation in clinopyroxenes from igneous rocks. American Journal of Science, 258(5): 548-554.
Lebas, N.J., 1962. The role of aluminous in igneous clinopyroxenes with relation to their parentage. American Journal of Science, 260(4): 267-88.
Leterrier, J., Maury, R.C., Thonon, P., Girard, D. and Marchal, M., 1982. Clinopyroxene composition as a method of identification of the magmatic affinities of paleo- volcanic series. Earth and Planetary Science Letters, 59(1): 139-154.
Morimoto, N., 1988. Nomenclature of pyroxenes. Fortschr mineral, 66: 237-252.
Nisbet, E.G. and Pearce, J.A., 1977. Clinopyroxene composition of mafic lavas from different tectonic settings. Contributions to Mineralogy and Petrology, 63(2): 161-173.
Schweitzer, E.L., Papike, J.J. and bence, A. E., 1979. Statitical analysis of clinopyroxenes from deep sea basalts. American Mineralogist, 64)2): 501-513.
Soesoo, A., 1997. A multivariate statistical analysis of clinopyroxene composition: empirical coordinates for the crystallisation PT-estimations. Geological Society of Sweden (Geologiska Föreningen), 119(1): 55-60.
Verhooge, J., 1962. Distribution of titanium between silicates and oxydes in igneous rocks. American Journal of Science, 260(2): 211-220.

Keywords

Clinopyroxene; Thermobarometry; Tectonomagmatic; Shurab; Qom

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