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Published August 2015 | Published
Journal Article Open

Crystal size and shape distribution systematics of plagioclase and the determination of crystal residence times in the micromonzogabbros of Qisir Dagh, SE of Sabalan volcano (NW Iran)


The Qisir Dagh igneous complex occurs as a combination of volcanic and intrusive rocks to the south-east of the Sabalan volcano, north-western Iran. Micromonzogabbroic rocks in the region consist of plagioclase, alkaline feldspar and clinopyroxene as the major mineral phases and orthopyroxene, olivine, apatite and opaque minerals as the accessory minerals. Microgranular and microporphyritic textures are well developed in these rocks. Considering the importance of plagioclase in reconstructing magma cooling processes, the size and shape distribution and chemical composition of this mineral were investigated. Based on microscopic studies, it is shown that the 2-dimensional size average of plagioclase in the micromonzogabbros is 538 micrometers and its 3-dimensional shape varies between tabular to prolate. Crystal size distribution diagrams point to the presence of at least two populations of plagioclase, indicating the occurrence of magma mixing and/or fractional crystallization during magma cooling. The chemical composition of plagioclase shows a wide variation in abundances of Anorthite-Albite-Orthoclase (An = 0.31—64.58, Ab = 29.26—72.13, Or = 0.9—66.97), suggesting a complex process during the crystal growth. This is also supported by the formation of antiperthite lamellae, which formed as the result of alkali feldspar exsolution in plagioclase. The calculated residence time of magma in Qisir Dagh, based on 3D crystal size distribution data, and using growth rate G = 10—10 mm/s, varies between 457 and 685 years, which indicates a shallow depth (near surface) magma crystallization and subvolcanic nature of the studied samples.

Additional Information

© 2015 Geologica Carpathica. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0 Manuscript received October 2, 2014; accepted in revised form June 23, 2015. The authors would like to thank D.J. Morgan and D.A. Jerram for providing CSDSlice spreadsheet and also H. Altafi, S. Nezamdoost and B. Arefi for their assistance with the field work. Dr. I. Petrík is thanked for editorial handing of the paper and for providing us with valuable scientific comments. Reviews and comments by Prof. H. Yu and an anonymous reviewer are very much appreciated.

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August 20, 2023
August 20, 2023