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Crystallographic orientation relationships between quartz and feldspar in myrmekite: a case study in monzodiorite from Meichuan pluton, China

Published online by Cambridge University Press:  19 April 2021

Yueting Song Open the ORCID record for Yueting Song [Opens in a new window] ,
Shanrong Zhao Open the ORCID record for Shanrong Zhao [Opens in a new window]  and
Chang Xu
Yueting Song
Affiliation:
School of Earth Sciences, China University of Geosciences, 430074Wuhan, China
Shanrong Zhao*
Affiliation:
School of Earth Sciences, China University of Geosciences, 430074Wuhan, China
Chang Xu
Affiliation:
School of Earth Sciences, China University of Geosciences, 430074Wuhan, China
*
*Author for correspondence: Shanrong Zhao, Email: [email protected]
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Abstract

Myrmekites occurring in monzodiorite from the Meichuan pluton in the Dabie ultrahigh-pressure metamorphic belt were investigated. The petrographic evidence demonstrates a metasomatic origin for myrmekite formation at the scale of individual alkali feldspar grains, and that the myrmekitic quartz and plagioclase matrix are generated simultaneously replacing precursor feldspar. Energy-dispersive X-ray spectroscopy and electron microprobe analysis indicate a low anorthite content in the narrow rim of host plagioclase near the myrmekite–alkali-feldspar interface. The Ca2+, Na+ proportion of hydrothermal fluids replacing precursor alkali feldspar is 1:5.4, calculated from the anorthite content of the inner part of the host plagioclase and the neighbouring alkali feldspar. Electron back-scattered diffraction was used to identify the crystallographic orientation of the myrmekitic quartz, plagioclase matrix and the precursor alkali feldspar. The crystallographic orientation relationships (110)Kfs//(11$\bar{2}\bar{1}$)Qtz, (20$\bar{1}$)Kfs//(11$\bar{2}$1)Qtz and [11$\bar{2}3]$Qtz//[001]Kfs between myrmekitic quartz and adjacent alkali feldspar were obtained from statistical analysis. No clear crystallographic orientation relationship between quartz and plagioclase was found. The growth of myrmekitic quartz is constrained by the precursor alkali feldspar rather than the simultaneously crystallised plagioclase. This research is helpful for understanding the intergrowth mechanism during metasomatism.

Keywords

myrmekitemetasomatismEMPAEDSEBSDcrystallographic orientation relationships
Type
Article
Information
Mineralogical Magazine , Volume 85 , Issue 3 , June 2021 , pp. 406 - 415
Copyright
Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland

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Footnotes

Associate Editor: Martin Lee

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