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Formation of Foliations and their Related Minerals from Diagenetic to Medium‐grade Metamorphic Rocks: A Case Study of the Hongyanjing and Liao‐Ji Backarc Basins, China
Author(s) -
TIAN Zhonghua,
WEN Fei,
LIU Fulai,
ZHU Haozhong,
YE Zhanghuang
Publication year - 2021
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.14790
Subject(s) - foliation (geology) , metamorphic rock , geology , metamorphism , crenulation , overprinting , geochemistry , staurolite , diagenesis , pressure solution , tectonic phase , recrystallization (geology) , cleavage (geology) , kyanite , petrology , tectonics , mineralogy , paleontology , shear zone , fracture (geology)
Deciphering the relationship between polyphase tectonic foliations and their associated mineral assemblages is significant in understanding the process from diagenesis to low‐/medium‐/high‐grade metamorphism. It can provide information related to strain, metamorphic conditions and overprinting relationships and so help reveal the tectonic evolution of orogenesis. In this study, we predominately focus on the formation of foliations and their related minerals, as developed in two separate basins. First of all, two stages of axial plane cleavages (S 1 and S 2 ) were recognized in the Hongyanjing inter‐arc basin, the formation of the S 1 axial plane cleavage is associated with mica rotation and elongation in mudstones in the local area. The pencil structure of S 2 formed during the refolding phase, the minerals in the sedimentary rocks not changing their shape and orientation. Secondly, in the Liao‐Ji backarc basin, foliations include diagenetic foliation (bedding parallel foliation), tectonic S 1 foliation (secondary foliation or axial plane cleavage of S 0 folding) and crenulation cleavage (S 2 ). The formation mechanism of foliation changes from mineral rotation or elongation and mineral solution transfer in S 1 to crystal‐plastic deformation, dynamic recrystallization and micro‐folding in S 2 . Many index metamorphic minerals formed from low‐grade to medium‐grade consist of biotites, garnets, staurolite and kyanite, constituting a typical Barrovian metamorphic belt. Accordingly, a new classification of foliation is presented in this study. The foliations can be divided into continuous and disjunctive foliations, based on the existence of microlithons, detectable with the aid of a microscope. Disjunctive foliation can be further sub‐divided into spaced foliation and crenulation cleavage, according to whether (or not) crenulation (micro‐folding) is present. The size of the mineral grains is also significant for classification of the foliations.