Magnetic fabric and rock magnetic studies of metasedimentary rocks in the central Okcheon Metamorphic Belt, Korea

Yong-Hee Park,Wonnyon Kim,Dongwoo Suk,Seong-Jae Doh

doi:10.1186/bf03351863

Yong-Hee Park, Wonnyon Kim + Show 2 more

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https://doi.org/10.1186/bf03351863

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Abstract

Abstract Anisotropy of magnetic susceptibility (AMS) and rock magnetic studies have been carried out for the metasedimentary rocks in the central Okcheon Metamorphic Belt. The study area is divided into three metamorphic zones: the biotite zone, the garnet zone, and the sillimanite+andalusite zone from southeast to northwest. Magnetic foliation dipping to the northwest is the dominant magnetic fabric in the biotite zone. Magnetic lineation plunging down-dip of the vertical cleavage plane is appeared in the southeastern part of the garnet zone, while magnetic lineation plunging to southeast prevails in the middle and northwestern parts of the garnet zone. It is interpreted that this apparent synform structure, defined by AMS fabric data, was formed by successive top-to-the-southeast vergent thrusting followed by back-steepening process during the regional metamorphism in the Late Paleozoic. In the sillimanite + andalusite zone, AMS fabric is clearly defined and may reflect a tectonic fabric, recorded during the thermal metamorphism in the middle Jurassic, despite the scarcity of rock fabric in the field. The spatial distribution of magnetic mineralogy, defined by the rock magnetic results, implies the 500°C isotherm at the boundary between the biotite and garnet zones, which is associated with the transformation of pyrrhotite into magnetite above 500°C under the oxidizing condition.

Highlights

Anisotropy of low field magnetic susceptibility (AMS)studies have been widely used to delineate the petrofabrics of metamorphic rocks in terms of finite strain during tectono-metamorphism (e.g., Graham, 1954; Hrouda, 1982; Borradaile, 1988)
Samples of meta-psammite, which are most frequently occurred throughout the study area, especially in the garnet and sillimanite + andalusite zones (Fig. 2(a)), do not show a large variation of Km values (10−3∼10−2 SI), indicating that the ferromagnetic (s.l.) concentration in metapsammite is relatively constant (Fig. 3). 4.2 Magnetic mineralogy
5.1 Magnetic mineralogy, Anisotropy of magnetic susceptibility (AMS) parameters, and metamorphic zonation The predominant ferromagnetic (s.l.) mineral assemblages for each metamorphic zone were outlined based on the results of rock magnetic experiments (Figs. 4, 5, and 11)

Summary

Introduction

Anisotropy of low field magnetic susceptibility (AMS)studies have been widely used to delineate the petrofabrics of metamorphic rocks in terms of finite strain during tectono-metamorphism (e.g., Graham, 1954; Hrouda, 1982; Borradaile, 1988). The main magnetic mineralogy and its relation with the measured AMS should be determined for magnetic fabric studies of the metamorphic rocks. Several studies showed that magnetic mineralogy can be used as a proxy for the metamorphic zonation in low-grade metamorphosed areas (e.g., Rochette, 1987; Robion et al, 1997). It was not verified whether the magnetic zonation can be applied to the medium- and high-grade metamorphosed areas with various lithologies

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