Magnetic Fabrics and Source Implications of Chisulryoung Ignimbrites, South Korea

Hoabin Hong,Jeongmin Kim,Yongjae Yu,Dongwoo Suk,Seong-Jae Doh

doi:10.3389/feart.2016.00079

Abstract

The anisotropy of magnetic susceptibility (AMS) of late Cretaceous ash-flow tuffs in Chisulryoung Volcanic Formation, southeastern Korea was studied to define the primary pyroclastic flow azimuth. AMS data revealed a dominant oblate fabric with a tight clustering of k3 (minimum axis of magnetic susceptibility) and shallow dispersal of k1 (maximum axis of magnetic susceptibility) and k2 (intermediate axis of magnetic susceptibility). Dominance of oblate fabrics indicates clast imbrications imposed by compaction and welding. Flow azimuth inferred from AMS data indicates the nearby intrusive welded tuff (IWT) as the source of calderas for ignimbrites. Such an inference is supported by geologic investigations, in which the IWT displays eutaxitic textures nearly parallel to its subvertical contacts. The results are compatible with a unique prolate fabric and an anomalously high inclination observed for the IWT, possibly produced by rheomorphic flows as the welded tuff is squeezed along the rough-surfaced dyke walls due to agglutination.

Highlights

Ignimbrites are the deposits of pyroclastic density flows ejected from explosive volcanic eruptions
When magnetic fabric was analyzed in terms of Pj vs. shape parameter (T) (Jelinek, 1981), results for A4 member are biased toward positive shape parameters, suggesting a prevalent oblate fabric due to welding process (Figure 7B)
Presence of predominant oblate fabrics in ignimbrites indicates clast imbrications imposed by compaction and welding

Summary

Introduction

Ignimbrites are the deposits of pyroclastic density flows ejected from explosive volcanic eruptions. AMS is sensitive indicator of fabric elements within welded tuffs to yield a primary flow azimuth (Wolff et al, 1989). Rheomorphic tuff showed well-developed eutaxitic textures and is composed predominantly of glass shards and pumice with minor amounts of lithic fragments and crystals in a reddish devitrified matrix (Figures 3G,H).

Results

Conclusion