무주-설천 지역 변성암류의 지질구조: 옥천벨트와 영남육괴의 경계부 고찰

Geological Structure of the Metamorphic Rocks in the Muju-Seolcheon Area, Korea: Consideration on the Boundary of Ogcheon Belt and Ryeongnam Massif

Abstract

옥천벨트와 영남육괴의 경계부에 위치하는 것으로 알려져 있는 무주-설천 지역은 시대미상 내지 선캠브리아기 변성암류[호상흑운모편마암, 변성퇴적암류(흑색천매암, 운모편암, 결정질석회암, 규암), 화강암질편마암, 각섬암], 중생대 퇴적암류와 화성암류로 구성되어 있다. 이 연구에서는 주요 변형된 암석구조의 기하학적 운동학적적 특성과 중첩된 변형구조들의 선후관계로부터 무주-설천 지역 변성암류의 변형단계별 구조적 특성을 규명하고, 기존에 보고된 연대학적, 지화학적, 구조지질학적 자료와 함께 옥천벨트와 영남육괴 사이의 경계 위치를 고찰하였다. 연구지역의 지질구조는 적어도 네 번의 변형단계(Dn-1, Dn, Dn+1, Dn+2 변형)를 걸쳐 형성되었다. Dn-1 변형은 광역엽리 Sn이 형성되기 이전에 발생하여 Fn 습곡에 의해 습곡되는 Sn-1 엽리를 형성시켰다. Dn 변형은 광역엽리 Sn을 형성시킨 변형작용이다. Sn 엽리는 변성암류의 대상 분포 방향과 일치하는 북동 방향으로 우세한 방향성을 보인다. Fn 습곡축이 신장선구조의 방향과 일치하는 A-형습곡 내지 칼집습곡은 결정질석회암에서 노두규모로 종종 관찰된다. Dn+1 변형은 광역엽리 Sn을 습곡시키는 변형작용으로 북북서~남북 방향의 압축응력 하에서 발생하여 동북동~동서 방향의 Fn+1 습곡을 형성시켰다. 광역엽리 Sn은 주로 Fn+1 습곡작용에 의해 재배열되어, 분산된 Sn 엽리의 극점배열의 파이-축의 방향성은 Fn+1 습곡축의 우세한 방향성과 거의 일치한다. Dn+2 변형은 Sn 엽리와 Sn+1 엽리를 습곡시키는 변형작용으로 동서 방향의 압축응력 하에서 발생하여 남북 방향의 Fn+2 개방습곡을 형성시켰다. 그리고 이러한 네 번의 변형작용은 무주-설천 지역의 옥천벨트와 영남육괴에서 모두 관찰되고, 무주-설천 지역에서 이들 지체구조구를 구분할 수 있는 구조적 특성과 차이점은 관찰할 수 없었다. 지화학적 자료와 연대학적 자료에 따르면, 무주-설천 지역 일대 변성암류의 형성시기 내지 변성시기는 중기~후기 고원생대이다. 이는 이 지역에서 산출되는 결정질석회암은 적어도 중기 고원생대 이전에 퇴적되었음을 지시하고, 이러한 퇴적시기는 최근에 보고된 옥천누층군의 지질시대(신원생대~후기 고생대)와 다르다. 따라서 지금까지의 연구결과를 고찰해 볼 때, 무주-설천 지역에서 결정질석회암을 포함하는 변성퇴적암류를 시대미상의 옥천층군으로 추정하여 설정된 옥천벨트와 영남육괴 사이의 지체구조구 구분은 재고될 필요가 있다.

The Muju-Seolcheon area, which is known to be located in the boundary of Ogcheon Belt and Ryeongnam Massif (OB-RM), consists of age unknown or Precambrian metamorphic rocks (MRs) [banded biotite gneiss, metasedimentary rocks (black phyllite, mica schist, crystalline limestone, quartzite), granitic gneiss, hornblendite], Mesozoic sedimentary and igneous rocks. In this paper are researched the structural characteristics of each deformation phase from the geometric and kinematic features and the developing sequence of multi-deformed rock structures of the MRs, and is considered the boundary location of OB-RM with the previous geochemical, radiometric, structure geological data. The geological structure of this area is at least formed through four phases (Dn-1, Dn, Dn+1, Dn+2) of deformation. The Dn-1 is the deformation which took place before the formation of Sn regional foliation and formed Sn-1 foliation folded by Fn fold. The Dn is that which formed the Sn regional foliation. The predominant Sn foliation shows a NE direction which matches the zonal distribution of MRs. A-type or sheath folds, in which the Fn fold axis is parallel to the direction of stretching lineation, are often observed in the crystalline limestone. The Dn+1 deformation, which folded the Sn foliation, took place under compression of NNW~NS direction and formed Fn+1 fold of ENE~EW trend. The Sn foliation is mainly rearranged by Fn+1 folding, and the ${\pi}$-axis of Sn foliation, which is dispersed, shows the nearly same direction as the predominant Fn+1 fold axis. The Dn+2 deformation, which folded the Sn and Sn+1 foliations, took place under compression of E-W direction, and formed open folds of N-S trend. And the four phases of deformation are recognized in all domains of the OB-RM, and the structural characteristics and differences to divide these tectonic provinces can not be observed in this area. According to the previous geochemical and radiometric data, the formation or metamorphic ages of the MRs in and around this area were Middle~Late Paleproterozoic. It suggests that the crystalline limestone was at least deposited before Middle Paleproterozoic. This deposition age is different in the geologic age of Ogcheon Supergroup which was recently reported as Neoproterozoic~Late Paleozoic. Therefore, the division of OB-RM tectonic provinces in this area, which regards the metasedimentary rocks containing crystalline limestone as age unknown Ogcheon Group, is in need of reconsideration.

Fig. 1. The locality of study area in the (a) tectonic map of Korea (1: 1,000,000) (KIGAM, 2001), (b) Daejon (left D) (Lee et al., 1996) and Andong (right A) (Hwang et al., 1996) sheets (1: 250,0000), and (c) Muju (left M) (Hong et al., 1980) and Seolcheon (right S) (Yun and Park, 1968) sheets (1: 50,000). For detailed explanation, see these sheets and map and https://mgeo.kigam.re.kr/map/geology. For reference, the boundary (bdy) of Ogcheon Belt (OB) and Ryeongnam Massif (RM) and the shear zones (SZ) on the 1: 250,000 and 1: 50,000 geological sheets modified after the 1: 1,000,000 tectonic map of Korea.

Fig. 2. Geological map of the study area [modified from Yun and Park(1968) and Hong et al.(1980)].

Fig. 3. Outcrop photographs of major rock structures deformed in the Dn-1 and Dn phases. (a) Fn fold folding Sn-1 foliation (No. 136: banded biotite gneiss). (b) Sn-1 foliation and FnAp refolded by Fn+1 fold (No. 137: banded biotite gneiss). (c) A-type Fn fold, (d) sheath Fn fold (arrow marks) (No. 113: crystalline limestone). Ap: fold axial plane, Ax: fold axis, Ln: stretching lineation. For detailed explanation and outcrop sites, see the text and Fig. 4a.

Fig. 5. Outcrop photographs of major rock structures deformed in the Dn+1 and Dn+2 phases in the crystalline limestone [(a-e) No. 111, (f) No. 113, (g) No. 110 outcrop sites]. (a) Whole photograph, and (b-e) partial enlarged photographs of (a) [(a) S-shaped asymmetric Fn+1 major fold folding the Sn regional foliation, (b, c) M-shaped symmetric and (d) Z-shaped asymmetric Fn+1 parasitic folds, (e) Sn foliation and Fn+1Ap refolded by Fn+2 fold]. (f) M-shaped symmetric Fn+1 fold. (g) Fn+2 fold folding the Sn foliation. Ap: fold axial plane. For detailed explanation and outcrop sites, see the text and Fig. 4a.

Fig. 6. Outcrop photographs of major rock structures deformed in the Dn+1 phase in the non-crystalline limestone metamorphic rocks. (a) Whole photograph, (b) partial enlarged photographs of (a) (No. 115). (c) Fn+1 box fold (No. 122). For detailed explanation and outcrop sites, see the text and Fig. 4a.

Fig. 8. Stereoplots of the Sn foliation dispersed by Fn+1 folding in the (a) No. 111 (crystalline limestone), (b) No. 113 (crystalline limestone), and (c) No. 115 (banded biotite gneiss) outcrop sites. (d) Stereoplots of the Sn foliation dispersed by Fn+2 folding in the No. 111 (crystalline limestone) outcrop site.

Fig. 4. (a) Orientations of the Dn-1 and Dn structural elements. (b-e) Stereoplots of the (b) Dn-1 and (c-e) Dn [(c) all, (d) crystalline limestone, (e) non-crystalline limestone metamorphic rocks] structural elements. Lowerhemisphere equal-area projection, Numbers (110, 111, 113, 115, 122, 136, 137): outcrop sites. For detailed explanation, see the text.

Fig. 7. (a) Orientations of the Dn+1 and Dn+2 structural elements. Stereoplots of the (b) Dn+1 and (c) Dn+2 structural elements. Lower-hemisphere equal-area projection. For detailed explanation, see the text.