Journal of the Geological Society of Korea (지질학회지)

The Geological Society of Korea (대한지질학회)
권호 표지

Petrogeneses and Evolution of Early Proterozoic Granitic Rocks in the Northeastern Ryeongnam Massif, Korea

한국 영남육괴 북동부에 분포하는 선캠브리아기 화강암질암류들의 성인 및 진화사연구

Hong, Young-Kook
홍영국

  • Published : 19921200

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Abstract

Petrogeneses and evolution of the lour distinctive early Proterozoic granitic rocks(1.64-2.10 Ga) at the northeastern Ryeongnam massif in Korea are studied by petrological and geochemical means Including trace element modelling. Major and trace element data are used to establish the nature of chemical variations in the plutons. The Buncheon granite gneiss(2.10 Ga) which is the eldest Proterozoic granite in the NE part of the massif could be generated from plagioclase-dominated fractional crystallization with up to 70% of I-type Intermediate parent magma under the high oxygen fugacity condition. This suggestion is generally supported by geochemical evidences such as high chondrite normalized(Ce/yb) ratio. abundant incompatible elements. strong negative Eu anomaly$(0.28{\pm}0.07)$. low compatible elements and $(^{87}Sr/^{86}Sr)$ Initial ratio of 0.7056-0.7082. According to Rb-Sr-Ba and REE modellings, the S-type Proterozoic granitoids(1.64-1.80 Ga) are found to be derived by wide range of fractional crystallization processes(about 25-75%) from a parental magma that is assumed as the product of 75% partial melting of the metapelitic protolith which is geochemically similar to the Yulri Group. The Hongjesa granite(1.71-1.83 Ca) may be produced under different oxygen fugacity conditions of parent magma. Geochemical evidences such as peraluminous nature, considerable variations of major. trace elements and $(Ce/Yb)_N\;of\;23.98{\pm}22.24$. and high $(^{87}Sr/^{86}Sr)$ initial ratio of 0.7174-0.7229 Indicate the S-type characteristics of the Hongjesa granite. The degree of fractionation and Eu negative anomalies(0.59-0.87) are Increasing from the central part of the Hongjesa granite in which approximatly 25% of fractional crystallization had been undergone. that Is assumed as a depocenter of the Yulri Group. to the marginal sides with 50-75% of fractionation. The Nonggeori(1.73-1.80 Ga) and Naedeogri(1.64-1.79 Ga) granites with rather small variation of chemical compositions. low CaO content, high normative corundum(3.90-4.36%) and primary muscovite may be generated by low degree(25%-50%) of fractional crystallization of the S-type magma derived from the Yulri Basin. Fractional crystallization is most probably responsible for the chemical variations observed in the early Proterozoic granitic rocks of NE Ryeongnam massif in Korea. But, the possible heterogeneity of the metapelitic rocks as source legion for the S-type granitic rocks is not completely excluded for explaining the variable petrochemical characteristics. Tectonic evolution of the early Proterozoic granitic rocks is generally summarized as: 1) Intrusion and emplacement of I-type Buncheon granite in 2.10 Ga: 2) Development of strong foliation with $N30^{\circ}-60^{\circ}E$ during 2.10 Ga to 1.80 Ga: 3) Development and sedimentation of the Yulri Basin: 4) Generation and Intrusion of the S-type granitoids during 1.70-1.80Ga in the Basin: and 5) Various thermal events exceeding $300\pm50^{\circ}C$ were undergone in the studied area during 1.2-1.3 Ga. 0.6-0.7 Ga and 0.3-0.4 Ga. The parental magmas for the Proterozoic granites are found to be generated in syn-collisional tectonic setting.

본 연구는 영남륙괴의 북동부에 분포하는 선캠브리아기 화강암류들의 암석지화학적 특징을 상호비교하고 미양원소 모델링을 실시하여 생성과정과 진화사를 밝히는데 목적이 있다. 1) 분천화강편마암(21억년)은 엽리가 가장 잘 발달된 중립-세립질 안구상 화강편마암 이며, 주 미양원소 변화폭이 크고, $(^{87}Sr/^{86}Sr)$ 초생치가 대체로 낮다(0.7056-0.7082). 엽리의 형성시기는 분천화강암의 관입 정치시기인 21억년으로부터, 매우 약한 엽리를 보이는 17-18억년의 홍제사-농거리-내덕리 화강암 형성시기 사이일 것으로 생각된다. 분천화강편마암은 지화학적으로 진화(evolve)한 화성암류가 대규모로 부분용융되어 생성된 마그마가 높은 정도(70%)의 분별결정작용을 거쳐서 형성되었다. 2) 홍제사화강암(17-18억년)은 함암회색 장석 조립질 복운모화강암이며, 주 미량원소의 변화폭이 크며, $(^{87}Sr/^{86}Sr)$ 초생치가 높다(0.7174-0.7229). 홍제사 화강암내에는 전기석, 하부 율리층군에서 유래되었다고 믿어지는 변성이질포획물과 페그마타이트가 매우 풍부하다. 엽리발달은 대체로 약하나 노옴 corundum 함양은 2.45%로 높다. 홍제사화강암은 평균적이고 대표적인 율리층군같은 이질(politic) 변성퇴적암류가 근원암의 지화학적 특징을 가진다고 가정하여 미양원소 모델링을 해 본 결과 근원암이 75% 부분용융 되어 만들어진 hydrous한 본원 마그마가 관입 정치하여 지역적으로 각기 다른 정도의 분별결정작용에 의하여 형성되었다. 특히, 홍제사화강암은 "율리" 퇴적분지의 중심부에서는 25% 그리고 가장자리부분으로 갈수록 50-75%의 다양한 분별결정작용을 거쳤음이 밝혀졌다. 3) 농거리 화강암(17-18억년)은 함암회색장석 중립-세립질 복운모 화강암이며, 내덕리 화강편마암(17억년)은 세립의 우백질 복운모화강편마암이다. 이 두 화강암류들은 주 미양원소변화폭이 작으며, 엽리는 거의 없거나 약하다. 노옴 corundum의 함양이 평균 3.65-3.97%로 peraluminous하다. 이 화강암류들은 홍제사화강암의 중심부에 나타나는 지화학적 특징과 유사하며 낮은 정도(25-50%)의 분별결정작용을 받았음이 희토류원소 암석성인 모델링에 의하여 밝혀졌다. 본 연구지역의 화성활동사는 약 21억년전에 I-형의 분천화강암이 관입 정치되고, 분천 화강암이 $N30^{\circ}-60^{\circ}E$의 강한 엽리를 보이며 편마암화되어 분천화강편마암으로 변화된 시기는 21억년에서 18억년 사이 일 것이다. 그 후, 분천화강편마암의 북익부에 퇴적두께가 적어도 10km 이상인 대규모의 "율리" 퇴적분지가 형성되었다. 홍제사-농거리-내덕리 화강암류 (17-18억년)들은 율리층군과 지화학적으로 유사한 퇴적분지 중심부에 풍부한 함수광물을 가진 이질 변성퇴적암이 75%의 부분용융에 의하여 형성된 S-형의 본원마그마가 여러 다른 정도(25-75%)의 분별 결정작용을 거침으로서 형성되었다고 생각된다. 홍제사-농거리-내덕리 화강암류들과 접촉하고 있는 율리층군의 저 변성작용은 이를 S-형 화강암질 마그마들이 어느정도의 거리를 관입하여 정치했기 때문이라고 판단된다. 그러나 S-형의 화강암들은 유리분지 퇴적물의 불균질성, 즉 분지 중심지는 이질이고 사면은 사질이 우세한 퇴적물로 구성되었다는 근원암의 이질성 때문일 것이라는 가정을 완전히 배제할 수는 없다. 또한, 선 캠브리아기 화강암류들은 syn-collision에 의한 하부지각의 부분용융으로 형성된 화강암질 마그마들로부터 결정화되었음이 밝혀졌다.

Keywords

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