영남육괴 번암-함양일대 트라이아스기 중기-말기 및 쥐라기 초기 심성암류에 대한 지질특성,지구화학 및 Nd-Sr 동위원소 연구

Abstract

The Yeongnam Massif is located in the southern part of the Korean Peninsula, between the Phanerozoic Okcheon Belt to the north and Cretaceous volcano-sedimentary rocks to the south. The Massif consists predominantly of Paleoproterozoic gneiss, including orthogneiss, banded paragneiss, migmatite, and quartzofeldspathic gneiss with minor slate, schist, and quartzite, intruded by Mesozoic plutonic rocks. Middle-Late Triassic and Early Jurassic plutonic rocks in the Yeongnam Massif are developed along the NE-trending massif (Daegang, Macheon, Sinwon, Sancheong, Bunam, Hamyang, Gerchang, Gimcheon, Seongju, Chilgok, Sangju, Seonsan) and have intruded Paleoproterozoic gneiss and schist sequences. Some Triassic and Jurassic granitoids are foliated resulted from Middle Jurassic dextral strike-slip fault motion along the Honam Shear Zone in the massif. The Bunam-Hamyang area, as a main study area, consists of a composite of distinct Middle and Late Triassic plutonic units. The Middle Triassic granitoids are coarse- to medium-grained K-feldspar megacryst-bearing granodiorite. The Late Triassic granitoids are mainly composed of coarse- to medium-grained leucocratic granite, biotite granite and K-feldspar megacryst-bearing alkali granite. On the other hand, the Middle Triassic Macheon gabbroic rocks, located south of the study area, occurs as a independent large stocks. On the other hand, the Early Jurassic granitoids, located south and northeast of the study area (Hamcheon, Sinwon, Sancheon, Gerchang, Gimcheon, Seongju and Chilgok), consist of hornblende-biotite granitoids (granodiorite to quartz monzodiorite), pophyric biotite granodiorite and biotite granite with small stocks of diorite, syenite and gabbro compositions. The Middle-Late Triassic and Early Jurassic plutonic rocks in the Yeongnam Massif are all strongly LREE enriched relative to chondrites. On trace element spider diagrams normalized to the primitive mantle, these plutonic rocks are depleted in Ba, Ta, Nb, Sr, P, and Ti and relatively enriched in Th, U, and K and show the typical Ta-Nb trough. These Ta-Nb troughs and the depletion of P and Ti with large ion lithophile element enrichments are characteristics of subduction-related granitoids and have also been found in granitic rocks formed from a crustal source that was derived from arc-related crust. The Middle-Late Triassic plutonic rocks in the Yeongnam Massif have moderate to high initial ^(87)Sr/^(86)Sr ratios (0.706590-0.713842) and negative εNd(t) (-19.19 to -3.81), indicating derivation of the magmas from sources in the middle to upper crust. In contrast, the Early Jurassic plutonic rocks in the Massif have low to moderate initial ^(87)Sr/^(86)Sr ratios (0.704860-0.710133) and moderate negative to weakly positive εNd(t) (-12.77 to 0.86), indicating derivation of the magmas from sources in the middle to upper crust. Although the Yeongnam Massif has variously been considered part of the South China Block, North China Block, or an independent crustal block by the previous studies, the distribution and characteristics of the Middle-Late Triassic plutonic rocks suggest a subduction-related Triassic magmatic episodes, and offer important clues to the link between South China and South Korea, particularly given that recent studies of the Indosinian orogeny in South China have indicated that collision belts in these terrains might extend into South Korea. In South China, the Indosinian orogeny is best recorded in and around the Qinling-Dabie-Sulu belt between the North China Block and the Yangtze Craton, and in the collision belt between the Yangtze Craton and the Cathaysian Block along the Chenzhou-Linwu fault. The orogeny was accompanied by voluminous Permian to Triassic magmatism. Therefore, the Triassic plutonic rocks in the Qinling-Dabie-Sulu belt and the broad intra-cratonic South China fold belt might be the products of continental collision and active continental margin magmatism, respectively. As a result, the Triassic plutonic rocks of the Yeongnam Massif, together with the Okcheon Belt are a possible eastern extension of the broad intra-cratonic South China fold belt along the active continental margin of South China (or the Cathaysian Block) into South Korea In contrast, the Triassic plutonic rocks of the Imjingang Belt and the Gyeonggi Massif should be considered an extension of the Qinling-Dabie-Sulu belt between North China and the Yangtze Craton in central China. However, regional-scale geochronological and geochemical studies of Triassic plutonic rocks in South Korea are required to confirm these postulations. Regarding the geographic distribution of Early and Middle Jurassic granitoids in South Korea, Early Jurassic granitoids (201-185 Ma) are mostly exposed in the Yeongnam Massif. This finding indicates that subduction along the Yeongnam Massif began around 201 Ma and continued until at least 185 Ma. In general, Early Jurassic granitoids can be related to steeper subduction-related primitive arc magmatism in the active continental margin, based on their geochemical characteristics and narrow distribution in the Yeongnam Massif. Steeper subduction would have resulted from combinations of slow or retrograde absolute upper plate motion; slow relative rates of plate convergence; and subduction of the old, dense oceanic lithosphere. This would have induced the development of a magmatic arc closer to the trench and extensional tectonics within and behind the arc. Steeper subduction ended at around 185 Ma and was followed by subduction, which caused the regional magmatism (>300-km wide) and formed the Middle Jurassic granitoids (180-160 Ma) in the Okcheon Belt, the Gyeonggi Massif, and the Imjingang Belt, as well as parts of North Korea. The similar age distribution of the granitoids in these areas may be related to shallow or moderate dipping subduction (compared to steeper Early Jurassic subduction) with similar activation age of subduction. Low to moderate dipping subduction could have resulted from a combination of rapid absolute upper plate motion toward the trench, relatively rapid plate convergence, subduction of low-density oceanic lithosphere, and subduction of the young oceanic lithosphere. During collision due to early moderate dipping subduction, the major shear zone (Honam shear zone) of the orogenic front (the Yeongnam Massif and the Okcheon Belt) activated during 180 to 170 Ma and propagated to central South Korea. The foreland basin also opened and propagated to central South Korea. Regional emplacements of granitoids occurred between the Imjingang and Okcheon belts during the period 170-160 Ma, caused by extensive moderate-dipping subduction with increasing gravitational pull of the slab. During this period, dextral movement of the transform fault reactivated pre-existing zones of weakness such as the Honam shear zone, one of the area’s important tectonic boundaries.