경기도 화성시 우음도 일원의 화강암 암맥군과 U-Pb 연령 = Granite Dike Swarm and U-Pb Ages in the Ueumdo, Hwaseong City, Korea

다국어 초록 (Multilingual Abstract)

The Middle Jurassic granite dike swarm intruding into the Paleoproterozoic banded gneiss is pervasively observed inUeumdo, Hwaseong City, mid-western Gyeonggi Massif. Based on their cross-cutting relationships in a representative outcrop,there are four dikes (UE-A, UE-C, UE-D, UE-E), and depending on the direction, there are three granite dike groups, whichare NW- (UE-A dike), NW to WNW- (UE-C dike), and NE-trending (UE-D and UE-E dikes). These granite dikes aremassive, medium-to coarse-grained biotite granites, and their relative ages observed in outcrops are in the order of UE-A, UE D (=UE-E), and UE-C. The geometric analysis of the dikes indicates that the UE-A and UE-C dikes intrude underapproximately NE–SW trending horizontal minimum stress fields. The UE-A dike, which showed a relatively low averageSiO2 content by major element analysis, is a product of early magma differentiation compared to other dikes; therefore, it isconsistent with the relative age of each dike. The 206Pb/238U weighted mean ages for each dike obtained from SHRIMP zirconU–Pb dating were calculated to be 167 Ma (UE-A), 164 Ma (UE-C), 167 Ma (UE-D), and 167 Ma (UE-E), respectively. Thesamples of the UE-A, UE-D, and UE-E dikes showed very similar ages. The UE-C dike shows the youngest age, which isconsistent with the results of the relative age in the outcrops and major element analysis. Therefore, the granite dikes intrudedinto the Middle Jurassic (approximately 167 and 164 Ma), coinciding with those of the Gyeonggi Massif, where the MiddleJurassic plutons are geographically widely distributed. This result indicates that the wide occurrence of the Middle Jurassicplutons on the Gyeonggi Massif was formed as a result of igneous activity moving in the northwest direction with theshallower subduction angle of the subducting oceanic plate during the Jurassic.

국문 초록 (Abstract)

경기육괴 중서부에 위치한 화성시 우음도 일대에는 고원생대 호상편마암을 관입하고 있는 중기 쥐라기의 화강암 암맥군이 발달한다. 우음도 일대의 대표 노두에서 야외 횡절관계를 근거하면 4개의 암맥들(UE-A, UE-C, UE-D,UE-E)로 구분되며, 방향성에 따라서는 북서 방향(UE-A 암맥), 북서 내지 서북서 방향(UE-C 암맥), 북동 방향(UE-D 및UE-E 암맥)의 3개의 암맥군으로 나타난다. 이들 화강암 암맥들은 괴상의 중립~조립질의 흑운모 화강암으로 야외에서관찰된 이들의 상대연령은 UE-A, UE-D (=UE-E), UE-C 순으로 젊어진다. 또한 암맥들의 기하학적 분석으로부터 UE A 및 UE-C 암맥은 대략 북동-남서 방향의 최소수평응력장 하에서 관입한 것으로 판단된다. 주원소 분석에 의한 SiO2평균 함량에서 비교적 낮은 값을 보인 UE-A 암맥은 다른 암맥들보다 초기 마그마 분화의 산물임을 지시하여 암맥들의상대연령과도 부합한다. SHRIMP 저어콘 U-Pb 연대측정으로부터 구한 암맥별 206Pb/238U 누적평균연령은 각각 약 167Ma (UE-A), 164 Ma (UE-C), 167 Ma (UE-D), 167 Ma (UE-E)로 UE-A, UE-D, UE-E 암맥들은 매우 유사한 연령을보이며 이들 암맥 중 가장 세립인 UE-C 암맥은 가장 젊은 연령을 나타내어 야외에서 관찰한 상호 횡절관계에 의한 상대연령과 주원소 분석 결과와도 일치한다. 따라서 연구지역의 화강암 암맥들은 중생대 중기 쥐라기(약 167 Ma와 164Ma)에 짧은 시간 간격을 두고 다양한 화강암질 마그마가 관입한 결과이며, 이들 관입 시기는 지리적으로 중기 쥐라기암체들이 널리 분포하고 있는 경기육괴의 심성암체들과 일치하는 연령이다. 따라서 연구지역의 화강암 암맥군은 지구조적으로 쥐라기 동안 섭입하는 해양판의 얕아지는 섭입각과 함께 북서 방향으로 이동하는 화성활동의 결과로 형성되었음을 의미한다.

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