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중부 옥천대에 분포하는 쥬라기 보은 화강섬록암의 암석화학과 마그마과정
좌용주 한국암석학회 1996 암석학회지 Vol.5 No.2
옥천대 내에서 옥천층군을 관입하고 있는 보은 화강섬록암은 그화학적 성질이 과알루미나질이며, 칼크-알칼리 계열을 따르는 화강암류이다.그리고 보은 암체를 형성한 마그마가 생성되고 관입한 환경은 활동성 대륙 주변부일 가능성이 있다. 한편, 옥천대의 보은암체를 동시대의 경기육괴의 인제-홍천암체와 비교하면 보은암체 ?이 인제-홍천암체 보다 성숙도가 낮은 대륙호의 환경에서 형성된 것임을 알 수 있다. 주성분원소, 미량원소 및 회토류원소의 함량 변화에 있어서 결정분화를 지시하는 경향성이 결여되어 나타나는 것은 화강암류의 화학적 성질을 지배하는 마그마과정이 단순한 분별작용의 모델 만으로는 설명되지 않음을 나타낸다. 또한 보은 화강암체를 형성시킨 마그마과장에 대해서 레일리 분별 및 주성분원소 함량변화에 따른 분별과정을 모델 계산을 통해 검토해 보았으나, 어느것도 보은암체이 지구화학적 경향성을 효과적으로 설명해 주지 않는다. 반면, 결정작용과 동시성 혼염작용(AFC)에 대한 모델 계산에서는 보은 화강섬록암을 형성시킨 화강암질 마그마는 옥천층군을관입하기 이전에 퇴적기원의 지각물질과의 혼염과정을 거쳤을 가능성이 제기되고, 그 때이 조건으로는 혼염되는 주변암석이 성분과 마그마의 성분이 유사하고 또한 혼염작용의 속도가 결정작용의 속도와 비슷하여 혼염되는 질량은 마그마 전체 질량의 10% 내외인 경우이다. 한편, 분별과정과 AFC모델 계산의 결과는 규장질암맥이 보은 화강섬록암으로 파생된 것이 아니라 다른 성인의 마그마로부터 유래되었을 가능성을 제시한다. Boeun granodiorite, which intruded into the metasedimentary rocks of the Ogcheon Group, show chemical natures of metaluminous and calc-alkaline. Generating and emplacing environment of the Boeun granodiorite would have been a active continental margin. Comparing to the contemporaneous Inje-Hongcheon granodiorite in the Gyeonggi massif, the Boeun granodiorite seems likely to have formed under more immature continental arc environment. Compositional changes of major, trace and rare earth elements in granodiorite and felsic dyke are not certain to indicate crystallization differentiation. From this fact, the simple fractional crystallization model would be in question to explain the magma process which controlled the formation of the Boeun granitic mass. The model calculations for Rayleigh fractionation, fractionation with variable major-component composition, assimilation-fractional crystallization (AFC) were carried out to examine the magma process of the mass. The results of former two models do not agree with the compositional variations in the mass. The AFC model can be, however, applied to the magma process. The conditions for AFC process are (1) composition of assimilated wallrock is similar to that of primary magma. (2) assimilating rate is similar to crystallizing rate, and (3) mass of assimilated wallrock is about 10% of that of the magma. These conditions deny a possibility that the assimilated wallrock was the metasedimentary rocks of the Ogcheon Group. This indicates that after having experienced the assimilation process in deeper crust, the granodiorite magma intruded into the Ogcheon group. Every model calculating suggests that the felsic dyke was differentiated not from the granodiorite magma, but from a different source magma.
좌용주,박충화 한국지구과학회 1992 한국지구과학회지 Vol.13 No.2
Uyeda and Kanamori (1979) classified subduction zones into two modes - Mariana type and Chilean type. Many features such as back-arc basin, stress field, dip angle of Benioff-Wadati zone, thrust earthquake and accretionary prism are different in both types. Kinds of stress field as well as the dip angle of Benioff-Wadati zone are important factors controlling the spatial distribution of volcanic activity. Mariana type represents tensional field and high dip angle, whereas Chilean type does compressional and gentle dip. From the relationship between the dip angle of Benioff-Wadati zone and volcanic spacing, it is recognized that as the dip steepens the linear density of volcanoes increases but the width of them decreases. As the dip angle increases, the stress field becomes more tensional and volcanic magmas rise more actively, Also, the dip angle has a close relation to the dip of partial melting zone located along the subducting slab. As the dip increases, the width of partial melting zone decreases and subsequently the spacing of volcanoes decreases. Volcanic rocks in the subduction zone represent geochemical differences due to the mode of subduction and degree of arc maturity. In continental arc and mature island arc (Chilean type) the volcanic rocks show the geochemical variation from tholeiitic to calc-alkaline and finally to alkaline, whereas in immature arc without continental crust (Mariana type) they generally show tholeiitic affinities. However, in addition to the inter-type differences in tectonic environments and geochemical natures, the infra-type ones are also prominent. It seems like that the change of evolutionary step in each type has caused the intra-type differences.
남극 킹죠지섬 필데스반도의 화산암류에 대한 지구화학적 연구
좌용주,이종익 한국지구과학회 1992 韓國地球科學會誌 Vol.13 No.2
Tertiary volcanic activity in the Fildes Peninsula can be divided into three stages (I, II, III) of magmatism. Evolutionary process of each stage was examined from the geochemical characteristics. For stage I, olivine, orthopyroxene and clinopyroxene of low Fe/Mg ratio as well as Ca-plagioclase begin to crystallize and fractionate. Stage II magma, which is more differentiated than stage I magma, shows the fractionation of Ca-plagioclase and olivine and orthopyroxene of high Fe/Mg ratio. Stage III magma, which is slightly more differentiated than stage I magma but less than stage II magma, intruded into the area when stage II magmatism was weakened. The volcanic rocks on the Fildes Peninsula show, as a whole, tholeiitic natures. From the previous studies on Sr isotopic ratio, the origin of the volcanic magma in the area was assumed to be upper mantle. Accordingly, the volcanic rocks in the area were originated from tholeiitic magma which generated by the partial melting of upper mantle under island-arc environment.