Oxygen Isotopic Compositions of Mesozoic Granitic Rocks in South Korea
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概要
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Mesozoic granitic rocks of South Korea are divided, on the basis of their ages, occurrences and petrochemical characteristics, into the Daebo granite of early Jurassic to early Cretaceous age and the Bulguksa granite of late Cretaceous to early Tertiary age. The whole rock δ <SUP>18</SUP>O values of the Daebo and Bulguksa granitic rocks range from +5.0 to +10.0‰ (+8.2‰ in average) and +2.4 to +9.1‰ (+7.0‰ in average), respectively. A wide variation of the δ <SUP>18</SUP>O values implies that 18O depleted meteoric waters are involved in the granite suites during the post magmatic alteration process. Successful discrimination on the δ <SUP>18</SUP>O value for quartz, and on the SiO<SUB>2</SUB>, TiO<SUB>2</SUB> and Sr concentrations was obtained between the Daebo and Bulguksa granitic rocks. Quartz-biotite geothermometric temperatures of the Daebo and Bulguksa granites are 500 to 630°C and 550 to 660°C, which are consistent with solidus temperatures of the granites, respectively. The Daebo granite probably results mainly from the partial melting of the <SUP>18</SUP>O and <SUP>87</SUP>Sr-enriched metasediment of lower continental crust and /or of the crustal contamination of upper mantle magma during the Daebo orogeny. The Bulguksa granitic and volcanic rocks characterized by low δ <SUP>18</SUP>O and initial <SUP>87</SUP>Sr/<SUP>86</SUP>Srr values and cogenetic characteristics with respect to REE and trace elements, are possibly emplaced from upper mantle magma without assimilation of sedimentary rocks in the Kyongsang basin.
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