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The rare earth element (REE) tetrad effect includes a large, strongly negative Eu anomaly, and appears in highly evolved, fractionated granite and/or A-type granite, which it is sometimes interpreted as evidence of fluid-rock interaction. Magnesium is a fluid-mobile element, and its isotopes show very little fractionation during igneous processes. In order to understand the formational processes that cause the REE tetrad effect in granitic magmas and the role of fluid-rock interaction in producing REE tetrad patterns, this study analyzed magnesium and oxygen isotope compositions along with REE patterns in three Cretaceous granites from the Okcheon belt, Korea. Samples exhibited a linear correlation between magnesium isotopic values (δ26Mg) and SiO2, MgO, TiO2, initial 87Sr/86Sr, 143Nd/144Nd values. Oxygen isotope values also showed some correlation with SiO2, TiO2, δ26Mg, initial 87Sr/86Sr, and 143Nd/144Nd values. Our data indicate that the three granites should be evolved from source magma derived from isotopically heterogeneous source materials within the crust. In addition, the relationship between initial 143Nd/144Nd and δ26Mg values strongly suggests that the REE tetrad effect may develop before the granitic magma evolved from crustal source material rather than during late stage of magma differentiation or hydrothermal alteration. [ABSTRACT FROM AUTHOR]