| Research Abstract |
To clarify the mode of differentiation of granitic magmas of South Korea and Southeast Japan, a neutron activation analysis has been made to measure the trace element abundance, especially rare earth element (REE) abundance and Ba, Rb and Sr relationships of the granitic rocks. The REE patterns of the Jurassic granitic rocks in South Korea are quite different from the patterns of the Cretaceous granitic rocks in South Korea. Namely, the [Eu] /[Eu*] ratio ranges from 1.1 to 0.62, the (La/Lu)cn ratio changes from 80 to 20, and total REE content varies from 100 to 180 ppm for Jurassic patterns, while the same ratios and the total REE content range from 1.1 to 0.04, from 38 to 2.4, and from 100 to 240, respectively, for the Cretaceous patterns. We used the Rayleigh fractionation model as a crystallization model of the granitic melts. Since the model calculation fit the REE data for the Cretaceous granitic rocks very well, it is suggested that the cretaceous granitic rocks in South Korea we
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re mainly differentiated by fractional crystallization. The REE patterns for the Jurassic granitic rocks suggest that participation of garnet or zircon as residues of partial melting of old metamorphic rocks (Tsusue et al., 1987a).
Four granite provinces have been recongnized in Southwest Japan on the basis of their distribution, mode of occurrence, age and chemical and mineralogical composition of granitic rocks. The REE patterns of the Mesozoic and Tertiary granitic rocks of Southwest Japan are somewhat similar to the REE patterns of the Cretaceous granitic rocks of South Korea. Namely, [Eu]/[Eu*] ratio ranges from 1.03 to 0.01, the (La/Lu)cn ratio changes from 28 to 1.2, and total REE content varies from 110 to 730 ppm for the granitic rocks of Southwest Japan. We employed the Rayleigh fractionation model to simulate the crystallization of granitic melts. Since the model calculation fit the observed REE patterns for the granitic rocks well, it is suggested that the granitic rocks of Southwest Japan were mainly differentiated by fractional crystallization (Tsusue et al., 1987b).
As suggested by McCarthy and Robb (1978), a change in liquidus mineralogy form plagioclase-quartz-biotite to plagioclase-quartz-K-feldspar-biotite during the in situ fractional crystallization of a granitic magma has a marked effect on the abundance of an interrelationships between Ba, Rb and Sr. During plagioclase-quartz-biotite fractionation, Ba and Rb enrich in successive solid and melt, while Sr is depleted. The incoming of K-feldspar as a solid phase causes Ba to be depleted along with Sr insuccessively formed solid, While Rb continues to be enriched. In general, trace element data from the granitic rocks of Southwest Japan fit the model well, although the observed scatter of the data is somewhat greater than predicted by simple variation in cumulus-intercumulus proportions - possibly due, at any rate mainly to a difference in Ba, Rb and Sr contents of original melts and to the effect assimilation. Less
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