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Takashi Nakajima, GSJ, Geological Survey of Japan (Japan)
Yuji Orihashi, ERI, University of Tokyo (Japan)
Kazuhiro Miyazaki, GSJ, Geological Survey of Japan (Japan)
Tohru Danhara, KFT, Kyoto Fission Track Co. Ltd. (Japan)
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The processes from migmatites to plutons have been discussed and modeled from the viewpoint of chemistry, dynamics, rheology, etc., to examine its physical reality and possible condition. But nobody has documented a natural occurrence of a transitional stage from migmatite to granitic pluton. So it has been regarded as a geologiocal missing link.
Cretaceous granitic province in Southwest Japan is a typical arc-type plutono-metamorphic terrane comprising high-level to deep-seated granitoids intruding Jurassic accretionary complexes and their metamorphic equivalents. The U-Pb isotopic ratios of zircons from leucosome and melanosome of migmatites, and accompanied granitic plutons in the Southwest Japan were examined using LA-ICP-MS. The results are, 1. The zircons from the leucosome and melanosome are mostly inherited ones, plotted far apart from the U-Pb concordia line, but forming a beautiful discordia line with an upper intercept of ca.1900Ma, with a small amount of Cretaceous magmatic zircons. 2. In contrast, the zircons from the granitic plutons are mostly Cretaceous magmatic zircons.
These results imply that the granitic plutons in the Southwest Japan are not originated mainly from the leucosomes, namely, the upper crustal migmatites cannot be the source of arc granites. This conclusion is consistent with the fact that the Cretaceous granitoids from Southwest Japan are mostly of I-type and also with Nakajima et al.(2004)'s geochemical discussion that concluded their main source is mantle-derived juvenile component.
Considering that the migmatites from Southwest Japan are pelitic migmatites, they cannot be a main source because if the granitic plutons were composed of the integrated such migmatitic melts, those granites must be of S-type. In other granitic provinces in Circum-Pacific area, such as Sierra Nevada and Andes, granitoids are mostly of I-type, too.
"From migmatites to plutons" may be valid in the case of middle-lower crustal melting of pyroxenite, gabbro and amphibolite. In the Kohistan paleo-arc crustal section in northern Pakistan, incipient melt pockets in the lower crustal horizon grow and integrate to form a melt network as they go to higher horizons, and finally form sheet-like tonalite intrusions. The tonalite has quite primitive chemical characters of low K2O, Rb, Zr, Y, Th and Nb content with low 87Sr/86Sr initial ratio, having magmatic epidotes which shows high-pressure crystallization (>5kb) and no inherited zircons, inferring juvenile tonalite magma generated from the lower crustal mafic source materials at the lower crust.
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