PRIMARY FORMATION AGES AND CIRCUMUSTANCES OF PRECRETACEOUS ROCKS MAKING UP THE RYOKE BELT IN HONSHU ARC

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Crustal evolution of the Ryoke Belt is as follows ; (1) 2000-1800Ma.Granitic upper crust was formed from depleted mantle-origin lower crust, which is a primitive basement of the Ryoke Belt. These age data were obtained from Nd model ages of sedimentary-origin metamorphic rocks and granites, which were calculated using depleted mantle parameters, and U-Pb zircons contained in Cretaceous Ryoke granites and Jurassic mafic rocks. (2) 1750-1600Ma. Ceologic events of this age range are not clear. These age data were obtained from Nd model calculations of sedimentary-origin metamorphic rocks and granites. (3)220-190Ma. Mafic volcanism and plutonism took place under continental circumustances. These magmas were derived from continental lithospheric mantle. Age data of this range were measured using Sm-Nd whole rock isochron and whole rock-mineral isochron of gabbros and metadiabases. (4) 170-160Ma. Some mafic to intermediate rocks were formed during this age range. Age data of this range were obtained from Sm-Nd and Rb-Sr whole rock isochrons and Sm-Nd whole rock-mineral isochron. (5) 125-70Ma. Voluminous felsic igneous activity took place during this age ranges under island arc and continental margin circumustances. These age data were obtained from Rb-Sr whole rock isochron. Sm-Nd and Rb-Sr whole rock-mineral isochronos. As initial Sr and Nd isotopic compositions of these rocks are the same as the 220-190Ma mafic rocks, the felsic magmas are considered to have derived from the continental lithosphere lower crust and upper mantle.The Cretaceous to Paleogene igneous rocks showing the same isotopic features with Ryoke felsic igneous rocks are widely distributed in the Honshu Arc, which means that the Ryoke-type continental lithosphere basically made up the Honshu Arc.