[1] Abstract: A new geochemical estimate of the young (mainly Paleozoic age to present) upper crust of the Japan Arc shows a dacitic composition in contrast to the idea that andesite is predominant in active orogenic arcs. Temporal changes in composition are not significant from the Paleozoic age to the present for the Japan Arc. The major element composition is similar to previous models of old cratonic upper crusts. The coincidence in the major elements between young and old crusts indicates that essential mechanisms during crust formation have not changed from the Archean era to the present. In trace element compositions the average young upper crust of the Japan Arc has higher Sb and As concentrations and lower concentrations of alkaline, light rare earth, and high field strength elements with respect to previous models of continental upper crusts. The large degree of constancy of trace element composition in marine sedimentary rocks is in contrast to the large variety in igneous rocks. However, the averages for both accretionary and nonaccretionary sedimentary rocks are almost identical to the average for the igneous rocks of the Japan Arc, with the exceptions of high Sb and As concentrations in unmetamorphosed sedimentary rocks. The compositional homogeneity among different types of rocks on an arc scale implies that recycling processes mechanically mix the arcderived igneous materials to homogenize the chemical composition during erosion, transportation, sedimentation, accretion, and uplifting. Since the contribution of oceanic crust to the composition of arc crust is small, the recycling processes have not changed the bulk upper crustal composition of the active continental margin except increase the Sb and As from sediments. Instead, the influx of differentiated acidic rocks from depth is essential to characterize the orogenic crust formation of the young Japan Arc. The characteristically low incompatible element content of the Japanese upper arc crust appears inherited from parental magmas derived from a mantle source depleted during a long-term evolution.

Optical and X-ray studies of carbonaceous material in the Tono contact metamorphic aureole, Kitakami Mountains, northeast Japan, have revealed that metamorphic graphitization proceeded through two discontinuous changes: first, optically anisotropic domains develop within the coaly phytoclast, forming transitional material, and then, ordered graphite crystallizes by the decornposition of pre-existing carbonaceous materials. Coaly material disappears in the uppermost chlorite zone. Transitional material appears in the middle of the lower chlorite zone. Graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite isograd to the cordierite isograd where all the carbonaceous materials have converted to graphite. The apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials. The temperature of graphitization in the present area is at least 100°C higher than the temperature in the Sanbagawa and New Caledonia high-pressure metamorphic terrains, probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism.

Margarite-paragonite-muscovite assemblages from the low-grade metapelites of the Tono metamorphic aureole, Kitakami Mountains, Northeast Japan YASUKO OKUYAMA-KUSUNOSE Fuel Geology Department, Geological Survey of Japan Ibaraki, 305, Japan.Margarite occurs in the low-grade zone of the contact aureole of the Tono granitic mass, Kitakami Mountains, Northeast Japan, coexisting with muscovite, paragonite, chlorite and quartz, but without carbonates and feldspars. The margarite is produced by the reaction between pyrophyllite and zoisite in the progressive metamorphism at a temperature near the upper stability limit of the assemblage margarite +quartz.The solubility of muscovite component in paragonite in the three white micas is suppressed as compared to that in the muscovite-paragonite assem blage, whereas the paragonite solubility in muscovite of the three-phase assemblage is similar to that of the two-phase assemblage.Paragonite component in margarite in three-phase assem blage of this area is about 17.3-22.5 mol% ; this value is the smallest for the margarite in three phase assemblages so far reported. The low paragonite solubility of margarite in this area is probably due to the low pressure condition of the contact metamorphism of the Tono area.

Mineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2-3 kbar. The bulk rock compositions of metapelites are aluminous with A' values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low-grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of AI,SiO, polymorphs is limited in typical pelitic rocks.The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5-0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K-feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite-sillimanite type aureoles. Rocks with FeO/(FeO + MgO) > 0.8 progressively develop chloritoid-bearing assemblages from Bt-Chl-Cld, And-Bt-Cld, toAnd-Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low-temperature side of the formation of cordierite by the terminal equilibrium of chlorite. A P-T model depicting the relative stability of chloritoid and staurolite at low-and medium-pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, chloritoid, garnet and staurolite.

No abstract