CRYSTAL CHEMISTRY OF MONAZITE FROM THE GRANITIC ROCKS OF JAPAN: PETROGENETIC IMPLICATIONS

Abstract

Electron-microprobe analyses were carried out on monazite from magnetite-series and ilmenite-series granitic rocks in Japan. All of the samples examined consist of monazite-(Ce), except for a unique case of monazite-(Sm) in the Shimo-ono ilmenite-series granitic pegmatite. The observed variations in their chemical composition are largely explained by the huttonite exchange [(Th + U) 4+ ↔Si 4+ ], and subordinately by the cheralite exchange [(Th + U) 4+ ↔2Ca 2+ ]. However, there are differences in the degree of contribution of the two exchanges between monazite grains from magnetite-series and ilmenite-series granitic rocks. The cheralite exchange is more important for monazite grains in ilmenite-series granitic rocks than for those in magnetite-series granitic rocks. Monazite grains in Japanese granitic rocks of these two series characteristically have a low cheralite component compared to those in S-type granites from various regions in the world, but chemical characteristics of ilmenite-series granitic rocks are more similar to S-type granitic rocks than those of the magnetite-series. The monazite grains of the Japanese granitic rocks show a continuous decrease in the slope of chondrite-normalized REE patterns from magnetite-series granites [(La/Sm)cn: 6.22–7.50] to ilmenite-series granitic pegmatite [(La/Sm)cn: 0.09–1.88] through magnetite-series granitic pegmatite [(La/Sm)cn: 2.25–3.70] and ilmenite-series granites [(La/Sm)cn: 2.13–4.56]. Thus, the REE compositions of the monazites reflect the degree of fractionation from granites to granitic pegmatites in both series. The same trend is confirmed by the (La/Nd)cn value, resulting in a positive correlation between (La/Nd)cn and (La/Sm)cn values of monazite from Japanese granitic rocks. Because the whole rock REE compositions of the magnetite-series and ilmenite-series granites show higher and lower (La/Sm)cn-(La/Nd)cn values, respectively, which are identical to the values of the respective granites-series monazites, we conclude that the chemical compositions (especially REE compositions) of monazite reflect the types (magnetite-series or ilmenite-series) and the degree of differentiation (granite or granitic pegmatite) of the granitic rocks in the subduction-related magmatic arcs.