MIDDLE PRECAMBRIAN PROVENANCE OF JURASSIC SANDSTONE IN THE MINO TERRANE, CENTRAL JAPAN - TH-U-TOTAL PB EVIDENCE FROM AN ELECTRON-MICROPROBE MONAZITE STUDY

Precise electron microprobe analyses have been made on detrital monazites in Jurassic sandstones of the Mino Terrane, central Japan, and their Th-U-Pb relations are examined in terms of PbO and ThO2* (ThO2* = ThO2 plus the equivalent of UO2). The PbO/ThO2* ratios of most detrital monazites (403 of 581 grains) cluster in the range from 0.0763 to 0.0618. Other grains show the ratio of about 0.0541 (46 grains), 0.0364 (29 grains), 0.0170 (24 grains) and 0.0116 (79 grains). The PbO/ThO2* ratio of 0.0763-0.0618 is nearly identical with that of monazines in a middle Precambrian (1740 Ma) gneiss clast from the Kamiaso Conglomerate of the Mino Terrane; this strongly suggests that the detrital monazites were derived from gneisses and/or granitoids of middle Precambrian age. The ratio of 0.0116 is nearly the same as that of monazites in a Permian (274 Ma) paragneiss of the Hida Terrane north of the Mino Terrane. The PbO/ThO2* monazite data, coupled with palaeocurrent analyses and petrological characteristics of sandstones, show that middle Precambrain high-grade metamorphic rocks and granitoids were widespread both in the northern and southern source regions of the E-W trending Jurassic basin of the Mino Terrane. The PbO/ThO2* ratio of detrital monazites is a useful new measure for the provenance analysis of clastic rocks.