The Chemical Th-U-total Pb Isochron Ages of Zircon and Monazite from the Gray Granite of the Hida Terrane, Japan

Abstract

Zircon and monazite from thirteen samples in the central part of the Hida terrane were analyzed precisely on an electron microprobe. The U-Th-Pb relation of zircon was examined in terms of PbO and UO2* (UO2* means measured UO2 plus UO2 equivalent of measured ThO2), and that of monazite in terms of PbO and ThO2* (measured ThO2 plus ThO2 equivalent of measured UO2). Most zircon grains from the Gray Granite form euhedral prisms with concentric growth zones; only a few exhibit rounded and/or overgrowth forms. Analytical data for clear portions of euhedral zircon grains from individual samples are arrayed linearly on the PbO-UO2* diagram and form a single isochron. The chemical Th-U-total Pb isochron ages of zircon are in the range from 230±10 to 250±20 Ma. Several zircon grains have rounded cores of Precambrian age (as old as 1270 Ma), suggesting contributions of recycled older materials to the formation of the Gray Granite. Monazite, recognized only in one sample, gives a chemical Th-U-total Pb isochron age of 230±10 Ma. These zircon and monazite ages strikingly contrast to previously measured Rb-Sr whole-rock isochron ages of 506±37 Ma for the Gray Granite (Shibata et al., 1989). Our microprobe observation of Sr- and Rbdistribution in granite samples discloses that the Gray Granite was open with respect to these elements during chloritization of biotite and sericitization of plagioclase. Consequently, the 506 Ma Rb-Sr isochron age as well as several Precambrian Rb-Sr model ages for the Gray Granite appears to be incorrect. Rather, we believe that the Gray Granite was emplaced into the Hida gneiss in early Triassic time (ca. 240 Ma).