Precise electron microprobe analysis makes it possible to determine the Th, U and Pb concentrations in an area less than 5 μm across in a single grain of monazite, zircon or xenotime; the detection limit of PbO at 2σ confidence level is 0.005–0.008 wt.% and the relative error is 3% for 0.2 wt.% concentration level. The subgrain analyses of monazite are plotted on the coordinates of PbO and ThO2* (ThO2 plus the equivalent of UO2), and those of zircon and xenotime on the coordinates of PbO and UO2* (UO2 plus the equivalent of ThO2). Data points are arrayed linearly and enable us to define an isochron which passes through the origin. The chemical Th-U-total Pb isochron ages coincide well with mineral and whole-rock ages isotopically determined for the same samples. The chemical Th-U-total Pb isochron method was applied to the age determination of monazite, zircon and xenotime from the Tsubonosawa paragneiss and the host Hikami granite in the South Kitakami terrane of Northeast Japan. The chemical ages show that (1) the sedimentation of the gneiss-protolith occurred soon after the emplacement of 500 Ma granitoids in the source region, where the 500 Ma granitoids were widespread together with Precambrian rocks possibly dated back to as old as Archean (3080 ± 180 Ma), (2) the gneiss-protolith was metamorphosed to the amphibolite facies grade about 430 ± 10 Ma ago, and (3) the gneiss was affected by multiple thermal events of 350, 260, 180 and 100 Ma. The 350 Ma event corresponds to the intrusion of the Hikami granite, and the 100 Ma one to the intrusion of the Cretaceous Kesengawa granite. The 260 and 180 Ma events may correspond in age to the main metamorphic event and subsequent igneous activity in the Hida terrane, central Japan. The chemical Th-U-total Pb isochron age in terms of the precise microprobe analysis of low-level Th, U and Pb will open a new vista on the Paleozoic-Mesozoic tectonics of the Japanese Islands.

Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime

Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites

CHIME (chemical Th–U-total Pb isochron method) monazite ages were determined for gneisses and granitoids from the eastern and western parts of the Ryoke belt separated by about 500 km. The monazite ages for the gneisses are concentrated between 102 and 98 Ma, and are interpreted as the time of monazite formation under lower amphibolite facies conditions. The peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c. 95 Ma in both the eastern and western parts. In the eastern part plutonism continued from c. 95 Ma to c. 68 Ma at intervals of 2–10 Ma, whereas in the western part it ceased at c. 85 Ma. The CHIME monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts. These lines of evidence are incompatible with a current view that the plutonometamorphism in the Ryoke belt becomes younger towards the east. The CHIME monazite ages, coupled with available data on the depth at which the Ryoke metamorphism took place and the emplacement of individual plutons, show that the western part was eroded more rapidly (about 1.5 mm year−1) than the eastern part (about 0.8 mm year−1) over the time span from 91 to 85 Ma. The denudation rates agree well with those in active orogenic belts like the Alps and Himalayas.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1998.00057.x

Denudation history of the high T/P Ryoke metamorphic belt, southwest Japan: constraints from CHIME monazite ages of gneisses and granitoids

CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data

A Rodinian suture in western India: New insights on India-Madagascar correlations

Standards for quantitative microprobe determination of uranium and thorium with additional information on the chemical formulae of davidite and euxenite—Polycrase

N. Cogger

Papers

Standards for quantitative microprobe determination of uranium and thorium with additional information on the chemical formulae of davidite and euxenite—Polycrase