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.

Abstract

Precise electron microprobe analysis makes it possible to determine the Th, U and Pb concentra tions in an area less than 5 pm across in a single grain of monazite, zircon or xenotime; the detection limit of PbO at 2a confidence level is 0.005-0.008 wt.% and the relative error is 3% for 0.2 wt.% concen tration level. The subgrain analyses of monazite are plotted on the coordinates of PbO and Th02* (Th02 plus the equivalent of U02), and those of zircon and xenotime on the coordinates of PbO and U02* (U02 plus the equivalent of Th02). 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.