RADIOMETRIC GEOCHRONOLOGY - SOME CONSTRAINTS TO THE ISOCHRON METHOD BY AN ITERATIVE LEAST-SQUARES APPROACH

An iterative procedure, based on the implemented Gauss method, has been applied to the radioactive decay law as used in geochronology, in order to refine the age and initial isotopic com position parameters. The method furnishes some statistical information about the fitting, obtained during the iterative path, such as the correlation coefficient between parameters and the residual/standard deviation ratio for each of the regressed points. In particular, the ratio between residuals and standard deviation is a tool to evaluate how each point influences the linear regression in relation to its actual scatter from the best-fit line. It is believed that this approach may be useful if coupled with standard isochron calculations. As an example, the method has been applied to date some Hercynic leucogranites outcropping in the Buddusò intrusive complex (NE Sardinia, Italy). The iterative least-squares procedure allows to remove from the fitting some samples which were perturbed with respect to the Rb-Sr systematic. The isochron calculated on the selected rocks yields an age of 274+/- 9 Ma, which dates the last isotopic homogenization event, and could represent the emplacement age of the leucogranites. Furthermore, biotite minerals separated from the leucogranites yield an isochron age of 272+/-7 M a, thus indicating a possible high cooling rate of the granitic intrusion.