| Phlogopite-biotite micas are among the most commonly-dated minerals by 40Ar/39Ar technique to determine the crystallization or cooling ages of rocks. Interpretation of the geological significance of these ages is largely dependent on understanding how 40Ar* can be retained or released from the mineral during the thermal history of a terrane.; Marble and metapelite outcrops located near Verona, Ontario have been metamorphosed to granulite facies during the Grenville orogeny (ca. 1200--950 Ma). Samples from this area show that there is an age discrepancy of over 100 Ma among coexisting metamorphic micas with a range of chemical compositions, all from the same outcrop. As a result, this field site has provided a unique opportunity to elucidate the various chemical controls on argon mobility in micas.; A wide variety of analytical laboratory techniques were used in an attempt to understand this enigmatic age difference, from the fields of crystal chemistry (the electron microprobe analysis, the Fourier transform infrared spectroscopy analysis, and the X-ray diffraction with corresponding structural refinements) and isotopes (the 40Ar/39Ar step-heating method, U/Pb dating, Rb/Sr dating, and the stable-isotope analyses).; Three main factors may account for the age discrepancies in micas, including thermal pulses from magmatic intrusions, excess argon incorporated from metamorphic or hydrothermal fluids, and variations in chemical composition among the metamorphic micas. However, none of these factors can successfully explain the analytical data. The pattern of Grenvillian-age distributions suggests that the micas have cooled rapidly; this is a distinctly different interpretation from previously inferred thermal histories in the southwestern Grenville province. |
