AR GEOCHRONOLOGY OF PSEUDOTACHYLYTES USING SUB-SAMPLE DATING, ILLUSTRATED BY DATING THE ST. LAWRENCE RIFT SYSTEM, SOUTHERN QUEBEC: 613-614Ma RIFT INITIATION OF THE IAPETUS OCEAN

Radiogenic dating of pseudotachylytes (PST) offers the potential for determining the absolute age of major, coseismic faulting, but is hindered by incomplete melting and associated resetting of host rock inclusions. Incorporation of clasts in PST veins, in particular, has given geologically ambiguous ages from ⁴⁰Ar/³⁹Ar dating, signifying that host rock inclusions are not equilibrated with the melt. In the Quebec Appalachians, the Montmorency fault (MF) of the St. Lawrence Rift System (SLRS) is a Late Proterozoic normal fault in association with the opening of the Iapetus Ocean. The MF is a NE-SW trending, steeply dipping normal fault that separates Grenville gneisses to the NW from Paleozoic sediments to the SE. PST specimens from this long-lived fault were collected from Grenville footwall rocks. Two ⁴⁰Ar/³⁹Ar techniques are used to determine the timing of displacement: encapsulation dating and multiple sub-sample dating. The total gas age (TGA) of encapsulation dating represents a minimum age for argon retention upon cooling, and the retention age (RA) provides a maximum age for the sample. Sub-sample analysis capitalizes on the inclusion of clasts, by incorporating clast/matrix proportions in the ages of sub-samples from a single vein. Extrapolating the proportion of clasts to zero (i.e., all matrix), the age of vein formation is dated; conversely, extrapolating to 100% clasts records the (cooling) age of the host rock. Laser-ablation ⁴⁰Ar/³⁹Ar step-heating analyses yield TGA and RA for encapsulated PST from the MF, are 610.3 ± 4.6 Ma and 619.0 +/- 2.5 Ma, reflecting the permissible age window. Linear regression analyses of 10 sub-samples from two PST veins with varying matrix/clast proportions intercept the y-axis (0% clasts) at 614.2 and 613.3 Ma. At 100% clasts, the regression lines for these samples intersect at 856 and 861 Ma, which match cooling ages of Grenville host rock in the region. These ages tightly constrain major motion along the SLRS, significantly improving on previous time constraints for rifting. We conclude that the time of major continental rifting along the northern Laurentian margin and initiation of the Iapetus Ocean occurred at 613-614Ma, and that sub-sample dating of pseudotachylyte is a robust Ar geochronology approach.