Ar-40-Ar-39 geochronometry of pseudotachylytes by vacuum encapsulation: North Cascade Mountains, Washington, USA

To determine a reliable means of dating pseudotachylytes, we obtained total gas ages, plateau ages and argon-retention ages on samples from the North Cascade Mountains, Washington. Quartz ampoule vacuum-encapsulation analysis of small grains (a few milligrams) from six samples allowed evaluation of Ar-39 loss through recoil during irradiation, and laser ablation on four sa mples allowed textural control, In microlitic samples, recoil-loss Ar-39 ra nges from 0.2% to 8.4%, The "total gas" ages incorporating this Ar-39 are i nconsistent, but retention ages are much more internally consistent, and wi thin analytical error they match or nearly match plateau ages from most of the microlitic samples at 54-55 Ma. Samples analyzed by laser ablation sugg est one episode of faulting and pseudotachylyte formation at 55-59 Ma, and an earlier episode at ca, 80 Ma, The partly glassy pseudotachylyte yielded laser-ablation, total gas, and retention ages of ca, 65 Ma. The microlitic pseudotachylytes indicate formation close to 55 Ma, with possible older fau lting and pseudotachylyte formation at ca, 81-84 Ma, The good consistency a mong the retention ages, the laser ages with highest precision, and especia lly the plateau dates, combined with the ages falling within the anticipate d time span, provides confidence in the geologic reality of the ages. Never theless, it is clear that large clasts and crystal fragments within these p articular veins did not completely degas or may have an anomalous inherited radiogenic Ar-40 component.