Abstract

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 39Ar loss through recoil during irradiation, and laser ablation on four samples allowed textural control. In microlitic samples, recoil-loss 39Ar ranges from 0.2% to 8.4%. The “total gas” ages incorporating this 39Ar are inconsistent, but retention ages are much more internally consistent, and within analytical error they match or nearly match plateau ages from most of the microlitic samples at 54–55 Ma. Samples analyzed by laser ablation suggest 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 faulting and pseudotachylyte formation at ca. 81–84 Ma. The good consistency among the retention ages, the laser ages with highest precision, and especially the plateau dates, combined with the ages falling within the anticipated time span, provides confidence in the geologic reality of the ages. Nevertheless, it is clear that large clasts and crystal fragments within these particular veins did not completely degas or may have an anomalous inherited radiogenic 40Ar component.

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