Contact relations and internal fabric suggest that penetrative metamorphism and deformation of parental rocks of the Okanogan gneiss dome culminated in their mobilization and diapiric intrusion into the country rocks. The gneiss dome cuts both plutonic rocks and eugeosynclinal low-grade metamorphic rocks, including rocks as young as Late Triassic, but not adjacent sedimentary deposits and associated volcanic rocks of Eocene age. The gneiss dome is itself cut by the Swimptkin Creek pluton, an epizonal quartz monzonite body that yields concordant biotite and hornblende K-Ar ages of 48.0 and 48.2 m.y. (Eocene), respectively. Thus, the field relations bracket the age of emplacement of the gneiss dome between Late Triassic and Eocene time.

A total of 21 age determinations were made using samples from seven localities within the gneiss dome. Zircon dated by U-Th-Pb methods gave the oldest ages, including Pb206/U238 and Pb207/U235 ages of 87.3 and 100.0 m.y., respectively. Fission-track ages of 66 m.y. (sphene), 63 m.y. (epidote), 59 m.y. (allanite), 53 m.y. (apatite), and 51 m.y. (apatite) were obtained, along with K-Ar ages ranging from 58 m.y. (hornblende) to 46 m.y. (biotite).

These data suggest that the gneiss dome was emplaced in Late Cretaceous time — probably between roughly 87 and 65 m.y. ago — then cooled slowly through the successive temperature thresholds for sphene, epidote-allanite, hornblende, and finally apatite-muscovite-biotite, below which either loss of argon or erasure of fission tracks ceased.

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