THE DETRITAL-ZIRCON TRAIL OF THE LEMHI ARCH: 500 MA ZIRCON POPULATION IN THE WORM CREEK QUARTZITE MEMBER, UPPER CAMBRIAN ST. CHARLES FORMATION, SE IDAHO

The Lemhi Arch, first documented by Larry Sloss (Sloss, 1954), is represented by thinning of lower Paleozoic strata above an unconformity in the Beaverhead and Lemhi Ranges of east-central Idaho. Specifically, Cambrian carbonates are absent and Ordovician quartzites of the Summerhouse and Kinnikinic Formations overlie Mesoproterozoic Lemhi Group or Neoproterozoic and Cambrian Wilbert and Tyler Peak siliciclastic units.

Detrital zircons from a fine-grained arkosic arenite in the Worm Creek Quartzite, a shallow marine sandstone within the Upper Cambrian (Dunderbergia and Elvinia Zones) carbonate platform of the SE Idaho miogeocline, reveal a very strong (90% of the grains) population with ages from 520 to 480 Ma (LA-ICPMS ages from Arizona LaserChron Center). In thin section, the sandstone contains altered microlitic volcanic rock fragments, abundant straight-extinction quartz and about 15% total orthoclase and albite feldspar.

Recent U-Pb zircon dating of the alkalic Beaverhead, Arnett Creek, and Deep Creek plutons in the general area of the Lemhi Arch (Lund et al., in press, GSA Bull) documents a 500 Ma to 485 Ma period of magmatism. Gillerman et al. (2008) document a 530 Ma syenite dike at Lemhi Pass in the Beaverhead Range.

We propose that the detrital zircons in the Worm Creek Quartzite represent this Cambrian magmatic pulse. An origin as reworked ash from volcanoes overlying the alkalic plutons is suggested by the overlap between the sandstone depositional age and zircon ages. Another option is rapid uplift of the plutons. The only other zircon population in the sandstone (~10% of total grains) is 1705 to 1780 Ma, and could be reworked from Mesoproterozoic Lemhi Group into which the plutons intruded. This suggests the Worm Creek Quartzite is the product of exposure of the Lemhi Arch during Late Cambrian time.