A HUGE EVOLVED 1730 MA MAGMATIC ARC TO THE SOUTHWEST OF THE MESOPROTEROZOIC LEMHI SUBBASIN, BELT SUPERGROUP, IDAHO AND MONTANA

In the Beaverhead and Lemhi Ranges of MT and ID, south-derived, thick and lithologically uniform Mesoproterozoic Lemhi subbasin strata contain identical detrital zircon age-populations. The rocks have sparse grains as old as 2800 Ma, a strong unimodal age-peak at 1720 to 1740 Ma, and sparse grains as young as 1400 Ma. The presence of these young zircons dictate that rocks of the Lemhi subbasin correlate with the upper Missoula Gp. Strata mapped as the quartzite of Jahnke Lake from 50 km north of the crest of the Beaverhead Range in the Skalkaho area of the Sapphire Range, MT. (J. Lonn, unpublished), share the same 1720-1740 Ma age-peak. This requires the source terrane for the Lemhi subbasin to be a major ca 1730 Ma magmatic arc system to the southwest. Depending on definition, this might be part of Mojave, or Selway, or an unnamed exotic basement province.

We have attempted a reconstruction of the volume of the eroded material from this arc system by estimating the volume of sediment in the Lemhi subbasin. We estimate a restored area for the Lemhi subbasin of 50,000 km², and an average thickness of 17 km, yielding a volume of 850,000 km³. Extensive chemical weathering during erosion may have resulted in the deposition of quartz-rich lithologies, such as the Swauger Fm. Preliminary estimates adding in the effects of chemical weathering suggest a volume of eroded material in excess of 1,000,000 km³. This volume is significantly larger than volumes for typical batholiths (100,000 to 450,000 km³), and may imply either a very large batholith supplied the sediment to the Lemhi subbasin, or extensive lower crustal melting occurred synchronously with batholith emplacement.

1675 to 1775 Ma detrital zircons in Lemhi Gp strata have εHf(i) between +8 and -7 suggesting they were derived from both melted Archean evolved crust, and from juvenile melts. Bulk rock Nd isotopes from Lemhi Gp argillites yield a range in εNd(i) between +1.1 and -5.4, also suggesting mixed derivation from juvenile and evolved crust.

In the Coyote Creek quad in southwest Montana, both Lemhi subbasin sandstones and Missoula Gp sandstones are present. The latter have a slightly older and less unimodal set of zircon age-populations, with age-peaks from 1750 to 1800 Ma, comparable to detrital zircons in the main Missoula Gp to the north and west.