NEWLY RECOGNIZED BASEMENT(?) TO THE BELT SUPERGROUP, NORTHERN IDAHO

Amphibolite facies metasedimentary rocks near the edge of the North American continental margin north of Orofino, Idaho, are unlike the 1400-1470 Ma Belt Supergroup with which they have been previously correlated. Four intervals compose this Syringa metamorphic sequence, but the top directions are not known. In succession from south to north, the intervals are dominated by the following rock types: garnet-muscovite-biotite gneiss, coarsely crystalline quartzite, calc-silicate gneiss, and muscovite-biotite schist; each interval also contains subordinate amounts of the other three rock types. The quartzite contains little or no feldspar, unlike that in the Belt Supergroup. Calc-silicate rocks are mostly thin alternating layers of diopside- or hornblende-rich quartz-plagioclase rocks with local garnet-rich zones. The schist contains up to 20 percent sillimanite but little garnet, whereas the opposite is true of similar grade schist of the Belt Supergroup (upper Wallace Formation). Overall, the Syringa sequence is more similar to amphibolite facies metasedimentary rocks in the northern Monashee Mountains of British Columbia, some of which are at least 1850 Ma in age. Similarities also exist with the 1700 Ma Willyama Supergroup of Australia. Four abraded zircon fractions from augen gneiss within the sequence yielded strongly discordant analyses that define a chord with upper and lower intercepts at 1372 ± 10 Ma and 64 ± 14 Ma respectively. The upper intercept age is consistent with ages on similar augen gneiss at Shoup and Elk City, Idaho. If the upper intercept represents an intrusive age, then the Syringa sequence is as old, or older, than the Belt Supergroup. Understanding the age and protolith of the Syringa metamorphic sequence is important to continental reconstructions of western North America.