NEW DETRITAL ZIRCON AGES CONSTRAIN THE ORIGIN AND EVOLUTION OF THE RIGGINS GROUP ASSEMBLAGE ALONG THE SALMON RIVER SUTURE ZONE, WESTERN IDAHO

Hornblende gneiss, calc-silicate quartzite, and uncommon marble of the Riggins Group assemblage comprise a nearly continuous high-grade metamorphic belt west of, and adjacent to, the north-south trending Salmon River suture zone in western Idaho. This assemblage, which attained lower-crustal metamorphic conditions during coeval intrusion of magmatic epidote-bearing tonalite-trondhjemite plutons in the Early Cretaceous (ca. 122 to 111 Ma), is sandwiched between lower metamorphic grade assemblages of the North American craton to the east and accreted Wallowa island arc terrane to the west. The origin and placement of the Riggins Group in the collage of Cordilleran tectonic terranes has been controversial for decades. Recent geologic mapping and new detrital zircon analyses provide constraints on the evolution of this important tectonic belt. Results from U/Pb LA-ICPMS detrital zircon analyses of two samples of amphibolite-grade hornblende gneiss yielded well-defined age modes of 202 Ma (+32, -50 Ma) and 198 Ma (+32, -19 Ma). In the former sample, zircon rims with low Th/U ratios yielded an additional mode of 113 Ma (+12, -8 Ma) and are as young as 83 Ma. This age mode is similar to Lu/Hf garnet ages in the same sample. Significantly, no reliable ages older than 235 Ma were recovered from either sample. We interpret these data as follows: 1) sedimentary protoliths of the Riggins Group are Early Jurassic to Early Cretaceous deposits; 2) the dominant ca. 200 Ma age mode represents detrital zircon derived from early Mesozoic igneous source rocks; 3) the ca. 113 Ma age mode represents peak metamorphism coincident with tonalite-trondhjemite pluton emplacement; and 4) younger ages represent continued rim growth during mid- to Late-Cretaceous plutonism. The Riggins Group contains clastic material eroded from igneous sources and deposited in a basin that was isolated from cratonal input. Likely protolith rocks in accreted terranes to the west include the Jurassic Weatherby Fm. of the Izee terrane and the Triassic–Jurassic Hurwal Fm. of the Wallowa terrane.