SHRIMP U-PB CONSTRAINTS ON AGE AND PROVENANCE OF THE RIGGINS GROUP AND ASSOCIATED METAMORPHIC ROCKS WEST OF THE SALMON RIVER SUTURE, WEST-CENTRAL IDAHO: TECTONIC IMPLICATIONS

In west-central Idaho, three undated, island-arc origin formations of the Riggins Group lie between the Salmon River suture on the east and Wallowa terrane on the west. Correlation of the Riggins Group with terranes of the Blue Mountains superterrane or with other Cordilleran terranes remains unresolved but is important to interpretations of tectonic evolution for this isolated accretionary-margin segment.

SHRIMP U-Pb analyses and mapping were undertaken to provide age, provenance, and structural constraints. Resulting data require complete revision of the two lower formations as well as of age, origin, and provenance of all three. From bottom to top, the formerly identified Fiddle Creek and Lightning Creek Schist include: (1) An undated, dominantly tonalite terrane that was faulted over the Wallowa terrane; (2) greenschist facies schists that depositionally overstepped the resulting terrane juncture, contain varying amounts of carbonate- and igneous-clast conglomerate, and contain Proterozoic Laurentian, Permian-Jurassic island-arc, and Cretaceous-Eocene zircons; (3) a thrust sheet containing amphibole-bearing metaflysch and garnet amphibolite having a maximum age of Middle Triassic (235 Ma); and (4) an undated sliver including thin interlayered ultramafic, carbonaceous slate, and debris flow units. Structurally above these is the island-arc derived, lower amphibolite facies, metaflysch of the Squaw Creek Schist (formerly upper formation of the Riggins Group) having a maximum age of Middle Jurassic (200 Ma).

Revised timing of tectonic events include: (1) amphibolite facies thrust sheets are fragments of several terranes that cooled during the Late Cretaceous after earlier tectonism, (2) greenschist facies schists are part of overstepping basin(s) that were deposited across earlier terrane junctures during the Cretaceous-Eocene, and (3) the overstepping rocks and terrane thrust sheets underwent metamorphism and deformation during structural stacking as late as Eocene. These data indicate that complex terrane interactions were ongoing west of the Salmon River suture through the Cretaceous into the Eocene, similar to the timing of terrane accretion and translation documented in British Columbia.