The Salmon River suture zone of western Idaho (USA) records mid-crustal metamorphism and deformation associated with orogenesis during Mesozoic accretion of volcanic arc terranes to western Laurentia. We present petrographic and microstructural observations, garnet geochemistry, pressure-temperature isochemical phase diagrams, and Sm-Nd garnet and U-Pb zircon ages to investigate the timing and conditions of metamorphism in the Salmon River suture zone. The Salmon River suture zone is comprised of three thrust sheets: from east to west, the amphibolite facies Pollock Mountain plate, upper greenschist to amphibolite facies Rapid River plate, and greenschist facies Heavens Gate plate. The Pollock Mountain plate was isothermally loaded from 6 to >8 kbar at ~700 °C between 141 and 124 Ma during northwest-southeast crustal shortening. The underlying Rapid River plate was isothermally loaded from 7 to ~10 kbar at 600-650 °C during ca. 124-112 Ma metamorphism, which is contemporaneous with late-to post-peak metamorphism and ca. 118 Ma exhumation of the overlying Pollock Mountain plate. In the Rapid River plate, thrust sheet emplacement induced high-strain ductile deformation and led to regional development of linear-planar fabrics. The 206 Pb/ 238 U zircon ages for syndeformational to postdeformational magmatism record ca. 117 Ma or younger juxtaposition of the two plates on the southeast-dipping Pollock Mountain thrust fault. Coeval 124-112 Ma metamorphism of the Rapid River plate, ca. 118 Ma exhumation of the Pollock Mountain plate, and ca. 117 Ma or younger movement along the Pollock Mountain fault suggest that metamorphism of the Rapid River plate was possibly driven in part by thrust juxtaposition and loading along the Pollock Mountain fault. In this context, we interpret that metamorphism records diachronous thrust stacking during prolonged (>30 m.y.) accretionary orogenesis in western Idaho.

The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time (P-T-t) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30±0.13 to 115.70±0.18 Ma (CA-ID-TIMS) and 121.4±2.0 to 109.8±1.8 Ma This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.