Abstract:
The metamorphic rocks of the Salmon River suture zone (SRSZ) in west-central Idaho provide a unique glimpse into mid-lower crustal processes during continental growth by island arc accretion. The SRSZ, which separates island arc terranes of the Blue Mountains Province (BMP) from the Mesozoic margin of North America, contains medium to high grade tectonites that record multiple metamorphic and deformation events. The SRSZ is divided by the Pollock Mountain thrust fault (PMtf) into two structural blocks: the higher-grade Pollock Mountain plate (PMp), and the lower-grade, underlying Rapid River plate (RRp). Previous studies interpreted pre-144 Ma metamorphism within the SRSZ related to assembly of the BMP. Counter-clockwise P-T paths for metamorphism within the RRp [peak=8-9 kbar ~600°C, retrograde=5-7 kbar, 450- 525°C] were inferred to include prograde garnet growth during pre-144 Ma loading followed by garnet growth during rapid cooling due to lithospheric delamination. The PMp was interpreted to have subsequently been buried to increasing depth and metamorphosed again at 128 Ma as a result of the BMP docking with North America. New P-T-t paths for the RRp and PMp constructed from geochronology, geothermobarometry, pseudosections, and petrography suggest that after loading, slow cooling rates caused diffusion in garnet rims, which produced counter-clockwise P-T paths. Garnet Sm-Nd ages of 112.5±1.5 Ma from the RRp, and 141-124 Ma from the PMp suggest that metamorphism within the SRSZ is diachronous and that crustal thickening was protracted occurring between 141-112 Ma. P-T-t paths between both plates indicate that the PMp reached peak metamorphism prior to peak metamorphism of the RRp. This suggests that the PMp was buried prior to the development of the PMtf. The RRp was subsequently buried along the PMtf, which was followed by development of the Rapid River thrust fault, which juxtaposed RRp schists onto the Wallowa terrane of the BMP. This model suggests that metamorphism in the SRSZ was controlled by individual thrust faults instead of recording collisions between terranes and is consistent with a prolonged burial of rocks in the SRSZ followed by slow cooling that does not require lithospheric delamination to account for retrograde P-T estimates.
