Regional Tectonic Evolution of the Pioneer Metamorphic Core Complex, South-Central Idaho

The Pioneer core complex (PCC) is one of lesser-studied Cordilleran metamorphic core complexes (MCCs) that is located north of Snake River Plain in Rocky Mountains. The PCC resides in a transitional zone separating Eocene MCCs to the north and Miocene MCCs to the south which is a key location to understand processes of subduction and continental rifting in western US. The project applied thermodynamic modeling, zircon U-Pb dating, and Ti-in-zircon and Ti-in-biotite thermometry to study the pressure-temperature-time-deformation (P-T-t-d) history of the PCC. Zircon ages from gneiss, granitic dike and quartzite are in Late Cretaceous to Early Eocene. The age of 109±4 Ma suggests that NE-SW D1 compression was on going 109 Ma. The ages of 83.9±5 and 85.6±1 Ma indicate that D2 compression was >86-84 Ma. The younger ages fall into 3 groups, 52 Ma, 48 Ma and 44 Ma. They indicate that NW-SE D3 extension and magmatism was ongoing 52 Ma and ceased 44 Ma; the emplacement of intrusive granodiorite at 48 Ma gave peak metamorphism of the PCC in upper amphibolite-facies. Ti-in-zircon thermometry calculated peak T of the PCC at 1100°C. Ti-in-biotite thermometry calculated peak T of biotite at 895°C and retrograde metamorphism following the path from 895°C down to 500°C finally. Thermodynamic modeling generated pseudosections of gneiss and phyllite on which the P-T-t-d path of the PCC follows a clockwise path that progrades to peak condition at 4 kbar, 850-900°C at 52-48 Ma, and followed by retrograde stage to 2 kbar, 500°C at 44 Ma. The metamorphic history of PCC started at >109 Ma (D1) in Late Cretaceous during the initiation of Sevier orogeny as the Farallon plate subducted at a shallow angle. Then the PCC underwent D2 Sevier compression 86-84 Ma when the Farallon plate became flat-subducting. However, due to lack of ages between 109 and 86 Ma, it is uncertain if D1 and D2 were one continuous event or two separate episodes in Pioneer region. Following the compression, the PCC underwent D3 extension and synchronous Challis magmatism 52-44 Ma in Eocene as a result of resurrection of spreading ridges on the Farallon plate. It created a micro-ocean lithosphere 55 Ma that stalled underneath the continent and opened a slab window under Idaho. The slab window triggered rapid Challis magmatism and high-rate extension in north of the Snake River Plain 50-45 Ma.