INSIGHTS INTO THE ORIGINS OF METAMORPHIC CORE COMPLEXES (MCC'S) FROM THE ALBION-RAFT RIVER-GROUSE CREEK (ARG) REGION

After more than 35 years of research, Cordilleran MCC’s still present challenges and controversies. Questions include: The high P’s of metamorphic rocks in MCC’s- how did they get so deep and how did they get back up? Where are the structures that accomplish this and when/how did they move? Did mylonitic stretching fabrics form due to early extensional collapse of thick crust or at younger times during regional extension? How are the multiple events at depth in MCC’s (a lot going on in the Cretaceous to Oligocene) related to structures at the earth’s surface (apparently not a lot going on)? And what do the spreads of 40Ar/39Ar mica ages in MCC’s mean?

The ARG metamorphic core complex provides some resolution of these questions and underscores the importance of Cenozoic magmatism in MCC genesis. Intrusion of lower plate plutons between 32 - 24 Ma post-date the onset of Eocene magmatism (~42 Ma), their emplacement ages corresponding to a hiatus in regional volcanism. Petrologic and isotopic studies indicate they formed by partial melting during continued input of mantle-derived magmas into the crust. Heating and partial melting led to crustal flow and diapiric rise of plutons encased in zones of extreme ductile extensional strains, the “granite-cored gneiss domes” of Armstrong and Compton. Final crystallization occurred at ~ triple point conditions. Surface topography remained subdued above the domes, possibly because of the mobility of the underlying crust but extensional “damage zones” likely developed above and along their margins as they formed. High angle normal faults (throws up to 8-10 km+) are responsible for the final uplift of the ARG, nearly 10 Ma later beginning with the onset of Snake River Plain magmatism. In sum, data from the ARG suggest that the origins of MCC’s are linked to increasing T’s in the crust during mantle-derived magmatism in the Cenozoic. Gneiss dome models for MCC’s imply far less regional extension than do low-angle normal fault models, but MCC’s formed significant relative structural relief within the crust prior to Basin and Range faulting. The spreads in 40Ar/39Ar ages are related to structural position in ARG and proximity to plutons and are the result of Ar loss from Mesozoic micas in the Oligocene and/or residence of minerals in their partial retention zones until their uplift in the Miocene, or both.