USING ZIRCON, MONAZITE, AND TITANITE U/PB GEOCHRONOLOGY AND PHASE EQUILIBRIUM MODELING TO CONTRAIN PRESSURE, TEMPERATURE, AND TIME CONDITIONS FOR DECOUPLED MID-CRUSTAL FLOW IN THE PIONEER MOUNTAINS METAMORPHIC CORE COMPLEX

The Pioneer Mountains metamorphic core complex (PMCC) in Idaho (US) formed during Eocene NW-SW extension. U-Pb zircon ages from deformed dikes and undeformed dikes that cut extensional ductile fabrics at all levels of the footwall suggest that ~49-46 Ma ductile strain recorded by fabrics with NNE-NE lineations at the deepest levels was synchronous with, and decoupled from ductile and brittle strain at higher levels in the footwall that record the regional NW extension. We interpret the strain at the deepest levels as isostatically induced decoupled crustal flow. We used pseudosections and other metamorphic constraints, combined with U-Pb dating in zircon, monazite and titanite from samples above and within the decoupled level, to constrain the P-T-t conditions that led to syn-extensional strain decoupling.

In the decoupled level titanite in calc-silicates yields ages of ~48-49 Ma and monazites from a schist yield ages of ~55-49 Ma. Monazite and zircon from two migmatite samples from above the decoupled level yield ages of ~50 Ma. A metapelite from above the decoupled level, preserves monazite as inclusions in garnet, in Crd-bearing coronas around garnet, and in the matrix. Monazites in coronas and matrix yield ages from ~53-48 Ma, and monazites within garnets give concordant Precambrian ages (~1.1-1.4 Ga). 52-49 Ma ages were found as individual inclusions and as rims on Precambrian monazite, suggesting garnet growth during Eocene high T-metamorphism. A pseudosection from this sample suggests peak metamorphic conditions at .

Our new ages show that above and within the decoupled zone, high-T metamorphism took place at ~55-48 Ma, synchronous with magmatism in the footwall. We found no evidence for monazite growth during Cretaceous metamorphism, suggesting that the decoupled zone remained at relatively low metamorphic grade during pre-extensional crustal thickening, consistent with our metamorphic pressure estimates. Our data indicate that the isostatically induced decoupled crustal flow in the PMCC occurred at anomalously shallow crustal levels as a result of weakening due to magmatic heating. The PMCC contrasts with many core complex models where the levels of decoupling is deeper and controlled by the thermal structure inherited from crustal thickening.