SHRIMP-RG ANALYSES OF IGNEOUS AND METAMORPHIC MINERALS FROM THE NORTHERN GROUSE CREEK MOUNTAINS, UTAH: NEW EVIDENCE FOR THE TIMING OF MESOZOIC AND CENOZOIC METAMORPHISM AND DEFORMATION

The Albion-Grouse Creek-Raft River (ARG) metamorphic core complex exposes ductiley deformed rocks that preserve multiple phases of deformation and are characterized by high strain, subhorizontal foliations. In Basin Creek, in the northern Grouse Creek Mountains, Archean augen gneiss and Neoproterozoic schists and quartzites are intruded by the Cenozoic Vipoint pluton; all units contain subhorizontal ductile fabrics with W-NW-plunging lineations. The Schist of Stevens Springs is in contact with the crystalline basement, with several older units missing at the contact, suggesting a pre- or syn-metamorphic younger-on-older structural juxtaposition. Previous workers have suggested a Cretaceous age for much of the metamorphism and deformation in this area, but U-Pb SHRIMP-RG ages of zircon and monazite also reveal significant Tertiary metamorphism. Zircons from the deformed Vipoint pluton have 28.3 ± 0.7 Ma igneous rims and 2556 ± 21 Ma inherited cores, and the Vipoint pluton was likely the source of heat and fluids for metamorphic mineral growth in the country rocks at the time of intrusion. The Vipoint pluton has a strong mylonitic foliation that developed during top-to-the-west, normal-sense shear. Zircons from the basement augen gneiss have 97 ± 13 Ma metamorphic rims and 2551 ± 14 Ma igneous cores. However, metamorphic monazites from the gneiss are 29.6 ± 0.3 Ma with no evidence for inheritance indicating either new growth or complete recrystallization at that time. Zircons from the Schist of Stevens Springs have narrow metamorphic rims that range in age from 30 Ma to 145 Ma, consistent with Cretaceous metamorphism that was overprinted by a Tertiary event. Metamorphic monazites from the schist are dominantly 34 ± 0.5 Ma, but also contain age domains of ~140 Ma and ~30 Ma. The Tertiary ages are similar to the ages of the syn-extensional plutons in both the Albion Mountains and the southern Grouse Creek Mountains. These data document that, although Mesozoic metamorphic events are preserved in the northern Grouse Creek Mountains, they are largely obscured by a major regional magmatic, thermal and deformational event or events that occurred in the middle Tertiary.