SYN-EXTENSIONAL MAGMATISM IN THE GROUSE CREEK AND ALBION MOUNTAINS METAMORPHIC CORE COMPLEXES, UTAH AND IDAHO: IMPLICATIONS FOR GNEISS DOME GENESIS

Key evidence linking magmatism to vertical rise of mid-crustal rocks during Cenozoic Basin and Range extension is found in core complexes/gneiss domes in the Grouse Creek Mts., Utah and the Albion Mts., Idaho.

In the Grouse Creek Mts., Precambrian gneisses and overlying PC-Cambrian sediments form the metamorphic core of the range and are intruded by the Red Butte granite (25.3 ± 0.5 Ma, SHRIMP U-Pb, zircon). A high-strain sub-horizontal foliation and NW-SE to E-W lineation overprints an older (undated) N-S lineation associated with an older sub-horizontal foliation in country rocks. Adjacent to the granite, textures indicate heating outpaced deformation during the second, younger event. Metamorphic assemblages in the contact aureole of the granite include STT+KY+MU+BI followed by SILL (after KY), and then andalusite. Late- growth of KY (w/ AND) is also observed. The younger fabric becomes best developed towards the Ingham Pass fault, which began its motion at peak metamorphic conditions and ceased moving in the brittle regime. The Red Butte pluton is deformed primarily in its western reaches, where final shear zone deformation post-dated its crystallization.

The Almo pluton in the Albion Mts. intrudes PC gneisses and overlying PC-Cambrian sediments and is 29 Ma (U/Pb, monazite). A subhorizontal foliation with NW-SE to E-W lineation is syn-metamorphic. Thinning of country rocks over the pluton has collapsed metamorphic isograds; within a structural section of several 100 m’s, chlorite schists give way to STT, and then SILL-bearing schists. Near the contact, textural relations indicate heating outpaced deformation and the pluton is undeformed; at higher structural levels and to the west, syn-metamorphic to late-metamorphic mylonitic fabrics involve the western border of the pluton within the (normal-sense) Middle Mountain Shear Zone.

In both localities described above, lower plate rocks were initially intruded by magmas at depths of 14 km or greater (KY+STT stable); temperatures rose during intrusion (SILL stable) as pressures decreased (AND stable). Thus, crystallizing magmas rose ~6 –10 km, entraining country rocks that underwent simultaneous vertical thinning. Normal-sense shear zones formed along the western side of the complexes as they rose into the brittle regime.