TEXTURAL EVIDENCE FROM GARNET SCHIST FOR A MAJOR SYNCONVERGENT EXHUMATION EVENT IN THE HINTERLAND OF THE SEVIER OROGENIC BELT

Detailed pressure-temperature (P-T) paths obtained from metamorphic rocks in the Sevier orogenic belt document episodes of burial and exhumation. Some of these events are inferred by the numerical simulation of growth zonation in garnet based on chemical thermodynamics. Additionally, in the Neoproterozoic schist of Stevens Spring from the Basin Creek area of the central Grouse Creek Mountains in northwest Utah, reaction textures record a segment of the P-T path not recorded by garnet growth. Garnet grains that grew from chlorite breakdown during isothermal compression in the upper greenschist facies are heavily corroded and surrounded by wide reaction rims consisting of quartz, plagioclase and biotite. Muscovite, abundant in the matrix, is absent from the reaction rims. Several lines of evidence point to the development of this texture during increasing temperatures and decreasing pressures. Prograde diffusive Fe-Mg reequilibration occurs narrowly along the corroded garnet rims and along cracks within garnet, requiring post-corrosion exposure to temperatures at least 80°C hotter than growth temperatures. This is consistent with the complete lack of chlorite anywhere in the rocks, which would have developed had the corrosion been due to retrogradation, and with fluid-absent net transfer reactions that consume garnet and muscovite and produce biotite and plagioclase as a result of decreasing pressures. We interpret this prograde drop in pressure to be the result of a major synconvergent exhumation event, permissibly correlated to an orogen-parallel extensional event dated at 105 ± 12 Ma by laserprobe Ar-Ar on strain fringes. Exhumation postdates 138-149 Ma garnet growth from chlorite breakdown (Lu-Hf ages on presumably age-correlative garnet from the area) and predates garnet+sillimanite growth from staurolite breakdown in another horizon at 600-630°C. Growth of the younger garnet is inferred to have begun ~60Ma, based on in situ Th-Pb dating of monazite inclusions, and thus the age of the exhumation is bracketed between 138 and 60Ma. The 30-40 m.y. time lag between major thrusting and exhumation suggests that the mid-Cretaceous orogen-parallel extension may be have been triggered by radiogenic heating at lower crustal depths, leading to increased ductility and orogenic collapse, following Late Jurassic-Early Cretaceous crustal shortening.