Abstract

Petrologic and thermochronological analysis of the Santaquin metamorphic complex (SMC) reveal that it differs in composition and age from Archean basement exposed 50 km to the north and may represent the northern most exposure of Colorado Plateau basement. The SMC consists of mildly strained garnet amphibolite and schists intruded by granitoid bodies, which are mildly deformed by mostly non-rotational shear. Hornblende separates yield 40Ar/39Ar plateau cooling ages of 1657 +/- 2 Ma for the host amphibolite and 1623 +/- 2 Ma for a mafic syenite intrusion. K-feldspar from the syenite indicates the SMC cooled to <100fC at around 750 Ma, which is near the age of the unconformably overlying Big Cottonwood Formation. Reheating to 325 +/- 30fC occurred from around 500-350 Ma. This event was most likely caused by burial of up to 10 km of sedimentary successions during passive margin and Oquirrh Basin development. The feldspar reached its closure temperature (~200fC) at around 180 Ma, most likely as a result of basin inversion associated with the Sevier orogen. The time/temperature history after this event is currently being explored by analysis of apatite fission tracks.