Thermobarometric constraints on the tectonothermal evolution of the East Humboldt Range metamorphic core complex, Nevada

The East Humboldt Range of Nevada provides a record of deep-crustal tectoni c processes in a classic Cordilleran metamorphic core complex located in th e hinterland of the Late Cretaceous to early Tertiary Sevier orogenic belt. New constraints reported here on the metamorphic history of this terrane s uggest an overall clockwise pressure-temperature-time (P-T-t) path that beg an with deep tectonic burial and metamorphism at kyanite + staurolite + gar net grade before Late Cretaceous time (possibly in Late Jurassic time?). Su bsequently, higher-temperature Late Cretaceous peak metamorphism overprinte d this event, resulting in widespread partial melting and leucogranite inje ction contemporaneous with emplacement of a large-scale recumbent fold (the Winchell Lake nappe), A new Pb-207/Pb-206* date of 84.8 +/- 2.8 Ma on synt ectonic leucogranite from the hinge zone of this fold constrains the age of this major phase of tectonism, Metamorphism at this time probably reached the second sillimanite isograd, at least at deep structural levels, with pe ak P-T conditions of 800 degrees C and >9 kbar, High-grade conditions persi sted during extensional tectonic denudation throughout much of Tertiary tim e. In conjunction with previously published work, the petrologic and thermo barometric results reported here for the northern East Humboldt Range delin eate a steeply decompressional P-T trend that extends from similar to 9 kba r and 800 degrees C to 5 kbar and 630 degrees C. In the light of decompress ional reaction textures, microstructural evidence, and previously published thermochronometric results, we interpret this trend as a P-T-t path for La te Cretaceous to Oligocene time. At least 2 kbar of this decompression (equ ivalent to at least 7 km of denudation) occurred in Late Cretaceous to earl y Tertiary time. This interpretation supports the idea that tectonic exhuma tion of deep-crustal rocks in northeastern Nevada began during or immediate ly after the closing stages of the Sevier orogeny, Finally, the steepness o f the proposed P-T-t path implies a thermal evolution from a colder to a mu ch hotter geotherm, a circumstance that probably requires plastic thinning of the lower plate in addition to brittle attenuation and removal of the up per plate during Tertiary extension.