HIGH THERMAL GRADIENT IN THE UPPER PLATE OF A CORE COMPLEX, DETERMINED BY CALCITE-DOLOMITE AND RSCM THERMOMETRY, PEQUOP MOUNTAINS, NV

The Ruby Mountains (RM), East Humbolt Range (EHR), Wood Hills (WH) and Pequop Mountains (PM) of northeastern Nevada are widely recognized as an exemplar metamorphic core complex. However, debates still persist about the timing and magnitude of extension and the evolving thermal structure of the crust in this area. The Pequop Mountains represent the structurally shallowest level of this core complex and the lowest grade of metamorphism ranging from unmetamorphosed sediments to lower amphibolite facies. Previous work suggests that peak metamorphism likely occurred during the Late Cretaceous, but to date, no quantitative paleothermomety has been reported for the Pequop Mountains. Insight into the thermal evolution and degree of metamorphism can be better defined by establishing a pre-extensional thermal gradient in the Pequop Mountains in order to enhance the overall understanding of the metamorphic core complex. In this study we attempt to construct a thermal gradient across the metamorphic rocks in the Pequop Mountains. SEM and petrographic observations combined with calcite-dolomite thermometry and RSCM thermometry were used to document metamorphic temperatures in the Pequop Mountains and to assess the relationship of the Independence thrust to the exhumation history of the core complex. Aluminous layers within the Prospect Mountain Quartzite show an assemblage of qtz+alm+ms+phl+chl with alm replacing chl at higher grades. The Toano Formation marbles contain cc+dol+qtz+phl+chl+tlc with tr present at higher grades. Metamorphic marbles can contain cc+dol+qtz+ms±chl±bt±ksp±an±alb and in certain units the presence of talc with the appearance of bt and chl at higher grades. Provisional calcite-dolomite temperature data indicates a temperature increase down section from the east (292 ± 21 °C) to the west (481 ± 10 °C), consistent with the appearance of alm and tr at the expense of tlc near the base of the section. The results suggest a steep paleogeothermal gradient (~50 ˚C / km ± 10) for the section below the Independence Thrust, consistent with previously published suggestions that this area may have resembled the modern Altiplano during Late Cretaceous peak metamorphism.