The P-T History of Anatectic Pelites of the Northern East Humboldt Range, Nevada: Evidence for Tectonic Loading, Decompression, and Anatexis

The migmatitic lower plate of the Ruby Mountains-East Humboldt Range (RM-EHR) metamorphic core complex of northeastern Nevada represents a deep section of the hinterland of the Sevier orogenic belt. New major and trace element (Zr-in-rutile, Ti-in-biotite) thermobarometry is aided by garnet zoning analysis and a thermodynamic modeling approach involving melt reintegration. The P-T history for rocks from the Winchell Lake nappe (WLN) is characterized by a phase of high-temperature, nearly isothermal, probably tectonic loading followed by decompression and melting during continued heating from similar to 675 to similar to 740 degrees C. Rocks of the Lizzies Basin Block (LBB) below the emplacement fault for the WLN allochthon record no evidence of a nearly isothermal phase of tectonic loading and associated metamorphism. Hence a different P-T path characterized by more widespread melting during prograde heating and contraction is compiled for the LBB. Different interpretations are presented for the exhumation history and the context of partial melting in the WLN versus the LBB. Anatexis in pelites of the WLN occurred as a result of dehydration melting reactions that took place in a decompressional phase of the prograde P-T path, in contrast to dehydration melting during contraction in the LBB. The emplacement of the WLN must have been a significant event in the tectonic history of the RM-EHR. These results have important implications regarding the relationship between crustal thickening, syncontractional exhumation, and anatexis within continental orogenic systems.