ZR-IN-TITANITE GEOTHERMOMETRY INTEGRATED WITH MAJOR PHASE THERMOBAROMETRY IN THE LIZZIES BASIN BLOCK OF THE NORTHERN EAST HUMBOLDT RANGE, NEVADA

The prograde metamorphic history of a mid-crustal block of the Ruby Mountains–East Humboldt Range, Nevada, is strongly obscured by pervasive anatexis, intrusions, and penetrative ductile deformation. Metamorphism and deformation are attributed to compressional followed by extensional tectonism in an exhumed metamorphic core complex. In order to read the cryptic record of regional metamorphism during Cordilleran crustal thickening, Zr was measured in titanite crystals from a garnet amphibolite exposed at the deepest levels of Lizzies Basin block in the northern East Humboldt Range. Backscattered electron (BSE) imaging of titanite from various textural settings shows minor patchy zoning. Zr concentrations analyzed by electron microprobe range from below detection limits (~40 ppm) to 277 ppm. No Zr correlation with darker or lighter BSE zoning is recognized. Zr was analyzed across titanite grains that occur as inclusions within hornblende and garnet, as well as in the matrix. Zoning in Zr is generally minimal and shows no regular core–rim trend. Applying the Zr-in-titanite thermometer of Hayden et al. (2008), with a TiO₂ activity of 0.85, gives temperatures in the range of 745±10ºC at 8 kbar pressure (estimated by garnet–plagioclase–hornblende–quartz barometry). No distinct difference in temperature/Zr is recognized for matrix vs. inclusion grains. The results imply that titanite in these rocks was a near-peak phase and that at least some garnet and hornblende growth occurred at or above ~745ºC. These results are consistent with a rapid burial and heating scenario for the prograde evolution of the Lizzies Basin crustal block, similar to a rapid loading and then heating interpretation for the Angel Lake nappe block ~5 km to the north. It is possible that these blocks represent underthrust tectonic slices that experienced multiple phases of exhumation.