FINAL COOLING AND UNROOFING OF THE PILOT AND NORTH TOANO METAMORPHIC TERRAINS IS MIDDLE MIOCENE IN AGE

New 40Ar/39Ar and (U-Th)/He cooling ages from the Pilot Range and northern Toano Range of northeastern Nevada record final unroofing of these terrrains during the Middle Miocene, although suggestions of an older cooling history are locally preserved. 40Ar/39Ar hornblende data from granodiorite at Bettridge Creek on the east flank of the Pilot Range yields saddle-shaped spectra indicating excess argon, but one sample yields a quasi-plateau at ~75 Ma. Biotite from the same sample yields an isochron age of 29.5 +/- 0.4 Ma suggesting a cooling event in Oligocene time, and K-feldspar from a nearby lecogranite yields a similar quasi-plateau at ~30 Ma in the higher temperature steps with a diffusive loss pattern stepping down to ~14 Ma in the lower temperature increments. (U-Th)/He cooling ages from zircon and apatite are 10.6 Ma and 9.1 Ma, respectively, confirming the Miocene age of final unroofing. Data from Patterson Pass at the northern end of the Pilot Range also indicate final unroofing in the Middle Miocene, with biotite yielding 40Ar/39Ar cooling ages of 36.3 to 37.6 Ma, probably recording conductive cooling after emplacement of the McGinty quartz monzonite. Similarly, Multi-diffusion domain modeling of K-feldspar from the McGinty quartz monzonite indicates initial cooling at ~ 35 Ma followed by steep final cooling between ~19 and 14 Ma. (U-Th)/He zircon ages from the same rocks are 11.8 to 12.2 Ma whereas Apatite cooling ages range from 8.2 to 10.6 Ma.

The North Toano Range farther west yields a similar but slightly older cooling history. The granodiorite of Silver Zone Pass yields 40Ar/39Ar hornblende plateau ages of ~ 155.5 Ma interpreted to record conductive cooling following pluton emplacement, but (U-Th)/He cooling ages for zircon from the same rocks range from 20.0 to 24.1 Ma, suggesting a slightly older, latest Oligocene to Early Miocene start to extension and cooling in this terrain. (U-Th)/He apatite cooling ages range from 13.6 to 16.9 Ma. Meanwhile, a clast of the distinctive Toano Springs 2-mica granite from Miocene conglomerate yields a muscovite plateau age of 74.7 +/- 0.2 Ma, a biotite cooling age of ~64.2 Ma, and a K-feldspar diffusive release spectrum stepping down from a high at ~47 Ma to a minimum at ~15 Ma, indicating that final unroofing of the North Toano Range must be Middle Miocene or later.