THE MAJOR AND TRACE ELEMENT ZONING RECORD OF GARNET GROWTH AND EQUILIBRATION IN METAPELITIC ROCKS OF THE CENTRAL RUBY MOUNTAINS, NEVADA

The Ruby – East Humboldt metamorphic core complex in northeastern Nevada exposes a mid-crustal section of the hinterland of the Sevier Orogen. High-grade metamorphism and anatexis are recorded in metasedimentary rocks of the East Humboldt Range and the northern Ruby Mountains. An apparent decrease in peak metamorphic pressure and temperature occurs in a ~20 km span southward from Lamoille Canyon to the Green Mountain area of the central Ruby Mountains. No evidence for in situ partial melting is found in the central Ruby Mountains. Metapelitic rocks intruded by the Jurassic Dawley Canyon granite east of Green Mountain in the central Ruby Mountains, preserve the assemblage garnet + biotite + staurolite + sillimanite + muscovite + quartz + plagioclase + ilmenite ± tourmaline + apatite + monazite ± zircon. Some samples contain relict poikiloblastic andalusite. In an andalusite-bearing sample, garnet cores are relatively enriched in X_Grs (0.02–0.03) and X_Sps (0.10–0.15) with a rim-ward step down in X_Grs (to <0.01), followed by an X_Grs increase. This is consistent with a shift in garnet phosphorus (P) zoning from moderate to high P compositions. While studied samples contain a low modal amount of feldspar, this shift in X_Grs may represent the onset of an accessory phase reaction that produced apatite. An andalusite-absent sample contains sillimanite and coarse garnet porphyroblasts with staurolite inclusions. Garnet + biotite thermometry indicates that garnet core growth occurred at temperatures above the typical staurolite isograd in the sillimanite field. Garnet rims are strongly enriched in X_Grs (up to 0.05). Using matrix plagioclase, the garnet + sillimanite + quartz + plagioclase barometer indicates pressures near 5.5 kbar, suggesting that perhaps the difference in peak pressure between the northern and central Ruby Mountains may be roughly 1 kbar. These results indicate a sillimanite grade overprint of formerly andalusite bearing assemblages and are consistent with previous interpretations of polyphase tectonism in Jurassic through Late Cretaceous time.