TRACE ELEMENTS DOCUMENT POLYMETAMORPHIC GARNET GROWTH DURING PALEOPROTEROZOIC OROGENY, IVANPAH MOUNTAINS, SOUTHEASTERN CALIFORNIA

The Ivanpah Mountains contain garnet + biotite ± sillimanite gneisses, granitic leucosomes, and hornblende-bearing mafic granulites. Garnets in the gneisses are unzoned in major elements, except for minor Fe/Mg zoning adjacent to biotite, interpreted as a retrograde diffusion feature. SHRIMP-RG analyses of monazite records two stages of growth at ca. 1740 and 1670 Ma; metamorphic zircon shows more age complexity, but ages are also 1740-1670 Ma. The 1740 Ma analyses are characterized by variable Yb/Gd and HREE concentrations, whereas all 1670 Ma analyses are characterized by low Yb/Gd and HREE concentrations. LA-ICP-MS analyses of garnet reveal high HREE in cores, low HREE in rims, interpreted as a growth feature. Garnet zoning matches the range of HREE observed in 1740 Ma monazite, indicating that these samples preserve garnet grown at 1740 Ma. Samples with abundant 1740 Ma monazite and/or zircon contain garnet with the highest HREE cores and largest range of HREE zoning. Samples dominated by 1670 Ma monazite and/or zircon contain garnets with lower HREE cores and a smaller range of zoning, suggesting that both garnet and monazite may have experienced more extensive recrystallization and/or new growth in these samples. Homogenization of garnet major elements by volume diffusion requires several 10's of m.y. for T = 800 °C; Ti-in-zircon thermometry indicates T > 700 °C for all 1740-1670 Ma analyses, and the absence of major element zoning in garnet indicates that homogenization was completed during the 1670 Ma event. The slow diffusion rates of the REE prevented significant homogenization of REE growth zoning. Dominant fabrics, mineral assemblages, and mineral compositions formed during the 1670 Ma event, but trace element observations indicate that garnet formed during both events, and that 1740 Ma garnet is partially to extensively preserved.