130 MILLION YEARS OF PALEOPROTEROZOIC HISTORY RECORDED BY U-PB MONAZITE AND ZIRCON AGES FROM THE IVANPAH MOUNTAINS, EASTERN CALIFORNIA

The Ivanpah Mountains of southeastern California expose Paleoproterozoic banded gneisses of igneous and supracrustal origin that belong to the Mojave crustal province. Metasedimentary rocks, dominantly metawackes, are characterized by detrital zircon ages of 1.8-2.0 (dominant) and 2.4-2.8 Ga, and were deposited by ~1.80 Ga. These rocks were intruded by a suite of granitoids (gabrro, tonalite, and porphyritic granite) at ~1.76 Ga. This combined suite of rocks is strongly deformed, migmatitic and isoclinally folded with a penetrative, N-S-trending foliation that dips steeply to the west. U-Pb zircon and monazite SHRIMP-RG ages record multiple periods of metamorphism and deformation. Migmatite formation of the metasedimentary rocks occurred at ~1.74 Ga, as indicated by U-Pb zircon ages of a leucocratic segregation and a biotite-bearing, granitic dike. Growth of metamorphic zircon and monazite in the metasediments also occurred at this time, and their REE patterns variably show the presence of garnet. Metamorphic zircon grew again at ~1.70 Ga in the metasedimentary rocks, which correlates to the regionally identified Ivanpah Orogeny, but no monazites of this age have been identified. The final major metamorphic event occurred at ~1.67 Ga. In metasedimentary rocks, metamorphic zircon growth was sparse, but monazite growth and/or recrystallization was widespread such that monazite grains contain age domains of both 1.74 and 1.67 Ga. Garnet-bearing, leucocratic pegmatites are aligned roughly parallel to the dominant, N-S trending foliation and yield U-Pb zircon ages of 1.67 Ga. REE patterns in both zircon and monazite suggest that the 1.67 Ga event involved garnet growth. Garnet is abundant and typically unzoned with respect to major elements, which may indicate prolonged residence at high temperatures during the final metamorphic event. New thermobarometry from a garnet + biotite + sill + cordierite + kspar + plag-bearing gneiss indicates a P-T of 4-6 kb and ~750 °C, which is interpreted to reflect the conditions of the 1.67 Ga event. This was a period of widespread magmatism and batholith emplacement across the region. Taken together, monazite and zircon ages from the Ivanpah Mountains preserve a complex history from initial deposition through multiple igneous and metamorphic events.