U-PB DISCORDANCE MODELING OF MOJAVE PROVINCE ZIRCON FROM THE IVANPAH MOUNTAINS (EASTERN MOJAVE DESERT, CALIFORNIA) MAY RECORD ARC-RELATED HYDROTHERMAL INDUCED LEAD-LOSS

The affinity and crustal history of rocks belonging to the Mojave Province is steadily growing in importance as isotopic and petrologic studies identify components of the crystalline basement incorporated into arc rocks, melts, and detritus through the southern North American Cordillera. U-Pb investigation of zircon separated from four samples of Mojave Province rock collected from the Ivanpah Mountains, eastern Mojave Desert, California, provide the opportunity to assess lead-loss in detrital zircon from previously a characterized crustal province, and to assess the post-metamorphism thermal history of the Mojave basement through discordance modeling (e.g., Reimink et al., 2016).

Included in this study are two garnet-rich meta-pelites, one meta-granitoid, and one meta-mafic lithology with a basaltic or gabbroic protolith. Discordance modeling yields consistent upper-intercept peaks between ~1.7 and ~1.8 Ga for all samples, despite the presence of occasional zircon with ages above ~2.0 Ga, as is consistent with characterizations of Mojave Province zircon. These results indicate discordance modeling of arc-proximal samples subjected to subsequent metamorphic or hydrothermal events may add clarity to traditional KDE plots of detrital zircon samples with an abundance of discordant grains.

Lower intercept ages are generally interpreted to reflect the timing of a geologic event resulting in sub-solidus preferential loss of Pb relative to U from a zircon. Three of the samples yield lower-intercept ages of ~60 Ma, which may be related to the transition from arc magmatism to Laramide-style tectonics. In contrast, zircon (n=16) separated from the fourth sample reveal a preliminary lower-intercept age of ~230 Ma. We interpret this lower-intercept age as documenting the minimum geographic reach and southeastward extent of hydrothermal activity in-board of the Permo-Triassic North American Cordilleran margin arc. The lead-loss achieved by this sample seemingly requires vigorous hydrothermal alteration as Permo-Triassic magmas were intruded into extending arc crust (Cecil et al., 2019; Clemens-Knott and Gevedon; 2023).