MULTIPLE ZIRCON CRYSTALLIZATION EVENTS WITHIN THE BLACK MOUNTAINS, DEATH VALLEY CALIFORNIA; NEW INSIGHTS FROM LA-ICPMS U-PB DEPTH-PROFILING

The Death Valley region is a type locality for continental extension in the Basin and Range Province, but extensional deformation may also be significantly influenced by fabric development, inherited structures, and melt products that formed during earlier Sevier-Laramide contractional deformation. Inherited fabrics and lithologic heterogeneity may significantly influence the style of later extension. Sevier-Laramide inheritance has been well documented in the Funeral and Panamint Mountains on the east and west sides of Death Valley, respectively, but has been less evident in the Black Mountains just to the south of the Funeral Mountains. New zircon LA-ICPMS U-Pb data collected using depth-profiling on unpolished mounts provides new information on multiple zircon crystallization events in the Black Mountains, and most significantly a previously unrecognized Late Cretaceous-Paleogene (K-Pg) zircon crystallization event. Samples were collected from the basement rocks of Mormon Point turtleback structure that prior to this study were interpreted to be entirely Proterozoic. LA-ICPMS depth-profiling allowed identification of clear U-Pb age zonation in Mormon Point zircons. Different age zones were identified as plateaus on U-Pb age vs ablation time plots used during data reduction. U-Pb depth profiling yielded Late Cretaceous-Paleogene ²⁰⁶Pb/²³⁸U ages of ~80-50 Ma ± Miocene rims of ~10-11 Ma, ± Proterozoic cores with ²⁰⁷Pb/²⁰⁶Pb ages of ~1.6-1.7 Ga. Numerous zircons from samples MP1, MP2, and MP3 yielded all three U-Pb age zones. Whereas most of the samples have several age zones, sample DV1 exclusively yielded Late Cretaceous-Paleogene ²⁰⁶Pb/²³⁸U ages (WMA=59.0 ± 1.3 Ma). The majority of K-Pg age zones have Th/U ratios between 0.35-3.4 with an average of ~1.9 that suggests magmatic overgrowth during melting. K-Pg zircon U-Pb ages with predominantly magmatic Th/U ratios suggests crustal melting in the late stages of Sevier-Laramide orogenesis and implies that the Black Mountains were part of a broader region that experienced local melting and deformation within the thickened orogenic wedge. The addition of late orogenic partial-melt products may have contributed to crustal weakening that favored the wide-rift mode of extension during subsequent Basin and Range deformation.