USING SKARN GARNET U-PB AGES AND OXYGEN ISOTOPES TO UNDERSTAND FLUID SOURCES AND THE PALEOGEOGRAPHY OF THE JURASSIC MOJAVE SEGMENT OF THE NORTH AMERICAN CORDILLERAN MARGIN ARC (Invited Presentation)

U-Pb laser ablation geochronology of andradite-rich garnet (e.g. Seman et al., 2017; Gevedon et al., 2018) provides opportunity to add temporal constraint to the myriad geochemical studies often applied to garnet. This method is well-suited to skarn garnet, which often fall within the grossular-andradite solid solution. Although traditional interest in skarns focuses on economically important minerals, skarns are excellent recorders of the conditions present at the interface between plutons and host rocks and bear witness to focused crustal fluid flow.

We present garnet U-Pb ages and oxygen isotope ratios from Jurassic skarns of the Mojave and Sierra Nevada segments of the Cordilleran arc that emphasize lateral differences in crustal fluid sources and emplacement conditions. Skarn garnet oxygen isotope ratios vary widely through the Jurassic arc, however, low δ¹⁸O values indicative of participation of meteoric fluids are limited to the west-central Mojave Desert, where isotopic heterogeneity exceeds 10‰ cross garnet zonations of ≤ 3mm, with values as low as –12.25‰ measured by secondary ion mass spectrometry. Garnet U-Pb ages of ~162 and ~149 Ma from 2 skarns (the Verde Antique and Whitehorse skarns, respectively) provide the temporal context required to interpret these δ¹⁸O data in terms of paleogeography and the changing tectono-magmatic regime. Strongly negative garnet δ¹⁸O values can only form in the presence of a surface-derived fluid and require formation in a sub-areal crustal column. In this way, garnet oxygen isotope ratios serve as a non-traditional paleoenvironmental indicator useful for deciphering the geographic, and even tectonic, conditions of the Middle to Late Jurassic arc.

Multiple lines of evidence indicate the region hosted extensional to transtensional tectonics and the migrating Jurassic paleoshoreline. The data require westward migration of the paleoshoreline by 162 Mya and provides a minimum age of emergence for the Mojave arc segment. Alternatively, skarn U-Pb and δ¹⁸O data from Jurassic arc to the northwest indicate domination by metamorphic- and magmatic-derived fluids, either as a result of greater emplacement depths or more voluminous plutonism. A review of global skarn garnet suggests low δ¹⁸O values are the hallmark of extensional tectonics; we propose coupled garnet U-Pb and δ¹⁸O values may provide means to recognize extensional settings in the detrital record.