Longstanding observations of the Peninsular Ranges Batholith (PRB) have established that granitoids of the Western and Eastern Zones (WZ, EZ) differ systematically in age, petrology, and isotope and rare earth element (REE) geochemistry; WZ magmas are more oxidized, with shallower melting and less crustal assimilation, while EZ magmas are more reduced, show isotopic and REE evidence for sediment assimilation, and show REE evidence for deeper melting. This is reflected in zircon REE and oxygen isotope geochemistry across the batholith, but zircon has the added advantage of tracking melt geochemistry during crystallization and can potentially be leveraged to show processes such as fractional crystallization, assimilation, and magma mixing. Some aspects of zircon geochemistry (e.g. Eu/Eu*, U/Yb) have been suggested as magma depth proxies, and they display differing relationships across the batholith. However, they also vary with magma redox, water content, and other confounding factors.

I propose that zircon-based depth indices which avoid some of these pitfalls can be defined for the PRB using relationships between geochemical ratios with differing behavior with respect to melting depth vs shallow fractionantion – e.g., Eu/Eu* decreases with plagioclase fractionation in the upper crust but should increase with magma melting depth since deeper melt derives from precursors with less plagioclase and more garnet.  U/Yb by contrast should increase with both melting depth and shallow fractionation.  A simple (uncalibrated) depth index of (Eu/Eu*)x(U/Yb) increases from the PRB’s WZ to the EZ as expected. It correlates positively with whole rock Sr/Y, a hypothesized proxy for Moho depth, in various magmatic units of the WZ and TZ, but two large batholiths in the EZ differ in their unit-to-unit behavior and require further scrutiny. Units in the zoned La Posta pluton show a positive correlation between Sr/Y and the depth index, but both quantities vary with oxygen isotopes suggesting a relationship to sediment assimilation. Future work will involve tying the depth index to thermobarometry of mineral inclusions in zircon for more direct calibration. The relationship with crustal assimilation must also be better assessed. Finally, such an index may be applicable to other segments of the Mesozoic California Arc, and may allow the use of detrital zircons for reconstruction of magma depth variations.