Effects of crustal assimilation and magma mixing on zircon trace element relationships across the Peninsular Ranges Batholith

Abstract Magmatic zircon tends to exhibit characteristic trends in trace element contents in response to magma cooling and fractionation, such that zircon may provide a window into melt evolution not accessible by whole rock geochemistry. The Peninsular Ranges Batholith of southern California and Baja California ranges from older rocks of oceanic arc affinity in the west to, moving eastward and younger, rocks derived from younger magmas in the east. This has been interpreted via whole rock chemistry to reflect (Transition Zone) deeper origins and (Eastern Zone) deeper origins with significant crustal assimilation compared to the western zone. The youngest magmas in the Eastern Zone form large, typically zoned plutons and are high in Sr/Y. We present zircon trace element and oxygen isotope measurements in selected Western and Transition Zone samples and in many units from the high-Sr/Y La Posta and San Jacinto plutons. Most Western and Transition Zone populations exhibit typical shallow fractionation-related trace element trends, while one Eastern Transition Zone sample and most samples from the high-Sr/Y Eastern Zone lack Eu/Eu* evidence for plagioclase fractionation. Oxygen isotopes, zircon P contents, and Ce-based redox estimates suggest that the most evolved unit (2-mica granodiorite) of the La Posta pluton is significantly contaminated by sediments, with other high-Sr/Y units displaying zircon oxygen isotope and redox estimates falling between the 2-mica granodiorite and the relatively uncontaminated Transition Zone in these parameters. However, most of these zircons lack unambiguous markers for supracrustal contamination that could be interpreted even in a detrital context. The contrasting behavior of zircon U/Yb and Eu/Eu* during shallow fractionation and their similar response to melting depth variations can be used to distinguish the effects of shallow fractional crystallization, depth of melting, and introduction of exotic materials by either crustal assimilation or magma mixing. On the basis of U/Yb vs Eu/Eu* relationships, we interpret the behavior of Western Zone and most Transition Zone zircons as reflecting mainly shallow fractionation. One Eastern Transition Zone tonalite appears to result from mixing of magma from different depths. We interpret the high-Sr/Y Eastern Zone magmas as resulting from mixing of magmas that have assimilating varying amounts of metasediment. Moderate levels of sediment assimilation may complicate magma depth interpretations which are based on zircon (Th, U)/Yb plots or detrital zircon Eu/Eu* – without yielding clear zircon geochemical evidence for supracrustal assimilants. Magma mixing is less problematic for regional-scale detrital zircon Eu/Eu* studies and may be easier to detect based on the relationships among (Th, U)/Yb and Eu/Eu*.