THE CASE FOR RAPID CRUSTAL RECYCLING IN ARCS BASED on ZIRCON INHERITANCE PATTERNS IN THE EASTERN PENINSULAR RANGES BATHOLITH OF SOUTHERN AND BAJA CALIFORNIA

Cordilleran batholiths are characterized by episodic flare-ups that produce the bulk of batholith volumes and the Late Cretaceous La Posta suite intrusions within the eastern Peninsular Ranges batholith are a type example. The La Posta belt is associated with an abrupt eastwards shift in the locus of arc magmatism, crustal shortening and thickening, and massive input of supracrustal materials in the deep melt source region. Ion microprobe and laser ablation ICPMS zircon U-Pb ages (n=66) reveal that the bulk of the intrusions were emplaced in a brief interval from 98-92 Ma along the entire 1400 km strike length of the batholith. An elevated δ¹⁸O domain comprising the northern 400 km segment of the belt correlates with synchronous forearc erosion and underthrusting of the high P/T Catalina Schist. In general, the La Posta zircon populations comprise quite well-behaved igneous grains, free from inheritance or other problems that might affect interpretation of igneous crystallization ages. Application of an outlier rejection algorithm used in calculating error weighted means excluded 57 grains from 29 of the samples, or 6.9% of the ~816 grains analyzed in this study; most of the 29 samples from which ages were rejected had just 1 or 2 grains excluded. Of the 57 outliers, 4 were excluded from the low side of the associated sample age distribution while the remaining 53 samples represent older grains. The intriguing feature of the 53 older grains is that while many of the grains have distinctly older Mesozoic or in a few cases Paleozoic apparent ²⁰⁶Pb*/²³⁸U ages, and thus are clearly either inherited or entrained grains, a large proportion of the grains are only a few million years older than their associated sample age distributions. Entrainment of zircon from shallow level sources is considered unlikely because of a lack of suitable wall rock materials to derive these grains as well as compositional data that indicate the magmas were unaffected by shallow crustal assimilation. The grains are therefore likely a reflection of source region inheritance. Age probability distribution of the inherited grains closely match detrital zircon age distributions from batholith derived forearc sediments as well as Catalina Schist metasedimentary rocks suggesting crustal recycling of forearc subduction erosion products on short time scales.