Over dispersion of zircon crystallization ages in the Cordillera Blanca batholith, Central Peru: Lead loss or prolonged magmatism and protracted crystallization?

Over dispersed zircon LA-ICP-MS U-Pb dates in single samples of igneous rock present challenges for age interpretation as the distribution of dates cannot be assumed to represent a single zircon population and formation event. Additionally, assessing the role of lead loss in young zircons is difficult due to the near-linear slope of the 1:1 concordia line. The Cordillera Blanca batholith in the central Peruvian Andes has a published TIMS age of 8.2 0.2 Ma from a single sample near the central portion of the >200-km granitic exposure. Other studies have demonstrated age heterogeneity (5 25 Ma) in single samples across the compositionally diverse intrusion. These previously reported ages have been presented without a corresponding mean square weighted deviation (MSWD) preventing thorough validation of reported ages. Here, we present 64 new concordant zircon LA-ICP-MS 206Pb*/235U dates paired with rare earth element concentrations (REE) from four samples spread across the Miocene Cordillera Blanca Batholith that yield mean ages with unacceptable MSWD (MSWD = 12.8 20.8). Collectively, these dates represent an age progression from ca. 5 to 23 Ma, suggesting either protracted crystallization or progressive lead loss of a uniform zircon population. Adding 169 concordant dates that have been previously reported without mean ages corroborates this trend and extends the maximum to 25 Ma. Cumulative age distribution plots of 233 new and existing data points result in peaks centered around 6, 13, and 23 Ma. Cumulative and individual sample age distribution plots are normally distributed and lack negatively skewed tails indicative of post-crystallization lead loss. CL textures, Th/U, light rare earth element indexes, and molar (REE+Y)/P indicate that hydrothermal or metamorphic alteration of zircon is largely restricted to grains that fall beyond 2- of concordia, supporting a magmatic origin for 57 of the 64 new zircon ages. We identified 4 grains of probable metamorphic origin and 3 that may have experienced post-crystallization hydrothermal modification and possible lead loss during the same event. We also identified 4 discordant analyses that support lead loss. Cumulatively, this framework indicates the over dispersion of zircon dates in the Cordillera Blanca batholith represents protracted crystallization.