Decoupled U-Pb date and chemical zonation of monazite in migmatites: The case for disturbance of isotopic systematics by coupled dissolution-reprecipitation

Abstract

Monazite is an abundant accessory mineral in metasedimentary rocks and their anatectic products. Trace element analysis combined with U-Pb dating of monazite is widely used to reconstruct P-T-t histories of high-grade metamorphic terranes. This approach relies on interpreting U-Pb dates, which requires understanding the processes that cause isotopic disturbances. We present in-situ laser ablation split-stream (LASS) results of U-Pb dating and trace element spot analyses of single monazite grains from anatectic granite samples from a HT-LP migmatitic turbidite terrane exposed in the Sierra de Quilmes, NW Argentina. This terrane was part of the back-arc and remained close to anatectic temperatures for most of the ∼510–440 Ma Famatinian Orogeny, in the Western Gondwana margin. Sampled outcrops display evidence of remelting of the anatectic granites, marked by leucosomes and confirmed by zircon chronology that define multiple thermal peaks and anatexis events during ∼60 Myr duration of the arc. U-Pb monazite chronology of these granite samples reveals a continuum of dates between ∼500 and 450 Ma recording most of the duration of the Famatinian event and matching the range of zircon growth ages, but without their multiple, well-defined age populations. Despite each sample yielding a reasonable Famatinian monazite mean date, the results reveal that the isotopic system has been perturbed. Data for each rock sample yield similar date spread for both the cores and the well-defined rims of single monazite grains, indicating a disconnect between U-Pb dates and chemical zonation. We interpret these random within-grain date variations as a result of coupled dissolution-reprecipitation reactions between monazite grains and ambient fluids, most likely silicate melts. These reactions occurred during a single orogeny marked by a long-lasting, high-temperature metamorphic history. During coupled dissolution-reprecipitation reactions, ingrown radiogenic Pb was remobilized and/or redistributed within the grain giving rise to unsupported Pb. This process was likely significant here because dissolution of apatite into silicate melts during migmatization stabilized monazite, preventing their dissolution but not preventing their reaction with the melt. Redistribution of radiogenic Pb caused by the coupled dissolution-reprecipitation reactions gave rise to meaningless individual dates spreading along the 50 Myr duration of the thermal event associated with the orogeny. In this case, rather than reacting with fluids from a distinct event, monazite dates were perturbed during the same orogeny.