Discriminating prolonged, episodic or disturbed monazite age spectra: An example from the Kalak Nappe Complex, Arctic Norway

Abstract

Monazite within a granite intruding the Kalak Nappe Complex (Norway) provides an informative example of a complex age spectrum in which U-Th-Pb data scatters for ~300 Ma along the Concordia curve. SIMS analyses yield 207Pb/235U ages (1s) of 876 ± 18 to 633 ± 15 Ma, and petrographically constrained age groupings suggest dates of 856 ± 16 Ma for oscillatory zoned cores, 761 ± 32 Ma for patchy domains, and 647 ± 21 Ma for rims. A grid of LA-ICPMS spots across a single grain resolve 207Pb/235U ages (1s) of 884 ± 23 to 564 ± 14 Ma, corroborating the spread in the SIMS dataset and highlighting its spatial relationship to BSE textures. Such Concordia patterns have led to diverse interpretations including prolonged growth or the influence of a variety of radiogenic-Pb mobility processes. In combination with U-Pb analyses, detailed chemical, EBSD, and Raman imaging are used to resolve the primary mechanism for this protracted age spread.The spread in monazite ages is not the result of deformation-induced radiogenic Pb loss because EBSD reveals that the grains are only weakly deformed and have no discernible microstructures. The age spread is not the result of thermally induced radiogenic Pb mobility either, as thermal diffusion time-temperature models fail to reproduce the observed age pattern, and even simplifications indicate diffusion domains that are vastly below the observed scale of dated domains. The age spread is not the result of metamictization as well-defined Raman peaks and the strong EBSPs indicate a well-ordered crystal structure. Younger monazite domains have smaller negative Eu/Eu* anomalies, elevated LaN/YbN ratios, and enhanced Th and U. The age distribution is primarily attributed to fluid-mediated element mass transfer driven by coupled substitution in the altered parts of monazite, consistent with the geochemical signatures in these domains. This process left the P-framework of the original c. 850 Ma magmatic crystal intact, as confirmed by EBSD, but variably purged of its radiogenic-Pb cargo, as demonstrated by spatial interpolation of LA-ICPMS U-Pb data. In some instances, removal of radiogenic Pb appears complete, consistent with the correlation between texturally-defined age domains and well-known tectonothermal events, as inferred from dated intrusive rocks in the region. These results indicate the susceptibility of monazite to fluid alteration, which allows even a single monazite grain to record several tectonothermal events and potentially chart much of an orogens history.