Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane

• Published in: Geosciences
• Main Authors: Vezinet, Adrien, Thomassot, Emilie, Sarkar, Chiranjeeb, Pearson, D. Graham, Luo, Yan
• Other Authors: University of Alberta, Institut des Sciences de la Terre (ISTerre), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Université Gustave Eiffel-Université Grenoble Alpes (UGA), Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Metal Earth contribution MERC-ME-2022-01, European Project: 0856555(2009)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDU.STU.PE]Sciences of the Universe [physics]/Earth Sciences/Petrography; North Atlantic
• Online Access: https://hal.science/hal-03539415; https://hal.science/hal-03539415/document; https://hal.science/hal-03539415/file/geosciences-12-00036-v3.pdf; https://doi.org/10.3390/geosciences12010036

International audience In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.