Differentiating between Inherited and Autocrystic Zircon in Granitoids

Inherited zircon, crystals that did not form in situ from their host magma but were incorporated from either the source region or assimilated from the wall-rock, is com m on but can be difficult to identify. Age, chemical and/or textural dissimilarity to the youngest zircon fraction are the primary mechanisms of distinguishing such grains. However, in Zr-undersaturated magmas, the entire zircon population may be inherited and, if not identifiable via textural constraints, can lead to erroneous interpretation of m agm atic crystallization age and magma source. Here, we present detailed field mapping of cross-cutting relationships, whole-rock geochem istry and zircon textural, U-Pb and trace element data for trondhjem ite, granodiorite and granite from tw o localities in a com plex Archean gneiss terrane in SW Greenland, w hich reveal cryptic zircon inheritance. Zircon textural, U-Pb and trace element data dem onstrate that, in both localities, trondhjem ite is the oldest rock (3011 +/- 5 Ma, 2 sigma), w hich is intruded by granodiorite (2978 +/- 4 Ma, 2 sigma). However, granite intrusions, constrained by cross-cutting relationships as the youngest com ponent, contain only inherited zircon derived from trondhjem ite and granodiorite based on ages and trace element concentrations. W ithout age constraints on the older tw o lithologies, it would be tem pting to consider the youngest zircon fraction as recording crystallization of the granite but this w ould be erroneous. Furthermore, whole-rock geochem istry indicates that the granite contains only 6 mu g g(-1) Zr, extremely low for a granitoid with similar to 77w t% SiO2- Such low Zr concentration explains the lack of autocrystic zircon in the granite. We expand on a differentiation tool that uses Th/U ratios in zircon versus that in the whole-rock to aid in the identification of inherited zircon. This work emphasizes the need for field observations, geochem istry, grain characterization, and precise geochronology to accurately determine igneous crystallization ages and differentiate between inherited and autocrystic zircon.