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 common 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 magmatic crystallization age and magma source. Here, we present detailed field mapping of cross-cutting relationships, whole-rock geochemistry and zircon textural, U–Pb and trace element data for trondhjemite, granodiorite and granite from two localities in a complex Archean gneiss terrane in SW Greenland, which reveal cryptic zircon inheritance. Zircon textural, U–Pb and trace element data demonstrate that, in both localities, trondhjemite is the oldest rock (3011 ± 5 Ma, 2σ), which is intruded by granodiorite (2978 ± 4 Ma, 2σ). However, granite intrusions, constrained by cross-cutting relationships as the youngest component, contain only inherited zircon derived from trondhjemite and granodiorite based on ages and trace element concentrations. Without age constraints on the older two lithologies, it would be tempting to consider the youngest zircon fraction as recording crystallization of the granite but this would be erroneous. Furthermore, whole-rock geochemistry indicates that the granite contains only 6 µg g–1 Zr, extremely low for a granitoid with ∼77 wt% 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, geochemistry, grain characterization, and precise geochronology to accurately determine igneous crystallization ages and differentiate between inherited and autocrystic zircon.

Geochronology of ophiolites of the Newfoundland Appalachians

Precise U/Pb zircon ages (2σ errors) are reported for four ophiolite complexes from the Newfoundland Appalachians.Zircons from trondhjemite from Blow Me Down Mountain, Bay of Islands Complex, give an age of [Formula: see text] (five points). Those from gabbro from Blow Me Down Brook give a minimum 207Pb/206Pb age of 480 Ma (one point), confirming that this is the age of the main plutonic episode. These data refute previously determined latest Cambrian U/Pb zircon and Sm/Nd (mineral–rock isochron) ages of 504 ± 10 and 505 Ma for the complex.Gabbro from the Betts Cove Complex yielded zircon that gives an age of [Formula: see text] (four points). This refutes the interpretation of an earlier U/Pb (zircon) age of 463 ± 6 Ma for a trondhjemite dyke as the age of the ophiolite. It was inconsistent with the presence of Arenigian graptolites in the overlying Snooks Arm Group.Two zircon ages from a pegmatitic and medium-grained trondhjemite of the Annieopsquotch Complex are [Formula: see text] (five points) and [Formula: see text] (two points). Zircon from trondhjemite of the Pipestone Pond Complex of central Newfoundland gives an age of [Formula: see text] (five points). Analysis of a single zircon fraction from the Coy Pond Complex gives a minimum 207pb/206pb age of 489 MaThe ages are comparable to those of the Lushes Bight Terrane, Newfoundland, the M'Clintock West Massif, Arctic Canada, and the Ballantrae Complex, Scotland, and together span 18 Ma of the Tremadocian and Arenigian stages. This short age range may imply that only young "hot" oceanic crust was emplaced and preserved in the Appalachian–Caledonian mountain belt.