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.

中文翻译：

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区分花岗岩中的继承锆石和自晶锆石

继承的锆石，未原位形成的晶体通常从它们的宿主岩浆中提取，但是从源岩地区掺入，或者从围岩中吸收，这是很常见的，但是可能很难识别。年龄，化学和/或质地上与最年轻的锆石馏分的不同是区分此类晶粒的主要机制。但是，在Zr欠饱和岩浆中，整个锆石种群可能是遗传的，如果不能通过构造约束加以识别，则可能导致对岩浆结晶年龄和岩浆源的错误解释。在这里，我们提供了西南格陵兰一个复杂的太古代片麻岩地层中两个地点的跨界关系，全岩地球化学和锆石质地，U–Pb以及痕量元素数据的长晶岩，花岗闪长岩和花岗岩的详细野外映射，揭示了神秘的锆石的继承。锆石质地 U–Pb和微量元素数据表明，在两个地方，长晶辉母岩都是最古老的岩石（3011±5 Ma，2σ），由花岗闪长岩（2978±4 Ma，2σ）侵入。然而，受年龄跨度和痕量元素浓度的限制，花岗岩侵入体受横切关系的限制，是最年轻的成分，仅含有衍生自硬硼铁矿和花岗闪长岩的遗传锆石。如果没有年龄上的两种岩性的年龄限制，那么将最年轻的锆石馏分记录为花岗岩的结晶结晶将很诱人，但这是错误的。此外，全岩石地球化学表明该花岗岩仅含6 µg g 受年龄跨度和痕量元素浓度的限制，跨界关系作为最年轻的成分，仅含有衍生自硬硼锰矿和花岗闪长岩的遗传锆石。在年龄较大的两种岩性上没有年龄限制的情况下，将最年轻的锆石馏分记录为花岗岩的结晶体将很诱人，但这将是错误的。此外，全岩石地球化学表明该花岗岩仅含6 µg g 受年龄跨度和痕量元素浓度的限制，跨界关系作为最年轻的成分，仅含有衍生自硬硼锰矿和花岗闪长岩的遗传锆石。如果没有年龄上的两种岩性的年龄限制，那么将最年轻的锆石馏分记录为花岗岩的结晶结晶将很诱人，但这是错误的。此外，全岩石地球化学表明该花岗岩仅含6 µg g^–1 Zr，对于具有约77 wt％SiO ₂的花岗岩来说极低。如此低的Zr浓度说明花岗岩中缺乏自晶锆石。我们扩展了一个区分工具，该工具使用锆石中的Th / U比与整个岩石中的Th / U比来帮助识别继承的锆石。这项工作强调需要现场观察，地球化学，晶粒表征和精确的年代学，以准确确定火成岩的结晶年龄并区分继承的和自晶的锆石。