We present new data for the ∼3.0 Ga Maniitsoq Norite Belt of the Akia Terrane, West Greenland, with the aim of understanding its petrogenesis. The Maniitsoq Norite Belt is hosted in regional tonalite-trondhjemite-granodiorite (TTG) and dioritic orthogneisses, intruded by later sheets of TTG and granite pegmatites, and comprises two main rock types: plagioclase-rich “norites” and pyroxene-rich “melanorites”. Both norites and melanorites have high SiO₂ contents (52–60 wt% SiO₂), high bulk rock Mg# (0.57–0.83), and low TiO₂ contents (0.1–0.7 wt%). Their trace element patterns are defined by depleted heavy Rare-Earth elements, highly enriched light Rare-Earth elements, negative anomalies in Nb, Ta, and Ti, and variable anomalies in Zr, Hf, and Eu. New zircon U-Pb geochronology data and previously published ages establish an emplacement age of 3,013 ± 1 Ma for the majority of the Maniitsoq Norite Belt, with magmatism continuing until 3,001 ± 3 Ma. This ∼12 Myr period of norite magmatism is coeval with an ongoing period of TTG production in the Akia Terrane. Norite Belt emplacement was closely followed by high temperature, low pressure granulite-facies metamorphism at ∼800°C and <9 kbar. These conditions imply high temperature gradients (>900°C/GPa) and that the norite magmas were emplaced into thin crust and lithosphere. Compositions of the norites and melanorites can be explained by derivation from a single mafic parental melt (∼13 wt% MgO), with the norites predominantly accumulating plagioclase and the melanorites predominantly accumulating pyroxene. Evidence from field relationships, the presence of xenocrystic zircon, major element compositions and combined trace element and Hf-isotope modelling suggests the norites were contaminated by assimilation of ∼20–30% continental TTG crust. Geochemical and Hf-Nd isotopic constraints indicate that the norite mantle source was depleted, and that this depletion occurred significantly before the emplacement of the norite magmas. Contemporaneous production of both TTGs and norite, their emplacement in thin crust, and the rapid transition to high temperature, low pressure granulite-facies metamorphism is best explained by their formation in an ultra-hot orogeny. Formation of norites in this setting may be restricted to >2.7 Ga, when geothermal gradients were higher on Earth.

我们介绍了西格陵兰岛Akia Terrane的〜3.0 Ga Maniitsoq Norite带的新数据，目的是了解其成因。Maniitsoq Norite带位于区域斜长石-长辉岩-花岗岩-闪长闪长岩和闪长岩正长片岩中，由后来的TTG和花岗岩伟晶岩侵入，包括两种主要的岩石类型：斜长石富含“ norites”和富辉石的富含“黑铁矿” 。褐铁矿和黑变石都具有较高的SiO ₂含量（52–60 wt％SiO ₂），较高的块体Mg＃（0.57–0.83）和较低的TiO ₂含量（0.1–0.7 wt％）。其痕量元素模式由贫化的重稀土元素，高度富集的轻稀土元素，Nb，Ta和Ti的负异常以及Zr，Hf和Eu的可变异常定义。新的锆石U-Pb年代学数据和先前公布的年龄确定了Maniitsoq Norite带大部分的沉积年龄为3,013±1 Ma，岩浆作用一直持续到3,001±3 Ma。这〜12 Myr的赤铁矿岩浆作用期与Akia Terrane正在进行的TTG生产期同时期。在约800°C和<9 kbar的高温，低压花岗石相变质作用之后，紧紧跟随着Norite带的沉积。这些条件意味着温度梯度较高（> 900°C / GPa），并且该赤铁矿岩浆已进入薄壳和岩石圈。可以通过从单一的铁镁质母体熔体（约13 wt％MgO）中衍生出镍辉石和黑色素岩的成分来解释，其中镍辉石主要聚积斜长石，而黑色素岩主要聚积辉石。磁场关系，异晶锆石，主要元素组成以及痕量元素和Hf同位素模型的结合表明，这些橄榄石被20％至30％的TTG地壳吸收。地球化学和Hf-Nd同位素约束表明，该赤铁矿地幔源已被耗尽，并且这种消耗明显发生在赤铁矿岩浆侵位之前。同时产生TTG和Norite，将它们置于薄皮中，并迅速转变为高温，低压花岗石相变质最好通过超热造山作用中的形成来解释。当地球上的地热梯度较高时，在这种情况下形成的橄榄石可能被限制在> 2.7 Ga。