Reworking of Hadean crust in the Acasta gneisses, northwestern Canada: Evidence from in-situ Lu–Hf isotope analysis of zircon
Reworking of Hadean crust in the Acasta gneisses, northwestern Canada: Evidence from in-situ Lu–Hf isotope analysis of zircon
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DOI：
10.1016/j.chemgeo.2008.11.007
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发表时间：
2009-02
期刊：
影响因子：
3.9
通讯作者：
T. Iizuka;T. Komiya;S. Johnson;Y. Kon;S. Maruyama;T. Hirata
中科院分区：
文献类型：
--
作者：
T. Iizuka;T. Komiya;S. Johnson;Y. Kon;S. Maruyama;T. Hirata
加拿大西北部的Acasta片麻岩杂岩由早太古宙的正片麻岩组成，其中包括已知的最古老的陆相岩石(3.94-4.03 Ga)。前人的锆石年代学研究表明，在最古老的岩石中存在年龄高达4.2 Ma的锆石包裹体，这表明早太古宙花岗岩的源区含有Hadean(>；4.03(Ga))地壳。在本研究中，我们测定了早太古代Acasta片麻岩中提取的锆石颗粒的Lu-Hf同位素组成，以评估Hadean地壳对这些花岗岩类形成的贡献程度。用原位激光烧蚀电感耦合等离子体质谱结合阴极发光颗粒内部结构的图像对Lu-Hf同位素进行了分析。两个约3.59aGa花岗岩类的εHf(T)分别为−2.4±2.2和−3.2±2.5，表明花岗岩源区早在3.8aGa时来自地幔。这与丰富的锆石捕虏晶的存在是一致的，年龄可达3.9Ga。3.72NaGa花岗岩类的Hf同位素组成明显不具有放射性，εHf(T)为−6.1±2.5。这表明花岗岩类是由非常古老的地壳重熔形成的。最古老的两种岩石--约3.97kgGa英云闪长岩的εHf(T)值分别为−1.2±3.3和−3.3±1.7，表明即使是已知的最古老的花岗岩类也含有改造的更古老的地壳物质。这些结果表明，Hadean地壳对一些太古代早期Acasta花岗岩类的成因有重要贡献。
The Acasta Gneiss Complex of northwestern Canada comprises early Archean orthogneisses and includes the oldest known terrestrial rocks (3.94–4.03 Ga). Previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4.2 Ga in the oldest rocks, indicating that the source of early Archean granitoids contained Hadean (>4.03 Ga) crust. In this study, we have determined the Lu–Hf isotopic compositions of zircon grains extracted from the early Archean Acasta gneisses to evaluate the extent of the Hadean crust contribution to the formation of these granitoids. Analyses of the Lu–Hf isotopes were carried out using in-situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains. Two ca. 3.59 Ga granitoids have εHf(T) of −2.4±2.2 and −3.2±2.5, suggesting that the source of the granitoids was extracted from the mantle as far back as 3.8 Ga. This is consistent with the presence of abundant zircon xenocrysts with ages up to 3.9 Ga. The Hf isotopic composition of a 3.72 Ga granitoid is significantly less radiogenic, with an εHf(T) of −6.1±2.5. This indicates that the granitoid formed by remelting of very old crust. The εHf(T) values for the two oldest rocks, the ca. 3.97 Ga tonalites, are −1.2±3.3 and −3.3±1.7, respectively, indicating that even the oldest known granitoids contain reworked older crustal materials. These results suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean Acasta granitoids.