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首页> 外文期刊>Contributions to Mineralogy and Petrology >approx 3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex
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approx 3,850 Ma tonalites in the Nuuk region, Greenland: geochemistry and their reworking within an Eoarchaean gneiss complex

机译：格陵兰岛努克地区约3,850个Ma色母岩：地球化学及其在Eoarchaean片麻岩复合体中的重制

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摘要

The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectono-thermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 mu m) younger (3,660-3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110-140 ppm Zr in the tonalites and likely magmatic temperatures of > 850 deg C, zircon solubility-melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble.
机译：西格陵兰南部的欧陶纪（> 3,600 Ma）Itsaq片麻岩复合体以具有古生代构造热历史复杂的多相正相片麻岩为主。一些原片麻杆菌有c。 3,850 Ma锆石，其种类从稀有的单相变质岩到更常见的复杂条带状辉钼矿。这是由于在较年轻的构造热事件中叠加了非均质应变，原位麻醉和花岗岩脉。在单相同色托氏石中。 3,850 Ma的锆石，振荡带状棱形锆石都具有3,850 Ma的历史，但显示出古老的放射源性Pb斑片状消失。 SHRIMP点分析和激光烧蚀ICP-MS深度分析表明，在这3,850 Ma锆石中，存在着较薄的（通常<10μm）年轻的低Th / U变质锆石壳（3,660-3,590 Ma和Neoarchaean）。带有这种简单锆石种群的几个样本出现在Akilia附近的岛屿上。相比之下，migmatite通常包含更复杂的锆石种群，通常存在多于一代的火成锆石。整个复合体中带状片麻岩的锆石定年显示，样品中c。 3,850 Ma火成锆石不仅限于Akilia和邻近岛屿。例如，来自Itilleq（距Akilia约65公里）的辉锰矿含有不同数量的振荡带3,850 Ma和3,650 Ma锆石，分别解释为岩石年龄和Eoarchaean变质作用下的地壳融化时间。锆石中Zr的含量只有110-140 ppm，岩浆温度可能超过850℃，锆石的溶解度与熔体组成的关系表明，锆石中锆石的饱和度仅为三分之一。因此，在前体岩浆中夹带的任何锆石都是高度可溶的。

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