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首页> 外文期刊>Contributions to Mineralogy and Petrology >Independent ages of magmatic and hydrothermal activity in alkaline igneous rocks: The Motzfeldt Centre, Gardar Province, South Greenland
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Independent ages of magmatic and hydrothermal activity in alkaline igneous rocks: The Motzfeldt Centre, Gardar Province, South Greenland

机译：碱性火成岩中岩浆和热液活动的独立年龄：南格陵兰加尔达省莫兹费尔特中心

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摘要

The Motzfeldt Centre is one of four major alkaline centres belonging to the Igaliko complex, part of the Mid-Proterozoic Gardar province of South Greenland. Motzfeldt comprises syenites and nepheline syenites displaying complex magmatic and subsolidus histories. Ta, Nb and REE-rich zones are associated with a pervasive hydrothermal alteration process in the North and East of the complex, part of a subdivision of the Centre called the Motzfeldt Sø Formation (MSF). The zircons from the MSF show textures that are both magmatic and altered, whereas the textures of pyrochlore have a predominantly subsolidus chemistry. We compare and contrast the radiogenic isotope geochemistry of the zircon and pyrochlore to obtain independent age estimates of the magmatic and hydrothermal episodes. Selected analyses of magmatic zircons are concordant with an age of 1,273 ± 6 Ma. Altered zircons are normally discordant with one intercept at 1,261 ± 28 Ma and the other at 349 ± 49 Ma. This younger age would appear on face value to date the hydrothermal event at Motzfeldt to ~900 Ma after the magmatism. However, the Pb–Pb isotope systematics from altered pyrochlore define an isochron age of 1,267 ± 6 Ma for isotopic closure of the pyrochlore following alteration during the subsolidus. There is no known magmatic event in Southern Greenland at ~350 Ma and discordant points have high common lead. We therefore infer that the younger intercept age is spurious. The overlap between the magmatic U–Pb zircon and the Pb–Pb pyrochlore age suggests that alteration of the Motzfeldt centre occurred shortly after magmatism.
机译：莫茨费尔特中心​​是属于Igaliko联合体的四个主要碱性中心之一，该联合体是南格陵兰中元古代加尔达省的一部分。莫兹费尔特由正岩和霞石正岩组成，具有复杂的岩浆和近固相线历史。富含Ta，Nb和REE的区域与该综合体北部和东部的普遍的热液蚀变过程有关，这是该中心分区MotzfeldtSø组（MSF）的一部分。来自MSF的锆石显示出岩浆质和蚀变质感，而烧绿石的质感主要是亚固相线化学。我们比较和对比了锆石和烧绿石的放射性同位素地球化学，以获得岩浆和热液事件的独立年龄估计。选定的岩浆锆石分析年龄为1,273±6 Ma。锆石蚀变通常不一致，其中一个截距为1,261±28 Ma，另一个截距为349±49 Ma。岩浆作用之后，这个较年轻的年龄会出现在Motzfeldt的热液事件发生之前，直到约900 Ma。然而，由改变的烧绿石形成的Pb–Pb同位素定义了亚固相线变化后烧绿石的同位素封闭的等时年龄为1,267±6 Ma。格陵兰岛南部约350 Ma尚无已知的岩浆事件，不协调点的铅含量较高。因此，我们推断年轻的拦截年龄是虚假的。岩浆U–Pb锆石与Pb–Pb烧绿石年龄之间的重叠表明，岩浆作用后不久就发生了Motzfeldt中心的变化。

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