Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss
Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss
复制标题
含金刚石片麻岩变质演化过程中独居石的行为
DOI：
--
复制
发表时间：
2020
期刊：
影响因子：
--
通讯作者：
T. Vaculovič
中科院分区：
文献类型：
--
作者：
I. Petrik;M. Janák;I. Klonowska;J. Majka;N. Froitzheim;Kenta Yoshida;V. Sasinková;P. Konečný;T. Vaculovič
<p>我们研究了斯堪的纳维亚喀里多尼德山脉 Seve Nappe 杂岩中 Saxnäs 超高压含金刚石片麻岩中的独居石行为（Petrík 等人，2019）。尽管岩石已在麻粒岩相和部分熔融条件下重新平衡，但金刚石的存在记录了超高压阶段。微金刚石以石榴石、蓝晶石和锆石中的包裹体形式原位生成，可以是单晶或多相包裹体，其中包含 Fe-Mg 碳酸盐、金红石和 CO<sub>2</sub>。已识别出两种石榴石类型：占主导地位的 Grt I，其钻石内含物主要存在于石榴石边缘，这表明最初大部分 Grt I 是在超高压条件下生长的。 Grt II，在 Grt I 上形成小晶体、过度生长或内部域，起源于脱水熔融反应，涉及减压过程中多硅白云石和单斜辉石的分解。独居石产于接近微金刚石的 Grt I 边缘，其中石榴石显示出最高的镁铝榴石含量和钇的次峰。这样的位置表明在进行变质作用结束时高温下的热激活扩散。基于这样的结构关系，我们认为独居石是在超高压条件下形成的。</p><p>独居石成分显示出负的Eu异常和中等的Y含量，这与通常的解释不一致，即超高压条件必然导致由于不存在斜长石而导致不存在Eu异常和低Y含量，并且石榴石含量较高。我们通过全岩成分的影响来解释这一点。 LA ICP MS 分析表明，全岩预算受独居石、磷灰石和石榴石控制，均具有负 Eu 异常。整个岩石成分成功地由（重量％）石榴石 16、磷灰石 3、独居石 0.06 模拟。我们的结论是Eu异常是从烃源岩遗传而来的，并不反映与斜长石和/或钾长石共存，而斜长石和/或钾长石在超高压条件下不稳定。通过伪截面模型预测，石榴石丰度均匀（16 vol.%）高于 20 kbars，这解释了在 UHP 条件下石榴石含量增加导致 Y 值没有减少的原因。我们的研究结果表明，全岩成分的影响可能比共存相的影响更重要。</p><p>独居石U-Th-Pb化学年龄测定得出等时质心年龄为472 ±3 Ma。我们将这个年龄解释为与早奥陶世波罗的海大陆边缘俯冲有关的超高压条件下独居石的生长。</p><p>这项工作得到了国家科学中心“CALSUB”2014/14/E/ST1/00321 项目 APVV-14-0278 和 APVV-18-0107 的支持</p><p>参考文献：Petrík, I., Janák, M.、Klonowska, I.、Majka, J.、Froitzheim, N.、Yoshida, K.、Sasinková, V.、Konečný, P.、Vaculovič, T. 2019。岩石学杂志 doi: 10.1093/petrology/egz051</p>
<p>We studied monazite behaviour in UHP diamond-bearing gneiss from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides (Petrík et al., 2019). Although the rock has been re-equilibrated under  granulite facies and partial melting conditions, the UHP stage is recorded by the presence of diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystal or polyphase inclusions with Fe-Mg carbonates, rutile and CO<sub>2</sub>. Two garnet types have been recognised: dominant Grt I  with inclusions of diamond found mostly in the garnet rims, which suggests that originally the bulk of Grt I grew at UHP conditions. Grt II, forming small crystals, overgrowths on, or domains within Grt I originated by dehydration melting reactions involving breakdown of phengite and clinopyroxene during decompression. Monazite occurs in the rims of Grt I close to microdiamond, where garnet shows the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Based on such textural relations, we argue that monazite formed at UHP conditions.</p><p>Monazite composition shows negative Eu anomalies and moderate Y contents, which is not in agreement with common interpretation that UHP conditions necessarily lead to the absence of Eu anomaly and low Y content due to absence of plagioclase and high garnet content. We explain this by the effect of whole-rock composition. LA ICP MS analyses show that whole-rock budget is controlled by monazite, apatite and garnet, all having negative Eu anomalies. Whole rock composition is successfully modelled by (wt. %) garnet 16, apatite 3, monazite 0.06. We conclude that the Eu anomaly is inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar, which are unstable at UHP conditions. Uniform garnet abundance (16 vol. %) above 20 kbars predicted by pseudo-section modelling explains the lack of Y decrease due to the increase of garnet content at UHP conditions. Our results suggest that the effect of the whole-rock composition may be more important than that of coexisting phases.</p><p>U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 ±3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltican continental margin in Early Ordovician time.</p><p>This work was supported by the projects APVV-14-0278 and APVV-18-0107, National Science Center “CALSUB” 2014/14/E/ST1/00321</p><p>Reference: Petrík, I., Janák, M., Klonowska, I., Majka, J., Froitzheim, N., Yoshida, K., Sasinková, V., Konečný, P., Vaculovič, T. 2019. Journal of Petrology doi: 10.1093/petrology/egz051</p>