Monazite behaviour during metamorphic evolution of a diamond-bearing gneiss: a case study from the Seve Nappe Complex, Scandinavian Caledonides
Monazite behaviour during metamorphic evolution of a diamond-bearing gneiss: a case study from the Seve Nappe Complex, Scandinavian Caledonides
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含金刚石片麻岩变质演化过程中独居石的行为：斯堪的纳维亚喀里多尼德塞韦推覆杂岩的案例研究
DOI：
10.1093/petrology/egz051
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发表时间：
2019
影响因子：
3.9
通讯作者：
Vaculovic T
中科院分区：
文献类型：
--
作者：
Petrik I;Janak M;Klonowska I;Majka J;Froitzheim N;Yoshida K;Sasinkova V;Konecny P;Vaculovic T
独居石是变质岩石中常见的矿物，包括那些经历超高压（UHP）变质作用的岩石。在变质演化过程中，独居石的成分适应了矿物组合的变化，特别是重稀土元素含量的变化。我们在斯堪的纳维亚加里东纪塞夫推覆复合体的萨克斯奈斯研究了含独居石的含金刚石片麻岩中的这一过程。虽然岩石已经在麻粒岩相和部分熔融条件下重新平衡，但它仍然保留了超高压阶段的矿物：石榴石，蓝晶石，金红石，特别是金刚石。微金刚石以包裹体的形式存在于石榴石、蓝晶石和锆石中，既可以是单晶，也可以是与铁镁碳酸盐、金红石和CO2形成的多相包裹体。独居石和金刚石都出现在石榴石的边缘，显示出最高的镁铝榴石含量和钇的第二个峰。这一位置反映了变质末期高温下的热激活扩散作用。独居石组合物显示负Eu异常，我们解释为继承源岩，不反映与斜长石和/或钾长石，这是不稳定的超高压条件下共存。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。超高压独居石很可能是由褐帘石在俯冲-俯冲变质作用中形成的。石榴子石和蓝晶石中的独居石大多未发生蚀变，而基质中的独居石则显示出由磷灰石、稀土-绿帘石/褐帘石和稀土-碳酸盐组成的破裂日冕，可能是由于压力降低和冷却而形成的。独居石的U-Th-Pb化学年龄为472 ± 3 Ma。我们解释这个年龄独居石生长在超高压条件下有关的俯冲的波罗的海大陆边缘在早奥陶世的时间。
Monazite is a common mineral in metapelitic rocks including those that underwent ultrahigh-pressure (UHP) metamorphism. During metamorphic evolution monazite adapts its composition to the changing mineral assemblage, especially in its heavy rare earth element contents. We studied this process in diamond-bearing gneiss containing monazite, from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides. Although the rock has been re-equilibrated under granulite-facies and partial melting conditions, it still preserves minerals from the UHP stage: garnet, kyanite, rutile, and especially diamond. Microdiamonds occurin situas inclusions in garnet, kyanite and zircon, either as single crystals or as polyphase inclusions with Fe–Mg carbonates, rutile and CO2. Both monazite and diamond occur in the rims of garnet showing 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. Monazite compositions show negative Eu anomalies, which we interpret to be inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar, which are unstable at UHP conditions. Our results suggest that the effect of whole-rock composition may be more important than that of coexisting phases. The UHP monazite was most probably formed from allanite during subduction and prograde metamorphism. The monazites included in garnet and kyanite are mostly unaltered, whereas those in the matrix show breakdown coronas consisting of apatite, REE-epidote/allanite and REE-carbonate, probably formed as a result of pressure decrease and cooling. 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 Baltica continental margin in Early Ordovician time.