Eberlei T, Habler G, Wegner W, Schuster R, Körner W, Thöni M, Abart R
Department of Lithospheric Research, University of Vienna, Althanstraße 14, A-1090 Vienna, Austria.
Austrian Geological Survey, Neulinggasse 38, A-1030 Vienna, Austria.
Lithos. 2015 Jun 15;227:161-178. doi: 10.1016/j.lithos.2015.04.007.
Permian metapegmatite muscovite from the Upper-Austroalpine Matsch Unit in Southern Tyrol (Italy) was investigated regarding its Rb/Sr and compositional retentivity during Cretaceous Upper-greenschist facies deformation. The data imply that microstructurally relic Permian magmatic muscovite largely maintained its major and trace element compositions during deformation, whereas the Rb/Sr geochronometer is strongly affected by a net loss of Sr. Lower Sr concentrations of muscovite correlate with higher Rb/Sr and Sr/Sr ratios. In most samples, the muscovite grain size- and magnetic-fractions with the lowest Rb/Sr and Sr/Sr ratios preserve a Permo-Triassic muscovite-whole rock Rb/Sr apparent age interpreted as to reflect formation during or cooling after pegmatite emplacement. Contrastingly, muscovite fractions with higher Rb/Sr and Sr/Sr ratios are arranged along a roughly linear array with a positive correlation of the Rb/Sr and Sr/Sr ratios in the Rb/Sr vs Sr/Sr space. They yield successively lower muscovite-whole rock Rb/Sr apparent ages. We explain the variations in the Rb/Sr isotopic character of microstructurally relic muscovite by a, presumably deformation-related, loss of Sr during the Cretaceous event. Contemporaneously, only very limited amounts of isotopically different Sr from the matrix reservoir might possibly have entered the muscovite. Consequently, the Rb/Sr of the relic muscovite is affected by a net loss of Sr. The results imply that at temperatures of < 500 °C, deformation is supposed to be the predominant factor in controlling the Rb/Sr geochronometer of relic muscovite, by significantly reducing the characteristic length scale for volume diffusion. However, variations of the major and trace element compositions within Permian relic muscovite are interpreted to rather reflect primary compositional instead of deformation-related variations.
对来自意大利南蒂罗尔上奥地利阿尔卑斯马特施单元的二叠纪变伟晶岩白云母进行了研究，以探讨其在白垩纪上绿片岩相变形过程中的铷/锶比值和成分保留情况。数据表明，微观结构上残留的二叠纪岩浆白云母在变形过程中基本保持了其主量和微量元素组成，而铷/锶地质年代计受到锶净损失的强烈影响。白云母较低的锶浓度与较高的铷/锶和锶/锶比值相关。在大多数样品中，铷/锶和锶/锶比值最低的白云母晶粒尺寸和磁性部分保留了一个二叠纪 - 三叠纪白云母 - 全岩铷/锶表观年龄，解释为反映伟晶岩侵位期间或之后冷却时的形成时间。相反，铷/锶和锶/锶比值较高的白云母部分在铷/锶与锶/锶空间中沿着大致线性的阵列排列，铷/锶和锶/锶比值呈正相关。它们产生的白云母 - 全岩铷/锶表观年龄依次降低。我们通过白垩纪事件期间可能与变形相关的锶损失来解释微观结构上残留白云母铷/锶同位素特征的变化。同时，只有非常少量来自基质储库的同位素不同的锶可能进入了白云母。因此，残留白云母的铷/锶受到锶净损失的影响。结果表明，在温度低于500°C时，变形被认为是控制残留白云母铷/锶地质年代计的主要因素，通过显著减小体积扩散的特征长度尺度。然而，二叠纪残留白云母内主量和微量元素组成的变化被解释为更多地反映原生成分变化而非与变形相关的变化。
