Tourmaline as a petrogenetic indicator in the Pfitsch Formation, Western Tauern Window, Eastern Alps
Tourmaline as a petrogenetic indicator in the Pfitsch Formation, Western Tauern Window, Eastern Alps
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DOI：
10.1016/j.lithos.2017.04.008
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发表时间：
2017-07
期刊：
影响因子：
3.5
通讯作者：
Eleanor J. Berryman;M. Kutzschbach;R. Trumbull;A. Meixner;V. Hinsberg;S. Kasemann;G. Franz
中科院分区：
文献类型：
--
作者：
Eleanor J. Berryman;M. Kutzschbach;R. Trumbull;A. Meixner;V. Hinsberg;S. Kasemann;G. Franz
电气石是位于东阿尔卑斯山陶恩窗Pfitscher Joch（帕索de Vizze）地区变质沉积岩Pfitsch组中的常见副矿物。这些后华力西变质沉积单元在阿尔卑斯造山运动期间经历了约550 °C，1.0 GPa的峰值变质条件。电气石在约25 m厚的钾长石片麻岩（约20-200 μg/g B）中含量最丰富，其自形晶体长度通常为10 mm。在粗粒石英长石偏析（~ 1200 μg/g B）附近，电气石晶体的丰度和尺寸增加，反映了变质流体对B的活化和富集。在偏析附近，单个的电气石晶体有多达三个生长区，记录了正向（~ 350-500 °C，0.7-1.0 GPa）和逆向（~ 400 °C，0.2 GPa）生长，这是通过结合纹理信息和扇形分区测温法确定的。退变质电气石以单个晶体形式出现，也可在逆冲电气石晶体上过度生长，特别是在区域减压过程中东西向伸展形成的伸展裂缝表面。Pfitsch组中的电气石是一种具有不同Fe含量的镁铁岩，其含量与其各自主体单元的Fe含量相关。电荷平衡计算表明，一个显着比例的铁在电气石是铁，支持陆上大陆沉积原岩的解释。分离附近的Tourwallet具有最高的推断三价铁含量，其在生长带之间降低，可能反映了变质作用期间流体的减少。Mg/（Mg + Fe）比值随逆生长而增大，随逆生长而减小。与此相反，随着变质作用的进行，Ca/（Ca + Na）比值从0.05逐渐增加到0.20，在退变质作用中继续增加到0.25，记录了阿尔卑斯变质作用期间流体的最大Ca/（Ca + Na）比值（− 14.1至− 33.6‰），最高值（− 17.7至− 14.1‰）出现在富含电气石的富硼样品（165-1200 μg/g B）中，最低值（− 24.2至− 33.6‰）出现在缺乏电气石的贫硼样品（21-40 μg/g）中。这一观察结果支持了进变质作用期间岩石中11 B的优先损失。在Pfitsch组中，带状的电气石晶体显示出B同位素比值在核部从− 7.8 ‰到− 11.2‰逐渐降低，在边缘从− 17.3 ‰到− 20.3‰逐渐降低。在Rayleigh分馏模型的支持下，寄主岩石和电气石晶体的B同位素值最容易被解释为在阿尔卑斯变质作用期间B从富B的前体矿物（如云母）到电气石的内部再分配。
Tourmaline is a common accessory mineral in the metasedimentary Pfitsch Formation located in the Pfitscher Joch (Passo de Vizze) area in the Tauern Window of the Eastern Alps. These post-Variscan metasedimentary units experienced peak metamorphic conditions of ~ 550 °C, 1.0 GPa during the Alpine orogeny. Tourmaline is most abundant in a ~ 25 m thick unit of feldspathic gneiss (~ 20–200 μg/g B), where it occurs as idiomorphic crystals typically 10 mm in length. The abundance and size of the tourmaline crystals increase near coarse-grained quartzofeldspathic segregations (~ 1200 μg/g B), reflecting the mobilization and concentration of B by metamorphic fluids. Near segregations, individual tourmaline crystals have up to three growth zones, recording pro- (~ 350–500 °C, 0.7–1.0 GPa) and retrograde (~ 400 °C, 0.2 GPa) growth as determined by combining textural information and sector-zoning thermometry. Retrograde tourmaline occurs as individual crystals as well as overgrowths on prograde tourmaline crystals, especially on the surface of extensional fractures formed by E–W extension during regional decompression.Tourmaline in the Pfitsch Formation is dravitic with a variable Fe content that correlates with the Fe content of its respective host unit. Charge balance calculations suggest that a significant proportion of Fe in tourmaline is ferric, supporting the interpretation of a subaerial continental sedimentary protolith. Tourmaline near segregations has the highest inferred ferric iron content, which decreases across growth zones, potentially reflecting a reduction of the fluid during metamorphism. The Mg/(Mg + Fe) ratio increases with prograde tourmaline growth and decreases in retrograde overgrowths. In contrast, the Ca/(Ca + Na) ratio increases gradually from 0.05 to 0.20 with prograde growth and continues to increase up to 0.25 in the retrograde overgrowths, recording the maximum Ca/(Ca + Na) ratio of the fluid during Alpine metamorphism.The metasediments of the Pfitsch Formation have very low and variable whole-rock δ11B values (− 14.1 to − 33.6‰), with the highest values (− 17.7 to − 14.1‰) found in B-rich samples (165–1200 μg/g B) containing abundant tourmaline, and the lowest values (− 24.2 to − 33.6‰) in B-depleted samples (21–40 μg/g), which lack tourmaline. This observation supports preferential loss of11B from the rocks during prograde metamorphism. Zoned tourmaline crystals in the Pfitsch formation show successively decreasing B isotope ratios from − 7.8 to − 11.2‰ in their cores and − 17.3 to − 20.3‰ in their rims. As supported by a Rayleigh fractionation model, the B-isotope values of the host rocks and the tourmaline crystals are most easily explained by the internal redistribution of B from a B-rich precursor mineral (e.g. mica) to the tourmaline during Alpine metamorphism.