Ta and Sn concentration by muscovite fractionation and degassing in a lens-like granite body: The case study of the Penouta rare-metal albite granite (NW Spain)
Ta and Sn concentration by muscovite fractionation and degassing in a lens-like granite body: The case study of the Penouta rare-metal albite granite (NW Spain)
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DOI：
10.1016/j.oregeorev.2016.11.027
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发表时间：
2017-04
影响因子：
3.3
通讯作者：
F. López-Moro;F. Polonio;T. González;J. Contreras;A. F. Fernández;M. Benito
中科院分区：
文献类型：
--
作者：
F. López-Moro;F. Polonio;T. González;J. Contreras;A. F. Fernández;M. Benito
Penouta过铝低磷花岗岩是伊比利亚地块最重要的低品位、高吨位含锡钽铌钠长石花岗岩。片或岩盖的形状，而不是股票，推断Penouta花岗岩，可能与含氟熔体的低粘度和高流动性。岩浆的近水平侧向伸展也通过垂直和水平地球化学变化推断。在变化图中没有成分空白，再加上相容和不相容元素随高度的连续演化趋势，放弃了多脉冲侵入，指向一个单一的岩浆脉冲。矿物化学、微量元素和最小二乘质量平衡模型支持侵位部位自下而上的分异过程。切换不兼容的行为（钟形趋势）在锡，铌和钽的变化图，再加上实验限制钽铁矿和钽铁矿饱和度的情况下，表明铌钽氧化物和可能的钽铁矿不分馏矿物相，其结晶被困堆间熔体内的浓度梯度。主要和微量元素建模支持的浓度以上的Ta和Ta/Nb比可能是矿物分馏的结果，具有关键作用的白云母（主要是原生）的Ta/Nb比，因为这种矿物具有较高的分配系数Nb比Ta。我们的研究结果表明，氟和peraluminosity在Ta/Nb比的变化有一个有限的影响。因此，Ta富集主要受分步结晶过程控制。在大多数情况下，锡的富集也伴随着钽，表明晶体-熔体分馏过程中也发挥了重要作用，锡的浓度。然而，最强的锡富集在花岗岩（如，花岗岩体中部）并不对应显著的Ta富集。Sn对流体的高亲和力和Ta对熔体的高分配可以解释这种解耦。然而，在这些强烈的锡富集区（中央部分的花岗岩体）的岩浆签名排除了这种流体的热液subsolidus起源。通过与在类似于岩床的岩体中进行的模型进行类比，似乎可能的是，在花岗岩体的中心部分的Sn富集与流体饱和/脱气发生在较低的边缘，作为冷却和结晶的主要无水矿物（即二次沸腾）的结果。出溶的蒸汽迁移到更热的熔体中，到达中心部分，在那里它可能被重新吸收，产生具有岩浆特征的镁橄榄石。此外，我们认为，在花岗岩体的上边缘的热损失也可能有助于形成第二流体饱和带。因此，发育了伟晶细晶岩和云英岩。
The Penouta peraluminous low-phosphorous granite is the most important low-grade, high-tonnage Sn-Ta-Nb-bearing albite granite from the Iberian Massif. A sheet or laccolith shape, instead of a stock, is inferred for the Penouta granite, maybe in relation with the low viscosity and high mobility of a fluorine-bearing melt. Subhorizontal lateral extension of the magma is also inferred via vertical and horizontal geochemical variations. The absence of compositional gaps in variation diagrams, coupled with continuous evolutionary trends of compatible and incompatible elements with height, discard a multi-pulse intrusion and point to a single magma pulse. Mineral chemistry, trace element and least-squares mass balance modelling support a differentiation process from bottom to top in the emplacement place. The absence of switch from incompatible to compatible behaviour (bell-shaped trends) in Sn, Nb and Ta variation diagrams, coupled to experimental constraints on tantalite and cassiterite saturation, suggest that Nb-Ta oxides and probably cassiterite were not fractionated mineral phases, their crystallisation being related to concentration gradients within a trapped intercumulus melt. Major and trace element modelling support that the concentration upwards of Ta and the Ta/Nb ratio could be a consequence of mineral fractionation, with a key role of muscovite (mainly primary) for the Ta/Nb ratio, as this mineral has a higher partition coefficient for Nb than Ta. Our results suggest that fluorine and peraluminosity had a limited effect in the Ta/Nb ratio variations. Hence, Ta enrichment is mainly controlled by fractional crystallisation processes. In most cases, Sn enrichment was also concomitant with Ta, indicating that crystal-melt fractionation processes also played an important role in Sn concentration. Nevertheless, the strongest Sn enrichment in the granite (e.g., central part of the granite body) does not correspond to a significant Ta enrichment. The high affinity of Sn for fluids and the high partitioning of Ta for melt could explain this decoupling. Nevertheless, the magmatic signature of cassiterites in these strongly Sn-enriched zones (central part of the granite body) rules out a hydrothermal subsolidus origin for this fluid. By analogy with models carried out in sill-like bodies it seems likely that the Sn enrichment in the central part of the granite body is related to fluid saturation/degassing occurred in the lower margin, as a consequence of cooling and crystallisation of mostly anhydrous minerals (i.e. second boiling). The vapour exsolved migrated into the hotter melt up to the central part, where it probably was reabsorbed, yielding cassiterite with a magmatic signature. Moreover, we suggest that heat loss in the upper margin of the granite body might also contribute to the formation of a second fluid-saturated zone. As a result, pegmo-aplites and greisen were developed.