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首页> 外文期刊>Contributions to Mineralogy and Petrology >The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source
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The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source

机译：S型花岗岩的痕量元素组成：源中不平衡熔融和副相夹带的证据

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Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton (Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals. Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates allow the residency time for the melt in the source to be conservatively estimated at less than 500 years. Keywords S-type granite - Trace element - Partial melting - Geochemical modelling - Accessory phases - Cape Granite Suite Communicated by T. L. Grove.
机译：在单个储集层中，S型花岗岩中的微量元素浓度通常随铁矿化度而增加（本研究中铁和镁的总含量并表示为原子Fe + Mg）；相同参数下痕量元素浓度的变化程度也显着扩大。半岛岩体（南非开普花岗岩套房）的强烈高铝质，高级S型花岗岩是在地壳底部附近的黑云母熔融沉积的沉积物。套件中的白花岗石代表了接近于来自高铝酸盐来源的纯熔体，而更多的铁镁质变种代表了熔体与包晶石榴石和钛铁矿的混合物。在熔化过程中集中在反应性矿物中的痕量元素（如Rb，Ba，Sr和Eu）在花岗岩中散布相当多。这被解释为反映来源中的成分变化。相比之下，诸如LREE，Zr和Hf之类的元素集中在耐火的辅助相（锆石和独居石）中，它们与SiO 2 的增加具有明确的负相关关系，并且随着磁导率的增加呈线性关系。这被解释为反映了辅助矿物和包晶相共混到熔体中的夹带：白垩纪岩石不可能通过分步结晶过程从套件中更多的镁铁质成分中演化出来，因为在这种情况下，它们将继承饱和的锆石。这个假设的早期岩浆的特征。通过平衡和不平衡微量元素熔化，对半岛岩体中花岗岩的微量元素行为进行了建模。在不平衡情况下，将熔体建模为使源具有可变比例的夹带包晶相和辅助矿物，但是在熔体溶解任何辅助矿物之前。因此，熔体中的痕量元素特征很大程度上来自于熔融反应中的反应物，而没有锆石和独居石溶解的贡献。在平衡情况下，在达到锆石和独居石饱和之后，熔体离开夹带晶体的源。半岛岩体中很大一部分岩石的痕量元素浓度低于锆石和独居石饱和度所预测的浓度。对于最白垩纪的岩石，所有成分都是锆石欠饱和的。而大多数镁铁质组成中的锆石过饱和。但是，在两种情况下，锆石通常都是异晶的。因此，白云岩代表的熔岩接近于纯熔岩，它们的逸出速度足够快，以至于有些熔岩与本研究中应用的微量元素不平衡熔岩模型非常接近。锆石的溶解速度可以保守地估计源中熔体的停留时间少于500年。关键词S型花岗岩-痕量元素-部分熔融-地球化学模拟-附属相-开普花岗岩套房由T. L. Grove通讯。

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