Magmatic-to-hydrothermal crystallization in the W–Sn mineralized Mole Granite (NSW, Australia): Part II: Evolving zircon and thorite trace element chemistry
Magmatic-to-hydrothermal crystallization in the W–Sn mineralized Mole Granite (NSW, Australia): Part II: Evolving zircon and thorite trace element chemistry
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DOI：
10.1016/j.chemgeo.2005.02.017
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发表时间：
2005-08
期刊：
影响因子：
3.9
通讯作者：
T. Pettke;A. Audétat;U. Schaltegger;C. Heinrich
中科院分区：
文献类型：
--
作者：
T. Pettke;A. Audétat;U. Schaltegger;C. Heinrich
在澳大利亚东部的锡-钨矿化的Mole花岗岩中，在一个漫长的岩浆-热液演化过程中，锆石在几个阶段结晶。34个元素已被量化的激光烧蚀电感耦合等离子体质谱显微分析与锆石的化学成分，其生长环境的目的。微量元素含量是高度可变的所有纹理发生。最早期石英斑晶中的锆石包裹体表明，锆石可能是在地壳深处结晶的液相。微量元素含量明显偏高，仅表现出轻微的Ce正异常，而表现出明显的Eu负异常。岩浆锆石的连续结晶阶段的特征是微量元素含量逐渐减少，特别是稀土元素，具有越来越重要的正Ce异常。这种演化反映了独居石、钍石、磷钇矿和磷灰石等稀土元素接受矿物的饱和度，受岩浆热液出溶作用的影响较小。被解释为从含锡钨石英脉中的含水流体中沉淀的锆石显示出与晚期岩浆锆石难以区分的稀土元素模式。当结合实验证据的流体-熔体分配的稀土元素，它表明，锆石/熔体和锆石/流体的稀土分配系数在很大程度上是可比的。热液锆石的第二个例子是在寄主花岗岩之后结晶了大约2 My。这些晶体揭示了从核心到边缘微量元素浓度增加的颗粒内环带。因此，稀土元素丰度和模式本身并不是锆石结晶的地质环境的决定性指标。尽管如此，微量元素含量的变化，涉及到锆石结晶的熔体或流体的化学成分，在同生熔体和流体包裹体测量，是有前途的未来成岩建模。热液锆石中铅和铯是强不相容的，估计的锆石-流体分配系数D≤0.001，而Sn和Li是中等不相容的，DSn <$0.6和DLi <$0.1，Ce是相容的，DCe <$14。此外，热液锆石具有更显著的负Eu异常和更高的Ta/Nb和U/Th比值。
The Sn–W mineralized Mole Granite in Eastern Australia hosts zircon populations that crystallized at several stages during a protracted magmatic to hydrothermal evolution. Thirty-four elements have been quantified by laser-ablation inductively-coupled-plasma mass-spectrometric microanalysis with the aim of relating the chemistry of zircon to its growth environment. Trace element contents are highly variable for all textural occurrences. Zircon inclusions in earliest quartz phenocryst suggest that zircon was a liquidus phase that crystallized probably deep in the crust. Trace element contents are conspicuously high, showing only a slight positive Ce anomaly but a pronounced negative Eu-anomaly. Successive crystallization stages of magmatic zircon are characterized by progressive depletion in trace element contents, notably the rare earth elements, with an increasingly important positive Ce-anomaly. This evolution reflects saturation of REE accepting minerals such as monazite, thorite, xenotime and possibly apatite and is affected little by the exsolution of a magmatic–hydrothermal fluid. Zircon that is interpreted to have precipitated from aqueous fluids in Sn–W-bearing quartz veins shows REE patterns indistinguishable from those of late magmatic zircon. When combined with experimental evidence on the fluid–melt partitioning of REE, it indicates that the REE distribution coefficients for zircon/melt and zircon/fluid are largely comparable. The second example of hydrothermal zircon crystallized some 2 My after the host granite. These crystals reveal an intragranular zonation of increasing trace element concentrations from core to rim. Therefore, REE abundances and patterns alone are not conclusive indicators of the geological environment in which zircon crystallized. Nevertheless, variations in trace element contents of zircon that relate to the chemistry of the melt or fluid from which zircon crystallized, as measured in cogenetic melt and fluid inclusions, are promising for future petrogenetic modeling. Lead and Cs are strongly incompatible in hydrothermal zircon, with estimated zircon–fluid distribution coefficients D≤0.001, while Sn and Li are moderately incompatible, DSn∼0.6 and DLi∼0.1, and Ce is compatible, DCe∼14. Moreover, hydrothermal zircon has a more pronounced negative Eu-anomaly and higher Ta/Nb and U/Th ratios than the magmatic zircons of the Mole Granite.