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首页> 外文期刊>Contributions to Mineralogy and Petrology >Boron metasomatism and behaviour of rare earth elements during formation of tourmaline rocks in the eastern Arunta Inlier, central Australia
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Boron metasomatism and behaviour of rare earth elements during formation of tourmaline rocks in the eastern Arunta Inlier, central Australia

机译：澳大利亚中部阿伦塔内陆东部电气石岩石形成过程中的硼交代作用和稀土元素的行为

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摘要

Tourmaline rocks of previously unclear genesis and spatially associated with W- (Cu)-bearing calc-silicate rocks occur in Palaeoproterozoic supracrustal and felsic intrusive rocks in the Bonya Hills in the eastern Arunta Inlier, central Australia. Tourmalinisation of metapelitic host rocks postdates the peak of regional low-pressure metamorphism (M1/D1, ~500 °C, ~0.2 GPa), and occurred synkinematically between the two main deformation events D1 and D2, coeval with emplacement of Late Strangways (~1.73 Ga) tourmaline-bearing leucogranites and pegmatites. Tourmaline is classified as schorl to dravite in tourmaline–quartz rocks and surrounding tourmaline-rich alteration zones, and as Fe-rich schorl to foitite in the leucogranites. Boron metasomatism resulted in systematic depletion of K, Li, Rb, Cs, Mn and enrichment of B, and in some samples of Na and Ca, in the tourmaline rocks compared to unaltered metasedimentary host rocks. Whole-rock REE concentrations and patterns of unaltered schist, tourmalinised schist and tourmaline–quartz veins—the latter were the zones of influx of the boron-rich hydrothermal fluid—are comparable to those of post-Archaean shales. Thus, the whole-rock REE patterns of these rocks are mostly controlled by the metapelitic precursor. In contrast, REE concentrations of leucogranitic rocks are low (≤10 times chondritic), and their flat REE patterns with pronounced negative Eu anomalies are typical for fractionated granitic melts coexisting with a fluid phase. REE patterns for tourmalines separated from metapelite-hosted tourmaline–quartz veins and tourmaline-bearing granites are very different from one another but each tourmaline pattern mirrors the REE distribution of its immediate host rock. Tourmalines occurring in tourmaline–quartz veins within tourmalinised metasediments have LREE-enriched (LaN/YbN=6.3–55), shale-like patterns with higher ΣREE (54–108 ppm). In contrast, those formed in evolved leucogranites exhibit flat REE patterns (LaN/YbN=1.0–5.6) with pronounced negative Eu anomalies and are lower in ΣREE (5.6–30 ppm). We therefore conclude that REE concentrations and patterns of tourmaline from the different tourmaline rocks studied are controlled by the host rock and not by the hydrothermal fluid causing boron metasomatism. From the similarity of the REE pattern of separated tourmaline with the host rock, we further conclude that incorporation of REEs in tourmaline is not intrinsically controlled (i.e. by crystal chemical factors). Tourmaline does not preferentially fractionate specific REEs or groups of REEs during crystallisation from evolved boron- and fluid-rich granitic melts or during alteration of clastic metasediments by boron-rich magmatic-hydrothermal fluids.
机译：在澳大利亚中部东部阿伦塔内里尔邦尼山的古元古代生界上，长英质侵入岩中，发生了以前不清楚的成因，并且在空间上与含W-（Cu）的钙硅酸盐岩相关的电气石岩。变质岩体的电气化作用晚于区域低压变质作用的峰值（M1 / D1 ，〜500°C，〜0.2 GPa），并且在两个主要变形事件D1 和D2 ，与晚期电气石道（〜1.73 Ga）的位置有关，托玛琳岩中含亮白花岗岩和伟晶岩。电气石被分类为在电气石-石英岩石和周围富含电气石的蚀变带中发生漂移的schorl，以及在白云石中的富铝酸盐中为富铝的schorl。硼的交代作用导致电气石系统中的K，Li，Rb，Cs，Mn和B以及某些Na和Ca样品中的K，Li，Rb，Rb的系统消耗，而未改变的准沉积基质岩。完整岩石的稀土元素浓度和未改变的片岩，电气化的片岩和电气石-石英脉（后者是富含硼的热液的流入区域）的模式与后阿尔凯斯页岩相当。因此，这些岩石的全岩REE模式主要由变质前驱体控制。相比之下，白垩质岩石的REE浓度较低（≤碎晶质岩的10倍），并且它们的扁平REE模式具有明显的Eu负异常，是与液相共存的分馏花岗岩熔体的典型特征。电气石的REE模式与含变质岩的电气石-石英脉和含电气石的花岗岩相距甚远，但每种电气石模式都反映了其直接寄主岩石的REE分布。在电气化的准沉积物中电气石-石英脉中发生的电气石富含LREE（LaN / YbN = 6.3–55），页岩状模式具有较高的REEREE（54–108 ppm）。相反，在演化的无色花岗岩中形成的那些表现出平坦的REE模式（LaN / YbN = 1.0–5.6），具有明显的Eu负异常，而其ΣREE较低（5.6–30 ppm）。因此，我们得出的结论是，所研究的不同电气石岩石中的REE浓度和电气石的模式受基质岩控制，而不受引起硼交代作用的热液控制。从分离的电气石与母岩的REE模式的相似性中，我们进一步得出结论，电气石中REE的掺入不受内在控制（即受晶体化学因素的影响）。在从富含硼和流体的花岗岩熔体中析出结晶过程中，或在富含硼的岩浆热液流体改变碎屑沉积物过程中，电气石不会优先分离特定的REE或REE组。

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