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Ultrapotassic magmatism in the heyday of the Variscan Orogeny: the story of the Třebíč Pluton, the largest durbachitic body in the Bohemian Massif
International Journal of Earth Sciences ( IF 2.3 ) Pub Date : 2020-05-29 , DOI: 10.1007/s00531-020-01872-2
Vojtěch Janoušek , Pavel Hanžl , Martin Svojtka , John M. Hora , Yulia V. Erban Kochergina , Petr Gadas , František V. Holub , Axel Gerdes , Kryštof Verner , Kristýna Hrdličková , J. Stephen Daly , David Buriánek

The Třebíč Pluton (TP) comprises plutonic rocks of the ‘durbachite series’, i.e., ultrapotassic biotite–amphibole quartz melasyenites–melagranites with conspicuous K-feldspar phenocrysts and abundant mafic enclaves. The TP with its satellite bodies intruded the eastern Moldanubian Zone (orogenic root of the Variscan Bohemian Massif) at c. 338 Ma. Overall, the entire TP durbachite series (SiO2 = 52–67 wt%) remains highly magnesian, reduced, and (except when strongly contaminated) metaluminous. The durbachitic rocks are markedly enriched in lithophile elements (esp. Cs, Rb, Th, U, K and Pb) and LREE; in contrast, the contents of high-field strength elements (Nb, Ta, and Ti) and Sr are much lower, resulting in conspicuous negative anomalies in Primitive mantle-normalized multielement plots. The proposed petrogenetic model requires re-enrichment of the depleted, harzburgitic lithospheric mantle by deeply subducted crustal material of the Saxothuringian lower plate, directly and/or via (U)HP melt/fluid. This process produced phlogopite-rich metasomes (phlogopite harzburgites and/or glimmerites). The metasomes re-melted soon thereafter by a short thermal pulse caused by slab break-off or decompression melting of rising diapirs of the relatively light metasomatized mantle, yielding primary ultrapotassic melts (87Sr/86Sr337 > 0.712 and \(\varepsilon_{{{\text{Nd}}}}^{337}\) ≤ 7.6). Upon their passage through the Moldanubian upper plate, ultrapotassic primary magmas mixed with felsic anatectic melts (87Sr/86Sr337 < 0.710; \(\varepsilon_{{{\text{Nd}}}}^{337}\) > – 6.5). K-feldspar-driven fractional crystallization ± accumulation represented only a late modifying influence. The occurrence of such ultrapotassic enriched/crustally contaminated mantle-derived magmatism complicates any conventional mass-balance assessments of relative roles of crustal recycling vs. growth in hot collisional orogens, e.g., Variscides or Himalayas.



中文翻译:

瓦里斯卡造山运动鼎盛时期的超钾盐岩浆作用:波希米亚地块最大的硬脊岩体特热比克·普鲁顿(TřebíčPluton)的故事

TřebíčPluton(TP)包含“杜巴石系列”的深成岩,即超钾黑云母-闪石石英黑云母-黑云母,具有明显的钾长石斑岩和丰富的镁铁质飞地。TP及其卫星体侵入了c的东部Moldanubian区(Variscan Bohemian断层的造山根)。338毫安。总体而言,整个TP钙铝矾石系列(SiO 2 = 52–67 wt%)仍然是高镁质,还原态的(金属元素除外)。硬质岩中的岩溶岩元素(尤其是Cs,Rb,Th,U,K和Pb)和LREE显着丰富。相比之下,高场强元素(Nb,Ta和Ti)和Sr的含量要低得多,从而在原始地幔归一化多元图中产生明显的负异常。拟议的成岩模式要求直接或通过(U)HP熔体/流体通过萨克斯图林下部板的深俯冲地壳物质重新富集贫乏的哈茨堡型岩石圈地幔。该过程产生富含金云母的变质体(金云母的harzburgites和/或闪闪石)。87 Sr / 86 Sr 337  > 0.712并且\(\ varepsilon _ {{{text {Nd}}}} ^ {337} \)  ≤7.6)。当它们穿过摩尔达努比亚上板时,超钾化的原岩浆与长英质的前熔融相混合(87 Sr / 86 Sr 337  <0.710; \(\ varepsilon _ {{{\ text {Nd}}}} ^ {337} \)  > – 6.5)。钾长石驱动的部分结晶±累积仅表示后期修饰影响。这种超钾质富集/地壳污染的地幔源岩浆作用的发生,使常规的质量平衡评估复杂化了地壳再循环与热碰撞造山带(如Variscides或喜马拉雅山)中的相对作用。

更新日期:2020-05-29
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