Calc-alkaline lamprophyres from Lusatia (Germany)¡ªEvidence for a repeatedly enriched mantle source

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• 作者：Kh.M. Abdelfadil ; R.L. Romer ; Th. Seifert ; R. Lobst
• 关键词：Tholeiitic gabbro ; Spessartite ; Mantle metasomatism ; Variscan orogeny
• 刊名：Chemical Geology
• 出版年：2013
• 出版时间：30 August, 2013
• 年：2013
• 卷：353
• 期：Complete
• 页码：230-245
• 全文大小：1669 K

文摘

Pre-Variscan mantle derived gabbros (c. 400 Ma) and late-Variscan calc-alkaline lamprophyres (c. 330 Ma) were emplaced within the Cadomian basement of Lusatia. They were sampled to characterize the effect of the Variscan orogeny onto the mantle beneath Lusatia. The tholeiitic gabbros originated from a mantle source that had been metasomatized during subduction beneath the Cadomian magmatic arc at c. 570 Ma, which led to enrichment of LREE, Ba/Nb, and LILE relative to primitive mantle. The late-Variscan calc-alkaline lamprophyres (spessartites) have high MgO, Cr, and Ni contents reflecting the mantle source. The spessartites, however, have distinctly higher Rb, Ba, Pb, Sr, Th, and Cs contents, higher La/Yb, ⁸⁷Sr/⁸⁶Sr, and ²⁰⁶Pb/²⁰⁴Pb ratios, and lower ¹⁴³Nd/¹⁴⁴Nd ratios than the gabbros, which indicates a second, Variscan event of mantle enrichment. In addition, the spessartites have trace element ratios (i.e., Ba/Nb, Nb/U, Th/U, and Th/Nb) that resemble continental crust and Sr, Nd, and Pb isotopic compositions that demonstrate involvement of crustal material by source enrichment during the Variscan orogeny. The calc-alkaline lamprophyres of the Lusatia occupy the same age range as calc-alkaline lamprophyres from the adjacent Erzgebirge and Sudetes. The trace-element signatures and Sr and Nd isotopic compositions of Lusatian spessartites, however, are less enriched than those of comparable dikes in the Sudetes and the Erzgebirge. This implies that the Variscan orogeny resulted in geochemically and isotopically heterogeneous lithospheric mantle on the regional scale, possibly reflecting the contrasting nature of the subducted rocks.