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Mesozoic mafic alkaline magmatism of southern Scandinavia

机译：斯堪的纳维亚南部中生代镁铁质碱性岩浆作用

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More than 100 volcanic necks in central Scania (southern Sweden) are the product of Jurassic continental rift-related mafic alkaline magmatism at the southwest margin of the Baltic Shield. They are mainly basanites, with rarer melanephelinites. Both rock groups display overlapping primitive Mg-numbers, Cr and Ni contents, steep chondrite-normalized rare earth element patterns (LaN /YbN = 17–27) and an overall enrichment in incompatible elements. However, the melanephelinites are more alkaline and have stronger high field strength element enrichment than the basanites. The existence of distinct primary magmas is also indicated by heterogeneity in highly incompatible element ratios (e.g. Zr/Nb, La/Nb). Trace element modelling indicates that the magmas were generated by comparably low degrees of melting of a heterogeneous mantle source. Such a source can best be explained by a metasomatic overprint of the mantle lithosphere by percolating evolved melts. The former existence of such alkaline trace element-enriched melts can be demonstrated by inversion of the trace element content of green-core clinopyroxenes and anorthoclase which occur as xenocrysts in the melanephelinites and are interpreted as being derived from crystallization of evolved mantle melts. Jurassic magmatic activity in Scania was coeval with the generation of nephelinites in the nearby Egersund Basin (Norwegian North Sea). Both Scanian and North Sea alkaline magmas share similar trace element characteristics. Mantle enrichment processes at the southwest margin of the Baltic Shield and the North Sea Basin generated trace element signatures similar to those of ocean island basalts (e.g. low Zr/Nb and La/Nb) but there are no indications of plume activity during the Mesozoic in this area. On the contrary, the short duration of rifting, absence of extensive lithospheric thinning, and low magma volumes argue against a Mesozoic mantle plume. It seems likely that the metasomatic imprint resulted from the earlier Permo-Carboniferous rifting episode which affected the entire study area and clearly was accompanied by plume activity (Ernst and Buchan in American Geophysical Union, pp 297–337, 1997). Renewed rifting in Jurassic times triggered decompression melting in the volatile-enriched lithospheric mantle and the alkaline melts generated inherited the earlier “stored” plume signature.
机译：斯堪尼亚中部（瑞典南部）有100多个火山颈，是波罗的海盾西南缘侏罗纪大陆裂谷相关的镁铁质碱性岩浆作用的产物。它们主要是玄武岩，稀有黑闪石。两个岩石组均显示出重叠的原始Mg数，Cr和Ni含量，陡峭的球粒晶归一化稀土元素模式（LaN / YbN = 17-27）和不相容元素的整体富集。然而，黑闪锌矿比碱性泡沫石更碱性，并且具有更强的高场强元素富集。高度不相容的元素比率（例如Zr / Nb，La / Nb）的异质性也表明了独特的原生岩浆的存在。痕量元素建模表明，岩浆是由相对较低的非均质地幔源融化产生的。可以通过渗出演化的熔体通过地幔岩石圈的交代交叠叠印来最好地解释这种来源。这类富含碱性痕量元素的熔体的存在可以通过将绿芯斜向辉石和顺正石的痕量元素含量反转来证明，它们在黑闪石中以异晶晶形式出现，并被解释为是由演化的地幔熔体结晶而得。斯堪尼亚的侏罗纪岩浆活动与附近的埃格松德盆地（挪威北海）的霞石岩生成同时期。斯堪尼亚和北海碱性岩浆都具有相似的微量元素特征。波罗的海盾西南缘和北海盆地的地幔富集过程产生了与海洋玄武岩类似的微量元素特征（例如低Zr / Nb和La / Nb），但没有迹象表明中生代有羽状活动。这片区域。相反，裂谷的持续时间短，岩石圈没有广泛变薄以及岩浆体积小，这与中生代地幔柱不符。易位标记似乎是由较早的Permo-Carboniferous裂谷事件引起的，它影响了整个研究区域，并明显伴随着羽流活动（Ernst and Buchan in American Geophysical Union，第297-337页，1997年）。侏罗纪时期重新裂谷导致富集岩石圈地幔的减压融化，生成的碱性熔体继承了较早的“储藏”羽状特征。

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