Geochemical signature of the Egersund basaltic dyke swarm, SW Norway, in the context of late-Neoproterozoic opening of the Iapetus Ocean
Geochemical signature of the Egersund basaltic dyke swarm, SW Norway, in the context of late-Neoproterozoic opening of the Iapetus Ocean
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挪威西南部埃格尔松玄武岩墙群的地球化学特征，在新元古代晚期土卫八洋开放的背景下
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
D. Demaiffe
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文献类型：
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作者：
B. Bingen;D. Demaiffe
斑晶和无斑晶岩脉。浅部岩脉岩浆分异作用有限！入侵。斑状岩墙和其中一个斑状岩墙具有亚碱性至弱碱性成分，而其他斑状岩墙具有碱性成分。最原始的岩浆出现在斑状岩墙中;它们的Mg值为56-61，TiO 2含量为2.0-2。富集了不相容的微量元素。初始同位素组成Sr_i和e（Nd）_i分别为0.7034-0.7039和+2.0-+3.1。碱性岩墙中演化最快的岩浆是贫SiO2（45- 46%）的铁玄武岩，非常富集TiO 2（3.2 - 3.4%）、P2 O 5（2.0-2.5%）和不相容微量元素。它们具有0.7058 - 0.7060的Sr i和± 1.0的e（Nd）i。最原始的岩浆和矿物成分表明，斑岩脉在壳幔边界形成了一个岩浆房。10千巴。结果表明，其中一条斑晶岩墙可能与斑状岩墙岩浆有关，在此压力下，它们经历了分离结晶过程，并伴随着有限的地壳同化作用。碱性岩墙代表了一个具有不同地幔源区的独特岩套。埃格松群（616 ± 3 Ma）形成于新元古代晚期裂谷作用导致土卫八洋开启的大地构造背景下。在Baltoscandian被动边缘的波罗的海，有一个第一级“向海洋”减少不兼容的微量元素含量和Sr;和增加的e（Nd）;值的基底连接岩墙群，从埃格松群在大陆基底，席状岩墙复合体位于大陆-{）Cean过渡。这种地球化学趋势被解释为从高压部分熔融的石榴橄榄岩残留石榴石在一个温和的亏损地幔，低压部分熔融亏损地幔的变化。
phenocrysts and aphyric dykes. Limited magmatic differentiation occurred in the dykes during shallow leve! intrusion. The porphyritic dykes and one of the aphyric dykes have subalkaline to mildly alkaline compositions, whereas the other aphyric dykes have alkaline compositions. The most primitive magmas occur in the porphyritic dykes; they have Mg numbers of 56-61, Ti02 coutents of 2.0-2. 1% and enriched incompatible trace element contents. Initial isotopic compositions, Sr; and e(Nd);, are 0.7034-0.7039 and +2.0-+3.1 respectively. The most evolved magmas in alkaline dykes are SiOz-poor (45--46%) ferrobasalts, very enriched in Ti02 (3.2 -3.4%), P205 (2.0-2.5%) and incompatible trace elements. They have Sr; of 0.7058 -0.7060 and e(Nd); of +.1.0. The compositions of the most primitive magmas and minerals suggest that the porphyritic dykes tapped a magma chamber situated at the crust -mantle boundary, at ca. 10 kbar. It is shown that one of the aphyric dykes is possibly related to the porphyritic dyke magma by a fractional crystallization process accompanied by limited crustal assimilation at this pressure. The alkaline dykes represent a distinct suite with a distinct mantle source. The 616 ± 3 Ma Egersund Swarm is placed in the geotectonic context of late-Neoproterozo ic rifting leading to opening of the Iapetus Ocean. At the Baltoscandian passive margin of Baltica, there is a first order 'oceanwards' decrease in incompatible trace element contents and Sr; and an increase of e(Nd); values of basal tie dyke swarms, from the Egersund Swarm in the continental basement, to the sheeted dyke complexes situated at the continent--{)Cean transition. This geochemical trend is interpreted as a change from high-pressure partial melting of garnet peridotite with residual garnet in a mildly depleted mantle, to lower-pressure partial metting of a depleted mantle.