A thick succession of mafic upper Asbian volcanic rocks is exposed as cap rocks in the Magdalen Islands of eastern Quebec. This succession records peak stages of a Late Devonian to Early Carboniferous magmatic system that regionally developed from transtensional tectonics above a mantle plume. In the upper part of the succession, tholeiitic basalts with enriched mid-oceanic ridge basalt (E-MORB) affinities are tightly interbedded with highly alkaline basalts with ocean island affinities, whereas the lower part is composed of moderately alkaline basalts and pyroclastic deposits that are interpreted as the products of mixing between these two compositionally distinct melts. Based on trace-element contents and geological constraints, the tholeiitic basalts were produced by decompressional melting in depleted uppermost asthenospheric mantle material enriched over the average (E-MORB-type source) at the top of the plume, whereas the highly alkaline basalts were produced subsequently when decompressional melting reached down into the enriched mantle (ocean island basalt–type source). These two primary melts may have been thermally stratified below the lithosphere before being tapped by transtensional structures to feed a thick magmatic underplating that regionally developed at the base of the crust, where magma mixing may have occurred. Interbedding of the two contrasting melts in the upper part of the succession implies that, although the two primary mantle sources were still being tapped into, magma mixing was by then no longer occurring. Above a small gap, the uppermost flow is a fluorine-rich tholeiitic basalt with arc affinities that suggest partial melting of the regional subcontinental lithospheric mantle source, which was previously enriched by nearly continuous subduction below eastern Canada in early to middle Paleozoic times. The latter setting seemingly corresponds to the local waning of the magmatic system in uppermost Asbian times, possibly because of the westward migration of the overridden plume toward New Brunswick, where alkaline volcanism from a sublithospheric source resumed in Brigantian times.

中文翻译：

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加拿大魁北克东部的上维森马格达伦群岛玄武岩：在地幔柱上方的横张盆地发育的高峰阶段喷发的对比性基性岩石类型的复杂组合

在魁北克东部的马格达伦群岛，一层厚厚的基性上阿斯比安火山岩作为盖层出露。这个序列记录了晚泥盆世到早石炭世岩浆系统的高峰阶段，该岩浆系统是从地幔柱上方的张拉构造区域发展而来的。在序列的上部，具有富集洋中脊玄武岩（E-MORB）亲合性的拉斑玄武岩与具有洋岛亲合性的强碱性玄武岩紧密互层，而下部则由中碱性玄武岩和火山碎屑沉积组成解释为这两种成分不同的熔体混合的产物。基于微量元素含量和地质条件，拉斑玄武岩是通过在羽流顶部的平均（E-MORB 型源）富集的耗尽的最上部软流圈地幔物质中减压熔融产生的，而高碱性玄武岩是在减压熔融向下进入富集的地幔时产生的（海洋岛玄武岩型来源）。这两种初级熔体可能在岩石圈下方被热分层，然后被横张结构挖掘以供给厚厚的岩浆底盘，该底盘在地壳底部区域发育，岩浆可能在那里发生混合。两个对比熔体在序列上部的互层意味着，虽然两个主要地幔源仍在被挖掘，但到那时岩浆混合已不再发生。在一个小缝隙之上，最上面的流是富含氟的拉斑玄武岩，具有弧亲和性，表明区域次大陆岩石圈地幔源部分熔融，以前在早到中古生代在加拿大东部下方通过近乎连续的俯冲而富集。后一种情况似乎对应于阿斯比安时代最上层岩浆系统的局部减弱，这可能是因为被覆盖的羽流向西迁移到新不伦瑞克，在布里甘特时代，来自亚岩石圈源的碱性火山活动恢复了。