Occurrence of inherited supra-subduction zone mantle in the oceanic lithosphere as inferred from mantle xenoliths from Dragon Seamount (southern Tore–Madeira Rise)

Spinel-bearing peridotite and pyroxenite xenoliths dredged from the Dragon Seamount (southern Tore–Madeira Rise, West Iberia
and Morocco margin) give an insight into the composition of the underlying lithosphere. These xenoliths are devoid of evidence
of strong host lava–peridotite interaction and re-equilibration or late impregnation in the plagioclase facies. The spinels
and pyroxenes from the Dragon peridotites have compositions distinct from those of both lherzolites and harzburgites from
the Iberia margin and the Mid-Atlantic Ridge. They display a highly depleted composition, in particular, high Cr-number, up
to 0.63 in the spinels, consistent with a melting degree between 12 and 19%. Because of the strong chemical similarities between
the Tore–Madeira Rise, Newfoundland peridotites, and peridotites from supra-subduction zones, we propose that the Dragon peridotites
formed in a similar context. The pyroxenites display a cumulate texture and are probably a high-temperature–high-pressure
cumulate formed by fractional crystallization from a melt. The Tore–Madeira Rise peridotites may represent a former mantle
wedge in an oceanic arc, later included into the continental lithosphere and finally tectonically disseminated within the
lithosphere during the rifting of the Newfoundland–Iberia continental lithosphere. As a consequence, rifting processes may
produce heterogeneities in the oceanic lithosphere and influence isotopic compositions of ocean island basalt-type lavas during
plume–lithosphere interactions, as inferred for the southern Tore–Madeira Rise.

承载石榴石辉石岩和辉长岩岩浆包体，经从龙牙海山（西伊比利亚托雷-马德拉脊南部及摩洛哥边缘）打捞而来，为我们揭示了下地壳的成分。这些岩浆包体缺乏与宿主岩浆-石榴石强烈相互作用及在斜长石相中的重新平衡或晚期充填的证据。来自龙牙海山的石榴石和辉石组成与伊比利亚边缘和中大西洋脊的橄榄岩和哈茨伯里岩明显不同。它们呈现出高度贫化的组成，特别是石榴石中的高铬含量，高达0.63，这与12%至19%的熔融程度相一致。鉴于托雷-马德拉脊、纽芬兰的橄榄岩以及上覆下俯冲带橄榄岩之间强烈的化学相似性，我们提出龙牙海山的橄榄岩形成于相似的环境。辉长岩呈现出堆积结构，可能是通过熔融物的分凝作用形成的高温高压堆积岩。托雷-马德拉脊的橄榄岩可能代表了一个曾经的海山弧地幔楔，后来被包含进大陆地壳中，最终在纽芬兰-伊比利亚大陆地壳的裂谷作用过程中在地壳中构造性地散布。因此，裂谷过程可能会产生海洋地壳的非均质性，并在柱状地幔与地壳相互作用期间影响洋岛玄武岩型岩浆的同位素组成，正如对南托雷-马德拉脊所推断的那样。