Subduction-related metasomatism in the lithospheric mantle beneath the Calatrava volcanic field (central Spain): constraints from lherzolite xenoliths of the Cerro Gordo volcano

Two types of lherzolite xenoliths appear in the Cerro Gordo maar from the Cenozoic Calatrava volcanic field. Hydrous lherzolites (group-1) show a major proportion of metasomatic amphibole (>5 modal%), whereas group-2 lherzolites have only accessory amounts or an absence of amphibole. Group-1 lherzolites show a higher LREE-Ba-Sr-Th content in whole-rock composition compared to group-2 lherzolites. The most depleted group-2 lherzolite xenoliths indicate a low degree of partial melting (<5%) of the mantle source, similarly to other Calatrava xenolith suites. Trace element content combined with Sr-Nd isotopic ratios suggests a protolith mantle source with the characteristics of a depleted MORB Mantle (DMM type). This mantle protolith was later refertilized by subduction-related metasomatism, causing the formation of modal amphibole and obliterating the original MORB-like chemical features in the most transformed xenoliths (group-1 lherzolites). The entrainment of these mantle fragments in the volcanic alkaline magma promoted amphibole breakdown reactions during its transport, generating silica-rich glasses with the associated unusual formation of secondary orthopyroxene. The suprasubduction character of the protolithic mantle combined with previously published Re-Os model ages in nearby lherzolite xenoliths suggests that the sampled lherzolites could represent the old Cadomian mantle wedge of the Central Iberian zone.

新生代卡拉特拉瓦火山岩场（Cenozoic Calatrava volcanic field）内的塞罗戈尔多玛珥火山（Cerro Gordo maar）产出两类二辉橄榄岩捕虏体（lherzolite xenoliths）。第1组为含水二辉橄榄岩（hydrous lherzolites），其以占比主体的交代成因角闪石为特征（角闪石含量＞5体积百分比（modal%））；第2组二辉橄榄岩则仅含微量角闪石或完全不含角闪石。相较于第2组二辉橄榄岩，第1组二辉橄榄岩的全岩成分（whole-rock composition）中轻稀土元素-钡-锶-钍（LREE-Ba-Sr-Th）含量更高。亏损程度最高的第2组二辉橄榄岩捕虏体指示其地幔源区的部分熔融程度较低（＜5%），这与卡拉特拉瓦火山岩场其他捕虏体组合的特征一致。微量元素含量（trace element content）与锶-钕同位素比值（Sr-Nd isotopic ratios）的联合分析表明，其原岩地幔源区具备亏损型洋中脊玄武岩地幔（depleted MORB Mantle, DMM型）的特征。该原岩地幔后续受到俯冲相关交代作用（subduction-related metasomatism）的再富集，形成了模态角闪石，并在改造程度最深的捕虏体（即第1组二辉橄榄岩）中抹去了原始的类洋中脊玄武岩地球化学特征。这些地幔岩屑被裹挟进入火山碱性岩浆（alkaline magma）中，在岩浆运移过程中触发了角闪石分解反应，生成了富硅玻璃（silica-rich glasses），并伴生了罕见的次生斜方辉石（secondary orthopyroxene）。原岩地幔的俯冲带上方属性（suprasubduction character）结合此前已发表的邻区二辉橄榄岩捕虏体铼-锇模式年龄（Re-Os model ages），表明本次采样的二辉橄榄岩可能代表伊比利亚中部地带（Central Iberian zone）古老的卡多米造山期地幔楔（Cadomian mantle wedge）。