Sub-volcanic xenoliths from Raoul Volcano: a window on petrogenetic processes in an intra-oceanic subduction system

Gabbroic and tonalitic xenoliths from c. 3 ka pyroclastic deposits on Raoul Island in the intra-oceanic Kermadec arc are samples of shallow crust formed 2 and incompatible trace element abundances are lower. The rare earth element (REE) patterns of the gabbros manifest strong positive Eu anomalies indicative of cumulate plagioclase. Tonalitic xenoliths comprise plagioclase, quartz, magnesiohornblende, magnetite, ilmenite and titanite. Their REE patterns feature strong negative Eu anomalies, which could indicate extensive plagioclase fractionation, but some have orthocumulate textures. In contrast to Raoul dacites, tonalitic xenoliths commonly contain amphibole. They also have higher total REE, and lower K, Ba and Rb abundances, and more evolved Sr isotope compositions. They represent a spectrum from highly fractionated felsic melts, through crystal mushes to felsic cumulates and were generated during an earlier phase in the evolution of the Raoul magmatic system.

采自洋内克马德克弧（intra-oceanic Kermadec arc）拉乌尔岛（Raoul Island）约3千年前（c. 3 ka）火山碎屑沉积（pyroclastic deposits）中的辉长岩质与英云闪长岩质捕虏体（gabbroic and tonalitic xenoliths），是形成于2（原文未标注具体单位）的浅地壳样品，且其不相容微量元素（incompatible trace element）丰度较低。该类辉长岩的稀土元素（Rare Earth Element，简称REE）配分模式呈现显著的正铕异常，指示堆积型斜长石（cumulate plagioclase）的存在。英云闪长岩质捕虏体由斜长石、石英、镁角闪石（magnesiohornblende）、磁铁矿、钛铁矿与榍石（titanite）组成。其稀土元素配分模式具有强烈的负铕异常，这可能指示大规模的斜长石分异作用（plagioclase fractionation），但部分样品具备正堆积结构（orthocumulate textures）。相较于拉乌尔岛英安岩（Raoul dacites），英云闪长岩质捕虏体通常含有角闪石。此类捕虏体的稀土元素总丰度更高，钾（K）、钡（Ba）与铷（Rb）丰度更低，且锶（Sr）同位素组成更为演化成熟。它们代表了从高度分异的长英质熔体、经由晶粥（crystal mush）过渡至长英质堆积岩（felsic cumulates）的连续演化序列，形成于拉乌尔岛岩浆系统（magmatic system）演化的早期阶段。