Plutonic xenoliths from Raoul Volcano, Kermadec Arc, south-west Pacific: a window on sub-arc processes

Raoul Island, in the Kermadec island group, south west Pacific, is the summit of a large arc-type volcano located in one of the simplest oceanic subduction settings on Earth (Smith and Price, 2006). Intra-oceanic volcanic arcs develop in tectonic, structural and chemical contexts that are simpler than is the case for their continental counterparts. However, because of the oceanic setting, sampling is commonly restricted to the subaerial summits of the volcanic edifices. Consequently, the temporal perspective is severely limited and the plutonic record is commonly inaccessible. Well rounded boulders of gabbro, diorite, quartz diorite and tonalite up to several meters in diameter are found along the northern coast of Raoul Island. Gabbroic boulders are orthocumulates composed of plagioclase, augite and orthopyroxene partly replaced by actinolite, magnetite and ilmenite. They have geochemical and petrological features indicating an affinity with young basaltic lavas erupted from Raoul Volcano but their chondrite normalized REE patterns are characterized by strong positive Eu anomalies indicative of plagioclase accumulation or crystallization under more reducing conditions. The alteration mineral assemblage, together with subtle mineralogical and geochemical differences compared with Raoul lavas, suggests that these rocks are representative of an earlier stage in the evolution of the volcano. Tonalitic boulders found with the gabbros are orthocumulates composed of plagioclase, quartz, magnesiohornblende, magnetite, ilmenite and titanite. Actinolite and epidote are alteration minerals. The tonalities have a wide range of compositions and in comparison with volcanic rocks and the gabbroic xenoliths suite, they show subtle mineralogical and compositional differences indicating that they represent a separate and different phase in the evolution of the volcano. The Raoul xenolith suite provides insights into the nature of magmatic processes by which intra-oceanic volcanic rocks are generated and evolve and they are also constitute a unique sample of the crustal section formed over time beneath the arc. Reference Smith IEM and Price RC (2006) The Tonga-Kermadec and Le Havre-Lau back arc system: their role in the development of tectonic and magmatic models for the Western Pacific. Journal of Volcanology and Geothermal Research 156, 315-331.