Mafic and ultramafic mantle and deep crustal xenoliths from banks peninsula, south Island, New Zealand

Abstract

Mafic and ultramafic xenoliths of spinel lherzolite, wehrlite, websterite, pyroxenite, and metagabbro (mafic granulite) occur in a basanite plug forming Le Bons Bay Peak on the eastern flanks of Akaroa volcano, Banks Peninsula. Spinel lherzolites are assigned to the Cr‐diopside xenolith suite and are interpreted to represent fragments of depleted upper mantle derived from a depth of c. 60 km. Pyroxenites belong to the wehrlite series Al‐augite xenolith suite and may represent basaltic cumulates derived from differentiation of mafic magma intruded at or near the crust/ mantle boundary. Mafic granulite of gabbroic composition represents a tectonised and partly recrystallised lower crustal basaltic cumulate. Mineral and bulk rock chemistry of the xenoliths suggests that chemical and mineralogical variation in Banks Peninsula magmas could reflect the development of a subcrustal basaltic “underplate”. This might explain the trend from extensively modified to relatively unmodified mantle‐derived magmas through time. The Le Bons Bay Peak xenolith suite may be explained in terms of rapidly rising basanite magma, with entrained spinel lherzolite xenoliths, which incorporated ultramafic cumulates and metagabbro while passing through the crust/mantle boundary region. © The Royal Society of New Zealand 1993.