Evolution of Phanerozoic eastern Australian lithosphere: Isotopic evidence for magmatic and tectonic underplating

Mafic granulite xenoliths occur in basaltic host rocks of Recent to Jurassic age throughout eastern Australia. The granulites were derived mainly from depths of 15-35 km, with some from up to 75 km depth. The crust-mantle boundary is at about 30 km beneath eastern Australia, so that some mafic granulite xenoliths were entrained within the upper mantle. The protoliths of the mafic granulites were derived from basaltic magmas but Sr and Nd isotopic data suggest at least three different petrogenetic histories for groups of xenoliths from various regions. The first group formed by the simple one-stage process of intrusion of basaltic magmas, mainly around the crust-mantle boundary and probably closely related to Jurassic-Tertiary volcanism. This group shows no obvious geochemical interaction with crustal wall-rocks. The second group is also a result of basaltic magmatic underplating, but shows evidence of mixing between basalts, with depleted isotopic signatures, and a Crustal or enriched-mantle component with⁸⁷Sr/⁸⁶Sr > 0·705. The third group shows a Sr-Nd isotopic mixing relationship suggesting a more complex petrogenesis involving a component of crustal material subducted into the upper mantle.