Island arc magmatism in relation to the evolution of the crust and mantle

Abstract

One aspect of the evolution of the Earth's mantle is the development through geologic time of the continental crust. A popular hypothesis is that the Earth's crust has developed through the magmatic events associated with plate movements. A two-stage melting process involving generation of ocean-ridge basalts followed by subduction and partial remelting of the oceanic lithosphere to form andesitic suites has been invoked as the major, continental crust-forming magmatic cycle. However, there is chemical and isotopic evidence for the composition of continental crust and arcs that is in conflict with the broad consequences predicted by this hypothesis. The average composition of the upper continental crust has been fairly readily determined as granodioritic, but the composition of the lower crust is less certain. Seismological and petrological evidence suggest that a substantial portion of the lower crust is basaltic in character. Therefore, it cannot be concluded that the bulk, average crust corresponds to andesite. Again, the rock type most frequently collected in many arcs is andesite but the conclusion that the whole arc pile is of this composition is illogical, and not supported by much field and petrologic evidence. The peripheral accretion of arcs to continents is well documented for post-Palaeozoic epochs so that the nature of arc magma sources is critically relevant to the processes of continental crustal growth. A large body of data on arc magmas can be explained by the hypothesis that the superjacent upper mantle wedge, rather than the subducted lithosphere itself is the major source of arc-erupted melts. For many oceanic arcs, isotopic evidence ( 87Sr 86Sr- 143Nd 144Nd) rules out the possibility that they constitute appropriate bulk crustal additions for the development of continents. Continental crustal growth may therefore either have changed in essential chemical/isotopic character since the Cambrian, or arc magmas derived from chondritic-type, or LIL-enriched sources may be the only volumetrically significant additions to continental crust. Show more