ANTITHETICAL MIGRATING PLUTONS IN EVOLVING ARCS

Continental arcs migrate (1-10 ky/myr), flare up, locally focus inwards into large intrusive complexes (1-5 ky/myr) and undergo extensive tectonism. Within such arcs, over 90% of the plutons and intrusive complexes show expected growth patterns. However, Gastil et al. (1991) noted that some intrusive complexes form asymmetrical growth patterns including migrating plutons. In these complexes, magmatism migrates enough that younger magmatic pulses only partially intrude into older warm(?) pulses and partially intrude into colder host rock. An examination of migrating plutons, including the Jack Main Canyon Intrusive Suite, Sierra Nevada, the Tarana Complex, Australia, and the Adamello Complex, Italy, indicates that final sizes (200 to 600 km²), migration durations (3-8 m.y.), and distances (14-50 km) give migration rates of ca 4-7 km/myr (rates comparable to arc migration and focusing). Migration directions are antithetical to arc migration and, when present, inward focusing directions. Preliminary geochemistry syntheses indicate that these migrating systems are not compositionally unique, consist of multiple, geochemically linked and possibly cyclic pulses, which is consistent with an origin from an underlying, evolving magma reservoir. Contacts of migrating pulses are highly discordant to both host rock structures and to contacts with previous pulses. Migration paths ignore host rock structure, and faults parallel to the migration paths are not preserved at exposed levels, implying that regional deformation is not the main cause of migration. The above characteristics suggest that migrating plutons may reflect a transition from deeper evolving plutons to shallow migrating ring complexes and ultimately cause the migration volcanic centers. The causes of migration and mechanics of emplacement remain puzzling: we speculate that they are partly a response of initial failure to establish hot magma pathways that ultimately (sometimes even in the final stages of migrating systems) lead to nesting of plutons. Migration may also reflect the crystallization pattern in underlying plutons and/or the attempt of rising magma to ascent around particularly resistant objects.