Differentiation of the continental crust by relamination

a r t i c l e i n f o Crust extracted from the mantle in arcs is refined into continental crust in subduction zones. During sediment subduction, subduction erosion, arc subduction, and continent subduction, mafic rocks become eclogite and may sink into the mantle, whereas more silica-rich rocks are transformed into felsic gneisses that are less dense than peridotite but more dense than the upper crust. These more felsic rocks rise buoyantly, undergo decompression melting and melt extraction, and are relaminated to the base of the crust. As a result of this process, such felsic rocks could form much of the lower crust. The lower crust need not be mafic and the bulk continental crust may be more silica rich than generally considered. The origin and composition of continental crust—particularly the lower crust—remain enigmatic. The principal conundrum to be resolved is how an andesitic to dacitic continental crust has formed when most mantle-derived magmas are basaltic. This differentiation has been explained as the result of lower crustal foundering (Arndt and Goldstein, 1989; Kay and Kay, 1991), crustal formation from primary mantle-derived andesitic magmas (Kelemen, 1995), or the mixing of basaltic rock with silicic magma derived by partial melting of mafic, subducting crust (Martin, 1986). In their seminal papers, Herzberg et al. (1983) and Ringwood and Green (1966) introduced the idea that igneous processes can lead to crustal differentiation by lower crustal foundering if mantle-derived basaltic magma intruded into the crust forms a buoyant differentiate that is retained in the crust, plus a dense, olivine-and pyroxene-rich residue that sinks into the mantle (Fig. 1A). Kay and Kay (1991) expanded this idea by postulating that differentiation and lower crustal foundering might also be driven by the formation of metamorphic garnet in mafic rock (Fig. 1B). Jull and Kelemen (2001) quantified aspects of lower crustal foundering, and noted that buoyancy and viscosity requirements restrict significant lower crustal foundering to relatively warm environments such as rifts or active volcanoplutonic arcs. Though foundering of dense garnet granulites and pyroxenites probably is recorded in some arc sections (DeBari and Sleep, 1991; Kelemen et al., 2003a), loss of these rocks still yielded a mafic arc crust very different from continental crust (DeBari and Sleep, 1991; Greene et al., 2006). This is so because garnet-free mafic rocks that are either density-stable or too viscous to founder remain in the cold, upper and middle crust. Whereas foundering …