Batholith formation and crustal growth

Intra-oceanic island arcs are often invoked as the source rocks for continental crust generation. Many have developed ideas related to this process (Patchett, 1992, for a review). Another proposal for magmatism from the mantle that initiates crustal growth is mantle plume activity (Stein and Goldstein, 1996). However, melting processes associated with these environments generate mafic rocks, not the intermediate and felsic compositions of the continental crust (Rudnick, 1995). If the mafic material ultimately involved in continental growth has been extracted at plume or island arc settings, a second, “distillation” stage, probably of tectonically accreted terranes in continental arcs, is required to refine the mafic material into a ~30 km thick crust with felsic-intermediate calc-alkaline composition (e.g. Fliedner et al., 2000 and Figure 5). Available data suggest that a large amount of pre-existing continental crust and lithospheric mantle is involved in the making of continental arcs. Continental magmatic arcs are thus, to a first order, important sites of crustal distillation and may form a critical step in the formation of stabilized continental crust.

A major unknown in deciphering crustal growth is the fate of the ultramafic residues (Chappell et al., 1987) that must complement granitoid magmatism. Several different mechanisms have been proposed to resolve the compositional paradox between basaltic magmatic flux in arcs and the andesitic bulk composition of continental crust (Rudnick, 1995), two of which are plausible in the specific case of Mesozoic-Early Cenozoic North American arcs. The first is that the complementary roots founder into the mantle while the arc is active, or after its demise, leaving behind a remnant felsic crust (Kay and Kay, 1991, 1993; Ducea and Saleeby, 1996; Lee et al., 2000; Jull and Kelemen, 2001). The negative buoyancy of the root presumably triggers this process (Conrad and Molnar, 1997), which is similar to the proposed convective removal of deep orogenic roots metamorphosed under eclogite facies (Dewey et al, 1993). The difference is that production of the high-density lower crustal arc root rocks is primarily a crystal-liquid fractionation process, which takes place in the magma source regime of regional composite batholiths (Saleeby et al., 2002), rather than purely a metamorphic process. The second hypothesis suggests that the mafic roots may be composed of ultramafic cumulates which reside below the Moho and are compositionally, but not seismologically, distinct from the surrounding peridotitic mantle (Griffin and O'Reilly, 1987). The composition of such a “hidden” root, when integrated with the overlying felsic crust would result in an average crustal composition of basalt. Discriminating between these end-member hypotheses, the main task of this project, would provide a major step forward in quantifying crustal evolution and differentiation processes in arc environments.