Origin and evolution of continental crust: Insights from igneous and metamorphic processes in subduction zones
Erdman, Monica Elizabeth
Lee, Cin-Ty A.
Doctor of Philosophy
Today, continental crust is primarily produced in subduction zone settings, where slab dehydration induces melting of the peridotitic mantle wedge. Mantle melting produces Mg-rich basalt (~50 wt. % SiO2, ~8 wt. % MgO). The average composition of the continents is far too Si-rich and Mg-poor (55–65 wt. % SiO2, 2–5 wt. % MgO) to have been derived from peridotite melting directly. Continental crust production therefore requires an additional differentiation stage to generate andesite from a basaltic source. This thesis investigates two processes by which density sorting can evolve more felsic compositions in the upper plate of subduction zones: delamination and relamination. Garnet pyroxenite xenoliths from central Arizona indicate significant fractionation occurs in the lower crust of continental arcs. Crystallization of mafic minerals from primary basalt drives the derivative liquids toward felsic compositions, resulting in the accumulation of dense, mafic cumulate minerals in the lower crust. On average 8% denser than the underlying mantle, this layer is expected to delaminate from the base of the crust and recycle back into the mantle. Preferential removal of mafic material will drive continental crust toward more felsic compositions, providing a simple explanation to the continental crust paradox. Comparisons with seismic profiles indicate that lower crustal delamination has occurred in the Basin and Range Province and along the edges of the Colorado Plateau of western North America. A second explanation for the andesitic composition of the continents arises when considering density sorting of subducted material. During subduction, material of all compositions enters the trench, however buoyancy controls what material is subducted and what returns to the surface. A compilation of high pressure–low temperature metamorphic terranes reveals that material exhumes in subduction zones in a predictable way. Oceanic-type terranes exhume from pressures less than 2.8 GPa, to be exposed near the subduction trench, whereas continental-type terranes return from greater pressures, and are exposed far inland of the trench. These observations suggest oceanic-type terranes exhume via channel flow within an inclined subduction channel and continental-type terranes exhume vertically through the mantle wedge via diapirism. Modeling of buoyancy-driven flow helps to predict where the transition from channel flow to diapirism occurs along the slab-mantle interface. Additionally, heat diffusion calculations predict that diapirs will undergo partial melting, suggesting they may buoyantly rise to relaminate to the base of the crust. Preferential sequestration of buoyant, felsic materials in the upper plate via relamination may drive continental crust toward more felsic compositions.