Refertilization of deep continental arc lithosphere: constraints from major element and trace element systematics in mantle xenoliths from the Sierra Nevada, California

Thickening of continental arc lithosphere, either by underplating, magmatic inflation, or other tectonic processes, is an important stage in the maturation of primitive island arcs into Phanerozoic continental crust. The Sierra Nevada continental arc is one example of a mature arc that has experienced significant thickening and recent orogenic collapse. Previous studies of mantle xenoliths from the Sierras point to delamination of a dense mafic root that represented the residua of the evolved crust distilled out of a >90 km thickened lithospheric column. What has not received much attention, however, is how this lithospheric column has been compositionally modified over the lifespan of the Sierra Nevada arc. As the interface between the crust and mantle, the base of the thickened lithospheric column (upper mantle and lowermost crust) is potentially a zone of refertilization, wherein previously depleted mantle is replenished with fertile components from ascending melts and/or fluids. Refertilization of deep lithosphere not only influences the composition of arc magmas in their early stages, but also has implications for the stability of highly melt-depleted, refractory mantle that may become “rejuvenated” by such refertilization. In this study, we evaluate the role of refertilization in continental arc evolution by investigating upper mantle xenoliths from the Sierra Nevada. Cr# in spinel cores is chosen as a baseline index of melt depletion, and is compared to whole-rock major- and trace-element concentrations (Yb, Ti, Mg, Al) to assess the degree of refertilization. Whole-rock trace element patterns indicate depletions in high field strength elements and complementary enrichments in fluid mobile elements, suggesting that the deep Sierran lithosphere was refertilized by a melt containing a large component of slab-derived fluid. Although high degrees of melting (>15%) are recorded by Cr# in spinel, many Sierran peridotites contain diffuse bands comprised of clinopyroxene, orthopyroxene, and garnet texturally superimposed on a matrix of equigranular olivine and orthopyroxene. This texture suggests that modal metasomatism of previously depleted harzburgites may have occurred. To evaluate the magnitude of such metasomatism, the amount of excess Al that is not present as exsolved garnet lamellae in orthopyroxene is determined. The rationale behind this approach is that “extraneous” Al in the form of matrix-hosted garnet (and clinopyroxene) may be metasomatically derived, whereas exsolved garnet in orthopyroxene originated in situ from an Al-rich precursor orthopyroxene. Our results have implications for the compositional evolution and modification of the deep lithosphere beneath mature continental arcs, which will be integrated within the broader context of the P - T evolution of deep arc lithosphere.