The origin and evolution of the deep lithosphere beneath continental arcs: constraints from lower crustal and mantle xenoliths from the Sierra Nevada, California

[摘要] Lower crustal and upper mantle xenoliths from the Sierra Nevada continental arc in California, USA reveal a complex P-T-X-t (pressure-temperature-composition-time) evolution of the deep lithosphere.Metasedimentary xenoliths, despite high quartz contents, preserve granulite-facies equilibration conditions, necessitating transport from the surface into the lower crust.U-Pb and Hf isotope data on detrital zircon in the metasediments support continental underthrusting coeval with the peak of arc magmatism.Mantle xenoliths, represented by spinel and garnet-bearing spinel peridotites, indicate that shallow (spinel-facies), high-degree melt residues underwent thickening, cooling, and metamorphism into refertilized garnet peridotites that equilibrated below 800 °C and ca. 3 GPa.The source of refertilizing melts appears to be the mantle wedge itself, rather than the subducting slab based on mass-balance models and oxygen isotopes.Textural disequilibria, particularly Al-depletion haloes between orthopyroxene and exsolved garnet in peridotites, are diagnostic features related to cooling and increasing pressure.Diffusion modeling indicate that such disequilibria formed within 6 to 8 Ma.Coupled Lu-Hf and Sm-Nd age data on garnet pyroxenites show a significant lag between closure of the two chronometers in a lower crustal pyroxenite, but negligible difference in the deepest websterites.As a whole, the Sierran xenolith data provide insights into the origin and evolution of a mature continental arc, particularly highlighting the importance of thickening in both the crust and mantle lithosphere.

[发布机构] Rice University

[效力级别] petrology