UC Santa Cruz

The continental crust is a chemically distinct geochemical reservoir of the bulk silicate Earth that has formed near continuously over the last 4 billion years and is preserved by its intrinsic buoyancy relative to its oceanic counterpart. Rare earth element patterns imply that continental crust is formed at subduction zone settings and yet the primary melts produced by flux melting of the mantle here are basaltic in composition. An additional process is necessary to differentiate these melts to the Si-rich character of continental crust. The models proposed for differentiation includes fractional crystallization and partial melt extraction, which differ greatly in the resulting lower crustal compositions and the timing of formation relative to Si-rich melts. Here we propose that the investigation of the lower crust composition and timing provides one way to distinguish between these two mechanisms. A study was undertaken to characterize lower crustal xenoliths that are inferred to be related to granitoid plutons from the Southern Sierra Nevadas; the Dinkey Creek and Red Lake granodiorites specifically. Thirteen lower to mid-crustal xenoliths were analyzed for major element chemistry and U-Pb geochronology. We suggest that the broad range in chemistry seen in these lower crustal xenoliths can only be derived by the multiple steps and continuous evolution of fractional crystallization. The ages of these xenoliths record magmatism contemporaneous with the upper crustal granitoid plutons. The mafic lower crustal rocks yield zircon U-Pb dates that are younger than their upper felsic counterparts. This is interpreted to reflect a difference in temperature and crystallization sequence anticipated for mafic and felsic bodies. A summary is developed for the genesis of these plutons and their corresponding mafic cumulate, leading to the generation of crust matching current bulk continental crust estimates.