ISOTOPIC EVIDENCE FOR CHANNEL FLOW DRIVEN LOWER CRUSTAL REFERTILIZATION BENEATH THE RUBY MOUNTAIN &NDASH; EAST HUMBOLDT METAMORPHIC CORE COMPLEX
Zircon from magmatic pegmatitic gneiss reveals two periods of crystallization at 70.7 ± 0.5 Ma and 60.6 ± 0.2 Ma. The older gneiss has an average εHf value of -30 (± 8 ε units). Heavy rare earth element (HREE) ratios range between (LuCN/GdCN) ~15 and ~33. Pegmatitic gneiss dated at 60.6 Ma has an average εHf value of -46.3 ± 2.4 (± 8 ε units). Heavy REE abundances are an order of magnitude higher than the older unit, with HREE ratios from (LuCN/GdCN) ~54 to ~83. Biotite monzogranite from the same field area gives a crystallization age of 33.7 ± 0.3. Hf analyses gave a mean εHf value of -40.1 ± 3.2 (± 6 ε unit). U-Pb analysis of zircon cores from very coarse-grained gneiss give ages of ~71 Ma, whereas rims yield a crystallization age of 34.2 ± 2.7 Ma. Hf analysis of the 34.2 Ma zircon rims give average εHf values of -27.1 ± 2.2 (± 6 ε unit). The U-Pb and Hf values overlap with those of the Late Cretaceous pegmatitic gneiss but show a more restricted range. Heavy REE ratios (LuCN/GdCN = ~39 to ~88) are similar to those from Late Cretaceous pegmatitic gneiss. It is proposed that partial melting of Late Cretaceous pegmatitic gneiss was the result of an increased thermal input, possibly from intruding magmas, during extensional deformation and core complex formation to produce migmatites.
These new data, when combined with εHf values of -14 in zircon from a 91-92 Ma equi-granular granitic gneiss suggest that Cretaceous to Eocene peraluminous granites in the Ruby Mountains have distinctly crustal, but unique, sources as well as temporally short-lived emplacement histories. The data support a model that requires refertilization of the lower crust, which is compatible with channel flow beneath an elevated plateau.