LINKING GRANULITE-FACIES METAMORPHISM, PARTIAL MELTING AND HIGH SR/Y MAGMATISM IN THE ROOT OF A CONTINENTAL ARC, CUCAMONGA TERRANE, CALIFORNIA
We report 40 new whole rock geochemical analyses and 6 new SHRIMP-RG zircon ages to document time scales of metamorphism and magmatism in the CT. Undeformed metamorphic zircons from garnet granulites display high U cores surrounded by lower U and Th/U (<0.1) rims. Both cores and rims show depletions in heavy-rare-earth elements suggesting coeval growth of garnet during prograde metamorphism. 206Pb/238U error-weighted average ages for zircon cores and rims are 86.2 ± 0.7 Ma and 83.8 ± 1.5 Ma, respectively. Metamorphic zircons from a calc-silicate yielded a similar error-weighted average age of 86.1 ± 2.5 Ma. Zircons from a mid-crustal tonalite yielded an error-weighted average age of 85.8 ± 0.6 Ma indicating coeval granulite-facies metamorphism and tonalite emplacement. Low U, metamorphic rims on tonalite zircons yielded an age of 75.5 ± 5.3 Ma, which overlaps with ages from two granitic dikes in the area (76.2 ± 0.5 and 74.0 ± 0.7 Ma).
We performed whole rock geochemical modeling to determine possible geochemical links between granulites and tonalitic magmatism. We use several granulite compositions as sources and model various partial melting scenarios. Our results indicate that high Sr/Y tonalite compositions are consistent with 20-30% partial melting of garnet granulites, which supports field observations and temporal links from zircon geochronology.