HF ISOTOPIC RECORD OF MAGMATIC SECULAR VARIATION IN THE CORDILLERAN CONTINENTAL ARC IN SOUTHERN CALIFORNIA

The volcanic and plutonic components of the Cordilleran magmatic arc in the western United States record both regional geochemical variation, reflecting the variable arc substrate and framework rocks, and secular geochemical variation, reflecting the dynamics of a long-lived, continental arc system. Isotopic variations over a range of time scales constrain secular change in geodynamic setting, thermal structure, and source inputs. We examined zircon Hf isotope variations in latest Permian to Late Cretaceous intrusive suites recording three temporal pulses of the Cordilleran continental margin magmatic arc. Host rock samples cover a spectrum of bulk compositions in each intrusive suite, including mafic diorite to quartz monzodiorite. Measured Hf(t) isotopic compositions span 55 epsilon units, ranging from +1 to -22 among magmatic zircons, with premagmatic cores extending measured values to as low as -54. Permo-Triassic zircons have less radiogenic average Hf compared to Jurassic and Cretaceous pulses. Within all three pulses Hf trends toward less radiogenic values with time, consistent with a shift to progressively greater crustal involvement during evolving pulses. The common presence, in a single sample, of both premagmatic zircon cores and variations in excess of 8 epsilon units among magmatic zircons indicate common operation of dynamic open-system magmatic processes operating over relatively short time scales in individual intrusive suites. The secular epsilon Hf arrays may be interpreted in terms of recurrent dynamic mixing of melts of a hybrid Paleozoic - Proterozoic mantle with Proterozoic crust containing recycled Archean components, represented by the Mojave crustal province. Recurring involvement of melts of a hybrid mantle is supported by the range of Hf isotopic compositions in magmatic zircons from mafic rocks. Mantle-derived melts were further hybridized in lower and middle crust storage zones by progressive assimilation of crustal melts.