LATE CRETACEOUS ARC FLARE-UP IN NORTHWESTERN NEVADA: ELEMENTAL, ZIRCON HF ISOTOPE, AND U-PB ZIRCON GEOCHONOLOGY OF THE SANTA ROSA RANGE AND BLOODY RUN HILLS GRANITOIDS
New U-Pb zircon geochronology via LA-ICP-MS yields two primary age groups within the SRR: the Santa Rosa/Andorno group (SRA) (Santa Rosa stock, 101.5 ± 1.6Ma; Andorno stock, 101.0 ± 1.0Ma) and the Granite Peak/Sawtooth group (GPS) (Granite Peak stock, 94.2 ± 1.4Ma; Sawtooth stock, 93.1 ± 1.2Ma). Within the BRH, however, U-Pb geochronology yields both an older pulse (Bloody Run Hill stock, 105.5 ± 1.3) and a pulse intermediate to the two primary pulses observed within the SRR (Flynn stock, 96.7 ± 1.5).
Despite within-sample variability, in-situ zircon Hf isotopic values via LA-ICP-MS show a consistent decrease in εHf(t) with time, mirroring the trend of whole-rock εNd values. Whereas zircon εHf(t) values for the older SRA pulse range from +8 to +1, the younger GPS pulse yields zircon εHf(t) values ranging from +3 to -5. Proterozoic inherited cores yield εHf values ranging from -14 to -25 at the time of SRA and GPS emplacement, suggesting that an ancient crustal component was involved in the evolution of these magmas.
Both pulses show high Sr (100-600ppm), high Sr/Y (30-90) and La/Yb ratios (15-65), low Y (8-15ppm), small to no Eu anomalies, and slightly concave-up REE patterns, consistent with signatures produced via melting of deep subarc crust in equilibrium with a feldspar-poor, garnet and amphibole-rich residue. Increasing whole-rock Sr(i), a correlated decrease in whole-rock εNd with zircon εHf, and an increase in inherited zircon cores within the younger GPS pulse, suggest a decreasing role for mantle derived melts and increasing involvement of crustal reservoirs through time. These same trends have been observed along much of the Cordilleran during high-magmatic flux periods (e.g. 120-80Ma), suggesting that magmatism in NW Nevada shared similar magmatic histories to the larger batholiths and was generated during the late Cretaceous arc flare-up (ca. 120-80 Ma).