CRETACEOUS GRANITIC MAGMATISM IN NORTHWESTERN NEVADA: PETROGENETIC INSIGHTS INTO REGIONAL CORDILLERAN ARC FLARE-UP

The Santa Rosa granitoids of northwestern Nevada are a newly recognized component of regional Late Cretaceous Cordilleran arc flare-up. Within the Santa Rosa Range, two distinct granitoid groups exist within close spatial association: the Santa Rosa/Andorno (SRA) and Granite Peak/Sawtooth (GPS) groups. The SRA granitoids (~105 - 96 Ma) are generally more mafic and metaluminous and the GPS granitoids (~95 - 92 Ma) are more felsic and peraluminous, indicating a compositional shift to more felsic magma production with time. The older SRA group has higher whole-rock εNd_(t) and zircon εHf_(t), lower initial ⁸⁷Sr/ ⁸⁶Sr ratios, and less radiogenic feldspar Pb isotope ratios than the younger GPS group. Both groups also have low bulk zircon (5-7 ‰) and sphene (4.5-6 ‰) δ¹⁸O values. Overall, the SRA group has isotope characteristics typical of granitoids emplaced west of the Sr_(i) = 0.706 isopleth that have a dominant mantle component, whereas the GPS group has isotope ratios more consistent with granitoids east of Sr_(i) = 0.706 isopleth that exhibit greater influence from crustal or aged subcontinental mantle reservoirs. Despite between group isotopic variations, both granitoid groups share a common set of elemental characteristics, namely high Sr and Ba, moderate to high Sr/Y and La/Yb ratios, low Rb/Sr ratios, small to no Eu anomalies, and slightly concave-up REE patterns. The consistency of these elemental signatures through time suggests that both granitoid groups shared a similar, but compositionally heterogeneous source region. Thus, the Santa Rosa granitoids are interpreted to have been generated within a lower crustal (~35-45 km) MASH zone with primary contributions from rising mantle wedge melts and dehydration-partial melts of sub-arc crustal sources in equilibrium with feldspar-poor, and garnet-, clinopyroxene-, and amphibole-rich residues. The close spatial association (<5 km) of these two granitoid groups indicates that this region contains compositional complexities that may reflect the transitional nature of the underlying lithosphere. The Santa Rosa intrusions share similar geochemical and isotopic traits with other Cordilleran intrusive suites, indicating certain commonalities of source contributions and petrogenetic processes along strike of the Late Cretaceous magmatic arc.