Three-million-year pulse: Pace and processes of Late Jurassic magmatism in the Sierra Nevada arc

Continental arcs commonly experience brief, high-flux magmatic pulses, and the Late Jurassic Sierra Nevada arc records one such event as a compositionally diverse magmatic system that includes highly silicic plutonic, hypabyssal, and volcanic rocks, associated cumulate gabbros, and a regionally extensive Independence dike swarm. This study presents new constraints on the timing, magma sources, differentiation, and genetic relationships of this diverse magmatic episode. High-precision chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) zircon geochronology, whole-rock Sr-Nd-Pb isotopic data, trace-element geochemistry, and zircon δ18O data were integrated to characterize the evolution of this magmatic system. Geochronological data constrain all magmatic activity to a narrow interval (152−149 Ma), indicating that the silicic rocks, cumulate gabbros, and Independence dike swarm are coeval features. Geochemical and isotopic data indicate that andesitic magma, best represented by the Independence dike swarm, served as the parental melt, from which high-silica, crystal-poor volcanic, hypabyssal, and plutonic rocks formed as interstitial melts, whereas the mafic cumulate gabbros represent the residuum. Geochemical patterns, including mantle-like zircon δ18O values, evolved Sr, Nd, and Pb isotopic signatures, and trace-element systematics, point to partial melting of the hydrated lower crust or subcontinental lithosphere to generate the parental andesitic magma, which then differentiated in shallow, deformation-influenced reservoirs to generate the bimodal suite of silicic rocks and mafic cumulates.