THE RADIOGENIC ISOTOPE RECORD OF CENOZOIC CONTINENTAL ARC MAGMATISM ACROSS EASTERN CALIFORNIA AND NORTHERN NEVADA: MANTLE VS. CRUSTAL SOURCES AND ISOTOPIC BOUNDARIES

Northern Nevada and northeastern California include a phenomenal record of continental arc magmatism from ca. 40 Ma to 3 Ma that progressed southwesterly across Nevada into eastern California, presumably due to rollback of the subducting Farallon Plate. Since 1996, we have collected over 800 samples of igneous rocks across northern Nevada to characterize mantle and crustal sources of these arc rocks and to test the influence of basement age and composition on magmatism. Here, we report radiogenic isotopic data (Sr, Nd, Pb, Hf) from this dataset. Arc magmatism was primarily mafic in the younger western arc but was more felsic in the older, eastern arc that includes the caldera complexes of the Ignimbrite Flare-up. Compared to the modern south Cascades Arc, all mafic igneous rocks of the Eocene-Pliocene arc required a more enriched mantle source, which we propose was the lithospheric mantle that was refertilized during low-angle subduction prior to the onset of arc activity. The 0.706 and 0.708 Sr isotope lines pass through central Nevada and separate Precambrian lithosphere to the east (Laurentia) from accreted Proterozoic lithosphere to the west, with an intervening Transition Zone. Mafic igneous rocks west of the 0.706 line generally have 87Sr/86Sr < 0.7055. Within the Transition Zone, mafic rocks have highly variable initial isotopic compositions (e.g., 87Sr/86Sr = 0.7055 to 0.708) suggesting that both Phanerozoic and Precambrian mantle lithosphere exists at the base of the Transition Zone. Mafic igneous rocks east of the 0.708 line rarely have 87Sr/86Sr < 0.708. Thus, mantle sources vary in isotopic composition from west to east depending on basement age but don’t always follow the existing isotopic lines. Felsic igneous rocks west of the Transition Zone commonly have isotopic compositions and Th/La that overlap with coeval mafic rocks, possibly due to similarities in mantle and crustal isotopic compositions, but felsic rocks emplaced within the Transition Zone and especially on Precambrian basement show a distinct Pb isotopic signature and higher Th/La ratios that are likely derived from Precambrian crust. We conclude that the use of isotopic data to construct “lines”, i.e., boundaries between lithospheric domains of differing age, must consider the composition of the igneous rocks used in their construction.