MESOZOIC SPATIOTEMPORAL EVOLUTION OF THE CENTRAL SIERRA NEVADA ARC LITHOSPHERE: INVESTIGATING MAGMATIC RECORDS OF DYNAMIC CONTINENTAL ARC ACTIVITY (Invited Presentation)

The Mesozoic Sierra Nevada arc spans >150 Myr and stitched together transitional oceanic to continental basements. Arc lithosphere evolution occurred during arc migration, multiple magmatic flare-ups, magma focusing, and evolving tectonism. Unraveling the mechanisms driving each process requires an understanding of the temporal and spatial patterns of arc behavior. We compiled age-controlled whole rock element and isotope geochemistry of plutonic, hypabyssal, and volcanic rocks between 250-85 Ma to map arc activity in 4D. Locally, slight differences in REE patterns and trace elements (e.g., Rb/Sr) indicates fractionation between magmas. Even so, at the arc scale, volcanic and plutonic compositions overlap considerably and show the same spatiotemporal patterns of arc behavior.

Each magmatic flare-up has a distinct ⁸⁷Sr/⁸⁶Sr_i, _ƐNd, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb signal, also seen in volcanic _ƐHf_zirc data. In a single flare-up, the largest-magnitude Sr, Nd and Pb isotopic changes occur from west to east. Further, W-E isotope patterns persist for the lifespan of the arc and are consistent with lateral variations in pre-existing arc framework crust and mantle [1, 2]. Cretaceous W-E trends are linked to eastward migration at ~2.7 km/Myr, which is not yet apparent for the Jurassic and Triassic arcs. Within defined arc-parallel slices, where spatial position is constant, smaller-scale isotope shifts through time are also observed. Temporal trends may represent changes in mantle input by volume and/or composition.

During the Cretaceous flare-up and eastward migration, magmatism localized by inward focusing, and crustal thickness rapidly increased. Decreasing Zr/Hf and Dy/Yb and narrowing in the range of Sr and Nd isotopes suggests that the Cretaceous arc produced voluminous, hydrous and fractionated magmas that were able to mix. This indicates that the Cretaceous arc reached a mature stage, not seen in the earlier arcs, involving the formation of melting, assimilation, storage and homogenization (MASH) zones at mid- to upper-crustal levels. Arc lithosphere was substantially modified, especially in the Cretaceous, when several processes primed the arc thermally and mechanically to enable transcrustal differentiation.

[1] Kistler (1990). GSA Mem. 174:271-282 [2] Lackey et al. (2008). J.Pet. 49(7):1397-1426