VOLCANIC-HYPABYSSAL-PLUTONIC CONNECTIONS FROM SYSTEM TO ARC-SEGMENT SCALE IN THE CENTRAL SIERRA NEVADA ARC, CALIFORNIA

Intrusion and exhumation of upper-crustal arc plutons present challenges in reconstructing the volcanic record of the Mesozoic Sierra Nevada arc system and thus connecting plutons to volcanic deposits. However, the tectonic juxtaposition of hypabyssal and underlying plutons with coeval volcanic units supports a broader, time-integrated view into the processes operating in shallow magmatic plumbing systems, and the superposition of magmatic and sub-solidus processes.

Hypabyssal, commonly porphyritic, intrusions capture the transitional zone between subvolcanic processes and possible eruption, seen in samples with volcanic clasts, glomerocrysts, stoped blocks, fractured crystals, and flow banding. Plagioclase phenocrysts preserve core-rim zoning, and crystallized from rhyolitic melts that are, in some cases, more felsic than the samples they are currently found in, consistent with melt loss. Hypabyssal and volcanic samples are regionally metamorphosed and are often hydrothermally altered to varying degrees, seen in the replacement of amphibole > biotite > chlorite and local-to-pervasive plagioclase sericitization, where timing suggests the hydrothermal system was synchronous with, or shortly postdates, emplacement. In addition, there is an increased abundance of mobile alkali elements and evidence for alkali exchange. Alteration processes are particularly striking at the Iron Mountain pendant, which displays widespread epidote-chlorite-tourmaline alteration and veining at the map scale.

Across the central Sierra Nevada arc segment, integrating data from multiple systems, there is substantial overlap between volcanic-hypabyssal-plutonic bulk rock compositions, although volcanic rocks are weakly bimodal (45-55 wt% SiO₂ and 65-78 wt% SiO₂). Each crustal level tracks flare-up, migration, and magma focusing patterns during Cretaceous arc maturation. Plutonic rocks display higher La/Yb, Dy/Yb, and Sr/Y ratios that suggest a greater role for crystal accumulation, while volcanic rocks contain higher Rb/Sr and Zr content with increasing SiO₂, indicative of eruption of fractionated melts. To probe magmatic processes within shallow plumbing systems in exhumed arc sections, the study of hypabyssal intrusions is key to closing gaps in the record, and alteration must be accounted for.