TRACKING VOLCANIC-PLUTONIC LINKS VIA HYPABYSSAL PORPHYRY INTRUSIONS: EXPLORING INDICATORS OF ERUPTION AND FELDSPAR ACCUMULATION IN TEXTURES AND GEOCHEMISTRY

Porphyritic hypabyssal intrusions in the central Sierra Nevada (CSN) record a snapshot of physical and chemical processes between extensive upper-crustal plutonism and poorly-exposed volcanic deposits. CSN hypabyssal intrusions, or porphyries, are between 10 m – 10 km in diameter and form various 3D shapes, including pipes, sheets, and laccoliths. Thus, they represent a unique way to examine volcanic-plutonic relationships in tectonically reorganized sections. We analyzed porphyries and adjacent broadly contemporaneous plutonic and volcanic rocks using field, petrographic, and geochemical tools to explore evidence for differentiation and possible eruption at the intersection of volcanic and plutonic fields.

Outcrop scale structures and textures within porphyries are consistent with shallow, subvolcanic emplacement and possible eruption. Observations include volcanic clasts, stoped blocks, fractured phenocrysts, crystal clots, and flow banding. Porphyries in the CSN are predominantly dacite and rhyolite composition, less variable than coeval plutons that experienced varying amounts of crystal accumulation (e.g., Barnes et al. 2020). Plagioclase phenocrysts were analyzed in porphyry, volcanic, and plutonic units at Tioga Pass and Beartrap Lake to provide greater resolution of the compositional variation at each level. Multiple populations of feldspar were found at all structural levels, permissive of open-system interaction. Additionally, common features across levels include: (1) normal core-rim zoning in An content and trace elements (e.g., Sr, Ce, Ti); (2) reverse zoning of high An mantles that could indicate magma recharge; and (3) crystallization over a broad range in temperature (965 – 622 °C; Caricchi and Blundy, 2015).

Plagioclase equilibrium melts (Scruggs and Putirka, 2018) are more felsic than plutonic and porphyry bulk rock samples, indicative of feldspar accumulation and melt loss in the intrusive units. In contrast, a small subset of volcanic feldspars from CSN deposits show minimal variance in temperature and Sr contents and overlap with equilibrium melt compositions. The Tioga Pass and Beartrap Lake porphyries show evidence for both melt loss and possible eruption, suggesting that they represent magma feeder systems rather than failed eruptions.