THE VOLCANIC-PLUTONIC CONNECTION FROM THE PERSPECTIVE OF THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA, CA (Invited Presentation)

Large, long-lived, intermediate to felsic intrusions in arcs, such as the 95-84.5 Ma Tuolumne intrusive complex (TIC, monzodiorites to granite), Sierra Nevada, CA, are often interpreted to feed volcanic clusters of similar composition, volume and longevity, such as the 11-0 Ma Aucanquilcha volcanic complex (AVC, basaltic andesite to dacite), Chile. The TIC (7-11 km emplacement depth) is characterized by an early lower volume, compositionally more mafic and diverse, but isotopically homogeneous episode that focused into a central locus after <2 myrs to produce a high-volume, isotopically more heterogeneous and more felsic maturation stage of several myrs. This peak of magmatism is marked by across-unit magma mixing and erosion that resulted in hybrid granodiorites and the recycling of antecrystic minerals (Memeti et al., session T23). These findings on timing, spatial focusing, waxing/waning volumes, recycled minerals, etc. are almost identical to those reported from the AVC. So, are the TIC and AVC equivalent systems preserved at different levels?

Mineral element studies in the TIC highlight one of the main differences: The majority of TIC rocks represent crystal cumulates that have lost up to 40% of dacitic to rhyolitic melt, while the AVC erupted melts containing antecrysts from a mush (Walker et al., 2013). Some of these “lost” felsic TIC melts likely fed shallow plutons or erupted. Other high-SiO₂ melts drained from the crystal-rich mush and pooled to become leucogranitic bodies. Leucogranites are found in all TIC units, but are most common and largest (<several km across) in the Cathedral Peak. The biggest body is the Johnson Peak Granite. Our calculations of melt compositions in equilibrium with hornblende and plagioclase largely overlap with the leucogranite whole rock data. Rhyolitic or rhyodacitic magmas are not reported from the AVC, which might imply that if they formed there, they solidified at depth or are yet to erupt.

We conclude that examples like the TIC and AVC represent different parts of transcrustal magmatic systems, and their magmatic evolution is mirrored in many physical and chemical traits. Still, these large volcanic and plutonic systems also have a fundamental complementary relationship with large volumes of crystal cumulates being left behind at depth to form extensive batholiths.