The Bear Valley Intrusive Suite (BVIS), located in the southernmost Sierra Nevada Batholith (SNB; California, USA) exposes a transcrustal magma system consisting of lower-crustal gabbros and volumetrically extensive middle- and upper-crustal tonalites. New chemical abrasion–isotope dilution–thermal ionization mass spectrometry U-Pb geochronology shows that the bulk of this ca. 100 Ma magmatic system crystallized in 1.39 ± 0.06 m.y. and was constructed with ultrahigh magmatic fluxes (∼250 km3/km/m.y.). This magmatic flux is roughly a factor of three greater than estimates for the SNB-wide flux during the Late Cretaceous flare-up, showing that individual magmatic systems can be constructed at extremely rapid rates. Further, the Hf isotopic composition of the BVIS (εHfi ∼–2 to +4) only allows for limited (∼25%) crustal assimilation. Our results show that the high magmatic fluxes recorded in the BVIS were dominantly derived from the mantle, and that “flare-up”–like local magmatic fluxes can be produced without extraordinary assimilation of crustal material.