FLARE-UP WITHOUT A CAUSE: THE BEAR VALLEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

The Bear Valley Intrusive Suite (BVIS), located in the southernmost Sierra Nevada Batholith, California, exposes a trans-crustal magmatic system comprising lower crustal gabbros and volumetrically extensive middle and upper crustal tonalites. Emplacement pressures within the BVIS vary continuously from >8 kbars to ~3 kbars, and the total exposed area of the BVIS is ~1900 km². We present here combined CA-ID-TIMS U-Pb geochronology and zircon Hf isotopic compositions for samples representing a wide range of lithologies and crustal depths within the BVIS. Our new data show that this entire magmatic system was emplaced and crystallized within 1.1 million years, and that the BVIS records the highest magmatic fluxes documented in the Sierra Nevada Batholith. However, these remarkable magmatic fluxes are not accompanied by any of the characteristic signatures of arc flare-ups: the BVIS was emplaced in the western (trenchward) portion of the Sierran arc, and the Hf isotopic composition of the BVIS is consistent with only limited incorporation of upper crustal material and is comparable to the Hf isotopic composition of other Sierra Nevada plutons emplaced during periods of lower magmatic flux. These observations indicate that the high magmatic fluxes observed in the BVIS are dominantly generated in the mantle, and support the conclusion that arc flare-ups need not be driven by upper-plate processes. Based on these observations, we explore the possibility that arc flare-ups should not be considered anomalous but instead represent typical levels of arc magmatic activity.