Thermochronology and cooling histories of plutons: implications for incremental pluton assembly

Zircon U-Pb geochronology results indicate that the John Muir Intrusive Suite of the central Sierra Nevada batholith, California, was assembled over a period of at least 9 m.y. between 96 and 87 Ma, and the nearby Mount Whitney Intrusive Suite was assembled over at least 7 m.y. between 90 and 83 Ma. Bulk mineral thermochronology (U-Pb titanite, 40Ar/39Ar hornblende and biotite) of rocks from both suites indicate rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure. Thermochronologic data are consistent with thermal cycling between hornblende and biotite closure temperatures for millions of years following intrusion. Assembly of intrusive suites over millions of years favors growth by incremental intrusion. Estimated long-term pluton assembly rates for the John Muir and Mount Whitney intrusive suites are on the order of 0.002 km3dotyr-1 which is inconsistent with the rapid magma fluxes that are necessary to form large-volume magma chambers capable of producing caldera-forming eruptions. If large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites, it is doubtful that the intrusive suites represent cumulates left behind following caldera-forming eruptions. K-feldspar multi-diffusion domain (MDD) thermal modeling for samples from the John Muir Intrusive Suite suggests that the central Sierra Nevada batholith underwent a period of accelerated cooling in the Late Cretaceous. In combination with previously published low-temperature thermochronologic data, the new data are consistent with a rapid cooling event that commenced around 76 Ma in the Sierra Nevada and Peninsular Ranges batholiths. Rapid cooling is apparently coincident with high erosion and sedimentation rates documented throughout this portion of the Cordilleran orogen. These observations, and the possibility that the Sierra Nevada range reached high elevations during the Late Cretaceous, indicate that the western edge of North America was tectonically active after the cessation of arc magmatism