The age and origin of a thick ma®c±ultrama®c keel from beneath the Sierra Nevada batholith

We present evidence for a thick ( 100 km) sequence of cogenetic rocks which make up the root of the Sierra Nevada batholith of California. The Sierran magmatism produced tonalitic and granodioritic magmas which reside in the Sierra Nevada upperto midcrust, as well as deep eclogite facies crust/upper mantle ma®c±ultrama®c cumulates. Samples of the ma®c±ultrama®c sequence are preserved as xenoliths in Miocene volcanic rocks which erupted through the central part of the batholith. We have performed Rb-Sr and Sm-Nd mineral geochronologic analyses on seven fresh, cumulate textured, olivine-free ma®c±ultrama®c xenoliths with large grainsize, one garnet peridotite, and one high pressure metasedimentary rock. The garnet peridotite, which equilibrated at 130 km beneath the batholith, yields a Miocene (10 Ma) Nd age, indicating that in this sample, the Nd isotopes were maintained in equilibrium up to the time of entrainment. All other samples equilibrated between 35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 80 and 120 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The Rb-Sr ages are generally consistent with the Sm-Nd ages, but are more scattered. The Sr/Sr and Nd/Nd intercepts of the igneoustextured xenoliths are similar to the ratios published for rocks outcroping in the central Sierra Nevada. We interpret the ma®c/ultrama®c xenoliths to be magmatically related to the upperand mid-crustal granitoids as cumulates and/or restites. This more complete view of the vertical dimension in a batholith indicates that there is a large mass of ma®c±ultrama®c rocks at depth which complement the granitic batholiths, as predicted by mass balance calculations and experimental studies. The Sierran magmatism was a large scale process responsible for segregating a column of 30 km thick granitoids from at least 70 km of mainly olivine free ma®c±ultrama®c residues/cumulates. These rocks have resided under the batholith as granulite and eclogite facies rocks for at least 70 million years. The presence of this thick ma®c±ultrama®c keel also calls into question the existence of a `` ̄at'' (i.e., shallowly subducted) slab at Central California latitudes during Late Cretaceous± Early Cenozoic, in contrast to the southernmost Sierra Nevada and Mojave regions.