Nd, Sr, and Pb isotopic characteristics of Cretaceous intrusive rocks from deep levels of the Sierra Nevada batholith, Tehachapi Mountains, California
The Cretaceous gabbroic to granitic intrusive rocks of the Tehachapi Mountains were emplaced at depths of 25–30 km and thus afford a view of deep processes in the Sierra Nevada batholith. They consist of the ∼115 Ma Tehachapi suite and the ∼100 Ma Bear Valley suite; new zircon U-Pb age data reveal the presence of the latter as far west as Grapevine Canyon. The Nd, Sr, Pb, and O isotopic whole-rock data and zircon Pb inheritance patterns for the bulk of the suites suggest an origin by mixing between depleted mantle-derived magmas and metasedimentary material with a substantial component of old continental material. However, this mixing is not evident in variations between isotopic ratios and chemical and lithologic parameters. This implies that isotopic hybridization of magmas took place deeper than 30 km, and that fractionation processes are likely responsible for the bulk of the chemical variation in this part of the Sierra Nevada batholith. Consideration of the isotopic data in the context of the Sierra Nevada batholith as a whole suggests that the well-known east-to-west isotopic gradients in the batholith may reflect a change in the average isotopic character of the preintrusive framework rather than a change in amount of crustal component. On the other hand, the lack of areal gradients in Sr and Nd isotopic ratios in the main study area may indicate a lack of pronounced gradation at deep levels, at least within the western batholith.
© 1994 Springer-Verlag. Received: 19 July 1993. Accepted: 16 March 1994. We thank G.J. Wasserburg, D.A. Papanastassiou, and H.H. Ngo for their support during establishment or our Nd-Sr capabilities. We benefitted from discussions with L.T. Silver and D. Clemens Knott (who also provided oxygen isotope data), comments on an early version or the paper by E.W. James and C.A. Manduca, and reviews by A.P. Barth and Anonymous. M.A. Fahnestock helped with the figures. Access to the field area was graciously provided by the Tejon Ranch Company. This research was supported by National Science Foundation grants EAR-8904063 and EAR-9105692. Division or Geological and Planetary Sciences contribution no. 5442.