Isotopic systematics of Pb/U (zircon) and 40Ar/39Ar (biotite-hornblende) from rocks of the central Foothills terrane, Sierra Nevada, California

Pb/U zircon and ⁴⁰Ar/³⁹Ar biotite-horn-blende studies in conjunction with detailed structural and textural studies clarify the age and structural relations of a unique cluster of plutons in the central Sierra Nevada western metamorphic belt. The plutons range from gabbro to tonalite/trondhjemite and include the Guadalupe Igneous Complex. Magmatic emplacement ages for the plutons are for the most part between ∼150 and ∼145 m.y., as recorded in both the zircon and hornblende isotopic systems. Additional ages of 138 and 123 m.y. were determined on small plutons having key structural settings. In general, the Pb/U and ⁴⁰Ar/³⁹Ar isotopic systems are well behaved. Second-order complications are locally present in the Pb/U systems, owing to the entrainment of older zircon from wall rocks, and in ⁴⁰Ar/³⁹Ar systems, owing to minor compositional differences in mica and amphibole phases.

The age data and structural relations of the plutons in conjunction with ⁴⁰Ar/³⁹Ar metamorphic wall-rock biotite and hornblende age data constrain regional ductile deformation in the study area to have occurred between ∼150 and ∼125 Ma. Such deformation was accompanied by upper greenschist- to lower amphibolite-facies metamorphism and is at least in part closely related to pluton emplacement. Such deformation and metamorphism has in the past been attributed to the Late Jurassic Nevadan orogeny. This study shows clearly, however, that ductile deformation and metamorphism in the study area are post-Nevadan if modern concepts of the Nevadan orogeny are strictly adopted.