Preliminary Report on Initial Lead and Strontium Isotopes from Ophiolitic and Batholithic Rocks Southwestern Foothills Sierra Nevada California

Initial lead and strontium isotopic ratios have been determined for three suites of gabbros from the mafic-ultramafic belt of the Kings and Kaweah Rivers in the southwest Sierra Nevada foothills. Geochronological control is derived from zircon studies on the same suites (Saleeby and Sharp, 1978). The oldest suite of gabbro is from the metamorphosed Kings-Kaweah ophiolite belt whose petrogenetic age is 250 to 300 m.y. The second suite is a 169 m.y. (Middle Jurassic) syntectonic gabbrodiorite complex which intrudes the ophiolite. The third suite is a posttectonic suite of voluminous olivine-hornblende gabbros, which are intimately related to norites, pyroxene diorites, and quartz diorites, all yielding ages from 115 to 125 m.y. (Early Cretaceous). The Jurassic and Cretaceous gabbros constitute the mafic members of the composite Sierra Nevada batholith. The ophiolitic gabbros are part of an allocthonous terrane, which forms part of the western metamorphic wall of the batholith.