Variations in lead-isotopic compositions in Mesozoic granitic rocks of California: A preliminary investigation

Six alkali feldspar and two whole-rock samples of granitic rocks from the Sierra Nevada batholith and adjacent Klamath Mountains were analyzed for their lead-isotope compositions. The samples represented each of three ⁸⁷Sr/⁸⁶Sr groupings (< 0.704, 0.704 to 0.706, and > 0.706) for granitic rocks north of the Garlock fault in California. The isotopic compositions of lead in the samples from the Sierra Nevada batholith range from 18.73 to 19.37 for ²⁰⁶Pb/²⁰⁴Pb, 15.61 to 15.71 for ²⁰⁷Pb/²⁰⁴Pb, and 38.44 to 39.10 for ²⁰⁸Pb/²⁰⁴Pb. A crude parallel correspondence was found between lead and strontium isotopes, in that the specimens with the most radiogenic strontium also tend to have the most radiogenic lead similar to the previously studied Boulder batholith of Montana. A parallel correspondence is thought to imply characteristics of the source rocks for the plutons rather than consequences of partial melting or natural contamination. Lead-isotopic compositions for the Sierra Nevada batholith and the Boulder batholith differ, average values of ²⁰⁶Pb/²⁰⁴Pb being at least 18.8 for the Sierra Nevada batholith and about 18 for the Boulder batholith.

In the Late Cretaceous part of the Sierra Nevada batholith, the secondary isochron “age” for the lead data in these rocks is about 2,900 m.y., far older than known Precambrian in California. Sources are proposed for these plutons from the lower continental crust and upper continental mantle or dominantly recycled continental materials, probably of intermediate composition and possibly carried down to the zone of melting by subduction. This source material may have been formed in Pre-cambrian times but did not undergo a Precambrian metamorphism greater than upper amphibolite facies which would have reduced the values of ²³⁸U/²⁰⁴Pb in the source rocks and resulted in Mesozoic leads like those found in the Boulder batholith and elsewhere in the Rocky Mountain region.

A trondhjemite from the Klamath Mountains has a lead-isotope composition (²⁰⁶Pb/²⁰⁴Pb, 18.57; ²⁰⁷Pb/²⁰⁴Pb, 15.50; ²⁰⁸Pb/²⁰⁴Pb, 38.08) similar to that of oceanic volcanic rocks, particularly like those of island volcanics on oceanic ridges. Derivation of this trondhjemite from an oceanic mantle or recycled mantle material is indicated by this observation and supports the conclusion of Kistler and Peterman (1973) based on its alkali abundances and ⁸⁷Sr/⁸⁶Sr value.