Rb-Sr whole-rock and mineral ages, K-Ar, 40Ar/39Ar, and U-Pb mineral ages, and strontium, lead, neodymium, and oxygen isotopic compositions for granitic rocks from the Salinian composite terrane, California

This report summarizes new and published age and isotopic data for whole-rocks and minerals from granitic rocks in the Salinian composite terrane, California. Rubidium-strontium whole-rock ages of plutons are in two groups, Early Cretaceous (122 to 100 Ma) and Late Cretaceous (95 to 82 Ma). Early Cretaceous plutons occur in all granitic rock exposures from Bodega Head in the north to those from the Santa Lucia and Gabilan Ranges in the central part of the terrane. Late Cretaceous plutons have been identified in the Point Reyes Peninsula, the Santa Lucia and the Gabilan Ranges, and in the La Panza Range in the southern part of the terrane. Ranges of initial values of isotopic compositions are ⁸⁷Sr/⁸⁶Sr, 0.7046-0.7147, δ¹⁸O, +8.5 to +12.5 per mil, ²⁰⁶Pb/²⁰⁴Pb, 18.901-19.860, ²⁰⁷Pb/²⁰⁴Pb, 15.618-15.814, ²⁰⁸Pb/²⁰⁴Pb, 38.569- 39.493, and εNd, +0.9 to -8.6.

The initial ⁸⁷Sr/⁸⁶Sr=0.706 isopleth is identified in the northern Gabilan Range and in the Ben Lomond area of the Santa Cruz Mountains, in Montara Mountain, in Bodega Head, and to the west of the Farallon Islands on the Cordell Bank. This isotopic boundary is offset about 95 miles (160km) by right-lateral displacements along the San Gregorio-Hosgri and San Andreas fault systems.