Variations in Sr, Rb, K, Na, and Initial Sr87/Sr86 in Mesozoic Granitic Rocks and Intruded Wall Rocks in Central California

Initial Sr⁸⁷/Sr⁸⁶ of granitic rocks which are exposed north of the Garlock fault in California, and which represent the entire 130-m.y. time span of emplacement during the Mesozoic, ranges mainly from 0.7031 to 0.7082, with one value of 0.7094. A systematic areal variation, independent of age, exists for initial Sr⁸⁷/Sr⁸⁶ in these granitic rocks and is the same as the areal variation in initial Sr⁸⁷/Sr⁸⁶ of superjacent upper Cenozoic basalts and andesites.

Two values of initial Sr⁸⁷/Sr⁸⁶, 0.7040 and 0.7060, mark natural separations of granitic rock data on K-Rb, K-Sr, and Rb/Sr-Rb variation diagrams, and also, when contoured, seem to represent geographic markers of paleo-geographic, geochemical, and physiographic significance. Upper Precambrian sedimentary and metamorphic rocks in California crop out only in the region where initial Sr⁸⁷/Sr⁸⁶ of granitic rocks is greater than 0.7060. A line of initial Sr⁸⁷/Sr⁸⁶ = 0.7060 is approximately coincident with the boundary between Paleozoic eugeosynclinal and miogeosynclinal rocks. Granitic rocks intruded into Paleozoic miogeosynclinal rocks have initial Sr⁸⁷/Sr⁸⁶ greater than 0.7060, whereas those intruded into eugeosynclinal Paleozoic rocks have initial Sr⁸⁷/Sr⁸⁶ less than 0.7060. The line of initial Sr⁸⁷/Sr⁸⁶ = 0.7040 is the eastern limit of principal exposures of ultramafic rocks, the western limit of Cretaceous granitic rocks, and is coincident with an abrupt change in “topographic expression” on the Bouguer gravity map of California. Correlation of the isotopic variations with these major crustal features suggests that there has been a sharp lateral contrast in crust-mantle chemistry across the region of study that has been fixed in position from the Precambrian to the present time.

The chemical and isotopic variations observed are best explained if the parent magmas of the majority of granitic rocks investigated were derived in a region that was laterally variable in composition and in a zone of melting that intersected both upper mantle and lower crust. However, some igneous rocks, such as Jurassic volcanic rocks in wall rocks and roof pendants and some granitic rocks with high strontium concentrations and low Rb-Sr ratios, suggest that deeper sources are also involved in the total spectrum of igneous rocks in the region.