Variations of Major Chemical Constituents across the Central Sierra Nevada Batholith
A study of 193 chemical analyses of plutonic rocks from 132 localities in the central Sierra Nevada shows convincingly that K2O decreases systematically westward and suggests that Fe2O3 and TiO2 may also decrease westward and that FeO, MgO, and CaO may increase. The ratio K2O/SiO2 obviously decreases westward across six of eight provisionally established sequences of granitic rocks. Plots of analyses of rocks from each sequence form discrete fields that are strongly elongate toward zero K2O at 40 to 45 percent SiO2. The boundaries between fields on these plots and between fields on plots of normative minerals on triangular diagrams are sharp. Compositional trends within sequences are different than the compositional changes that take place across the batholith—rocks in the western Sierra Nevada probably are not compositionally identical with rocks that are present at depth beneath the eastern Sierra Nevada.

Progressive decrease of K2O in the Paleozoic and Mesozoic country rocks westward across the batholith is consistent with the anatectic model for its origin. However, it also is consistent with the hypothesis developed to explain chemical patterns in volcanic island arcs—that K2O increases toward continental land masses because of increasing depth of magma generation along landward-dipping seismic (Benioff) zones. The seismic-zone hypothesis encounters several difficulties, but it cannot be ruled out.