Franklin C. W. Dodge

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.

Nd and Sr isotopic study of crustal and mantle inclusions from the Sierra Nevada and implications for batholith petrogenesis

Sm-Nd and Rb-Sr systematics have been examined for eight upper-mantle and lower-crustal xenoliths, the xenolith-bearing trachyandesite, and the granodiorite intruded by the trachyandesite in the Sierra Nevada batholith. Isotopic heterogeneity is shown by the xenoliths with 143 Nd/ 144 Nd ranging from 0.51169 to 0.51306, and 87 Sr/ 86 Sr ranging from 0.7031 to 0.7333. These ranges are similar to those of the plutonic rocks of the batholith. This similarity suggests that the xenoliths, believed to be representative of mantle and crustal reservoirs, may approximate the diverse source regions from which the varied granitic rocks of the batholith were derived and that the Sr-Nd isotopic correlation seen in granitic rocks may not be due to simple mixing of two end-member components. The subgranitic crust in this part of the Sierra Nevada apparently consists of metamorphosed sedimentary and igneous rocks which are highly variable isotopically. The mantle beneath the Sierra Nevada is also isotopically heterogeneous both vertically and laterally.

Fission-Track Ages of Accessory Minerals from Granitic Rocks of the Central Sierra Nevada Batholith, California

Ages of apatite, sphene, allanite, epidote, and garnet from plutonic rocks of the central Sierra Nevada and Inyo Mountains have been determined by the fission-track method. Ages of 44 specimens of apatite range from 54 to 128 m.y. Oldest apatites generally occur in rocks from the western portion of the batholith; youngest are from granitic rocks along the eastern slope of the Sierra Nevada. Thirty-four specimens of sphene have been dated and range in age from 71 to 118 m.y. Within experimental error, all sphenes are as old as, or older than, coexisting apatites. The oldest sphene is from granitic rock of the Inyo Mountains; however, sampled rocks from the western Sierra Nevada do not contain sphene. The youngest sphenes are generally from rocks slightly west of the Sierran crest. Fission-track ages of two allanites are 86 and 87 ± 9 m.y. Two epidotes yielded ages of 84 ± 8 m.y. and a third is 140 ± 14 m.y. Age of a garnet was determined to be 114 ± 11 m.y. Several of the apatites and sphenes used for fission-track dating are from the same samples that previous workers had used for potassium-argon dating of biotites and hornblendes. Hornblende K-Ar ages appear to be the most insensitive to heat, apatite fission-track ages, the least, and biotite K-Ar and sphene fission-track ages fall between the two extremes and show the closest concordance of all pairs.

The Stability of Phlogopite in the Presence of Quartz and Diopside

The stability of phlogopite in the presence of quartz and diopside is of interest in the interpretation of biotite-bearing assemblages in granitic rocks and of biotite-clinopyroxene-amphibole assemblages in metamorphic rocks. This study, begun to compliment the studies of Luth (1967), Schairer (1954), and Shaw (1963), is now being abandoned and the results to date are presented herein. Numerous persons have contributed to the study. Starting materials were contributed by W.T. Pecora, J.F. Schairer, and H.S. Yoder. Laboratory assistance was provided by J.V. Chernosky, C.J. Duffy, H.R. Shaw, and J. Whitney. N. Chatterjee, W.C. Luth, and D.A. Hewitt discussed many aspects of the theoretical and applied aspects of the study. J.S. Huebner and M.E. Woodruff were particularly helpful in the acquisition of x-ray powder intensity data. J.V. Chernosky provided library access and working space at the Univ. of Maine, Orono, so that this manuscript could be finished in the midst of an active field season. Critical reviews by J.S. Huebner, R.A. Robie, D.B. Stewart and E-an Zen helped to improve the manuscript.

Radiogenic Heat Production of Contrasting Magma Series: Bearing on Interpretation of Heat Flow

Variation in radiogenic heat production of rocks of diverse magma series (representative of calcic, calc-alkalic, alkali-calcic, and alkalic petrographic provinces on a worldwide basis) is better correlated with some form of magmatic differentiation index, rather than simply with potassium content alone as commonly supposed. The “lime-alkali” (Peacock) index generally gives the best correlation of the various indices tested. Plots of heat production for igneous suites versus indices of differentiation (“heat production trends”) demonstrate systematic correlations of heat productivity as functions of fractionation trends and/or spatial-temporal associations. In general, heat production trends of calcic suites tend to indicate lower radiogenic heat productivity than those of more alkalic suites for a given silica or potassium content. Because many recent measurements of heat flow in plutons can be linearly related to heat production of surface rocks, these “heat production trends” have important bearing on the interpretation of crustal heat flow. Our data suggest possible alternative interpretations of the heat production-heat flow pattern for the Sierra Nevada which differ from that in Lachen-bruchs (1968) preliminary geothermal model.