Paul Charles Bateman

Crystallization, fractionation, and solidification of the Tuolumne Intrusive Series, Yosemite National Park, California

Study of the Tuolumne Intrusive Series, a concentric texturally and compositionally zoned plutonic sequence in the eastern part of Yosemite National Park, was undertaken to develop and test a model for the origin of comagmatic plutonic sequences in the Sierra Nevada batholith. The granitoid units that make up the sequence are progressively younger and more felsic inward. The bulk of the rocks are granodiorite, but the outermost formation is quartz diorite, and the innermost one is granite porphyry. The compositional gradient changes both gradually within formations and abruptly between them. The change is greatest in the outer 1 km and lower toward the center of the sequence. Hornblende and biotite, abundant in the marginal rocks, decrease rapidly inward for 1 km as K-feldspar and quartz increase, but farther inward, they decrease slowly. The most conspicuous chemical changes are shown by the elements that are enriched in the mafic minerals. The compositional zoning indicates that with decreasing temperature, the sequence solidified from the margins inward. Solidification was interrupted repeatedly by surges of fluid core magma. The magma eroded the adjacent solidifying rock, and it expanded the area of the magma chamber at the exposed level by crowding the wall and roof rocks outward and upward and by breaking through the solidifying carapace into the wall rocks. The compositional zonation resulted from crystal fractionation that could have involved (1) preferential accretion of crystalline material present in the magma to the margins of the magma chamber, thus displacing the melt phase progressively inward, and/or (2) downward settling of crystals, probably accompanied by upward movement of melt and volatiles; the residual magma solidifying to form the granitoids. Although either mechanism can explain the observed relations, they lead to very different interpretations of the composition of the magma when the first exposed granitoids solidified at the margins of the magma chamber and as the sequence solidified inward.

Isotopic variation in the Tuolumne Intrusive Suite, central Sierra Nevada, California

Granitoid rocks of the compositionally zoned Late Cretaceous Toulumne Intrusive Suite in the central Sierra Nevada, California, have initial87Sr/86Sr values (Sri) and143Nd/144Nd values (Ndi) that vary from 0.7057 to 0.7067 and from 0.51239 to 0.51211 respectively. The observed variation of both Sri and Ndi and of chemical composition in rocks of the suite cannot be due to crystal fractionation of magma solely under closed system conditons. The largest variation in chemistry, Ndi, and Sri is present in the outer-most equigranular units of the Tuolumne Intrusive Suite. Sri varies positively with SiO2, Na2O, K2O, and Rb concentrations, and negatively with Ndi, Al2O3, Fe2O3, MgO, FeO, CaO, MnO, P2O5, TiO2, and Sr concentrations. This covariation of Sri, Ndi and chemistry can be modeled by a process of simple mixing of basaltic and granitic magmas having weight percent SiO2 of 48.0 and 73.3 respectively. Isotopic characteristic of the mafic magma are Sri=0.7047, Ndi=0.51269 andδ18O=6.0, and of the felsic magma are Sri=0.7068, Ndi=0.51212 andδ18O=8.9. The rocks sampled contain from 50 to 80% of the felsic component. An aplite in the outer equigranular unit of the Tuolumne Intrusive Suite apparently was derived by fractional crystallization of plagioclase and hornblende from magma with granudiorite composition that was a product of mixing of the magmas described above. Siliceous magmas derived from the lower crust, having a maximum of 15 percent mantle-derived mafic component, are represented by the inner prophyritic units of the Tuolumne Intrusive Suite.

Sierra Nevada Batholith The batholith was generated within a synclinorium

The Sierra Nevada batholith is localized in the axial region of a complex faulted synclinorium that coincides with a downfold in the Mohorovicic discontinuity and in P-wave velocity boundaries within the crust. Observed P-wave velocities are compatible with downward increase in the proportion of diorite, quartz diorite, and calcic granodiorite relative to quartz monzonite and granite in the upper crust, with amphibolite or gabbro-basalt in the lower crust, and with periodotite in the upper mantle. The synclinorium was formed in Paleozoic and Mesozoic strata during early and middle Mesozoic time in a geosyncline marginal to the continent. Granitic magmas are believed to have formed in the lower half of the crust at depths of 25 to 45 kilometers or more, primarily as a result of high radiogenic heat production in the thickened prism of crustal rocks. Magma was generated at different times in different places as the locus of down-folding shifted. It rose into the upper crust because it was less dense than rock of the same composition or residual refractory rocks. Refractory rocks and crystals that were not melted and early crystallized mafic minerals that settled from the rising magma thickened the lower crust. Wall and roof rocks settled around, and perhaps through, the rising magma and provided space for its continued rise. Erosion followed each magmatic episode, and 10 to 12 kilometers of rock may have been eroded away since the Jurassic and 7 to 10 kilometers since the early Late Cretaceous.

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.

Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith

Chemical analyses of 167 typical specimens indicate that about 95 percent of the intrusive rocks of the central Sierra Nevada contain more than 79 percent normative Ab + An + Or + Qz. If the composition of the lower continental crust is similar to or slightly more felsic than andesite, as seems likely, the system NaAlSi 3 O 8 -CaAl 2 Si 2 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O provides an excellent chemical model for testing various schemes of fusion of the lower crust and crystallization of the resulting magmas. From consideration of this system in conjunction with field and petrographic data, we conclude that the intrusive rocks are best explained by repeated episodes of equilibrium fusion corresponding to magmatic sequences defined by field, petrologic, chemical, and geochronologic data. Fractional crystallization of the crystal-liquid mush generated by equilibrium fusion, coupled with periodic upward or lateral movement of the less crystallized central part of the magma, would produce the characteristic mafic to felsic sequence of intrusion; each mafic to felsic sequence corresponds to a separate equilibrium fusion event. In contrast, a close approach to fractional fusion of the lower crust is inadequate for obtaining most of the plutonic rocks, because rock compositions capable of being produced by this process do not match those observed. Normal amounts of conductive heat from the mantle and from radioactive decay in the crust may have been capable of causing fusion in the deepest parts of a thickened crust under the central part of the Sierra Nevada without the aid of a transient heat source from the mantle, but would have been inadequate where the crust was thin in the western Sierra Nevada. However, upward transport of andesitic and basaltic magmas generated along a Mesozoic subduction zone dipping beneath the Sierra Nevada would have provided sufficient additional heat to make fusion of the lower crust unavoidable. This implies that a major portion of the present batholith must have been derived from the lower crust.

Isotopic Ages of Minerals from Granitic Rocks of the Central Sierra Nevada and Inyo Mountains, California

Potassium-argon ages of biotite and hornblende from specimens of 17 granitic plutons in the central Sierra Nevada and the western Inyo Mountains, California, range from 69 to 183 m. y. The Mount Givens, Lamarck. and Round Valley Peak Granodiorites and related younger and more felsic quartz monzonites represent a pulse of magma emplaced in the general time interval of 80-90 million years ago, during Cretaceous time. Mineral ages of granitic rocks that flank these plutons on both the east and the west have been reduced during the emplacement of the Cretaceous intrusive rocks and are minimum ages for the time of crystallization. The ages of hornblende from the Tinemaha Granodiorite (150 to 180 m. y.) may approach crystallization dates. In conjunction with ages for other intrusive rocks in the Sierra Nevada and adjacent desert ranges they strongly suggest a magmatic episode during the Early Jurassic.

Investigation of Initial Sr87 / Sr86 Ratios in the Sierra Nevada Plutonic Province

One to three whole-rock samples from each of more than a dozen discrete plutonic intrusions in the east-central Sierra Nevada batholith have been analyzed for Sr 87 /Sr 86 and Rb/Sr ratios to obtain information on initial Sr 87 abundances. The initial Sr 87 /Sr 86 ratios in the rock magmas forming this province appear to have been in the range 0.7073 ± .0010 in the majority of cases. This range is definitely higher than that found for modern alkali-type and tholeiite-type basalt magmas of oceanic regions, which commonly range between 0.703 and 0.705. However, it is much lower than the average Sr 87 /Sr 86 ratios found in Precambrian sialic regions which range from 0.71 to 0.73. It seems clear therefore that the Sierra Nevada magmas were not derived solely either from the typical source regions of oceanic basalt or from the melting of ancient crustal sial. It is possible that these magmas represent a mixture of oceanic basalt and crustal sial, as would be the case of anatexis in a geosyncline containing much volcanic material of fairly recent origin and some terrigenous sialic detritus. They may instead be of mantle derivation with admixtures of crustal material assimilated during their rise. The whole-rock Rb-Sr age results derivec from the study indicate that the Lamarck and Mount Givens Granodiorites and the alaskite of Evolution Basin and porphyritic biotite granite of Dinkey Lakes form a younger group of intrusive rocks of 90 ± 10 m.y. Although the sampling was not designed for isochron age studies, it appears that most of the remaining rock units are considerably older.

Granitic Formations in the East-Central Sierra Nevada Near Bishop, California

This report establishes lithologic units among the granitic rocks of the east-central Sierra Nevada near Bishop, California. In this area the Sierra Nevada batholith is composed chiefly of quartz-bearing plutonic rocks ranging in composition from quartz diorite to alaskite but includes scattered small masses of darker and older plutonic rocks and remnants of metamorphosed sedimentary and volcanic rocks. The granitic rocks are in discrete plutons, either in sharp contact with one another or separated by thin septa of metamorphic or mafic igneous rock or by late aplitic dikes. The granitic rocks are grouped into lithologic units on the basis of composition, texture, and intrusive relations. The units include six new formations, three informal units made up of the rocks in several plutons, and four informal units that include the rocks in single plutons. The new formations are the Inconsolable Granodiorite, Tinemaha Granodiorite, Wheeler Crest Quartz Monzonite, Round Valley Peak Granodiorite, Lamarck Granodiorite, and Tungsten Hills Quartz Monzonite.

Cretaceous deformation in the western foothills of the Sierra Nevada, California

Deformation of metamorphic rocks, the tonalite of Blue Canyon, several bodies of granite and granodiorite, and the leucotonalite of Ward Mountain within an area of about 600 km 2 in the western foothills of the Sierra Nevada, just south of the south end of the western metamorphic belt, is attributed to emplacement of the leucotonalite in a nearly solid state. The east flanks of two plutons of leucotonalite of Ward Mountain in the west-central part of the deformed area dip gently beneath the area of deformed rocks, whereas their west flanks are vertical or dip steeply east. Cata-clastic planar and linear structures in the metamorphic rocks and in the tonalite of Blue Canyon form coherent and related patterns. The planar structures generally strike parallel to contacts with the leucotonalite and dip outward; between the two plutons of leucotonalite, they form a synform. Lineations generally plunge outward from the leucotonalite but trend parallel to contacts within the synform. Heat and possibly volatiles that accompanied emplacement of the rising and westward-moving mass of leucotonalite may have increased the susceptibility of the overlying rocks to attenuate normal to the planar structures and to stretch in the direction of the linear structures. Isotopic U-Pb and K-Ar determinations indicate that the age of the tonalite of Blue Canyon is about 114 m.y. and that the age of the leucotonalite of Ward Mountain coincides with the time of deformation at about 113 m.y. ago. The isotopic age of the granodiorite of Knowles, which truncates cataclastic structures in the tonalite of Blue Canyon and the leucotonalite of Ward Mountain, is 110 m.y.

Aeromagnetic Investigation of Crustal Structure for a Strip Across the Western United States

This report represents part of a larger study undertaken to interpret the gross features of the earth9s crust by aeromagnetic methods. The larger survey covers a 100-mile-wide strip along a great circle arc from Washington, D.C., to San Francisco, California. The area considered extends from about 200 miles east of the Rocky Mountains to 150 miles west of the coast of the Pacific Ocean. Inferences on gross structure and lithology are made from the magnetic patterns and trends shown on the map. At the continental margin, the magnetic data strongly suggest that the oceanic crust becomes much more deeply buried as the continent is approached. A large magnetic anomaly extending northwestward along the center of the Great Valley is probably caused by mafic intrusive rocks. Broad wavelength magnetic anomalies over the Sierra Nevada are consistent with results obtained from gravity and seismic observations, indicating the batholith is underlain by gabbroic rocks. The Basin and Range province is characterized by an east-west and northeasterly magnetic grain, whereas the structural grain is nearly north-south. A complex belt of circular and elliptical anomalies extends southwesterly from the Utah line south of Wendover to a point beyond Ely, Nevada. These anomalies are due to magnetic basement rocks and small intrusive bodies that extend diagonally across several ranges. From the Wasatch fault block to the Rocky Mountain area the predominant grain is conspicuously east-west. The trend becomes more northwesterly as one progresses from north to south. The aeromagnetic data, supported by geologic mapping, suggest the existence of a regional pattern of en echelon northeast-trending shear zones across the Rocky Mountains and Denver basin. East-west magnetic lineaments, some as long as 400 miles, may represent fundamental fractures or fracture zones, most of which originate deep in the earth9s crust. These fracture zones may be genetically related to the system of east-west strike slip faults which have been discovered beneath the oceans by various oceanographic institutions.