Systematics of the Distribution of Uranium and Lead in Relation to the Petrology of the Mt. Rubidoux Granites, Riverside County, California.

Citation

Banks, Philip Oren (1963) Systematics of the Distribution of Uranium and Lead in Relation to the Petrology of the Mt. Rubidoux Granites, Riverside County, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/PB8W-1F55. https://resolver.caltech.edu/CaltechTHESIS:06272011-104837127

Abstract

Field and petrologic studies and uranium-lead isotope geochemistry investigations have been made of the Mt. Rubidoux granites, Riverside Co., Calif. The coarse-grained granite was emplaced earlier than the fine-grained granite, and both were intruded by later dikes of pyroxene granodiorite. The two rocks are very similar quartz monzonites with an unusual assemblage of iron-rich silicates including biotite, hornblende, hypersthene, and fayalite. The composition of the fine granite corresponds very nearly to a natural minimum in the Q-Or-Ab-water system. Data from published experimental studies suggest a temperature of 700-760°C and a water vapor pressure of 1000-1500 bars during crystallization. The presence of fayalite and hypersthene is attributed to late-stage loss of water pressure. U-Pb isotopic analyses of accessory minerals, potash feldspar, and whole rock samples were performed to investigate the behavior of these isotopes in the coarse granite. Zircon, uranothorite, allanite, and apatite constitute the principal sites of U and radiogenic Pb in the rock. Apatite was not analyzed. Systematic analyses of the other minerals have provided a criterion for recognizing discordance in young U-Pb systems. The granites are at least 116 m.y. old and possibly as old as 130 m.y. Variations in the degree of discordance for equivalent zircon fractions from different localities suggest that an unrecognized episodic event less than 100 m.y. ago may have caused the discordance. Zircon fractions from other rocks of the southern California batholith gave the following minimum ages: Woodson Mtn. granodiorite 119 m.y.; pyroxene granodiorite 108 m.y.; Crestmore quartz monzonite porphyry 106 m.y. Zircons from a soil profile over the coarse granite contained older zircons which have a minimum age of 1730 m.y. and are thought to be from metasedimentary wall rocks. The isotopic composition of common lead was determined by several analyses of potash feldspar. Analyses of whole rock material and acid leaches of the fresh granite indicate that the total rock is discordant to the same extent as the principal minerals, and that the abundance and leachability of U and radiogenic Pb in the rock are governed strongly by the mineral uranothorite. Similar analyses of C-zone weathered granite indicate that the chief effect of weathering is to make available to acid leaching a higher proportion of common lead from the feldspar and radiogenic Pb^(208) from allanite. Isotopic analyses of radiogenic Pb have been utilized to evaluate the decay constant of U^(235). The presently accepted constant is believed to be accurate to within 1%, relative to the accepted constant for U^(238). The effects of intermediate daughter losses in contributing to discordance in natural U-Pb systems have been considered in a series of model calculations. It is suggested that such losses may be of significance where the degree of discordance is relatively small and/or where the apparent time of episodic disturbance is very young.

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