Textural Studies in Igneous Rocks near Twentynine Palms, California

Citation

Rogers, John James William (1955) Textural Studies in Igneous Rocks near Twentynine Palms, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/A05M-NQ50. https://resolver.caltech.edu/CaltechETD:etd-12082003-112407

Abstract

A description of a sequence of igneous rocks south of Twentynine Palms has been undertaken for the purpose of describing and determining the origin of the major textural variations within individual igneous bodies. The work involved field mapping and sampling, petrographic study, and spectrochemical analysis. The Pinto gneiss, a middle rank metamorphic rock, is the oldest formation in the area. It is intruded by a sequence of plutonic rocks including (in chronologic order) the Gold Park gabbro-diorite (oldest intrusive rock), Palms quartz monzonite, monzonitic porphyry, and White Tank quartz monzonite. Other igneous rocks include basalt and silicic and basic dikes. The major rocks, and the ones most intensively studied, are the Palms quartz monzonite-monzonitic porphyry complex and the White Tank quartz monzonite. The Palms quartz monzonite is divided into three units on the basis of slight differences in mineralogy and texture. Most of the quartz monzonite is characterized by very complex, irregular intergrowths between potash feldspar and plagioclase; these intergrowths are possibly the result of crystallization from a relatively dry melt during the latter stages of formation of the rock. Widespread reaction between the quartz monzonite melt and portions of the Pinto gneiss has caused the formation of a monzonitic porphyry along some contacts between quartz monzonite and gneiss. Plots have been made of mineral compositions and grain sizes of a sequence of rocks from gneiss through porphyry to quartz monzonite. Potash and soda have been added to the gneiss by the melt, and development of coarse crystals of potash feldspar in the porphyry is caused by incorporation of solid gneiss in the melt. Some contacts (ranging from relatively abrupt to broadly gradational) between quartz monzonite and gneiss do not exhibit development of porphyry. The White Tank quartz monzonite is divided into four units, two of which have two separate facies. The units are distinguished by differences in mineralogic composition and texture. The different facies and units are grouped into two differentiation sequences: the trend in each sequence is toward a more silicic rock, but one sequence results in the formation of large crystals of microcline, whereas the other sequence results in the formation of muscovite and garnet. The trace element content of potash feldspar varies throughout each sequence roughly in accordance with differentiation trends previously established by other workers. Measurements of the grain sizes of potash feldspar (in the field) and quartz and plagioclase (in thin section) also reflect the sequence of differentiation. The White Tank quartz monzonite is believed to have formed by the intrusion, differentiation, and solidification of a magma.

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