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Bull. Geol. Soc. Am., Vol. 50 Osborn, Pl. 1 BULL. GEOL. SOC. AM., VOL. 50 OSBORN, PL. 1 F i g u r e 1 . G a b b r o a t C o n t a c t w i t h T o n a l i t e Poikilitic crystals of hornblende (dark) enclose andesine. W ithout analyzer (x 25). F i g u r e 2 . T o n a l i t e Tongue-like projections of biotite cut andesine. Crossed nicols (x 80). PHOTOMICROGRAPHS OF GABBRO AND TONALITE Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/50/6/921/3415803/BUL50_6-0921.pdf by California Inst of Technology user on 21 November 2019 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA STRUCTURAL PETROLOGY OF THE YAL YERDE TONALITE, SOUTHERN CALIFORNIA BY E. F. OSBORN CONTENTS Page Abstract...................................................................................................................................... 922 Introduction.............................................................................................................................. 922 Acknowledgments.................................................................................................................... 923 Earlier publications................................................................................................................. 923 Location of district.................................................................................................................. 923 General geologic setting......................................................................................................... 924 General statement........................................................................................................... 924 Metamorphic rocks......................................................................................................... 924 Igneous rocks.................................................................................................................... 925 General statement................................................................................................... 925 Gabbro....................................................................................................................... 925 Tonalite..................................................................................................................... 926 Inclusions in tonalite.............................................................................................. 927 Granite...................................................................................................................... 927 D ikes.......................................................................................................................... 928 Age and correlation................................................................................................ 928 Structural geology........................................................................................................... 928 Micrometric and radioactivity measurements.................................................................. 929 General statement........................................................................................................... 929 Errors in sampling Val Verde tonalite....................................................................... 929 Variation in mineralogic composition of tonalite..................................................... 933 Correlation of mineralogic data with radioactivity determinations..................... 933 Radioactivity determinations............................................................................... 933 Earlier studies of radioactivity of rocks............................................................ 934 Source of radioactivity.......................................................................................... 935 Origin of dark inclusions in tonalite........................................................................... 935 Petrofabric analyses................................................................................................................ 937 General statement..................................................................................)........................ 937 Petrofabric analysis of schist....................................................................................... 938 Axes of reference..................................................................................................... 938 Fabric diagrams....................................................................................................... 938 Joints and faults...................................................................................................... 938 Interpretation.......................................................................................................... 938 Petrofabric analysis of tonalite.................................................................................... 939 Axes of reference..................................................................................................... 939 Fabric diagrams....................................................................................................... 942 Interpretation........................................................................................................... 944 Conclusions....................................................................................................................... 948 (921) Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/50/6/921/3415803/BUL50_6-0921.pdf by California Inst of Technology user on 21 November 2019 922 E. F . OSBORN----PETROLOGY OF VAL VERDE TONALITE ILLUSTRATIONS Figure Page 1. Map showing location of Yal Verde district............................................................. 924 2. Geologic map of the Val Verde district..................................................................... 925 3. Graph of variation in mineral composition oftonalité............................................ 932 4. Graphs of albite content of plagioclase and radioactivity of tonalité................ 934 5. Diagrams illustrating relation of axes of reference to dimensional orientation of grains in tonalité.......................................................................................................... 939 6. Fabric diagrams of schist.............................................................................................. 940 7. Fabric diagrams of tonalité.......................................................................................... 941 8. Fabric diagrams of tonalité.......................................................................................... 942 9. Fabric diagrams of tonalité and an inclusion........................................................... 943 Plate Facing page 1. Photomicrographs of gabbro and tonalité................................................................. 921 2. Photomicrographs of tonalité....................................................................................... 926 3. Hand specimen oriented with respect to axes of reference...................................... 927 ABSTRACT Micrometric analyses of the Val Verde tonalité were made on samples taken along a line extending from the border of the intrusive to a point 5 miles out in the intrusive. An attempt was made to evaluate sampling errors and to deter­ mine variation in minéralogie composition and radioactivity. The border of the intrusive was found to be more sodic and higher in radioactivity than the central part, and this is thought to be due to assimilation of quartz-biotite schist. Inclusions in the tonalité are believed to be xenoliths of gabbro and quartz- biotite schist. Petrofabric analyses of the schist country rock and the tonalité indicate that : (1) the schist was deformed primarily by rotation of mineral grains about a hori­ zontal axis; (2) during intrusion of the tonalité a gneissoid structure developed in it parallel to the contact with the country rock with a linear element lying in the plane of foliation and paralleling the dip; (3) later stresses directed parallel to the strike of the foliation caused rotation of mineral grains in the tonalité about an axis parallel to the dip. The orientation of the minerals in the tonalité is thus believed to be due to a combination of magmatic flow and post-magmatic deforma­ tion. INTRODUCTION Two phases of the problem of batholiths and their origin are of special interest. The first concerns variation in composition of intrusive bodies, and the second deals with the origin and interpretation of primary flow structure. A unique opportunity to investigate these problems as applied to one intrusive was furnished by the construction of the Yal Yerde tunnel, one of a series composing the Colorado River Aqueduct which will carry water from Parker Dam on the Colorado River to Los Angeles and vicinity. For slightly over 5 miles this tunnel passes through an apparently uni­ form tonalité intrusive at nearly right angles to the strike of the flow structure in the mass, and crosses the contact between the tonalité and quartz-biotite schist. The tonalité was examined along the line of the Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/50/6/921/3415803/BUL50_6-0921.pdf by California Inst of Technology user on 21 November 2019 INTRODUCTION 923 tunnel for variations in mineralogic composition and radioactivity; petro- fabric data were obtained from oriented specimens in an attempt to determine the origin of the gneissoid structure. ACKNOWLEDGMENTS
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