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Pdf/52/6/817/3416039/BUL52 6-0817.Pdf by Guest on 01 October 2021 BULLETIN of the GEOLOGICAL SOCIETY of AMERICA BULL. GEOL. SOC. AM., VOL. 52 PECORA, PL. 1 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/52/6/817/3416039/BUL52_6-0817.pdf by guest on 01 October 2021 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA STRUCTURE AND PETROLOGY OF THE BOXELDER LACCOLITH, BEARPAW MOUNTAINS, MONTANA BY WILLIAM T. PECORA CONTENTS Page Abstract................................................................................................................................... 818 Introduction........................................................................................................................... 819 Location and general statement................................................................................. 819 Field work and acknowledgments.............................................................................. 819 Topography of Boxelder Butte................................................................................... 820 Sedimentary rocks................................................................................................................. 821 Geology of the laccolith....................................................................................................... 821 Form and size................................................................................................................. 821 Exposed vertical section............................................................................................... 822 Ribboned shonkinite..................................................................................................... 824 Rubbled shonkinite....................................................................................................... 825 Tongues of ribboned shonkinite................................................................................. 825 Fringing sills of ribboned shonkinite......................................................................... 825 Inclusions........................................................................................................................ 827 Horizontal injection series........................................................................................... 828 Illustrated sections of cliff........................................................................................... 828 Upper ribboned shonkinite.......................................................................................... 829 Syenite and mafic syenite............................................................................................ 829 General statement................................................................................................. 829 Distribution and thickness.................................................................................. 830 Contact of syenite with shonkinite.................................................................... 830 Field description of the syenite.......................................................................... 831 Aplite dikes in mafic syenite....................................................................................... 832 Pegmatitic syenite......................................................................................................... 833 Friable shonkinite......................................................................................................... 833 Brim shonkinite............................................................................................................. 835 Distribution and thickness.................................................................................. 835 General description............................................................................................... 835 Petrography and mineralogy.............................................................................................. 835 General petrography..................................................................................................... 835 Mineralogy..................................................................................................................... 837 Quantitative lithologie relations......................................................................................... 841 Specific gravity variations........................................................................................... 841 Heavy mineral variations............................................................................................ 841 Discrepancies in data................................................................................................... 842 Calculated averages...................................................................................................... 842 Related shonkinitic bodies................................................................................................... 843 Dikes of ribboned shonkinite...................................................................................... 843 Duck Creek laccolith (?).............................................................................................. 843 (817) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/52/6/817/3416039/BUL52_6-0817.pdf by guest on 01 October 2021 818 W. T. PECORA— BOXELDER LACCOLITH, MONTANA Page Movement of magma........................................................................................................... 844 Feeder for laccolith....................................................................................................... 844 Critical horizon and depth of cover........................................................................... 844 Mechanics of intrusion................................................................................................. 845 Petrogenesis............................................................................................................................ 846 Ribbons of plagioclase syenite.................................................................................... 846 Segregation plagioclase syenite................................................................................... 847 Dikelets of plagioclase-free syenite............................................................................ 847 Friable shonkinite......................................................................................................... 847 Significance of brim shonkinite.................................................................................. 848 Plagioclase syenite......................................................................................................... 848 Dikes of plagioclase-free syenite................................................................................ 850 Hydrothermal mineralization..................................................................................... 850 Construction of the laccolith............................................................................................... 851 Conclusions............................................................................................................................. 853 Works to which reference is made....................................................................................... 853 ILLUSTRATIONS Figure Page 1. Index map of part of north-central Montana.......................................................... 820 2. Border of Boxelder laccolith near triangulation station T ..................................... 822 3. Columnar section of igneous rocks of Boxelder laccolith....................................... 823 4. Tongues of fine-grained shonkinite............................................................................. 826 5. Multiple shonkinite sill................................................................................................. 827 6. Inclusions of shonkinite................................................................................................ 828 7. Face of lower cliff east of triangulation station A................................................... 829 8. Face of lower cliff west of triangulation station N ................................................. 830 9. Face of lower cliff west of triangulation station G.................................................. 831 10. Irregular syenite-friable shonkinite contact near triangulation station S.......... 832 11. Specific gravity curves.................................................................................................. 840 12. Heavy mineral curve..................................................................................................... 840 13. Idealized section of Boxelder compound laccolith................................................... 852 14. Specific gravity curve for idealized section............................................................... 852 Plate Facing page 1. Boxelder Butte from the northwest............................................................................ 817 2. Northwest escarpment and West Ledge.................................................................... 820 3. Ribbons (layers) of syenite in shonkinite.................................................................. 821 4. Aplite dike and skeletal biotite................................................................................... 834 5. Syenite segregations and syenite pipe........................................................................ 835 6. Topographic and geologic
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