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Metamorphism and the Origin of Granitic Rocks Northgate District Colorado

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Metamorphism and the Origin of Granitic Rocks Northgate District Colorado Metamorphism and the Origin of Granitic Rocks Northgate District Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-M Metamorphism and the Origin of Granitic Rocks Northgate District Colorado By T. A. STEVEN SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-M A discussion of the progressive metamorphism, granitixation, and local rheomorphism of a layered sequence of rocks, and of the later emplacement and deuteric alteration of an unrelated granitic stock UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. CONTENTS Page Page Abstract_________________________________ 335 Pre-Cambrian geology—Continued Introduction-_______________________________________ 335 Dacite porphyry—____ ——— __ —— _____________ 364 Acknowledgments__ ___--_____-____-_____-______-_ 336 Intrusive quartz monzonite_-____--_-__-_--_-_-_. 365 Geologic setting._______ — _________________________ 336 Petrography ________—— —— _______________ 365 Pre-Cambrian geology—___________________________ 337 Main body of the stock____________— 366 Hornblende gneiss___-_________-_-_____-________ 338 Marginal dikes_________-____-__-__——— 366 Quartz monzonite gneiss_________________________ 342 Satellitic dikes___-___.__________ 367 Biotite-garnet gneiss___________________________ 345 Wall-rock alteration_________ _ __——_ 368 Pegmatite_________________________________ 350 Origin_______________________ 368 Hornblende-biotite gneiss____________-__-________ 353 Emplacement. ___-----_---------_-----_ 368 Mylonite gneiss___----_--__---___--_-__---_--__- 354 Original rock______________ —— __ 370 Rheomorphic quartz monzonite gneiss-____________ 357 Deuteric alteration—________________-„ 370 Petrography. ______________________________ 359 Late magmatic solutions.______ —— _ _ _ _ 371 Origin__________________________ 361 Summary and conclusions———______ — — ___________.„ 372 Condition of rocks________--__----_-____ 362 Granitic rocks in the gneiss complex———________„ 373 Method of movement._________-_-__-__. 362 Granitic rocks of magmatic origin——____________„ 374 Direction of movement-__________________ 363 Literature cited__________________________________ 374 Cause of movement---___--__-____--____ 364 Index.____________________________________________ 377 ILLUSTRATIONS [Plates 48-49 in pocket; plates 50-55 following page 378] PLATE 48. Geologic map of the Northgate district, Wyoming and Colorado. 49. Geologic map of the northwest part of the Northgate district. 50. Photomicrographs of hornblende gneiss. 51. View and photomicrographs of quartz monzonite gneiss. 52. Photomicrographs of biotite-garnet gneiss. 53. Photomicrographs of hornblende-biotite gneiss and mylonite gneiss. 54. Photomicrographs of rheomorphic quartz monzonite gneiss and intrusive quartz monzonite. 55. Photomicrographs of intrusive quartz monzonite. FIGURE 67. Index map showing location of Northgate district_____________ ——— _______ — _______ 336 68. Lineation in hornblende gneiss___-___________-________-__--___--__--_---------_--_- 341 69. Lineation in hornblende-biotite gneiss and mylonite gneiss __________ _ _____________ — 356 70. Lineation in rheomorphic quartz monzonite gneiss._____________________— ______ — 358 TABLE TABLE 1. Approximate specific gravities of pre-Cambrian rocks from the Northgate district. OT SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY METAMORPHISM AND THE ORIGIN OF GRANITIC^ROCKS, NORTHGATE DISTRICT, COLORADO By T. A. STEVEN ABSTRACT alkali- and silica-bearing magmatic solutions which converted The pre-Cambrian rocks in the Northgate district on the the main body of the stock into a biotite-quartz monzonite. western flank of the Medicine Bow Mountains, Colorado, Much of the biotite in the peripheral parts of the stock is chlori- consist of a gneiss, complex invaded by a granitic stock. A few tized, and the associated plagioclase is significantly more sodic than that in the central part of the stock. Wall-rock alteration dacite porphyry dikes cut the gneiss complex and are older than the stock. was minor. Dynamothermal metamorphism converted the parent rocks INTRODUCTION of the gneiss complex into a hornblende-plagioclase gneiss (hornblende gneiss), and was closely followed by widespread This report deals largely with the progressive dynamo- metasomatic transformations. Alkali and silica metasomatism thermal, metasomatic, and dynamic metamorDhism, converted relatively large masses of hornblende gneiss to quartz monzonite gneiss in the northern and southeastern parts of the culminating in local rheomorphism, of a layered series district; smaller bodies of quartz monzonite gneiss were formed of rocks, and with the later, unrelated emplacement through the central part of the district where they are associated of a dioritic stock and the deuteric alteration of this with abundant pegmatite of related metasomatic origin. Bio- rock to quartz monzonite. All of these metamorphic tite-garnet-quartz-plagioclase gneiss (biotite-garnet gneiss) is a and igneous rocks are of pre-Cambrian age; they common associate of the pegmatite and was formed by local "basic" transformation of hornblende gneiss in a zone peripheral comprise about two-thirds of the rocks exposed in the to the main zone of alkali metaspmatism. Hydrothermal Northgate district, Colorado. metamorphism of small ultramafic bodies developed a number of The Northgate district is in Jackson County, Colo., zoned deposits of serpentine, chlorite, tremolite-actinolite, and near the north end of North Park, a broad inter- vermiculite. montane basin between the Medicine Bow Mountains Shearing during and after granitization reduced much of the rock in the gneiss complex to mylonite along an eastward- and the Park Range of the southern Rocky Mountains trending zone through the south-central part of the district. (fig. 67). The district is largely on the western flank Farther north, where shearing was less intense, only hornblende of the Medicine Bow Mountains, but also includes the gneiss was much affected, and irregular bodies of hornblende- northeast corner of North Park. The area covered biotite-quartz-plagioclase gneiss (hornblende-biotite gneiss) were by this report—about 65 square miles—is bounded formed. Some quartz monzonite gneiss in the large bodies in the north­ roughly on the north by the Colorado State line, on western and southeastern parts of the district became mobile the west by the North Platte River, on the south by after transformation and invaded the surrounding rocks. Rela­ the township line between Tps. 10 and 11 N., and on tions are clearest near the northwest corner of the district the east by the range line between Rs. 78 and 79 W., where a funnel-shaped mass more than a mile in diameter forcibly sixth principal meridian. injected and greatly deformed the adjacent rocks. Several fine-grained dacite porphyry dikes, definitely older This report presents part of the results of an in­ than the granitic stock, cut the gneiss complex in the vicinity of vestigation by the U. S. Geological Survey centering lower Camp Creek. The dikes follow tension fissures that do not on the fluorspar deposits of the district. The work conform to the structural pattern shown by the gneiss complex, began in 1943 as a strategic minerals investigation, and the dacite porphyry is believed to be unrelated to the other when D. C. Cox, assisted by J. O. Fisher and J. W. pre-Cambrian rocks in the district. An intrusive granitic rock forms a stock and associated dikes Odell, made a preliminary study of the larger fluorspar in the central part of the Northgate district and several related deposits of the area. The vein zones were studied dikes near the east edge of the district. Similar rocks are com­ in more detail during 1944 and 1945 by D. O. Cox mon in the Rocky Mountains of southern Wyoming and northern and W. E. Benson, assisted by D. M. Henderson, Colorado; and the stock is believed to be a cupola on a much when the Geological Survey was working in coopera­ larger underlying body. The original magma, which apparently was dioritic or quartz dioritic in composition, made way for tion with an exploratory program conducted by the itself by magmatic stoping. After solidification, the dioritic U. S. Bureau of Mines. The writer visited tie area rock was pervasively deformed on a minor scale and invaded by briefly during the winter of 1945-46, and spent about 335 336 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY p. 28-31; 1927, p. 116-119), Burchard (1933, p. 12-14), Cox (1945, p. 277), Cox, Benson, Steven, and Van Alstine,2 and Steven (1953, 1954). A note on the mineralogy of one of the fluorspar deposits was pub­ lished by Goldring (1942). At the time of field work there was no adequate base map covering the Northgate district. Therefore the writer did most of the regional mapping on en­ largements of U. S. Forest Service aerial photographs taken in 1937; some mapping in 1948 was done on aerial photographs taken for the Geological Survey in 1947. Section corners set during the General Land Office resurvey made in 1938-39 were located in the field, and a planimetric map was contracted using the General Land Office township plates for control. The resurvey did not cover the northern part of the Northgate district, and here the radial line method was • used for map compilation. Control along the north edge of the map
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