Geology of the Igneous Rocks of the Spanish Peaks Region Colorado
Total Page：16
File Type：pdf, Size：1020Kb
Geology of the Igneous Rocks of the Spanish Peaks Region Colorado GEOLOGICAL SURVEY PROFESSIONAL PAPER 594-G GEOLOGY OF THE IGNEOUS ROCKS OF THE SPANISH PEAKS REGION, COLORADO The Spanish Peaks (Las Cumbres Espanolas). Geology of the Igneous Rocks of the Spanish Peaks Region Colorado By ROSS B. JOHNSON SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 594-G A study of the highly diverse igneous rocks and structures of a classic geologic area, with a discussion on the emplacement of these features UNITED STATES GOVERNl\1ENT PRINTING OFFICE, WASHINGTON : 1968 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract__________________________________________ _ G1 Petrography-Continued Introduction ______________________________________ _ 1 Syenite_______________________________________ _ G19 Geologic setting ___________________________________ _ 3 Porphyry _________________________________ _ 19 General features _______________________________ _ 3 Aphanite---------------------------------- 20 Sedhnentaryrocks _____________________________ _ 4 Lamprophyre _____________________________ _ 20 Structure _____________________________________ _ 4 Syenodiorite __________________________________ _ 22 Thrust and reverse faults ___________________ _ 5 Phanerite_________________________________ _ 22 Normal faults _____________________________ _ 5 Porphyry _________________________________ _ 22 Folds_____________________________________ _ 6 Aphanite _________________________________ _ 26 Joints ____________________________________ _ 6 Lamprophyre _____________________________ _ 27 General features of igneous rocks ____________________ _ 7 Diorite _____________________________________ - __ 28 Intrusive bodies ___________________________________ _ 28 7 PorphyrY----------------------------------Aphanne _________________________________ _ Stocks________________________________________ _ 29 Laccoliths ____________________________________ _ 10 Lamprophyre _____________________________ _ 29 Sole injections_________________________________ _ 11 Syenogabbro __________________________________ _ 30 Plugs _________________________________________ _ 11 Porphyry _________________________________ _ 30 13 Aphanne _________________________________ _ Dikes ________________________________________ _ 30 Sills __________________________________________ _ 16 Lamprophyre _____________________________ _ 30 Extrusive bodies ___________________________________ _ 16 Gabbro_______________________________________ _ 31 Petrography ______________________________________ _ 16 Porphyry _________________________________ _ 31 Terminology __________________________________ _ 16 Aphanne _________________________________ _ 32 Granite_______________________________________ _ 17 Lamprophyre _____________________________ _ 33 Porphyry _________________________________ _ 17 Chemical composition of the various rock types ________ _ 34 Aphanne _________________________________ _ 17 Discussion ____________________________ ------------- 36 Granodiorite __________________________________ _ 17 Age and order of magmatic intrusion _______ - _____ - 36 Porphyry _________________________________ _ 17 Origin of the radial dike patterns ________________ _ 36 Aphanne _________________________________ _ 18 References cited _______________________ ------------- 37 ILLUSTRATIONS Page PLATE 1. Geologic map of the Spanish Peaks region, Colorado ___________________________________________ _ In pocket FRoNTISPIECE. The Spanish Peaks (Las Cumbres Espanolas). FIGURE 1. Index map of the Spanish Peaks region______________________________________________________________ G3 2-9. Photographs: 2. Profile. Rock, granodiorite porphyry dike _____________________________________________________ - 3 3. Bleached sandstone adhering to Devils Stairway granite porphyry dike ____________________ ------- 4 4. Two granite sills at northwest termination of East Spanish Peak intrusive_________________________ 8 5. Little Sheep Mountain, showing base of laccolith(?) __________________________________________ -- 11 6. Goemmer Butte, a latite plug________________________________________________________________ 12 7. Gneissic cobble-sized xenoliths in scoriaceous basalt plug _____________________________________ --- 13 8. Dike with archway cut through it for road along Apishapa River southeast of West Spanish Peak____ 13 9. Parallel walls formed by indurated sandstone on margins of a lamprophyre dike that forms trough_-- 14 v VI CONTENTS TABLES Page TABLE 1. Chemical analyses of granites __________________________________________________________________ ---- 040 2. Semiquantitative spectrographic analyses of granites _______________________________________________ --- 40 3. Normative composition of granites ________________________________________________________________ _ 40 4. Chemical analyses of radial dikes of granodiorites ____________________________________________________ - 40 5. Semiquantitative spectrographic analyses of radial dikes of granodiorites _____________________________ -_- 41 6. Normative composition of radial dikes of granodiorites _________________________________________ - ___ --- 41 7. Chemical analyses of syenites _____________________________________________________________________ _ 41 8. Semiquantitative spectrographic analyses of syenites ___________________________________________ -_----- 42 9. Normative composition of syenites ________________________________________________________________ - 42 10. Chemical analyses of syenodiorites ________________________________________________________________ _ 43 11. Semiquantitative spectrographic analyses of syenodiorites ____________________________________________ _ 43 12. Normative composition of syenodiorites ____________________________________________________________ - 44 13. Chemical analyses of diorites _____________________________________________________________________ _ 45 14. Semiquantitative spectrographic analyses of diorites _________________________________________________ _ 45 15. Normative composition of diorites _________________________________________________________________ _ 45 16. Chemical analyses of syenogabbros ________________________________________________________________ _ 45 17. Semiquantitative spectrographic analyses of syenogabbros ____________________________________________ _ 46 18. Normative composition of syenogabbros ____________________________________________________________ _ 46 19. Chemical analyses of gabbros _____________________________________________________________________ _ 46 20. Semiquantitative spectrographic analyses of gabbros _________________________________________________ _ 47 21. Normative composition of gabbros ________________________________________________________________ _ 47 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGY OF THE IGNEOUS ROCKS OF THE SPANISH PEAKS REGION, COLORADO By Ross B. JOHNSON ABSTRACT Chemically, the rocks range from felsic to ultramafic and from The Spanish Peaks region includes nearly 2,000 square miles oversaturated to undersaturated types. They are also peralumi in Las Animas and Huerfano as well as small parts of Costilla nous to subaluminous and calcic to alkalic. and Custer Counties in south-central Colorado. The region is mainly an upland area in the westernmost part of the Great INTRODUCTION Plains, and it borders the eastern foothills of the Sangre de Cristo Mountains. Maximum altitudes are 13,623 feet on West September 10.-We soon came in .sight of "Las Cumbres Espan Spanish Peak and 12,708 feet on East Spanish Peak. Regionally, olas" or the Spanish Peaks, their twinned summits towering the land slopes in all directions from the Spanish Peaks, but above the clouds that drifted midway up their sides. Our route the major streams drain northeastward to the Arkansas River. bore direct for the peaks. The Spanish Peaks region and adjoining areas in Colorado and September· 14.-I made a sketch of the Spanish Peaks; there New Mexico are well known to geologists for highly diverse were light clouds hanging around them, but although they lent igneous rocks and structures. The topography is dominated great beauty to the mountains by ever v·arying contour of their mainly by intrusive and extrusive igneous rocks ; stocks, lacco shadows, that curved about in "mazes intricate, eccentric, in liths, sole injections, plugs, dikes, and sills have invaded the tervolved, yet regular, when most irregular they .seem," and the sedimentary rocks over the entire region, and many of these rays of light that pierced these clouds were ever changing; thus, intrusives form mountains, hills, or buttes that dominate the the same scene presented endless variety. landscape. Basaltic lava sheets cap high mesas east of Lt. J. W. Abert ... 1846 (1848) Trinidad, Colo., and Raton, N. Mex. Lava sheets also cap sedi mentary rocks north of Huerfano Park and in the San Luis The beauty of the Spanish Peaks of southern Colo Valley as well as sedimentary rocks and Precambrian igneous rado has inspired men for untold centuries. With the and metamorphic rocks in the Sangre de Cristo and Wet Moun majestic Sangre de Cristo Mountains as their backdrop tains. Volcanic cones are scattered throughout the plains east the summits of these twin peaks tower more than 6,500 of Raton and occur