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Title: Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado

Abstract

The Guffey volcanic center is the largest within the 2000 km{sup 2} mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which present the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the Guffey center was followed by extrusion of a thick series of basalt, trachybasalt, and shoshonite flows and lahars. Plugs, dikes, and vents ranging from basalt to rhyolite cut the thick mafic deposits, and felsic tuffs breccias chemically identical to the small rhyolitic plutons are locally preserved. Whole-rack major and trace element analyses of 80 samples, ranging almost continuously from 47% to 78% SiO{sub 2}, indicate that the rocks of the Guffey center are among the most highly enriched in K{sub 2}O (up to 6%) and rare earth elements (typically 200-300 ppm) of any volcanic rocks in Colorado. These observations, along with the relatively high concentrations of Ba and Rb and the depletion of Cr and Ni, suggest an appreciable contribution of lower crustal material to the magmas that produced the Thirtynine Mile volcanic rocks.

Authors:
;  [1]; ; ;  [2]; ;  [3]; ;  [4];  [5];  [6];  [7]
  1. (Williams College, Williamstown, MA (USA))
  2. (Franklin Marshall College, Lancaster, PA (USA))
  3. (Colorado College, Colorado Springs (USA))
  4. (Carleton College, Northfield, MN (USA))
  5. (Smith College, Northampton, MA (USA))
  6. (Beloit College, WI (USA))
  7. (Whitman College, Walla Walla, WA (USA))
Publication Date:
OSTI Identifier:
6140437
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geology; (USA); Journal Volume: 18:7
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; COLORADO; VOLCANIC REGIONS; GEOCHEMISTRY; BARIUM; BASALT; CHEMICAL ANALYSIS; CHROMIUM; CONTINENTAL CRUST; DIKES; ENRICHMENT; EROSION; EXTRUSION; MAGMA; MIXING; MULTI-ELEMENT ANALYSIS; NICKEL; POTASSIUM OXIDES; RARE EARTHS; RHYOLITES; RUBIDIUM; SILICON OXIDES; SOLIDS FLOW; TERTIARY PERIOD; TRACE AMOUNTS; TUFF; VOLCANIC ROCKS; ALKALI METAL COMPOUNDS; ALKALI METALS; ALKALINE EARTH METALS; CENOZOIC ERA; CHALCOGENIDES; CHEMISTRY; EARTH CRUST; ELEMENTS; FABRICATION; FEDERAL REGION VIII; FLUID FLOW; GEOLOGIC AGES; GEOLOGIC STRUCTURES; IGNEOUS ROCKS; MATERIALS WORKING; METALS; NORTH AMERICA; OXIDES; OXYGEN COMPOUNDS; POTASSIUM COMPOUNDS; ROCKS; SILICON COMPOUNDS; TRANSITION ELEMENTS; USA; 580000* - Geosciences

Citation Formats

Wobus, R.A., Mochel, D.W., Mertzman, S.A., Eide, E.A., Rothwarf, M.T., Loeffler, B.M., Johnson, D.A., Keating, G.N., Sultz, K., Benjamin, A.E., Venzke, E.A., and Filson, T.. Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado. United States: N. p., 1990. Web. doi:10.1130/0091-7613(1990)018<0642:GOHPRF>2.3.CO;2.
Wobus, R.A., Mochel, D.W., Mertzman, S.A., Eide, E.A., Rothwarf, M.T., Loeffler, B.M., Johnson, D.A., Keating, G.N., Sultz, K., Benjamin, A.E., Venzke, E.A., & Filson, T.. Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado. United States. doi:10.1130/0091-7613(1990)018<0642:GOHPRF>2.3.CO;2.
Wobus, R.A., Mochel, D.W., Mertzman, S.A., Eide, E.A., Rothwarf, M.T., Loeffler, B.M., Johnson, D.A., Keating, G.N., Sultz, K., Benjamin, A.E., Venzke, E.A., and Filson, T.. 1990. "Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado". United States. doi:10.1130/0091-7613(1990)018<0642:GOHPRF>2.3.CO;2.
@article{osti_6140437,
title = {Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado},
author = {Wobus, R.A. and Mochel, D.W. and Mertzman, S.A. and Eide, E.A. and Rothwarf, M.T. and Loeffler, B.M. and Johnson, D.A. and Keating, G.N. and Sultz, K. and Benjamin, A.E. and Venzke, E.A. and Filson, T.},
abstractNote = {The Guffey volcanic center is the largest within the 2000 km{sup 2} mid-Tertiary Thirtynine Mile volcanic field of central Colorado. This study is the first to provide extensive chemical data for these alkalic volcanic and subvolcanic rocks, which present the eroded remnants of a large stratovolcano of Oligocene age. Formation of early domes and flows of latite and trachyte within the Guffey center was followed by extrusion of a thick series of basalt, trachybasalt, and shoshonite flows and lahars. Plugs, dikes, and vents ranging from basalt to rhyolite cut the thick mafic deposits, and felsic tuffs breccias chemically identical to the small rhyolitic plutons are locally preserved. Whole-rack major and trace element analyses of 80 samples, ranging almost continuously from 47% to 78% SiO{sub 2}, indicate that the rocks of the Guffey center are among the most highly enriched in K{sub 2}O (up to 6%) and rare earth elements (typically 200-300 ppm) of any volcanic rocks in Colorado. These observations, along with the relatively high concentrations of Ba and Rb and the depletion of Cr and Ni, suggest an appreciable contribution of lower crustal material to the magmas that produced the Thirtynine Mile volcanic rocks.},
doi = {10.1130/0091-7613(1990)018<0642:GOHPRF>2.3.CO;2},
journal = {Geology; (USA)},
number = ,
volume = 18:7,
place = {United States},
year = 1990,
month = 7
}
  • A wide range of rock types (abyssal tholeiite, Fe-Ti-rich basalt, andesite, and rhyodacite) were dredged from near 95/sup 0/ W and 85/sup 0/ W on the Galapagos spreading center. Computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization. However, the P/sub 2/O/sub 5//K/sub 2/O ratio averages 0.83 at 95/sup 0/W and 1.66 at 85/sup 0/W and implies distinct mantle source compositions for the two areas. These source regions also have different rare earth element (REE) abundance patterns, with (La/Sm)/sub EF/ = 0.67 at 95/sup 0/Wmore » and 0.46 at 85/sup 0/W. The sequence of fractional lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95/sup 0/W. Incompatible trace element abundances in 26 samples are used to infer that the range of Fe-Ti-rich basalt from 85/sup 0/W represents 19 to 35% residual liquid following crystal fractionation of a mineral assemblage of plagioclase, clinopyroxene, and lesser olivine. Most samples from 85/sup 0/W can be related to a common parental magma that contained approximately 9 wt %FeO*, 1 wt % TiO/sub 2/, and had an Mg number (Mg/sup 3/ = 100 Mg/(Mg+Fe/sup 2 +/)) of about 65. Although the samples from 95/sup 0/W cannot all be derived from a common parental magma, the inferred parental magmas may have been derived by varying degrees of partial melting of a common source. The fractionation sequence consists of two parts: an initial iron enrichment trend followed by a silica enrichment trend. The most iron rich lavas represent about 32% residual liquid derived by crystal fractionation of plagioclase, clinopyroxene, and lesser olivine from a parental magma with an Mg number of about 66. The silicic enrichment trend results from crystallization of titanomagnetite and some apatite.« less
  • The possibility of using volcanic glasses for potassium-argon dating is investigated. The following properties must be available in the mineral: it should be relatively widespread in ratural settings; it should be completely outgassed of argon at the time of the geological event it is to mark; it should undergo little loss of argon by diffusion; and it should contain a reasonable amount of potassium. From the mass spectrometer measurements made in this preliminary investigation it is concluded that volcanic glasses have a good possibility of being a useful mineral for K--Ar dating, provided that samples are taken only from fairlymore » large specimens. (N.W.R.)« less
  • The San Juan sag, concealed by the vast San Juan volcanic field of south-central Colorado, has only recently benefited from oil and gas wildcat drilling and evaluations. Sound geochemical analyses and maturation modeling are essential elements for successful exploration and development. Oil has been produced in minor quantities from an Oligocene sill in the Mancos Shale within the sag, and major oil and gas production occurs from stratigraphically equivalent rocks in the San Juan basin to the southwest and in the Denver basin to the northeast. The objectives of this study were to identify potential source rocks, assess thermal maturity,more » and determine hydrocarbon-source bed relationships. Source rocks are present in the San Juan sag in the upper and lower Mancos Shale (including the Niobrara Member), which consists of about 666 m (2184 ft) of marine shale with from 0.5 to 3.1 wt. % organic carbon. Pyrolysis yields (S{sub 1} + S{sub 2} = 2000-6000 ppm) and solvent extraction yields (1000-4000 ppm) indicate that some intervals within the Mancos Shale are good potential source rocks for oil, containing type II organic matter, according to Rock-Eval pyrolysis assay.« less
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