Holocene and Paleogene arkoses of the Massif Central, France; mineralogy, chemistry, provenance, and hydrothermal alteration of the type arkose

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doi: 10.1306/D4267CF3-2B26-11D7-8648000102C1865D
Authors:van de Kamp, Peter C.; Helmold, Kenneth P.; Leake, Bernard E.
Author Affiliations:Primary:
Cornelis Corporation, Helena, CA, United States
Other:
ARCO Alaska, United States
University of Glasgow, United Kingdom
Volume Title:Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes
Source:Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes, 64(1), p.17-33. Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States. ISSN: 1073-130X
Publication Date:1994
Note:In English. 84 refs.; illus. incl. 3 tables, geol. sketch map
Summary:Paleogene arkoses of the Auvergne region of France represent the type arkose originally described by Brongniart (1826). They are alluvial-fan and fluvial deposits including traction-current sandstones (arenites) and matrix-rich debris-flow deposits (wackes). Locally, they have been extensively altered by geothermal waters related to nearby Tertiary-Holocene volcanic activity. The alteration is typified by leaching of detrital grains and precipitation of ubiquitous chert cement. The average Gazzi-Dickinson composition of unaltered arenites is Q40F60L0, with K/F (potash feldspar/total feldspar) = 0.63. Altered arenites have an average composition of Q55F43L2, with K/F= 0.73, reflecting loss of plagioclase due to intense alteration. When secondary chert is included with detrital quartz and chert, the altered rocks have an average composition of Q66F32L1, emphasizing silica enrichment associated with alteration. Holocene sands derived from crystalline basement of the Massif Central have the same provenance as the Paleogene sandstones. They have an average Gazzi-Dickinson composition of Q46F49L5 with K/F = 0.52 and compare favorably with their unaltered ancient counterparts. Arkosic alluvium at Roosevelt Hot Springs, Utah is believed to represent a modern analog for alteration of the Auvergne sandstones. The alluvium has been altered and cemented with silica by geothermal waters at near-surface conditions in the epithermal zone. Chemical analyses of 130 sand and rock samples demonstrate original sediment compositions and changes due to hydrothermal alteration. Holocene sands and unaltered Paleogene clastics are compositionally similar and show variation trends similar to feldspathic clastics derived from granitic basement in other basins. Silica enrichment in the altered sandstones causes dilution of all other elements as an effect of constant summation. Consequently, most element abundances decline proportionately with silica dilution. Na and Rb, however, are reduced below the levels predicted by dilution due to plagioclase and biotite destruction. As a result, K/Rb ratios are higher in the altered sandstones. Ba, S, SO4, As, and Sb are enriched in the altered rocks by precipitation from hydrothermal solutions. Barium is in barite as fracture-filling veins and scattered patches in sandstone matrix. Destruction of mafic minerals during alteration released Fe, Co, Cr, Ni, Zn, Pb, and Th that later precipitated as oxides and sulfides in fractures and locally in chert matrix in the sandstones. [K.A.R.]
Subjects:Arenite; Arkose; Cenozoic; Chemical composition; Clastic rocks; Holocene; Hydrothermal alteration; Metasomatism; Mineral composition; Paleogene; Provenance; Quaternary; Sedimentary rocks; Tertiary; Central Massif; Europe; France; Western Europe
Coordinates:N440000 N470000 E0050000 E0004500
Abstract Numbers:94M/3784
Record ID:1994014295
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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