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Research Article| February 01, 1976 The 1.7- to 1.8-b.y.-old trondhjemites of southwestern Colorado and northern New Mexico: Geochemistry and depths of genesis FRED BARKER; FRED BARKER 1U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar J. G. ARTH; J. G. ARTH 1U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Z. E. PETERMAN; Z. E. PETERMAN 1U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar IRVING FRIEDMAN IRVING FRIEDMAN 1U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1976) 87 (2): 189–198. https://doi.org/10.1130/0016-7606(1976)87<189:TTBTOS>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation FRED BARKER, J. G. ARTH, Z. E. PETERMAN, IRVING FRIEDMAN; The 1.7- to 1.8-b.y.-old trondhjemites of southwestern Colorado and northern New Mexico: Geochemistry and depths of genesis. GSA Bulletin 1976;; 87 (2): 189–198. doi: https://doi.org/10.1130/0016-7606(1976)87<189:TTBTOS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Four trondhjemitic bodies — three of intrusive and one of extrusive origin — 1.7 to 1.8 b.y. in age occur in the Precambrian rocks of northern New Mexico and southwestern Colorado. These are the metamorphosed plutonic or hypabyssal trondhjemite of Rio Brazos, New Mexico, the interlayered quartzofeldspathic and metabasaltic metavolcanic Twilight Gneiss of the West Needle Mountains, Colorado, the syntectonic Pitts Meadow Granodiorite of the Black Canyon of the Gunnison River, Colorado, and the late syntectonic to posttectonic Kroenke Granodiorite of the Central Sawatch Range, Colorado.From south to north, over a distance of 235 km, the four rock units show systematic increases in average Al2O3 from 13.7 to 16.1 percent, in K2O from 1.5 to 2.6 percent, in Rb from 28 to 76 ppm, and in Sr from 101 to 547 ppm. Initial Sr87/Sr86 ratios are low — 0.7015 to 0.7027 — and suggest a mafic or ultramafic source. All four trondhjemite bodies have similar light rare-earth element (REE) contents. The trondhjemites of Rio Brazos and the Twilight Gneiss have relatively flat patterns (Ce/Yb < 5) and large negative Eu anomalies. The Pitts Meadow and Kroenke Granodiorites have fractionated REE patterns (Ce/Yb > 10) with low heavy rare earth content and small or no Eu anomalies. Whole-rock δO18 values for siliceous rocks of three of the bodies range from 5.8 to 8.0 per mil, although the Pitts Meadow Granodiorite gives values of 8.5 to 9.4 per mil.The parent magmas for these bodies were probably generated from a parental basaltic source, either by partial melting or fractional crystallization. Fractional crystallization mechanisms would operate at crustal levels where crystallization of plagioclase and clinopyroxene or hornblende would produce the Rio Brazos and Twilight magmas, and crystallization of hornblende, plagioclase, and biotite would produce the Kroenke and Pitts Meadows magmas. The preferred partial melting mechanism would produce the Rio Brazos and Twilight magmas at shallow depth (< 50 km), leaving a residue of plagioclase and clinopyroxene or amphibole; the Pitts Meadow magma at 50 to 60 km, where hornblende, garnet, clinopyroxene, and plagioclase would be residual; and the Kroenke magma at greater than 60 km leaving a residue of garnet and clinopyroxene.The magmas probably formed in a ridge-and-basin complex that lay between the early Precambrian craton to the north and the contemporaneous quartzite-rhyolite-tholeiite terrane to the south. A northward-dipping subduction zone can be postulated from the variation in compositions and inferred depths of melting, but complete modern analogues of similar setting are not known. A better tectonic analogue might be the Archean regimes, in which vertical motion is dominant and trondhjemitic magmas may have formed by melting at the base of foundering thick volcanic piles. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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